diff --git a/src/lib/demo-data.ts b/src/lib/demo-data.ts
index 7b0f7e49f..9929c1789 100644
--- a/src/lib/demo-data.ts
+++ b/src/lib/demo-data.ts
@@ -331,7 +331,25 @@ function scoreChunk(query: string, chunk: SearchResult) {
   return Math.min(0.98, chunk.similarity * 0.6 + keywordScore + tokenScore);
 }
 
+// Fail closed: synthetic demo content must never reach a live production request. Reads
+// process.env directly (not isDemoMode) so it cannot be silently disabled by a test that mocks
+// @/lib/env, and so every current/future caller of demoSearch is covered — this is what prevents
+// a recurrence of the /api/search/universal leak. In production isLocalNoAuthMode and the
+// Supabase-config demo fallback are already disabled, so an explicit NEXT_PUBLIC_DEMO_MODE=true
+// deploy is the sole legitimate case. Mirrors the fail-closed requireOwnerScope retrieval guard.
+function assertDemoDataAllowed(context: string) {
+  if (process.env.NODE_ENV === "production" && process.env.NEXT_PUBLIC_DEMO_MODE !== "true") {
+    throw new Error(
+      `Refusing to serve synthetic demo data (${context}) to a live production request. ` +
+        "Demo fixtures are only permitted when NEXT_PUBLIC_DEMO_MODE=true.",
+    );
+  }
+}
+
 export function demoSearch(query: string, topK = 8, documentId?: string, documentIds?: string[]) {
+  // Single choke point for synthetic document search (demoAnswer routes through here too), so the
+  // guard covers any current or future caller, not just the routes audited today.
+  assertDemoDataAllowed("demoSearch");
   const filters = documentIds?.length ? documentIds : documentId ? [documentId] : null;
   return demoChunks
     .filter((chunk) => !filters || filters.includes(chunk.document_id))
diff --git a/tests/demo-data.test.ts b/tests/demo-data.test.ts
index a932c5823..677f6f6d1 100644
--- a/tests/demo-data.test.ts
+++ b/tests/demo-data.test.ts
@@ -1,4 +1,4 @@
-import { describe, expect, it } from "vitest";
+import { afterEach, describe, expect, it, vi } from "vitest";
 import { demoAnswer, demoDocuments, demoSearch, getDemoDocumentPayload } from "../src/lib/demo-data";
 
 describe("demo data mode", () => {
@@ -83,3 +83,25 @@ describe("demo data mode", () => {
     expect(payload?.images[0].caption).toContain("monitoring table");
   });
 });
+
+// Class-level guard so a future route cannot reintroduce the /api/search/universal leak: demo
+// fixtures must fail closed in a live production request and only run under an explicit demo deploy.
+describe("demoSearch production safety guard", () => {
+  afterEach(() => {
+    vi.unstubAllEnvs();
+    vi.resetModules();
+  });
+
+  it("refuses to serve synthetic demo data to a live production request", async () => {
+    vi.stubEnv("NODE_ENV", "production");
+    const { demoSearch: guarded } = await import("../src/lib/demo-data");
+    expect(() => guarded("lithium toxicity")).toThrow(/synthetic demo data/i);
+  });
+
+  it("still serves fixtures for an explicit NEXT_PUBLIC_DEMO_MODE=true deploy", async () => {
+    vi.stubEnv("NODE_ENV", "production");
+    vi.stubEnv("NEXT_PUBLIC_DEMO_MODE", "true");
+    const { demoSearch: guarded } = await import("../src/lib/demo-data");
+    expect(() => guarded("lithium toxicity")).not.toThrow();
+  });
+});