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Streamable HTTP: Content-Type handling is order dependent and accepts invalid values

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@jayzuccarelli 已經在處理了。

開始於 2026年10月12日。

評估

難度
2/5
預估耗時
1-3 小時
新手友好度
76/100
Issue 類型
缺陷
描述清晰度
描述清楚
活躍度
活躍
技術堆疊
python
領域
backend-api-design, security

研究方向

先閱讀 src/mcp/server/transport_security.py 和 src/mcp/server/streamable_http.py 中的 Content-Type 檢查，接著追蹤對應的 modern transport 檢查。為列出的 Content-Type 值新增或更新測試，包括帶參數的值和以逗號分隔的值。完成標準是所有 transport 都一致地只接受 application/json，參數可選。

由索引模型根據 Issue 內容生成。

描述

bug v1 v2
Initial Checks
Release line

2.x (current stable)

Description

The Content-Type header:

  • accepts values that are not JSON media types. application/json, text/plain is accepted and the request is executed, although a comma cannot appear inside a media type. On the modern transport application/jsonx is accepted as well
  • The answer depends on order. application/json, text/plain is accepted while text/plain, application/json is rejected, so the same invalid value gets opposite answers.

The first check is a prefix test in the middleware:

# src/mcp/server/transport_security.py
return content_type is not None and content_type.lower().startswith("application/json")

The second splits the value on ; and , and compares the result:

# src/mcp/server/streamable_http.py
content_type_parts = [part.strip() for part in content_type.split(";")[0].split(",")]
return any(part == CONTENT_TYPE_JSON for part in content_type_parts)

The second check rejects application/jsonx. The 2.x modern transport has no equivalent, so it accepts that value.

For reference, the TypeScript SDK answers 415 for application/json, text/plain and for application/jsonx, and mcp-go answers 400 for both.

Reproduction

Against a server started with stateless_http=True, so no session handshake is needed:

curl -s http://127.0.0.1:8901/mcp \
  -H 'Content-Type: application/json, text/plain' \
  -H 'Accept: application/json, text/event-stream' \
  -d '{"jsonrpc":"2.0","id":1,"method":"tools/call","params":{"name":"ping","arguments":{}}}'

Observed: 200 with a normal JSON-RPC result, and the tool's handler runs.

Content-Type sent Expected handshake transport (1.x and 2.x) modern transport (2.x only)
application/json accepted 200 200
application/json; charset=utf-8 accepted 200 200
application/json, text/plain rejected 200, body executed 200, body executed
application/jsonx rejected 415 200, body executed
text/plain, application/json rejected 400 400
text/plain rejected 400 400

For modern transport calls were sent with MCP-Protocol-Version: 2026-07-28 plus the Mcp-Method and Mcp-Name headers.

Fix

Parse the media type, the segment before the first ;, and compare it exactly to application/json. Parameters keep working, so application/json; charset=utf-8 is unaffected, while the values above are rejected.

Python & MCP Python SDK
Reproduced on the 2.x line (`2.2.0`, `2.3.0`) and the 1.x line (`1.30.0`)
主要語言
Python
星號
24.5k
分支
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平均合併
18 小時 31 分鐘
30 天內合併 PR
38

環境準備

從這裡開始

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  2. 在 Issue 下留言說明你要接手 —— 這能避免兩個人做同樣的事。
  3. Fork 儲存庫，在一個分支上完成修改。
  4. 送出 Pull Request，並在描述裡引用這個 Issue 編號。

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