Unsupported Media Type: Content-Type must be application/json (415)

What it means

We sent a POST with Content-Type: text/plain:

Server	Response
TypeScript SDK v2	415 {"jsonrpc":"2.0","error":{"code":-32000,"message":"Unsupported Media Type: Content-Type must be application/json"},"id":null}
Python SDK 2.3.0	400 Invalid Content-Type header

The request is not sent as JSON

curl -d without a -H 'content-type: application/json' header sends application/x-www-form-urlencoded. Some HTTP libraries send text/plain for string bodies. Set the header explicitly:

curl -X POST http://127.0.0.1:8000/mcp \
  -H 'content-type: application/json' \
  -H 'accept: application/json, text/event-stream' \
  -H 'MCP-Protocol-Version: 2026-07-28' \
  -H 'Mcp-Method: tools/list' \
  -d '{"jsonrpc":"2.0","id":1,"method":"tools/list","params":{"_meta":{"io.modelcontextprotocol/protocolVersion":"2026-07-28","io.modelcontextprotocol/clientCapabilities":{}}}}'

With the header set, both servers returned 200 and the tool list.

A body that is labeled as JSON but is not valid JSON gets a different answer: 400 with JSON-RPC code -32700. The TypeScript server said Parse error: Invalid JSON, the Python server just Parse error. Check the quoting of the -d argument, especially on Windows, where cmd.exe does not treat single quotes as quotes.

Why the two servers answer differently

The examples in the Streamable HTTP specification send Content-Type: application/json, but the specification names no status code for a request without it (we checked on 2026-10-29). The TypeScript SDK uses the HTTP status meant for it, 415, with a JSON-RPC error body. The Python SDK answers 400 with a plain-text body, so a client that only shows the status code says “Bad Request” and nothing more. If you see a bare 400 from a Python server, check Content-Type along with the other headers.

Related

• 406 Not Acceptable: the other header every request needs.