fix(core): clarify plan mode constraints and exit mechanism

[jerop]

Fixes #19437

• Added explicit Rule 5 to system prompt to refuse direct code modification in Plan Mode and proactively explain the restriction.
• Enhanced exit_plan_mode tool description to emphasize its role as the mandatory approval and exit mechanism, reducing "policy surprise" failures.
• Improved test signal in plan_mode.eval.ts with explicit user approval to stabilize model behavior during transitions.

[gemini-code-assist]

Summary of Changes

Hello @jerop, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request clarifies the operational boundaries and exit procedures for the system's 'Plan Mode'. The changes aim to prevent the model from attempting direct code modifications when it should be planning, and to ensure a clear, user-approved transition from planning to implementation. This improves the model's adherence to its operational policies and enhances the user experience by making the workflow more predictable.

Highlights

• Plan Mode Constraints: An explicit Rule 5 has been added to the system prompt, instructing the model to refuse direct code modification while in Plan Mode and to proactively explain this restriction to the user.
• Exit Mechanism Clarity: The description of the exit_plan_mode tool has been enhanced to clearly state its mandatory role as the approval and exit mechanism for Plan Mode, aiming to reduce 'policy surprise' failures.
• Improved Test Signal: The test signal in plan_mode.eval.ts has been improved by adding explicit user approval to the prompt, which helps stabilize model behavior during transitions from planning to implementation.

Changelog

• evals/plan_mode.eval.ts
  • Updated the test prompt to explicitly include user approval for plan implementation.
• packages/core/src/prompts/snippets.ts
  • Added Rule 5 to the system prompt, instructing the model to refuse direct code modification in Plan Mode and use the exit_plan_mode tool.
• packages/core/src/tools/definitions/dynamic-declaration-helpers.ts
  • Enhanced the description of the exit_plan_mode tool to emphasize its mandatory use for exiting Plan Mode and enabling source code edits.

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[github-actions]

Size Change: +467 B (0%)

Total Size: 24.5 MB

ℹ️ View Unchanged

Filename	Size	Change
./bundle/gemini.js	24.5 MB	+467 B (0%)
./bundle/sandbox-macos-permissive-open.sb	890 B	0 B
./bundle/sandbox-macos-permissive-proxied.sb	1.31 kB	0 B
./bundle/sandbox-macos-restrictive-open.sb	3.36 kB	0 B
./bundle/sandbox-macos-restrictive-proxied.sb	3.56 kB	0 B
./bundle/sandbox-macos-strict-open.sb	4.82 kB	0 B
./bundle/sandbox-macos-strict-proxied.sb	5.02 kB	0 B

_{compressed-size-action}

[gemini-code-assist]

Code Review

This pull request effectively clarifies the constraints and exit mechanism for the experimental 'Plan Mode'. The changes include updating the system prompt with a new rule to handle requests for direct code modification, enhancing the description of the exit_plan_mode tool to be more explicit about its purpose, and strengthening a test case to improve its reliability. These modifications are well-aligned with the goal of making the model's behavior more predictable and robust when operating in Plan Mode. The changes are clear, targeted, and represent a solid improvement.