diff --git a/src/index.ts b/src/index.ts
index 7c0d94c..0c48de5 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -790,16 +790,19 @@ export class LlmGateNode {
 
   /**
    * Intercepts preliminary evaluations to log heuristic latency.
-   * @returns Express-compatible request handler.
+   * @returns Express-compatible request handler that returns true if authorized to continue, false if response was sent.
    */
-  public middleware() {
-    return async (req: any, res: any, next: any) => {
+  public middleware(): (req: any, res: any, next: any) => Promise<boolean> {
+    return async (req: any, res: any, next: any): Promise<boolean> => {
       const start = Date.now();
       try {
         const canonical = await this.fetchCoreDecision(req.body);
         if (!canonical) {
           if (this.requireCoreDecision) {
-            return res.status(503).json({ error: 'Routing decision unavailable or denied.' });
+            return (
+              res.status(503).json({ error: 'Routing decision unavailable or denied.' }),
+              false
+            );
           }
           // Compatibility path: allow heuristic routing only when explicitly opted out
         } else {
@@ -809,7 +812,8 @@ export class LlmGateNode {
             decision: { ...adapted, latencyMs: Date.now() - start },
             executionEnvelope: envelope,
           };
-          return next();
+          next();
+          return true;
         }
 
         const body = req.body || {};
@@ -825,9 +829,13 @@ export class LlmGateNode {
           },
         };
         next();
+        return true;
       } catch (_err) {
         if (this.requireCoreDecision) {
-          return res.status(503).json({ error: 'Routing decision unavailable or denied.' });
+          return (
+            res.status(503).json({ error: 'Routing decision unavailable or denied.' }),
+            false
+          );
         }
         req.llmRouter = {
           decision: {
@@ -839,6 +847,7 @@ export class LlmGateNode {
           },
         };
         next();
+        return true;
       }
     };
   }
@@ -977,6 +986,7 @@ export class LlmGateNode {
   /**
    * Next.js /api route handler for POST /api/* style routes.
    * Mirrors Express composition: evaluate routing metadata, then proxy.
+   * Only calls proxy() if middleware() authorizes continuation (returns true).
    */
   public nextApiHandler(): NextApiHandlerLike {
     const middleware = this.middleware();
@@ -989,11 +999,14 @@ export class LlmGateNode {
         return;
       }
 
-      await middleware(req, res, (error: unknown) => {
+      const authorized = await middleware(req, res, (error: unknown) => {
         if (error) {
           throw error;
         }
       });
+      if (!authorized) {
+        return; // Response already sent by middleware (e.g., 503)
+      }
       await proxy(req, res, (error: unknown) => {
         if (error) {
           throw error;
diff --git a/tests/router.test.ts b/tests/router.test.ts
index 4ae47e7..f6a7177 100644
--- a/tests/router.test.ts
+++ b/tests/router.test.ts
@@ -827,8 +827,8 @@ describe('LlmGateNode', () => {
   });
 
   describe('Next.js /api compatibility', () => {
-    it('handles a Next.js-like /api route without Express next()', async () => {
-      const handler = createNextApiHandler({ apiKey: 'secret-token' });
+    it('handles a Next.js-like /api route without Express next() (compatibility mode)', async () => {
+      const handler = createNextApiHandler({ apiKey: 'secret-token', requireCoreDecision: false });
       jest.spyOn(globalThis, 'fetch').mockImplementation(async url => {
         if (String(url).endsWith('/models')) {
           return new Response(JSON.stringify({ data: [] }), { status: 200 });
@@ -860,6 +860,69 @@ describe('LlmGateNode', () => {
       expect(recorder.headers.get('Allow')).toBe('POST');
       expect(recorder.jsonPayload).toEqual({ error: 'Method Not Allowed' });
     });
+
+    it('returns 503 and does NOT call upstream when Core decision unavailable (requireCoreDecision=true)', async () => {
+      const handler = createNextApiHandler({ requireCoreDecision: true });
+      const recorder = createProxyResponseRecorder();
+      const fetchCalls: string[] = [];
+
+      jest.spyOn(globalThis, 'fetch').mockImplementation(async (url: string | URL | Request) => {
+        const urlStr = String(url);
+        fetchCalls.push(urlStr);
+        if (urlStr.endsWith('/chat/completions')) {
+          return new Response(JSON.stringify(validResponse), { status: 200 });
+        }
+        // Core decision endpoint returns failure/no decision
+        return new Response(JSON.stringify({ error: 'no decision' }), { status: 500 });
+      });
+
+      await handler(
+        {
+          method: 'POST',
+          body: validRequest,
+          headers: { accept: 'application/json' },
+        },
+        recorder.res
+      );
+
+      expect(recorder.statusCode).toBe(503);
+      expect(recorder.jsonPayload).toEqual({ error: 'Routing decision unavailable or denied.' });
+      // Verify NO upstream fetch to /chat/completions was made
+      expect(fetchCalls.some(c => c.endsWith('/chat/completions'))).toBe(false);
+    });
+
+    it('returns 503 and does NOT call upstream when Core decision denied (requireCoreDecision=true)', async () => {
+      const handler = createNextApiHandler({ requireCoreDecision: true });
+      const recorder = createProxyResponseRecorder();
+      const fetchCalls: string[] = [];
+
+      jest.spyOn(globalThis, 'fetch').mockImplementation(async (url: string | URL | Request) => {
+        const urlStr = String(url);
+        fetchCalls.push(urlStr);
+        if (urlStr.endsWith('/chat/completions')) {
+          return new Response(JSON.stringify(validResponse), { status: 200 });
+        }
+        // Core decision endpoint returns denied
+        return new Response(
+          JSON.stringify({ selected_route: { decision: 'denied', availability: 'unavailable' } }),
+          { status: 200 }
+        );
+      });
+
+      await handler(
+        {
+          method: 'POST',
+          body: validRequest,
+          headers: { accept: 'application/json' },
+        },
+        recorder.res
+      );
+
+      expect(recorder.statusCode).toBe(503);
+      expect(recorder.jsonPayload).toEqual({ error: 'Routing decision unavailable or denied.' });
+      // Verify NO upstream fetch to /chat/completions was made
+      expect(fetchCalls.some(c => c.endsWith('/chat/completions'))).toBe(false);
+    });
   });
 
   describe('OpenAI chat completion request parser', () => {