Side chat doesn't restore the parent thread's model_provider, so it fails with invalid_encrypted_content

What version of the Codex App are you using (From “About Codex” dialog)?

26.911.61220

What subscription do you have?

Pro

What platform is your computer?

Darwin 25.6.0 arm64 arm

What issue are you seeing?

Opening a side chat on a thread created on the ChatGPT plan fails on the first message once the default provider is Azure:

{
  "error": {
    "message": "The encrypted content for item rs_...79a could not be verified. Reason: Encrypted content could not be decrypted or parsed.",
    "type": "invalid_request_error",
    "param": null,
    "code": "invalid_encrypted_content"
  }
}

The side chat's first request goes to the Azure endpoint (POST https://<resource>.openai.azure.com/openai/responses?api-version=2025-04-01-preview → 400 Bad Request). That request carries reasoning item rs_...79a, which the parent thread received from the ChatGPT backend.

What steps can reproduce the bug?

1. With model_provider = "openai" (ChatGPT sign-in), start a thread in Codex Desktop and run a few turns with a reasoning model so the history contains encrypted reasoning items.
2. Add a custom Azure provider (wire_api = "responses"), set model_provider = "azure", and restart the app.
3. Reopen the thread from step 1. It resumes on openai, as expected.
4. Open a side chat on that thread and send any message.

Result: the first turn fails with invalid_encrypted_content, and the request goes to the Azure endpoint.

What is the expected behavior?

A side chat should run on its parent thread's provider unless the client explicitly asks for a different one. This is the rule thread/resume followed since #19287. If the client does explicitly switch providers, encrypted reasoning and compaction items from the other provider should be dropped, not sent to an endpoint that can't decrypt them.

Additional information

No response

Start by tracing side-chat creation and the thread/resume provider-handling path, using the rule described in #19287 as a comparison. Done means a side chat inherits its parent provider by default, while an explicit provider switch does not send incompatible encrypted reasoning or compaction items to the new endpoint.