Skip to content

MCP Inspector

npx @modelcontextprotocol/inspector # connect later to a mcp server e.g., https://docs.langchain.com/mcp
npx @modelcontextprotocol/inspector <command> # start a server stdio

Exploring MCP with curl

  • Remote MCP uses the Streamable HTTP transport: a single POST endpoint speaking JSON-RPC 2.0
  • Responses come back as text/event-stream (SSE-framed): each message arrives as event: message\ndata: {...json...}
  • Required headers on every POST:
  • Content-Type: application/json
  • Accept: application/json, text/event-stream
  • Some servers issue an Mcp-Session-Id response header on initialize that must be echoed back on subsequent requests
  • Handshake order: initialize → notifications/initialized (HTTP 202, no body) → any operation (tools/list, tools/call, ...)

1. Initialize

curl -s -X POST https://docs.langchain.com/mcp \
  -H "Content-Type: application/json" \
  -H "Accept: application/json, text/event-stream" \
  -d '{
    "jsonrpc": "2.0",
    "id": 1,
    "method": "initialize",
    "params": {
      "protocolVersion": "2025-06-18",
      "capabilities": {},
      "clientInfo": {"name": "curl", "version": "0.1"}
    }
  }'

Server replies with its protocolVersion, advertised capabilities (tools/resources/prompts), and serverInfo.

2. Send the initialized notification

Notifications have no id; the server responds with HTTP 202 and an empty body.

curl -s -X POST https://docs.langchain.com/mcp \
  -H "Content-Type: application/json" \
  -H "Accept: application/json, text/event-stream" \
  -d '{"jsonrpc":"2.0","method":"notifications/initialized"}'

3. List and call tools

# list tools
curl -s -X POST https://docs.langchain.com/mcp \
  -H "Content-Type: application/json" \
  -H "Accept: application/json, text/event-stream" \
  -d '{"jsonrpc":"2.0","id":2,"method":"tools/list"}'

# call a tool
curl -s -X POST https://docs.langchain.com/mcp \
  -H "Content-Type: application/json" \
  -H "Accept: application/json, text/event-stream" \
  -d '{
    "jsonrpc": "2.0",
    "id": 3,
    "method": "tools/call",
    "params": {
      "name": "search_docs_by_lang_chain",
      "arguments": {"query": "what is langgraph"}
    }
  }'

Other useful methods: resources/list, resources/read, prompts/list, prompts/get, ping.