[Plan Mode] Claude Sonnet 4.6 executes tool calls instead of outputting text-only plan

[nicolasbustosv]

• Copilot Chat Extension Version: 0.37.9
• VS Code Version: 1.109.5
• OS Version: Ubuntu 24.04 LTS (x64), kernel 6.14.0-35-generic
• Feature: Plan mode
• Selected model: Claude Sonnet 4.6
• Logs: (attach Help > Toggle Developer Tools > Console output)

Steps to Reproduce:

1. Open the Copilot Chat panel and select Plan mode from the mode dropdown in the chat input bar.
2. Ensure the selected model is Claude Sonnet 4.6.
3. Ask a question that would normally require file edits or tool use (e.g., "How should I restructure this module?").
4. Observe that instead of outputting a text-only plan and stopping, the model proceeds to execute tool calls — writing files and/or running terminal commands — without pausing for user confirmation.

Expected behavior: In Plan mode, the model should only output a text plan describing proposed changes. No tools should execute, no files should be written, and no terminal commands should run.

Actual behavior: Claude Sonnet 4.6 executes tool calls (file edits, terminal commands) while in Plan mode, bypassing the read-only constraint that Plan mode is supposed to enforce.

[varunsharma11-coder]

I am facing the same issue, plan mode updates all the files. Claude integration is not really good.