diff --git a/packages/subagent/subagent/src/index.ts b/packages/subagent/subagent/src/index.ts
index ec66118ec6..ee6540f44a 100644
--- a/packages/subagent/subagent/src/index.ts
+++ b/packages/subagent/subagent/src/index.ts
@@ -207,8 +207,20 @@ export class SubagentService extends Service {
         // the SAME array reference the caller consumes would let a mutating
         // `subagent/end` listener corrupt the caller's SubagentResult.output —
         // breaking the observe-only contract. A snapshot makes the event a
-        // read-only view, not a shared handle.
-        this.emitLifecycle('subagent/end', { provider: name, id: run.id, ...agentType, stopReason: result.stopReason, lastAssistantMessage: structuredClone(result.output) })
+        // read-only view, not a shared handle. The clone is wrapped: it runs
+        // inside `onFulfilled`, OUTSIDE emitLifecycle's per-listener containment,
+        // so an uncloneable value (a future non-serializable content-block type,
+        // or a contract-violating result with no `output`) would otherwise become
+        // an unhandled rejection on this detached `.then`. On clone failure, log
+        // and emit the event WITHOUT lastAssistantMessage rather than dropping the
+        // whole `subagent/end`.
+        let lastAssistantMessage: SubagentResult['output'] | undefined
+        try {
+          lastAssistantMessage = structuredClone(result.output)
+        } catch (error: unknown) {
+          this.ctx.logger.warn(`subagent: could not clone ${name} output for subagent/end: ${String(error)}`)
+        }
+        this.emitLifecycle('subagent/end', { provider: name, id: run.id, ...agentType, stopReason: result.stopReason, ...lastAssistantMessage !== undefined ? { lastAssistantMessage } : {} })
       },
       () => { this.emitLifecycle('subagent/end', { provider: name, id: run.id, ...agentType, stopReason: 'error' }) },
     )
diff --git a/packages/subagent/subagent/tests/service.spec.ts b/packages/subagent/subagent/tests/service.spec.ts
index 2e5a3ef8a7..9db02ebe34 100644
--- a/packages/subagent/subagent/tests/service.spec.ts
+++ b/packages/subagent/subagent/tests/service.spec.ts
@@ -277,6 +277,41 @@ describe('SubagentService', () => {
     expect('lastAssistantMessage' in endInfo).toBe(false) // but no output exists
   })
 
+  it('contains a structuredClone failure: emits subagent/end without lastAssistantMessage (no unhandled rejection)', async () => {
+    // The clone runs inside onFulfilled, OUTSIDE emitLifecycle's per-listener
+    // containment. An uncloneable output (here a content block carrying a
+    // function) would otherwise throw and become an unhandled rejection on the
+    // detached `.then`. The handler must instead log and emit the event WITHOUT
+    // lastAssistantMessage, still carrying the real stopReason/agentType.
+    const ctx = new Context()
+    await ctx.plugin(SubagentService)
+    const warn = vi.fn(); ctx.logger.warn = warn as never
+    // An output value structuredClone cannot handle (a function is uncloneable).
+    const uncloneable = [{ type: 'text', text: 'x', evil: () => 0 }] as unknown as SubagentResult['output']
+    ctx.subagents.registerProvider({
+      name: 'unclone',
+      capabilities: NO_CAPS,
+      start: () => ({
+        id: AgentId('unclone-child'),
+        result: Promise.resolve({ output: uncloneable, stopReason: 'completed' } as SubagentResult),
+        cancel() {},
+        dispose: async () => {},
+      }),
+    })
+
+    const ended = vi.fn()
+    ctx.on('subagent/end', ended)
+    const run = ctx.subagents.start('unclone', baseRequest({ agentType: 'researcher' }))
+    await run.result
+    await Promise.resolve()
+
+    const endInfo = ended.mock.calls[0]![0] as Record<string, unknown>
+    expect(endInfo.stopReason).toBe('completed')        // the real outcome is preserved
+    expect(endInfo.agentType).toBe('researcher')
+    expect('lastAssistantMessage' in endInfo).toBe(false) // clone failed → omitted, not crashed
+    expect(warn).toHaveBeenCalledWith(expect.stringContaining('could not clone'))
+  })
+
   it('emits subagent/end with stopReason "error" when the run result promise rejects', async () => {
     const ctx = new Context()
     await ctx.plugin(SubagentService)