diff --git a/src/test/e2e/architect-workflow.test.ts b/src/test/e2e/architect-workflow.test.ts
index 6c59f85..2c01596 100644
--- a/src/test/e2e/architect-workflow.test.ts
+++ b/src/test/e2e/architect-workflow.test.ts
@@ -149,6 +149,93 @@ describe('Architect Workflow E2E', () => {
     });
   });
 
+  describe('breakdown conflict detection', () => {
+    it('should reject if a breakdown agent is already running', async () => {
+      vi.useFakeTimers();
+
+      const initiative = await harness.createInitiative('Auth System');
+
+      // Set up a long-running breakdown agent (never completes during this test)
+      harness.setArchitectBreakdownComplete('first-breakdown', [
+        { number: 1, name: 'Phase 1', description: 'First', dependencies: [] },
+      ]);
+      // Use a delay so it stays running
+      harness.setAgentScenario('first-breakdown', { status: 'done', delay: 999999 });
+
+      await harness.caller.spawnArchitectBreakdown({
+        name: 'first-breakdown',
+        initiativeId: initiative.id,
+      });
+
+      // Agent should be running
+      const agents = await harness.caller.listAgents();
+      expect(agents.find(a => a.name === 'first-breakdown')?.status).toBe('running');
+
+      // Second breakdown should be rejected
+      await expect(
+        harness.caller.spawnArchitectBreakdown({
+          name: 'second-breakdown',
+          initiativeId: initiative.id,
+        }),
+      ).rejects.toThrow(/already running/);
+    });
+
+    it('should auto-dismiss stale breakdown agents before checking', async () => {
+      vi.useFakeTimers();
+
+      const initiative = await harness.createInitiative('Auth System');
+
+      // Set up a breakdown agent that crashes immediately
+      harness.setAgentScenario('stale-breakdown', { status: 'error', error: 'crashed' });
+
+      await harness.caller.spawnArchitectBreakdown({
+        name: 'stale-breakdown',
+        initiativeId: initiative.id,
+      });
+      await harness.advanceTimers();
+
+      // Should be crashed
+      const agents = await harness.caller.listAgents();
+      expect(agents.find(a => a.name === 'stale-breakdown')?.status).toBe('crashed');
+
+      // New breakdown should succeed (stale one gets auto-dismissed)
+      harness.setArchitectBreakdownComplete('new-breakdown', [
+        { number: 1, name: 'Phase 1', description: 'First', dependencies: [] },
+      ]);
+
+      const agent = await harness.caller.spawnArchitectBreakdown({
+        name: 'new-breakdown',
+        initiativeId: initiative.id,
+      });
+      expect(agent.mode).toBe('breakdown');
+    });
+
+    it('should allow breakdown for different initiatives', async () => {
+      vi.useFakeTimers();
+
+      const init1 = await harness.createInitiative('Initiative 1');
+      const init2 = await harness.createInitiative('Initiative 2');
+
+      // Long-running agent on initiative 1
+      harness.setAgentScenario('breakdown-1', { status: 'done', delay: 999999 });
+      await harness.caller.spawnArchitectBreakdown({
+        name: 'breakdown-1',
+        initiativeId: init1.id,
+      });
+
+      // Breakdown on initiative 2 should succeed
+      harness.setArchitectBreakdownComplete('breakdown-2', [
+        { number: 1, name: 'Phase 1', description: 'First', dependencies: [] },
+      ]);
+
+      const agent = await harness.caller.spawnArchitectBreakdown({
+        name: 'breakdown-2',
+        initiativeId: init2.id,
+      });
+      expect(agent.mode).toBe('breakdown');
+    });
+  });
+
   describe('full workflow', () => {
     it('should complete discuss -> breakdown -> phases workflow', async () => {
       vi.useFakeTimers();