fix(agent): the IME log tracker verifies and rewinds when a concurrent IME process overwrote bytes behind its bookmark; executor lines belong to the invocation at their file position (D-246)

IME's trace listener opens every log once per process with FileMode.Append and FileShare.ReadWrite
and writes at its own stream position afterwards. AgentExecutor.exe (two processes per user-context
script, one per detection script) has such a listener on AgentExecutor.log and on
IntuneManagementExtension.log, so a process that resumes logging after another one appended
overwrites those bytes: the final file holds the last writer, a byte-position tailer has read the
first version. Session 46749560 lost 6 of 16 platform-script results and 4 of 16 executor end
blocks with every tracker health counter at zero — results fell back to the exit code (one Success
where IME said Failed), one finished script stayed Incomplete. The agentexecutor_fallback results
in the fleet are this race, not late IME logging.

The tracker now keeps a fingerprint ledger of every entry it processed from the two multi-writer
files (FNV-1a 64 over the decoded bytes, one CmTraceEntryAssembler shared by the read loop and the
verification so entry boundaries are identical by construction), verifies it on every script end
signal, on a byte-budgeted cadence while an invocation is in flight (256 KB/s, at least 1 s
apart), in a 4 KB guard zone on every growth pass and on a fragment at the bookmark, and rewinds
to the first changed entry; entries whose (offset, hash) still read unchanged are skipped,
everything else is processed as backlog (never fresh, never a calibration anchor). The
policyId-less executor lines are owned by the nearest preceding invocation marker in their file
(PS-AGENT-ARG opens, PS-AGENT-SCRIPT-START upgrades to the platform script, PS-AGENT-INVOCATION and
PS-AGENT-COMPLETED close) instead of a last-started pointer, so an end block a rewind uncovers
reaches its own script. Restart seeds the ledger from the 64 KB behind the persisted bookmark;
a rollover clears it. Four restart-safe counters (ime_overwrite_rewinds,
ime_overwrite_bytes_reprocessed, ime_verify_passes, ime_verified_bytes) ride agent_metrics_snapshot
and ime_pattern_hits — recovered data, not skipped work, no new event type.

Tests: ImeLogTrackerOverwriteRewindTests (15) drive the positional-overwrite write pattern against
both files, including the two field scenarios with the shipped patterns; agent suite 2921 green,
backend known-actions list extended for the two new patterns.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

rules/dist/ime-log-patterns.json

@@ -646,6 +646,22 @@
       "description": "Detects WinGet app installation progress with Operation Phase = Installing (format variant 2).",
       "enabled": true
     },
+    {
+      "patternId": "PS-AGENT-ARG",
+      "category": "always",
+      "pattern": "^Adding argument (?<kind>[A-Za-z0-9]+) with value ",
+      "action": "agentInvocationArgument",
+      "description": "The first argument line of every AgentExecutor.exe invocation (powershell, remediationScript, detectionScript, requirementScript, ...). Opens the invocation that owns the policyId-less exit/output lines following it in AgentExecutor.log; PS-AGENT-SCRIPT-START upgrades a powershell invocation to its platform script. Also keeps the tracker's overwrite check active while the invocation runs.",
+      "enabled": true
+    },
+    {
+      "patternId": "PS-AGENT-COMPLETED",
+      "category": "always",
+      "pattern": "^Agent executor completed\\.",
+      "action": "agentExecutorCompleted",
+      "description": "Last line of an AgentExecutor.exe invocation. Closes the invocation's ownership of the exit/output lines in AgentExecutor.log and triggers the tracker's overwrite check: the executor wrote its end block at its own stream position, which may lie behind the tracker's bookmark when another process appended meanwhile.",
+      "enabled": true
+    },
     {
       "patternId": "PS-AGENT-ERROR",
       "category": "always",

rules/ime-log-patterns/PS-AGENT-ARG.json

@@ -0,0 +1,9 @@
+{
+  "$schema": "../schema/ime-log-pattern.schema.json",
+  "patternId": "PS-AGENT-ARG",
+  "category": "always",
+  "pattern": "^Adding argument (?<kind>[A-Za-z0-9]+) with value ",
+  "action": "agentInvocationArgument",
+  "description": "The first argument line of every AgentExecutor.exe invocation (powershell, remediationScript, detectionScript, requirementScript, ...). Opens the invocation that owns the policyId-less exit/output lines following it in AgentExecutor.log; PS-AGENT-SCRIPT-START upgrades a powershell invocation to its platform script. Also keeps the tracker's overwrite check active while the invocation runs.",
+  "enabled": true
+}

rules/ime-log-patterns/PS-AGENT-COMPLETED.json

@@ -0,0 +1,9 @@
+{
+  "$schema": "../schema/ime-log-pattern.schema.json",
+  "patternId": "PS-AGENT-COMPLETED",
+  "category": "always",
+  "pattern": "^Agent executor completed\\.",
+  "action": "agentExecutorCompleted",
+  "description": "Last line of an AgentExecutor.exe invocation. Closes the invocation's ownership of the exit/output lines in AgentExecutor.log and triggers the tracker's overwrite check: the executor wrote its end block at its own stream position, which may lie behind the tracker's bookmark when another process appended meanwhile.",
+  "enabled": true
+}

rules/schema/ime-log-pattern.schema.json

@@ -71,6 +71,8 @@
             "scriptOutput",
             "scriptCompleted",
             "resetPlatformScriptContext",
+            "agentInvocationArgument",
+            "agentExecutorCompleted",
             "healthScriptResult",
             "healthScriptDetectionResult",
             "captureExitCode",

src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Monitoring/Ime/ImeLogTrackerOverwriteRewindTests.cs

@@ -0,0 +1,527 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Text;
+using System.Text.RegularExpressions;
+using System.Threading;
+using System.Threading.Tasks;
+using AutopilotMonitor.Agent.V2.Core.Logging;
+using AutopilotMonitor.Agent.V2.Core.Monitoring.Enrollment.Ime;
+using AutopilotMonitor.Agent.V2.Core.Tests.Harness;
+using AutopilotMonitor.Shared.Models;
+using Newtonsoft.Json;
+using Xunit;
+
+namespace AutopilotMonitor.Agent.V2.Core.Tests.Monitoring.Ime
+{
+    /// <summary>
+    /// Session 46749560: IME's trace listener writes every log through one FileMode.Append
+    /// stream per process, so a process that resumes logging after another one appended
+    /// overwrites those bytes at its own stale position. The tracker had read the first version
+    /// and never saw the second — 6 of 16 script results and 4 of 16 executor end blocks were
+    /// lost while the final files looked intact. These tests drive that exact write pattern
+    /// (positional overwrite behind the bookmark) against the two multi-writer logs and pin the
+    /// contract: unchanged bytes never rewind, changed bytes are processed exactly once, and the
+    /// executor lines a rewind uncovers belong to the invocation that owns their file position.
+    /// </summary>
+    public sealed class ImeLogTrackerOverwriteRewindTests
+    {
+        private static readonly DateTime T0 = new DateTime(2026, 9, 10, 19, 30, 0, DateTimeKind.Utc);
+
+        private static List<ImeLogPattern> MarkerPatterns() => new List<ImeLogPattern>
+        {
+            new ImeLogPattern
+            {
+                PatternId = "T-MARK", Category = "always", Enabled = true,
+                Pattern = @"^marker (?<n>\d+)", Action = "noop",
+                Parameters = new Dictionary<string, string>(),
+            },
+        };
+
+        private static readonly string[] ScriptPatternIds =
+        {
+            "PS-AGENT-INVOCATION", "PS-AGENT-ARG", "PS-AGENT-SCRIPT-START", "PS-AGENT-EXITCODE",
+            "PS-AGENT-OUTPUT", "PS-AGENT-COMPLETED", "PS-SCRIPT-GENERATED", "PS-SCRIPT-CONTEXT", "PS-SCRIPT-RESULT",
+        };
+
+        /// <summary>The shipped pattern JSON — the same source combine.js embeds — so the contract is the real one.</summary>
+        private static List<ImeLogPattern> ScriptPatterns()
+        {
+            DirectoryInfo? dir = new DirectoryInfo(AppContext.BaseDirectory);
+            string? patternDir = null;
+            while (dir != null)
+            {
+                var candidate = Path.Combine(dir.FullName, "rules", "ime-log-patterns");
+                if (Directory.Exists(candidate)) { patternDir = candidate; break; }
+                dir = dir.Parent;
+            }
+            Assert.NotNull(patternDir);
+
+            var byId = new Dictionary<string, ImeLogPattern>(StringComparer.OrdinalIgnoreCase);
+            foreach (var file in Directory.GetFiles(patternDir, "*.json"))
+            {
+                var pattern = JsonConvert.DeserializeObject<ImeLogPattern>(File.ReadAllText(file));
+                if (pattern?.PatternId != null) byId[pattern.PatternId] = pattern;
+            }
+            return ScriptPatternIds.Select(id =>
+            {
+                Assert.True(byId.TryGetValue(id, out var p), $"Shipped IME pattern '{id}' not found under {patternDir}.");
+                return p!;
+            }).ToList();
+        }
+
+        private static string Entry(string message, string thread = "1")
+            => $"<![LOG[{message}]LOG]!><time=\"12:00:00.0000000\" date=\"9-10-2026\" component=\"X\" context=\"\" type=\"1\" thread=\"{thread}\" file=\"\">\r\n";
+
+        private const string PlatformId = "d39eeef3-e88e-41a0-82d7-5e28ee7b5acf";
+        private const string PlatformStartLine =
+            @"Adding argument powershell with value C:\Program Files (x86)\Microsoft Intune Management Extension\Policies\Scripts\73d664e4-0886-4a73-b745-c694da45ddb4_d39eeef3-e88e-41a0-82d7-5e28ee7b5acf.ps1 to the named argument list.";
+        private const string DetectionStartLine =
+            @"Adding argument detectionScript with value C:\Windows\IMECache\DetectionScripts\App_cc46aa62-e93a-41a1-bdd0-44f57e92a66d.ps1 to the named argument list.";
+        private const string GeneratedLine =
+            @"Script file C:\Program Files (x86)\Microsoft Intune Management Extension\Policies\Scripts\73d664e4-0886-4a73-b745-c694da45ddb4_d39eeef3-e88e-41a0-82d7-5e28ee7b5acf.ps1 is generated.";
+        private const string ResultLine =
+            "[PowerShell] User Id = 73d664e4-0886-4a73-b745-c694da45ddb4, Policy id = " + PlatformId + ", policy result = Success";
+
+        private sealed class Harness : IDisposable
+        {
+            private readonly TempDirectory _tmp = new TempDirectory();
+            private readonly string _matchLog;
+            private readonly List<ImeLogPattern> _patterns;
+            public ImeLogTracker Tracker { get; private set; }
+            public DateTime Now { get; set; } = T0;
+            public string LogName { get; }
+            public string LogPath => Path.Combine(_tmp.Path, LogName);
+            public string StateDir { get; }
+            public List<ScriptExecutionState> Completed { get; } = new List<ScriptExecutionState>();
+
+            public Harness(string logName, List<ImeLogPattern> patterns)
+            {
+                LogName = logName;
+                _patterns = patterns;
+                var logDir = Path.Combine(_tmp.Path, "agent");
+                Directory.CreateDirectory(logDir);
+                _matchLog = Path.Combine(logDir, "ime-pattern-matches.log");
+                StateDir = Path.Combine(_tmp.Path, "state");
+                Tracker = Build();
+            }
+
+            private ImeLogTracker Build()
+            {
+                var t = new ImeLogTracker(_tmp.Path, _patterns, new AgentLogger(Path.Combine(_tmp.Path, "agent"), AgentLogLevel.Debug),
+                    matchLogPath: _matchLog, stateDirectory: StateDir);
+                t.UtcNowProvider = () => Now;
+                t.OnScriptCompleted = s => Completed.Add(s);
+                return t;
+            }
+
+            /// <summary>Agent restart: dispose, rebuild over the same state directory, restore the bookmark.</summary>
+            public void Restart()
+            {
+                Tracker.SaveStateForTest();
+                Tracker.Dispose();
+                Tracker = Build();
+                Tracker.LoadStateForTest();
+            }
+
+            public void Append(string text) => File.AppendAllText(LogPath, text, new UTF8Encoding(false));
+
+            /// <summary>What IME's stale writer does: write at its own position, over whatever is there.</summary>
+            public void Overwrite(long offset, string text)
+            {
+                using (var fs = new FileStream(LogPath, FileMode.Open, FileAccess.Write, FileShare.ReadWrite))
+                {
+                    fs.Seek(offset, SeekOrigin.Begin);
+                    var bytes = new UTF8Encoding(false).GetBytes(text);
+                    fs.Write(bytes, 0, bytes.Length);
+                }
+            }
+
+            public long Length => new FileInfo(LogPath).Length;
+
+            public Task Pass(CancellationToken token = default) => Tracker.CheckLogFilesAsync(token);
+
+            /// <summary>Marker numbers matched so far, in match order.</summary>
+            public List<int> Matched()
+            {
+                if (!File.Exists(_matchLog)) return new List<int>();
+                return File.ReadAllLines(_matchLog)
+                    .Select(l => Regex.Match(l, @"\[T-MARK\] <!\[LOG\[marker (\d+)"))
+                    .Where(m => m.Success).Select(m => int.Parse(m.Groups[1].Value)).ToList();
+            }
+
+            public ImeTrackerHealth Health => Tracker.GetHealthSnapshot();
+
+            public void Dispose()
+            {
+                Tracker.Dispose();
+                _tmp.Dispose();
+            }
+        }
+
+        // -----------------------------------------------------------------------
+        // Marker-level contract on AgentExecutor.log
+        // -----------------------------------------------------------------------
+
+        [Fact]
+        public async Task Unchanged_bytes_never_rewind_however_often_they_are_verified()
+        {
+            using var h = new Harness("AgentExecutor.log", MarkerPatterns());
+            h.Append(Entry("marker 1") + "<![LOG[marker 2\nsecond line\n]LOG]!><time=\"12:00:00.0000000\" date=\"9-10-2026\" component=\"X\" context=\"\" type=\"1\" thread=\"1\" file=\"\">\r\n" + Entry("marker 3"));
+            await h.Pass();
+            Assert.Equal(new[] { 1, 2, 3 }, h.Matched());
+
+            for (var i = 0; i < 5; i++)
+            {
+                h.Tracker.RequestOverwriteCheckForTest();
+                h.Now = h.Now.AddSeconds(2);
+                await h.Pass();
+            }
+
+            Assert.Equal(new[] { 1, 2, 3 }, h.Matched());
+            Assert.Equal(0, h.Health.OverwriteRewinds);
+            Assert.Equal(5, h.Health.VerifyPasses);
+            Assert.Equal(3, h.Tracker.LedgerEntryCountForTest("AgentExecutor.log"));
+        }
+
+        [Fact]
+        public async Task Non_multi_writer_files_keep_no_ledger_and_are_never_verified()
+        {
+            using var h = new Harness("AppWorkload.log", MarkerPatterns());
+            h.Append(Entry("marker 1") + Entry("marker 2"));
+            await h.Pass();
+            h.Tracker.RequestOverwriteCheckForTest();
+            h.Now = h.Now.AddSeconds(2);
+            await h.Pass();
+
+            Assert.Equal(new[] { 1, 2 }, h.Matched());
+            Assert.Equal(0, h.Tracker.LedgerEntryCountForTest("AppWorkload.log"));
+            Assert.Equal(0, h.Health.VerifyPasses);
+            Assert.Equal(0, h.Health.VerifiedBytes);
+        }
+
+        [Fact]
+        public async Task Block_written_over_read_bytes_is_processed_once_and_the_surviving_tail_is_not_repeated()
+        {
+            using var h = new Harness("AgentExecutor.log", MarkerPatterns());
+            var one = Entry("marker 1");
+            h.Append(one + Entry("marker 2") + Entry("marker 3"));
+            await h.Pass();
+            Assert.Equal(new[] { 1, 2, 3 }, h.Matched());
+
+            // A stale writer lays two entries over the first two (same length: marker 3 survives
+            // intact at its old offset). No growth, no fragment — only the end signal finds it.
+            h.Overwrite(0, Entry("marker 7") + Entry("marker 8"));
+            Assert.Equal(3 * one.Length, h.Length);
+            h.Tracker.RequestOverwriteCheckForTest();
+            h.Now = h.Now.AddMilliseconds(100);
+            await h.Pass();
+
+            Assert.Equal(new[] { 1, 2, 3, 7, 8 }, h.Matched());
+            Assert.Equal(1, h.Health.OverwriteRewinds);
+            Assert.Equal(3 * one.Length, h.Health.OverwriteBytesReprocessed);
+            Assert.Equal(3, h.Tracker.LedgerEntryCountForTest("AgentExecutor.log"));
+
+            // Nothing left to find.
+            h.Tracker.RequestOverwriteCheckForTest();
+            h.Now = h.Now.AddSeconds(2);
+            await h.Pass();
+            Assert.Equal(new[] { 1, 2, 3, 7, 8 }, h.Matched());
+            Assert.Equal(1, h.Health.OverwriteRewinds);
+        }
+
+        [Fact]
+        public async Task Block_that_cuts_an_old_entry_leaves_a_fragment_that_is_never_matched()
+        {
+            using var h = new Harness("AgentExecutor.log", MarkerPatterns());
+            h.Append(Entry("marker 1") + Entry("marker 2") + Entry("marker 3"));
+            await h.Pass();
+
+            // Longer than two old entries, shorter than three: the tail of marker 3's line survives
+            // as a fragment behind the new block.
+            h.Overwrite(0, Entry("marker 7") + Entry("marker 8 is a little longer"));
+            h.Tracker.RequestOverwriteCheckForTest();
+            h.Now = h.Now.AddMilliseconds(100);
+            await h.Pass();
+
+            Assert.Equal(new[] { 1, 2, 3, 7, 8 }, h.Matched());
+            Assert.Equal(1, h.Health.OverwriteRewinds);
+        }
+
+        [Fact]
+        public async Task Guard_zone_catches_a_short_block_when_the_file_grows_without_any_signal()
+        {
+            using var h = new Harness("AgentExecutor.log", MarkerPatterns());
+            h.Append(Entry("marker 1") + Entry("marker 2") + Entry("marker 3"));
+            await h.Pass();
+
+            // Same-length replacement of marker 1 (no fragment), then an ordinary append by
+            // another writer: the growth pass re-checks the last 4 KB and finds the change.
+            h.Overwrite(0, Entry("marker 7"));
+            h.Append(Entry("marker 4"));
+            h.Now = h.Now.AddMilliseconds(100);
+            await h.Pass();
+
+            Assert.Equal(new[] { 1, 2, 3, 7, 4 }, h.Matched());
+            Assert.Equal(1, h.Health.OverwriteRewinds);
+            Assert.Equal(1, h.Health.VerifyPasses); // the full check that follows a guard hit
+            Assert.True(h.Health.VerifiedBytes > 0);
+        }
+
+        [Fact]
+        public async Task Guard_hit_rewinds_to_the_true_start_of_the_block_not_the_guard_zone()
+        {
+            using var h = new Harness("AgentExecutor.log", MarkerPatterns());
+            // ~6 KB of entries so the guard zone covers only the last few.
+            var sb = new StringBuilder();
+            for (var i = 1; i <= 60; i++) sb.Append(Entry($"marker {i}"));
+            h.Append(sb.ToString());
+            await h.Pass();
+            Assert.Equal(60, h.Matched().Count);
+
+            // A block over everything plus one more entry: the guard sees a change at its
+            // zone start, the full check must place the rewind at offset 0.
+            var block = new StringBuilder();
+            for (var i = 101; i <= 161; i++) block.Append(Entry($"marker {i}"));
+            h.Overwrite(0, block.ToString());
+            h.Now = h.Now.AddMilliseconds(100);
+            await h.Pass();
+
+            var matched = h.Matched();
+            Assert.Equal(60 + 61, matched.Count);
+            Assert.Equal(Enumerable.Range(101, 61), matched.Skip(60));
+            Assert.Equal(1, h.Health.OverwriteRewinds);
+        }
+
+        [Fact]
+        public async Task Fragment_at_the_bookmark_triggers_the_check_when_no_ledger_entry_starts_in_the_guard_zone()
+        {
+            using var h = new Harness("AgentExecutor.log", MarkerPatterns());
+            var one = Entry("marker 1");
+            var big = Entry("marker 2 " + new string('x', 6000));
+            h.Append(one + big);
+            await h.Pass();
+            Assert.Equal(new[] { 1, 2 }, h.Matched());
+
+            // The 6 KB entry is the last ledger entry and starts before the 4 KB guard zone, so
+            // the guard has nothing to check. The stale writer's block is longer than that entry:
+            // the pass after it starts mid-line — a fragment — and must not match it raw.
+            h.Overwrite(one.Length, Entry("marker 7 " + new string('y', 3000)) + Entry("marker 8 " + new string('z', 3100)));
+            h.Now = h.Now.AddMilliseconds(100);
+            await h.Pass();
+            Assert.Equal(new[] { 1, 2 }, h.Matched());
+
+            h.Now = h.Now.AddMilliseconds(100);
+            await h.Pass();
+            Assert.Equal(new[] { 1, 2, 7, 8 }, h.Matched());
+            Assert.Equal(1, h.Health.OverwriteRewinds);
+        }
+
+        [Fact]
+        public async Task Hidden_block_is_found_by_the_cadence_while_a_script_is_in_flight_and_the_cadence_is_throttled()
+        {
+            using var h = new Harness("AgentExecutor.log", MarkerPatterns());
+            h.Append(Entry("marker 1") + Entry("marker 2") + Entry("marker 3"));
+            await h.Pass();
+
+            // A platform script is pending → contention. First pass in contention verifies at once.
+            h.Tracker.SeedPendingPlatformScriptForTesting("policyX", exitCode: null, exitObservedAtUtc: null, startedAtUtc: T0);
+            h.Now = T0.AddMilliseconds(100);
+            await h.Pass();
+            Assert.Equal(1, h.Health.VerifyPasses);
+
+            // Same-length replacement, no growth, no signal: only the cadence can find it —
+            // and not before a second has passed since the last verification.
+            h.Overwrite(0, Entry("marker 7"));
+            h.Now = T0.AddMilliseconds(500);
+            await h.Pass();
+            Assert.Equal(new[] { 1, 2, 3 }, h.Matched());
+            Assert.Equal(1, h.Health.VerifyPasses);
+
+            h.Now = T0.AddMilliseconds(1200);
+            await h.Pass();
+            Assert.Equal(new[] { 1, 2, 3, 7 }, h.Matched());
+            Assert.Equal(2, h.Health.VerifyPasses);
+            Assert.Equal(1, h.Health.OverwriteRewinds);
+        }
+
+        [Fact]
+        public async Task Without_contention_or_signal_nothing_is_verified()
+        {
+            using var h = new Harness("AgentExecutor.log", MarkerPatterns());
+            h.Append(Entry("marker 1") + Entry("marker 2"));
+            await h.Pass();
+
+            for (var i = 0; i < 20; i++)
+            {
+                h.Now = h.Now.AddSeconds(1);
+                await h.Pass();
+            }
+
+            Assert.Equal(0, h.Health.VerifyPasses);
+            Assert.Equal(0, h.Health.VerifiedBytes);
+        }
+
+        [Fact]
+        public async Task Restart_seeds_the_ledger_so_an_overwrite_after_the_restart_is_still_found()
+        {
+            using var h = new Harness("AgentExecutor.log", MarkerPatterns());
+            h.Append(Entry("marker 1") + Entry("marker 2") + Entry("marker 3"));
+            await h.Pass();
+            h.Restart();
+            Assert.Equal(0, h.Tracker.LedgerEntryCountForTest("AgentExecutor.log"));
+
+            // First pass after the restart: nothing new to read, but the ledger is seeded from
+            // the bytes behind the persisted bookmark.
+            h.Now = h.Now.AddMilliseconds(100);
+            await h.Pass();
+            Assert.Equal(3, h.Tracker.LedgerEntryCountForTest("AgentExecutor.log"));
+
+            h.Overwrite(0, Entry("marker 7"));
+            h.Tracker.RequestOverwriteCheckForTest();
+            h.Now = h.Now.AddMilliseconds(100);
+            await h.Pass();
+
+            Assert.Equal(new[] { 1, 2, 3, 7 }, h.Matched());
+            Assert.Equal(1, h.Health.OverwriteRewinds);
+
+            // The counter is part of the persisted health.
+            h.Restart();
+            Assert.Equal(1, h.Health.OverwriteRewinds);
+        }
+
+        [Fact]
+        public async Task Rollover_clears_the_ledger_and_the_new_file_is_read_without_a_false_rewind()
+        {
+            using var h = new Harness("AgentExecutor.log", MarkerPatterns());
+            h.Append(Entry("marker 1") + Entry("marker 2") + Entry("marker 3"));
+            await h.Pass();
+
+            File.WriteAllText(h.LogPath, Entry("marker 7") + Entry("marker 8"), new UTF8Encoding(false));
+            h.Now = h.Now.AddMilliseconds(100);
+            await h.Pass();
+            Assert.Equal(new[] { 1, 2, 3, 7, 8 }, h.Matched());
+
+            h.Tracker.RequestOverwriteCheckForTest();
+            h.Now = h.Now.AddSeconds(2);
+            await h.Pass();
+            Assert.Equal(new[] { 1, 2, 3, 7, 8 }, h.Matched());
+            Assert.Equal(0, h.Health.OverwriteRewinds);
+            Assert.Equal(2, h.Tracker.LedgerEntryCountForTest("AgentExecutor.log"));
+        }
+
+        [Fact]
+        public async Task Cancelled_pass_does_not_lose_a_pending_check()
+        {
+            using var h = new Harness("AgentExecutor.log", MarkerPatterns());
+            h.Append(Entry("marker 1") + Entry("marker 2") + Entry("marker 3"));
+            await h.Pass();
+            h.Overwrite(0, Entry("marker 7") + Entry("marker 8"));
+            h.Tracker.RequestOverwriteCheckForTest();
+            h.Now = h.Now.AddMilliseconds(100);
+
+            using (var cts = new CancellationTokenSource())
+            {
+                cts.Cancel();
+                await h.Pass(cts.Token);
+            }
+
+            // The cancelled pass touched nothing after the rewind; the next one reads it all.
+            h.Now = h.Now.AddMilliseconds(100);
+            await h.Pass();
+            Assert.Equal(new[] { 1, 2, 3, 7, 8 }, h.Matched());
+        }
+
+        // -----------------------------------------------------------------------
+        // The field scenario with the shipped patterns
+        // -----------------------------------------------------------------------
+
+        [Fact]
+        public async Task Executor_end_block_written_over_a_detection_invocation_belongs_to_its_own_script()
+        {
+            using var h = new Harness("AgentExecutor.log", ScriptPatterns());
+
+            // Executor E starts its platform script and waits for PowerShell.
+            h.Append(Entry("ExecutorLog AgentExecutor gets invoked") + Entry(PlatformStartLine) + Entry("[Executor] created powershell with process id 12708"));
+            await h.Pass();
+            var executorPosition = h.Length;
+
+            // Detection executor D appends its whole run while E waits — the tracker reads it.
+            var detectionBlock = Entry("ExecutorLog AgentExecutor gets invoked") + Entry(DetectionStartLine)
+                + Entry("Powershell exit code is 3") + Entry("write output done. output = detected nothing, error = ")
+                + Entry("Agent executor completed.") + Entry(@"Writing result: False to resultFilePath: C:\x\App_cc46aa62.txt");
+            h.Append(detectionBlock);
+            h.Now = h.Now.AddMilliseconds(100);
+            await h.Pass();
+
+            // E ends: its end block lands at its own stale position, over D's lines.
+            var endBlock = Entry("Powershell exit code is 0") + Entry("write output done. output = Hello from E, error = ") + Entry("Agent executor completed.");
+            Assert.True(endBlock.Length < detectionBlock.Length);
+            h.Overwrite(executorPosition, endBlock);
+
+            // Nothing visible signalled the end; the cadence (E is pending) finds the block.
+            h.Now = h.Now.AddSeconds(1.5);
+            await h.Pass();
+            Assert.Equal(1, h.Health.OverwriteRewinds);
+
+            // IME's result line (in the other file) completes the script with E's own data.
+            h.Tracker.ProcessLogMessageForTest(ResultLine);
+            var script = Assert.Single(h.Completed);
+            Assert.Equal(PlatformId, script.PolicyId);
+            Assert.Equal("Success", script.Result);
+            Assert.Equal("ime_policy_result", script.ResultSource);
+            Assert.Equal(0, script.ExitCode);
+            Assert.Equal("Hello from E", script.Stdout);
+        }
+
+        [Fact]
+        public async Task Detection_exit_lines_read_before_the_overwrite_never_reach_the_platform_script()
+        {
+            using var h = new Harness("AgentExecutor.log", ScriptPatterns());
+            h.Append(Entry("ExecutorLog AgentExecutor gets invoked") + Entry(PlatformStartLine) + Entry("[Executor] created powershell with process id 1"));
+            await h.Pass();
+            h.Append(Entry("ExecutorLog AgentExecutor gets invoked") + Entry(DetectionStartLine)
+                + Entry("Powershell exit code is 3") + Entry("write output done. output = detected nothing, error = ") + Entry("Agent executor completed."));
+            h.Now = h.Now.AddMilliseconds(100);
+            await h.Pass();
+
+            // The platform result arrives without any end block ever becoming visible.
+            h.Tracker.ProcessLogMessageForTest(ResultLine);
+            var script = Assert.Single(h.Completed);
+            Assert.Equal("Success", script.Result);
+            Assert.Null(script.ExitCode);
+            Assert.Null(script.Stdout);
+        }
+
+        [Fact]
+        public async Task Ime_result_line_hidden_by_an_executor_start_line_is_recovered_from_the_ime_log()
+        {
+            using var h = new Harness("IntuneManagementExtension.log", ScriptPatterns());
+            h.Append(Entry(GeneratedLine, "17") + Entry("Launch powershell executor in user session", "17") + Entry("process id = 10608", "17"));
+            await h.Pass();
+            var servicePosition = h.Length;
+
+            // The executor process appends its telemetry start line at the true end of file.
+            h.Append(Entry("[Telemetry] Telemetry is enabled. Starting mananger...", "1"));
+            h.Now = h.Now.AddMilliseconds(100);
+            await h.Pass();
+            Assert.Empty(h.Completed);
+
+            // The service's first write after the script wait lands at ITS position — over the
+            // executor's line; the block is longer, so the pass sees a fragment at the bookmark.
+            h.Overwrite(servicePosition, Entry("Execution is done, collecting result", "17") + Entry(ResultLine, "17") + Entry("[PowerShell] Success, Result details length: 315", "17"));
+            h.Now = h.Now.AddMilliseconds(100);
+            await h.Pass();
+            h.Now = h.Now.AddMilliseconds(100);
+            await h.Pass();
+
+            var script = Assert.Single(h.Completed);
+            Assert.Equal(PlatformId, script.PolicyId);
+            Assert.Equal("Success", script.Result);
+            Assert.Equal("ime_policy_result", script.ResultSource);
+            Assert.Equal("User", script.RunContext);
+            Assert.Equal(1, h.Health.OverwriteRewinds);
+        }
+    }
+}

src/Agent/AutopilotMonitor.Agent.V2.Core/Monitoring/Enrollment/Ime/BoundedLineReader.cs

@@ -33,6 +33,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Enrollment.Ime
 
         private readonly Stream _stream;
         private readonly int _maxLineBytes;
+        private readonly bool _hashLines;
         private readonly byte[] _readBuffer = new byte[ReadBufferSize];
         private int _readPos;
         private int _readLen;
@@ -41,21 +42,32 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Enrollment.Ime
 
         private byte[] _line = new byte[4096];
         private int _lineLen;
+        private ulong _lineHash;
 
         /// <param name="stream">Seekable stream positioned at the first byte to read.</param>
         /// <param name="maxLineBytes">Upper bound of bytes kept per line; the remainder is discarded.</param>
-        public BoundedLineReader(Stream stream, int maxLineBytes)
+        /// <param name="hashLines">Fingerprint the raw bytes of every line (<see cref="LastLineHash"/>) — the
+        /// tracker's byte ledger needs it for multi-writer files only.</param>
+        public BoundedLineReader(Stream stream, int maxLineBytes, bool hashLines = false)
         {
             if (stream == null) throw new ArgumentNullException(nameof(stream));
             if (maxLineBytes <= 0) throw new ArgumentOutOfRangeException(nameof(maxLineBytes));
             _stream = stream;
             _maxLineBytes = maxLineBytes;
+            _hashLines = hashLines;
             _readBufferFileOffset = stream.Position;
         }
 
         /// <summary>Cap in bytes applied to every line.</summary>
         public int MaxLineBytes => _maxLineBytes;
 
+        /// <summary>
+        /// FNV-1a 64 over the bytes the most recently returned line was decoded from (BOM and
+        /// line terminator excluded; the capped prefix of a truncated line). Zero unless the
+        /// reader was created with hashing on.
+        /// </summary>
+        public ulong LastLineHash => _lineHash;
+
         /// <summary>Absolute file offset of the next unread byte (exact — no decoder read-ahead).</summary>
         public long Position => _readBufferFileOffset + _readPos;
 
@@ -78,6 +90,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Enrollment.Ime
             LastLineTerminated = false;
             LastLineTruncated = false;
             _lineLen = 0;
+            _lineHash = _hashLines ? Fnv1a64.OffsetBasis : 0UL;
             var sawAnyByte = false;
 
             while (true)
@@ -154,6 +167,16 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Enrollment.Ime
             // tail (and thus any CR) discarded, so the check is skipped there.
             if (LastLineTerminated && !LastLineTruncated && len > 0 && _line[start + len - 1] == (byte)'\r')
                 len--;
+            if (_hashLines)
+            {
+                // Fingerprint exactly the bytes the string is decoded from, so a tail that was
+                // first read unterminated and later gained its CRLF hashes the same.
+                var h = Fnv1a64.OffsetBasis;
+                var end = start + len;
+                for (var i = start; i < end; i++)
+                    h = (h ^ _line[i]) * Fnv1a64.Prime;
+                _lineHash = h;
+            }
             return len == 0 ? string.Empty : Encoding.UTF8.GetString(_line, start, len);
         }
     }

src/Agent/AutopilotMonitor.Agent.V2.Core/Monitoring/Enrollment/Ime/CmTraceEntryAssembler.cs

@@ -0,0 +1,171 @@
+using System.Text;
+
+namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Enrollment.Ime
+{
+    /// <summary>FNV-1a 64-bit — the entry fingerprint used by the tracker's byte ledger.</summary>
+    internal static class Fnv1a64
+    {
+        public const ulong OffsetBasis = 14695981039346656037UL;
+        public const ulong Prime = 1099511628211UL;
+
+        public static ulong Fold(ulong hash, ulong value) => (hash ^ value) * Prime;
+    }
+
+    /// <summary>One assembled CMTrace entry: its byte range in the file, the fingerprint of those bytes and the text.</summary>
+    internal struct AssembledEntry
+    {
+        public long Offset;
+        public int Length;
+        public ulong Hash;
+        public string Text;
+    }
+
+    /// <summary>
+    /// Turns physical lines into CMTrace entries: a line is one entry, unless it opens
+    /// <c>&lt;![LOG[</c> without its <c>]LOG]!&gt;</c> close — then the continuation lines up to the
+    /// closing one belong to it. Oversized lines and capped entries are dropped together with
+    /// their remaining continuation lines, so no fragment is ever matched as an entry.
+    /// <para>
+    /// The read loop and the ledger verification (<c>ImeLogTracker.Overwrite.cs</c>) both feed
+    /// this class, which is what makes their entry boundaries — and therefore the fingerprints
+    /// they compare — identical by construction.
+    /// </para>
+    /// </summary>
+    internal sealed class CmTraceEntryAssembler
+    {
+        public enum Outcome
+        {
+            /// <summary>The line was absorbed into an open multiline entry.</summary>
+            Buffered,
+            /// <summary>An entry is complete.</summary>
+            Entry,
+            /// <summary>A line over the byte cap was dropped (its entry with it).</summary>
+            OversizedDropped,
+            /// <summary>An open multiline entry hit the line/char cap and was dropped.</summary>
+            CapDropped,
+            /// <summary>A continuation line of a dropped entry was skipped.</summary>
+            Skipped,
+        }
+
+        private const string LogOpen = "<![LOG[";
+        private const string LogClose = "]LOG]!>";
+
+        private readonly int _maxLines;
+        private readonly int _maxChars;
+
+        private StringBuilder _buffer;
+        private int _lineCount;
+        private long _entryStart;
+        private ulong _entryHash;
+        private bool _skippingDroppedEntry;
+
+        public CmTraceEntryAssembler(int maxMultiLineBufferLines, int maxEntryBytes)
+        {
+            _maxLines = maxMultiLineBufferLines;
+            _maxChars = maxEntryBytes;
+        }
+
+        /// <summary>True while a multiline entry is being assembled.</summary>
+        public bool HasOpenEntry => _buffer != null;
+
+        /// <summary>File offset of the open multiline entry (valid while <see cref="HasOpenEntry"/>).</summary>
+        public long OpenEntryStart => _entryStart;
+
+        /// <summary>Line count of the entry dropped by the last <see cref="Outcome.CapDropped"/>.</summary>
+        public int DroppedLines { get; private set; }
+
+        /// <summary>Char count of the entry dropped by the last <see cref="Outcome.CapDropped"/>.</summary>
+        public int DroppedChars { get; private set; }
+
+        /// <summary>
+        /// Feeds one physical line. <paramref name="lineStart"/> / <paramref name="positionAfter"/>
+        /// are the line's file offsets (start, next unread byte), <paramref name="lineHash"/> the
+        /// reader's raw-byte fingerprint of the line.
+        /// </summary>
+        public Outcome Feed(string line, long lineStart, long positionAfter, bool truncated, ulong lineHash, out AssembledEntry entry)
+        {
+            entry = default(AssembledEntry);
+
+            if (truncated)
+            {
+                // The capped prefix only tells whether the line opened an entry whose remaining
+                // lines must be skipped rather than matched as raw text.
+                if (_buffer != null)
+                {
+                    _buffer = null;
+                    _lineCount = 0;
+                    _skippingDroppedEntry = true;
+                }
+                else if (line.StartsWith(LogOpen) && !line.Contains(LogClose))
+                {
+                    _skippingDroppedEntry = true;
+                }
+                return Outcome.OversizedDropped;
+            }
+
+            if (_skippingDroppedEntry)
+            {
+                // A line opening a new entry ends the skip and is processed itself — checked
+                // BEFORE the close-tag test, because a complete single-line entry contains both.
+                if (line.StartsWith(LogOpen))
+                    _skippingDroppedEntry = false;
+                else
+                {
+                    if (line.Contains(LogClose)) _skippingDroppedEntry = false;
+                    return Outcome.Skipped;
+                }
+            }
+
+            if (_buffer != null)
+            {
+                _buffer.Append('\n').Append(line);
+                _lineCount++;
+                _entryHash = Fnv1a64.Fold(_entryHash, lineHash);
+
+                if (line.Contains(LogClose))
+                {
+                    entry = new AssembledEntry
+                    {
+                        Offset = _entryStart,
+                        Length = (int)(positionAfter - _entryStart),
+                        Hash = _entryHash,
+                        Text = _buffer.ToString(),
+                    };
+                    _buffer = null;
+                    _lineCount = 0;
+                    return Outcome.Entry;
+                }
+
+                if (_lineCount >= _maxLines || _buffer.Length >= _maxChars)
+                {
+                    DroppedLines = _lineCount;
+                    DroppedChars = _buffer.Length;
+                    _buffer = null;
+                    _lineCount = 0;
+                    _skippingDroppedEntry = true;
+                    return Outcome.CapDropped;
+                }
+
+                return Outcome.Buffered;
+            }
+
+            if (line.StartsWith(LogOpen) && !line.Contains(LogClose))
+            {
+                _buffer = new StringBuilder(line);
+                _lineCount = 1;
+                _entryStart = lineStart;
+                _entryHash = lineHash;
+                return Outcome.Buffered;
+            }
+
+            entry = new AssembledEntry
+            {
+                Offset = lineStart,
+                Length = (int)(positionAfter - lineStart),
+                Hash = lineHash,
+                Text = line,
+            };
+            return Outcome.Entry;
+        }
+    }
+}

src/Agent/AutopilotMonitor.Agent.V2.Core/Monitoring/Enrollment/Ime/ImeLogTracker.Handlers.cs

@@ -222,19 +222,14 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Enrollment.Ime
         // PowerShell script tracking handlers
         // -----------------------------------------------------------------------
 
-        // Returns the current platform-script accumulator, or null when no platform script is active.
+        // Returns the platform-script accumulator that owns the entry being processed, or null.
         // Health-script (remediation) state is no longer tracked line-by-line — the HS-NEW-RESULT
         // pattern delivers the full pre-detection / remediation / post-detection JSON in one shot
         // via HandleHealthScriptResult. The platform branch keeps its line-by-line accumulator
         // because PS-* patterns (PS-SCRIPT-CONTEXT / PS-SCRIPT-EXITCODE / PS-AGENT-OUTPUT / …) still
-        // arrive across multiple log lines.
-        private ScriptExecutionState GetCurrentPlatformScript()
-        {
-            if (!string.IsNullOrEmpty(_lastPlatformScriptPolicyId) &&
-                _pendingPlatformScripts.TryGetValue(_lastPlatformScriptPolicyId, out var state))
-                return state;
-            return null;
-        }
+        // arrive across multiple log lines; ownership is by file position (see
+        // ImeLogTracker.Overwrite.cs), never by which script happened to start last.
+        private ScriptExecutionState GetCurrentPlatformScript() => ResolvePlatformScriptForCurrentEntry();
 
         private void HandleScriptStarted(Match match, Dictionary<string, string> parameters)
         {
@@ -320,7 +315,8 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Enrollment.Ime
                 try { OnScriptStarted?.Invoke(new ScriptStartedInfo { PolicyId = id, ScriptType = "platform" }); }
                 catch (Exception ex) { _logger.Warning($"ImeLogTracker: OnScriptStarted handler threw: {ex.Message}"); }
             }
-            _lastPlatformScriptPolicyId = id;
+            RecordInvocationMarker(id, isClose: false);
+            NoteInvocationActivity();
             // Started lines fire twice per script (agentexecutor + ime source) and carry no
             // outcome — the matching `platform script completed` line below carries result+exit
             // and stays on Info. Keep starts on Debug so Info reflects script outcomes only.
@@ -338,33 +334,34 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Enrollment.Ime
                 : GetCurrentPlatformScript();
         }
 
-        // Session 6b4993e5 fix — platform-script stdout/exit cross-contamination.
-        //
-        // AgentExecutor.exe hosts BOTH platform scripts (…\Policies\Scripts\<id>.ps1) and
-        // proactive-remediation scripts (…\IMECache\HealthScripts\<id>\detect.ps1), and writes
-        // their lines interleaved into the shared AgentExecutor.log. The policyId-less
-        // PS-AGENT-EXITCODE ("Powershell exit code is N") and PS-AGENT-OUTPUT ("write output
-        // done. output = …") lines are routed to whichever platform script
-        // _lastPlatformScriptPolicyId points at. A remediation invocation's start
-        // ("Adding argument remediationScript …") does NOT match PS-AGENT-SCRIPT-START, so it
-        // never moves the pointer — its exit/output then bleed into the last-started PLATFORM
-        // script's slot (observed: platform c3e0124c emitted result=Failed but with exit 0 +
-        // stdout "[Compliant] No Classic Teams found" — the Teams remediation's output).
-        //
-        // Each AgentExecutor process logs "ExecutorLog AgentExecutor gets invoked" first, so a
-        // new banner means the previous invocation's line-capture context is over. Clearing the
-        // pointer here scopes line-capture to a single invocation: the immediately following
-        // "Adding argument powershell …\Policies\Scripts\<id>.ps1" (PS-AGENT-SCRIPT-START)
-        // re-establishes it for a platform invocation, while a remediation invocation leaves it
-        // null so its exit/output are dropped (the remediation's authoritative data still
-        // arrives via HS-NEW-RESULT). The platform script's final result is unaffected — it is
-        // keyed by policyId via PS-SCRIPT-RESULT, not by this pointer.
-        private void HandleAgentExecutorInvocationBoundary()
+        // AgentExecutor.exe hosts platform scripts (…\Policies\Scripts\<id>.ps1) as well as
+        // detection, requirement and proactive-remediation scripts, and its policyId-less
+        // PS-AGENT-EXITCODE / PS-AGENT-OUTPUT lines belong to whichever invocation owns that
+        // file position. The banner closes the previous invocation; the argument line that
+        // follows opens the next one — as a platform script when PS-AGENT-SCRIPT-START matched
+        // it, otherwise as an invocation whose exit/output are not captured (a remediation's
+        // authoritative data arrives via HS-NEW-RESULT; session 6b4993e5 saw its output bleed
+        // into a platform slot). "Agent executor completed" closes the invocation again.
+        private void HandleAgentExecutorInvocation()
         {
-            if (_lastPlatformScriptPolicyId == null) return;
-            _logger.Debug(
-                $"ImeLogTracker: AgentExecutor invocation boundary — clearing platform-script line-capture pointer (was {_lastPlatformScriptPolicyId}).");
-            _lastPlatformScriptPolicyId = null;
+            RecordInvocationMarker(null, isClose: true);
+            NoteInvocationActivity();
+        }
+
+        private void HandleAgentInvocationArgument(string kind)
+        {
+            RecordInvocationMarker(null, isClose: false);
+            NoteInvocationActivity();
+            if (!string.IsNullOrEmpty(kind) && !string.Equals(kind, "powershell", StringComparison.OrdinalIgnoreCase))
+                _logger.Debug($"ImeLogTracker: AgentExecutor invocation of kind '{kind}' — exit/output lines not captured as a platform script");
+        }
+
+        private void HandleAgentExecutorCompleted()
+        {
+            RecordInvocationMarker(null, isClose: true);
+            // The end block was written at the executor's own stream position — if that lay
+            // behind the bookmark, the ledger check on the next pass finds it.
+            RequestOverwriteCheck();
         }
 
         private void HandleScriptContext(Match match, Dictionary<string, string> parameters)
@@ -372,6 +369,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Enrollment.Ime
             var context = match.Groups["context"]?.Value;
             if (string.IsNullOrEmpty(context)) return;
 
+            NoteInvocationActivity();
             var runContext = string.Equals(context, "machine", StringComparison.OrdinalIgnoreCase) ? "System" : "User";
             var script = GetCurrentScriptForLineUpdate(parameters);
             if (script != null)
@@ -398,6 +396,10 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Enrollment.Ime
                     script.ExitObservedAtUtc = LastMatchedLogTimestamp ?? DateTime.UtcNow;
                 _logger.Debug($"ImeLogTracker: script exit code {exitCode} for {script.PolicyId}");
             }
+            // An exit code means the executor just wrote its end block and IME is about to
+            // write the result — both land at a stale stream position when another process
+            // appended meanwhile. Check the ledgers on the next pass.
+            RequestOverwriteCheck();
         }
 
         private void HandleScriptOutput(Match match, Dictionary<string, string> parameters)
@@ -455,8 +457,6 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Enrollment.Ime
             {
                 _logger.Debug($"ImeLogTracker: platform script {id} PS-SCRIPT-RESULT arrived after fallback emit — skipping duplicate");
                 _pendingPlatformScripts.Remove(id);
-                if (string.Equals(_lastPlatformScriptPolicyId, id, StringComparison.OrdinalIgnoreCase))
-                    _lastPlatformScriptPolicyId = null;
                 return;
             }
 
@@ -496,17 +496,18 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Enrollment.Ime
             _platformScriptResultEmitted.Add(id);
             _pendingPlatformScripts.Remove(id);
             _stateDirty = true;
-            if (string.Equals(_lastPlatformScriptPolicyId, id, StringComparison.OrdinalIgnoreCase))
-                _lastPlatformScriptPolicyId = null;
+            // The result block is IME's first write after the script wait — the executor's end
+            // block in the other file may have landed behind the bookmark at the same time.
+            RequestOverwriteCheck();
         }
 
         /// <summary>
         /// Grace period after a platform script's AgentExecutor exit code is observed before
         /// <see cref="FlushStalePlatformScriptResults"/> emits a completion from that exit code.
-        /// IME logs its authoritative <c>PS-SCRIPT-RESULT</c> line within ~1 s of exit on healthy
-        /// reporting cycles, so 15 s lets the normal path win virtually always — the fallback is a
-        /// true safety net for the case where IME has not flushed its batch-send to the Microsoft
-        /// service before the (often short) Autopilot enrollment ends and the agent terminates.
+        /// IME logs its authoritative <c>PS-SCRIPT-RESULT</c> line within ~1 s of exit; when it
+        /// is missing after 15 s it was hidden by a concurrent writer (the ledger check in
+        /// <c>ImeLogTracker.Overwrite.cs</c> recovers that line) or the enrollment ended before
+        /// IME wrote it — this fallback is the safety net for the remaining cases.
         /// </summary>
         private static readonly TimeSpan PlatformScriptResultGrace = TimeSpan.FromSeconds(15);
 
@@ -578,11 +579,8 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Enrollment.Ime
                 // the state file before a restart or the restarted tracker re-emits (H1).
                 _stateDirty = true;
                 foreach (var key in toRemove)
-                {
                     _pendingPlatformScripts.Remove(key);
-                    if (string.Equals(_lastPlatformScriptPolicyId, key, StringComparison.OrdinalIgnoreCase))
-                        _lastPlatformScriptPolicyId = null;
-                }
+                RequestOverwriteCheck();
             }
         }
 
@@ -602,7 +600,8 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Enrollment.Ime
                 Stdout = stdout,
                 StartedAtUtc = startedAtUtc,
             };
-            _lastPlatformScriptPolicyId = policyId;
+            NextTestEntry();
+            RecordInvocationMarker(policyId, isClose: false);
         }
 
         /// <summary>Test seam: simulate the authoritative IME PS-SCRIPT-RESULT path for a platform
@@ -616,8 +615,6 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Enrollment.Ime
             if (_platformScriptResultEmitted.Contains(policyId))
             {
                 _pendingPlatformScripts.Remove(policyId);
-                if (string.Equals(_lastPlatformScriptPolicyId, policyId, StringComparison.OrdinalIgnoreCase))
-                    _lastPlatformScriptPolicyId = null;
                 return;
             }
 
@@ -642,8 +639,6 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Enrollment.Ime
             EmitScriptEvent(script);
             _platformScriptResultEmitted.Add(policyId);
             _pendingPlatformScripts.Remove(policyId);
-            if (string.Equals(_lastPlatformScriptPolicyId, policyId, StringComparison.OrdinalIgnoreCase))
-                _lastPlatformScriptPolicyId = null;
             _stateDirty = true;
         }
 
@@ -794,6 +789,8 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Enrollment.Ime
                 return;
             }
             HandleHealthScriptResultJson(json);
+            // A remediation executor just ended too — its end block may sit behind the bookmark.
+            RequestOverwriteCheck();
         }
 
         /// <summary>

src/Agent/AutopilotMonitor.Agent.V2.Core/Monitoring/Enrollment/Ime/ImeLogTracker.LogProcessing.cs

@@ -78,243 +78,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Enrollment.Ime
 
                 try
                 {
-                    var fileInfo = new FileInfo(filePath);
-                    if (!fileInfo.Exists) continue;
-
-                    // Captured BEFORE MarkChecked below stamps this pass. Growth measured
-                    // against a previously observed state is what makes this pass's lines valid
-                    // "written now" anchors — for the per-file measurement AND per-line anchoring.
-                    var hadPreviousObservation = _positionTracker.HasSeen(filePath);
-                    var lastCheckedUtc = _positionTracker.GetLastCheckedUtc(filePath);
-                    var passNowUtc = UtcNowProvider();
-
-                    var startPos = _positionTracker.GetSafePosition(filePath, fileInfo.Length);
-
-                    // Every look counts, including "no new data" and an empty first sight: the
-                    // NEXT pass's freshness window is measured from here. Restored bookmarks
-                    // deliberately carry no LastCheckedUtc — the first pass after a restart reads
-                    // downtime backlog and must never count as fresh.
-                    _positionTracker.MarkChecked(filePath, passNowUtc);
-
-                    if (startPos >= fileInfo.Length)
-                    {
-                        // M2: guard the interpolated string so it isn't built every 100 ms tick
-                        // (per file) when Trace is off — which is the production default (Info).
-                        if (_logger.LogLevel >= AgentLogLevel.Trace)
-                            _logger.Trace($"ImeLogTracker: {Path.GetFileName(filePath)} — no new data (pos={startPos}, size={fileInfo.Length})");
-                        continue;
-                    }
-                    if (_logger.LogLevel >= AgentLogLevel.Trace)
-                        _logger.Trace($"ImeLogTracker: reading {Path.GetFileName(filePath)} from pos {startPos} (size={fileInfo.Length}, delta={fileInfo.Length - startPos})");
-
-                    _currentSourceFileName = Path.GetFileName(filePath);
-                    _currentPassLinesAreFresh = hadPreviousObservation
-                        && lastCheckedUtc > DateTime.MinValue
-                        && (passNowUtc - lastCheckedUtc) <= FreshLineMaxAge;
-
-                    // Newest bias-less line of this pass — the calibration anchor. Bias-carrying
-                    // lines are skipped: they already state the writer's offset, so they need no
-                    // measurement and must not overwrite one.
-                    CmTraceLogEntry calibrationAnchor = null;
-
-                    using (var stream = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
-                    {
-                        stream.Seek(startPos, SeekOrigin.Begin);
-
-                        var reader = new BoundedLineReader(stream, MaxEntryBytes);
-                        var stats = new PassStats();
-
-                        // Buffer for multiline CMTrace entries (e.g. AgentExecutor.log
-                        // "write output done. output = ..." spans many lines)
-                        StringBuilder multiLineBuffer = null;
-                        int multiLineCount = 0;
-                        // Set after a capped entry is dropped: its remaining physical lines
-                        // are skipped instead of being matched as raw text, until the entry
-                        // closes or a new entry begins (so no real entry is lost).
-                        bool skippingDroppedEntry = false;
-                        // File offset where the entry currently being assembled/processed
-                        // begins — the bookmark to fall back to when that entry is NOT
-                        // consumed (cancellation, or a held-back unterminated tail).
-                        long entryStart = startPos;
-                        // Where the next pass resumes. EOF unless an entry is left unconsumed.
-                        long resumePos;
-
-                        while (true)
-                        {
-                            // CancellationToken.None: cancellation is honoured by the explicit
-                            // check below (exact bookmark) rather than by an exception out of
-                            // the read, which the poll loop would log as an error.
-                            var line = await reader.ReadLineAsync(CancellationToken.None);
-                            if (line == null)
-                            {
-                                // EOF. An in-flight multiline entry here is an entry the writer
-                                // has not finished — hold it back (see HoldBackTail) instead of
-                                // dropping it and raw-matching its remaining lines next pass.
-                                if (multiLineBuffer != null && HoldBackTail(filePath, entryStart, fileInfo.Length, passNowUtc))
-                                {
-                                    _heldTailCount++;
-                                    resumePos = entryStart;
-                                    break;
-                                }
-                                resumePos = reader.Position;
-                                break;
-                            }
-
-                            if (token.IsCancellationRequested)
-                            {
-                                // Exact bookmark: nothing of this line (or of the multiline
-                                // entry it belongs to) has been handled.
-                                resumePos = multiLineBuffer != null ? entryStart : reader.LastLineStart;
-                                break;
-                            }
-
-                            if (multiLineBuffer == null)
-                                entryStart = reader.LastLineStart;
-
-                            _linesRead++;
-
-                            // A physical line over the cap is never matched: its captures would be
-                            // cut and the regexes would run over attacker-sized input. The capped
-                            // prefix is enough to tell whether it opened an entry whose remaining
-                            // lines must be skipped rather than raw-matched.
-                            if (reader.LastLineTruncated)
-                            {
-                                stats.OversizedLines++;
-                                _oversizedLines++;
-                                if (multiLineBuffer != null)
-                                {
-                                    multiLineBuffer = null;
-                                    multiLineCount = 0;
-                                    skippingDroppedEntry = true;
-                                }
-                                else if (line.StartsWith("<![LOG[") && !line.Contains("]LOG]!>"))
-                                {
-                                    skippingDroppedEntry = true;
-                                }
-                                continue;
-                            }
-
-                            if (!reader.LastLineTerminated && multiLineBuffer == null
-                                && HoldBackTail(filePath, reader.LastLineStart, fileInfo.Length, passNowUtc))
-                            {
-                                // Unterminated single line at EOF: the writer is mid-line.
-                                _heldTailCount++;
-                                resumePos = reader.LastLineStart;
-                                break;
-                            }
-
-                            if (skippingDroppedEntry)
-                            {
-                                // A line opening a new entry ends the skip and is processed
-                                // itself — checked BEFORE the close-tag test, because a complete
-                                // single-line entry contains both and must not be consumed as
-                                // the dropped entry's close.
-                                if (line.StartsWith("<![LOG["))
-                                    skippingDroppedEntry = false;
-                                else
-                                {
-                                    if (line.Contains("]LOG]!>")) skippingDroppedEntry = false;
-                                    continue;
-                                }
-                            }
-
-                            // --- Multiline CMTrace buffering ---
-                            // CMTrace entries: <![LOG[message]LOG]!><time=...>
-                            // When message contains newlines, the entry spans multiple lines.
-                            // We buffer until we find the closing ]LOG]!> tag.
-                            if (multiLineBuffer != null)
-                            {
-                                // Continuing a multiline entry
-                                multiLineBuffer.Append('\n').Append(line);
-                                multiLineCount++;
-
-                                if (line.Contains("]LOG]!>"))
-                                {
-                                    // Entry complete — use the assembled line
-                                    line = multiLineBuffer.ToString();
-                                    multiLineBuffer = null;
-                                    multiLineCount = 0;
-                                }
-                                else if (multiLineCount >= MaxMultiLineBufferLines || multiLineBuffer.Length >= MaxEntryBytes)
-                                {
-                                    // Safety limit — discard to bound memory and parser work. Warning
-                                    // (not Debug) so a capped entry is visible in the client log at the
-                                    // default level: a real IME entry this large would be news.
-                                    _logger.Warning($"ImeLogTracker: discarding multiline CMTrace buffer in {Path.GetFileName(filePath)} after {multiLineCount} lines / {multiLineBuffer.Length} chars (cap {MaxMultiLineBufferLines} lines / {MaxEntryBytes} chars) — entry dropped");
-                                    multiLineBuffer = null;
-                                    multiLineCount = 0;
-                                    skippingDroppedEntry = true;
-                                    continue;
-                                }
-                                else
-                                {
-                                    // Still accumulating — read next line
-                                    continue;
-                                }
-                            }
-                            else if (line.StartsWith("<![LOG[") && !line.Contains("]LOG]!>"))
-                            {
-                                // Start of a multiline CMTrace entry
-                                multiLineBuffer = new StringBuilder(line);
-                                multiLineCount = 1;
-                                continue;
-                            }
-
-                            // --- Normal processing (single-line or completed multiline) ---
-                            CmTraceLogEntry entry;
-                            string messageToMatch;
-                            _currentEntryOffset = entryStart;
-                            if (CmTraceLogParser.TryParseLine(line, out entry))
-                            {
-                                messageToMatch = entry.Message;
-                                if (entry.HasTimestamp && !entry.BiasMinutes.HasValue)
-                                    calibrationAnchor = entry;
-                            }
-                            else
-                            {
-                                // Non-CMTrace line - match raw
-                                messageToMatch = line;
-                                entry = null;
-                            }
-
-                            if (string.IsNullOrEmpty(messageToMatch)) continue;
-
-                            // Simulation mode delay
-                            if (SimulationMode && entry != null)
-                            {
-                                await ApplySimulationDelay(ResolveEntryUtc(entry), token);
-                            }
-
-                            MatchLine(filePath, line, messageToMatch, entry, stats);
-                        }
-
-                        ClearHeldTailIfConsumed(filePath, resumePos);
-                        _positionTracker.SetPosition(filePath, resumePos);
-                        _stateDirty = true;
-                        backlogBytes += Math.Max(0, fileInfo.Length - resumePos);
-
-                        // One line per pass and file, never per hostile line: the counters are
-                        // the operator's only trace that matching was skipped or cut short.
-                        if (stats.RegexTimeouts > 0 || stats.BudgetBreaks > 0 || stats.OversizedLines > 0)
-                        {
-                            _logger.Warning(
-                                $"ImeLogTracker: {Path.GetFileName(filePath)} pass skipped work — " +
-                                $"oversizedLines={stats.OversizedLines} (cap {MaxEntryBytes} bytes), " +
-                                $"regexTimeouts={stats.RegexTimeouts}, lineBudgetBreaks={stats.BudgetBreaks}" +
-                                (stats.FirstSkippedPatternId != null ? $", firstSkippedPattern={stats.FirstSkippedPatternId}" : string.Empty));
-                            RaiseTrackerDegradedOnce(Path.GetFileName(filePath), stats.FirstSkippedPatternId);
-                        }
-
-                        // Calibrate AFTER the pass: this pass's lines were resolved with the
-                        // offset established previously, at most one poll (100 ms) old. Buffering
-                        // the pass to calibrate first is not an option — the first pass of
-                        // AppWorkload.log can be hundreds of MB. The cost is a warm-up of one
-                        // growing pass, during which lines fall back to the reader zone and are
-                        // flagged as such.
-                        if (hadPreviousObservation && calibrationAnchor != null)
-                            CalibrateFrom(_currentSourceFileName, Path.GetFileName(filePath), calibrationAnchor);
-                        _currentEntryOffset = -1;
-                    }
+                    backlogBytes += await ProcessFileAsync(filePath, token);
                 }
                 catch (FileNotFoundException) { }
                 catch (IOException ex)
@@ -330,6 +94,246 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Enrollment.Ime
             }
         }
 
+        /// <summary>
+        /// One pass over one file: the bytes behind the bookmark, on a multi-writer file preceded
+        /// by the overwrite check that may move the bookmark back first (see
+        /// <c>ImeLogTracker.Overwrite.cs</c>). Returns the file's remaining backlog in bytes.
+        /// </summary>
+        private async Task<long> ProcessFileAsync(string filePath, CancellationToken token)
+        {
+            var fileInfo = new FileInfo(filePath);
+            if (!fileInfo.Exists) return 0;
+
+            // Captured BEFORE MarkChecked below stamps this pass. Growth measured
+            // against a previously observed state is what makes this pass's lines valid
+            // "written now" anchors — for the per-file measurement AND per-line anchoring.
+            var hadPreviousObservation = _positionTracker.HasSeen(filePath);
+            var lastCheckedUtc = _positionTracker.GetLastCheckedUtc(filePath);
+            var passNowUtc = UtcNowProvider();
+
+            var startPos = _positionTracker.GetSafePosition(filePath, fileInfo.Length);
+
+            // Every look counts, including "no new data" and an empty first sight: the
+            // NEXT pass's freshness window is measured from here. Restored bookmarks
+            // deliberately carry no LastCheckedUtc — the first pass after a restart reads
+            // downtime backlog and must never count as fresh.
+            _positionTracker.MarkChecked(filePath, passNowUtc);
+
+            var fileName = Path.GetFileName(filePath);
+            var ledger = IsMultiWriterLogFile(fileName) ? GetLedger(fileName) : null;
+
+            // Set by a rewind: entries that still read unchanged behind the old bookmark are
+            // skipped, and nothing below the old bookmark is fresh or a calibration anchor.
+            HashSet<LedgerEntry> rewoundEntries = null;
+            long oldBookmark = -1;
+
+            if (ledger != null)
+            {
+                var rewindTo = await PrepareLedgerAsync(filePath, fileName, ledger, startPos, passNowUtc, token);
+                if (rewindTo >= 0)
+                {
+                    rewoundEntries = ApplyRewind(filePath, fileName, ledger, rewindTo, startPos);
+                    oldBookmark = startPos;
+                    startPos = rewindTo;
+                }
+            }
+
+            if (startPos >= fileInfo.Length)
+            {
+                // M2: guard the interpolated string so it isn't built every 100 ms tick
+                // (per file) when Trace is off — which is the production default (Info).
+                if (_logger.LogLevel >= AgentLogLevel.Trace)
+                    _logger.Trace($"ImeLogTracker: {fileName} — no new data (pos={startPos}, size={fileInfo.Length})");
+                return 0;
+            }
+            if (_logger.LogLevel >= AgentLogLevel.Trace)
+                _logger.Trace($"ImeLogTracker: reading {fileName} from pos {startPos} (size={fileInfo.Length}, delta={fileInfo.Length - startPos})");
+
+            _currentSourceFileName = fileName;
+            var passLinesAreFresh = hadPreviousObservation
+                && lastCheckedUtc > DateTime.MinValue
+                && (passNowUtc - lastCheckedUtc) <= FreshLineMaxAge;
+            _currentPassLinesAreFresh = passLinesAreFresh;
+
+            // Newest bias-less line of this pass — the calibration anchor. Bias-carrying
+            // lines are skipped: they already state the writer's offset, so they need no
+            // measurement and must not overwrite one.
+            CmTraceLogEntry calibrationAnchor = null;
+
+            using (var stream = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
+            {
+                // Guard zone: the file grew and is open anyway — re-check the entries just behind
+                // the bookmark, where a stale writer's block lands when it is short.
+                if (ledger != null && rewoundEntries == null && ledger.Entries.Count > 0)
+                {
+                    var guardFrom = ledger.OffsetOfEntryAtOrAfter(startPos - OverwriteGuardBytes);
+                    if (guardFrom >= 0 && guardFrom < startPos)
+                    {
+                        var divergence = await FindOverwriteAsync(stream, ledger, guardFrom, startPos, token);
+                        _verifiedBytes += startPos - guardFrom;
+                        if (divergence >= 0)
+                        {
+                            // The guard saw only the tail of the ledger; the block may start earlier.
+                            var earliest = await FindOverwriteAsync(stream, ledger, ledger.StartOffset, startPos, token);
+                            _verifiedBytes += startPos - ledger.StartOffset;
+                            _verifyPasses++;
+                            if (earliest >= 0) divergence = earliest;
+                            rewoundEntries = ApplyRewind(filePath, fileName, ledger, divergence, startPos);
+                            oldBookmark = startPos;
+                            startPos = divergence;
+                        }
+                    }
+                }
+
+                stream.Seek(startPos, SeekOrigin.Begin);
+
+                var reader = new BoundedLineReader(stream, MaxEntryBytes, hashLines: ledger != null);
+                var assembler = new CmTraceEntryAssembler(MaxMultiLineBufferLines, MaxEntryBytes);
+                var stats = new PassStats();
+                var firstEntry = true;
+                // Where the next pass resumes. EOF unless an entry is left unconsumed.
+                long resumePos;
+
+                while (true)
+                {
+                    // CancellationToken.None: cancellation is honoured by the explicit
+                    // check below (exact bookmark) rather than by an exception out of
+                    // the read, which the poll loop would log as an error.
+                    var line = await reader.ReadLineAsync(CancellationToken.None);
+                    if (line == null)
+                    {
+                        // EOF. An in-flight multiline entry here is an entry the writer
+                        // has not finished — hold it back (see HoldBackTail) instead of
+                        // dropping it and raw-matching its remaining lines next pass.
+                        if (assembler.HasOpenEntry && HoldBackTail(filePath, assembler.OpenEntryStart, fileInfo.Length, passNowUtc))
+                        {
+                            _heldTailCount++;
+                            resumePos = assembler.OpenEntryStart;
+                            break;
+                        }
+                        resumePos = reader.Position;
+                        break;
+                    }
+
+                    if (token.IsCancellationRequested)
+                    {
+                        // Exact bookmark: nothing of this line (or of the multiline
+                        // entry it belongs to) has been handled.
+                        resumePos = assembler.HasOpenEntry ? assembler.OpenEntryStart : reader.LastLineStart;
+                        break;
+                    }
+
+                    _linesRead++;
+
+                    if (!reader.LastLineTruncated && !reader.LastLineTerminated && !assembler.HasOpenEntry
+                        && HoldBackTail(filePath, reader.LastLineStart, fileInfo.Length, passNowUtc))
+                    {
+                        // Unterminated single line at EOF: the writer is mid-line.
+                        _heldTailCount++;
+                        resumePos = reader.LastLineStart;
+                        break;
+                    }
+
+                    AssembledEntry entry;
+                    var outcome = assembler.Feed(line, reader.LastLineStart, reader.Position, reader.LastLineTruncated, reader.LastLineHash, out entry);
+                    if (outcome == CmTraceEntryAssembler.Outcome.OversizedDropped)
+                    {
+                        // A physical line over the cap is never matched: its captures would be
+                        // cut and the regexes would run over attacker-sized input.
+                        stats.OversizedLines++;
+                        _oversizedLines++;
+                        continue;
+                    }
+                    if (outcome == CmTraceEntryAssembler.Outcome.CapDropped)
+                    {
+                        // Warning (not Debug) so a capped entry is visible in the client log at the
+                        // default level: a real IME entry this large would be news.
+                        _logger.Warning($"ImeLogTracker: discarding multiline CMTrace buffer in {fileName} after {assembler.DroppedLines} lines / {assembler.DroppedChars} chars (cap {MaxMultiLineBufferLines} lines / {MaxEntryBytes} chars) — entry dropped");
+                        continue;
+                    }
+                    if (outcome != CmTraceEntryAssembler.Outcome.Entry) continue;
+
+                    if (firstEntry)
+                    {
+                        firstEntry = false;
+                        // A fragment right at the bookmark of a multi-writer file (the previous
+                        // entry ended exactly here, this one does not open an entry) is the tail of
+                        // a block a stale writer laid over already-read bytes: verify before reading on.
+                        if (ledger != null && rewoundEntries == null && entry.Offset == ledger.EndOffset
+                            && ledger.LastFragmentOffset != entry.Offset && !entry.Text.StartsWith("<![LOG["))
+                        {
+                            ledger.VerifyRequested = true;
+                            ledger.LastFragmentOffset = entry.Offset;
+                            resumePos = entry.Offset;
+                            break;
+                        }
+                    }
+
+                    if (ledger != null) ledger.Record(entry);
+                    if (rewoundEntries != null && rewoundEntries.Contains(new LedgerEntry { Offset = entry.Offset, Hash = entry.Hash }))
+                        continue; // unchanged behind the old bookmark — processed before the rewind
+
+                    _currentPassLinesAreFresh = passLinesAreFresh && entry.Offset >= oldBookmark;
+
+                    // --- Normal processing (single-line or completed multiline) ---
+                    CmTraceLogEntry parsed;
+                    string messageToMatch;
+                    _currentEntryOffset = entry.Offset;
+                    if (CmTraceLogParser.TryParseLine(entry.Text, out parsed))
+                    {
+                        messageToMatch = parsed.Message;
+                        if (parsed.HasTimestamp && !parsed.BiasMinutes.HasValue && entry.Offset >= oldBookmark)
+                            calibrationAnchor = parsed;
+                    }
+                    else
+                    {
+                        // Non-CMTrace line - match raw
+                        messageToMatch = entry.Text;
+                        parsed = null;
+                    }
+
+                    if (string.IsNullOrEmpty(messageToMatch)) continue;
+
+                    // Simulation mode delay
+                    if (SimulationMode && parsed != null)
+                    {
+                        await ApplySimulationDelay(ResolveEntryUtc(parsed), token);
+                    }
+
+                    MatchLine(filePath, entry.Text, messageToMatch, parsed, stats);
+                }
+
+                ClearHeldTailIfConsumed(filePath, resumePos);
+                _positionTracker.SetPosition(filePath, resumePos);
+                _stateDirty = true;
+                if (ledger != null) ledger.TrimBelow(RetentionFloor(fileName, resumePos));
+
+                // One line per pass and file, never per hostile line: the counters are
+                // the operator's only trace that matching was skipped or cut short.
+                if (stats.RegexTimeouts > 0 || stats.BudgetBreaks > 0 || stats.OversizedLines > 0)
+                {
+                    _logger.Warning(
+                        $"ImeLogTracker: {fileName} pass skipped work — " +
+                        $"oversizedLines={stats.OversizedLines} (cap {MaxEntryBytes} bytes), " +
+                        $"regexTimeouts={stats.RegexTimeouts}, lineBudgetBreaks={stats.BudgetBreaks}" +
+                        (stats.FirstSkippedPatternId != null ? $", firstSkippedPattern={stats.FirstSkippedPatternId}" : string.Empty));
+                    RaiseTrackerDegradedOnce(fileName, stats.FirstSkippedPatternId);
+                }
+
+                // Calibrate AFTER the pass: this pass's lines were resolved with the
+                // offset established previously, at most one poll (100 ms) old. Buffering
+                // the pass to calibrate first is not an option — the first pass of
+                // AppWorkload.log can be hundreds of MB. The cost is a warm-up of one
+                // growing pass, during which lines fall back to the reader zone and are
+                // flagged as such.
+                if (hadPreviousObservation && calibrationAnchor != null)
+                    CalibrateFrom(_currentSourceFileName, fileName, calibrationAnchor);
+                _currentEntryOffset = -1;
+
+                return Math.Max(0, fileInfo.Length - resumePos);
+            }
+        }
+
         private sealed class PassStats
         {
             public int RegexTimeouts;
@@ -828,10 +832,18 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Enrollment.Ime
                         break;
 
                     case "resetplatformscriptcontext":
-                        // Session 6b4993e5 fix: a fresh AgentExecutor invocation banner
-                        // ("ExecutorLog AgentExecutor gets invoked") ends the previous
-                        // invocation's line-capture context. See HandleAgentExecutorInvocationBoundary.
-                        HandleAgentExecutorInvocationBoundary();
+                        // "ExecutorLog AgentExecutor gets invoked": a new process — the executor
+                        // lines that follow belong to its own argument line, never to the
+                        // previous invocation (session 6b4993e5).
+                        HandleAgentExecutorInvocation();
+                        break;
+
+                    case "agentinvocationargument":
+                        HandleAgentInvocationArgument(match.Groups["kind"]?.Value);
+                        break;
+
+                    case "agentexecutorcompleted":
+                        HandleAgentExecutorCompleted();
                         break;
 
                     case "healthscriptresult":
@@ -864,6 +876,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Enrollment.Ime
         internal void ProcessLogMessageForTest(string message, DateTime? sourceTimestampUtc = null)
         {
             if (string.IsNullOrEmpty(message)) return;
+            NextTestEntry();
             // The test seam hands in an already-resolved UTC instant, so it takes the same route a
             // writer-declared bias does: TimestampUtc set, no zone left to guess.
             var entry = sourceTimestampUtc.HasValue

src/Agent/AutopilotMonitor.Agent.V2.Core/Monitoring/Enrollment/Ime/ImeLogTracker.Overwrite.cs

@@ -0,0 +1,465 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Enrollment.Ime
+{
+    /// <summary>
+    /// Partial: overwrite-aware tailing of the two IME logs that several processes append to.
+    /// <para>
+    /// IME's trace listener opens each log once per process with FileMode.Append and
+    /// FileShare.ReadWrite and writes at its own stream position afterwards. AgentExecutor.exe
+    /// (two processes per user-context script, one per detection or remediation script) has such
+    /// a listener on AgentExecutor.log AND on IntuneManagementExtension.log. Whenever one process
+    /// resumes writing after another one appended, it overwrites those bytes: the final file
+    /// holds the last writer, while a tailer that trusts its bookmark read the first version and
+    /// never sees the second (session 46749560: 6 of 16 script results, 4 of 16 exit blocks).
+    /// </para>
+    /// <para>
+    /// So the bytes behind the bookmark are not immutable. The tracker keeps a fingerprint ledger
+    /// of every entry it read from these files, re-verifies it while script invocations are in
+    /// flight or right after a signal that a stale writer just wrote, and on the first changed
+    /// entry rewinds the bookmark — processing only entries it has not processed before.
+    /// </para>
+    /// </summary>
+    public partial class ImeLogTracker
+    {
+        /// <summary>Live logs with more than one writing process; archived rotations are closed files.</summary>
+        internal static readonly string[] MultiWriterLogFiles = { "AgentExecutor.log", "IntuneManagementExtension.log" };
+
+        /// <summary>Ledger depth behind the bookmark while no script is in flight.</summary>
+        internal const int LedgerRetentionBytes = 1024 * 1024;
+
+        /// <summary>Hard cap on the ledger depth, whatever is in flight.</summary>
+        internal const int LedgerMaxBytes = 4 * 1024 * 1024;
+
+        /// <summary>Entries this close to the bookmark are re-checked on every pass that found growth (the file is open anyway).</summary>
+        internal const int OverwriteGuardBytes = 4 * 1024;
+
+        /// <summary>After a restart the ledger is seeded from this many bytes before the persisted bookmark.</summary>
+        internal const int RestartSeedBytes = 64 * 1024;
+
+        /// <summary>Verification work per second while in contention — a deep ledger is checked less often, never skipped.</summary>
+        internal const long VerifyBudgetBytesPerSecond = 256 * 1024;
+
+        internal static readonly TimeSpan VerifyMinInterval = TimeSpan.FromSeconds(1);
+
+        /// <summary>How long an observed invocation keeps the files in contention on its own (detection executors have no pending slot).</summary>
+        internal static readonly TimeSpan InvocationContentionWindow = TimeSpan.FromSeconds(120);
+
+        /// <summary>A pending script older than this no longer holds the files in contention.</summary>
+        internal static readonly TimeSpan MaxPendingScriptContention = TimeSpan.FromHours(2);
+
+        private const int MaxInvocationMarkersPerFile = 512;
+        private const string TestSourceFileName = "TEST.log";
+        private const string LogOpenTag = "<![LOG[";
+
+        /// <summary>One processed entry: where it starts, how far it reaches, what its bytes hashed to.</summary>
+        internal struct LedgerEntry : IEquatable<LedgerEntry>
+        {
+            public long Offset;
+            public int Length;
+            public ulong Hash;
+
+            // Identity is (offset, hash): a tail first read unterminated and later completed
+            // keeps its hash but grows by its terminator.
+            public bool Equals(LedgerEntry other) => Offset == other.Offset && Hash == other.Hash;
+            public override bool Equals(object obj) => obj is LedgerEntry other && Equals(other);
+            public override int GetHashCode() => unchecked(((int)Offset * 397) ^ (int)Hash ^ (int)(Hash >> 32));
+        }
+
+        internal sealed class FileLedger
+        {
+            public readonly List<LedgerEntry> Entries = new List<LedgerEntry>();
+            public bool VerifyRequested;
+            public DateTime NextCadenceVerifyUtc = DateTime.MinValue;
+            public long LastFragmentOffset = -1;
+            public bool Seeded;
+            public bool RewindLogged;
+
+            public long StartOffset => Entries.Count > 0 ? Entries[0].Offset : -1;
+            public long EndOffset => Entries.Count > 0 ? Entries[Entries.Count - 1].Offset + Entries[Entries.Count - 1].Length : -1;
+
+            public int IndexOfFirstAtOrAfter(long offset)
+            {
+                int lo = 0, hi = Entries.Count;
+                while (lo < hi)
+                {
+                    var mid = (lo + hi) >> 1;
+                    if (Entries[mid].Offset < offset) lo = mid + 1; else hi = mid;
+                }
+                return lo;
+            }
+
+            /// <summary>Entry offset at or after <paramref name="offset"/>, or -1.</summary>
+            public long OffsetOfEntryAtOrAfter(long offset)
+            {
+                var idx = IndexOfFirstAtOrAfter(offset);
+                return idx < Entries.Count ? Entries[idx].Offset : -1;
+            }
+
+            public void Record(AssembledEntry e)
+            {
+                var idx = IndexOfFirstAtOrAfter(e.Offset);
+                if (idx < Entries.Count) Entries.RemoveRange(idx, Entries.Count - idx);
+                Entries.Add(new LedgerEntry { Offset = e.Offset, Length = e.Length, Hash = e.Hash });
+            }
+
+            /// <summary>Drops everything from <paramref name="offset"/> on and returns it — the entries a rewind must not process twice.</summary>
+            public HashSet<LedgerEntry> TruncateFrom(long offset)
+            {
+                var idx = IndexOfFirstAtOrAfter(offset);
+                var removed = new HashSet<LedgerEntry>();
+                for (var i = idx; i < Entries.Count; i++) removed.Add(Entries[i]);
+                if (idx < Entries.Count) Entries.RemoveRange(idx, Entries.Count - idx);
+                return removed;
+            }
+
+            public void TrimBelow(long floor)
+            {
+                var idx = IndexOfFirstAtOrAfter(floor);
+                if (idx > 0) Entries.RemoveRange(0, idx);
+            }
+
+            public void Clear()
+            {
+                Entries.Clear();
+                LastFragmentOffset = -1;
+            }
+        }
+
+        // Keyed by file name (unique within the log folder). Poll thread only.
+        private readonly Dictionary<string, FileLedger> _ledgers = new Dictionary<string, FileLedger>(StringComparer.OrdinalIgnoreCase);
+        private DateTime _contentionUntilUtc = DateTime.MinValue;
+
+        // Cumulative, restart-safe like the other health counters.
+        private long _overwriteRewinds;
+        private long _overwriteBytesReprocessed;
+        private long _verifyPasses;
+        private long _verifiedBytes;
+
+        internal static bool IsMultiWriterLogFile(string fileName)
+        {
+            foreach (var f in MultiWriterLogFiles)
+                if (string.Equals(f, fileName, StringComparison.OrdinalIgnoreCase)) return true;
+            return false;
+        }
+
+        private FileLedger GetLedger(string fileName)
+        {
+            FileLedger ledger;
+            if (!_ledgers.TryGetValue(fileName, out ledger))
+            {
+                ledger = new FileLedger();
+                _ledgers[fileName] = ledger;
+            }
+            return ledger;
+        }
+
+        /// <summary>A stale writer writes exactly when its script ends: every end signal asks both files for a check on the next pass.</summary>
+        private void RequestOverwriteCheck()
+        {
+            foreach (var ledger in _ledgers.Values) ledger.VerifyRequested = true;
+        }
+
+        private void NoteInvocationActivity()
+        {
+            _contentionUntilUtc = UtcNowProvider() + InvocationContentionWindow;
+        }
+
+        private bool InContention(DateTime nowUtc)
+        {
+            if (nowUtc < _contentionUntilUtc || _pendingHealthScript != null) return true;
+            foreach (var script in _pendingPlatformScripts.Values)
+            {
+                if (script.Result != null) continue;
+                if (!script.StartedAtUtc.HasValue || nowUtc - script.StartedAtUtc.Value < MaxPendingScriptContention) return true;
+            }
+            return false;
+        }
+
+        /// <summary>
+        /// Runs before a multi-writer file is read: rollover reset, restart seeding, then the
+        /// overwrite check when one is due. Returns the offset to rewind to, or -1.
+        /// </summary>
+        private async Task<long> PrepareLedgerAsync(string filePath, string fileName, FileLedger ledger, long bookmark, DateTime nowUtc, CancellationToken token)
+        {
+            if (ledger.Entries.Count > 0 && bookmark < ledger.EndOffset)
+            {
+                // The bookmark moved below what was read: the file was replaced (rollover).
+                ledger.Clear();
+                InvalidateInvocationMarkersFrom(fileName, 0);
+            }
+
+            if (!ledger.Seeded)
+            {
+                ledger.Seeded = true;
+                if (ledger.Entries.Count == 0 && bookmark > 0)
+                    await SeedLedgerAsync(filePath, ledger, bookmark, token).ConfigureAwait(false);
+            }
+
+            var due = ledger.VerifyRequested || (InContention(nowUtc) && nowUtc >= ledger.NextCadenceVerifyUtc);
+            if (!due || ledger.Entries.Count == 0) return -1;
+
+            var from = ledger.StartOffset;
+            var bytes = bookmark - from;
+            if (bytes <= 0)
+            {
+                ledger.VerifyRequested = false;
+                return -1;
+            }
+
+            long divergence;
+            using (var stream = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
+                divergence = await FindOverwriteAsync(stream, ledger, from, bookmark, token).ConfigureAwait(false);
+            if (token.IsCancellationRequested) return -1;
+
+            ledger.VerifyRequested = false;
+            var paceSeconds = Math.Max(VerifyMinInterval.TotalSeconds, (double)bytes / VerifyBudgetBytesPerSecond);
+            ledger.NextCadenceVerifyUtc = nowUtc.AddSeconds(paceSeconds);
+            _verifyPasses++;
+            _verifiedBytes += bytes;
+            return divergence;
+        }
+
+        /// <summary>
+        /// Re-reads [fromOffset, toOffset) with the read loop's assembler and compares against the
+        /// ledger. Returns the offset of the first entry whose bytes changed — a ledger entry not
+        /// reproduced, or an entry the ledger never had — or -1 when everything reads as before.
+        /// </summary>
+        private async Task<long> FindOverwriteAsync(FileStream stream, FileLedger ledger, long fromOffset, long toOffset, CancellationToken token)
+        {
+            var entries = ledger.Entries;
+            var idx = ledger.IndexOfFirstAtOrAfter(fromOffset);
+            if (idx >= entries.Count) return -1;
+
+            stream.Seek(entries[idx].Offset, SeekOrigin.Begin);
+            var reader = new BoundedLineReader(stream, MaxEntryBytes, hashLines: true);
+            var assembler = new CmTraceEntryAssembler(MaxMultiLineBufferLines, MaxEntryBytes);
+
+            while (reader.Position < toOffset)
+            {
+                var line = await reader.ReadLineAsync(CancellationToken.None).ConfigureAwait(false);
+                if (line == null) break;
+                if (token.IsCancellationRequested) return -1;
+
+                AssembledEntry e;
+                if (assembler.Feed(line, reader.LastLineStart, reader.Position, reader.LastLineTruncated, reader.LastLineHash, out e)
+                    != CmTraceEntryAssembler.Outcome.Entry)
+                    continue;
+                if (e.Offset >= toOffset) break;
+
+                if (idx < entries.Count && entries[idx].Offset < e.Offset) return entries[idx].Offset;
+                if (idx < entries.Count && entries[idx].Offset == e.Offset)
+                {
+                    if (entries[idx].Hash != e.Hash) return e.Offset;
+                    idx++;
+                    continue;
+                }
+                return e.Offset; // an entry the ledger never had: the bytes changed here
+            }
+
+            return idx < entries.Count && entries[idx].Offset < toOffset ? entries[idx].Offset : -1;
+        }
+
+        /// <summary>After a restart the ledger is empty: fingerprint the entries just behind the persisted bookmark without processing them.</summary>
+        private async Task SeedLedgerAsync(string filePath, FileLedger ledger, long bookmark, CancellationToken token)
+        {
+            var from = Math.Max(0, bookmark - RestartSeedBytes);
+            using (var stream = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
+            {
+                stream.Seek(from, SeekOrigin.Begin);
+                var reader = new BoundedLineReader(stream, MaxEntryBytes, hashLines: true);
+                var assembler = new CmTraceEntryAssembler(MaxMultiLineBufferLines, MaxEntryBytes);
+                var aligned = from == 0;
+                while (reader.Position < bookmark)
+                {
+                    var line = await reader.ReadLineAsync(CancellationToken.None).ConfigureAwait(false);
+                    if (line == null || token.IsCancellationRequested) break;
+                    if (!aligned)
+                    {
+                        // The seek may have landed mid-entry: wait for an entry boundary.
+                        if (!line.StartsWith(LogOpenTag)) continue;
+                        aligned = true;
+                    }
+                    AssembledEntry e;
+                    if (assembler.Feed(line, reader.LastLineStart, reader.Position, reader.LastLineTruncated, reader.LastLineHash, out e)
+                            == CmTraceEntryAssembler.Outcome.Entry
+                        && e.Offset + e.Length <= bookmark)
+                        ledger.Record(e);
+                }
+            }
+        }
+
+        /// <summary>Moves the bookmark back to the first changed entry; returns the entries to skip if they still read unchanged behind it.</summary>
+        private HashSet<LedgerEntry> ApplyRewind(string filePath, string fileName, FileLedger ledger, long rewindTo, long bookmark)
+        {
+            var removed = ledger.TruncateFrom(rewindTo);
+            _positionTracker.SetPosition(filePath, rewindTo);
+            _heldTails.Remove(filePath);
+            InvalidateInvocationMarkersFrom(fileName, rewindTo);
+            _overwriteRewinds++;
+            _overwriteBytesReprocessed += bookmark - rewindTo;
+
+            var message = $"ImeLogTracker: {fileName} changed behind the bookmark at offset {rewindTo} " +
+                          $"(a concurrent writer overwrote already-read bytes) — rewinding {bookmark - rewindTo} bytes";
+            if (!ledger.RewindLogged)
+            {
+                ledger.RewindLogged = true;
+                _logger.Info(message);
+            }
+            else
+            {
+                _logger.Debug(message);
+            }
+            return removed;
+        }
+
+        private long RetentionFloor(string fileName, long bookmark)
+        {
+            var floor = bookmark - LedgerRetentionBytes;
+            var oldest = OldestPendingScriptMarkerOffset(fileName);
+            if (oldest >= 0 && oldest < floor) floor = oldest;
+            var hardFloor = bookmark - LedgerMaxBytes;
+            return floor < hardFloor ? hardFloor : floor;
+        }
+
+        // -----------------------------------------------------------------------
+        // Invocation markers: who owns the policyId-less executor lines at a file position
+        // -----------------------------------------------------------------------
+        //
+        // AgentExecutor.log carries "Powershell exit code is N" / "write output done" without a
+        // policy id. Their owner is decided by FILE POSITION, not by the order lines were read:
+        // an executor writes its end block at its own stream position, i.e. directly after its
+        // own start block in the final bytes, so the nearest preceding invocation marker names
+        // it — and stays right after a rewind re-reads that region. A marker is a platform start
+        // (policy id), some other invocation (detection, remediation, requirement: null) or a
+        // close ("gets invoked" banner, "Agent executor completed").
+
+        private sealed class InvocationMarker
+        {
+            public long Offset;
+            public string PolicyId;
+            public bool IsClose;
+        }
+
+        private readonly Dictionary<string, List<InvocationMarker>> _invocationMarkers =
+            new Dictionary<string, List<InvocationMarker>>(StringComparer.OrdinalIgnoreCase);
+        private InvocationMarker _lastPlatformMarker;
+        private long _testEntryOrdinal;
+
+        private void RecordInvocationMarker(string policyId, bool isClose)
+        {
+            if (_currentEntryOffset < 0) return;
+            var file = _currentSourceFileName ?? TestSourceFileName;
+            List<InvocationMarker> list;
+            if (!_invocationMarkers.TryGetValue(file, out list))
+            {
+                list = new List<InvocationMarker>();
+                _invocationMarkers[file] = list;
+            }
+
+            if (list.Count > 0 && list[list.Count - 1].Offset == _currentEntryOffset)
+            {
+                // Two patterns on one line: the platform start outranks the generic argument line.
+                var last = list[list.Count - 1];
+                if (!isClose && policyId != null)
+                {
+                    last.PolicyId = policyId;
+                    last.IsClose = false;
+                    _lastPlatformMarker = last;
+                }
+                return;
+            }
+
+            if (list.Count >= MaxInvocationMarkersPerFile) list.RemoveAt(0);
+            var marker = new InvocationMarker { Offset = _currentEntryOffset, PolicyId = policyId, IsClose = isClose };
+            list.Add(marker);
+            if (!isClose && policyId != null) _lastPlatformMarker = marker;
+        }
+
+        /// <summary>The pending platform script that owns the entry being processed, or null.</summary>
+        private ScriptExecutionState ResolvePlatformScriptForCurrentEntry()
+        {
+            InvocationMarker marker = null;
+            List<InvocationMarker> list = null;
+            if (_currentSourceFileName != null && _currentEntryOffset >= 0 && _invocationMarkers.TryGetValue(_currentSourceFileName, out list))
+            {
+                for (var i = list.Count - 1; i >= 0; i--)
+                {
+                    if (list[i].Offset <= _currentEntryOffset)
+                    {
+                        marker = list[i];
+                        break;
+                    }
+                }
+            }
+
+            // A file without any marker (a context line after a rotation): the last platform start wins.
+            if (marker == null && (list == null || list.Count == 0)) marker = _lastPlatformMarker;
+            if (marker == null || marker.IsClose || marker.PolicyId == null) return null;
+
+            ScriptExecutionState state;
+            return _pendingPlatformScripts.TryGetValue(marker.PolicyId, out state) ? state : null;
+        }
+
+        private void InvalidateInvocationMarkersFrom(string fileName, long offset)
+        {
+            List<InvocationMarker> list;
+            if (!_invocationMarkers.TryGetValue(fileName, out list) || list.Count == 0) return;
+            var removedLast = false;
+            for (var i = list.Count - 1; i >= 0 && list[i].Offset >= offset; i--)
+            {
+                if (ReferenceEquals(list[i], _lastPlatformMarker)) removedLast = true;
+                list.RemoveAt(i);
+            }
+            if (removedLast)
+            {
+                _lastPlatformMarker = null;
+                for (var i = list.Count - 1; i >= 0; i--)
+                {
+                    if (!list[i].IsClose && list[i].PolicyId != null)
+                    {
+                        _lastPlatformMarker = list[i];
+                        break;
+                    }
+                }
+            }
+        }
+
+        private long OldestPendingScriptMarkerOffset(string fileName)
+        {
+            List<InvocationMarker> list;
+            if (!_invocationMarkers.TryGetValue(fileName, out list)) return -1;
+            long oldest = -1;
+            foreach (var marker in list)
+            {
+                if (marker.IsClose || marker.PolicyId == null || !_pendingPlatformScripts.ContainsKey(marker.PolicyId)) continue;
+                if (oldest < 0 || marker.Offset < oldest) oldest = marker.Offset;
+            }
+            return oldest;
+        }
+
+        // -----------------------------------------------------------------------
+        // Test seams
+        // -----------------------------------------------------------------------
+
+        /// <summary>Same effect as an end signal: both multi-writer ledgers are verified on the next pass.</summary>
+        internal void RequestOverwriteCheckForTest() => RequestOverwriteCheck();
+
+        internal int LedgerEntryCountForTest(string fileName)
+        {
+            FileLedger ledger;
+            return _ledgers.TryGetValue(fileName, out ledger) ? ledger.Entries.Count : 0;
+        }
+
+        /// <summary>Gives a seam-fed message a file position of its own so positional attribution behaves like in a real pass.</summary>
+        internal void NextTestEntry()
+        {
+            if (_currentSourceFileName == null) _currentSourceFileName = TestSourceFileName;
+            _currentEntryOffset = ++_testEntryOrdinal * 4096;
+        }
+    }
+}

src/Agent/AutopilotMonitor.Agent.V2.Core/Monitoring/Enrollment/Ime/ImeLogTracker.cs

@@ -181,7 +181,6 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Enrollment.Ime
         // arrive across multiple log entries and IME may interleave platform scripts).
         private readonly Dictionary<string, ScriptExecutionState> _pendingPlatformScripts =
             new Dictionary<string, ScriptExecutionState>(StringComparer.OrdinalIgnoreCase);
-        private string _lastPlatformScriptPolicyId;
 
         // Health-script (remediation) line-by-line accumulator — single slot because IME
         // executes health scripts SEQUENTIALLY within a session (verified across multiple
@@ -367,6 +366,10 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Enrollment.Ime
                     FilesTailed = _filesTailed,
                     BacklogBytes = _backlogBytes,
                     ImeAgentVersion = _imeAgentVersionSeen,
+                    OverwriteRewinds = _overwriteRewinds,
+                    OverwriteBytesReprocessed = _overwriteBytesReprocessed,
+                    VerifyPasses = _verifyPasses,
+                    VerifiedBytes = _verifiedBytes,
                     PatternHits = new Dictionary<string, int>(_patternHits, StringComparer.OrdinalIgnoreCase),
                 };
             }
@@ -1140,6 +1143,10 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Enrollment.Ime
                     HealthScriptResultParseFailures = state.Health.HealthScriptResultParseFailures;
                     _imeAgentVersionSeen = state.Health.ImeAgentVersion;
                     _trackerDegradedFired = state.Health.TrackerDegradedFired;
+                    _overwriteRewinds = state.Health.OverwriteRewinds;
+                    _overwriteBytesReprocessed = state.Health.OverwriteBytesReprocessed;
+                    _verifyPasses = state.Health.VerifyPasses;
+                    _verifiedBytes = state.Health.VerifiedBytes;
                 }
                 if (state.PatternHits != null)
                 {
@@ -1209,6 +1216,10 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Enrollment.Ime
                     HealthScriptResultParseFailures = HealthScriptResultParseFailures,
                     ImeAgentVersion = _imeAgentVersionSeen,
                     TrackerDegradedFired = _trackerDegradedFired,
+                    OverwriteRewinds = _overwriteRewinds,
+                    OverwriteBytesReprocessed = _overwriteBytesReprocessed,
+                    VerifyPasses = _verifyPasses,
+                    VerifiedBytes = _verifiedBytes,
                 };
                 state.PatternHits = new Dictionary<string, int>(_patternHits, StringComparer.OrdinalIgnoreCase);
             }

src/Agent/AutopilotMonitor.Agent.V2.Core/Monitoring/Enrollment/Ime/ImeTrackerHealth.cs

@@ -46,6 +46,23 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Enrollment.Ime
         /// <summary>IME version as logged by IME itself ("Agent version is: …"), null until seen.</summary>
         public string ImeAgentVersion { get; set; }
 
+        /// <summary>
+        /// Times the bookmark of a multi-writer log (AgentExecutor.log, IntuneManagementExtension.log)
+        /// was moved back because bytes behind it had been overwritten by a concurrent IME
+        /// process. Expected nonzero whenever user-context scripts or detection scripts ran —
+        /// recovered data, not skipped work.
+        /// </summary>
+        public long OverwriteRewinds { get; set; }
+
+        /// <summary>Bytes re-read by those rewinds (only entries that changed are processed again).</summary>
+        public long OverwriteBytesReprocessed { get; set; }
+
+        /// <summary>Full ledger verifications run (cadence while scripts are in flight, end signals, fragments).</summary>
+        public long VerifyPasses { get; set; }
+
+        /// <summary>Bytes re-read for verification, guard-zone checks included — the cost of the overwrite detection.</summary>
+        public long VerifiedBytes { get; set; }
+
         /// <summary>Match count per enabled pattern ID — every enabled pattern is present, zeros included.</summary>
         public IReadOnlyDictionary<string, int> PatternHits { get; set; }
 

src/Agent/AutopilotMonitor.Agent.V2.Core/Monitoring/Enrollment/Ime/ImeTrackerStatePersistence.cs

@@ -165,6 +165,11 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Enrollment.Ime
         public int HealthScriptResultParseFailures { get; set; }
         public string ImeAgentVersion { get; set; }
         public bool TrackerDegradedFired { get; set; }
+        // Overwrite detection counters (null-defaulting on older state files).
+        public long OverwriteRewinds { get; set; }
+        public long OverwriteBytesReprocessed { get; set; }
+        public long VerifyPasses { get; set; }
+        public long VerifiedBytes { get; set; }
     }
 
     public class PackageStateData

src/Agent/AutopilotMonitor.Agent.V2.Core/Monitoring/Telemetry/Periodic/AgentSelfMetricsCollector.cs

@@ -209,6 +209,10 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Periodic
                         data["ime_line_budget_breaks"] = health.BudgetBreaks;
                         data["ime_held_tails"] = health.HeldTails;
                         data["ime_unanchored_patterns"] = health.UnanchoredPatterns;
+                        data["ime_overwrite_rewinds"] = health.OverwriteRewinds;
+                        data["ime_overwrite_bytes_reprocessed"] = health.OverwriteBytesReprocessed;
+                        data["ime_verify_passes"] = health.VerifyPasses;
+                        data["ime_verified_bytes"] = health.VerifiedBytes;
                     }
                 }
                 catch (Exception ex)

src/Agent/AutopilotMonitor.Agent.V2.Core/Termination/EnrollmentTerminationHandler.cs

@@ -584,6 +584,10 @@ namespace AutopilotMonitor.Agent.V2.Core.Termination
                     ["heldTails"] = health.HeldTails,
                     ["unanchoredPatterns"] = health.UnanchoredPatterns,
                     ["hsParseFailures"] = health.HealthScriptResultParseFailures,
+                    ["overwriteRewinds"] = health.OverwriteRewinds,
+                    ["overwriteBytesReprocessed"] = health.OverwriteBytesReprocessed,
+                    ["verifyPasses"] = health.VerifyPasses,
+                    ["verifiedBytes"] = health.VerifiedBytes,
                     ["agentVersion"] = _agentVersion ?? string.Empty,
                 };
                 if (!string.IsNullOrEmpty(health.ImeAgentVersion))

src/Backend/AutopilotMonitor.Functions.Tests/BuiltInRulesTests.cs

@@ -175,7 +175,7 @@ public class BuiltInRulesTests
             "cancelStuckAndSetCurrent", "imeSessionChange", "imeImpersonation", "imeTokenFailure",
             "enrollmentCompleted", "userSessionZeroApps", "updateDoTelemetry", "scriptStarted",
             "scriptContext", "scriptExitCode", "scriptOutput", "scriptCompleted",
-            "resetPlatformScriptContext",
+            "resetPlatformScriptContext", "agentInvocationArgument", "agentExecutorCompleted",
             "healthScriptResult", "healthScriptDetectionResult",
             "captureExitCode", "captureHResult",
             "captureAppVersion", "captureAppTypeWinGet", "captureAppTypeMsi",