Fix double read of json response when content-length header is missing

pkg/server/handlers.go

@@ -131,11 +131,16 @@ func (h *Handler) ListInstances() http.HandlerFunc {
 			return
 		}
 
-		w.Header().Set("Content-Type", "application/json")
-		if err := json.NewEncoder(w).Encode(instances); err != nil {
+		// Marshal to bytes first to set Content-Length header
+		data, err := json.Marshal(instances)
+		if err != nil {
 			http.Error(w, "Failed to encode instances: "+err.Error(), http.StatusInternalServerError)
 			return
 		}
+
+		w.Header().Set("Content-Type", "application/json")
+		w.Header().Set("Content-Length", strconv.Itoa(len(data)))
+		w.Write(data)
 	}
 }
 

webui/src/lib/api.ts

@@ -49,11 +49,8 @@ async function apiCall<T>(
     } else {
       // Handle empty responses for JSON endpoints
       const contentLength = response.headers.get('content-length');
-      if (contentLength === '0' || contentLength === null) {
-        const text = await response.text();
-        if (text.trim() === '') {
-          return {} as T; // Return empty object for empty JSON responses
-        }
+      if (contentLength === '0') {
+        return {} as T; // Return empty object for empty JSON responses
       }
       const data = await response.json() as T;
       return data;

webui/src/lib/errorUtils.ts

@@ -26,7 +26,8 @@ export async function handleApiError(response: Response): Promise<void> {
   }
 
   if (!response.ok) {
-    const errorMessage = await parseErrorResponse(response)
+    // Clone the response before reading to avoid consuming the body stream
+    const errorMessage = await parseErrorResponse(response.clone())
     throw new Error(errorMessage)
   }
 }