langsmith -js

LangSmith MCP Server for TypeScript provides a full MCP implementation that bridges LangSmith's APIs with a local or self-hosted environment. It exposes tools for conversation history, prompt management, traces and runs, datasets, experiments, and usage/billing data. After starting the server, you can integrate it with LangSmith by pointing your MCP client configuration at the langsmith server and supplying your API key. The included tools let you fetch thread histories, list and fetch prompts, query runs and traces, list datasets and examples, and inspect billing usage. The server handles pagination, filtering, and schema validation to ensure consistent interactions with LangSmith services.

To use it, start the server with the required API key in the environment, then invoke tools via the MCP client protocol (JSON-RPC). For example, you can request thread history for a specific thread, fetch the latest runs from a project, or pull prompts by name. The server is designed to work with the MCP inspector for interactive exploration, which helps you browse tool schemas and invoke them from a UI.

Prerequisites:

• Node.js 18+ installed on your machine
• npm or yarn installed
• Access to a LangSmith API key

1. Clone or install the MCP server package

# Install directly via npx (no local install required for usage)
# You can also install locally if preferred
npm install -g langsmith-mcp-server  # optional for global install

2. Set up environment variables

• LANGSMITH_API_KEY: Your LangSmith API key (required)
• LANGSMITH_WORKSPACE_ID: Optional, if your key has access to multiple workspaces
• LANGSMITH_ENDPOINT: Optional, for custom LangSmith endpoints or EU region

3. Run the server

LANGSMITH_API_KEY=your-key npx langsmith-mcp-server

4. (Optional) Run the MCP Inspector for interactive exploration

LANGSMITH_API_KEY=your-key npx @modelcontextprotocol/inspector npx .

5. Verify startup

• The server should listen on STDIO transport by default. You can verify by sending an initialize RPC using your MCP client.

Tips and common issues:

• Ensure LANGSMITH_API_KEY is valid and has permissions for the actions you perform.
• If you encounter network or endpoint issues, set LANGSMITH_ENDPOINT to the correct LangSmith API URL (e.g., EU region endpoints).
• The server uses character-budget pagination for certain tools (e.g., thread history and runs). Always supply page_number on every request and respect limit/page_number semantics.
• When testing via MCP Inspector, you can directly browse and invoke the tools to understand required schemas.
• If you upgrade to a different Node.js version, re-build if you are using a local build workflow (npm run build).
• The package name used in npx is langsmith-mcp-server, consistent with the Quick Start instructions.