Fix #28227: add support for AGENTS.md out of the box

[vichu3d-dev]

Fixes #28227

Description

This PR fixes an issue where the AGENTS.md context file was ignored by default unless explicitly listed in the user's ~/.gemini/settings.json.

Changes made:

• Updated the core memoryTool to use the array ['GEMINI.md', 'AGENTS.md'] as the default context filenames, ensuring GEMINI.md retains its priority.
• Simplified resetGeminiMdFilename() usage in the CLI initialization to cleanly fallback to this new core default.
• Updated the testing suite expectations to match the new behavior.

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[github-actions]

📊 PR Size: size/M

• Lines changed: 77
• Additions: +40
• Deletions: -37
• Files changed: 6

[github-actions]

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[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 enhances the system's context awareness by adding 'AGENTS.md' as a default configuration file alongside 'GEMINI.md'. By updating the core memory tool and associated initialization logic, the system now automatically prioritizes these files without requiring manual user configuration in settings.json, while maintaining backward compatibility and improving test coverage.

Highlights

• Support for AGENTS.md: Updated the core memory tool to include 'AGENTS.md' alongside 'GEMINI.md' as a default context filename, ensuring it is recognized out of the box.
• Refactored Configuration: Simplified the CLI initialization and reset logic for context filenames by defaulting to the new core array, reducing the need for explicit configuration.
• Updated Documentation and Tests: Updated system prompt documentation and adjusted test suites to reflect the new default context file behavior.

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[gemini-code-assist]

Code Review

This pull request adds AGENTS.md as a default context file alongside GEMINI.md by introducing DEFAULT_CONTEXT_FILENAMES and updating the default configuration, system prompts, and associated tests. The reviewer noted an inconsistency in resetGeminiMdFilename where the fallback logic for an empty array still references the single DEFAULT_CONTEXT_FILENAME instead of the new DEFAULT_CONTEXT_FILENAMES array.

[gemini-code-assist]

In resetGeminiMdFilename, when the filtered cleaned array is empty (for example, if an empty array [] or only invalid/non-existent paths are passed), the function falls back to DEFAULT_CONTEXT_FILENAME (which is 'GEMINI.md') on line 69:

if (cleaned.length === 0) {
  currentGeminiMdFilename = DEFAULT_CONTEXT_FILENAME;
}

Since the new default context filenames are DEFAULT_CONTEXT_FILENAMES (['GEMINI.md', 'AGENTS.md']), this fallback should be updated to DEFAULT_CONTEXT_FILENAMES to ensure consistency and that AGENTS.md is not ignored in fallback scenarios.

[vichu3d-dev]

@google-gemini/gemini-cli-maintainers @google-gemini/gemini-cli-prompt-approvers

Hi team!

This PR fixes the issue where AGENTS.md was ignored by default. I've updated the core fallback logic, adjusted the snapshot tests, and addressed the feedback from the gemini-code-assist[bot] to ensure all fallback scenarios correctly map to the new DEFAULT_CONTEXT_FILENAMES array.

All CI checks are currently passing. Whenever you have a moment, I'd appreciate a review so we can get this merged in! Thank you!

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