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MCP Server won't work with wildcard in "Accept" header and therefore is non‑compliant with HTTP spec #1641
Description
Question
Use Case
My client sends a request to MCP Server with Accept header */* (Same for application/*, text/event-stream). Server rejects this request because it expected strictly application/json or application/json and text/event-stream if json_response=False.
Expected
According to the HTTP/1.1 spec (RFC 7231, section 5.3.2), the Accept header allows wildcard so values */* and application/* should be valid, therefore server should accept them
from mcp.server.fastmcp import FastMCP
from starlette.applications import Starlette
...
mcp_server: FastMCP = FastMCP(streamable_http_path="/", stateless_http=True, json_response=True)
...
app: Starlette = mcp_server.streamable_http_app()
Start server: uvicorn mcp-server:app --host 127.0.0.1 --port 8000 --log-level trace
Example of Call
### Request
POST / HTTP/1.1.
Host: localhost:8000.
Accept-Encoding: gzip, deflate, br, zstd.
Connection: keep-alive.
User-Agent: python-httpx/0.28.1.
Content-Type: application/json.
Accept: */*.
Content-Length: 159.
.
{"jsonrpc":"2.0","id":0,"method":"initialize","params":{"protocolVersion":"2025-06-18","capabilities":{},"clientInfo":{"name":"mcp-client","version":"1.0.0"}}}
### Response
HTTP/1.1 406 Not Acceptable.
date: Tue, 18 Nov 2025 16:16:17 GMT.
server: uvicorn.
content-type: application/json.
content-length: 125.
.
T 127.0.0.1:8000 -> 127.0.0.1:61346 [AP] #107
{"jsonrpc":"2.0","id":"server-error","error":{"code":-32600,"message":"Not Acceptable: Client must accept application/json"}}
Additional Context
- mcp 1.18.0
- Python 3.11.13
- uvicorn 0.38.0
- starlette 0.49.1
Activity
int0x03 commentedon Dec 3, 2025
here is the code for the client
Acceptheader: https://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/server/streamable_http.py#L385-L391fix: Add middleware to handle missing Accept header from Claude Code
Fix 406 Not Acceptable error for ChatGPT MCP client
fix: accept wildcard media types in Accept header per RFC 7231
fix: accept wildcard media types in Accept header per RFC 7231
fix: accept wildcard media types in Accept header per RFC 7231
shivama205 commentedon Mar 8, 2026
Submitted a fix for this in #2152 — adds wildcard media type support per RFC 7231. CI is green and review feedback has been addressed.
Fix 406 Not Acceptable error for ChatGPT MCP client