diff --git a/apps/api/src/routes/health.ts b/apps/api/src/routes/health.ts
index d74082e..bb2dee0 100644
--- a/apps/api/src/routes/health.ts
+++ b/apps/api/src/routes/health.ts
@@ -118,53 +118,63 @@ export async function healthRoutes(fastify: FastifyInstance): Promise<void> {
     },
   );
 
-  // Stub route for testing validation errors
-  fastify.post(
-    '/api/_test/validation-error',
-    {
-      schema: {
-        hide: true,
-        body: {
-          type: 'object',
-          properties: {
-            trigger: { type: 'string' },
+  // ---------------------------------------------------------------------------
+  // Test-harness routes — registered only when NODE_ENV !== 'production'.
+  //
+  // These exist purely to exercise the error-mapping + idempotency code
+  // paths from CI tests. Gating them off in production is defense in
+  // depth: there's no reason production callers should be able to hit
+  // /api/_test/internal-error and force a 500. See issue #116.
+  // ---------------------------------------------------------------------------
+  if (fastify.config.NODE_ENV !== 'production') {
+    // Stub route for testing validation errors
+    fastify.post(
+      '/api/_test/validation-error',
+      {
+        schema: {
+          hide: true,
+          body: {
+            type: 'object',
+            properties: {
+              trigger: { type: 'string' },
+            },
           },
         },
       },
-    },
-    async () => {
-      const { ApiValidationError } = await import('../lib/errors.js');
-      throw new ApiValidationError('Test validation failed', { field: 'required' });
-    },
-  );
+      async () => {
+        const { ApiValidationError } = await import('../lib/errors.js');
+        throw new ApiValidationError('Test validation failed', { field: 'required' });
+      },
+    );
 
-  // Stub route for testing unknown/500 errors
-  fastify.post(
-    '/api/_test/internal-error',
-    { schema: { hide: true } },
-    async () => {
-      throw new Error('Deliberate internal error — should not leak to client');
-    },
-  );
+    // Stub route for testing unknown/500 errors
+    fastify.post(
+      '/api/_test/internal-error',
+      { schema: { hide: true } },
+      async () => {
+        throw new Error('Deliberate internal error — should not leak to client');
+      },
+    );
 
-  // Stub route for testing idempotency
-  fastify.post(
-    '/api/_test/idempotency',
-    { schema: { hide: true } },
-    async (request, reply) => {
-      const idempotencyKey = request.headers['idempotency-key'];
-      if (typeof idempotencyKey === 'string' && idempotencyKey.length > 0) {
-        const personId = 'test-person';
-        const cached = request.server.idempotency.check(personId, idempotencyKey);
-        if (cached) {
-          return reply.code(cached.status).send(cached.body);
-        }
+    // Stub route for testing idempotency
+    fastify.post(
+      '/api/_test/idempotency',
+      { schema: { hide: true } },
+      async (request, reply) => {
+        const idempotencyKey = request.headers['idempotency-key'];
+        if (typeof idempotencyKey === 'string' && idempotencyKey.length > 0) {
+          const personId = 'test-person';
+          const cached = request.server.idempotency.check(personId, idempotencyKey);
+          if (cached) {
+            return reply.code(cached.status).send(cached.body);
+          }
 
-        const body = ok({ echoed: idempotencyKey, at: new Date().toISOString() });
-        request.server.idempotency.store(personId, idempotencyKey, { status: 200, body });
-        return reply.code(200).send(body);
-      }
-      return ok({ echoed: null });
-    },
-  );
+          const body = ok({ echoed: idempotencyKey, at: new Date().toISOString() });
+          request.server.idempotency.store(personId, idempotencyKey, { status: 200, body });
+          return reply.code(200).send(body);
+        }
+        return ok({ echoed: null });
+      },
+    );
+  }
 }
diff --git a/apps/api/tests/api-skeleton.test.ts b/apps/api/tests/api-skeleton.test.ts
index 0562f8d..142c2b9 100644
--- a/apps/api/tests/api-skeleton.test.ts
+++ b/apps/api/tests/api-skeleton.test.ts
@@ -127,6 +127,43 @@ describe('error mapper', () => {
   });
 });
 
+// ---------------------------------------------------------------------------
+// /api/_test/* gating (issue #116)
+//
+// The test-harness routes exist only to exercise the error-mapping +
+// idempotency code paths from CI. In NODE_ENV=production they must NOT
+// be registered — no reason a prod caller should be able to hit
+// /api/_test/internal-error and force a 500.
+// ---------------------------------------------------------------------------
+
+describe('/api/_test/* route gating', () => {
+  it('returns 404 for all three test-harness routes when NODE_ENV=production', async () => {
+    // Close the default (NODE_ENV=test) app so the prod-mode app gets
+    // a clean fixture. The base afterEach takes care of the rest.
+    if (app) {
+      await app.close();
+      app = undefined;
+    }
+    const prodApp = await buildTestApp({ NODE_ENV: 'production' });
+    try {
+      const paths = [
+        '/api/_test/validation-error',
+        '/api/_test/internal-error',
+        '/api/_test/idempotency',
+      ];
+      for (const url of paths) {
+        const res = await prodApp.inject({ method: 'POST', url });
+        expect(res.statusCode, `expected ${url} to 404 in production`).toBe(404);
+      }
+      // Sanity: real routes still respond.
+      const health = await prodApp.inject({ method: 'GET', url: '/api/health' });
+      expect(health.statusCode).toBe(200);
+    } finally {
+      await prodApp.close();
+    }
+  });
+});
+
 // ---------------------------------------------------------------------------
 // Rate limiting
 // ---------------------------------------------------------------------------