fix(cli): prevent informational logs from polluting json output by cocosheng-g · Pull Request #26264 · google-gemini/gemini-cli

cocosheng-g · 2026-04-30T15:42:25Z

Summary

This PR prevents informational logs (e.g. MCP discovery messages, debug logs) from polluting the stdout stream when the CLI is running in non-interactive mode with JSON output.

Details

The issue occurred because patchStdio() starts buffering output very early, but the full ConsolePatcher (which redirects console.log to stderr) is initialized later. Any logs during this window were captured as stdout and then flushed to the real stdout during the pre-session drain.

Key changes:

Early Redirection: Immediately after starting the capture buffer, console.log and console.info are shallow-patched to point to console.error if in headless mode.
Smart Draining: coreEvents.drainBacklogs now supports an optional transformer. initializeOutputListenersAndFlush uses this to force any buffered stdout items to stderr if JSON output is active.
Robustness: Refactored gemini.tsx to manage the config variable more reliably during the initialization and cleanup phases.

Related Issues

Fixes #19198

How to Validate

Add a console.log('STRAY') at the beginning of main() in gemini.tsx.
Run npm run bundle.
Run node bundle/gemini.js --prompt 'hi' --output-format json > out.json.
Observe that out.json is valid JSON and does not contain 'STRAY', while 'STRAY' appears in stderr.

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

gemini-code-assist · 2026-04-30T15:42:34Z

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 an issue where informational logs were polluting stdout when the CLI was running in non-interactive mode with JSON output enabled. By introducing early console redirection and a flexible event transformation mechanism during the backlog flushing process, the changes ensure that diagnostic messages are correctly routed to stderr, preserving the integrity of the JSON output stream.

Highlights

Early Console Redirection: Implemented shallow patching of console.log and console.info to redirect to stderr during early CLI initialization in headless mode.
Enhanced Event Draining: Updated coreEvents.drainBacklogs to support an optional transformation function, allowing for dynamic modification of events during the flush process.
JSON Output Protection: Configured the output flushing mechanism to automatically redirect buffered stdout events to stderr when the CLI is configured for JSON output.

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github-actions · 2026-04-30T15:48:04Z

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gemini-code-assist

Code Review

This pull request introduces console output redirection to stderr in headless mode and enhances the event backlog flushing mechanism to support event transformations. The initializeOutputListenersAndFlush function was updated to force output to stderr based on the CLI configuration, specifically for JSON output. Feedback suggests defaulting this behavior to true when the configuration is not yet initialized to prevent stdout pollution during early initialization failures.

cocosheng-g · 2026-04-30T15:55:10Z

/gemini review

gemini-code-assist

Code Review

This pull request implements output redirection to stderr for headless mode and JSON output formats to prevent stdout pollution. Key changes include patching early console output, updating initializeOutputListenersAndFlush to handle configuration-based redirection, and enhancing the core event emitter's drainBacklogs method to support event transformation. A review comment identifies a logic issue where listener registration for console logs and user feedback is incorrectly dependent on the absence of an output listener, which could lead to lost information.

cocosheng-g · 2026-04-30T15:59:33Z

/gemini review

gemini-code-assist

Code Review

This pull request introduces a mechanism to redirect buffered output to stderr when the CLI is in headless mode or using JSON output format. This is achieved by enhancing the core event emitter's drainBacklogs method to support event transformation and updating the CLI initialization to utilize this feature. Feedback points out a critical Temporal Dead Zone (TDZ) risk where the config variable is captured by a cleanup closure before its declaration, potentially causing a ReferenceError. Additionally, manual patching of console methods was flagged as redundant and potentially disruptive to the existing ConsolePatcher logic.

cocosheng-g · 2026-04-30T16:16:55Z

/gemini review

gemini-code-assist

Code Review

This pull request introduces a mechanism to redirect output to stderr when the CLI is configured for JSON output or during early initialization failures. This is achieved by enhancing the drainBacklogs method in CoreEventEmitter to support an optional transformation function. The initializeOutputListenersAndFlush function in the CLI package now utilizes this transformation to force output chunks to stderr when necessary. New unit tests have been added to verify the redirection logic and the event transformation capabilities. I have no feedback to provide.

…le-gemini#26264)

fix(cli): prevent informational logs from polluting json output

924ea4c

cocosheng-g requested a review from a team as a code owner April 30, 2026 15:42

gemini-code-assist Bot reviewed Apr 30, 2026

View reviewed changes

Comment thread packages/cli/src/gemini.tsx Outdated

test: add unit tests for stdout pollution fix

8f12ac7

gemini-code-assist Bot reviewed Apr 30, 2026

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Comment thread packages/cli/src/gemini.tsx

fix(cli): ensure independent event listener attachment and add tests

5b9b813

cocosheng-g force-pushed the fix-stdout-pollution-19198 branch from 65648ff to 5b9b813 Compare April 30, 2026 16:00

gemini-cli Bot added the area/non-interactive Issues related to GitHub Actions, SDK, 3P Integrations, Shell Scripting, Command line automation label Apr 30, 2026

gemini-code-assist Bot reviewed Apr 30, 2026

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Comment thread packages/cli/src/gemini.tsx Outdated

fix(cli): address TDZ and redundant console patching in gemini.tsx

310660d

gemini-code-assist Bot reviewed Apr 30, 2026

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Adib234 approved these changes Apr 30, 2026

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Adib234 added this pull request to the merge queue Apr 30, 2026

Merged via the queue into main with commit 8c1e255 Apr 30, 2026
46 of 47 checks passed

Adib234 deleted the fix-stdout-pollution-19198 branch April 30, 2026 18:47

TirthNaik-99 pushed a commit to TirthNaik-99/gemini-cli that referenced this pull request May 4, 2026

fix(cli): prevent informational logs from polluting json output (goog…

ac4de7a

…le-gemini#26264)

kimjune01 pushed a commit to kimjune01/gemini-cli-claude that referenced this pull request May 6, 2026

fix(cli): prevent informational logs from polluting json output (goog…

7b2e185

…le-gemini#26264)

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cocosheng-g commented Apr 30, 2026

Summary

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Related Issues

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