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LibreChat has a plugin system similar to its native Langfuse integration. The LangSight plugin intercepts MCP tool calls and sends spans — with no changes to LibreChat’s core code.

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Setup

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1. Copy the plugin

cp integrations/librechat/langsight-plugin.js /path/to/librechat/plugins/

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2. Register in librechat.yaml

plugins:
  - name: langsight
    path: ./plugins/langsight-plugin.js

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3. Set environment variables

# Required
LANGSIGHT_URL=http://localhost:8000

# Optional
LANGSIGHT_API_KEY=your-api-key
LANGSIGHT_AGENT_NAME=librechat

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4. Restart LibreChat

All MCP tool calls from LibreChat agents are now traced in LangSight.

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What you see

trace.span_received  server=postgres-mcp  tool=query  status=success  latency_ms=42
trace.span_received  server=confluence-mcp  tool=search  status=error   latency_ms=1240

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How it works

The plugin hooks into LibreChat’s MCP client callTool() method before agents use it. It records timing, wraps the original call, and POSTs a span to POST /api/traces/spans as a fire-and-forget task. Fail-open: if LangSight is unreachable, the plugin logs a debug message and the tool call proceeds normally.

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Requirements

  • Node.js 18+ (uses native fetch)
  • LibreChat with plugin support
  • langsight serve running