fix(cli): wrap terminal capability queries in hidden sequence

[srithreepo]

Summary

Wrap terminal capability queries in a hidden mode sequence and clear the line afterwards to prevent a stray "m" character from appearing on some terminals (e.g., Apple Terminal, some tmux versions) during startup.

Details

When the CLI starts, it queries the terminal for various capabilities (Kitty protocol, background color, etc.). Some terminals might echo part of the response or malform it such that a stray "m" remains on the command line. By wrapping these queries in \x1b[8m (hidden mode) and following up with \x1b[2K (clear line and return) and \x1b[0m (reset attributes), we ensure any echoed or malformed output is not visible to the user.

Related Issues

Fixes #19081

None.

How to Validate

1. Run the project in a terminal that was previously showing the stray "m".
2. Observe that the "m" no longer appears during startup.
3. Run unit tests: npm test -w @google/gemini-cli -- src/ui/utils/terminalCapabilityManager.test.ts.

Pre-Merge Checklist

• Updated relevant documentation and README (if needed) - Not needed
• Added/updated tests (if needed)
• Noted breaking changes (if any) - None
• Validated on required platforms/methods:
  • MacOS
    • npm run
    • npx
    • Docker
    • Podman
    • Seatbelt
  • Windows
  • Linux

[gemini-cli]

Hi there! Thank you for your contribution to Gemini CLI.

To improve our contribution process and better track changes, we now require all pull requests to be associated with an existing issue, as announced in our recent discussion and as detailed in our CONTRIBUTING.md.

This pull request is being closed because it is not currently linked to an issue. Once you have updated the description of this PR to link an issue (e.g., by adding Fixes #123 or Related to #123), it will be automatically reopened.

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[gemini-cli]

Hi @srithreepo, thank you so much for your contribution to Gemini CLI! We really appreciate the time and effort you've put into this.

We're making some updates to our contribution process to improve how we track and review changes. Please take a moment to review our recent discussion post: Improving Our Contribution Process & Introducing New Guidelines.

Key Update: Starting January 26, 2026, the Gemini CLI project will require all pull requests to be associated with an existing issue. Any pull requests not linked to an issue by that date will be automatically closed.

Thank you for your understanding and for being a part of our community!

[gemini-code-assist]

Summary of Changes

Hello @srithreepo, 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 user experience of the CLI by addressing a visual artifact that could appear during startup on specific terminal emulators. By strategically applying ANSI escape codes, the CLI now gracefully handles terminal capability queries, ensuring a clean and uninterrupted display for users.

Highlights

• Terminal Output Glitch Fix: Implemented a fix to prevent a stray "m" character from appearing on some terminals (e.g., Apple Terminal, certain tmux versions) during CLI startup.
• Hidden Mode for Terminal Queries: Wrapped terminal capability queries in ANSI escape sequences (\x1b[8m for hidden mode, \x1b[2K\r to clear line and return, \x1b[0m to reset attributes) to hide the output and prevent malformed responses from being visible.
• Test Coverage: Added a new unit test to verify that terminal capability queries are correctly wrapped in the hidden/clear sequences.

Changelog

• packages/cli/src/ui/utils/terminalCapabilityManager.test.ts
  • Imported the node:fs module.
  • Added a new test case to assert that fs.writeSync is called with the correct hidden and clear sequences.
• packages/cli/src/ui/utils/terminalCapabilityManager.ts
  • Modified the fs.writeSync call to prepend \x1b[8m (hidden mode) and append \x1b[2K\r\x1b[0m (clear line, carriage return, reset attributes) to the terminal capability queries.
  • Added comments explaining the purpose of the new ANSI escape sequences.

Activity

• The author has completed pre-merge checklist items, including adding/updating tests and validating on MacOS.
• No breaking changes were noted.
• No documentation updates were deemed necessary.

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[gemini-code-assist]

Code Review

This pull request effectively resolves the issue of a stray character appearing in certain terminals during startup by wrapping capability queries in a hidden mode sequence. The implementation is solid and the addition of a test case is appreciated. I have one high-severity suggestion to improve the new test, making it more specific to ensure the atomicity of the write operation and prevent potential future regressions, which aligns with our guidelines for atomic fs.writeSync operations.

[gemini-cli]

Thank you for linking an issue! This pull request has been automatically reopened.

[github-actions]

Size Change: +629 B (0%)

Total Size: 24.4 MB

ℹ️ View Unchanged

Filename	Size	Change
./bundle/gemini.js	24.4 MB	+629 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

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