Reply with a parse error on non-UTF-8 lines in MCP serve

Found while testing crates/mcp/src/server.rs:62-64 (Server::serve)

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

crates/mcp/src/server.rs

@@ -59,10 +59,17 @@ impl Server {
 
     /// Serve newline-delimited JSON-RPC until `input` closes. Logs go to stderr only (stdout is
     /// the protocol stream).
-    pub fn serve(&mut self, input: impl BufRead, mut output: impl Write) -> std::io::Result<()> {
-        for line in input.lines() {
-            let line = line?;
-            if let Some(reply) = self.handle_line(&line) {
+    pub fn serve(&mut self, mut input: impl BufRead, mut output: impl Write) -> std::io::Result<()> {
+        let mut buf = Vec::new();
+        loop {
+            buf.clear();
+            if input.read_until(b'\n', &mut buf)? == 0 {
+                break;
+            }
+            // Invalid UTF-8 becomes U+FFFD, so the line gets a parse error instead of ending the stream.
+            let line = String::from_utf8_lossy(&buf);
+            let line = line.trim_end_matches(['\r', '\n']);
+            if let Some(reply) = self.handle_line(line) {
                 output.write_all(reply.as_bytes())?;
                 output.write_all(b"\n")?;
                 output.flush()?;

crates/mcp/src/tests.rs

@@ -76,6 +76,28 @@ fn framing_basics() {
     assert_eq!(rpc(&mut s, 5, "tools/call", json!({}))["error"]["code"], -32602);
 }
 
+#[test]
+fn serve_survives_invalid_utf8_line() {
+    let mut s = server();
+    let mut input = Vec::new();
+    input.extend_from_slice(json!({"jsonrpc":"2.0","id":1,"method":"ping"}).to_string().as_bytes());
+    input.extend_from_slice(b"\n\xff\xfe not utf-8\n");
+    input.extend_from_slice(json!({"jsonrpc":"2.0","id":2,"method":"ping"}).to_string().as_bytes());
+    input.extend_from_slice(b"\n\r\n");
+    input.extend_from_slice(json!({"jsonrpc":"2.0","id":3,"method":"ping"}).to_string().as_bytes());
+    input.extend_from_slice(b"\r\n");
+    let mut out = Vec::new();
+    let res = s.serve(input.as_slice(), &mut out);
+    assert!(res.is_ok(), "serve stopped: {res:?}");
+    let lines: Vec<Value> = String::from_utf8(out).unwrap().lines().map(|l| serde_json::from_str(l).unwrap()).collect();
+    assert_eq!(lines.len(), 4, "{lines:?}");
+    assert_eq!(lines[0]["id"], 1);
+    assert_eq!(lines[1]["error"]["code"], -32700);
+    assert!(lines[1]["id"].is_null());
+    assert_eq!(lines[2]["id"], 2);
+    assert_eq!(lines[3]["id"], 3);
+}
+
 #[test]
 fn protocol_round_trip_over_serve() {
     let mut s = server();