diff --git a/test/github-installation-token-stateless-format.test.ts b/test/github-installation-token-stateless-format.test.ts
new file mode 100644
index 000000000..5662ef471
--- /dev/null
+++ b/test/github-installation-token-stateless-format.test.ts
@@ -0,0 +1,184 @@
+import { afterEach, beforeAll, describe, expect, it, vi } from "vitest";
+import worker, { type Env } from "../src/index";
+
+/**
+ * GitHub's stateless installation tokens retain the `ghs_` prefix, are around
+ * 520 characters, and carry a JWT-shaped suffix with two dot separators.
+ * Clients are expected to treat that suffix as opaque rather than decoding or
+ * validating its claims. `createInstallationToken` in `src/index.ts` already
+ * accepts printable ASCII up to 4096 characters, so this regression exercises
+ * the changed transport shape without adding a second token parser.
+ */
+
+const configuredWorkflowRef =
+  "ContextualWisdomLab/.github/.github/workflows/noema-review.yml@refs/heads/main";
+const configuredWorkflowSha = "a".repeat(40);
+const targetRepository = "ContextualWisdomLab/installation-token-stateless-format";
+const installationId = "93001";
+const signingKid = "installation-token-stateless-format";
+
+/** Representative 520-character `ghs_APPID_JWT` value, intentionally opaque to Noema. */
+const statelessFormatToken =
+  `ghs_12345_${"A".repeat(80)}.${"B".repeat(300)}.${"C".repeat(128)}`;
+
+/** Return a replay-guard namespace that accepts every claim, as a real first-use would. */
+function acceptingReplayGuard(): DurableObjectNamespace {
+  return {
+    idFromName(name: string) {
+      return { toString: () => name } as DurableObjectId;
+    },
+    get() {
+      return {
+        fetch: async (_input: RequestInfo | URL, init?: RequestInit) => {
+          const body = JSON.parse(String(init?.body ?? "{}"));
+          return Response.json(
+            { accepted: true, expires_at_epoch_seconds: body.expires_at_epoch_seconds },
+            { status: 201 },
+          );
+        },
+      } as unknown as DurableObjectStub;
+    },
+  } as unknown as DurableObjectNamespace;
+}
+
+const env: Env = {
+  ALLOWED_ISSUER: "https://token.actions.githubusercontent.com",
+  ALLOWED_AUDIENCE: "cwl-noema-review",
+  ALLOWED_REPOSITORY_OWNER: "ContextualWisdomLab",
+  ALLOWED_WORKFLOW_REPOSITORY: "ContextualWisdomLab/.github",
+  ALLOWED_WORKFLOW_REF_PREFIX: configuredWorkflowRef,
+  ALLOWED_WORKFLOW_SHA: configuredWorkflowSha,
+  GITHUB_API_BASE: "https://api.github.com",
+  GITHUB_APP_ID: "1",
+  GITHUB_APP_PRIVATE_KEY_PEM: "initialized-in-beforeAll",
+  NOEMA_RATE_LIMIT_PER_MINUTE: "1000",
+  NOEMA_OIDC_REPLAY_GUARD: acceptingReplayGuard(),
+};
+
+let oidcKeyPair: CryptoKeyPair;
+let oidcPublicJwk: JsonWebKey;
+let appPrivateKeyPem: string;
+
+function encodeSegment(value: unknown): string {
+  return Buffer.from(JSON.stringify(value)).toString("base64url");
+}
+
+function encodeBytes(bytes: ArrayBuffer): string {
+  return Buffer.from(bytes).toString("base64url");
+}
+
+function pemFromPkcs8(pkcs8: ArrayBuffer): string {
+  const base64 = Buffer.from(pkcs8).toString("base64");
+  const lines = base64.match(/.{1,64}/g)?.join("\n") ?? base64;
+  return `-----BEGIN PRIVATE KEY-----\n${lines}\n-----END PRIVATE KEY-----`;
+}
+
+async function generateRsaKeyPair(): Promise<CryptoKeyPair> {
+  return crypto.subtle.generateKey(
+    {
+      name: "RSASSA-PKCS1-v1_5",
+      modulusLength: 2048,
+      publicExponent: new Uint8Array([1, 0, 1]),
+      hash: "SHA-256",
+    },
+    true,
+    ["sign", "verify"],
+  );
+}
+
+async function signedOidcToken(): Promise<string> {
+  const now = Math.floor(Date.now() / 1000);
+  const header = encodeSegment({ alg: "RS256", kid: signingKid, typ: "JWT" });
+  const payload = encodeSegment({
+    iss: env.ALLOWED_ISSUER,
+    aud: env.ALLOWED_AUDIENCE,
+    repository_owner: env.ALLOWED_REPOSITORY_OWNER,
+    repository_owner_id: "295022177",
+    repository: "ContextualWisdomLab/.github",
+    repository_id: "1274066402",
+    job_workflow_ref: configuredWorkflowRef,
+    job_workflow_sha: configuredWorkflowSha,
+    sub: "repo:ContextualWisdomLab/.github:ref:refs/heads/main",
+    jti: crypto.randomUUID(),
+    exp: now + 300,
+    nbf: now - 30,
+    iat: now - 30,
+  });
+  const signature = await crypto.subtle.sign(
+    "RSASSA-PKCS1-v1_5",
+    oidcKeyPair.privateKey,
+    new TextEncoder().encode(`${header}.${payload}`),
+  );
+  return `${header}.${payload}.${encodeBytes(signature)}`;
+}
+
+async function exchange(clientIp: string): Promise<Response> {
+  return worker.fetch(
+    new Request("https://noema.example/exchange", {
+      method: "POST",
+      headers: {
+        authorization: `Bearer ${await signedOidcToken()}`,
+        "content-type": "application/json",
+        "cf-connecting-ip": clientIp,
+      },
+      body: JSON.stringify({ target_repository: targetRepository }),
+    }),
+    { ...env, GITHUB_APP_PRIVATE_KEY_PEM: appPrivateKeyPem },
+  );
+}
+
+beforeAll(async () => {
+  oidcKeyPair = await generateRsaKeyPair();
+  oidcPublicJwk = await crypto.subtle.exportKey("jwk", oidcKeyPair.publicKey);
+  const appKeyPair = await generateRsaKeyPair();
+  appPrivateKeyPem = pemFromPkcs8(
+    await crypto.subtle.exportKey("pkcs8", appKeyPair.privateKey),
+  );
+});
+
+afterEach(() => {
+  vi.restoreAllMocks();
+});
+
+describe("GitHub installation-token stateless format", () => {
+  it("round-trips a representative 520-character stateless token without inspecting it", async () => {
+    expect(statelessFormatToken.length).toBe(520);
+    expect(statelessFormatToken.match(/\./g)).toHaveLength(2);
+
+    vi.spyOn(globalThis, "fetch").mockImplementation(async (input) => {
+      const url = String(input);
+      if (url === "https://token.actions.githubusercontent.com/.well-known/openid-configuration") {
+        return Response.json({
+          jwks_uri: "https://token.actions.githubusercontent.com/.well-known/jwks",
+        });
+      }
+      if (url === "https://token.actions.githubusercontent.com/.well-known/jwks") {
+        return Response.json({
+          keys: [{ ...oidcPublicJwk, kid: signingKid, kty: "RSA" }],
+        });
+      }
+      if (url === `https://api.github.com/repos/${targetRepository}/installation`) {
+        return Response.json({ id: Number(installationId) });
+      }
+      if (url === `https://api.github.com/app/installations/${installationId}/access_tokens`) {
+        return Response.json({
+          token: statelessFormatToken,
+          expires_at: new Date(Date.now() + 60 * 60_000).toISOString(),
+        }, { status: 201 });
+      }
+      return new Response("unexpected privileged egress", { status: 500 });
+    });
+
+    const response = await exchange("203.0.113.220");
+
+    expect(response.status).toBe(200);
+    const payload = await response.json();
+    expect(payload).toMatchObject({
+      ok: true,
+      data: {
+        repository: targetRepository,
+        token: statelessFormatToken,
+      },
+    });
+  });
+});