Issue on call FastMCP Server using Postman

Question

I have this simple MCP Server:

from fastmcp import FastMCP, Context

mcp = FastMCP("Demo 🚀")

@mcp.tool
def add(a: int, b: int) -> int:
    """Add two numbers"""
    return a + b

@mcp.tool
def hello(ctx: Context):
    ctx.info("in Hello!")
    return {"Respone": "Hello!"}


# Static resource
@mcp.resource("config://version")
def get_version(ctx: Context):
    ctx.info("Sono in get_version!")
    return "2.0.1"


if __name__ == "__main__":
    mcp.run(transport='streamable-http')

And I wish to test it using a REST call, but when I try:

curl --request POST \
  --url http://localhost:8000/mcp/tool/hello \
  --header 'accept: application/json, text/event-stream' \
  --header 'content-type: application/json' \
  --header 'x-session-id: my-test-session-123' \
  --data '{
  "jsonrpc": "2.0",
  "method": "hello",
  "id": 1
}

I got this error:

{
  "jsonrpc": "2.0",
  "id": "server-error",
  "error": {
    "code": -32600,
    "message": "Bad Request: Missing session ID"
  }
}

How can I resolve it?

Answer 1

You should understand how MCP life cycle works

MCP Protocol Lifecycle

1. Initialize - Establishes session, exchanges capabilities

2. Initialized notification - Tells server "I'm ready to use tools"

3. Tool calls - Now you can actually call your functions

The code correction

from fastmcp import FastMCP, Context

mcp = FastMCP("Demo 🚀")

@mcp.tool
def add(a: int, b: int) -> int:
    """Add two numbers"""
    return a + b

@mcp.tool
def hello(ctx: Context = None):
    """Say hello"""
    if ctx:
        ctx.info("in Hello!")
    return {"Response": "Hello!"}  #typo

# Static resource
@mcp.resource("config://version")
def get_version(ctx: Context):
    ctx.info("Sono in get_version!")
    return "2.0.1"

if __name__ == "__main__":
    mcp.run(transport='streamable-http')

Powershell
1. Initialize session

$init = Invoke-WebRequest -Uri "http://localhost:8000/mcp" -Method POST -Headers @{'accept'='application/json, text/event-stream'; 'content-type'='application/json'} -Body '{"jsonrpc":"2.0","method":"initialize","params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"test","version":"1.0.0"}},"id":1}'

$sessionId = $init.Headers['mcp-session-id']

Outcomes - sessionId

2. Send initialized notification - pass sessionID here

Invoke-RestMethod -Uri "http://localhost:8000/mcp" -Method POST -Headers @{'accept'='application/json, text/event-stream'; 'content-type'='application/json'; 'Mcp-Session-Id'=$sessionId} -Body '{"jsonrpc":"2.0","method":"notifications/initialized"}' | Out-Null

3. call you function.

$response = Invoke-RestMethod -Uri "http://localhost:8000/mcp" -Method POST -Headers @{'accept'='application/json, text/event-stream'; 'content-type'='application/json'; 'Mcp-Session-Id'=$sessionId} -Body '{"jsonrpc":"2.0","method":"tools/call","params":{"name":"add","arguments":{"a":10,"b":15}},"id":2}'

4. Parse result

$lines = $response -split "`n"
$dataLine = $lines | Where-Object {$_ -like "data: *"}
$jsonData = $dataLine.Substring(6)
$result = $jsonData | ConvertFrom-Json
Write-Host "result: $($result.result.structuredContent.result)"

The output
Result: 25

Python version

import requests
import json

def test_fastmcp_server():
   base_url = "http://localhost:8000/mcp"
   headers = {
       'accept': 'application/json, text/event-stream',
       'content-type': 'application/json'
   }
   
   print("check 1")
   
   init_payload = {
       "jsonrpc": "2.0",
       "method": "initialize",
       "params": {
           "protocolVersion": "2024-11-05",
           "capabilities": {},
           "clientInfo": {
               "name": "python-client",
               "version": "1.0.0"
           }
       },
       "id": 1
   }
   
   response = requests.post(base_url, headers=headers, json=init_payload)
   session_id = response.headers.get('mcp-session-id')
   print(f"Session ID: {session_id}")
   
   if not session_id:
       print("No session ID received")
       return
   
   headers['Mcp-Session-Id'] = session_id
   
   init_complete_payload = {
       "jsonrpc": "2.0",
       "method": "notifications/initialized"
   }
   
   requests.post(base_url, headers=headers, json=init_complete_payload)
   print("Initialization complete")
   
   add_payload = {
       "jsonrpc": "2.0",
       "method": "tools/call",
       "params": {
           "name": "add",
           "arguments": {
               "a": 10,
               "b": 15
           }
       },
       "id": 2
   }
   
   response = requests.post(base_url, headers=headers, json=add_payload)
   
   lines = response.text.split('\n')
   data_line = next((line for line in lines if line.startswith('data: ')), None)
   
   if data_line:
       json_data = data_line[6:]
       result = json.loads(json_data)
       answer = result['result']['structuredContent']['result']
       print(f"Add result: {answer}")
   else:
       print("No data found in response")
       print("Raw response:", response.text)
   
   hello_payload = {
       "jsonrpc": "2.0",
       "method": "tools/call",
       "params": {
           "name": "hello",
           "arguments": {}
       },
       "id": 3
   }
   
   response = requests.post(base_url, headers=headers, json=hello_payload)
   lines = response.text.split('\n')
   data_line = next((line for line in lines if line.startswith('data: ')), None)
   
   if data_line:
       json_data = data_line[6:]
       result = json.loads(json_data)
       hello_response = result['result']['content'][0]['text']
       print(f"Hello response: {hello_response}")
   
   print("Test complete")

if __name__ == "__main__":
   try:
       test_fastmcp_server()
   except requests.exceptions.ConnectionError:
       print("Check MCP its not working on 8000")
   except Exception as e:
       print(f"Error: {e}")

The output

check 1
Session ID: c15a125556ae4ea6bb12a5d62107e27a
Initialization complete
Add result: 25
Hello response: {"Response":"Hello!"}
Test complete