diff --git a/packages/api/session-controller/src/agent.ts b/packages/api/session-controller/src/agent.ts
index 9e916d03bb..2836289e37 100644
--- a/packages/api/session-controller/src/agent.ts
+++ b/packages/api/session-controller/src/agent.ts
@@ -10,7 +10,7 @@ import type {} from '@deepseek-ai/dsh-agent-default-model'
 import type {} from '@deepseek-ai/dsh-agent-presets'
 import { ReasoningEffortId } from '@deepseek-ai/dsh-llm'
 import type { Session, SessionId } from '@deepseek-ai/dsh-session'
-import { SessionAlreadyOwnedError, type SessionInspection } from '@deepseek-ai/dsh-session-persistence'
+import type { SessionInspection } from '@deepseek-ai/dsh-session-persistence'
 import { SessionQueryError, type SessionObservation } from '@deepseek-ai/dsh-session-query'
 import { RemoteError } from '@deepseek-ai/dsh-typert-protocol'
 import type {} from '@deepseek-ai/dsh-typert-registry'
@@ -215,9 +215,12 @@ export class ApiSessionAgentController {
       if (racedSession !== undefined && hasApiSessionSubagentOwner(this.ctx, racedSession, undefined)) {
         return { error: apiSessionSubagentOwnershipError(sessionId) }
       }
-      if (error instanceof SessionAlreadyOwnedError) {
-        return {
-          error: new RemoteError('session/agent-busy', error.message, { reason: 'session-already-owned' }),
+      if (this.ctx.get('sessionPersistence') !== undefined) {
+        const { SessionAlreadyOwnedError } = await import('@deepseek-ai/dsh-session-persistence')
+        if (error instanceof SessionAlreadyOwnedError) {
+          return {
+            error: new RemoteError('session/agent-busy', error.message, { reason: 'session-already-owned' }),
+          }
         }
       }
       return {
diff --git a/packages/api/session-controller/tests/agent.host.spec.ts b/packages/api/session-controller/tests/agent.host.spec.ts
index a202d9281f..7861f324d7 100644
--- a/packages/api/session-controller/tests/agent.host.spec.ts
+++ b/packages/api/session-controller/tests/agent.host.spec.ts
@@ -255,6 +255,16 @@ describe('ApiSession Agent lookup and recovery', () => {
     })
   })
 
+  it('retains resume diagnostics without a persistence service', async () => {
+    const { ctx, agents } = await harness()
+    const meta = header('memory-only-resume')
+    ctx.sessions.create(meta.id, { meta })
+    vi.spyOn(ctx.agents, 'resume').mockRejectedValue(new Error('factory unavailable'))
+    await expect(agents.resolveAgent(meta.id)).resolves.toMatchObject({
+      error: { code: 'gateway/internal', message: expect.stringContaining('factory unavailable') as string },
+    })
+  })
+
   it('requires projected observations before activation', async () => {
     const { agents } = await harness()
     const meta = header('unprojected-observation')