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

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

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

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

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

* fix(rmcp): only accept JsonRpcMessage::Error on non-success responses

crates/rmcp/Cargo.toml

@@ -219,6 +219,11 @@ name = "test_streamable_http_json_response"
 required-features = ["server", "client", "transport-streamable-http-server", "reqwest"]
 path = "tests/test_streamable_http_json_response.rs"
 
+[[test]]
+name = "test_streamable_http_4xx_error_body"
+required-features = ["transport-streamable-http-client", "transport-streamable-http-client-reqwest"]
+path = "tests/test_streamable_http_4xx_error_body.rs"
+
 
 [[test]]
 name = "test_custom_request"

crates/rmcp/src/transport/common/reqwest/streamable_http_client.rs

@@ -6,7 +6,7 @@ use reqwest::header::ACCEPT;
 use sse_stream::{Sse, SseStream};
 
 use crate::{
-    model::{ClientJsonRpcMessage, ServerJsonRpcMessage},
+    model::{ClientJsonRpcMessage, JsonRpcMessage, ServerJsonRpcMessage},
     transport::{
         common::http_header::{
             EVENT_STREAM_MIME_TYPE, HEADER_LAST_EVENT_ID, HEADER_MCP_PROTOCOL_VERSION,
@@ -59,6 +59,15 @@ fn apply_custom_headers(
     Ok(builder)
 }
 
+/// Attempts to parse `body` as a JSON-RPC error message.
+/// Returns `None` if the body is not parseable or is not a `JsonRpcMessage::Error`.
+fn parse_json_rpc_error(body: &str) -> Option<ServerJsonRpcMessage> {
+    match serde_json::from_str::<ServerJsonRpcMessage>(body) {
+        Ok(message @ JsonRpcMessage::Error(_)) => Some(message),
+        _ => None,
+    }
+}
+
 impl StreamableHttpClient for reqwest::Client {
     type Error = reqwest::Error;
 
@@ -190,21 +199,40 @@ 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());
+        // 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 parse_json_rpc_error(&body) {
+                    Some(message) => {
+                        return Ok(StreamableHttpPostResponse::Json(message, session_id));
+                    }
+                    None => tracing::warn!(
+                        "HTTP {status}: could not parse JSON body as a JSON-RPC error"
+                    ),
+                }
+            }
             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))
@@ -226,9 +254,7 @@ 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))
             }
         }
     }
@@ -308,8 +334,8 @@ fn extract_scope_from_header(header: &str) -> Option<String> {
 
 #[cfg(test)]
 mod tests {
-    use super::extract_scope_from_header;
-    use crate::transport::streamable_http_client::InsufficientScopeError;
+    use super::{extract_scope_from_header, parse_json_rpc_error};
+    use crate::{model::JsonRpcMessage, transport::streamable_http_client::InsufficientScopeError};
 
     #[test]
     fn extract_scope_quoted() {
@@ -356,4 +382,36 @@ mod tests {
         assert!(!without_scope.can_upgrade());
         assert_eq!(without_scope.get_required_scope(), None);
     }
+
+    #[test]
+    fn parse_json_rpc_error_returns_error_variant() {
+        let body =
+            r#"{"jsonrpc":"2.0","id":1,"error":{"code":-32600,"message":"Invalid Request"}}"#;
+        assert!(matches!(
+            parse_json_rpc_error(body),
+            Some(JsonRpcMessage::Error(_))
+        ));
+    }
+
+    #[test]
+    fn parse_json_rpc_error_rejects_non_error_request() {
+        // A valid JSON-RPC request (method + id) must not be accepted as an error.
+        let body = r#"{"jsonrpc":"2.0","id":1,"method":"ping"}"#;
+        assert!(parse_json_rpc_error(body).is_none());
+    }
+
+    #[test]
+    fn parse_json_rpc_error_rejects_notification() {
+        // A notification (method, no id) must not be accepted as an error.
+        let body =
+            r#"{"jsonrpc":"2.0","method":"notifications/cancelled","params":{"requestId":1}}"#;
+        assert!(parse_json_rpc_error(body).is_none());
+    }
+
+    #[test]
+    fn parse_json_rpc_error_rejects_malformed_json() {
+        assert!(parse_json_rpc_error("not json at all").is_none());
+        assert!(parse_json_rpc_error("").is_none());
+        assert!(parse_json_rpc_error(r#"{"broken":"#).is_none());
+    }
 }

crates/rmcp/tests/test_streamable_http_4xx_error_body.rs

@@ -0,0 +1,121 @@
+#![cfg(all(
+    feature = "transport-streamable-http-client",
+    feature = "transport-streamable-http-client-reqwest",
+    not(feature = "local")
+))]
+
+use std::{collections::HashMap, sync::Arc};
+
+use rmcp::{
+    model::{ClientJsonRpcMessage, ClientRequest, PingRequest, RequestId},
+    transport::streamable_http_client::{
+        StreamableHttpClient, StreamableHttpError, StreamableHttpPostResponse,
+    },
+};
+
+/// Spin up a minimal axum server that always responds with the given status,
+/// content-type, and body — no MCP logic involved.
+async fn spawn_mock_server(status: u16, content_type: &'static str, body: &'static str) -> String {
+    use axum::{Router, body::Body, http::Response, routing::post};
+
+    let router = Router::new().route(
+        "/mcp",
+        post(move || async move {
+            Response::builder()
+                .status(status)
+                .header("content-type", content_type)
+                .body(Body::from(body))
+                .unwrap()
+        }),
+    );
+
+    let listener = tokio::net::TcpListener::bind("127.0.0.1:0").await.unwrap();
+    let addr = listener.local_addr().unwrap();
+    tokio::spawn(async move {
+        axum::serve(listener, router).await.unwrap();
+    });
+
+    format!("http://{addr}/mcp")
+}
+
+fn ping_message() -> ClientJsonRpcMessage {
+    ClientJsonRpcMessage::request(
+        ClientRequest::PingRequest(PingRequest::default()),
+        RequestId::Number(1),
+    )
+}
+
+/// HTTP 4xx with Content-Type: application/json and a valid JSON-RPC error body
+/// must be surfaced as `StreamableHttpPostResponse::Json`, not swallowed as a
+/// transport error.
+#[tokio::test]
+async fn http_4xx_json_rpc_error_body_is_surfaced_as_json_response() {
+    let body = r#"{"jsonrpc":"2.0","id":1,"error":{"code":-32600,"message":"Invalid Request"}}"#;
+    let url = spawn_mock_server(400, "application/json", body).await;
+
+    let client = reqwest::Client::new();
+    let result = client
+        .post_message(
+            Arc::from(url.as_str()),
+            ping_message(),
+            None,
+            None,
+            HashMap::new(),
+        )
+        .await;
+
+    match result {
+        Ok(StreamableHttpPostResponse::Json(msg, _)) => {
+            let json = serde_json::to_value(&msg).unwrap();
+            assert_eq!(json["error"]["code"], -32600);
+            assert_eq!(json["error"]["message"], "Invalid Request");
+        }
+        other => panic!("expected Json response, got: {other:?}"),
+    }
+}
+
+/// HTTP 4xx with non-JSON content-type must still return `UnexpectedServerResponse`
+/// (no regression on the original error path).
+#[tokio::test]
+async fn http_4xx_non_json_body_returns_unexpected_server_response() {
+    let url = spawn_mock_server(400, "text/plain", "Bad Request").await;
+
+    let client = reqwest::Client::new();
+    let result = client
+        .post_message(
+            Arc::from(url.as_str()),
+            ping_message(),
+            None,
+            None,
+            HashMap::new(),
+        )
+        .await;
+
+    match result {
+        Err(StreamableHttpError::UnexpectedServerResponse(_)) => {}
+        other => panic!("expected UnexpectedServerResponse, got: {other:?}"),
+    }
+}
+
+/// HTTP 4xx with Content-Type: application/json but a body that is NOT a valid
+/// JSON-RPC message must fall back to `UnexpectedServerResponse`.
+#[tokio::test]
+async fn http_4xx_malformed_json_body_falls_back_to_unexpected_server_response() {
+    let url = spawn_mock_server(400, "application/json", r#"{"error":"not jsonrpc"}"#).await;
+
+    let client = reqwest::Client::new();
+    let result = client
+        .post_message(
+            Arc::from(url.as_str()),
+            ping_message(),
+            None,
+            None,
+            HashMap::new(),
+        )
+        .await;
+
+    match result {
+        Err(StreamableHttpError::UnexpectedServerResponse(_)) => {}
+        other => panic!("expected UnexpectedServerResponse, got: {other:?}"),
+    }
+}