diff --git a/src/Agent/AutopilotMonitor.DecisionCore.Tests/AppInstallSignalHandlerTests.cs b/src/Agent/AutopilotMonitor.DecisionCore.Tests/AppInstallSignalHandlerTests.cs
new file mode 100644
index 00000000..8eb3d511
--- /dev/null
+++ b/src/Agent/AutopilotMonitor.DecisionCore.Tests/AppInstallSignalHandlerTests.cs
@@ -0,0 +1,261 @@
+using System;
+using System.Collections.Generic;
+using AutopilotMonitor.DecisionCore.Engine;
+using AutopilotMonitor.DecisionCore.Signals;
+using AutopilotMonitor.DecisionCore.State;
+using AutopilotMonitor.Shared.Models;
+using Xunit;
+
+namespace AutopilotMonitor.DecisionCore.Tests
+{
+    /// <summary>
+    /// Codex follow-up #4 — coverage for the AppInstall observation handlers. Until this
+    /// PR both <see cref="DecisionSignalKind.AppInstallCompleted"/> and
+    /// <see cref="DecisionSignalKind.AppInstallFailed"/> fell through to
+    /// <c>HandleUnhandledSignal</c>, producing dead-end transitions and discarding the
+    /// per-app outcome. These tests pin the new contract: bookkeeping advances, the
+    /// <see cref="DecisionState.AppInstallFacts"/> aggregate updates correctly, and the
+    /// transitions come out as taken (not dead-ends).
+    /// </summary>
+    public sealed class AppInstallSignalHandlerTests
+    {
+        private const string SessionId = "session-appinstall";
+        private const string TenantId = "tenant-appinstall";
+
+        private static DecisionSignal MakeCompleted(long ordinal, string appId, string newState)
+        {
+            return new DecisionSignal(
+                sessionSignalOrdinal: ordinal,
+                sessionTraceOrdinal: ordinal,
+                kind: DecisionSignalKind.AppInstallCompleted,
+                kindSchemaVersion: 1,
+                occurredAtUtc: new DateTime(2026, 4, 24, 10, 0, 0, DateTimeKind.Utc).AddSeconds(ordinal),
+                sourceOrigin: "IME",
+                evidence: new Evidence(EvidenceKind.Derived, $"app:{appId}", $"App {appId} → {newState}",
+                    derivationInputs: new Dictionary<string, string>
+                    {
+                        ["appId"] = appId,
+                        ["newState"] = newState,
+                    }),
+                payload: new Dictionary<string, string>
+                {
+                    ["appId"] = appId,
+                    ["newState"] = newState,
+                });
+        }
+
+        private static DecisionSignal MakeFailed(long ordinal, string appId)
+        {
+            return new DecisionSignal(
+                sessionSignalOrdinal: ordinal,
+                sessionTraceOrdinal: ordinal,
+                kind: DecisionSignalKind.AppInstallFailed,
+                kindSchemaVersion: 1,
+                occurredAtUtc: new DateTime(2026, 4, 24, 10, 0, 0, DateTimeKind.Utc).AddSeconds(ordinal),
+                sourceOrigin: "IME",
+                evidence: new Evidence(EvidenceKind.Derived, $"app:{appId}", $"App {appId} → Error",
+                    derivationInputs: new Dictionary<string, string>
+                    {
+                        ["appId"] = appId,
+                        ["newState"] = "Error",
+                    }),
+                payload: new Dictionary<string, string>
+                {
+                    ["appId"] = appId,
+                    ["newState"] = "Error",
+                });
+        }
+
+        // ---- Completed handler -----------------------------------------------------------
+
+        [Fact]
+        public void Completed_Installed_increments_Completed_and_Installed_breakdown_only()
+        {
+            var engine = new DecisionEngine();
+            var state = DecisionState.CreateInitial(SessionId, TenantId);
+
+            var step = engine.Reduce(state, MakeCompleted(0, "app-1", "Installed"));
+
+            Assert.True(step.Transition.Taken);
+            Assert.Null(step.Transition.DeadEndReason);
+            Assert.Equal(nameof(DecisionSignalKind.AppInstallCompleted), step.Transition.Trigger);
+
+            var facts = step.NewState.AppInstallFacts;
+            Assert.Equal(1, facts.CompletedCount);
+            Assert.Equal(1, facts.InstalledCount);
+            Assert.Equal(0, facts.SkippedCount);
+            Assert.Equal(0, facts.PostponedCount);
+            Assert.Equal(0, facts.FailedCount);
+        }
+
+        [Fact]
+        public void Completed_Skipped_increments_Completed_and_Skipped_breakdown_only()
+        {
+            var engine = new DecisionEngine();
+            var step = engine.Reduce(
+                DecisionState.CreateInitial(SessionId, TenantId),
+                MakeCompleted(0, "app-1", "Skipped"));
+
+            var facts = step.NewState.AppInstallFacts;
+            Assert.Equal(1, facts.CompletedCount);
+            Assert.Equal(0, facts.InstalledCount);
+            Assert.Equal(1, facts.SkippedCount);
+            Assert.Equal(0, facts.PostponedCount);
+        }
+
+        [Fact]
+        public void Completed_Postponed_increments_Completed_and_Postponed_breakdown_only()
+        {
+            var engine = new DecisionEngine();
+            var step = engine.Reduce(
+                DecisionState.CreateInitial(SessionId, TenantId),
+                MakeCompleted(0, "app-1", "Postponed"));
+
+            var facts = step.NewState.AppInstallFacts;
+            Assert.Equal(1, facts.CompletedCount);
+            Assert.Equal(1, facts.PostponedCount);
+        }
+
+        [Fact]
+        public void Completed_unknown_newState_still_increments_CompletedCount_but_no_breakdown()
+        {
+            // Forward-compat: an adapter adding a future terminal label (e.g. "Replaced")
+            // should not throw; the bucket count goes up even if no breakdown matches.
+            var engine = new DecisionEngine();
+            var step = engine.Reduce(
+                DecisionState.CreateInitial(SessionId, TenantId),
+                MakeCompleted(0, "app-1", "Replaced"));
+
+            var facts = step.NewState.AppInstallFacts;
+            Assert.Equal(1, facts.CompletedCount);
+            Assert.Equal(0, facts.InstalledCount);
+            Assert.Equal(0, facts.SkippedCount);
+            Assert.Equal(0, facts.PostponedCount);
+        }
+
+        [Fact]
+        public void Completed_advances_step_bookkeeping_without_changing_stage_or_outcome()
+        {
+            var engine = new DecisionEngine();
+            var state = DecisionState.CreateInitial(SessionId, TenantId)
+                .ToBuilder()
+                .WithStage(SessionStage.AwaitingHello)
+                .WithStepIndex(4)
+                .WithLastAppliedSignalOrdinal(3)
+                .Build();
+
+            var step = engine.Reduce(state, MakeCompleted(4, "app-1", "Installed"));
+
+            Assert.Equal(SessionStage.AwaitingHello, step.NewState.Stage);
+            Assert.Null(step.NewState.Outcome);
+            Assert.Equal(5, step.NewState.StepIndex);
+            Assert.Equal(4, step.NewState.LastAppliedSignalOrdinal);
+        }
+
+        // ---- Failed handler --------------------------------------------------------------
+
+        [Fact]
+        public void Failed_increments_FailedCount_and_records_appId()
+        {
+            var engine = new DecisionEngine();
+            var step = engine.Reduce(
+                DecisionState.CreateInitial(SessionId, TenantId),
+                MakeFailed(0, "app-broken-1"));
+
+            Assert.True(step.Transition.Taken);
+            Assert.Equal(nameof(DecisionSignalKind.AppInstallFailed), step.Transition.Trigger);
+
+            var facts = step.NewState.AppInstallFacts;
+            Assert.Equal(1, facts.FailedCount);
+            Assert.Single(facts.FailedAppIds, "app-broken-1");
+            Assert.Equal(0, facts.CompletedCount);
+        }
+
+        [Fact]
+        public void Failed_without_appId_still_increments_count_but_does_not_pollute_list()
+        {
+            // Defensive: a malformed payload without "appId" should not crash or poison the
+            // list with empty entries.
+            var engine = new DecisionEngine();
+            var signal = new DecisionSignal(
+                sessionSignalOrdinal: 0,
+                sessionTraceOrdinal: 0,
+                kind: DecisionSignalKind.AppInstallFailed,
+                kindSchemaVersion: 1,
+                occurredAtUtc: new DateTime(2026, 4, 24, 10, 0, 0, DateTimeKind.Utc),
+                sourceOrigin: "IME",
+                evidence: new Evidence(EvidenceKind.Synthetic, "app:unknown", "test"),
+                payload: new Dictionary<string, string>()); // no "appId"
+
+            var step = engine.Reduce(DecisionState.CreateInitial(SessionId, TenantId), signal);
+
+            var facts = step.NewState.AppInstallFacts;
+            Assert.Equal(1, facts.FailedCount);
+            Assert.Empty(facts.FailedAppIds);
+        }
+
+        [Fact]
+        public void Failed_deduplicates_repeated_appId()
+        {
+            // The adapter posts terminal once per app, but a defensive dedup guards against
+            // any replay-injection that might double-feed the same id.
+            var engine = new DecisionEngine();
+            var state = DecisionState.CreateInitial(SessionId, TenantId);
+
+            var step1 = engine.Reduce(state, MakeFailed(0, "app-x"));
+            var step2 = engine.Reduce(step1.NewState, MakeFailed(1, "app-x"));
+
+            var facts = step2.NewState.AppInstallFacts;
+            Assert.Equal(2, facts.FailedCount);
+            Assert.Single(facts.FailedAppIds); // still just one entry
+            Assert.Equal("app-x", facts.FailedAppIds[0]);
+        }
+
+        [Fact]
+        public void Failed_caps_FailedAppIds_at_MaxFailedAppIds_limit()
+        {
+            var engine = new DecisionEngine();
+            DecisionState state = DecisionState.CreateInitial(SessionId, TenantId);
+
+            // Post MaxFailedAppIds + 10 distinct failures; the tail must be dropped.
+            var total = AppInstallFacts.MaxFailedAppIds + 10;
+            for (var i = 0; i < total; i++)
+            {
+                state = engine.Reduce(state, MakeFailed(i, $"app-{i}")).NewState;
+            }
+
+            var facts = state.AppInstallFacts;
+            Assert.Equal(total, facts.FailedCount);
+            Assert.Equal(AppInstallFacts.MaxFailedAppIds, facts.FailedAppIds.Count);
+            // First-observed wins — the cap drops later arrivals, not earlier ones.
+            Assert.Equal("app-0", facts.FailedAppIds[0]);
+            Assert.Equal($"app-{AppInstallFacts.MaxFailedAppIds - 1}",
+                facts.FailedAppIds[AppInstallFacts.MaxFailedAppIds - 1]);
+        }
+
+        // ---- Integration -----------------------------------------------------------------
+
+        [Fact]
+        public void Mixed_stream_of_completed_and_failed_accumulates_facts_correctly()
+        {
+            var engine = new DecisionEngine();
+            var state = DecisionState.CreateInitial(SessionId, TenantId);
+
+            state = engine.Reduce(state, MakeCompleted(0, "a1", "Installed")).NewState;
+            state = engine.Reduce(state, MakeCompleted(1, "a2", "Skipped")).NewState;
+            state = engine.Reduce(state, MakeFailed(2, "a3")).NewState;
+            state = engine.Reduce(state, MakeCompleted(3, "a4", "Installed")).NewState;
+            state = engine.Reduce(state, MakeFailed(4, "a5")).NewState;
+
+            var facts = state.AppInstallFacts;
+            Assert.Equal(3, facts.CompletedCount);
+            Assert.Equal(2, facts.InstalledCount);
+            Assert.Equal(1, facts.SkippedCount);
+            Assert.Equal(0, facts.PostponedCount);
+            Assert.Equal(2, facts.FailedCount);
+            Assert.Equal(new[] { "a3", "a5" }, facts.FailedAppIds);
+            Assert.Equal(5, state.StepIndex);
+            Assert.Equal(4, state.LastAppliedSignalOrdinal);
+        }
+    }
+}
diff --git a/src/Agent/AutopilotMonitor.DecisionCore.Tests/HarnessTests.cs b/src/Agent/AutopilotMonitor.DecisionCore.Tests/HarnessTests.cs
index 70d50436..a39d38c7 100644
--- a/src/Agent/AutopilotMonitor.DecisionCore.Tests/HarnessTests.cs
+++ b/src/Agent/AutopilotMonitor.DecisionCore.Tests/HarnessTests.cs
@@ -84,20 +84,22 @@ namespace AutopilotMonitor.DecisionCore.Tests
         }
 
         [Fact]
-        public void Replay_unknownSignalKind_recordsDeadEnd_stateUnchanged()
+        public void Replay_unknownSchemaVersion_recordsDeadEnd_stateUnchanged()
         {
-            // An unhandled signal advances bookkeeping but leaves stage/outcome alone
-            // and records a DeadEndReason — plan §2.5 fail-safe shape for non-handlers.
-            // AppInstallCompleted is scheduled for v11.1; still unhandled in M3.3.
+            // Codex follow-up #4 wired AppInstallCompleted / AppInstallFailed, so every
+            // DecisionSignalKind now has at least one handler. The dead-end path remains
+            // reachable through an unknown schema version — the forward-compat surface the
+            // reducer guards per plan §2.5. A v2 SessionStarted arriving at a v1-aware
+            // backend is treated as an unhandled kind dispatch.
             var harness = new ReplayHarness(new DecisionEngine());
             var signal = new DecisionSignal(
                 sessionSignalOrdinal: 0,
                 sessionTraceOrdinal: 0,
-                kind: DecisionSignalKind.AppInstallCompleted,
-                kindSchemaVersion: 1,
+                kind: DecisionSignalKind.SessionStarted,
+                kindSchemaVersion: 99,
                 occurredAtUtc: DateTime.UtcNow,
                 sourceOrigin: "harness",
-                evidence: new Evidence(EvidenceKind.Raw, "App:test", "test"));
+                evidence: new Evidence(EvidenceKind.Raw, "session:future", "test"));
 
             var result = harness.Replay("s", "t", new List<DecisionSignal> { signal });
 
diff --git a/src/Agent/AutopilotMonitor.DecisionCore.Tests/ReducerFailSafeTests.cs b/src/Agent/AutopilotMonitor.DecisionCore.Tests/ReducerFailSafeTests.cs
index 5f82ce21..5302a3ad 100644
--- a/src/Agent/AutopilotMonitor.DecisionCore.Tests/ReducerFailSafeTests.cs
+++ b/src/Agent/AutopilotMonitor.DecisionCore.Tests/ReducerFailSafeTests.cs
@@ -55,18 +55,20 @@ namespace AutopilotMonitor.DecisionCore.Tests
         }
 
         [Fact]
-        public void Reduce_unhandledKind_producesDeadEnd_withVersionedReason()
+        public void Reduce_unhandledSchemaVersion_producesDeadEnd_withVersionedReason()
         {
-            // Pick a kind that no sub-milestone has wired yet. AppInstallCompleted is
-            // scheduled for v11.1 app-install observability; update this test when wired.
+            // After Codex follow-up #4 every DecisionSignalKind has at least one handler,
+            // so the dead-end path is now only reachable through an unknown schema version
+            // on a known kind. This is the realistic forward-compat failure mode: the
+            // backend sees a v2 signal it doesn't know how to interpret yet.
             var engine = new DecisionEngine();
             var state = DecisionState.CreateInitial("s", "t");
-            var signal = MakeSignal(DecisionSignalKind.AppInstallCompleted, schemaVersion: 1);
+            var signal = MakeSignal(DecisionSignalKind.SessionStarted, schemaVersion: 99);
 
             var step = engine.Reduce(state, signal);
 
             Assert.False(step.Transition.Taken);
-            Assert.Equal("unhandled_signal_kind:AppInstallCompleted:v1", step.Transition.DeadEndReason);
+            Assert.Equal("unhandled_signal_kind:SessionStarted:v99", step.Transition.DeadEndReason);
             Assert.Equal(SessionStage.SessionStarted, step.NewState.Stage);
             Assert.Equal(1, step.NewState.StepIndex);
         }
diff --git a/src/Shared/AutopilotMonitor.DecisionCore/Engine/DecisionEngine.Shared.cs b/src/Shared/AutopilotMonitor.DecisionCore/Engine/DecisionEngine.Shared.cs
index ece2f3fb..ed47ad4b 100644
--- a/src/Shared/AutopilotMonitor.DecisionCore/Engine/DecisionEngine.Shared.cs
+++ b/src/Shared/AutopilotMonitor.DecisionCore/Engine/DecisionEngine.Shared.cs
@@ -159,6 +159,73 @@ namespace AutopilotMonitor.DecisionCore.Engine
             return new DecisionStep(newState, transition, new[] { failEffect });
         }
 
+        /// <summary>
+        /// Handle <see cref="DecisionSignalKind.AppInstallCompleted"/>. Codex follow-up #4 —
+        /// observation-only: advance bookkeeping, roll the terminal outcome into
+        /// <see cref="DecisionState.AppInstallFacts"/>, record a taken transition. Does NOT
+        /// affect <see cref="SessionStage"/> or <see cref="SessionOutcome"/>; downstream
+        /// consumers (Inspector UI, future classifiers) read the aggregate directly.
+        /// <para>
+        /// Payload contract (from <c>ImeLogTrackerAdapter</c>): <c>appId</c>, <c>newState</c>
+        /// ∈ {<c>Installed</c>, <c>Skipped</c>, <c>Postponed</c>}. Unknown <c>newState</c>
+        /// values still count toward <see cref="AppInstallFacts.CompletedCount"/> but do not
+        /// update any breakdown counter.
+        /// </para>
+        /// </summary>
+        private DecisionStep HandleAppInstallCompletedV1(DecisionState state, DecisionSignal signal)
+        {
+            var newStatePayload = signal.Payload != null && signal.Payload.TryGetValue("newState", out var v)
+                ? v
+                : null;
+            var updatedFacts = state.AppInstallFacts.WithCompleted(newStatePayload);
+
+            var nextStep = state.StepIndex + 1;
+            var newState = state.ToBuilder()
+                .WithStepIndex(nextStep)
+                .WithLastAppliedSignalOrdinal(signal.SessionSignalOrdinal)
+                .WithAppInstallFacts(updatedFacts)
+                .Build();
+
+            var transition = BuildTakenTransition(
+                before: state,
+                signal: signal,
+                toStage: state.Stage,
+                nextStepIndex: nextStep,
+                trigger: nameof(DecisionSignalKind.AppInstallCompleted));
+
+            return new DecisionStep(newState, transition, Array.Empty<DecisionEffect>());
+        }
+
+        /// <summary>
+        /// Handle <see cref="DecisionSignalKind.AppInstallFailed"/>. Codex follow-up #4 —
+        /// observation-only: advance bookkeeping, roll the failure into
+        /// <see cref="AppInstallFacts"/> (increments counter, appends <c>appId</c> up to
+        /// <see cref="AppInstallFacts.MaxFailedAppIds"/>), record a taken transition.
+        /// </summary>
+        private DecisionStep HandleAppInstallFailedV1(DecisionState state, DecisionSignal signal)
+        {
+            var appId = signal.Payload != null && signal.Payload.TryGetValue("appId", out var v)
+                ? v
+                : null;
+            var updatedFacts = state.AppInstallFacts.WithFailed(appId);
+
+            var nextStep = state.StepIndex + 1;
+            var newState = state.ToBuilder()
+                .WithStepIndex(nextStep)
+                .WithLastAppliedSignalOrdinal(signal.SessionSignalOrdinal)
+                .WithAppInstallFacts(updatedFacts)
+                .Build();
+
+            var transition = BuildTakenTransition(
+                before: state,
+                signal: signal,
+                toStage: state.Stage,
+                nextStepIndex: nextStep,
+                trigger: nameof(DecisionSignalKind.AppInstallFailed));
+
+            return new DecisionStep(newState, transition, Array.Empty<DecisionEffect>());
+        }
+
         /// <summary>
         /// Handle <see cref="DecisionSignalKind.SessionAborted"/>.
         /// <para>
diff --git a/src/Shared/AutopilotMonitor.DecisionCore/Engine/DecisionEngine.cs b/src/Shared/AutopilotMonitor.DecisionCore/Engine/DecisionEngine.cs
index 6c584554..acfebde8 100644
--- a/src/Shared/AutopilotMonitor.DecisionCore/Engine/DecisionEngine.cs
+++ b/src/Shared/AutopilotMonitor.DecisionCore/Engine/DecisionEngine.cs
@@ -98,7 +98,11 @@ namespace AutopilotMonitor.DecisionCore.Engine
                 // ----- Informational pass-through (DecisionEngine.Shared.cs) — single-rail §1.3 -----
                 (DecisionSignalKind.InformationalEvent, 1)                 => HandleInformationalEventV1(state, signal),
 
-                // ----- Still unwired (AppInstall — v11.1+) -----
+                // ----- App-install observations (DecisionEngine.Shared.cs) — Codex follow-up #4 -----
+                (DecisionSignalKind.AppInstallCompleted, 1)                => HandleAppInstallCompletedV1(state, signal),
+                (DecisionSignalKind.AppInstallFailed, 1)                   => HandleAppInstallFailedV1(state, signal),
+
+                // ----- Fall-through: unknown (kind, schemaVersion) pair → dead-end journal entry -----
                 _ => HandleUnhandledSignal(state, signal),
             };
         }
diff --git a/src/Shared/AutopilotMonitor.DecisionCore/State/AppInstallFacts.cs b/src/Shared/AutopilotMonitor.DecisionCore/State/AppInstallFacts.cs
new file mode 100644
index 00000000..d0400c5d
--- /dev/null
+++ b/src/Shared/AutopilotMonitor.DecisionCore/State/AppInstallFacts.cs
@@ -0,0 +1,141 @@
+using System;
+using System.Collections.Generic;
+
+namespace AutopilotMonitor.DecisionCore.State
+{
+    /// <summary>
+    /// Aggregated facts about app-install terminal outcomes observed during a session.
+    /// Codex follow-up #4 — <see cref="Signals.DecisionSignalKind.AppInstallCompleted"/> and
+    /// <see cref="Signals.DecisionSignalKind.AppInstallFailed"/> are now first-class
+    /// observation signals; this value holds their rolled-up counts so the reducer, the
+    /// journal (via <see cref="DecisionState"/>) and any future classifier have structured
+    /// evidence about which apps finished and which blocked.
+    /// <para>
+    /// <b>Invariants</b>:
+    /// <list type="bullet">
+    ///   <item><see cref="CompletedCount"/> == <see cref="InstalledCount"/> + <see cref="SkippedCount"/> + <see cref="PostponedCount"/>.</item>
+    ///   <item><see cref="FailedAppIds"/> is capped at <see cref="MaxFailedAppIds"/>; overflow is discarded so DecisionState cannot grow unbounded.</item>
+    ///   <item>Instances are immutable; <see cref="WithCompleted"/> / <see cref="WithFailed"/> return new instances.</item>
+    /// </list>
+    /// </para>
+    /// </summary>
+    public sealed class AppInstallFacts
+    {
+        /// <summary>Cap on <see cref="FailedAppIds"/> so pathological sessions cannot bloat state.</summary>
+        public const int MaxFailedAppIds = 50;
+
+        public static readonly AppInstallFacts Empty = new AppInstallFacts(
+            completedCount: 0,
+            installedCount: 0,
+            skippedCount: 0,
+            postponedCount: 0,
+            failedCount: 0,
+            failedAppIds: Array.Empty<string>());
+
+        public AppInstallFacts(
+            int completedCount,
+            int installedCount,
+            int skippedCount,
+            int postponedCount,
+            int failedCount,
+            IReadOnlyList<string> failedAppIds)
+        {
+            if (completedCount < 0) throw new ArgumentOutOfRangeException(nameof(completedCount));
+            if (installedCount < 0) throw new ArgumentOutOfRangeException(nameof(installedCount));
+            if (skippedCount < 0) throw new ArgumentOutOfRangeException(nameof(skippedCount));
+            if (postponedCount < 0) throw new ArgumentOutOfRangeException(nameof(postponedCount));
+            if (failedCount < 0) throw new ArgumentOutOfRangeException(nameof(failedCount));
+
+            CompletedCount = completedCount;
+            InstalledCount = installedCount;
+            SkippedCount = skippedCount;
+            PostponedCount = postponedCount;
+            FailedCount = failedCount;
+            FailedAppIds = failedAppIds ?? Array.Empty<string>();
+        }
+
+        /// <summary>Installed + Skipped + Postponed (everything the adapter maps to <c>AppInstallCompleted</c>).</summary>
+        public int CompletedCount { get; }
+
+        /// <summary>Apps with terminal state == <c>Installed</c>.</summary>
+        public int InstalledCount { get; }
+
+        /// <summary>Apps with terminal state == <c>Skipped</c> (policy-skipped, not failed).</summary>
+        public int SkippedCount { get; }
+
+        /// <summary>Apps with terminal state == <c>Postponed</c>.</summary>
+        public int PostponedCount { get; }
+
+        /// <summary>Apps with terminal state == <c>Error</c> (i.e. <c>AppInstallFailed</c>).</summary>
+        public int FailedCount { get; }
+
+        /// <summary>
+        /// Identifiers of failed apps, first <see cref="MaxFailedAppIds"/> in order of
+        /// observation. Duplicate <c>appId</c>s are ignored (terminal is posted once per app,
+        /// but the guard is defensive).
+        /// </summary>
+        public IReadOnlyList<string> FailedAppIds { get; }
+
+        /// <summary>
+        /// Produce a new facts value with the completed sub-category incremented.
+        /// <paramref name="newStatePayload"/> is the adapter's payload value — one of
+        /// <c>"Installed"</c>, <c>"Skipped"</c>, <c>"Postponed"</c>. Unknown values still
+        /// increment <see cref="CompletedCount"/> but don't contribute to any breakdown.
+        /// </summary>
+        public AppInstallFacts WithCompleted(string? newStatePayload)
+        {
+            var installed = InstalledCount;
+            var skipped = SkippedCount;
+            var postponed = PostponedCount;
+
+            if (string.Equals(newStatePayload, "Installed", StringComparison.OrdinalIgnoreCase)) installed++;
+            else if (string.Equals(newStatePayload, "Skipped", StringComparison.OrdinalIgnoreCase)) skipped++;
+            else if (string.Equals(newStatePayload, "Postponed", StringComparison.OrdinalIgnoreCase)) postponed++;
+
+            return new AppInstallFacts(
+                completedCount: CompletedCount + 1,
+                installedCount: installed,
+                skippedCount: skipped,
+                postponedCount: postponed,
+                failedCount: FailedCount,
+                failedAppIds: FailedAppIds);
+        }
+
+        /// <summary>
+        /// Produce a new facts value with <see cref="FailedCount"/> incremented and
+        /// <paramref name="appId"/> appended to <see cref="FailedAppIds"/> (dedup + cap).
+        /// </summary>
+        public AppInstallFacts WithFailed(string? appId)
+        {
+            var newIds = FailedAppIds;
+            if (!string.IsNullOrEmpty(appId))
+            {
+                var alreadyTracked = false;
+                for (var i = 0; i < FailedAppIds.Count; i++)
+                {
+                    if (string.Equals(FailedAppIds[i], appId, StringComparison.Ordinal))
+                    {
+                        alreadyTracked = true;
+                        break;
+                    }
+                }
+
+                if (!alreadyTracked && FailedAppIds.Count < MaxFailedAppIds)
+                {
+                    var copy = new List<string>(FailedAppIds.Count + 1);
+                    copy.AddRange(FailedAppIds);
+                    copy.Add(appId!);
+                    newIds = copy;
+                }
+            }
+
+            return new AppInstallFacts(
+                completedCount: CompletedCount,
+                installedCount: InstalledCount,
+                skippedCount: SkippedCount,
+                postponedCount: PostponedCount,
+                failedCount: FailedCount + 1,
+                failedAppIds: newIds);
+        }
+    }
+}
diff --git a/src/Shared/AutopilotMonitor.DecisionCore/State/DecisionState.cs b/src/Shared/AutopilotMonitor.DecisionCore/State/DecisionState.cs
index 865d0052..d1f3bc12 100644
--- a/src/Shared/AutopilotMonitor.DecisionCore/State/DecisionState.cs
+++ b/src/Shared/AutopilotMonitor.DecisionCore/State/DecisionState.cs
@@ -51,6 +51,7 @@ namespace AutopilotMonitor.DecisionCore.State
             IReadOnlyList<ActiveDeadline> deadlines,
             long lastAppliedSignalOrdinal,
             int stepIndex,
+            AppInstallFacts? appInstallFacts = null,
             string? schemaVersion = null)
         {
             if (string.IsNullOrEmpty(sessionId))
@@ -93,6 +94,7 @@ namespace AutopilotMonitor.DecisionCore.State
             Deadlines = deadlines ?? throw new ArgumentNullException(nameof(deadlines));
             LastAppliedSignalOrdinal = lastAppliedSignalOrdinal;
             StepIndex = stepIndex;
+            AppInstallFacts = appInstallFacts ?? AppInstallFacts.Empty;
             SchemaVersion = schemaVersion ?? CurrentSchemaVersion;
         }
 
@@ -164,6 +166,13 @@ namespace AutopilotMonitor.DecisionCore.State
 
         public int StepIndex { get; }
 
+        /// <summary>
+        /// Rolled-up terminal outcomes from <see cref="Signals.DecisionSignalKind.AppInstallCompleted"/>
+        /// and <see cref="Signals.DecisionSignalKind.AppInstallFailed"/> signals — Codex
+        /// follow-up #4. Never null; defaults to <see cref="State.AppInstallFacts.Empty"/>.
+        /// </summary>
+        public AppInstallFacts AppInstallFacts { get; }
+
         public string SchemaVersion { get; }
 
         /// <summary>
@@ -208,6 +217,7 @@ namespace AutopilotMonitor.DecisionCore.State
                 skipDeviceEsp: null,
                 deadlines: Array.Empty<ActiveDeadline>(),
                 lastAppliedSignalOrdinal: -1,
-                stepIndex: 0);
+                stepIndex: 0,
+                appInstallFacts: AppInstallFacts.Empty);
     }
 }
diff --git a/src/Shared/AutopilotMonitor.DecisionCore/State/DecisionStateBuilder.cs b/src/Shared/AutopilotMonitor.DecisionCore/State/DecisionStateBuilder.cs
index dd09d821..20353e28 100644
--- a/src/Shared/AutopilotMonitor.DecisionCore/State/DecisionStateBuilder.cs
+++ b/src/Shared/AutopilotMonitor.DecisionCore/State/DecisionStateBuilder.cs
@@ -49,6 +49,7 @@ namespace AutopilotMonitor.DecisionCore.State
             Deadlines = new List<ActiveDeadline>(source.Deadlines);
             LastAppliedSignalOrdinal = source.LastAppliedSignalOrdinal;
             StepIndex = source.StepIndex;
+            AppInstallFacts = source.AppInstallFacts;
             SchemaVersion = source.SchemaVersion;
         }
 
@@ -82,6 +83,7 @@ namespace AutopilotMonitor.DecisionCore.State
         public List<ActiveDeadline> Deadlines { get; set; }
         public long LastAppliedSignalOrdinal { get; set; }
         public int StepIndex { get; set; }
+        public AppInstallFacts AppInstallFacts { get; set; } = AppInstallFacts.Empty;
         public string SchemaVersion { get; set; }
 
         // ---------- fluent helpers for the most common reducer operations ----------
@@ -125,6 +127,12 @@ namespace AutopilotMonitor.DecisionCore.State
 
         public DecisionStateBuilder ClearDeadlines() { Deadlines.Clear(); return this; }
 
+        public DecisionStateBuilder WithAppInstallFacts(AppInstallFacts facts)
+        {
+            AppInstallFacts = facts ?? throw new ArgumentNullException(nameof(facts));
+            return this;
+        }
+
         public DecisionState Build() =>
             new DecisionState(
                 sessionId: SessionId,
@@ -157,6 +165,7 @@ namespace AutopilotMonitor.DecisionCore.State
                 deadlines: Deadlines.ToArray(),
                 lastAppliedSignalOrdinal: LastAppliedSignalOrdinal,
                 stepIndex: StepIndex,
+                appInstallFacts: AppInstallFacts,
                 schemaVersion: SchemaVersion);
     }
 }
diff --git a/tests/fixtures/signal-kinds/app-install-completed-v1.json b/tests/fixtures/signal-kinds/app-install-completed-v1.json
new file mode 100644
index 00000000..dfa3a7e7
--- /dev/null
+++ b/tests/fixtures/signal-kinds/app-install-completed-v1.json
@@ -0,0 +1,6 @@
+# AppInstallCompleted v1 — Codex follow-up #4.
+# Posted by ImeLogTrackerAdapter once per app on terminal Installed/Skipped/Postponed.
+# Reducer: HandleAppInstallCompletedV1 → bookkeeping advance + AppInstallFacts update
+# (CompletedCount++, and Installed/Skipped/Postponed breakdown when newState matches).
+# No Stage/Outcome change.
+{"SessionSignalOrdinal":1,"SessionTraceOrdinal":1,"Kind":"AppInstallCompleted","KindSchemaVersion":1,"OccurredAtUtc":"2026-04-24T10:00:00.0000000Z","SourceOrigin":"IME","Evidence":{"Kind":"Derived","Identifier":"ime-log-tracker-v1","Summary":"App app-guid-001 → Installed","DerivationInputs":{"appId":"app-guid-001","appName":"Company Portal","previousState":"Installing","newState":"Installed"}},"Payload":{"appId":"app-guid-001","newState":"Installed"}}
diff --git a/tests/fixtures/signal-kinds/app-install-failed-v1.json b/tests/fixtures/signal-kinds/app-install-failed-v1.json
new file mode 100644
index 00000000..e133a30d
--- /dev/null
+++ b/tests/fixtures/signal-kinds/app-install-failed-v1.json
@@ -0,0 +1,5 @@
+# AppInstallFailed v1 — Codex follow-up #4.
+# Posted by ImeLogTrackerAdapter once per app on terminal Error state.
+# Reducer: HandleAppInstallFailedV1 → bookkeeping advance + AppInstallFacts.FailedCount++
+# + FailedAppIds.Add(appId) (dedup, capped at MaxFailedAppIds). No Stage/Outcome change.
+{"SessionSignalOrdinal":2,"SessionTraceOrdinal":2,"Kind":"AppInstallFailed","KindSchemaVersion":1,"OccurredAtUtc":"2026-04-24T10:01:00.0000000Z","SourceOrigin":"IME","Evidence":{"Kind":"Derived","Identifier":"ime-log-tracker-v1","Summary":"App app-guid-002 → Error","DerivationInputs":{"appId":"app-guid-002","appName":"Broken Tool","previousState":"Installing","newState":"Error"}},"Payload":{"appId":"app-guid-002","newState":"Error"}}