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MCP Rust SDK 3.4.0 splits ServerInfo/ClientInfo into config types and tightens three HTTP transport edge cases
rmcp v3.4.0, released 2026-09-15, adds ServerConfig and ClientConfig (#1266) and deprecates the ServerInfo/ClientInfo aliases. The fixes are transport-correctness work: Origin validation semantics are now enforced (#1192), a malformed-JSON 200 is no longer treated as Accepted for requests (#1208), handler-generated HeaderMismatch maps to HTTP 400 in the streamable-http server (#1259), URL origin decides protected-resource-metadata discovery (#1264), and non-metadata JSON is ignored when probing for PRM (#1204). The first pre-init request now runs in the service loop rather than out of band.
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