Update crates/rmcp/src/transport/streamable_http_server/tower.rs

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: jokemanfire

crates/rmcp/src/transport/streamable_http_server/tower.rs

@@ -209,16 +209,26 @@ fn host_is_allowed(host: &NormalizedAuthority, allowed_hosts: &[String]) -> bool
         })
 }
 
+fn bad_request_response(message: &str) -> BoxResponse {
+    let body = Full::from(message.to_string()).boxed();
+
+    http::Response::builder()
+        .status(http::StatusCode::BAD_REQUEST)
+        .header(http::header::CONTENT_TYPE, "text/plain; charset=utf-8")
+        .body(body)
+        .expect("failed to build bad request response")
+}
+
 fn parse_host_header(headers: &HeaderMap) -> Result<NormalizedAuthority, BoxResponse> {
     let Some(host) = headers.get(http::header::HOST) else {
-        return Err(forbidden_response("Forbidden:missing_host header"));
+        return Err(bad_request_response("Bad Request: missing Host header"));
     };
 
     let host = host
         .to_str()
-        .map_err(|_| forbidden_response("Forbidden: Invalid Host header encoding"))?;
+        .map_err(|_| bad_request_response("Bad Request: Invalid Host header encoding"))?;
     let authority = http::uri::Authority::try_from(host)
-        .map_err(|_| forbidden_response("Forbidden: Invalid Host header"))?;
+        .map_err(|_| bad_request_response("Bad Request: Invalid Host header"))?;
     Ok(normalize_authority(authority.host(), authority.port_u16()))
 }