fix(clerk-js): Remove double slash from FAPI client urls

.changeset/fluffy-comics-peel.md

@@ -0,0 +1,5 @@
+---
+'@clerk/clerk-js': patch
+---
+
+Fix double slash in FAPI client URLs when using a proxy configuration (avoids 308 redirects).

packages/clerk-js/src/core/__tests__/fapiClient.spec.ts

@@ -21,6 +21,13 @@ const fapiClientWithProxy = createFapiClient({
   proxyUrl,
 });
 
+const proxyUrlWithTrailingSlash = 'https://clerk.com/api/__clerk/';
+
+const fapiClientWithProxyTrailingSlash = createFapiClient({
+  ...baseFapiClientOptions,
+  proxyUrl: proxyUrlWithTrailingSlash,
+});
+
 type RecursivePartial<T> = {
   [P in keyof T]?: RecursivePartial<T[P]>;
 };
@@ -79,6 +86,20 @@ describe('buildUrl(options)', () => {
     );
   });
 
+  it('returns the correct URL when proxy URL has a trailing slash', () => {
+    // The expected URL should NOT have double slashes after __clerk
+    expect(fapiClientWithProxyTrailingSlash.buildUrl({ path: '/foo' }).href).toBe(
+      `https://clerk.com/api/__clerk/v1/foo?__clerk_api_version=${SUPPORTED_FAPI_VERSION}&_clerk_js_version=test`,
+    );
+  });
+
+  it('handles complex paths correctly with proxy URL with trailing slash', () => {
+    const path = '/client/sign_ins/sia_123/prepare_first_factor';
+    expect(fapiClientWithProxyTrailingSlash.buildUrl({ path }).href).toBe(
+      `https://clerk.com/api/__clerk/v1${path}?__clerk_api_version=${SUPPORTED_FAPI_VERSION}&_clerk_js_version=test`,
+    );
+  });
+
   it('uses domain from options if production', () => {
     expect(
       createFapiClient({ ...baseFapiClientOptions, domain: 'clerk.other.com', instanceType: 'production' }).buildUrl({

packages/clerk-js/src/core/fapiClient.ts

@@ -149,7 +149,10 @@ export function createFapiClient(options: FapiClientOptions): FapiClient {
 
     if (options.proxyUrl) {
       const proxyBase = new URL(options.proxyUrl);
-      const proxyPath = proxyBase.pathname.slice(1, proxyBase.pathname.length);
+      let proxyPath = proxyBase.pathname.slice(1);
+      if (proxyPath.endsWith('/')) {
+        proxyPath = proxyPath.slice(0, -1);
+      }
       return buildUrlUtil(
         {
           base: proxyBase.origin,