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rmcp (official Rust MCP SDK): default Streamable HTTP client never follows spec-permitted redirects - redirecting MCP servers hard-fail with Err(UnexpectedServerResponse)

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@dawNotPoi 已经在做这个了。

开始于 2026年8月23日。

评估

难度
4/5
预计耗时
3-5 天
新手友好度
45/100
Issue 类型
缺陷
描述清晰度
基本清楚
活跃度
活跃
技术栈
rust
领域
api, networking

调研方向

从 crates/rmcp/src/transport/common/reqwest/streamable_http_client.rs 中的 StreamableHttpClientTransport::default_http_client() 开始，然后检查现有的单元测试，该测试涵盖重定向时自定义标头泄漏的问题。复现 issue 中的 307 情况，并确定预期的是安全的重定向策略还是可配置的策略。完成标准是：MCP 服务器的重定向能够正常工作，且不会不安全地重新发送自定义标头，同时现有行为和相关测试也已相应更新。

由索引模型根据 Issue 内容生成。

描述

P2 T-enhancement T-transport

rmcp (official Rust MCP SDK): default Streamable HTTP client never follows spec-permitted redirects

Type: functional / MCP conformance
Component: crates/rmcp/src/transport/common/reqwest/streamable_http_client.rs — StreamableHttpClientTransport::default_http_client()

Summary

The official Rust SDK's default Streamable HTTP client is built with:

reqwest::Client::builder()
    .pool_max_idle_per_host(0)
    .redirect(reqwest::redirect::Policy::none())
    .build()

so it never follows HTTP redirects. The MCP spec explicitly permits servers to respond to transport requests with 301/302/307/308 (load balancing, canonical URL / trailing-slash normalization — see e.g. https://github.com/IBM/mcp-context-forge#4441 for a real 307 on /mcp/). Against any redirecting server, every request degrades to:

Err(StreamableHttpError::UnexpectedServerResponse("HTTP 307: ..."))

and the client cannot talk to that server at all.

The truncation is deliberate (there is a unit test asserting no custom-header leak to a redirect target — sensible given reqwest's default policy would replay custom headers), but the consequence is that a spec-compliant redirecting MCP server is unsupported out of the box.

Repro (no build needed)

Point any rmcp StreamableHttpClientTransport (default client) at a server that answers POST /mcp with 307 Location: <same-origin>/v2/mcp. Every post_message returns UnexpectedServerResponse("HTTP 307: ...") instead of issuing a follow-up request to the new Location.

A self-contained crate reproducing this (and the goose-side contrast) is at https://github.com/trickyfalcon/cve-hunt/blob/main/poc-goose-rust/ — scenario B prints the Err(UnexpectedServerResponse("HTTP 307: ...")) directly.

Suggested direction

Follow redirects safely rather than never: only follow cross-host/cross-lo*...* leftovers to be trimmed — or, at minimum, make redirect policy configurable on StreamableHttpClientTransportConfig while keeping the default strict (no-follow) for backwards compatibility. An explicit redirect policy override (as an associated constructor option) would let security-conscious users opt in to same-origin redirect following.

Environment

  • rmcp via modelcontextprotocol/rust-sdk (checked out 2026-08-22, rmcp crate 3.x)
  • Reproducible with the PoC referenced above

Happy to be credited in the advisory/release notes as: Mo (@trickyfalcon, https://trickyfalcon.com)

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