Not Acceptable: Client must accept both application/json and text/event-stream

What it means

The Streamable HTTP specification says the client must send an Accept header that lists both application/json and text/event-stream, because the server may answer either way. Servers built with the official SDKs enforce it. Without the header we got HTTP 406:

Server	Response body
TypeScript SDK v2, 2025-style request	{"jsonrpc":"2.0","error":{"code":-32000,"message":"Not Acceptable: Client must accept both application/json and text/event-stream"},"id":null}
Python SDK 2.3.0, 2025-style request	{"jsonrpc":"2.0","id":null,"error":{"code":-32600,"message":"Not Acceptable: Client must accept both application/json and text/event-stream"}}
Python SDK 2.3.0, 2026-07-28 request with Accept: application/json	406 with an empty body

Which requests get checked differs between servers. In our tests:

• A TypeScript server built with createMcpHandler checked only 2025-style requests (with initialize). A 2026-07-28 request passed with any Accept, or none.
• Python 2.3.0 also rejected 2026-07-28 requests whose Accept named only one of the two types, with no message at all. The same request with no Accept header, or with */*, passed.
• A TypeScript server wired with the older per-request transport pattern checked every request.

So a request that works against one server can get 406 from another. The empty-body case is the hard one to recognise: the client usually reports just the status code.

The request has no or a narrow Accept header

Typical with curl, Postman, or a client written by hand that sends only Accept: application/json. Send both types:

curl -X POST http://127.0.0.1:8000/mcp \
  -H 'content-type: application/json' \
  -H 'accept: application/json, text/event-stream' \
  ...

The official client SDKs and the MCP Inspector already send this header, so if they work and your own request does not, compare the headers.

A proxy rewrites the header

If a client that works locally fails only through an API gateway or reverse proxy, check that the proxy forwards Accept unchanged. Some gateways replace it with their own default.

Related

• From HTTP+SSE to Streamable HTTP: complete curl requests that work.
• Run a server behind Docker and a reverse proxy