Bug: Gemini CLI bridge crashes on large MCP tool responses (64KB StreamReader limit)

[onmete]

🐛 Bug Description

Summary: The Gemini CLI bridge crashes with Separator is not found and chunk exceed the limit errors when an MCP tool returns a large response (e.g., mcp_kubernetes_events_list).

Expected Behavior: Large MCP tool responses should be processed without crashing the Gemini CLI bridge.

Actual Behavior: asyncio.StreamReader.readline() raises asyncio.LimitOverrunError because a single NDJSON line exceeds the default 64 KB buffer limit.

🔄 Steps to Reproduce

1. Start a session using the Gemini CLI bridge
2. Trigger a tool call that returns a large response (e.g., mcp_kubernetes_events_list on a busy cluster)
3. The bridge crashes with Separator is not found / chunk exceed the limit

🌍 Environment

Component: Ambient Agentic Runner — Gemini CLI bridge

File: components/runners/ambient-runner/ambient_runner/bridges/gemini_cli/session.py

📋 Additional Context

Root Cause: In ambient_runner/bridges/gemini_cli/session.py, the asyncio.create_subprocess_exec() call uses Python's default StreamReader buffer limit of 64 KB. The Gemini CLI outputs NDJSON lines to stdout — when a single line exceeds 64 KB (common with large tool results), asyncio.StreamReader.readline() raises asyncio.LimitOverrunError.

Error Messages:

asyncio.LimitOverrunError: Separator is not found, and chunk exceed the limit

🔍 Possible Solution

Add limit=10 * 1024 * 1024 (10 MB) to the create_subprocess_exec() call in session.py (around line 149). One-line change:

self._process = await asyncio.create_subprocess_exec(
    *cmd,
    stdout=asyncio.subprocess.PIPE,
    stderr=asyncio.subprocess.PIPE,
    cwd=self._cwd,
    env=env,
    limit=10 * 1024 * 1024,  # Default 64KB is too small for large MCP tool responses
)

✅ Acceptance Criteria

• Bug is reproduced and root cause identified
• Fix is implemented and tested
• Regression tests added to prevent future occurrences
• Documentation updated if needed

🏷️ Labels

• Priority: high
• Complexity: trivial
• Component: backend

[Gkrumbach07]

AI triage: Created session triage-gh-1126-gemini-buffer-limit to investigate and fix this issue.