feat: EnableGatherRuleDebugLog — per-tenant gather-rule debug trace (ConfigVersion 36)

Customers cannot tell why a gather rule produces nothing in the timeline
(e.g. a logparser rule with emit_mode on_change that never emits). New
per-tenant boolean EnableGatherRuleDebugLog (EnableImeMatchLog-style
plumbing end to end): when enabled, the agent writes
%ProgramData%\AutopilotMonitor\Logs\gather_rules_debug.log with one
pipe-delimited line per evaluation decision across all previously silent
"produced nothing" exits — disabled rules, interval without
intervalSeconds, first-tick-after-one-full-interval, phase-scope skips
incl. currentPhase=Unknown, per-(rule, phase) dedup, empty collector
results (emitOnlyIfExists), on_change hash suppression (hash prefix +
streak), guard blocks, logparser per-file outcomes (position skips,
format=text hint when every line fails CMTrace parsing, maxLines and
wildcard caps), and full exception stacks the agent log drops.

- Backend: ConfigVersion 35 -> 36; TenantConfiguration/AgentConfigResponse
  field + TableConfigRepository Store+Map
- Agent: GatherRuleDebugLog writer (10 MB cap, one .old generation,
  failures swallowed, optional console echo), GatherRuleContext.DebugLog
  (null-safe no-op), trace points in executor + collectors; never writes
  under _scopeLock (reasons captured inside, written after release);
  merger bool->path projection with --gather-debug-log CLI override
  tolerance; deliberately NOT in the offline-cache strip lists;
  --run-gather-rules always traces; zero-rules delivery leaves a one-line
  file instead of nothing
- Side fix: GatherRuleExecutorHost now passes the logger into
  InformationalEventPost so gather-event emissions hit the DEBUG log
- Web: tenant settings toggle (Settings -> Agent) + admin tenant-config
  report rows
- Tests: merger on/off/override, writer format+rotation, executor debug
  lines, logparser debug lines (agent 2131, backend 3561, tsc green)
- Docs: docs/agent/gather-rule-debug-log.md + index/log entries

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/agent/gather-rule-debug-log.md

@@ -0,0 +1,108 @@
+---
+type: Concept
+title: Gather Rule Debug Log — the per-tenant evaluation trace for rules that produce nothing
+description: EnableGatherRuleDebugLog (ConfigVersion 36) makes the agent write a local trace file explaining every gather-rule evaluation decision — scope skips, missing intervalSeconds, on_change suppression, empty collector results, logparser position/parse-mode/no-match outcomes — so customers can self-diagnose rules that never show up in the timeline.
+resource: /src/Agent/AutopilotMonitor.Agent.V2.Core/Monitoring/Telemetry/Gather/GatherRuleDebugLog.cs
+tags:
+  - agent
+  - gather-rules
+  - diagnostics
+  - tenant-configuration
+timestamp: 2026-07-28T00:00:00+02:00
+---
+
+# Gather Rule Debug Log
+
+## Schema
+
+### The problem
+
+A gather rule that produces no timeline events is indistinguishable from a rule that
+never ran. The pipeline has many *silent-nothing* exits — before this feature, none of
+them were visible to the customer:
+
+| # | Decision point | Where |
+|---|---|---|
+| 1 | Rule delivered but `Enabled=false` | `GatherRuleExecutor.UpdateRules` |
+| 2 | `trigger=interval` without `intervalSeconds` — never runs | `UpdateRules` timer filter |
+| 3 | Interval timer's first tick comes only after one **full** interval | `UpdateRules` timer creation |
+| 4 | Out of phase scope (`activePhases` / `activeFromPhase` not latched); incl. `currentPhase=Unknown` before the first phase signal | `IsRuleInScopeLocked` |
+| 5 | phase_change / phase_exit dedup — once per (rule, phase) | `OnPhaseChanged` |
+| 6 | Collector returned an **empty** result (e.g. `emitOnlyIfExists=true` and target absent) | `ExecuteRule` |
+| 7 | `emit_mode=on_change` — result hash unchanged, emit suppressed | `ShouldEmitOnChange` |
+| 8 | Guard blocked the target (emits `security_warning`, but the rule yields no data) | `GatherRuleContext.EmitSecurityWarning` |
+| 9 | logparser: no files matched, position tracker says nothing new, regex no-match, plain-text file parsed in default `cmtrace` mode, `maxLines` cap, wildcard capped at 20 files | `LogParserCollector` |
+| 10 | Rule execution threw (agent log had only the message, no stack) | `ExecuteRule` catch |
+
+### The setting
+
+`TenantConfiguration.EnableGatherRuleDebugLog` (`bool?`, default null→false), served to the
+agent as `AgentConfigResponse.EnableGatherRuleDebugLog` — **ConfigVersion 36**. Portal:
+Settings → Agent → Settings, toggle "Gather Rule Debug Log", stored via the usual
+tenant-config roundtrip (`TableConfigRepository` Store+Map). Also shown in the admin
+tenant-config report.
+
+`RemoteConfigMerger` projects the bool onto a runtime path (same pattern as
+`EnableImeMatchLog`): `true` → `Constants.GatherRuleDebugLogPath` =
+`%ProgramData%\AutopilotMonitor\Logs\gather_rules_debug.log`; `false` → null, unless a
+`--gather-debug-log <path>` CLI override supplied a non-default path. The field is not
+security-sensitive and deliberately **survives the offline config cache**.
+
+### The trace file
+
+`GatherRuleDebugLog` is an append-only writer held by `GatherRuleExecutor` and exposed
+to collectors via `GatherRuleContext.DebugLog(ruleId, stage, message)` (null-safe no-op
+when disabled — zero cost on the normal path). Line format:
+
+```
+2026-07-28T09:15:02.123Z | RULE-ID | stage | message
+```
+
+Stages: `config` (registration summary per rule, header, zero-rules note), `trigger`
+(timer scheduled, phase/event fired, dedup), `scope` (skips with reason incl. current
+phase), `exec` (execution start), `collector` (null/empty results with likely cause),
+`guard` (allowlist block), `emit` (event emitted, hash change), `suppress` (on_change
+suppression with hash prefix + streak), `logparser` (per-file outcome: lines read,
+position range, match count, parse failures, regex timeouts, mode; plus the
+`format=text` hint when every line fails CMTrace parsing), `error` (full exception
+**with stack trace** — the agent log keeps only the message).
+
+Volume control: 10 MB cap with a single `.old` rotation generation (~20 MB worst case).
+Every write failure is swallowed — tracing must never break gathering. The trace never
+leaves the device on its own — but because it lives in the agent log folder
+(`Constants.LogDirectory`), the diagnostics package (`DiagnosticsPackageService`,
+`*.log` pattern, `AgentLogs/` section) picks it up automatically, so a customer-initiated
+diagnostics upload hands the trace to support without manual copying.
+
+### Semantics
+
+* **No hot-reload** — the flag is consumed once when `DefaultComponentFactory` builds
+  the `GatherRuleExecutorHost` at session start (per-enrollment lifecycle, same as
+  `EnableImeMatchLog`). Enable it in the tenant, then the *next* enrollment writes the
+  trace.
+* **Zero-rules case is traced too** — flag on + backend delivered no gather rules →
+  a one-shot standalone `config` line, so the file is never just absent.
+* **`--run-gather-rules` always traces** (operator diagnostic mode; CLI override or
+  default path) and mirrors every line to the console under `--console`.
+
+## Examples
+
+A logparser rule with `emit_mode=on_change` that "delivers nothing" becomes:
+
+```
+... | LP-01 | config | registered: trigger=interval, collector=logparser, target=C:\...\app.log, unscoped, emitMode=on_change, interval=300s
+... | LP-01 | trigger | interval timer scheduled every 300s (first run after one full interval — no immediate execution)
+... | LP-01 | exec | executing: collector=logparser, target=C:\...\app.log
+... | LP-01 | logparser | app.log: read 1000 lines from position 0->81234, matched 0, parseFailures=1000, regexTimeouts=0, mode=cmtrace
+... | LP-01 | logparser | every line failed CMTrace parsing — if this is a plain-text log, set parameter format=text
+```
+
+— the answer ("plain-text log parsed in CMTrace mode") is in the file.
+
+## Citations
+
+* Executor + trace points: `../../src/Agent/AutopilotMonitor.Agent.V2.Core/Monitoring/Telemetry/Gather/GatherRuleExecutor.cs`
+* Writer: `../../src/Agent/AutopilotMonitor.Agent.V2.Core/Monitoring/Telemetry/Gather/GatherRuleDebugLog.cs`
+* Config projection: `../../src/Agent/AutopilotMonitor.Agent.V2.Core/Configuration/RemoteConfigMerger.cs`
+* Backend serve + ConfigVersion: `../../src/Backend/AutopilotMonitor.Functions/Functions/Config/GetAgentConfigFunction.cs`
+* Phase scoping / emit mode background: `../rules/gather-rule-phase-scoping.md`

docs/index.md

@@ -20,6 +20,7 @@ https://docs.autopilotmonitor.com (separate repository).
 * [V2 Agent](agent/index.md) - How the on-device agent works: [runtime overview](agent/overview.md), [decision engine](agent/decision-engine.md) (reducer, signals, completion arms), [logs & persistence](agent/logs-and-persistence.md) (signal log, snapshot, crash recovery), [Hello wizard un-skip](agent/hello-wizard-unskip.md) (why a single "WHfB disabled" read never decides completion), [Autopilot ZTD diagnostics](agent/autopilot-ztd-diagnostics.md) (event IDs, diagnostic registry, endpoints, error-code map).
 * [Build Counter Blob](agent/build-counter-blob.md) - How agent build numbers are minted: shared counter blob with ETag-CAS so local and CI builds never collide; CI additionally attests build provenance (Sigstore).
 * [Agent Endpoint Migration](agent/endpoint-migration.md) - How the backend re-homes agents to a new API base URL via the config channel (backend move or per-tenant region move): live-fetch-only, allowlist-validated on both sides, one hop, kill wins.
+* [Gather Rule Debug Log](agent/gather-rule-debug-log.md) - EnableGatherRuleDebugLog (ConfigVersion 36): the per-tenant local trace file that explains every gather-rule evaluation decision — scope skips, on_change suppression, empty collector results, logparser position/parse-mode outcomes — for rules that produce no timeline events.
 * [MDM Reboot Coalescing](agent/mdm-reboot-coalescing.md) - Attributing the mid-ESP coalesced reboot ("second sign-in") to the device-assigned policy URIs that forced it: aggregated DM-Enterprise 2800 observations (neutral, flush-time watermark) + the ANALYZE-ESP-005 advisory gated on an actually-observed reboot.
 * [Secure Boot CA-2023 Detection](agent/secureboot-ca2023-detection.md) - Why ANALYZE-SEC-001 v3 reads the UEFI db/KEK variables directly instead of trusting the SecureBoot\Servicing registry, and how the one-sided uefiCa2023FirmwareConfirmed marker plus a not_exists precondition give absence-tolerant suppression across old and new agents.
 * [ESP Policy-Provider Stall Detection](agent/esp-policy-provider-stall.md) - The EnrollmentStatusTracking CSP's no-timeout Setup/Apps wait is keyed to the Intune-registered "Sidecar" provider by NAME: co-management's "ConfigMgr" parks the user ESP at "Apps (Identifying)" for days even with TrackingPoliciesCreated=1 (issue #106); the always-on two-arm esp_policy_provider_stalled tripwire (provider_incomplete | sidecar_provider_missing) and the SOFTWARE\Microsoft\Windows\Autopilot gather-rule allowlist entry that surface it.

docs/log.md

@@ -1,5 +1,9 @@
 # Log
 
+## 2026-07-28
+
+* **Creation**: Added `agent/gather-rule-debug-log.md` — the new per-tenant setting `EnableGatherRuleDebugLog` (ConfigVersion 35 → 36, portal toggle under Settings → Agent, `EnableImeMatchLog`-style bool→path projection in `RemoteConfigMerger`). When enabled, the agent writes `%ProgramData%\AutopilotMonitor\Logs\gather_rules_debug.log`: one pipe-delimited line per evaluation decision across ten previously silent "produced nothing" exits — disabled rules, `interval` without `intervalSeconds` (never runs), the first-tick-after-one-full-interval window, phase-scope skips incl. `currentPhase=Unknown`, per-(rule, phase) dedup, empty collector results (`emitOnlyIfExists`), `on_change` hash suppression (hash prefix + streak), guard blocks, logparser per-file outcomes (position skips, parse failures with a `format=text` hint when every line fails CMTrace parsing, `maxLines`/wildcard caps), and full exception stacks that the agent log drops. 10 MB cap + one `.old` generation; no hot-reload (per-enrollment lifecycle); flag survives the offline config cache; `--run-gather-rules` always traces and echoes under `--console`; zero-rules delivery leaves a one-line file instead of nothing. Side fix: `GatherRuleExecutorHost` now passes the logger into `InformationalEventPost`, so gather-event emissions appear in the agent DEBUG log.
+
 ## 2026-07-27
 
 * **Creation**: Added `web/portal-navigation-prefetch.md` (and a `Web Portal` section to the index) — root cause of the minute-long portal freezes. `GlobalSidebar` and `Navbar` render the whole navigation on every page, so the App Router's default viewport prefetch issued one `?_rsc=` request per visible link against `portal.autopilotmonitor.com` (~10,400 in 14 days). That origin serves the document, the route chunks and the RSC payloads over **HTTP/1.1**, so the six-connection-per-origin cap applies; when the Static Web Apps managed runtime queued the prefetches (p50 91 ms, p95 896 ms, but max 234 s / 21 samples over 30 s), the in-flight prefetches held every socket and the navigation the user wanted could not get one. Measured on the 2026-07-27 17:25 CEST incident: `/admin/presence?_rsc=` 151 s, three siblings finishing within 340 ms of each other at ~37.5 s, all `200`, while the backend API answered in 1.6–2.2 s and `browserTimings` showed `networkDuration` 16 ms against `processingDuration` 225 s. Every navigation Link now sets `prefetch={false}` — these routes are client components behind `ProtectedRoute` that fetch their own data, so the prefetched RSC payload carried no benefit. The doc also records that browser telemetry lives in a **separate** App Insights resource (`AUTOPILOT_MONITOR_WEB_APPINSIGHTS_ID`) that the MCP `query_backend_logs` tool cannot reach, which is why the freezes left no trace in the backend logs.

src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Configuration/RemoteConfigMergerTests.cs

@@ -285,6 +285,54 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Configuration
             Assert.Equal(@"C:\Temp\custom-ime.log", agent.ImeMatchLogPath);
         }
 
+        [Fact]
+        public void Merge_maps_enable_gather_rule_debug_log_true_to_default_path()
+        {
+            var agent = NewAgentConfig();
+            agent.GatherRuleDebugLogPath = null;
+
+            var remote = FullRemote();
+            remote.EnableGatherRuleDebugLog = true;
+
+            RemoteConfigMerger.Merge(agent, remote);
+
+            Assert.False(string.IsNullOrEmpty(agent.GatherRuleDebugLogPath));
+            Assert.Equal(
+                Environment.ExpandEnvironmentVariables(Constants.GatherRuleDebugLogPath),
+                agent.GatherRuleDebugLogPath);
+        }
+
+        [Fact]
+        public void Merge_maps_enable_gather_rule_debug_log_false_clears_path()
+        {
+            var agent = NewAgentConfig();
+            agent.GatherRuleDebugLogPath = Environment.ExpandEnvironmentVariables(Constants.GatherRuleDebugLogPath);
+
+            var remote = FullRemote();
+            remote.EnableGatherRuleDebugLog = false;
+
+            RemoteConfigMerger.Merge(agent, remote);
+
+            Assert.Null(agent.GatherRuleDebugLogPath);
+        }
+
+        [Fact]
+        public void Merge_preserves_custom_gather_rule_debug_log_path_when_remote_disables()
+        {
+            // A dev-time --gather-debug-log <custom> override seeds a non-default path on the
+            // AgentConfiguration. When the tenant remote flips EnableGatherRuleDebugLog off we
+            // keep the custom path rather than clobbering a deliberate dev override.
+            var agent = NewAgentConfig();
+            agent.GatherRuleDebugLogPath = @"C:\Temp\custom-gather.log";
+
+            var remote = FullRemote();
+            remote.EnableGatherRuleDebugLog = false;
+
+            RemoteConfigMerger.Merge(agent, remote);
+
+            Assert.Equal(@"C:\Temp\custom-gather.log", agent.GatherRuleDebugLogPath);
+        }
+
         // ============================================================= Null / partial tolerance
 
         [Fact]

src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Monitoring/Gather/GatherRuleDebugLogTests.cs

@@ -0,0 +1,104 @@
+using System;
+using System.IO;
+using System.Linq;
+using AutopilotMonitor.Agent.V2.Core.Logging;
+using AutopilotMonitor.Agent.V2.Core.Monitoring.Enrollment.Ime;
+using AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather;
+using AutopilotMonitor.Agent.V2.Core.Tests.Harness;
+using Xunit;
+
+namespace AutopilotMonitor.Agent.V2.Core.Tests.Monitoring.Gather
+{
+    /// <summary>
+    /// Writer unit tests for <see cref="GatherRuleDebugLog"/>: line format, lazy directory
+    /// creation, size-cap rotation, and the null-writer no-op path on the context.
+    /// </summary>
+    public sealed class GatherRuleDebugLogTests : IDisposable
+    {
+        private readonly TempDirectory _tmp = new TempDirectory();
+        private readonly AgentLogger _logger;
+
+        public GatherRuleDebugLogTests()
+        {
+            _logger = new AgentLogger(_tmp.Path, AgentLogLevel.Info);
+        }
+
+        public void Dispose() => _tmp.Dispose();
+
+        private string LogPath => Path.Combine(_tmp.Path, "sub", "gather_rules_debug.log");
+
+        [Fact]
+        public void Write_produces_pipe_delimited_line_and_creates_directory_lazily()
+        {
+            var log = new GatherRuleDebugLog(LogPath, _logger);
+            Assert.False(File.Exists(LogPath)); // lazy — nothing until the first write
+
+            log.Write("RULE-1", GatherRuleDebugLog.StageScope, "skipped: out of scope");
+
+            var lines = File.ReadAllLines(LogPath);
+            Assert.Equal(2, lines.Length); // header + entry
+            Assert.Contains("| - | config | -- gather rule debug log started --", lines[0]);
+            var parts = lines[1].Split(new[] { " | " }, StringSplitOptions.None);
+            Assert.Equal(4, parts.Length);
+            Assert.Equal("RULE-1", parts[1]);
+            Assert.Equal("scope", parts[2]);
+            Assert.Equal("skipped: out of scope", parts[3]);
+        }
+
+        [Fact]
+        public void Write_flattens_newlines_except_for_error_stage()
+        {
+            var log = new GatherRuleDebugLog(LogPath, _logger);
+            log.Write("RULE-1", GatherRuleDebugLog.StageCollector, "line1\r\nline2");
+            log.Write("RULE-1", GatherRuleDebugLog.StageError, "boom\r\n   at Frame()");
+
+            var content = File.ReadAllText(LogPath);
+            Assert.Contains("line1 line2", content);
+            Assert.Contains("   at Frame()", content); // error stage keeps the stack multi-line
+        }
+
+        [Fact]
+        public void Write_null_ruleId_renders_dash()
+        {
+            var log = new GatherRuleDebugLog(LogPath, _logger);
+            log.Write(null, GatherRuleDebugLog.StageConfig, "no rules delivered");
+
+            Assert.Contains(" | - | config | no rules delivered", File.ReadAllText(LogPath));
+        }
+
+        [Fact]
+        public void Write_rotates_once_when_file_exceeds_cap()
+        {
+            var log = new GatherRuleDebugLog(LogPath, _logger, maxBytes: 512);
+            for (int i = 0; i < 20; i++)
+                log.Write("RULE-1", GatherRuleDebugLog.StageExec, new string('x', 100));
+
+            var oldPath = Path.Combine(Path.GetDirectoryName(LogPath), "gather_rules_debug.old.log");
+            Assert.True(File.Exists(oldPath), "expected one .old rotation generation");
+            Assert.True(File.Exists(LogPath));
+            Assert.Contains("-- rotated", File.ReadAllText(LogPath) + File.ReadAllText(oldPath));
+            // Never more than one .old generation
+            Assert.Single(Directory.GetFiles(Path.GetDirectoryName(LogPath), "*.old.log"));
+        }
+
+        [Fact]
+        public void WriteStandalone_writes_single_config_line()
+        {
+            GatherRuleDebugLog.WriteStandalone(LogPath, "no gather rules delivered by backend — nothing to execute", _logger);
+
+            var lines = File.ReadAllLines(LogPath);
+            Assert.Contains(lines, l => l.Contains("no gather rules delivered by backend"));
+        }
+
+        [Fact]
+        public void Context_DebugLog_is_noop_without_writer()
+        {
+            var context = new GatherRuleContext(
+                _logger, "sess", "tenant", _ => { }, null, new LogFilePositionTracker());
+
+            context.DebugLog("RULE-1", GatherRuleDebugLog.StageScope, "should not throw, should not write");
+
+            Assert.False(File.Exists(LogPath));
+        }
+    }
+}

src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Monitoring/Gather/GatherRuleExecutorDebugLogTests.cs

@@ -0,0 +1,156 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using AutopilotMonitor.Agent.V2.Core.Logging;
+using AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather;
+using AutopilotMonitor.Agent.V2.Core.Tests.Harness;
+using AutopilotMonitor.Shared.Models;
+using Xunit;
+
+namespace AutopilotMonitor.Agent.V2.Core.Tests.Monitoring.Gather
+{
+    /// <summary>
+    /// Debug-trace coverage of <see cref="GatherRuleExecutor"/> (EnableGatherRuleDebugLog):
+    /// every "silently produced nothing" outcome must leave an explanatory line in the trace
+    /// file — registration summary, missing interval, scope skips, on_change suppression,
+    /// empty collector results.
+    /// </summary>
+    [Collection("SerialThreading")] // startup rules execute on the shared ThreadPool
+    public sealed class GatherRuleExecutorDebugLogTests : IDisposable
+    {
+        private readonly TempDirectory _tmp = new TempDirectory();
+        private readonly GatherRuleExecutor _executor;
+        private readonly string _debugLogPath;
+
+        private const string AbsentPath =
+            "HKLM\\SOFTWARE\\AutopilotMonitorTests\\DefinitelyAbsent_7e3a1f4b-0000-0000-0000-000000000000";
+
+        public GatherRuleExecutorDebugLogTests()
+        {
+            var logger = new AgentLogger(_tmp.Path, AgentLogLevel.Info);
+            _debugLogPath = Path.Combine(_tmp.Path, "gather_rules_debug.log");
+            _executor = new GatherRuleExecutor(
+                "sess", "tenant", _ => { }, logger,
+                debugLogPath: _debugLogPath)
+            {
+                UnrestrictedMode = true
+            };
+        }
+
+        public void Dispose()
+        {
+            _executor.Dispose();
+            _tmp.Dispose();
+        }
+
+        private string TraceContent() => File.Exists(_debugLogPath) ? File.ReadAllText(_debugLogPath) : string.Empty;
+
+        private static GatherRule Rule(string id, Action<GatherRule>? mutate = null)
+        {
+            var rule = new GatherRule
+            {
+                RuleId = id,
+                Title = id,
+                CollectorType = "registry",
+                Target = AbsentPath,
+                Trigger = "startup",
+                OutputEventType = "gather_test",
+                Enabled = true,
+            };
+            mutate?.Invoke(rule);
+            return rule;
+        }
+
+        [Fact]
+        public void UpdateRules_writes_registration_summary_per_rule()
+        {
+            _executor.UpdateRules(new List<GatherRule>
+            {
+                Rule("GATHER-REG-001", r => { r.Trigger = "interval"; r.IntervalSeconds = 3600; r.EmitMode = "on_change"; }),
+                Rule("GATHER-REG-002", r => r.Enabled = false),
+            });
+            _executor.WaitForStartupRules(30);
+
+            var trace = TraceContent();
+            Assert.Contains("GATHER-REG-001 | config | registered: trigger=interval, collector=registry", trace);
+            Assert.Contains("emitMode=on_change", trace);
+            Assert.Contains("interval=3600s", trace);
+            Assert.Contains("GATHER-REG-002 | config | rule disabled — will never run", trace);
+            Assert.Contains("interval timer scheduled every 3600s (first run after one full interval", trace);
+        }
+
+        [Fact]
+        public void UpdateRules_flags_interval_rule_without_intervalSeconds()
+        {
+            _executor.UpdateRules(new List<GatherRule>
+            {
+                Rule("GATHER-NOINT-001", r => { r.Trigger = "interval"; r.IntervalSeconds = null; }),
+            });
+
+            Assert.Contains("GATHER-NOINT-001 | error | trigger=interval but intervalSeconds is missing — rule will NEVER run",
+                TraceContent());
+        }
+
+        [Fact]
+        public void ScopedRule_skip_traces_unknown_phase_reason()
+        {
+            var rule = Rule("GATHER-SCOPE-001", r => { r.Trigger = "interval"; r.IntervalSeconds = 3600; r.ActivePhases = new List<string> { "DeviceSetup" }; });
+            _executor.UpdateRules(new List<GatherRule> { rule });
+
+            // Synchronous internal check mirrors the timer-tick gate; the reason must name Unknown.
+            string reason;
+            Assert.False(_executor.IsRuleInScope(rule, out reason));
+            Assert.Contains("currentPhase=Unknown", reason);
+        }
+
+        [Fact]
+        public void ScopedStartupRule_deferral_is_traced()
+        {
+            _executor.UpdateRules(new List<GatherRule>
+            {
+                Rule("GATHER-DEFER-001", r => r.ActivePhases = new List<string> { "DeviceSetup" }),
+            });
+
+            Assert.Contains("GATHER-DEFER-001 | scope | startup rule deferred — currentPhase=Unknown", TraceContent());
+        }
+
+        [Fact]
+        public void OnChange_suppression_and_reemission_are_traced()
+        {
+            var rule = Rule("GATHER-OC-001", r => { r.Trigger = "interval"; r.IntervalSeconds = 3600; r.EmitMode = "on_change"; });
+            _executor.UpdateRules(new List<GatherRule> { rule });
+
+            string hashPrefix;
+            int streak;
+            var first = new Dictionary<string, object> { ["exists"] = false };
+            Assert.True(_executor.ShouldEmitOnChange(rule, first, out hashPrefix, out streak));
+            Assert.Equal(0, streak);
+            Assert.False(string.IsNullOrEmpty(hashPrefix));
+
+            Assert.False(_executor.ShouldEmitOnChange(rule, new Dictionary<string, object> { ["exists"] = false }, out hashPrefix, out streak));
+            Assert.Equal(1, streak);
+            Assert.False(_executor.ShouldEmitOnChange(rule, new Dictionary<string, object> { ["exists"] = false }, out hashPrefix, out streak));
+            Assert.Equal(2, streak);
+
+            var changed = new Dictionary<string, object> { ["exists"] = true };
+            Assert.True(_executor.ShouldEmitOnChange(rule, changed, out hashPrefix, out streak));
+            Assert.Equal(2, streak); // the streak that just ended
+            Assert.Equal(2, changed["suppressedPolls"]);
+        }
+
+        [Fact]
+        public void EmptyCollectorResult_is_traced_via_startup_execution()
+        {
+            _executor.UpdateRules(new List<GatherRule>
+            {
+                Rule("GATHER-EMPTY-001", r => r.Parameters = new Dictionary<string, string> { ["emitOnlyIfExists"] = "true" }),
+            });
+            Assert.True(_executor.WaitForStartupRules(30));
+
+            var trace = TraceContent();
+            Assert.Contains("GATHER-EMPTY-001 | exec | executing: collector=registry", trace);
+            Assert.Contains("GATHER-EMPTY-001 | collector | registry key/value not found", trace);
+            Assert.Contains("GATHER-EMPTY-001 | collector | collector returned EMPTY result — nothing emitted", trace);
+        }
+    }
+}

src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Monitoring/Gather/LogParserCollectorDebugLogTests.cs

@@ -0,0 +1,121 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using AutopilotMonitor.Agent.V2.Core.Logging;
+using AutopilotMonitor.Agent.V2.Core.Monitoring.Enrollment.Ime;
+using AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather;
+using AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather.Collectors;
+using AutopilotMonitor.Agent.V2.Core.Tests.Harness;
+using AutopilotMonitor.Shared.Models;
+using Xunit;
+
+namespace AutopilotMonitor.Agent.V2.Core.Tests.Monitoring.Gather
+{
+    /// <summary>
+    /// Debug-trace coverage of <see cref="LogParserCollector"/>: the classic
+    /// "rule delivers nothing" cases must be explained in the trace — zero matches,
+    /// position-tracker skips, and plain-text logs parsed in the default cmtrace mode.
+    /// </summary>
+    public sealed class LogParserCollectorDebugLogTests : IDisposable
+    {
+        private readonly TempDirectory _tmp = new TempDirectory();
+        private readonly List<EnrollmentEvent> _events = new List<EnrollmentEvent>();
+        private readonly GatherRuleContext _context;
+        private readonly string _debugLogPath;
+
+        // %TEMP% lives under C:\Users (hard-blocked) — parse targets sit beside the
+        // test assembly instead, admitted via UnrestrictedMode.
+        private readonly string _outsideUsers = Path.Combine(
+            AppContext.BaseDirectory, "gather-debug-tests-" + Guid.NewGuid().ToString("N"));
+
+        public LogParserCollectorDebugLogTests()
+        {
+            Directory.CreateDirectory(_outsideUsers);
+            var logger = new AgentLogger(_tmp.Path, AgentLogLevel.Info);
+            _debugLogPath = Path.Combine(_tmp.Path, "gather_rules_debug.log");
+            _context = new GatherRuleContext(
+                logger, "sess", "tenant",
+                evt => _events.Add(evt),
+                null,
+                new LogFilePositionTracker(),
+                new GatherRuleDebugLog(_debugLogPath, logger))
+            {
+                UnrestrictedMode = true
+            };
+        }
+
+        public void Dispose()
+        {
+            _tmp.Dispose();
+            try { Directory.Delete(_outsideUsers, recursive: true); } catch { /* best effort */ }
+        }
+
+        private string TraceContent() => File.Exists(_debugLogPath) ? File.ReadAllText(_debugLogPath) : string.Empty;
+
+        private static GatherRule Rule(string target, string pattern, string? format = null) => new GatherRule
+        {
+            RuleId = "GATHER-LP-DEBUG",
+            Title = "logparser debug test",
+            CollectorType = "logparser",
+            Target = target,
+            Parameters = format == null
+                ? new Dictionary<string, string> { ["pattern"] = pattern }
+                : new Dictionary<string, string> { ["pattern"] = pattern, ["format"] = format },
+            Trigger = "startup",
+            OutputEventType = "logparser_debug_test",
+        };
+
+        [Fact]
+        public void ZeroMatches_writes_per_file_outcome_line()
+        {
+            var path = Path.Combine(_outsideUsers, "app.log");
+            File.WriteAllText(path, "nothing interesting\nstill nothing\n");
+
+            new LogParserCollector().Execute(Rule(path, "WILL_NEVER_MATCH_[0-9]+", format: "text"), _context);
+
+            Assert.Empty(_events);
+            var trace = TraceContent();
+            Assert.Contains("app.log: read 2 lines", trace);
+            Assert.Contains("matched 0", trace);
+            Assert.Contains("mode=text", trace);
+        }
+
+        [Fact]
+        public void SecondRun_without_new_content_traces_position_skip()
+        {
+            var path = Path.Combine(_outsideUsers, "twice.log");
+            File.WriteAllText(path, "one line\n");
+            var rule = Rule(path, "one", format: "text");
+
+            var collector = new LogParserCollector();
+            collector.Execute(rule, _context);   // reads the file, advances the position
+            collector.Execute(rule, _context);   // nothing new
+
+            Assert.Contains("twice.log: no new content (position", TraceContent());
+        }
+
+        [Fact]
+        public void PlainTextFile_in_default_cmtrace_mode_gets_format_hint()
+        {
+            var path = Path.Combine(_outsideUsers, "plain.log");
+            File.WriteAllText(path, "plain line one\nplain line two\n");
+
+            new LogParserCollector().Execute(Rule(path, "plain"), _context); // default = cmtrace mode
+
+            Assert.Empty(_events);
+            var trace = TraceContent();
+            Assert.Contains("parseFailures=2", trace);
+            Assert.Contains("every line failed CMTrace parsing — if this is a plain-text log, set parameter format=text", trace);
+        }
+
+        [Fact]
+        public void MissingFile_traces_no_files_matched()
+        {
+            var path = Path.Combine(_outsideUsers, "does-not-exist.log");
+
+            new LogParserCollector().Execute(Rule(path, "x", format: "text"), _context);
+
+            Assert.Contains("no files matched target", TraceContent());
+        }
+    }
+}

src/Agent/AutopilotMonitor.Agent.V2.Core/Configuration/AgentConfiguration.cs

@@ -121,6 +121,13 @@ namespace AutopilotMonitor.Agent.V2.Core.Configuration
         /// </summary>
         public string ImeMatchLogPath { get; set; }
 
+        /// <summary>
+        /// Optional path to write a gather-rule evaluation trace (for debugging).
+        /// If empty, no trace is written. Set from remote config (EnableGatherRuleDebugLog)
+        /// or the --gather-debug-log CLI override.
+        /// </summary>
+        public string GatherRuleDebugLogPath { get; set; }
+
         /// <summary>
         /// Path to directory with real IME log files for log replay.
         /// When set, the agent replays these logs with time compression instead of reading live logs.

src/Agent/AutopilotMonitor.Agent.V2.Core/Configuration/RemoteConfigMerger.cs

@@ -135,6 +135,24 @@ namespace AutopilotMonitor.Agent.V2.Core.Configuration
                 }
             }
 
+            // ---------------- GatherRuleDebugLog: same bool→path projection as ImeMatchLog
+            //                  above. `true` expands Constants.GatherRuleDebugLogPath, `false`
+            //                  nulls the runtime path unless a --gather-debug-log CLI override
+            //                  supplied a non-default path.
+            if (remoteConfig.EnableGatherRuleDebugLog)
+            {
+                agentConfig.GatherRuleDebugLogPath = Environment.ExpandEnvironmentVariables(Constants.GatherRuleDebugLogPath);
+            }
+            else
+            {
+                var defaultGatherDebugPath = Environment.ExpandEnvironmentVariables(Constants.GatherRuleDebugLogPath);
+                if (string.IsNullOrEmpty(agentConfig.GatherRuleDebugLogPath)
+                    || string.Equals(agentConfig.GatherRuleDebugLogPath, defaultGatherDebugPath, StringComparison.OrdinalIgnoreCase))
+                {
+                    agentConfig.GatherRuleDebugLogPath = null;
+                }
+            }
+
             // ---------------- Upload interval / batch — direct, V1 parity (no > 0 gate so a
             //                  misconfigured tenant can in theory break uploads, but this
             //                  surfaces the config bug instead of silently masking it).

src/Agent/AutopilotMonitor.Agent.V2.Core/Configuration/RemoteConfigService.cs

@@ -327,6 +327,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Configuration
                 KeepLogFile = false,
                 EnableGeoLocation = true,
                 EnableImeMatchLog = false,
+                EnableGatherRuleDebugLog = false,
                 Collectors = CollectorConfiguration.CreateDefault(),
                 GatherRules = new System.Collections.Generic.List<GatherRule>(),
                 ImeLogPatterns = new System.Collections.Generic.List<ImeLogPattern>()

src/Agent/AutopilotMonitor.Agent.V2.Core/Monitoring/Telemetry/Gather/Collectors/LogParserCollector.cs

@@ -25,14 +25,21 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather.Collectors
         {
             var filePath = rule.Target;
             if (string.IsNullOrEmpty(filePath))
+            {
+                context.DebugLog(rule.RuleId, GatherRuleDebugLog.StageLogParser, "empty target — nothing to parse");
                 return null;
+            }
 
             // Expand custom tokens (%LOGGED_ON_USER_PROFILE%) and standard environment variables
             var userProfilePath = UserProfileResolver.ContainsUserProfileToken(filePath)
                 ? UserProfileResolver.GetLoggedOnUserProfilePath() : null;
             filePath = UserProfileResolver.ExpandCustomTokens(filePath);
             if (filePath == null)
+            {
+                context.DebugLog(rule.RuleId, GatherRuleDebugLog.StageLogParser,
+                    "target contains %LOGGED_ON_USER_PROFILE% but no user is logged on — skipped this run");
                 return null; // Token present but no user logged on — skip silently
+            }
 
             // Apply IME log path override if set. The override comes from the local
             // --ime-log-path flag only (never from remote config), so the operator
@@ -66,6 +73,8 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather.Collectors
                 string.IsNullOrEmpty(patternStr))
             {
                 context.Logger.Warning($"LogParser rule {rule.RuleId} has no 'pattern' parameter");
+                context.DebugLog(rule.RuleId, GatherRuleDebugLog.StageError,
+                    "missing 'pattern' parameter — rule can never match");
                 return null;
             }
 
@@ -93,6 +102,8 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather.Collectors
             catch (Exception ex)
             {
                 context.Logger.Warning($"LogParser rule {rule.RuleId} has invalid regex: {ex.Message}");
+                context.DebugLog(rule.RuleId, GatherRuleDebugLog.StageError,
+                    $"invalid regex pattern — rule can never match: {ex.Message}");
                 return null;
             }
 
@@ -101,6 +112,8 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather.Collectors
             if (resolvedPaths.Count == 0)
             {
                 context.Logger.Debug($"LogParser rule {rule.RuleId}: no files found for: {filePath}");
+                context.DebugLog(rule.RuleId, GatherRuleDebugLog.StageLogParser,
+                    $"no files matched target {filePath}" + (HasWildcard(Path.GetFileName(filePath)) ? " (wildcard)" : "") + " — nothing to parse");
                 return null;
             }
 
@@ -144,7 +157,11 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather.Collectors
             try
             {
                 // Return matched files sorted by last write time (newest first), capped at 20
-                return Directory.GetFiles(directory, fileNamePart)
+                var allMatches = Directory.GetFiles(directory, fileNamePart);
+                if (allMatches.Length > 20)
+                    context.DebugLog(ruleId, GatherRuleDebugLog.StageLogParser,
+                        $"wildcard matched {allMatches.Length} files — capped to the 20 newest");
+                return allMatches
                     .OrderByDescending(f => new FileInfo(f).LastWriteTimeUtc)
                     .Take(20)
                     .ToList();
@@ -152,6 +169,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather.Collectors
             catch (Exception ex)
             {
                 context.Logger.Warning($"LogParser rule {ruleId}: wildcard expansion failed for {filePath}: {ex.Message}");
+                context.DebugLog(ruleId, GatherRuleDebugLog.StageError, $"wildcard expansion failed for {filePath}: {ex.Message}");
                 return new List<string>();
             }
         }
@@ -168,10 +186,16 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather.Collectors
 
                 // Nothing new to read
                 if (startPosition >= fileInfo.Length)
+                {
+                    context.DebugLog(rule.RuleId, GatherRuleDebugLog.StageLogParser,
+                        $"{Path.GetFileName(filePath)}: no new content (position {startPosition} >= length {fileInfo.Length}) — trackPosition=true, nothing re-read");
                     return;
+                }
 
                 int matchCount = 0;
                 int linesRead = 0;
+                int parseFailCount = 0;
+                int timeoutCount = 0;
                 long endPosition = startPosition;
 
                 using (var stream = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
@@ -195,6 +219,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather.Collectors
                                 }
                                 catch (RegexMatchTimeoutException)
                                 {
+                                    timeoutCount++;
                                     continue;
                                 }
                                 if (!match.Success)
@@ -239,7 +264,10 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather.Collectors
                                 // CMTrace mode: parse line as CMTrace format, match regex against message
                                 CmTraceLogEntry entry;
                                 if (!CmTraceLogParser.TryParseLine(line, out entry))
+                                {
+                                    parseFailCount++;
                                     continue;
+                                }
 
                                 Match match;
                                 try
@@ -248,6 +276,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather.Collectors
                                 }
                                 catch (RegexMatchTimeoutException)
                                 {
+                                    timeoutCount++;
                                     continue;
                                 }
                                 if (!match.Success)
@@ -301,10 +330,24 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather.Collectors
 
                 if (matchCount > 0)
                     context.Logger.Debug($"LogParser rule {rule.RuleId}: {matchCount} matches from {linesRead} lines in {Path.GetFileName(filePath)}");
+
+                // Per-file outcome — always written so a "matched 0" run is visible too.
+                context.DebugLog(rule.RuleId, GatherRuleDebugLog.StageLogParser,
+                    $"{Path.GetFileName(filePath)}: read {linesRead} lines from position {startPosition}->{endPosition}, " +
+                    $"matched {matchCount}, parseFailures={parseFailCount}, regexTimeouts={timeoutCount}, mode={(isTextMode ? "text" : "cmtrace")}");
+
+                if (!isTextMode && linesRead > 0 && parseFailCount == linesRead)
+                    context.DebugLog(rule.RuleId, GatherRuleDebugLog.StageLogParser,
+                        "every line failed CMTrace parsing — if this is a plain-text log, set parameter format=text");
+
+                if (linesRead == maxLines)
+                    context.DebugLog(rule.RuleId, GatherRuleDebugLog.StageLogParser,
+                        $"stopped at maxLines={maxLines} — remaining content is deferred to the next run");
             }
             catch (Exception ex)
             {
                 context.Logger.Warning($"LogParser rule {rule.RuleId} failed reading {filePath}: {ex.Message}");
+                context.DebugLog(rule.RuleId, GatherRuleDebugLog.StageError, $"failed reading {filePath}: {ex}");
             }
         }
 

src/Agent/AutopilotMonitor.Agent.V2.Core/Monitoring/Telemetry/Gather/Collectors/RegistryCollector.cs

@@ -113,7 +113,11 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather.Collectors
 
                         // Suppress the "not found" event when the rule only wants a positive signal.
                         if (emitOnlyIfExists)
+                        {
+                            context.DebugLog(rule.RuleId, GatherRuleDebugLog.StageCollector,
+                                $"registry key/value not found: {path} — emitOnlyIfExists=true, empty result");
                             return new Dictionary<string, object>(); // empty → executor emits nothing
+                        }
 
                         data["exists"] = false;
                         data["path"] = path;

src/Agent/AutopilotMonitor.Agent.V2.Core/Monitoring/Telemetry/Gather/GatherRuleContext.cs

@@ -22,13 +22,20 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather
         public LogFilePositionTracker FilePositionTracker { get; }
         public bool UnrestrictedMode { get; set; }
 
+        /// <summary>
+        /// Optional gather-rule debug trace writer (EnableGatherRuleDebugLog). Null when
+        /// tracing is disabled — use <see cref="DebugLog"/> for null-safe writes.
+        /// </summary>
+        public GatherRuleDebugLog DebugWriter { get; }
+
         public GatherRuleContext(
             AgentLogger logger,
             string sessionId,
             string tenantId,
             Action<EnrollmentEvent> onEventCollected,
             string imeLogPathOverride,
-            LogFilePositionTracker filePositionTracker)
+            LogFilePositionTracker filePositionTracker,
+            GatherRuleDebugLog debugLog = null)
         {
             Logger = logger ?? throw new ArgumentNullException(nameof(logger));
             SessionId = sessionId ?? throw new ArgumentNullException(nameof(sessionId));
@@ -36,8 +43,14 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather
             OnEventCollected = onEventCollected ?? throw new ArgumentNullException(nameof(onEventCollected));
             ImeLogPathOverride = imeLogPathOverride;
             FilePositionTracker = filePositionTracker ?? throw new ArgumentNullException(nameof(filePositionTracker));
+            DebugWriter = debugLog;
         }
 
+        /// <summary>
+        /// Null-safe write to the gather-rule debug trace. No-op when tracing is disabled.
+        /// </summary>
+        public void DebugLog(string ruleId, string stage, string message) => DebugWriter?.Write(ruleId, stage, message);
+
         /// <summary>
         /// Emits a security_warning event and returns an empty result dictionary.
         /// Called when a gather rule targets a path/query not on the allowlist.
@@ -45,6 +58,8 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather
         public Dictionary<string, object> EmitSecurityWarning(GatherRule rule, string collectorType, string target)
         {
             Logger.Warning($"SECURITY: {collectorType} path blocked by guard: {target} (Rule: {rule.RuleId})");
+            DebugLog(rule.RuleId, GatherRuleDebugLog.StageGuard,
+                $"{collectorType} target blocked by allowlist guard: {target} — security_warning emitted, rule produced no data");
 
             var data = new Dictionary<string, object>
             {

src/Agent/AutopilotMonitor.Agent.V2.Core/Monitoring/Telemetry/Gather/GatherRuleDebugLog.cs

@@ -0,0 +1,115 @@
+using System;
+using System.IO;
+using AutopilotMonitor.Agent.V2.Core.Logging;
+
+namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather
+{
+    /// <summary>
+    /// Append-only trace file for gather-rule evaluation, enabled per tenant via
+    /// EnableGatherRuleDebugLog (or --gather-debug-log). Explains every evaluation
+    /// decision — including the outcomes that produce no event — so customers can
+    /// diagnose rules that never show up in the timeline. Every write failure is
+    /// swallowed: tracing must never break gathering.
+    /// </summary>
+    public sealed class GatherRuleDebugLog
+    {
+        public const string StageConfig = "config";
+        public const string StageTrigger = "trigger";
+        public const string StageScope = "scope";
+        public const string StageExec = "exec";
+        public const string StageCollector = "collector";
+        public const string StageGuard = "guard";
+        public const string StageEmit = "emit";
+        public const string StageSuppress = "suppress";
+        public const string StageLogParser = "logparser";
+        public const string StageError = "error";
+
+        private const long DefaultMaxBytes = 10 * 1024 * 1024;
+
+        private readonly string _path;
+        private readonly long _maxBytes;
+        private readonly Action<string> _echo;
+        private readonly object _lock = new object();
+        private bool _headerWritten;
+
+        public GatherRuleDebugLog(string path, AgentLogger logger, Action<string> echo = null, long maxBytes = DefaultMaxBytes)
+        {
+            if (string.IsNullOrEmpty(path)) throw new ArgumentNullException(nameof(path));
+            _path = path;
+            _maxBytes = maxBytes;
+            _echo = echo;
+            logger?.Info($"GatherRuleDebugLog: enabled -> {path}");
+        }
+
+        /// <summary>
+        /// Writes one trace line: <c>{utc} | {ruleId} | {stage} | {message}</c>.
+        /// Newlines in <paramref name="message"/> are flattened except for the error
+        /// stage, where exception stack traces stay multi-line.
+        /// </summary>
+        public void Write(string ruleId, string stage, string message)
+        {
+            try
+            {
+                if (stage != StageError && message != null && message.IndexOf('\n') >= 0)
+                    message = message.Replace("\r", "").Replace('\n', ' ');
+
+                var line = $"{DateTime.UtcNow:yyyy-MM-ddTHH:mm:ss.fffZ} | {ruleId ?? "-"} | {stage} | {message}";
+
+                lock (_lock)
+                {
+                    EnsureFileReadyLocked();
+                    File.AppendAllText(_path, line + Environment.NewLine);
+                }
+
+                _echo?.Invoke(line);
+            }
+            catch { }
+        }
+
+        /// <summary>
+        /// One-shot helper for callers that have no executor (e.g. the flag is on but the
+        /// backend delivered zero gather rules) — the customer's trace file should explain
+        /// that too instead of silently not existing.
+        /// </summary>
+        public static void WriteStandalone(string path, string message, AgentLogger logger)
+        {
+            if (string.IsNullOrEmpty(path)) return;
+            try
+            {
+                new GatherRuleDebugLog(path, logger).Write(null, StageConfig, message);
+            }
+            catch { }
+        }
+
+        // Lazy on first write: create the directory and emit a header line. Before every
+        // append, rotate when the file exceeds maxBytes — keeping exactly one .old
+        // generation (worst case ~2x maxBytes on disk).
+        private void EnsureFileReadyLocked()
+        {
+            if (!_headerWritten)
+            {
+                var dir = Path.GetDirectoryName(_path);
+                if (!string.IsNullOrEmpty(dir))
+                    Directory.CreateDirectory(dir);
+            }
+
+            var info = new FileInfo(_path);
+            if (info.Exists && info.Length > _maxBytes)
+            {
+                var oldPath = Path.ChangeExtension(_path, null) + ".old.log";
+                if (File.Exists(oldPath))
+                    File.Delete(oldPath);
+                File.Move(_path, oldPath);
+                File.AppendAllText(_path,
+                    $"{DateTime.UtcNow:yyyy-MM-ddTHH:mm:ss.fffZ} | - | {StageConfig} | -- rotated (previous file exceeded {_maxBytes} bytes) --" + Environment.NewLine);
+            }
+
+            if (!_headerWritten)
+            {
+                File.AppendAllText(_path,
+                    $"{DateTime.UtcNow:yyyy-MM-ddTHH:mm:ss.fffZ} | - | {StageConfig} | -- gather rule debug log started --" + Environment.NewLine);
+                _headerWritten = true;
+            }
+        }
+    }
+}

src/Agent/AutopilotMonitor.Agent.V2.Core/Monitoring/Telemetry/Gather/GatherRuleExecutor.cs

@@ -22,6 +22,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather
     public class GatherRuleExecutor : IDisposable
     {
         private readonly AgentLogger _logger;
+        private readonly GatherRuleDebugLog _debug;   // null unless EnableGatherRuleDebugLog / --gather-debug-log
         private readonly GatherRuleContext _context;
         private readonly Dictionary<string, IGatherRuleCollector> _collectors;
 
@@ -61,7 +62,8 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather
         }
 
         public GatherRuleExecutor(string sessionId, string tenantId, Action<EnrollmentEvent> onEventCollected,
-            AgentLogger logger, string imeLogPathOverride = null)
+            AgentLogger logger, string imeLogPathOverride = null, string debugLogPath = null,
+            Action<string> debugEcho = null)
         {
             if (sessionId == null) throw new ArgumentNullException(nameof(sessionId));
             if (tenantId == null) throw new ArgumentNullException(nameof(tenantId));
@@ -69,9 +71,10 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather
             if (logger == null) throw new ArgumentNullException(nameof(logger));
 
             _logger = logger;
+            _debug = string.IsNullOrEmpty(debugLogPath) ? null : new GatherRuleDebugLog(debugLogPath, logger, debugEcho);
 
             var filePositionTracker = new LogFilePositionTracker();
-            _context = new GatherRuleContext(logger, sessionId, tenantId, onEventCollected, imeLogPathOverride, filePositionTracker);
+            _context = new GatherRuleContext(logger, sessionId, tenantId, onEventCollected, imeLogPathOverride, filePositionTracker, _debug);
 
             // Register all collector strategies
             var collectorList = new IGatherRuleCollector[]
@@ -96,6 +99,12 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather
             _collectors["command"] = _collectors["command_allowlisted"];
         }
 
+        /// <summary>
+        /// Null-safe write to the gather-rule debug trace. Never call while holding
+        /// <see cref="_scopeLock"/> — capture the message under the lock, write after release.
+        /// </summary>
+        private void DebugLog(string ruleId, string stage, string message) => _debug?.Write(ruleId, stage, message);
+
         /// <summary>
         /// Updates the active rules and starts/stops execution accordingly
         /// </summary>
@@ -111,11 +120,43 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather
 
             _activeRules = rules.Where(r => r.Enabled).ToList();
 
+            // Registration summary for the debug trace: one line per delivered rule so the
+            // customer can see exactly what the agent received and how it will be triggered.
+            if (_debug != null)
+            {
+                foreach (var rule in rules)
+                {
+                    if (!rule.Enabled)
+                    {
+                        DebugLog(rule.RuleId, GatherRuleDebugLog.StageConfig, "rule disabled — will never run");
+                        continue;
+                    }
+
+                    var scope = rule.ActivePhases != null && rule.ActivePhases.Count > 0
+                        ? $"activePhases=[{string.Join(",", rule.ActivePhases)}]"
+                        : !string.IsNullOrEmpty(rule.ActiveFromPhase)
+                            ? $"activeFromPhase={rule.ActiveFromPhase}"
+                            : "unscoped";
+                    DebugLog(rule.RuleId, GatherRuleDebugLog.StageConfig,
+                        $"registered: trigger={rule.Trigger}" +
+                        (string.IsNullOrEmpty(rule.TriggerPhase) ? "" : $" triggerPhase={rule.TriggerPhase}") +
+                        (string.IsNullOrEmpty(rule.TriggerEventType) ? "" : $" triggerEventType={rule.TriggerEventType}") +
+                        $", collector={rule.CollectorType}, target={rule.Target}, {scope}" +
+                        $", emitMode={(string.IsNullOrEmpty(rule.EmitMode) ? "always" : rule.EmitMode)}" +
+                        (rule.IntervalSeconds.HasValue ? $", interval={rule.IntervalSeconds}s" : ""));
+
+                    if (rule.Trigger == "interval" && !rule.IntervalSeconds.HasValue)
+                        DebugLog(rule.RuleId, GatherRuleDebugLog.StageError,
+                            "trigger=interval but intervalSeconds is missing — rule will NEVER run");
+                }
+            }
+
             // Execute startup rules — track completion via CountdownEvent so callers can wait.
             // Scoped startup rules that are not yet in scope are deferred: they run once from
             // OnPhaseChanged when their scope activates (same _startupRulesExecuted dedup — a
             // rule runs either here or there, never both). Unscoped rules behave as before.
             List<GatherRule> pendingStartup;
+            var deferredTraces = _debug != null ? new List<KeyValuePair<string, string>>() : null;
             lock (_scopeLock)
             {
                 // A config refresh may deliver new from-phase rules after their phase was
@@ -124,7 +165,15 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather
 
                 pendingStartup = _activeRules
                     .Where(r => r.Trigger == "startup" && !_startupRulesExecuted.Contains(r.RuleId))
-                    .Where(r => IsRuleInScopeLocked(r))
+                    .Where(r =>
+                    {
+                        string reason;
+                        var inScope = IsRuleInScopeLocked(r, out reason);
+                        if (!inScope && deferredTraces != null)
+                            deferredTraces.Add(new KeyValuePair<string, string>(r.RuleId,
+                                $"startup rule deferred — {reason}; runs once when its scope activates"));
+                        return inScope;
+                    })
                     .ToList();
 
                 foreach (var rule in pendingStartup)
@@ -133,6 +182,12 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather
                 }
             }
 
+            if (deferredTraces != null)
+            {
+                foreach (var trace in deferredTraces)
+                    DebugLog(trace.Key, GatherRuleDebugLog.StageScope, trace.Value);
+            }
+
             if (pendingStartup.Count > 0)
             {
                 _startupRulesLatch?.Dispose();
@@ -158,8 +213,12 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather
                 var timer = new Timer(
                     _ =>
                     {
-                        if (!IsRuleInScope(intervalRule))
+                        string skipReason;
+                        if (!IsRuleInScope(intervalRule, out skipReason))
+                        {
+                            DebugLog(intervalRule.RuleId, GatherRuleDebugLog.StageScope, $"interval tick skipped: {skipReason}");
                             return;
+                        }
                         ExecuteRule(intervalRule);
                     },
                     null,
@@ -168,6 +227,8 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather
                 );
                 _intervalTimers[intervalRule.RuleId] = timer;
                 _logger.Info($"  Interval rule {intervalRule.RuleId} scheduled every {intervalRule.IntervalSeconds}s");
+                DebugLog(intervalRule.RuleId, GatherRuleDebugLog.StageTrigger,
+                    $"interval timer scheduled every {intervalRule.IntervalSeconds}s (first run after one full interval — no immediate execution)");
             }
 
             _logger.Info($"GatherRuleExecutor: {_activeRules.Count(r => r.Trigger == "startup")} startup, " +
@@ -187,6 +248,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather
             // _currentPhase moves below. A transition INTO Failed still fires the exit rules of the
             // phase that failed — that snapshot is the whole point at a failure boundary.
             List<GatherRule> exitRules = null;
+            var exitTraces = _debug != null ? new List<KeyValuePair<string, string>>() : null;
             lock (_scopeLock)
             {
                 var previousPhase = _currentPhase;
@@ -204,12 +266,22 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather
 
                         // Scope gate before the dedup add — an out-of-scope pass must not consume
                         // the once-per-(rule, phase) slot. IsRuleInScopeLocked still sees the old phase.
-                        if (!IsRuleInScopeLocked(rule)) continue;
+                        string scopeReason;
+                        if (!IsRuleInScopeLocked(rule, out scopeReason))
+                        {
+                            if (exitTraces != null)
+                                exitTraces.Add(new KeyValuePair<string, string>(rule.RuleId,
+                                    $"phase_exit of {previousPhaseName} skipped: {scopeReason}"));
+                            continue;
+                        }
 
                         // "exit:" prefix keeps this key space disjoint from the phase_change keys.
                         if (!_phaseRulesExecuted.Add($"exit:{rule.RuleId}|{previousPhaseName}"))
                         {
                             _logger.Debug($"Phase-exit rule {rule.RuleId} already executed for exit of {previousPhaseName}, skipping");
+                            if (exitTraces != null)
+                                exitTraces.Add(new KeyValuePair<string, string>(rule.RuleId,
+                                    $"phase_exit already executed for exit of {previousPhaseName} — once per (rule, phase)"));
                             continue;
                         }
 
@@ -219,6 +291,12 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather
                 }
             }
 
+            if (exitTraces != null)
+            {
+                foreach (var trace in exitTraces)
+                    DebugLog(trace.Key, GatherRuleDebugLog.StageScope, trace.Value);
+            }
+
             if (exitRules != null)
             {
                 foreach (var rule in exitRules)
@@ -252,6 +330,8 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather
             {
                 var deferredRule = rule;
                 _logger.Info($"Phase {phaseName} activated deferred startup rule {deferredRule.RuleId}");
+                DebugLog(deferredRule.RuleId, GatherRuleDebugLog.StageTrigger,
+                    $"phase {phaseName} activated deferred startup rule");
                 ThreadPool.QueueUserWorkItem(_ => ExecuteRule(deferredRule));
             }
 
@@ -262,18 +342,26 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather
                 {
                     // Scope gate BEFORE the dedup add — an out-of-scope pass must not consume
                     // the once-per-(rule, phase) execution slot.
-                    if (!IsRuleInScope(rule))
+                    string scopeReason;
+                    if (!IsRuleInScope(rule, out scopeReason))
+                    {
+                        DebugLog(rule.RuleId, GatherRuleDebugLog.StageScope,
+                            $"phase_change to {phaseName} skipped: {scopeReason}");
                         continue;
+                    }
 
                     // Deduplicate: only fire once per (ruleId, phase) combination
                     var deduplicationKey = $"{rule.RuleId}|{phaseName}";
                     if (!_phaseRulesExecuted.Add(deduplicationKey))
                     {
                         _logger.Debug($"Phase rule {rule.RuleId} already executed for phase {phaseName}, skipping");
+                        DebugLog(rule.RuleId, GatherRuleDebugLog.StageTrigger,
+                            $"phase_change already executed for phase {phaseName} — once per (rule, phase)");
                         continue;
                     }
 
                     _logger.Info($"Phase change triggered rule {rule.RuleId} (phase: {phaseName})");
+                    DebugLog(rule.RuleId, GatherRuleDebugLog.StageTrigger, $"phase_change to {phaseName} fired");
                     ThreadPool.QueueUserWorkItem(_ => ExecuteRule(rule));
                 }
             }
@@ -288,10 +376,16 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather
             {
                 if (string.Equals(rule.TriggerEventType, eventType, StringComparison.OrdinalIgnoreCase))
                 {
-                    if (!IsRuleInScope(rule))
+                    string scopeReason;
+                    if (!IsRuleInScope(rule, out scopeReason))
+                    {
+                        DebugLog(rule.RuleId, GatherRuleDebugLog.StageScope,
+                            $"on_event {eventType} skipped: {scopeReason}");
                         continue;
+                    }
 
                     _logger.Info($"Event triggered rule {rule.RuleId} (event: {eventType})");
+                    DebugLog(rule.RuleId, GatherRuleDebugLog.StageTrigger, $"on_event {eventType} fired");
                     ThreadPool.QueueUserWorkItem(_ => ExecuteRule(rule));
                 }
             }
@@ -302,6 +396,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather
             try
             {
                 _logger.Info($"Executing gather rule: {rule.RuleId} ({rule.Title})");
+                DebugLog(rule.RuleId, GatherRuleDebugLog.StageExec, $"executing: collector={rule.CollectorType}, target={rule.Target}");
 
                 var collectorType = rule.CollectorType?.ToLower();
 
@@ -309,6 +404,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather
                 if (collectorType == null || !_collectors.TryGetValue(collectorType, out collector))
                 {
                     _logger.Warning($"Unknown collector type: {rule.CollectorType} for rule {rule.RuleId}");
+                    DebugLog(rule.RuleId, GatherRuleDebugLog.StageError, $"unknown collector type '{rule.CollectorType}' — nothing emitted");
                     return;
                 }
 
@@ -316,7 +412,19 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather
 
                 // LogParser (and potentially others) emit events directly and return null
                 if (result == null)
+                {
+                    DebugLog(rule.RuleId, GatherRuleDebugLog.StageCollector,
+                        collectorType == "logparser"
+                            ? "logparser emits per-match events directly — see logparser lines above for the per-file outcome"
+                            : "collector returned null — nothing emitted");
                     return;
+                }
+
+                if (result.Count == 0)
+                {
+                    DebugLog(rule.RuleId, GatherRuleDebugLog.StageCollector,
+                        "collector returned EMPTY result — nothing emitted (typical causes: target not found with emitOnlyIfExists, or guard block — see guard line if present)");
+                }
 
                 if (result.Count > 0)
                 {
@@ -325,8 +433,19 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather
                     // empty result never reaches this point, so an emitOnlyIfExists miss neither
                     // emits nor updates the hash — composing to "one event on appearance, then
                     // only on change".
-                    if (IsOnChangeMode(rule) && !ShouldEmitOnChange(rule, result))
-                        return;
+                    if (IsOnChangeMode(rule))
+                    {
+                        string hashPrefix;
+                        int suppressedStreak;
+                        if (!ShouldEmitOnChange(rule, result, out hashPrefix, out suppressedStreak))
+                        {
+                            DebugLog(rule.RuleId, GatherRuleDebugLog.StageSuppress,
+                                $"result unchanged (hash {hashPrefix}) — emit suppressed (on_change), suppressed streak={suppressedStreak}");
+                            return;
+                        }
+                        DebugLog(rule.RuleId, GatherRuleDebugLog.StageEmit,
+                            $"result changed (hash {hashPrefix})" + (suppressedStreak > 0 ? $" after {suppressedStreak} suppressed polls" : ""));
+                    }
 
                     result["ruleId"] = rule.RuleId;
                     result["ruleTitle"] = rule.Title;
@@ -357,11 +476,15 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather
                         Message = $"Gather: {rule.Title}",
                         Data = result
                     });
+                    DebugLog(rule.RuleId, GatherRuleDebugLog.StageEmit,
+                        $"emitted {eventType} (severity={severity}, {result.Count} fields)");
                 }
             }
             catch (Exception ex)
             {
                 _logger.Warning($"Gather rule {rule.RuleId} failed: {ex.Message}");
+                // Full exception incl. stack trace only in the debug trace — the agent log stays terse.
+                DebugLog(rule.RuleId, GatherRuleDebugLog.StageError, $"rule execution failed: {ex}");
             }
         }
 
@@ -375,15 +498,33 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather
 
         // Internal so the scope matrix is testable synchronously (no timer/ThreadPool races).
         internal bool IsRuleInScope(GatherRule rule)
+        {
+            string reason;
+            return IsRuleInScope(rule, out reason);
+        }
+
+        /// <summary>
+        /// Scope gate with a human-readable skip reason for the debug trace. The reason is
+        /// captured under the lock but must be WRITTEN outside it (never trace under _scopeLock).
+        /// </summary>
+        internal bool IsRuleInScope(GatherRule rule, out string reason)
         {
             lock (_scopeLock)
             {
-                return IsRuleInScopeLocked(rule);
+                return IsRuleInScopeLocked(rule, out reason);
             }
         }
 
         private bool IsRuleInScopeLocked(GatherRule rule)
         {
+            string reason;
+            return IsRuleInScopeLocked(rule, out reason);
+        }
+
+        private bool IsRuleInScopeLocked(GatherRule rule, out string reason)
+        {
+            reason = null;
+
             if (!HasPhaseScope(rule))
                 return true; // unscoped — legacy behavior
 
@@ -400,7 +541,10 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather
                 // No phase signal yet (or scope tokens that never match, e.g. "Unknown"):
                 // scoped rules stay inactive.
                 if (_currentPhase == EnrollmentPhase.Unknown)
+                {
+                    reason = "currentPhase=Unknown — scoped rules stay inactive until the first phase signal";
                     return false;
+                }
 
                 var phaseName = _currentPhase.ToString();
                 foreach (var phase in rule.ActivePhases)
@@ -408,10 +552,15 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather
                     if (string.Equals(phase, phaseName, StringComparison.OrdinalIgnoreCase))
                         return true;
                 }
+                reason = $"out of scope (currentPhase={phaseName}, activePhases=[{string.Join(",", rule.ActivePhases)}])";
                 return false;
             }
 
-            return _fromPhaseLatched.Contains(rule.RuleId);
+            if (_fromPhaseLatched.Contains(rule.RuleId))
+                return true;
+
+            reason = $"activeFromPhase={rule.ActiveFromPhase} not latched yet (currentPhase={_currentPhase})";
+            return false;
         }
 
         /// <summary>
@@ -457,8 +606,23 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather
         /// Internal so the state machine is testable synchronously (no timer/ThreadPool races).
         /// </summary>
         internal bool ShouldEmitOnChange(GatherRule rule, Dictionary<string, object> result)
+        {
+            string hashPrefix;
+            int suppressedStreak;
+            return ShouldEmitOnChange(rule, result, out hashPrefix, out suppressedStreak);
+        }
+
+        /// <summary>
+        /// Overload exposing the hash prefix and suppression streak for the debug trace:
+        /// on suppress, <paramref name="suppressedStreak"/> is the updated streak count;
+        /// on emit, it is the streak that just ended (0 when none).
+        /// </summary>
+        internal bool ShouldEmitOnChange(GatherRule rule, Dictionary<string, object> result,
+            out string hashPrefix, out int suppressedStreak)
         {
             var hash = ComputeCanonicalHash(result);
+            hashPrefix = hash.Length > 8 ? hash.Substring(0, 8) : hash;
+            suppressedStreak = 0;
             bool emit;
             lock (_scopeLock)
             {
@@ -469,6 +633,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather
                     _suppressed[rule.RuleId] = _suppressed.TryGetValue(rule.RuleId, out streak)
                         ? (streak.Count + 1, streak.SinceUtc)
                         : (1, DateTime.UtcNow);
+                    suppressedStreak = _suppressed[rule.RuleId].Count;
                     emit = false;
                 }
                 else
@@ -479,6 +644,7 @@ namespace AutopilotMonitor.Agent.V2.Core.Monitoring.Telemetry.Gather
                     {
                         result["suppressedPolls"] = streak.Count;
                         result["suppressedSinceUtc"] = streak.SinceUtc.ToString("o", CultureInfo.InvariantCulture);
+                        suppressedStreak = streak.Count;
                     }
                     _suppressed.Remove(rule.RuleId);
                     emit = true;

src/Agent/AutopilotMonitor.Agent.V2.Core/Orchestration/DefaultComponentFactory.cs

@@ -483,7 +483,16 @@ namespace AutopilotMonitor.Agent.V2.Core.Orchestration
                     logger: logger,
                     rules: _remoteConfig.GatherRules,
                     imeLogPathOverride: _agentConfig.ImeLogPathOverride,
-                    unrestrictedMode: _agentConfig.UnrestrictedMode));
+                    unrestrictedMode: _agentConfig.UnrestrictedMode,
+                    gatherDebugLogPath: _agentConfig.GatherRuleDebugLogPath));
+            }
+            else if (!string.IsNullOrEmpty(_agentConfig.GatherRuleDebugLogPath))
+            {
+                // Debug trace is on but the backend delivered zero gather rules — the trace
+                // file must say so instead of silently not existing.
+                Monitoring.Telemetry.Gather.GatherRuleDebugLog.WriteStandalone(
+                    _agentConfig.GatherRuleDebugLogPath,
+                    "no gather rules delivered by backend — nothing to execute", logger);
             }
 
             return new CollectorSurfaces(hosts, imeLogHost, espAndHelloHost, aadJoinHost);

src/Agent/AutopilotMonitor.Agent.V2.Core/Orchestration/Hosts/GatherRuleExecutorHost.cs

@@ -39,7 +39,8 @@ namespace AutopilotMonitor.Agent.V2.Core.Orchestration
             AgentLogger logger,
             List<GatherRule> rules,
             string? imeLogPathOverride,
-            bool unrestrictedMode = false)
+            bool unrestrictedMode = false,
+            string? gatherDebugLogPath = null)
         {
             if (ingress == null) throw new ArgumentNullException(nameof(ingress));
             if (clock == null) throw new ArgumentNullException(nameof(clock));
@@ -55,9 +56,10 @@ namespace AutopilotMonitor.Agent.V2.Core.Orchestration
             // session mode we wrap post.Emit so every session-mode gather event still
             // flows through the InformationalEvent ingress pipe before hitting the
             // telemetry spool — Rail-A semantics for ordering / replay determinism.
-            var post = new InformationalEventPost(ingress, clock);
+            var post = new InformationalEventPost(ingress, clock, logger);
             _executor = new Monitoring.Telemetry.Gather.GatherRuleExecutor(
-                sessionId, tenantId, evt => post.Emit(evt), logger, imeLogPathOverride);
+                sessionId, tenantId, evt => post.Emit(evt), logger, imeLogPathOverride,
+                debugLogPath: gatherDebugLogPath);
             _observableIngress = ingress as SignalIngress;
         }
 

src/Agent/AutopilotMonitor.Agent.V2/Program.GatherRulesMode.cs

@@ -221,8 +221,18 @@ namespace AutopilotMonitor.Agent.V2
                         },
                     });
 
-                    // Step 4 — run the executor and wait.
-                    using (var executor = new GatherRuleExecutor(sessionId, tenantId, emitEvent, logger, config.ImeLogPathOverride))
+                    // Step 4 — run the executor and wait. Diagnostic mode always writes the
+                    // gather-rule debug trace (CLI override or default path) and mirrors it
+                    // to the console in --console mode.
+                    var gatherDebugLogPath = !string.IsNullOrEmpty(config.GatherRuleDebugLogPath)
+                        ? config.GatherRuleDebugLogPath
+                        : Environment.ExpandEnvironmentVariables(Constants.GatherRuleDebugLogPath);
+                    logger.Info($"--run-gather-rules: debug trace -> {gatherDebugLogPath}");
+                    if (consoleMode) Console.WriteLine($"Debug trace: {gatherDebugLogPath}");
+
+                    using (var executor = new GatherRuleExecutor(sessionId, tenantId, emitEvent, logger, config.ImeLogPathOverride,
+                        debugLogPath: gatherDebugLogPath,
+                        debugEcho: consoleMode ? (Action<string>)(line => Console.WriteLine($"  {line}")) : null))
                     {
                         // Diagnostic mode has no enrollment phase context — scoped rules
                         // (activePhases / activeFromPhase) execute unconditionally here.

src/Agent/AutopilotMonitor.Agent.V2/Program.cs

@@ -303,6 +303,7 @@ namespace AutopilotMonitor.Agent.V2
             var apiBaseUrl = GetArgValue(args, "--api-url", "--backend-api") ?? Constants.ApiBaseUrl;
             var imeLogPathOverride = GetArgValue(args, "--ime-log-path");
             var imeMatchLogPath = GetArgValue(args, "--ime-match-log");
+            var gatherDebugLogPath = GetArgValue(args, "--gather-debug-log");
 
             // Dev / test — IME log replay (V1 compat mode). Feeds ImeLogTracker.SimulationMode
             // + SpeedFactor so recorded raw IME logs are replayed at an accelerated rate; signal
@@ -373,6 +374,7 @@ namespace AutopilotMonitor.Agent.V2
                 EnableGeoLocation = !disableGeoLocation,
                 ImeLogPathOverride = imeLogPathOverride,
                 ImeMatchLogPath = imeMatchLogPath,
+                GatherRuleDebugLogPath = gatherDebugLogPath,
                 KeepLogFile = keepLogFile,
                 SelfDestructOnComplete = !noCleanup,
                 CommandLineArgs = FormatArgsForLog(args),
@@ -485,6 +487,7 @@ namespace AutopilotMonitor.Agent.V2
             Console.Out.WriteLine("  --api-url <URL>                   Override backend API base URL (alias: --backend-api)");
             Console.Out.WriteLine("  --ime-log-path <PATH>             Override IME logs directory");
             Console.Out.WriteLine("  --ime-match-log <PATH>            Write matched IME log lines to file (debug)");
+            Console.Out.WriteLine("  --gather-debug-log <PATH>         Write gather-rule evaluation trace to file (debug)");
             Console.Out.WriteLine();
             Console.Out.WriteLine("Dev / Test:");
             Console.Out.WriteLine("  --replay-log-dir <PATH>           Replay IME logs from this directory (simulation mode)");

src/Backend/AutopilotMonitor.Functions/DataAccess/TableStorage/TableConfigRepository.cs

@@ -413,6 +413,7 @@ namespace AutopilotMonitor.Functions.DataAccess.TableStorage
                 { "EnableTimezoneAutoSet", config.EnableTimezoneAutoSet },
                 { "NtpServer", config.NtpServer },
                 { "EnableImeMatchLog", config.EnableImeMatchLog },
+                { "EnableGatherRuleDebugLog", config.EnableGatherRuleDebugLog },
                 { "LogLevel", config.LogLevel },
                 { "MaxBatchSize", config.MaxBatchSize },
                 { "DiagnosticsBlobSasUrl", config.DiagnosticsBlobSasUrl },
@@ -514,6 +515,7 @@ namespace AutopilotMonitor.Functions.DataAccess.TableStorage
                 EnableTimezoneAutoSet = entity.GetBoolean("EnableTimezoneAutoSet"),
                 NtpServer = string.IsNullOrWhiteSpace(entity.GetString("NtpServer")) ? "time.windows.com" : entity.GetString("NtpServer"),
                 EnableImeMatchLog = entity.GetBoolean("EnableImeMatchLog"),
+                EnableGatherRuleDebugLog = entity.GetBoolean("EnableGatherRuleDebugLog"),
                 LogLevel = entity.GetString("LogLevel"),
                 MaxBatchSize = entity.GetInt32("MaxBatchSize"),
                 DiagnosticsBlobSasUrl = entity.GetString("DiagnosticsBlobSasUrl"),

src/Backend/AutopilotMonitor.Functions/Functions/Config/GetAgentConfigFunction.cs

@@ -270,12 +270,13 @@ namespace AutopilotMonitor.Functions.Functions.Config
             var response = req.CreateResponse(HttpStatusCode.OK);
             await response.WriteAsJsonAsync(new AgentConfigResponse
             {
-                ConfigVersion = 35, // MDM reboot-policy watcher (mdm_policy_reboot_required / esp_reboot_coalescing)
+                ConfigVersion = 36, // EnableGatherRuleDebugLog (local gather-rule evaluation trace)
                 UploadIntervalSeconds = 10,
                 SelfDestructOnComplete = tenantConfig.SelfDestructOnComplete ?? true,
                 KeepLogFile = tenantConfig.KeepLogFile ?? false,
                 EnableGeoLocation = tenantConfig.EnableGeoLocation ?? true,
                 EnableImeMatchLog = tenantConfig.EnableImeMatchLog ?? false,
+                EnableGatherRuleDebugLog = tenantConfig.EnableGatherRuleDebugLog ?? false,
                 MaxAuthFailures = tenantConfig.MaxAuthFailures ?? 5,
                 AuthFailureTimeoutMinutes = tenantConfig.AuthFailureTimeoutMinutes ?? 0,
                 LogLevel = tenantConfig.LogLevel ?? "Info",

src/Shared/AutopilotMonitor.Shared/Constants.cs

@@ -62,6 +62,13 @@ namespace AutopilotMonitor.Shared
         /// </summary>
         public const string ImeMatchLogPath = @"%ProgramData%\AutopilotMonitor\Logs\ime_pattern_matches.log";
 
+        /// <summary>
+        /// Default path for the gather rule debug trace file (debugging/diagnostics).
+        /// Written when the tenant enables EnableGatherRuleDebugLog; explains every
+        /// gather-rule evaluation decision, including outcomes that produce no event.
+        /// </summary>
+        public const string GatherRuleDebugLogPath = @"%ProgramData%\AutopilotMonitor\Logs\gather_rules_debug.log";
+
         /// <summary>
         /// Scheduled Task name used to run the agent as SYSTEM
         /// </summary>

src/Shared/AutopilotMonitor.Shared/Models/Config/AgentConfigResponse.cs

@@ -40,6 +40,12 @@ namespace AutopilotMonitor.Shared.Models
         /// </summary>
         public bool EnableImeMatchLog { get; set; } = false;
 
+        /// <summary>
+        /// Whether to write a local gather-rule evaluation trace file.
+        /// When true, the default path is used: Constants.GatherRuleDebugLogPath.
+        /// </summary>
+        public bool EnableGatherRuleDebugLog { get; set; } = false;
+
         public CollectorConfiguration Collectors { get; set; } = default!;
 
         /// <summary>

src/Shared/AutopilotMonitor.Shared/Models/Config/TenantConfiguration.cs

@@ -334,6 +334,15 @@ namespace AutopilotMonitor.Shared.Models
         /// </summary>
         public bool? EnableImeMatchLog { get; set; }
 
+        /// <summary>
+        /// Whether the agent writes a local gather-rule evaluation trace file so customers
+        /// can diagnose rules that produce no timeline events (scope skips, on_change
+        /// suppression, empty collector results, logparser details). When true, the agent
+        /// writes to Constants.GatherRuleDebugLogPath. The trace never leaves the device.
+        /// null = use agent default (false).
+        /// </summary>
+        public bool? EnableGatherRuleDebugLog { get; set; }
+
         /// <summary>
         /// Log verbosity level override for this tenant's agents.
         /// null = use agent default ("Info"). Values: "Info", "Debug", "Verbose", "Trace".

src/Web/autopilot-monitor-web/app/admin/tenants/sections/SectionTenantConfigReport.tsx

@@ -58,6 +58,7 @@ interface TenantConfig {
   enableTimezoneAutoSet?: boolean | null;
   ntpServer?: string | null;
   enableImeMatchLog?: boolean | null;
+  enableGatherRuleDebugLog?: boolean | null;
   logLevel?: string | null;
   maxBatchSize?: number | null;
   showEnrollmentSummary?: boolean | null;
@@ -130,6 +131,7 @@ const DEFAULTS: Record<string, unknown> = {
   enableTimezoneAutoSet: null,
   ntpServer: 'time.windows.com',
   enableImeMatchLog: null,
+  enableGatherRuleDebugLog: null,
   logLevel: null,
   maxBatchSize: null,
   showEnrollmentSummary: null,
@@ -168,6 +170,7 @@ const RUNTIME_DEFAULTS: Record<string, unknown> = {
   enableTimezoneAutoSet: false,
   ntpServer: 'time.windows.com',
   enableImeMatchLog: false,
+  enableGatherRuleDebugLog: false,
   maxAuthFailures: 5,
   authFailureTimeoutMinutes: 0,
   logLevel: 'Info',
@@ -296,6 +299,7 @@ function computeRuntime(c: TenantConfig): Record<string, unknown> {
     enableTimezoneAutoSet: c.enableTimezoneAutoSet ?? false,
     ntpServer: c.ntpServer ?? 'time.windows.com',
     enableImeMatchLog: c.enableImeMatchLog ?? false,
+    enableGatherRuleDebugLog: c.enableGatherRuleDebugLog ?? false,
     maxAuthFailures: c.maxAuthFailures ?? 5,
     authFailureTimeoutMinutes: c.authFailureTimeoutMinutes ?? 0,
     logLevel: c.logLevel ?? 'Info',
@@ -575,6 +579,7 @@ export function SectionTenantConfigReport() {
                 <ConfigRow label="Timezone Auto-Set" value={config.enableTimezoneAutoSet} configKey="enableTimezoneAutoSet" defaults={DEFAULTS} />
                 <ConfigRow label="NTP Server" value={config.ntpServer} configKey="ntpServer" defaults={DEFAULTS} />
                 <ConfigRow label="IME Match Log" value={config.enableImeMatchLog} configKey="enableImeMatchLog" defaults={DEFAULTS} />
+                <ConfigRow label="Gather Rule Debug Log" value={config.enableGatherRuleDebugLog} configKey="enableGatherRuleDebugLog" defaults={DEFAULTS} />
                 <ConfigRow label="Log Level" value={config.logLevel} configKey="logLevel" defaults={DEFAULTS} />
                 <ConfigRow label="Max Batch Size" value={config.maxBatchSize} configKey="maxBatchSize" defaults={DEFAULTS} />
                 <ConfigRow label="Show Script Output" value={config.showScriptOutput} configKey="showScriptOutput" defaults={DEFAULTS} />
@@ -647,6 +652,7 @@ export function SectionTenantConfigReport() {
                     <ConfigRow label="Timezone Auto-Set" value={runtime.enableTimezoneAutoSet} configKey="enableTimezoneAutoSet" defaults={RUNTIME_DEFAULTS} />
                     <ConfigRow label="NTP Server" value={runtime.ntpServer} configKey="ntpServer" defaults={RUNTIME_DEFAULTS} />
                     <ConfigRow label="IME Match Log" value={runtime.enableImeMatchLog} configKey="enableImeMatchLog" defaults={RUNTIME_DEFAULTS} />
+                    <ConfigRow label="Gather Rule Debug Log" value={runtime.enableGatherRuleDebugLog} configKey="enableGatherRuleDebugLog" defaults={RUNTIME_DEFAULTS} />
                     <ConfigRow label="Log Level" value={runtime.logLevel} configKey="logLevel" defaults={RUNTIME_DEFAULTS} />
                     <ConfigRow label="Send Trace Events" value={runtime.sendTraceEvents} configKey="sendTraceEvents" defaults={RUNTIME_DEFAULTS} />
                     <ConfigRow label="Unrestricted Mode" value={runtime.unrestrictedMode} configKey="unrestrictedMode" defaults={RUNTIME_DEFAULTS} />

src/Web/autopilot-monitor-web/app/settings/TenantConfigContext.tsx

@@ -167,6 +167,8 @@ interface TenantConfigContextValue {
   setEnableTimezoneAutoSet: (v: boolean) => void;
   enableImeMatchLog: boolean;
   setEnableImeMatchLog: (v: boolean) => void;
+  enableGatherRuleDebugLog: boolean;
+  setEnableGatherRuleDebugLog: (v: boolean) => void;
   logLevel: string;
   setLogLevel: (v: string) => void;
   showScriptOutput: boolean;
@@ -384,6 +386,7 @@ export function TenantConfigProvider({ children }: { children: React.ReactNode }
   const [enableGeoLocation, setEnableGeoLocation] = useState(true);
   const [enableTimezoneAutoSet, setEnableTimezoneAutoSet] = useState(false);
   const [enableImeMatchLog, setEnableImeMatchLog] = useState(false);
+  const [enableGatherRuleDebugLog, setEnableGatherRuleDebugLog] = useState(false);
   const [logLevel, setLogLevel] = useState("Info");
   const [showScriptOutput, setShowScriptOutput] = useState(true);
   const [showEnrollmentSummary, setShowEnrollmentSummary] = useState(false);
@@ -503,6 +506,7 @@ export function TenantConfigProvider({ children }: { children: React.ReactNode }
         setEnableGeoLocation(data.enableGeoLocation ?? true);
         setEnableTimezoneAutoSet(data.enableTimezoneAutoSet ?? false);
         setEnableImeMatchLog(data.enableImeMatchLog ?? false);
+        setEnableGatherRuleDebugLog(data.enableGatherRuleDebugLog ?? false);
         setLogLevel(data.logLevel ?? "Info");
         setShowScriptOutput(data.showScriptOutput ?? true);
         setShowEnrollmentSummary(data.showEnrollmentSummary ?? false);
@@ -700,6 +704,7 @@ export function TenantConfigProvider({ children }: { children: React.ReactNode }
         enableGeoLocation,
         enableTimezoneAutoSet,
         enableImeMatchLog,
+        enableGatherRuleDebugLog,
         logLevel,
         showScriptOutput,
         showEnrollmentSummary,
@@ -777,7 +782,7 @@ export function TenantConfigProvider({ children }: { children: React.ReactNode }
     manufacturerWhitelist, modelWhitelist, validateAutopilotDevice, validateCorporateIdentifier, validateDeviceAssociation,
     dataRetentionDays, sessionTimeoutHours, enablePerformanceCollector, performanceCollectorInterval,
     helloWaitTimeoutSeconds, selfDestructOnComplete, keepLogFile, rebootOnComplete, rebootDelaySeconds,
-    contactEmail, enableGeoLocation, enableTimezoneAutoSet, enableImeMatchLog, logLevel, showScriptOutput, showEnrollmentSummary,
+    contactEmail, enableGeoLocation, enableTimezoneAutoSet, enableImeMatchLog, enableGatherRuleDebugLog, logLevel, showScriptOutput, showEnrollmentSummary,
     enrollmentSummaryTimeoutSeconds, enrollmentSummaryBrandingImageUrl, enrollmentSummaryLaunchRetrySeconds,
     notificationChannels,
     slaTargetSuccessRate, slaTargetMaxDurationMinutes, slaTargetAppInstallSuccessRate,
@@ -1113,6 +1118,7 @@ export function TenantConfigProvider({ children }: { children: React.ReactNode }
     setEnableGeoLocation(config.enableGeoLocation ?? true);
     setEnableTimezoneAutoSet(config.enableTimezoneAutoSet ?? false);
     setEnableImeMatchLog(config.enableImeMatchLog ?? false);
+    setEnableGatherRuleDebugLog(config.enableGatherRuleDebugLog ?? false);
     setLogLevel(config.logLevel ?? "Info");
     setShowScriptOutput(config.showScriptOutput ?? true);
     setShowEnrollmentSummary(config.showEnrollmentSummary ?? false);
@@ -1542,6 +1548,7 @@ export function TenantConfigProvider({ children }: { children: React.ReactNode }
       enableGeoLocation, setEnableGeoLocation,
       enableTimezoneAutoSet, setEnableTimezoneAutoSet,
       enableImeMatchLog, setEnableImeMatchLog,
+      enableGatherRuleDebugLog, setEnableGatherRuleDebugLog,
       logLevel, setLogLevel,
       showScriptOutput, setShowScriptOutput,
       showEnrollmentSummary, setShowEnrollmentSummary,

src/Web/autopilot-monitor-web/app/settings/agent/sections/SectionAgentSettings.tsx

@@ -17,6 +17,7 @@ export function SectionAgentSettings() {
     enableGeoLocation, setEnableGeoLocation,
     enableTimezoneAutoSet, setEnableTimezoneAutoSet,
     enableImeMatchLog, setEnableImeMatchLog,
+    enableGatherRuleDebugLog, setEnableGatherRuleDebugLog,
     logLevel, setLogLevel,
     showScriptOutput, setShowScriptOutput,
     showEnrollmentSummary, setShowEnrollmentSummary,
@@ -51,6 +52,8 @@ export function SectionAgentSettings() {
         setEnableTimezoneAutoSet={setEnableTimezoneAutoSet}
         enableImeMatchLog={enableImeMatchLog}
         setEnableImeMatchLog={setEnableImeMatchLog}
+        enableGatherRuleDebugLog={enableGatherRuleDebugLog}
+        setEnableGatherRuleDebugLog={setEnableGatherRuleDebugLog}
         logLevel={logLevel}
         setLogLevel={setLogLevel}
         showScriptOutput={showScriptOutput}

src/Web/autopilot-monitor-web/app/settings/components/AgentSettingsSection.tsx

@@ -24,6 +24,8 @@ interface AgentSettingsSectionProps {
   setEnableTimezoneAutoSet: (value: boolean) => void;
   enableImeMatchLog: boolean;
   setEnableImeMatchLog: (value: boolean) => void;
+  enableGatherRuleDebugLog: boolean;
+  setEnableGatherRuleDebugLog: (value: boolean) => void;
   logLevel: string;
   setLogLevel: (value: string) => void;
   showScriptOutput: boolean;
@@ -64,6 +66,8 @@ export default function AgentSettingsSection({
   setEnableTimezoneAutoSet,
   enableImeMatchLog,
   setEnableImeMatchLog,
+  enableGatherRuleDebugLog,
+  setEnableGatherRuleDebugLog,
   logLevel,
   setLogLevel,
   showScriptOutput,
@@ -199,6 +203,21 @@ export default function AgentSettingsSection({
             </button>
           </div>
 
+          {/* Gather Rule Debug Log */}
+          <div className="flex items-center justify-between p-4 rounded-lg border border-gray-200 hover:border-violet-200 transition-colors">
+            <div>
+              <p className="font-medium text-gray-900">Gather Rule Debug Log</p>
+              <p className="text-sm text-gray-500">
+                Trace gather rule evaluation to a local file to diagnose rules that produce no timeline events. The trace never leaves the device.
+                {enableGatherRuleDebugLog && <span className="block text-xs text-gray-400 mt-0.5 font-mono">%ProgramData%\AutopilotMonitor\Logs\gather_rules_debug.log</span>}
+              </p>
+            </div>
+            <button onClick={() => setEnableGatherRuleDebugLog(!enableGatherRuleDebugLog)}
+              className={`relative inline-flex h-6 w-11 flex-shrink-0 items-center rounded-full transition-colors ${enableGatherRuleDebugLog ? 'bg-violet-500' : 'bg-gray-300'}`}>
+              <span className={`inline-block h-4 w-4 transform rounded-full bg-white transition-transform ${enableGatherRuleDebugLog ? 'translate-x-6' : 'translate-x-1'}`} />
+            </button>
+          </div>
+
           {/* Show Script Output */}
           <div className="flex items-center justify-between p-4 rounded-lg border border-gray-200 hover:border-violet-200 transition-colors">
             <div className="flex-1">

src/Web/autopilot-monitor-web/app/settings/types.ts

@@ -60,6 +60,7 @@ export interface TenantConfiguration {
   enableGeoLocation?: boolean;
   enableTimezoneAutoSet?: boolean;
   enableImeMatchLog?: boolean;
+  enableGatherRuleDebugLog?: boolean;
   logLevel?: string;
   // Teams notifications (legacy)
   teamsWebhookUrl?: string;