[BUG] Vertex/Bedrock subagents silently downgraded to older models (not fully fixed)

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Re previous bug report (now closed): https://github.com/anthropics/claude-code/issues/30815

I see this is listed as fixed in the relnotes for 2.1.73 but it's stills screwy.

For /model I get:
Default Sonnet 4.5
Sonnet 4.6
Sonnet 4.6 1M
Opus 4.1
Opus 4.6
Opus 4.6 1M
Haiku.

Next test spawning an opus and sonnet subagent with different main model selection:
Sonnet 4.5: Opus 4.6, Sonnet 4.5@20250929
Sonnet 4.6: Opus 4.6, Sonnet 4.6.
Sonnet 4.6 1M: Opus 4.6, Sonnet 4.6 1M.
Opus 4.1: Opus 4.6, Sonnet 4.5@20250929
Opus 4.6: Opus 4.6, Sonnet 4.5@20250929
Opus 4.6 1M: Opus 4.6 1M, Sonnet 4.5@20250929
Haiku: Opus 4.6, Sonnet Sonnet 4.5@20250929

So Opus is ok in that you get 4.6 or 4.6-1M if that's the parent, but Sonnet is still screwy - you get 4.6/4.6-1M if that's the parent, but otherwise still getting 4.5.

What Should Happen?

Should use same model as parent.

Error Messages/Logs

Steps to Reproduce

Via Vertex AI or other 3rd party provider:

Set /model, ask to spawn a sonnet sub-agent and report back its model, e.g. opus 4.6 spawns sonnet 4.5 not 4.6.

Claude Model

Opus

Is this a regression?

No, this never worked

Last Working Version

_No response_

Claude Code Version

2.1.76

Platform

Google Vertex AI

Operating System

Ubuntu/Debian Linux

Terminal/Shell

VS Code integrated terminal

Additional Information

_No response_