diff --git a/src/agent/mock-manager.ts b/src/agent/mock-manager.ts
index 6e6cd2f..9dad8c7 100644
--- a/src/agent/mock-manager.ts
+++ b/src/agent/mock-manager.ts
@@ -16,7 +16,7 @@ import type {
   PendingQuestions,
   QuestionItem,
 } from './types.js';
-import type { Decision, PhaseBreakdown } from './schema.js';
+import type { Decision, PhaseBreakdown, TaskBreakdown } from './schema.js';
 import type {
   EventBus,
   AgentSpawnedEvent,
@@ -30,10 +30,11 @@ import type {
  * Scenario configuration for mock agent behavior.
  * Uses discriminated union on status to match agent output schema.
  *
- * Supports all three agent modes:
+ * Supports all four agent modes:
  * - execute: done/questions/unrecoverable_error
  * - discuss: questions/context_complete/unrecoverable_error
  * - breakdown: questions/breakdown_complete/unrecoverable_error
+ * - decompose: questions/decompose_complete/unrecoverable_error
  */
 export type MockAgentScenario =
   // Execute mode statuses
@@ -66,6 +67,12 @@ export type MockAgentScenario =
       status: 'breakdown_complete';
       phases: PhaseBreakdown[];
       delay?: number;
+    }
+  // Decompose mode status
+  | {
+      status: 'decompose_complete';
+      tasks: TaskBreakdown[];
+      delay?: number;
     };
 
 /**
@@ -329,6 +336,30 @@ export class MockAgentManager implements AgentManager {
           this.eventBus.emit(event);
         }
         break;
+
+      case 'decompose_complete':
+        // Decompose mode completion - decomposed phase into tasks
+        record.result = {
+          success: true,
+          message: `Decomposed into ${scenario.tasks.length} tasks`,
+        };
+        record.info.status = 'idle';
+        record.info.updatedAt = new Date();
+
+        if (this.eventBus) {
+          const event: AgentStoppedEvent = {
+            type: 'agent:stopped',
+            timestamp: new Date(),
+            payload: {
+              agentId,
+              name: info.name,
+              taskId: info.taskId,
+              reason: 'decompose_complete',
+            },
+          };
+          this.eventBus.emit(event);
+        }
+        break;
     }
   }