fix(core) : Resolve relative URLs when enhancing fetch error messages

packages/core/src/instrument/fetch.ts

@@ -136,16 +136,18 @@ function instrumentFetch(onFetchResolved?: (response: Response) => void): void {
                 error.message === 'NetworkError when attempting to fetch resource.')
             ) {
               try {
-                const url = new URL(handlerData.fetchData.url);
+                const url = new URL(handlerData.fetchData.url, GLOBAL_OBJ.location?.href);
                 const hostname = url.host;
 
-                if (enhanceOption === 'always') {
-                  // Modify the error message directly
-                  error.message = `${error.message} (${hostname})`;
-                } else {
-                  // Store hostname as non-enumerable property for Sentry-only enhancement
-                  // This preserves the original error message for third-party packages
-                  addNonEnumerableProperty(error, '__sentry_fetch_url_host__', hostname);
+                if (hostname) {
+                  if (enhanceOption === 'always') {
+                    // Modify the error message directly
+                    error.message = `${error.message} (${hostname})`;
+                  } else {
+                    // Store hostname as non-enumerable property for Sentry-only enhancement
+                    // This preserves the original error message for third-party packages
+                    addNonEnumerableProperty(error, '__sentry_fetch_url_host__', hostname);
+                  }
                 }
               } catch {
                 // ignore it if errors happen here

packages/core/src/utils/worldwide.ts

@@ -19,6 +19,7 @@ import type { SdkSource } from './env';
 /** Internal global with common properties and Sentry extensions  */
 export type InternalGlobal = {
   navigator?: { userAgent?: string; maxTouchPoints?: number };
+  location?: { href?: string };
   console: Console;
   PerformanceObserver?: any;
   Sentry?: any;

packages/core/test/lib/instrument/fetch.test.ts

@@ -121,4 +121,40 @@ describe('instrument > addFetchInstrumentationHandler', () => {
 
     expect(error.message).toBe('Failed to fetch (example.com)');
   });
+
+  describe('relative URLs', () => {
+    const globalWithLocation = GLOBAL_OBJ as typeof GLOBAL_OBJ & { location?: { href: string } };
+
+    afterEach(() => {
+      Reflect.deleteProperty(globalWithLocation, 'location');
+    });
+
+    async function failWith(target: string): Promise<TypeError> {
+      const error = new TypeError('Failed to fetch');
+      globalThis.fetch = vi.fn<typeof fetch>().mockRejectedValue(error);
+      addFetchInstrumentationHandler(() => undefined);
+      await expect(globalThis.fetch(target)).rejects.toBe(error);
+      return error;
+    }
+
+    it.each([
+      ['a relative path', 'https://app.example.com/some/page?x=1', '/api/users', 'Failed to fetch (app.example.com)'],
+      ['an empty URL (`fetch("")`)', 'https://app.example.com/some/page?x=1', '', 'Failed to fetch (app.example.com)'],
+      ['a relative path with no current page', undefined, '/api/users', 'Failed to fetch'],
+      ['a relative path on a `file://` page', 'file:///C:/app/index.html', '/api/users', 'Failed to fetch'],
+      ['a relative path with an opaque origin', 'null', '/api/users', 'Failed to fetch'],
+      [
+        'an absolute URL, ignoring the current page',
+        'https://app.example.com/some/page',
+        'https://other.example.org/x',
+        'Failed to fetch (other.example.org)',
+      ],
+    ])('handles %s', async (_name, href, target, expected) => {
+      if (href) {
+        globalWithLocation.location = { href };
+      }
+
+      expect((await failWith(target)).message).toBe(expected);
+    });
+  });
 });