feat(plan): enforce read-only constraints in Plan Mode

[mattKorwel]

This PR enforces strict read-only constraints when the agent is in Plan Mode.

Key changes:

• Updates the Plan Mode policy to deny and on source code.
• Restricts the to read-only tools (+ plan-specific writes) during Plan Mode.
• Updates system prompts to clearly communicate these constraints to the model.
• Adds explicit warnings in tool schemas when in Plan Mode.

[gemini-code-assist]

Summary of Changes

Hello @mattKorwel, 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 significantly strengthens the read-only constraints within Plan Mode. It ensures that the agent cannot inadvertently modify source code by explicitly denying write operations, guiding the model through updated prompts, and dynamically filtering available tools. This change aims to improve the safety and predictability of the agent's behavior when operating in a planning context, preventing unintended direct code modifications.

Highlights

• Policy Enforcement: Explicitly denied write_file and replace operations on source code within Plan Mode through a new policy rule, ensuring that the agent cannot inadvertently modify production code.
• Prompt Updates: Enhanced system prompts to guide the model on handling direct source code modification requests in Plan Mode, emphasizing the necessity of creating a detailed plan first.
• Tool Filtering: Implemented logic to restrict available tools in Plan Mode to a predefined set of approved read-only tools, while still allowing specific write operations exclusively for plan files.
• Tool Schema Warnings: Modified the descriptions of write_file and replace tools to include explicit warnings about their limited use (only for plans) when the agent is operating in Plan Mode.

Changelog

• packages/core/src/policy/policies/plan.toml
  • Added a new policy rule to explicitly deny write_file and replace tools for source code modifications in Plan Mode, including a custom deny message.
• packages/core/src/prompts/snippets.ts
  • Updated the "Read-Only" rule in the system prompt to instruct the model to explain Plan Mode restrictions and require a detailed plan for source code changes before any modifications.
• packages/core/src/tools/tool-registry.ts
  • Imported ApprovalMode and specific tool names (PLAN_MODE_TOOLS, WRITE_FILE_TOOL_NAME, EDIT_TOOL_NAME).
  • Introduced logic within getActiveTools to filter tools, allowing only designated read-only tools and specific write tools for plans when in ApprovalMode.PLAN.
  • Modified getFunctionDeclarations to dynamically update the descriptions of write_file and replace tools, adding a clear warning about their restricted use to only writing plans in Plan Mode.

Ignored Files

• Ignored by pattern: .gemini/** (1)
  • .gemini/commands/strict-development-rules.md

Activity

• No human activity (comments, reviews, etc.) has been recorded for this pull request yet.

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[github-actions]

Size Change: +1.43 kB (+0.01%)

Total Size: 25.2 MB

Filename	Size	Change
./bundle/gemini.js	25.1 MB	+1.43 kB (+0.01%)

ℹ️ View Unchanged

Filename	Size
./bundle/sandbox-macos-permissive-open.sb	890 B
./bundle/sandbox-macos-permissive-proxied.sb	1.31 kB
./bundle/sandbox-macos-restrictive-open.sb	3.36 kB
./bundle/sandbox-macos-restrictive-proxied.sb	3.56 kB
./bundle/sandbox-macos-strict-open.sb	4.82 kB
./bundle/sandbox-macos-strict-proxied.sb	5.02 kB

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[gemini-code-assist]

Code Review

This pull request aims to enforce read-only constraints in Plan Mode. The changes to the system prompts and tool registry correctly implement the desired restrictions at the prompt and tool availability levels. However, there is a critical issue in the plan.toml policy file where an incorrect tool name (replace instead of edit) is used. This will cause the policy rules for editing files in Plan Mode to be ineffective, undermining the goal of this PR. The suggested fix also aligns with the repository's rule for consolidating policy rules for multiple tools into a single rule using an array for toolName.

[mattKorwel]

I have updated the PR to address the intent of the review. Regarding the tool name suggestion: the canonical tool name in this codebase is 'replace' (as defined in coreTools.ts). I have verified that 'replace' is the correct name to use in policy files to match the model's tool calls. I've restored the explicit deny rules using the correct 'replace' name and re-applied the other improvements to tool schemas and prompts.