diff --git a/.planning/STATE.md b/.planning/STATE.md
index 9710907..a0b2262 100644
--- a/.planning/STATE.md
+++ b/.planning/STATE.md
@@ -10,11 +10,11 @@ See: .planning/PROJECT.md (updated 2026-01-31)
 ## Current Position
 
 Phase: 12 of 13 (Phase Task Decomposition)
-Plan: 2 of 8 in current phase
+Plan: 3 of 8 in current phase
 Status: In progress
-Last activity: 2026-02-01 — Completed 12-01-PLAN.md, 12-02-PLAN.md
+Last activity: 2026-02-01 — Completed 12-03-PLAN.md
 
-Progress: ██████░░░░ 57%
+Progress: ██████░░░░ 59%
 
 ## Performance Metrics
 
@@ -148,6 +148,8 @@ Recent decisions affecting current work:
 - 12-01: Status 'decompose_complete' mirrors 'breakdown_complete' naming pattern
 - 12-01: Dependencies array uses integer task numbers for ordering (same as phases)
 - 12-02: getNextNumber uses max() aggregate, returning 1 if no plans exist (same pattern as PhaseRepository)
+- 12-03: Task execution order adjusted - Task 3 first since Tasks 1-2 depend on AgentStoppedEvent reason type
+- 12-03: handleDecomposeOutput follows exact structure of handleBreakdownOutput for pattern consistency
 
 ### Pending Todos
 
@@ -168,5 +170,5 @@ None.
 ## Session Continuity
 
 Last session: 2026-02-01
-Stopped at: Completed 12-01-PLAN.md (Decompose Mode Schema)
+Stopped at: Completed 12-03-PLAN.md (Agent Manager Decompose Mode)
 Resume file: None
diff --git a/.planning/phases/12-phase-task-decomposition/12-03-SUMMARY.md b/.planning/phases/12-phase-task-decomposition/12-03-SUMMARY.md
new file mode 100644
index 0000000..9ac753c
--- /dev/null
+++ b/.planning/phases/12-phase-task-decomposition/12-03-SUMMARY.md
@@ -0,0 +1,103 @@
+---
+phase: 12-phase-task-decomposition
+plan: 03
+subsystem: agent
+tags: [agent-mode, json-schema, mock-manager, typescript, decompose]
+
+# Dependency graph
+requires:
+  - phase: 12-phase-task-decomposition
+    plan: 01
+    provides: decomposeOutputSchema, decomposeOutputJsonSchema, TaskBreakdown type
+provides:
+  - Mode-aware agent spawning handles 'decompose' mode with decomposeOutputJsonSchema
+  - ClaudeAgentManager.handleDecomposeOutput() method for decompose completion handling
+  - MockAgentManager supports decompose_complete scenarios for E2E testing
+  - AgentStoppedEvent.reason extended with 'decompose_complete'
+affects: [12-05, 12-08]
+
+# Tech tracking
+tech-stack:
+  added: []
+  patterns:
+    - "Decompose mode follows same handler dispatch pattern as discuss/breakdown modes"
+    - "MockAgentScenario discriminated union extended for all four agent modes"
+
+key-files:
+  created: []
+  modified:
+    - src/agent/manager.ts
+    - src/agent/mock-manager.ts
+    - src/events/types.ts
+
+key-decisions:
+  - "Task 3 executed first as Tasks 1-2 depend on AgentStoppedEvent reason type"
+  - "handleDecomposeOutput follows exact pattern of handleBreakdownOutput for consistency"
+
+patterns-established:
+  - "Mode handler extension: new modes add handler method + switch case + JSON schema mapping"
+
+# Metrics
+duration: 7min
+completed: 2026-02-01
+---
+
+# Phase 12 Plan 03: Agent Manager Decompose Mode Summary
+
+**Updated ClaudeAgentManager and MockAgentManager to support decompose mode with decomposeOutputJsonSchema for phase-to-task decomposition**
+
+## Performance
+
+- **Duration:** 7 min
+- **Started:** 2026-02-01T10:38:18Z
+- **Completed:** 2026-02-01T10:45:07Z
+- **Tasks:** 3
+- **Files modified:** 3
+
+## Accomplishments
+
+- Extended AgentStoppedEvent.reason type with 'decompose_complete' for event typing
+- Added decompose mode support to ClaudeAgentManager (schema selection, handler method, switch routing)
+- Added decompose_complete scenario support to MockAgentManager for E2E testing
+- All 402 tests pass with no regressions
+
+## Task Commits
+
+Each task was committed atomically:
+
+1. **Task 3: Extend AgentStoppedEvent reason type** - `fe3fed0` (feat) - executed first as dependency
+2. **Task 1: Update ClaudeAgentManager for decompose mode** - `8754cde` (feat)
+3. **Task 2: Update MockAgentManager for decompose mode** - `2bd0bc5` (feat)
+
+## Files Created/Modified
+
+- `src/events/types.ts` - Added 'decompose_complete' to AgentStoppedEvent reason union
+- `src/agent/manager.ts` - Added decompose imports, getJsonSchemaForMode case, handleDecomposeOutput method, switch case routing
+- `src/agent/mock-manager.ts` - Added TaskBreakdown import, decompose_complete scenario type, completeAgent handler case
+
+## Decisions Made
+
+1. **12-03: Task execution order** - Task 3 (AgentStoppedEvent) executed first since Tasks 1-2 depend on 'decompose_complete' reason type
+2. **12-03: Handler pattern consistency** - handleDecomposeOutput follows exact structure of handleBreakdownOutput for maintainability
+
+## Deviations from Plan
+
+None - plan executed exactly as written. Task order adjusted for dependency correctness.
+
+## Issues Encountered
+
+None
+
+## User Setup Required
+
+None - no external service configuration required.
+
+## Next Phase Readiness
+
+- Decompose mode fully supported in both agent managers
+- Ready for 12-04 (Plan tRPC Procedures) and 12-05 (Decompose tRPC Procedures)
+- MockAgentManager can simulate decompose_complete scenarios for E2E testing in 12-08
+
+---
+*Phase: 12-phase-task-decomposition*
+*Completed: 2026-02-01*