fixed an error related to patched SFR responses when running theme dev

Author: EvilGenius13

.changeset/fair-parents-reply.md

@@ -0,0 +1,5 @@
+---
+'@shopify/theme': patch
+---
+
+Fixed theme dev error on HTTP 304 patched responses

packages/theme/src/cli/utilities/theme-environment/proxy.test.ts

@@ -197,6 +197,26 @@ describe('dev proxy', () => {
       // Stores _shopify_essential for the following requests
       expect(ctx.session.sessionCookies).toHaveProperty('_shopify_essential', ':AZFbAlZ..yAAH:')
     })
+
+    test('handles 304 Not Modified responses without crashing', async () => {
+      // Create 304 response with no body as per HTTP spec
+      const notModifiedResponse = new Response(null, {
+        status: 304,
+        statusText: 'Not Modified',
+        headers: {
+          etag: '"abc123"',
+          'cache-control': 'max-age=3600',
+          'x-request-id': 'test-123',
+        },
+      })
+
+      const result = await patchRenderingResponse(ctx, notModifiedResponse)
+
+      expect(result.status).toBe(304)
+      expect(result.statusText).toBe('Not Modified')
+      expect(result.headers.get('etag')).toBe('"abc123"')
+      expect(result.body).toBeNull()
+    })
   })
 
   describe('getProxyStorefrontHeaders', () => {

packages/theme/src/cli/utilities/theme-environment/proxy.ts

@@ -171,6 +171,11 @@ export async function patchRenderingResponse(
   rawResponse: Response,
   patchCallback?: (html: string) => string | undefined,
 ): Promise<Response> {
+  // 3xx responses should be returned
+  if (rawResponse.status >= 300 && rawResponse.status < 400) {
+    return rawResponse
+  }
+
   const response = patchProxiedResponseHeaders(ctx, rawResponse)
 
   // Only set HTML content-type for actual HTML responses, preserve JSON content-type: