[BUG] Sonnet 5 / Opus 4.8 marked as Org default but rejected as legacy/outdated

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?

Model "Sonnet 5" (claude-sonnet-5) is shown in /model as the Org default and is selected with a checkmark. Despite this, every request using it fails with a 400 error saying the model is legacy/outdated. The same happens with Opus 4.8 (claude-opus-4-8). Both models are active per Anthropic's own documentation, not legacy.

What Should Happen?

Sonnet 5 and Opus 4.8 should work normally, since they are shown as available/default options in the /model picker and are documented as currently active models.

Error Messages/Logs

API Error: 400 Model "claude-sonnet-5" is legacy/outdated and no longer supported. Use /model to switch to a current model.

API Error: 400 Model "claude-opus-4-8" is legacy/outdated and no longer supported. Use /model to switch to a current model.

Steps to Reproduce

1. Open Claude Code (Desktop app) in a project folder
2. Start a new session or resume an existing one
3. Send any message
4. Error appears: "claude-sonnet-5" is legacy/outdated
5. Run /model — Sonnet 5 is shown with "Org default" label and a checkmark
6. Selecting Opus 4.8 instead produces the same error with "claude-opus-4-8"
7. Reproduced also via Google Vertex AI (claude --version in terminal) and via the Desktop app separately

Claude Model

Not sure / Multiple models

Is this a regression?

Yes, this worked in a previous version

Last Working Version

Unknown - never worked with these models on this account

Claude Code Version

2.1.201 (Claude Code)

Platform

Google Vertex AI

Operating System

macOS

Terminal/Shell

Other

Additional Information

Account is under monday.com organization. /feedback command is disabled because the account uses Google Vertex AI as the backend, so filing this directly on GitHub instead. Issue occurs both via Vertex AI (in terminal) and via the Desktop app.

Reproduced in macOS Terminal.app (zsh) and in the Claude Desktop app (Code tab) - neither is listed as an option above.