[BUG] Plan acceptance does not auto-rename session when using Vertex AI provider

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?

When accepting a plan in plan mode, the session should auto-rename based on the plan content. This works correctly on a personal Claude subscription account but does NOT work when using Claude Code via Vertex AI (API key mode).

Environment:

• Claude Code version: 2.1.96
• Provider: Vertex AI (Google Cloud)
• OS: macOS (Darwin 25.4.0)
• Same version on personal subscription account works correctly

What Should Happen?

Session auto-renames based on plan content (as it does on subscription-based accounts)

Error Messages/Logs

Steps to Reproduce

1. Use Claude Code v2.1.96 connected via Vertex AI
2. Enter plan mode and create a plan
3. Accept the plan
4. Observe: session name does not update — stays as the default

Claude Model

None

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.96

Platform

Google Vertex AI

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_