diff --git a/README.md b/README.md
index dfae1e0..3d09c3a 100644
--- a/README.md
+++ b/README.md
@@ -179,6 +179,8 @@ binary execution. The compatibility record is in
 - [smoke-contextstream.mjs](smoke-contextstream.mjs) — real stdio MCP and
   hosted-grounding smoke test
 - [demo-script.md](demo-script.md) — reproducible 60–90 second flagship demo
+- [demo/fixture](demo/fixture) — disposable signed-invitation repository used
+  for the Claude-to-Codex continuity proof
 - [community-runbook.md](community-runbook.md) — public ContextStream Builders
   community launch checklist
 - [block-outreach.md](block-outreach.md) — concise proof-first outreach and
diff --git a/demo-script.md b/demo-script.md
index f08a01f..ce9422b 100644
--- a/demo-script.md
+++ b/demo-script.md
@@ -4,8 +4,12 @@ Target length: 75 seconds. This is a continuity proof, not a feature tour.
 
 ## Before recording
 
-- Create one ContextStream project with a small repository, a product
-  requirement, one architecture decision, and one known constraint.
+- Copy [demo/fixture](demo/fixture) into a disposable working directory and
+  initialize it as a fresh Git repository. The baseline deliberately verifies
+  token signatures without enforcing the ContextStream-backed expiry policy.
+- Create one ContextStream project for that fixture with a product requirement,
+  one architecture decision, and one known constraint. Label all records as
+  synthetic demo knowledge, not production guidance.
 - Start a Claude Code Buzz agent and a Codex Buzz agent through
   [run-agent.sh](run-agent.sh), each with a distinct Buzz identity and the same
   ContextStream project.
diff --git a/demo/fixture/README.md b/demo/fixture/README.md
new file mode 100644
index 0000000..ed072a2
--- /dev/null
+++ b/demo/fixture/README.md
@@ -0,0 +1,19 @@
+# Invitation continuity fixture
+
+This is the synthetic repository used by the flagship ContextStream × Buzz
+demonstration. Copy it to a disposable directory before each run; do not let a
+recording mutate this baseline.
+
+The fixture issues HMAC-signed invitations and verifies their signatures. It
+intentionally omits expiry enforcement so Claude can start the implementation
+from approved ContextStream requirements. After a human approves a changed
+admin-invitation policy, Codex must retrieve that durable decision in another
+Buzz room and finish the implementation without being rebriefed.
+
+Run the baseline tests with:
+
+```bash
+npm test
+```
+
+Everything here is demo-only and must not be treated as production guidance.
diff --git a/demo/fixture/package.json b/demo/fixture/package.json
new file mode 100644
index 0000000..10d824d
--- /dev/null
+++ b/demo/fixture/package.json
@@ -0,0 +1,8 @@
+{
+  "name": "contextstream-buzz-invitation-fixture",
+  "private": true,
+  "type": "module",
+  "scripts": {
+    "test": "node --test"
+  }
+}
diff --git a/demo/fixture/src/invitations.mjs b/demo/fixture/src/invitations.mjs
new file mode 100644
index 0000000..dec932d
--- /dev/null
+++ b/demo/fixture/src/invitations.mjs
@@ -0,0 +1,43 @@
+import { createHmac, timingSafeEqual } from "node:crypto";
+
+function sign(encodedPayload, secret) {
+  return createHmac("sha256", secret).update(encodedPayload).digest("base64url");
+}
+
+export function issueInvitation({
+  role = "member",
+  issuedAtMs,
+  nonce,
+  secret,
+}) {
+  const payload = Buffer.from(
+    JSON.stringify({ role, issuedAtMs, nonce }),
+    "utf8",
+  ).toString("base64url");
+
+  return `${payload}.${sign(payload, secret)}`;
+}
+
+export function verifyInvitation(token, { secret, nowMs }) {
+  const [payload, suppliedSignature] = token.split(".");
+  if (!payload || !suppliedSignature) {
+    return { valid: false, reason: "malformed" };
+  }
+
+  const expectedSignature = sign(payload, secret);
+  const supplied = Buffer.from(suppliedSignature);
+  const expected = Buffer.from(expectedSignature);
+  if (
+    supplied.length !== expected.length ||
+    !timingSafeEqual(supplied, expected)
+  ) {
+    return { valid: false, reason: "invalid_signature" };
+  }
+
+  const claims = JSON.parse(Buffer.from(payload, "base64url").toString("utf8"));
+
+  // Deliberately incomplete. Demo agents must retrieve and implement the
+  // approved expiry policy from the shared ContextStream project.
+  void nowMs;
+  return { valid: true, claims };
+}
diff --git a/demo/fixture/test/invitations.test.mjs b/demo/fixture/test/invitations.test.mjs
new file mode 100644
index 0000000..e576b1f
--- /dev/null
+++ b/demo/fixture/test/invitations.test.mjs
@@ -0,0 +1,39 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+
+import { issueInvitation, verifyInvitation } from "../src/invitations.mjs";
+
+const secret = "demo-secret";
+const issuedAtMs = Date.UTC(2026, 7, 5, 12, 0, 0);
+
+test("accepts an untampered signed invitation", () => {
+  const token = issueInvitation({
+    role: "member",
+    issuedAtMs,
+    nonce: "invite-1",
+    secret,
+  });
+
+  const result = verifyInvitation(token, {
+    secret,
+    nowMs: issuedAtMs + 1_000,
+  });
+
+  assert.equal(result.valid, true);
+  assert.equal(result.claims.role, "member");
+});
+
+test("rejects a tampered invitation", () => {
+  const token = issueInvitation({
+    role: "member",
+    issuedAtMs,
+    nonce: "invite-2",
+    secret,
+  });
+  const tampered = `${token.slice(0, -1)}x`;
+
+  assert.deepEqual(
+    verifyInvitation(tampered, { secret, nowMs: issuedAtMs + 1_000 }),
+    { valid: false, reason: "invalid_signature" },
+  );
+});
diff --git a/package.json b/package.json
index e506415..8bedf7f 100644
--- a/package.json
+++ b/package.json
@@ -5,7 +5,7 @@
   "type": "module",
   "description": "Shared ContextStream project context for agents in Buzz",
   "scripts": {
-    "test": "node --test reference.test.mjs",
+    "test": "node --test reference.test.mjs demo/fixture/test/invitations.test.mjs",
     "smoke": "node smoke-contextstream.mjs"
   },
   "engines": {