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

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

@@ -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.
         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) {
+                    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))
@@ -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))
             }
         }
     }