fix(rmcp): surface JSON-RPC error bodies on HTTP 4xx responses#748
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DaleSeo merged 3 commits intomodelcontextprotocol:mainfrom Mar 17, 2026
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When a server returns a 4xx status with Content-Type: application/json, attempt to deserialize the body as a ServerJsonRpcMessage before falling back to UnexpectedServerResponse. This allows JSON-RPC error payloads carried on HTTP error responses to be surfaced as McpError instead of being lost in a transport-level error string. Fixes modelcontextprotocol#724
When a server returns a 4xx status with Content-Type: application/json, attempt to deserialize the body as a ServerJsonRpcMessage before falling back to UnexpectedServerResponse. This allows JSON-RPC error payloads carried on HTTP error responses to be surfaced as McpError instead of being lost in a transport-level error string. Fixes modelcontextprotocol#724
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Thanks for tackling this, @kakarot-dev! I have a couple of comments below. It looks like you've done some manual testing, but it would be great if you could add some automated tests to avoid any regressions.
crates/rmcp/src/transport/common/reqwest/streamable_http_client.rs
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When a server returns a 4xx status with Content-Type: application/json, attempt to deserialize the body as a ServerJsonRpcMessage before falling back to UnexpectedServerResponse. This allows JSON-RPC error payloads carried on HTTP error responses to be surfaced as McpError instead of being lost in a transport-level error string.
Fixes #724
Attempt to deserialize HTTP 4xx response bodies as ServerJsonRpcMessage when Content-Type: application/json, surfacing JSON-RPC errors as McpError instead of discarding them as UnexpectedServerResponse.
How Has This Been Tested?
Tested locally with a mock axum server against three scenarios: valid JSON-RPC error body on 400, non-JSON 400, and malformed JSON on 400. All pass.
Breaking Changes
None.
Types of changes
Checklist
Additional context
I created a test file locally, Test file available if needed