fix: mcp-proxy process leak

[phact]

The /api/v2/mcp/servers?action_count=true endpoint in src/backend/base/langflow/api/v2/mcp.py calls update_tools() for each server to count tools which spanws mcp-proxy subprocesses but never cleans them up.

Noticed this when langflow was using 13gb of memory:

about 500 procs each w/ 10's of mb of RAM allocated after a couple of days of langflow being up.

user@f58f3aae4e79:/app$ count=0
for p in /proc/[0-9]*; do
  [ -r "$p/cmdline" ] || continue
  if tr '\0' ' ' < "$p/cmdline" 2>/dev/null | grep -q 'mcp-proxy'; then
    count=$((count+1))
  fi
done
echo "mcp-proxy processes: $count"
mcp-proxy processes: 554

Summary by CodeRabbit

• Bug Fixes
  • Improved resource management to prevent potential leaks and ensure proper cleanup of server connections.

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Walkthrough

The get_servers function now explicitly manages MCP client instances (MCPStdioClient and MCPStreamableHttpClient) by creating them within the per-server check workflow and ensuring their cleanup via a finally block, preventing resource and process leaks.

Changes

Cohort / File(s)	Summary
MCP Client Lifecycle Management src/backend/base/langflow/api/v2/mcp.py	Modified get_servers function to create and own MCP client instances per server check, pass them to update_tools, and add finally block cleanup logic to disconnect both clients, preventing resource leaks. Added local imports for MCPStdioClient and MCPStreamableHttpClient.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Areas requiring extra attention:

• Verify that both MCP clients are properly disconnected in all code paths (success and error scenarios)
• Confirm the finally block executes reliably within concurrent per-server checks without blocking operations
• Check that the local client creation doesn't introduce any unintended side effects with the update_tools function interface
• Validate that no other code paths in get_servers still create or reference client instances that might cause duplicate resource allocation

Suggested labels

bug

Suggested reviewers

• edwinjosechittilappilly

Pre-merge checks and finishing touches

Important

Pre-merge checks failed

Please resolve all errors before merging. Addressing warnings is optional.

❌ Failed checks (1 error, 3 warnings)

Check name	Status	Explanation	Resolution
Test Coverage For New Implementations	❌ Error	Bug fix for MCP resource leak lacks any regression tests to verify cleanup behavior and prevent future regressions.	Add regression tests in src/backend/tests/unit/api/v2/ to verify disconnect() is called in success and error scenarios.
Docstring Coverage	⚠️ Warning	Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%.	You can run @coderabbitai generate docstrings to improve docstring coverage.
Test Quality And Coverage	⚠️ Warning	PR introduces MCP client resource cleanup logic in finally block but lacks tests validating this cleanup behavior and resource leak fixes.	Add pytest tests using @pytest.mark.asyncio and unittest.mock.AsyncMock to verify disconnect() calls in finally block for both success and error scenarios.
Test File Naming And Structure	⚠️ Warning	PR modifies src/backend/base/langflow/api/v2/mcp.py to fix a process leak but adds no corresponding test files for this critical v2 API change.	Create test_mcp.py in src/backend/tests/unit/api/v2/ with tests for get_servers endpoint, client cleanup, disconnect() calls, and error scenarios.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'fix: mcp-proxy process leak' directly and accurately summarizes the main change: fixing a resource leak where mcp-proxy processes were not being cleaned up in the MCP servers endpoint.
Excessive Mock Usage Warning	✅ Passed	The PR modifies only src/backend/base/langflow/api/v2/mcp.py to fix resource leak by adding cleanup in finally block. No test files are modified.

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[github-actions]

Frontend Unit Test Coverage Report

Coverage Summary

Lines	Statements	Branches	Functions
	16.42% (4606/28041)	9.73% (2106/21644)	10.76% (664/6166)

Unit Test Results

Tests	Skipped	Failures	Errors	Time
1803	0 💤	0 ❌	0 🔥	24.462s ⏱️

[coderabbitai]

Actionable comments posted: 0

🧹 Nitpick comments (1)

src/backend/base/langflow/api/v2/mcp.py (1)

209-213: Wrap each disconnect in try/except to ensure both are attempted.

If mcp_stdio_client.disconnect() raises an exception, mcp_streamable_http_client.disconnect() will be skipped, potentially still causing leaks. As per coding guidelines, use try/except for proper cleanup.

         finally:
             # Always disconnect clients to prevent mcp-proxy process leaks
             # These clients spawn subprocesses that need to be explicitly terminated
-            await mcp_stdio_client.disconnect()
-            await mcp_streamable_http_client.disconnect()
+            try:
+                await mcp_stdio_client.disconnect()
+            except Exception:
+                await logger.aexception(f"Error disconnecting stdio client for {server_name}")
+            try:
+                await mcp_streamable_http_client.disconnect()
+            except Exception:
+                await logger.aexception(f"Error disconnecting http client for {server_name}")

📜 Review details

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Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between e9a537b and 271a230.

📒 Files selected for processing (1)

• src/backend/base/langflow/api/v2/mcp.py (2 hunks)

🧰 Additional context used

📓 Path-based instructions (2)

src/backend/**/*.py

📄 CodeRabbit inference engine (.cursor/rules/backend_development.mdc)

src/backend/**/*.py: Use FastAPI async patterns with await for async operations in component execution methods
Use asyncio.create_task() for background tasks and implement proper cleanup with try/except for asyncio.CancelledError
Use queue.put_nowait() for non-blocking queue operations and asyncio.wait_for() with timeouts for controlled get operations

Files:

• src/backend/base/langflow/api/v2/mcp.py

src/backend/base/langflow/api/**/*.py

📄 CodeRabbit inference engine (.cursor/rules/backend_development.mdc)

Backend API endpoints should be organized by version (v1/, v2/) under src/backend/base/langflow/api/ with specific modules for features (chat.py, flows.py, users.py, etc.)

Files:

• src/backend/base/langflow/api/v2/mcp.py

🧬 Code graph analysis (1)

src/backend/base/langflow/api/v2/mcp.py (1)

src/lfx/src/lfx/base/mcp/util.py (3)

• MCPStdioClient (1021-1231)
• MCPStreamableHttpClient (1234-1506)
• update_tools (1514-1659)

⏰ Context from checks skipped due to timeout of 90000ms. You can increase the timeout in your CodeRabbit configuration to a maximum of 15 minutes (900000ms). (15)

• GitHub Check: Run Backend Tests / Unit Tests - Python 3.10 - Group 5
• GitHub Check: Lint Backend / Run Mypy (3.12)
• GitHub Check: Run Backend Tests / Unit Tests - Python 3.10 - Group 2
• GitHub Check: Run Backend Tests / Unit Tests - Python 3.10 - Group 1
• GitHub Check: Run Backend Tests / Unit Tests - Python 3.10 - Group 4
• GitHub Check: Run Backend Tests / Unit Tests - Python 3.10 - Group 3
• GitHub Check: Lint Backend / Run Mypy (3.11)
• GitHub Check: Lint Backend / Run Mypy (3.10)
• GitHub Check: Run Backend Tests / Integration Tests - Python 3.10
• GitHub Check: Run Backend Tests / LFX Tests - Python 3.10
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🔇 Additional comments (2)

src/backend/base/langflow/api/v2/mcp.py (2)

152-157: Good approach: Creating locally-owned client instances.

Creating the MCP client instances within the function scope and passing them to update_tools gives this code explicit control over their lifecycle, enabling proper cleanup in the finally block.

---

159-164: Correct: Passing owned client instances to update_tools.

This ensures update_tools uses the caller-provided instances rather than creating its own, which previously caused the leak.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 33.02%. Comparing base (a54e508) to head (87d54c9).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/backend/base/langflow/api/v2/mcp.py	0.00%	5 Missing ⚠️

❌ Your patch status has failed because the patch coverage (0.00%) is below the target coverage (40.00%). You can increase the patch coverage or adjust the target coverage.
❌ Your project status has failed because the head coverage (39.26%) is below the target coverage (60.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10988      +/-   ##
==========================================
- Coverage   33.02%   33.02%   -0.01%     
==========================================
  Files        1388     1388              
  Lines       65544    65549       +5     
  Branches     9680     9680              
==========================================
+ Hits        21648    21649       +1     
- Misses      42798    42802       +4     
  Partials     1098     1098              

Flag	Coverage Δ
backend	52.29% <0.00%> (-0.01%)	⬇️
frontend	15.16% <ø> (ø)
lfx	39.26% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/backend/base/langflow/api/v2/mcp.py	57.14% <0.00%> (-2.24%)	⬇️

... and 2 files with indirect coverage changes

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[jordanrfrazier]

lgtm. I say we get this in for v1.7.

Let's add a follow up to the update_tools function that reminds us to refactor how it creates clients, so nobody can call it in the future without cleanup.

[phact]

that's a good point Jordan

[phact]

@copilot can you think of a clean refactor where it cleans up after itself?