[Bug]: MCP-launched threads briefly flash in Inbox before moving to Working #36681

Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

Area

apps/web

Steps to reproduce

1. Enable the Working sidebar section.
2. Ask an agent to launch several threads through t3_thread_launch, each with an initial message.
3. Watch the sidebar while the launch calls execute.

Concurrent launches make the issue easier to see. Reproduced with a Codex parent launching eight free Replay threads; individual launches did not consistently expose it. create_threads with prompts has the same underlying launch gap.

Expected behavior

Threads launched with an initial message appear directly in Working.

Actual behavior

New threads briefly appear as idle Inbox rows, then disappear into Working. The sidebar movement is visually distracting. Observed idle intervals were about 50–150 ms per launch in the real-parent recording.

The server commits thread creation and the first message separately, so a client can receive a thread with no run even though work was requested in the same MCP call.

Related: #24477 covers a thread leaving Working between completed and continuation runs. This happens before the first run exists; #24760's end-of-run grace period does not cover it.

Impact

Minor bug or occasional failure

Version or commit

Originally reproduced on 4574c6013d. Also reproduced by the automated real-app scenario on upstream main 179c032b3f and 121f0cf014 on 2026-11-14.

Environment

Linux, web client. A Codex parent launches free Replay threads through the T3 Code MCP. Also reproduced through the CLI, which calls t3_thread_launch over MCP.

Screenshots, recordings, or supporting files

Full-app recording: Send, parent MCP launch calls, then the sidebar flashes around 9–11 seconds:

Consecutive sidebar frames, read left to right across each row:

Workaround

None needed to continue work; the threads start normally after the flash.

Reported on Matt's behalf by GPT-6.1-Sol in the Codex harness through T3 Code.