[nextjs] The next-intl middleware does not work in composition with the Content SDK proxy.

.changeset/chatty-days-lie.md

@@ -0,0 +1,5 @@
+---
+'@sitecore-content-sdk/nextjs': patch
+---
+
+[nextjs] Short-circuit the proxy chain when a handler returns 403 or a redirect (`redirected` or HTTP 3xx), so upstream redirects (e.g. next-intl locale negotiation) are preserved when composed with `defineProxy`

packages/nextjs/src/proxy/proxy.test.ts

@@ -756,6 +756,98 @@ describe('defineProxy', () => {
     expect(result).to.equal(forbidden);
   });
 
+  it('should short-circuit the chain once a proxy sets a location header', async () => {
+    const redirect = {
+      status: 307,
+      headers: new Headers({ location: '/en' }),
+    } as unknown as NextResponse;
+
+    const nextIntlProxy: ProxyHandler = {
+      handle: sinon.stub().resolves(redirect),
+    };
+    const localeProxy: ProxyHandler = {
+      handle: sinon.stub().resolves({ status: 200 } as unknown as NextResponse),
+    };
+    const downstreamProxy: ProxyHandler = {
+      handle: sinon.stub().resolves({ status: 200 } as unknown as NextResponse),
+    };
+
+    const req = {} as NextRequest;
+    const res = { status: 200, headers: new Headers() } as unknown as NextResponse;
+
+    const result = await defineProxy(nextIntlProxy, localeProxy, downstreamProxy).exec(req, res);
+
+    expect(nextIntlProxy.handle).to.have.been.calledOnce;
+    expect(localeProxy.handle).to.not.have.been.called;
+    expect(downstreamProxy.handle).to.not.have.been.called;
+    expect(result).to.equal(redirect);
+    expect(result.headers.get('location')).to.equal('/en');
+  });
+
+  it('should short-circuit the chain once a proxy returns a 3xx response without redirected flag', async () => {
+    const redirect = {
+      status: 302,
+      redirected: false,
+      headers: new Headers({ location: '/target' }),
+    } as unknown as NextResponse;
+
+    const redirectsProxy: ProxyHandler = {
+      handle: sinon.stub().resolves(redirect),
+    };
+    const downstreamProxy: ProxyHandler = {
+      handle: sinon.stub().resolves({ status: 200 } as unknown as NextResponse),
+    };
+
+    const req = {} as NextRequest;
+    const res = { status: 200 } as unknown as NextResponse;
+
+    const result = await defineProxy(redirectsProxy, downstreamProxy).exec(req, res);
+
+    expect(redirectsProxy.handle).to.have.been.calledOnce;
+    expect(downstreamProxy.handle).to.not.have.been.called;
+    expect(result).to.equal(redirect);
+  });
+
+  it('should short-circuit the chain once a proxy sets redirected on the response', async () => {
+    const redirect = {
+      status: 301,
+      redirected: true,
+    } as unknown as NextResponse;
+
+    const redirectsProxy: ProxyHandler = {
+      handle: sinon.stub().resolves(redirect),
+    };
+    const downstreamProxy: ProxyHandler = {
+      handle: sinon.stub().resolves({ status: 200 } as unknown as NextResponse),
+    };
+
+    const req = {} as NextRequest;
+    const res = { status: 200 } as unknown as NextResponse;
+
+    const result = await defineProxy(redirectsProxy, downstreamProxy).exec(req, res);
+
+    expect(downstreamProxy.handle).to.not.have.been.called;
+    expect(result).to.equal(redirect);
+  });
+
+  it('should preserve redirect responses passed as the initial response', async () => {
+    const redirect = {
+      status: 307,
+      headers: new Headers({ location: '/en' }),
+    } as unknown as NextResponse;
+
+    const localeProxy: ProxyHandler = {
+      handle: sinon.stub().resolves({ status: 200 } as unknown as NextResponse),
+    };
+
+    const req = {} as NextRequest;
+
+    const result = await defineProxy(localeProxy).exec(req, redirect);
+
+    expect(localeProxy.handle).to.not.have.been.called;
+    expect(result).to.equal(redirect);
+  });
+
   it('should pass context to proxies when generateContext is true', async () => {
     const proxiesContext: ProxiesContext = new Map();
     const successfulExecution: { marker: string } & SuccessfulProxyExecution = {

packages/nextjs/src/proxy/proxy.ts

@@ -283,6 +283,15 @@ export abstract class ProxyBase extends ProxyHandler {
   }
 }
 
+/**
+ * Returns true when the proxy chain should stop (403, redirect, or HTTP 3xx).
+ * @param {NextResponse} res response from a proxy handler
+ * @returns {boolean} true when remaining handlers should be skipped
+ */
+function shouldShortCircuitProxyChain(res: NextResponse): boolean {
+  return res.status === 403 || !!res.redirected || (res.status >= 300 && res.status <= 399);
+}
+
 /**
  * Define a proxy with a list of proxy handlers
  * @param {ProxyHandler[]} proxies List of proxy handlers to execute
@@ -306,9 +315,9 @@ export const defineProxy = (...proxies: ProxyHandler[]) => {
       const proxyResponse = await proxies.reduce(
         (p, proxy) =>
           p.then((res) => {
-            // Short-circuit the remaining proxies once a previous one
-            // denied the request (e.g. PreviewProxy returning 403).
-            if (res.status === 403) return res;
+            if (shouldShortCircuitProxyChain(res)) {
+              return res;
+            }
 
             return proxy.handle(req, res, proxiesContext);
           }),