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fix(rmcp): surface JSON-RPC error bodies on HTTP 4xx responses #748

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fix(rmcp): surface JSON-RPC error bodies on HTTP 4xx responses #748

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7e03278
fix(rmcp): surface JSON-RPC error bodies on HTTP 4xx responses
kakarot-dev Mar 12, 2026
c09c6a3
fix(rmcp): surface JSON-RPC error bodies on HTTP 4xx responses
kakarot-dev Mar 12, 2026
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37 changes: 28 additions & 9 deletions crates/rmcp/src/transport/common/reqwest/streamable_http_client.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -190,21 +190,42 @@ impl StreamableHttpClient for reqwest::Client {
if status == reqwest::StatusCode::NOT_FOUND && session_was_attached {
return Err(StreamableHttpError::SessionExpired);
}
let content_type = response
.headers()
.get(reqwest::header::CONTENT_TYPE)
.map(|ct| String::from_utf8_lossy(ct.as_bytes()).to_string());
let session_id = response
.headers()
.get(HEADER_SESSION_ID)
.and_then(|v| v.to_str().ok())
.map(|s| s.to_string());
// For non-success responses, attempt to parse JSON-RPC error bodies
// before falling back to a transport error. HTTP 4xx responses with
// Content-Type: application/json may carry valid JSON-RPC error
// payloads that should be surfaced as McpError, not TransportSend.
Comment on lines +202 to +205
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@DaleSeo DaleSeo Mar 12, 2026

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The comment says "HTTP 4xx responses" but the guard is !status.is_success(), which also covers 1xx, 3xx, and 5xx. This could be clearer and simpler.

Suggested change
// For non-success responses, attempt to parse JSON-RPC error bodies
// before falling back to a transport error. HTTP 4xx responses with
// Content-Type: application/json may carry valid JSON-RPC error
// payloads that should be surfaced as McpError, not TransportSend.
// Non-success responses may carry valid JSON-RPC error payloads that
// should be surfaced as McpError rather than lost in TransportSend.

if !status.is_success() {
let body = response
.text()
.await
.unwrap_or_else(|_| "<failed to read response body>".to_owned());
if content_type
.as_deref()
.is_some_and(|ct| ct.as_bytes().starts_with(JSON_MIME_TYPE.as_bytes()))
{
match serde_json::from_str::<ServerJsonRpcMessage>(&body) {
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@DaleSeo DaleSeo Mar 12, 2026

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ServerJsonRpcMessage can parse any valid JSON-RPC message including Request, Response, Notification, not just Error. The deserialization should verify the parsed message is actually a JsonRpcMessage::Error.

Ok(message) => {
return Ok(StreamableHttpPostResponse::Json(message, session_id));
}
Err(e) => tracing::warn!(
"HTTP {status}: could not parse JSON response as ServerJsonRpcMessage: {e}"
),
}
}
return Err(StreamableHttpError::UnexpectedServerResponse(Cow::Owned(
format!("HTTP {status}: {body}"),
)));
}
let content_type = response.headers().get(reqwest::header::CONTENT_TYPE);
let session_id = response.headers().get(HEADER_SESSION_ID);
let session_id = session_id
.and_then(|v| v.to_str().ok())
.map(|s| s.to_string());
match content_type {
match content_type.as_deref() {
Some(ct) if ct.as_bytes().starts_with(EVENT_STREAM_MIME_TYPE.as_bytes()) => {
let event_stream = SseStream::from_byte_stream(response.bytes_stream()).boxed();
Ok(StreamableHttpPostResponse::Sse(event_stream, session_id))
Expand All @@ -226,9 +247,7 @@ impl StreamableHttpClient for reqwest::Client {
_ => {
// unexpected content type
tracing::error!("unexpected content type: {:?}", content_type);
Err(StreamableHttpError::UnexpectedContentType(
content_type.map(|ct| String::from_utf8_lossy(ct.as_bytes()).to_string()),
))
Err(StreamableHttpError::UnexpectedContentType(content_type))
}
}
}
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