MCP Server won't work with wildcard in "Accept" header and therefore is non‑compliant with HTTP spec

[rh-fr]

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