fix(core): improve diagnostics for malformed streaming responses

[junaiddshaukat]

Summary

Improves robustness and diagnostics in the streaming pipeline by making malformed or incomplete streamed responses easier to detect and debug.

Details

This change makes three focused improvements in the core streaming path:

• In packages/core/src/core/turn.ts, stream events with no response body are no longer skipped silently. They now emit a warning and continue processing later chunks.
• In packages/core/src/core/geminiChat.ts, invalid streaming response chunks are explicitly warned about before being skipped, making stream degradation easier to diagnose.
• In packages/core/src/code_assist/server.ts, the SSE parser now flushes the final buffered data: chunk even when the stream ends without a trailing blank line.

This keeps behavior unchanged for healthy streams while improving debuggability and correctness for malformed / incomplete streaming scenarios.

Related Issues

Fixes #22351

How to Validate

1. Run the focused test suites:
   npm test -w @google/gemini-cli-core -- src/core/turn.test.ts src/core/geminiChat.test.ts src/code_assist/server.test.ts

2. Confirm the tests pass, including coverage for:
   warning and skipping stream events without a response body
   warning when invalid stream chunks are skipped before a valid completion
   parsing a final SSE data chunk without a trailing blank line

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

Local Tests

[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 significantly enhances the diagnostic capabilities and robustness of the streaming pipeline. By introducing explicit warnings for malformed or incomplete streamed responses and refining the Server-Sent Events (SSE) parser's behavior, it makes it much easier to detect and debug issues in streaming interactions, ensuring more reliable operation without altering behavior for healthy streams.

Highlights

• Improved Stream Event Handling: Stream events in turn.ts that lack a response body now emit a warning instead of being silently skipped, enhancing debuggability.
• Explicit Warning for Invalid Chunks: The geminiChat.ts module now explicitly warns when invalid streaming response chunks are encountered and skipped, aiding in diagnosing stream degradation.
• Robust SSE Parsing: The SSE parser in server.ts was updated to correctly flush the final buffered data: chunk even if the stream ends without a trailing blank line, improving correctness for incomplete streaming scenarios.

Changelog

• packages/core/src/code_assist/server.test.ts
  • Added a test case to verify that the SSE parser correctly handles a final data chunk without a trailing blank line.
• packages/core/src/code_assist/server.ts
  • Modified the SSE parser to process any remaining buffered lines as a final chunk when the stream ends, even if no trailing blank line is present, and logs a warning for malformed JSON.
• packages/core/src/core/geminiChat.test.ts
  • Imported debugLogger.
  • Added a test case to confirm that a warning is issued when invalid stream chunks are skipped before a valid completion.
• packages/core/src/core/geminiChat.ts
  • Imported debugLogger.
  • Implemented a warning log when an invalid streaming response chunk is skipped.
• packages/core/src/core/turn.test.ts
  • Imported debugLogger.
  • Added a test case to ensure a warning is logged and events are skipped when a stream event lacks a response body.
• packages/core/src/core/turn.ts
  • Imported debugLogger.
  • Added a warning log when a stream event is received without a response body, preventing silent skipping.

Activity

• The author, junaiddshaukat, created this pull request to improve diagnostics and robustness for streaming responses.
• The pull request includes new and updated tests to cover the enhanced error handling and parsing logic.
• The author has validated the changes on MacOS using npm run.

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[gemini-code-assist]

Code Review

This pull request improves the robustness and diagnostics of the streaming pipeline by adding warnings for malformed or incomplete stream data and correctly handling streams that end without a final blank line. The changes are well-tested and improve debuggability. I have one suggestion regarding code duplication in the SSE parser to enhance maintainability and ensure error logging adheres to best practices.

[gemini-cli]

Hi there! Thank you for your interest in contributing to Gemini CLI.

To ensure we maintain high code quality and focus on our prioritized roadmap, we have updated our contribution policy (see Discussion #17383).

We only guarantee review and consideration of pull requests for issues that are explicitly labeled as 'help wanted'. All other community pull requests are subject to closure after 14 days if they do not align with our current focus areas. For this reason, we strongly recommend that contributors only submit pull requests against issues explicitly labeled as 'help-wanted'.

This pull request is being closed as it has been open for 14 days without a 'help wanted' designation. We encourage you to find and contribute to existing 'help wanted' issues in our backlog! Thank you for your understanding and for being part of our community!

[cocosheng-g]

@junaiddshaukat, apologies for the bot closing this PR! We have reopened it. Please sync your branch to the latest main and we will have someone review it shortly.

[junaiddshaukat]

@cocosheng-g i have resolved the merge confilcts.

[cocosheng-g]

/gemini review

[gemini-code-assist]

Code Review

This pull request improves the robustness of the Gemini CLI by adding debug logging when skipping invalid streaming response chunks or events without a response body. It refactors the SSE chunk processing logic in CodeAssistServer to ensure proper handling of final data chunks and adds corresponding unit tests to verify these improvements across the core components.

[cocosheng-g]

Please fix test

[junaiddshaukat]

@cocosheng-g I have fixed CI tests, pleaes have a look now