fix(core): preserve dollar sequences in prompt template substitutions

[nramanath]

Summary

This PR fixes a bug in system prompt template substitutions that corrupts content containing $ sequences (e.g. $$, $', $&) inside skill, sub-agent, or tool descriptions.

Details

applySubstitutions() in packages/core/src/prompts/utils.ts was performing template substitutions using the string form of String.prototype.replace() (e.g. result.replace(/\${AgentSkills}/g, skillsPrompt)). Because the replacement values are strings, JavaScript parses special sequences like $' (duplicates the tail of the prompt), $$ (collapses to $), and $& (inserts the match itself) inside the descriptions.

We fixed this by passing callback functions instead (e.g. () => skillsPrompt), which forces JavaScript to treat the replacement strings literally.

We also added regression unit tests in packages/core/src/prompts/utils.test.ts to ensure this behavior remains robust in the future.

Related Issues

Fixes #27993

How to Validate

1. Run the prompt utility unit tests:

   npm test -w @google/gemini-cli-core -- src/prompts/utils.test.ts

2. Run the workspace-wide validation checks:

   npm run build && npm run lint

Pre-Merge Checklist

• Updated relevant documentation and README (if needed)
• Added/updated tests (if needed)
• Noted breaking changes (if any)
• Validated on required platforms/methods:
  • MacOS
    • npm run
    • npx
    • Docker
    • Podman
    • Seatbelt
  • Windows
    • npm run
    • npx
    • Docker
  • Linux
    • npm run
    • npx
    • Docker

[gemini-code-assist]

Summary of Changes

Hello, 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 addresses a template substitution bug where content containing dollar-sign sequences was being incorrectly parsed by JavaScript's replace method. By switching to functional replacements, the system now treats injected descriptions as literal strings, ensuring that prompt templates remain intact regardless of their content.

Highlights

• Bug Fix in Prompt Substitutions: Updated applySubstitutions to use callback functions in String.prototype.replace() calls, preventing JavaScript from misinterpreting special dollar sequences (like $$, $', $&) within dynamic prompt content.
• Regression Testing: Added comprehensive unit tests to verify that dollar sequences are preserved verbatim when substituting ${AgentSkills}, ${SubAgents}, and ${AvailableTools}.

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[github-actions]

📊 PR Size: size/M

• Lines changed: 62
• Additions: +57
• Deletions: -5
• Files changed: 2

[github-actions]

🛑 Action Required: Evaluation Approval

Steering changes have been detected in this PR. To prevent regressions, a maintainer must approve the evaluation run before this PR can be merged.

Maintainers:

1. Go to the Workflow Run Summary.
2. Click the yellow 'Review deployments' button.
3. Select the 'eval-gate' environment and click 'Approve'.

Once approved, the evaluation results will be posted here automatically.

[gemini-code-assist]

Code Review

This pull request updates applySubstitutions in packages/core/src/prompts/utils.ts to use replacer functions instead of replacement strings in String.prototype.replace(), ensuring that special dollar sequences (like $$ and $&) in substituted content are preserved verbatim. Corresponding unit tests are added to verify this behavior. The review feedback points out an issue with the dynamic RegExp construction for tool-specific placeholders (e.g., ${toolName}_ToolName). Specifically, if a tool name contains special characters, the unescaped interpolation, the use of word boundaries (\b), and unescaped braces can cause matching failures or regex syntax errors. A suggestion is provided to escape the dynamically generated variable name and remove the word boundaries.

[nramanath]

@googlebot I signed it.