fix: drop server error bodies from mcp transport errors

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/windmill-mcp/src/client/redacting.rs

@@ -1,4 +1,4 @@
-use std::{collections::HashMap, future::Future, sync::Arc};
+use std::{borrow::Cow, collections::HashMap, future::Future, sync::Arc};
 
 use futures::stream::BoxStream;
 use reqwest::header::{HeaderName, HeaderValue};
@@ -20,35 +20,19 @@ type Error = StreamableHttpError<reqwest::Error>;
 #[derive(Clone)]
 pub(super) struct UrlRedactingClient(pub(super) reqwest::Client);
 
-async fn redacted<T>(
-    uri: Arc<str>,
-    request: impl Future<Output = Result<T, Error>>,
-) -> Result<T, Error> {
+async fn redacted<T>(request: impl Future<Output = Result<T, Error>>) -> Result<T, Error> {
     match request.with_subscriber(Dispatch::none()).await {
         Err(StreamableHttpError::Client(e)) => Err(StreamableHttpError::Client(e.without_url())),
-        Err(StreamableHttpError::UnexpectedServerResponse(body)) => {
-            Err(StreamableHttpError::UnexpectedServerResponse(
-                without_request_target(&uri, &body).into(),
-            ))
+        // An owned message is `HTTP <status>: <body>`, and the body is the server's
+        // to write: a common one echoes the request target (`Cannot POST /mcp/<key>`).
+        Err(StreamableHttpError::UnexpectedServerResponse(Cow::Owned(message))) => {
+            let status = message.split(": ").next().unwrap_or_default().to_string();
+            Err(StreamableHttpError::UnexpectedServerResponse(status.into()))
         }
         r => r,
     }
 }
 
-/// The body of an error response is the server's to write, and a common one
-/// echoes the request target (`Cannot POST /mcp/<key>`).
-fn without_request_target(uri: &str, body: &str) -> String {
-    let mut body = body.replace(uri, "<url>");
-    if let Ok(url) = reqwest::Url::parse(uri) {
-        for part in [Some(url.path()).filter(|p| p.len() > 1), url.query()] {
-            if let Some(part) = part.filter(|p| !p.is_empty()) {
-                body = body.replace(part, "<redacted>");
-            }
-        }
-    }
-    body
-}
-
 impl StreamableHttpClient for UrlRedactingClient {
     type Error = reqwest::Error;
 
@@ -61,7 +45,6 @@ impl StreamableHttpClient for UrlRedactingClient {
         custom_headers: HashMap<HeaderName, HeaderValue>,
     ) -> Result<StreamableHttpPostResponse, Error> {
         redacted(
-            uri.clone(),
             self.0
                 .post_message(uri, message, session_id, auth_header, custom_headers),
         )
@@ -77,17 +60,14 @@ impl StreamableHttpClient for UrlRedactingClient {
         custom_headers: HashMap<HeaderName, HeaderValue>,
         max_sse_event_size: usize,
     ) -> Result<StreamableHttpPostResponse, Error> {
-        redacted(
-            uri.clone(),
-            self.0.post_message_with_max_sse_event_size(
-                uri,
-                message,
-                session_id,
-                auth_header,
-                custom_headers,
-                max_sse_event_size,
-            ),
-        )
+        redacted(self.0.post_message_with_max_sse_event_size(
+            uri,
+            message,
+            session_id,
+            auth_header,
+            custom_headers,
+            max_sse_event_size,
+        ))
         .await
     }
 
@@ -99,7 +79,6 @@ impl StreamableHttpClient for UrlRedactingClient {
         custom_headers: HashMap<HeaderName, HeaderValue>,
     ) -> Result<(), Error> {
         redacted(
-            uri.clone(),
             self.0
                 .delete_session(uri, session_id, auth_header, custom_headers),
         )
@@ -115,7 +94,6 @@ impl StreamableHttpClient for UrlRedactingClient {
         custom_headers: HashMap<HeaderName, HeaderValue>,
     ) -> Result<BoxStream<'static, Result<Sse, SseError>>, Error> {
         redacted(
-            uri.clone(),
             self.0
                 .get_stream(uri, session_id, last_event_id, auth_header, custom_headers),
         )
@@ -131,42 +109,14 @@ impl StreamableHttpClient for UrlRedactingClient {
         custom_headers: HashMap<HeaderName, HeaderValue>,
         max_sse_event_size: usize,
     ) -> Result<BoxStream<'static, Result<Sse, SseError>>, Error> {
-        redacted(
-            uri.clone(),
-            self.0.get_stream_with_max_sse_event_size(
-                uri,
-                session_id,
-                last_event_id,
-                auth_header,
-                custom_headers,
-                max_sse_event_size,
-            ),
-        )
+        redacted(self.0.get_stream_with_max_sse_event_size(
+            uri,
+            session_id,
+            last_event_id,
+            auth_header,
+            custom_headers,
+            max_sse_event_size,
+        ))
         .await
     }
 }
-
-#[cfg(test)]
-mod tests {
-    use super::without_request_target;
-
-    #[test]
-    fn error_body_loses_the_request_target() {
-        let uri = "https://mcp.example.com/mcp/KEY?token=T0K";
-        for body in [
-            "Cannot POST /mcp/KEY",
-            "no route for /mcp/KEY?token=T0K",
-            uri,
-        ] {
-            let scrubbed = without_request_target(uri, &format!("HTTP 404: {body}"));
-            assert!(
-                !scrubbed.contains("KEY") && !scrubbed.contains("T0K"),
-                "{scrubbed}"
-            );
-        }
-        assert_eq!(
-            without_request_target("https://mcp.example.com/", "a/b"),
-            "a/b"
-        );
-    }
-}