diff --git a/src/test/e2e/decompose-workflow.test.ts b/src/test/e2e/decompose-workflow.test.ts
index 077dcb3..84df8a2 100644
--- a/src/test/e2e/decompose-workflow.test.ts
+++ b/src/test/e2e/decompose-workflow.test.ts
@@ -163,6 +163,90 @@ describe('Decompose Workflow E2E', () => {
     });
   });
 
+  describe('decompose conflict detection', () => {
+    it('should reject if a decompose agent is already running for the same phase', async () => {
+      vi.useFakeTimers();
+
+      const initiative = await harness.createInitiative('Test Project');
+      const phases = await harness.createPhasesFromBreakdown(initiative.id, [
+        { name: 'Phase 1' },
+      ]);
+
+      // Long-running decompose agent
+      harness.setAgentScenario('decomposer-1', { status: 'done', delay: 999999 });
+
+      await harness.caller.spawnArchitectDecompose({
+        name: 'decomposer-1',
+        phaseId: phases[0].id,
+      });
+
+      // Second decompose for same phase should be rejected
+      await expect(
+        harness.caller.spawnArchitectDecompose({
+          name: 'decomposer-2',
+          phaseId: phases[0].id,
+        }),
+      ).rejects.toThrow(/already running/);
+    });
+
+    it('should auto-dismiss stale decompose agents before checking', async () => {
+      vi.useFakeTimers();
+
+      const initiative = await harness.createInitiative('Test Project');
+      const phases = await harness.createPhasesFromBreakdown(initiative.id, [
+        { name: 'Phase 1' },
+      ]);
+
+      // Decompose agent that crashes immediately
+      harness.setAgentScenario('stale-decomposer', { status: 'error', error: 'crashed' });
+
+      await harness.caller.spawnArchitectDecompose({
+        name: 'stale-decomposer',
+        phaseId: phases[0].id,
+      });
+      await harness.advanceTimers();
+
+      // New decompose should succeed
+      harness.setArchitectDecomposeComplete('new-decomposer', [
+        { number: 1, name: 'Task 1', content: 'Do it', type: 'auto', dependencies: [] },
+      ]);
+
+      const agent = await harness.caller.spawnArchitectDecompose({
+        name: 'new-decomposer',
+        phaseId: phases[0].id,
+      });
+      expect(agent.mode).toBe('decompose');
+    });
+
+    it('should allow decompose for different phases simultaneously', async () => {
+      vi.useFakeTimers();
+
+      const initiative = await harness.createInitiative('Test Project');
+      const phases = await harness.createPhasesFromBreakdown(initiative.id, [
+        { name: 'Phase 1' },
+        { name: 'Phase 2' },
+      ]);
+
+      // Long-running agent on phase 1
+      harness.setAgentScenario('decomposer-p1', { status: 'done', delay: 999999 });
+      await harness.caller.spawnArchitectDecompose({
+        name: 'decomposer-p1',
+        phaseId: phases[0].id,
+      });
+
+      // Decompose on phase 2 should succeed
+      harness.setArchitectDecomposeComplete('decomposer-p2', [
+        { number: 1, name: 'Task 1', content: 'Do it', type: 'auto', dependencies: [] },
+      ]);
+
+      const agent = await harness.caller.spawnArchitectDecompose({
+        name: 'decomposer-p2',
+        phaseId: phases[1].id,
+      });
+      expect(agent.mode).toBe('decompose');
+    });
+  });
+
   describe('task persistence', () => {
     it('should create tasks from decomposition output', async () => {
       const initiative = await harness.createInitiative('Test Project');