How to Use Curl or Postman to Interact With a MCP Server

I wanted to understand how the data and transport layers work in MCP. To do that, I mimicked what a MCP client would do by using curl and then Postman to connect to a MCP server. This allowed me to step through the lifecycle of a connection and get a peek into how messages are exchanged via JSON-RPC.

Overview

MCP clients will handle client-server connections for you but what is actually happening?

The MCP docs state that:

The Model Context Protocol (MCP) defines a rigorous lifecycle for client-server connections that ensures proper capability negotiation and state management.

MCP Overview

Using curl and Postman, I was able to get a better sense of what happens in the initialization and operation phases (see diagram above).

The following guide assumes you:

have access to curl or Postman
have a MCP server running at http://127.0.0.1:8000
exposed a MCP endpoint at /mcp

curl

Send Initialize Request

# Save the response headers to get the session ID
curl -X POST http://127.0.0.1:8000/mcp \
  -H "Content-Type: application/json" \
  -H "Accept: application/json, text/event-stream" \
  -D headers.txt \
  -d '{
    "jsonrpc": "2.0",
    "id": 1,
    "method": "initialize",
    "params": {
      "protocolVersion": "2024-11-05",
      "capabilities": {
        "tools": {},
        "resources": {},
        "prompts": {}
      },
      "clientInfo": {
        "name": "curl-client",
        "version": "1.0.0"
      }
    }
  }'

The server responds immediately with initialization details:

event: message
data: {
  "jsonrpc": "2.0",
  "id": 1,
  "result": {
    "protocolVersion": "2024-11-05",
    "capabilities": {
      "tools": {
        "listChanged": false
      },
      "resources": {
        "subscribe": false,
        "listChanged": false
      },
      "prompts": {
        "listChanged": false
      }
    },
    "serverInfo": {
      "name": "Demo",
      "version": "1.12.3"
    }
  }
}

Extract the Session ID

# Extract the session ID from headers
SESSION_ID=$(grep -i "mcp-session-id" headers.txt | cut -d' ' -f2 | tr -d '\r')
echo "Session ID: $SESSION_ID"

Send Initialized Notification

curl -X POST http://127.0.0.1:8000/mcp \
  -H "Content-Type: application/json" \
  -H "Accept: application/json, text/event-stream" \
  -H "Mcp-Session-Id: $SESSION_ID" \
  -d '{
    "jsonrpc": "2.0",
    "method": "notifications/initialized"
  }'

Operational Communication

Once initialized, you can immediately send operational requests. Each request receives an immediate response.

List Available Tools

curl -X POST http://127.0.0.1:8000/mcp \
  -H "Content-Type: application/json" \
  -H "Accept: application/json, text/event-stream" \
  -H "Mcp-Session-Id: $SESSION_ID" \
  -d '{
    "jsonrpc": "2.0",
    "id": 2,
    "method": "tools/list"
  }'

Response includes all available tools with their schemas:

event: message
data: {
  "jsonrpc": "2.0",
  "id": 2,
  "result": {
    "tools": [
      {
        "name": "add",
        "description": "Add two numbers",
        "inputSchema": {
          "properties": {
            "a": {
              "title": "A",
              "type": "integer"
            },
            "b": {
              "title": "B",
              "type": "integer"
            }
          },
          "required": [
            "a",
            "b"
          ],
          "type": "object"
        },
        "outputSchema": {
          "properties": {
            "result": {
              "title": "Result",
              "type": "integer"
            }
          },
          "required": [
            "result"
          ],
          "type": "object"
        }
      }
    ]
  }
}

Call a Tool

curl -X POST http://127.0.0.1:8000/mcp \
  -H "Content-Type: application/json" \
  -H "Accept: application/json, text/event-stream" \
  -H "Mcp-Session-Id: $SESSION_ID" \
  -d '{
    "jsonrpc": "2.0",
    "id": 3,
    "method": "tools/call",
    "params": {
      "name": "add",
      "arguments": {
        "a": 5,
        "b": 3
      }
    }
  }'

Session Management

Streamable HTTP handles session management differently than SSE:

No explicit session IDs: The server may use connection-based or stateless session management
Optional session headers: Some servers support Mcp-Session-Id headers for explicit session tracking
Immediate responses: All responses come back in the same HTTP request