[BUG] Opus 4.6 hangs indefinitely when prompt is entered directly in Plan Mode (blue input box) in VS Code extension

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 using Opus 4.6 in the VS Code extension, entering a prompt directly in Plan Mode (the blue input box) causes the model to hang indefinitely on "Unravelling..." and never produce a response. I waited over 8 minutes with no output.

The same exact prompt works immediately when entered in the regular implementation mode (orange input box). In that case, Opus 4.6 self-selects into Plan Mode and begins producing a plan within seconds.

This only started happening with Opus 4.6. I did not experience this behavior with Opus 4.5 or Sonnet.

What Should Happen?

Opus 4.6 should process the prompt and return a plan within 1-2 minutes when the prompt is entered directly in Plan Mode, the same way it does when it self-selects into Plan Mode from the orange implementation input.

Error Messages/Logs

Steps to Reproduce

1. Open Claude Code in VS Code extension
2. Run /model and select Opus 4.6
3. Toggle to Plan Mode (blue input box at bottom of chat)
4. Enter a complex planning prompt (e.g., "Plan the implementation of a shapefile download ingestion approach for 5 NYC Open Data GIS datasets")
5. Observe that it shows "Unravelling..." indefinitely and never produces output
6. Stop the generation
7. Switch back to the regular orange implementation input box
8. Paste the same prompt
9. Observe that Opus 4.6 enters Plan Mode on its own and responds normally within seconds

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

Opus 4.6 VS Code Extention

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

VS Code integrated terminal

Additional Information

Workaround: Enter prompts in the regular (orange) implementation input instead of the blue Plan Mode input. Opus 4.6 will self-select into Plan Mode when appropriate and responds normally.

Environment: Windows 11, VS Code, Opus 4.6 selected via /model

Attaching two screenshots:

1. Hanging state: prompt entered in blue Plan Mode box, stuck on "Unravelling..."
2. Working state: same prompt entered in orange implementation box, Opus enters Plan Mode itself and responds immediately

<img width="1389" height="868" alt="Image" src="https://github.com/user-attachments/assets/8ac5a2ec-28d2-467a-9dc4-6e9d3d6f60b4" />

<img width="1389" height="868" alt="Image" src="https://github.com/user-attachments/assets/bf9a9066-ee6e-4de3-aca0-6c9ae707885c" />