fix: limit browser route cache sniffing

Only parse cloned JSON responses for browser metadata endpoints so unrelated API calls don't pay the route cache warm-up cost, and cover the regression with a focused routing fetch test.

Made-with: Cursor

Author: rgarcia

src/lib/browser-routing.ts

@@ -29,6 +29,7 @@ export class BrowserRouteCache {
 
 const BROWSER_ROUTING_SUBRESOURCES_ENV = 'KERNEL_BROWSER_ROUTING_SUBRESOURCES';
 const DEFAULT_BROWSER_ROUTING_SUBRESOURCES = ['curl'];
+const BROWSER_ROUTE_CACHEABLE_PATH = /^\/(?:v\d+\/)?browsers(?:\/[^/]+)?\/?$/;
 
 export function browserRoutingSubresourcesFromEnv(): string[] {
   const raw = readBrowserRoutingSubresourcesEnv();
@@ -63,12 +64,20 @@ export function createRoutingFetch(
 
   return async (input, init) => {
     const request = new Request(input, init);
+    const shouldSniff = shouldSniffAndPopulateCache(request, apiOrigin);
     const response = await routeRequest(innerFetch, { input, init, request }, apiOrigin, allowed, cache);
-    await sniffAndPopulateCache(response, cache);
+    if (shouldSniff) {
+      await sniffAndPopulateCache(response, cache);
+    }
     return response;
   };
 }
 
+function shouldSniffAndPopulateCache(request: Request, apiOrigin: string): boolean {
+  const url = new URL(request.url);
+  return url.origin === apiOrigin && BROWSER_ROUTE_CACHEABLE_PATH.test(url.pathname);
+}
+
 function browserRouteFromValue(value: unknown): BrowserRoute | undefined {
   if (!value || typeof value !== 'object') {
     return undefined;

tests/lib/browser-routing.test.ts

@@ -1,6 +1,10 @@
 import Kernel from '@onkernel/sdk';
 
-import { browserRoutingSubresourcesFromEnv } from '../../src/lib/browser-routing';
+import {
+  BrowserRouteCache,
+  browserRoutingSubresourcesFromEnv,
+  createRoutingFetch,
+} from '../../src/lib/browser-routing';
 
 describe('browser routing', () => {
   const browserRoutingEnv = 'KERNEL_BROWSER_ROUTING_SUBRESOURCES';
@@ -127,6 +131,32 @@ describe('browser routing', () => {
     });
   });
 
+  test('skips cache sniffing for non-browser JSON responses', async () => {
+    let cloneCalled = false;
+    const wrappedFetch = createRoutingFetch(
+      async () => {
+        const response = Response.json({ ok: true });
+        const clone = response.clone.bind(response);
+        Object.defineProperty(response, 'clone', {
+          value: () => {
+            cloneCalled = true;
+            return clone();
+          },
+        });
+        return response;
+      },
+      {
+        apiBaseURL: 'https://api.example/',
+        subresources: ['process'],
+        cache: new BrowserRouteCache(),
+      },
+    );
+
+    await wrappedFetch('https://api.example/deployments');
+
+    expect(cloneCalled).toBe(false);
+  });
+
   test('preserves custom fetch options for both API and routed VM requests', async () => {
     await withBrowserRoutingEnv('process', async () => {
       const dispatcher = Symbol('dispatcher');