Background agent result causes “200 Failed to process successful response”

Summary

I was running a session with two background agents. One finished normally, but the second one ran for a very long time and eventually stopped with an error.

When I continued the main session, it tried to retrieve the result of that agent with AGENT(output). The result actually appeared in the UI (... +12 lines), but immediately after that Command Code returned:

Error: 200 Failed to process successful response

Typing continue just brings me back to the same problem, so the session is effectively stuck.

Trace ID: bb9fe6a5d866dd91c2d545423130da57

The main model was GLM-5.3-Flash through Fireworks and was working normally before this happened. I’m attaching the session JSONL as well.

Expected Behavior

If a background agent finishes or fails, the main agent should be able to retrieve whatever result/status is available and continue working.

A failed background agent should not make the parent session unusable.

Actual Behavior

The background agent result seems to be available because AGENT(output) displays it in the UI, but Command Code then fails with:

200 Failed to process successful response

The current turn stops and continuing the session causes the same issue again.

Steps to reproduce the issue

1. Start a session and let the main agent launch a background agent.
2. In my case the background agent ran for a long time (~139 minutes) and eventually errored.
3. Continue the parent session and ask it to retrieve the background agent result.
4. Command Code calls AGENT(output).
5. The agent output appears to be returned, but the parent turn immediately fails with 200 Failed to process successful response.
6. Type continue and the session gets stuck on the same point again.

Command Code Version

1.39.3

Operating System

macOS

Terminal/IDE

Terminal

Shell

zsh

Session file (optional)

3075aad8-c5ec-4898-a3ec-7a736d98ad02.jsonl.zip
For context, the problematic background agent had run for ~139 minutes, used ~1.88M tokens over 23 turns, and then errored.

Fix prompt (optional)

No response

Additional context

OS: macOS Tahoe 26.6.2

首先检查附加的 session JSONL 中 AGENT(output) 调用及后续 Command Code error 附近的内容。如果可能，复现长时间运行的后台代理失败，然后跟踪返回的结果和失败状态是如何被处理的。当父 session 可以获取结果或状态，并且无需重复 200 错误即可继续时，即表示完成。