[Bug]: Resume retries Claude after selecting Codex, while Send hands off successfully #15555

Before submitting

• I searched existing issues and did not find a duplicate of this Resume behavior.
• I included a real affected thread, reproduction steps, and source references.

Area

apps/web, also used by the desktop client.

Steps to reproduce

1. Start a Claude conversation and let a turn fail with a usage-limit error.
2. In the same thread, select Codex in the composer model picker.
3. Click Resume without sending a new message.
4. Observe which provider runs the generated Continue where you left off. message.
5. For comparison, keep Codex selected and send an ordinary message in the same thread.

Expected behavior

Resume should use the provider/model currently shown in the composer and carry the prior conversation through the existing handoff path, as Send does. Please confirm the intended behavior if Resume is deliberately tied to the previous provider.

Actual behavior

Resume starts another Claude turn and immediately hits Claude's limit again. Sending an ordinary message starts Codex and successfully hands off the same conversation.

Real example

This occurred on October 4, 2026 while investigating a floating-preview drag problem. The reporter confirmed that they selected Codex and then clicked Resume. The following records were read from that thread's persisted timeline and run metadata. Times are America/Los_Angeles, UTC-07:00.

The successful run recorded a provider context handoff covering app runs 1-3. Codex's first response explicitly picked up the preview-drag investigation from that handoff. This comparison happened in the running app; the local source patch had not been installed.

Impact

Minor bug or occasional failure. Resume cannot move this usage-limited work to the selected provider, but sending a message works.

Version or commit

• Installed desktop app at investigation: 0.0.46-nightly.20261004.2644.
• Local source inspected: e19a48d220a18ef198178b48d12aa1a034596489.
• The same omission is present on upstream main at d60a71ef6e371d8b345ecb1f7575117d675187c7.
• The running app's exact source SHA was not independently verified.

Environment

macOS 27.0.1, Apple silicon, T3 Code Nightly desktop. Claude to Codex within one existing thread.

Logs or stack traces

Claude usage limit reached. This turn is paused until the 5-hour limit resets in 3h 41m.
You've hit your session limit · resets 4am (America/Los_Angeles)
Claude API rate limit reached. Try again later.

Source evidence and verification limits

• Resume calls startThreadTurn without modelSelection.
• The client command forwards modelSelection only when supplied.
• The server falls back to the thread's saved selection.

Four local integration cases verify that an explicit Codex selection on a manual continuation of usage-limited Claude work preserves the original request and partial response, for new/existing Codex conversations and native/inline history delivery. The focused hands off tests passed, 20 tests total. These tests exercise the server path, not the browser click. No automated end-to-end UI reproduction has been performed.

This differs from #10967, where the picker blocked provider switching entirely. Here Codex is selectable and the same conversation successfully switches through Send.

Workaround

Select Codex, then send a normal message such as Continue where you left off. instead of clicking Resume. The real successful comparison above used testing.