Bug fix: Gemini CLI bridge crashes on large MCP tool responses - raise gemini cli stream buffer limit to 10 MB

[vishsanghishetty]

Summary

Raises the asyncio.StreamReader buffer limit from the default 64 KB to 10 MB when spawning the Gemini CLI subprocess. Prevents asyncio.LimitOverrunError crashes when an MCP tool (e.g. mcp_kubernetes_events_list) returns a large NDJSON line.

Closes #1126

What changed

One-line fix in session.py — added limit=10 * 1024 * 1024 to the asyncio.create_subprocess_exec call.

How I tested

Unit test — test_stream_buffer_limit_raised mocks create_subprocess_exec and asserts the limit kwarg is 10 * 1024 * 1024.

Integration test — test_large_stdout_line_does_not_crash spawns a real subprocess that outputs 100 KB on a single line (well above the old 64 KB default) and reads it with the 10 MB limit. This would crash with ValueError: Separator is found, but chunk is longer than limit under the old default. Passes with our fix.

tests/test_gemini_session.py — 30 passed

Full E2E reproduction (triggering an actual Gemini CLI session with a large MCP tool response) requires a running ACP instance with Gemini API keys, so that's not included here. The integration test proves the mechanism at the asyncio level.

Summary by CodeRabbit

• Bug Fixes

  • Improved handling of large output streams to prevent buffer overflow errors.

• Tests

  • Added test coverage for subprocess buffer configuration and large payload handling.

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ℹ️ Review info

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Run ID: e2e4872d-5b08-415b-a4da-7cb4586542fe

📥 Commits

Reviewing files that changed from the base of the PR and between 49ca627 and 191c091.

📒 Files selected for processing (2)

• components/runners/ambient-runner/ambient_runner/bridges/gemini_cli/session.py
• components/runners/ambient-runner/tests/test_gemini_session.py

📝 Walkthrough

Walkthrough

Sets the subprocess StreamReader buffer limit to 10 MB by passing limit=10 * 1024 * 1024 to asyncio.create_subprocess_exec() in the Gemini CLI bridge to avoid asyncio.LimitOverrunError on large NDJSON stdout lines.

Changes

Cohort / File(s)	Summary
Subprocess Buffer Limit Configuration components/runners/ambient-runner/ambient_runner/bridges/gemini_cli/session.py	Added limit=10 * 1024 * 1024 argument to asyncio.create_subprocess_exec() in GeminiSessionWorker.query() to raise the StreamReader buffer limit from the 64KB default to 10MB.
Stream Buffer Limit Verification Tests components/runners/ambient-runner/tests/test_gemini_session.py	Added TestWorkerSubprocessConfig with test_stream_buffer_limit_raised() that patches asyncio.create_subprocess_exec to assert the limit kwarg is set to 10 * 1024 * 1024, and test_large_stdout_line_does_not_crash() which runs a real subprocess emitting ~100KB to verify reading succeeds and process exits normally.

🚥 Pre-merge checks | ✅ 7 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Title check	⚠️ Warning	Title does not follow Conventional Commits format (missing type prefix and parenthetical scope).	Reformat title as: 'fix(gemini-cli): raise stream buffer limit to 10 MB for large MCP tool responses'

✅ Passed checks (7 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Linked Issues check	✅ Passed	PR fully implements all coding requirements from issue #1126: increased buffer limit, regression tests for the fix.
Out of Scope Changes check	✅ Passed	All changes directly address issue #1126—buffer limit increase and related test coverage, no extraneous modifications.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Performance And Algorithmic Complexity	✅ Passed	Bounded 10MB subprocess buffer limit prevents crashes on large payloads with no algorithmic complexity or unbounded growth introduced.
Security And Secret Handling	✅ Passed	Pull request adds single-line subprocess buffer limit parameter with no secrets, proper environment variable handling, safe list-based command construction, and maintained security posture.
Kubernetes Resource Safety	✅ Passed	This check is not applicable to the provided pull request. The PR exclusively modifies Python source code to increase asyncio subprocess buffer limits and add unit tests, containing no Kubernetes manifests or deployment-related files.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

✨ Simplify code

• Create PR with simplified code

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[vishsanghishetty]

@ambient-code

[vishsanghishetty]

@ambient-code