diff --git a/pkg/server/handlers.go b/pkg/server/handlers.go
index 594c273..98514ec 100644
--- a/pkg/server/handlers.go
+++ b/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)
 	}
 }
 
diff --git a/webui/src/lib/__tests__/api.test.ts b/webui/src/lib/__tests__/api.test.ts
index 87e8ac7..2eda209 100644
--- a/webui/src/lib/__tests__/api.test.ts
+++ b/webui/src/lib/__tests__/api.test.ts
@@ -11,11 +11,13 @@ describe('API Error Handling', () => {
   })
 
   it('converts HTTP errors to meaningful messages', async () => {
-    mockFetch.mockResolvedValue({
+    const mockResponse = {
       ok: false,
       status: 409,
-      text: () => Promise.resolve('Instance already exists')
-    })
+      text: () => Promise.resolve('Instance already exists'),
+      clone: function() { return this }
+    }
+    mockFetch.mockResolvedValue(mockResponse)
 
     await expect(instancesApi.create('existing', {}))
       .rejects
@@ -23,11 +25,13 @@ describe('API Error Handling', () => {
   })
 
   it('handles empty error responses gracefully', async () => {
-    mockFetch.mockResolvedValue({
+    const mockResponse = {
       ok: false,
       status: 500,
-      text: () => Promise.resolve('')
-    })
+      text: () => Promise.resolve(''),
+      clone: function() { return this }
+    }
+    mockFetch.mockResolvedValue(mockResponse)
 
     await expect(instancesApi.list())
       .rejects
diff --git a/webui/src/lib/api.ts b/webui/src/lib/api.ts
index f7ecdf2..4e180e0 100644
--- a/webui/src/lib/api.ts
+++ b/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;
diff --git a/webui/src/lib/errorUtils.ts b/webui/src/lib/errorUtils.ts
index 1860bf9..85cdf03 100644
--- a/webui/src/lib/errorUtils.ts
+++ b/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)
   }
 }
\ No newline at end of file