diff --git a/apps/api/src/auth/introspection.ts b/apps/api/src/auth/introspection.ts
new file mode 100644
index 0000000..667a4f8
--- /dev/null
+++ b/apps/api/src/auth/introspection.ts
@@ -0,0 +1,80 @@
+import type { FastifyBaseLogger } from "fastify";
+
+export interface IntrospectionResult {
+  active: boolean;
+  sub?: string;
+  aud?: string[] | string;
+  scope?: string;
+  reason?: string;
+}
+
+export interface IntrospectionDeps {
+  issuer: string;
+  clientId: string;
+  clientSecret: string;
+  expectedAudience?: string;
+  fetchImpl?: typeof fetch;
+  timeoutMs?: number;
+  logger?: FastifyBaseLogger;
+}
+
+function audienceMatches(result: IntrospectionResult, expected: string): boolean {
+  if (result.aud === undefined) {
+    // RFC 7662: aud may be absent; only an asserted aud is enforced.
+    return true;
+  }
+  const aud = Array.isArray(result.aud) ? result.aud : [result.aud];
+  return aud.includes(expected);
+}
+
+export function createIntrospector(deps: IntrospectionDeps) {
+  const fetchImpl = deps.fetchImpl ?? fetch;
+  const timeoutMs = deps.timeoutMs ?? 3000;
+  const basic = Buffer.from(`${deps.clientId}:${deps.clientSecret}`).toString("base64");
+  const endpoint = `${deps.issuer.replace(/\/$/, "")}/oauth/v2/introspect`;
+
+  // Fail-closed by construction: every non-active outcome returns active=false
+  // with a reason, and callers must treat any reason as "no access".
+  return async function introspect(token: string): Promise<IntrospectionResult> {
+    let response: Response;
+    try {
+      response = await fetchImpl(endpoint, {
+        method: "POST",
+        headers: {
+          Authorization: `Basic ${basic}`,
+          "Content-Type": "application/x-www-form-urlencoded",
+        },
+        body: new URLSearchParams({ token }),
+        signal: AbortSignal.timeout(timeoutMs),
+      });
+    } catch (err) {
+      deps.logger?.warn({ err }, "zitadel introspection unreachable — fail-closed");
+      return { active: false, reason: "introspection_unreachable" };
+    }
+
+    if (!response.ok) {
+      // Zitadel returns 400 for bad client auth; any non-200 denies.
+      deps.logger?.warn({ status: response.status }, "zitadel introspection non-200 — fail-closed");
+      return { active: false, reason: `introspection_http_${response.status}` };
+    }
+
+    let body: IntrospectionResult;
+    try {
+      body = (await response.json()) as IntrospectionResult;
+    } catch (err) {
+      deps.logger?.warn({ err }, "zitadel introspection invalid JSON — fail-closed");
+      return { active: false, reason: "introspection_invalid_response" };
+    }
+
+    if (body.active !== true) {
+      return { active: false, reason: "token_inactive" };
+    }
+
+    if (deps.expectedAudience && !audienceMatches(body, deps.expectedAudience)) {
+      deps.logger?.warn({ sub: body.sub }, "zitadel introspection audience mismatch — fail-closed");
+      return { active: false, reason: "audience_mismatch" };
+    }
+
+    return { active: true, sub: body.sub, aud: body.aud, scope: body.scope };
+  };
+}
diff --git a/apps/api/src/index.ts b/apps/api/src/index.ts
index 9187b0e..b4c877a 100644
--- a/apps/api/src/index.ts
+++ b/apps/api/src/index.ts
@@ -1,5 +1,6 @@
 import Fastify from "fastify";
 import { envSchema } from "@siax/c0py-config";
+import { createIntrospector } from "./auth/introspection.js";
 
 async function main() {
   const env = envSchema.parse(process.env);
@@ -11,6 +12,24 @@ async function main() {
     timestamp: new Date().toISOString(),
   }));
 
+  const introspectionConfigured =
+    env.ZITADEL_INTROSPECTION_CLIENT_ID !== undefined &&
+    env.ZITADEL_INTROSPECTION_CLIENT_SECRET !== undefined;
+  if (!introspectionConfigured && env.NODE_ENV === "production") {
+    server.log.warn(
+      "ZITADEL_INTROSPECTION_CLIENT_ID/SECRET not set — running Bearer-presence-only auth",
+    );
+  }
+  const introspect = introspectionConfigured
+    ? createIntrospector({
+        issuer: env.ZITADEL_ISSUER,
+        clientId: env.ZITADEL_INTROSPECTION_CLIENT_ID as string,
+        clientSecret: env.ZITADEL_INTROSPECTION_CLIENT_SECRET as string,
+        expectedAudience: env.ZITADEL_EXPECTED_AUDIENCE,
+        logger: server.log,
+      })
+    : null;
+
   server.addHook("preHandler", async (request, reply) => {
     if (request.url === "/health") return;
     const auth = request.headers.authorization;
@@ -18,6 +37,18 @@ async function main() {
       await reply.code(401).send({ error: "Unauthorized" });
       return;
     }
+    if (introspect) {
+      const token = auth.slice("Bearer ".length).trim();
+      const result = await introspect(token);
+      if (!result.active) {
+        await reply
+          .code(401)
+          .header("WWW-Authenticate", `Bearer error="invalid_token"`)
+          .send({ error: "Unauthorized" });
+        return;
+      }
+      (request as never as { user?: { sub?: string } }).user = { sub: result.sub };
+    }
   });
 
   server.get("/", async () => ({
diff --git a/apps/api/tests/auth-flow.test.ts b/apps/api/tests/auth-flow.test.ts
new file mode 100644
index 0000000..717cdb4
--- /dev/null
+++ b/apps/api/tests/auth-flow.test.ts
@@ -0,0 +1,90 @@
+import { describe, it, expect, vi } from "vitest";
+import Fastify from "fastify";
+import { envSchema } from "@siax/c0py-config";
+import { createIntrospector } from "../src/auth/introspection.js";
+import type { IntrospectionDeps } from "../src/auth/introspection.js";
+
+const env = envSchema.parse({
+  CL0UD_BASE_URL: "https://cl0ud.siax.io",
+  ZITADEL_ISSUER: "https://id-customers.siax.io",
+  ZITADEL_AUDIENCE: "c0py-api.siax.io",
+  DATABASE_URL: "postgresql://localhost:5432/c0py",
+  ZITADEL_INTROSPECTION_CLIENT_ID: "cid",
+  ZITADEL_INTROSPECTION_CLIENT_SECRET: "csec",
+  ZITADEL_EXPECTED_AUDIENCE: "proj-1",
+  NODE_ENV: "test",
+});
+
+function mockIntrospect(body: unknown) {
+  return vi.fn().mockResolvedValue(body);
+}
+
+function buildApp(introspectImpl: (token: string) => Promise<unknown>) {
+  const server = Fastify({ logger: false });
+  server.get("/health", async () => ({ status: "ok" }));
+  server.addHook("preHandler", async (request, reply) => {
+    if (request.url === "/health") return;
+    const auth = request.headers.authorization;
+    if (!auth || !auth.startsWith("Bearer ")) {
+      await reply.code(401).send({ error: "Unauthorized" });
+      return;
+    }
+    const result = (await introspectImpl(auth.slice("Bearer ".length).trim())) as {
+      active: boolean;
+      sub?: string;
+    };
+    if (!result.active) {
+      await reply.code(401).send({ error: "Unauthorized" });
+      return;
+    }
+    (request as never as { user?: { sub?: string } }).user = { sub: result.sub };
+  });
+  server.get("/v1/c0py/registries", async (request) => ({
+    user: (request as never as { user?: { sub?: string } }).user,
+  }));
+  return server;
+}
+
+describe("preHandler auth (fail-closed)", () => {
+  it("401 without Bearer", async () => {
+    const app = buildApp(mockIntrospect({ active: true }));
+    const res = await app.inject({ method: "GET", url: "/v1/c0py/registries" });
+    expect(res.statusCode).toBe(401);
+  });
+
+  it("exempts /health", async () => {
+    const app = buildApp(mockIntrospect({ active: false }));
+    const res = await app.inject({ method: "GET", url: "/health" });
+    expect(res.statusCode).toBe(200);
+  });
+
+  it("401 when introspection denies", async () => {
+    const app = buildApp(mockIntrospect({ active: false, reason: "audience_mismatch" }));
+    const res = await app.inject({
+      method: "GET",
+      url: "/v1/c0py/registries",
+      headers: { Authorization: "Bearer tok" },
+    });
+    expect(res.statusCode).toBe(401);
+  });
+
+  it("200 with valid token and sub propagated", async () => {
+    const app = buildApp(mockIntrospect({ active: true, sub: "svc-user" }));
+    const res = await app.inject({
+      method: "GET",
+      url: "/v1/c0py/registries",
+      headers: { Authorization: "Bearer tok" },
+    });
+    expect(res.statusCode).toBe(200);
+    expect(res.json()).toEqual({ user: { sub: "svc-user" } });
+  });
+
+  it("env schema accepts new introspection vars", () => {
+    expect(env.ZITADEL_INTROSPECTION_CLIENT_ID).toBe("cid");
+    expect(env.ZITADEL_EXPECTED_AUDIENCE).toBe("proj-1");
+  });
+
+  it("createIntrospector is importable and typed", async () => {
+    expect(typeof createIntrospector).toBe("function");
+  });
+});
diff --git a/apps/api/tests/introspection.test.ts b/apps/api/tests/introspection.test.ts
new file mode 100644
index 0000000..16476ea
--- /dev/null
+++ b/apps/api/tests/introspection.test.ts
@@ -0,0 +1,97 @@
+import { describe, it, expect, vi, afterEach } from "vitest";
+import { createIntrospector } from "../src/auth/introspection.js";
+import type { IntrospectionDeps } from "../src/auth/introspection.js";
+
+function mockFetch(status: number, body: unknown) {
+  return vi.fn().mockResolvedValue(
+    new Response(JSON.stringify(body), {
+      status,
+      headers: { "Content-Type": "application/json" },
+    }),
+  );
+}
+
+const baseDeps: IntrospectionDeps = {
+  issuer: "https://id-customers.siax.io",
+  clientId: "cid",
+  clientSecret: "csec",
+};
+
+afterEach(() => {
+  vi.restoreAllMocks();
+});
+
+describe("createIntrospector", () => {
+  it("accepts an active token", async () => {
+    const fetchImpl = mockFetch(200, { active: true, sub: "u1", aud: ["cid"] });
+    const introspect = createIntrospector({ ...baseDeps, fetchImpl });
+    const r = await introspect("tok");
+    expect(r.active).toBe(true);
+    expect(r.sub).toBe("u1");
+    expect(fetchImpl).toHaveBeenCalledWith(
+      "https://id-customers.siax.io/oauth/v2/introspect",
+      expect.objectContaining({ method: "POST" }),
+    );
+  });
+
+  it("rejects an inactive token (fail-closed)", async () => {
+    const introspect = createIntrospector({ ...baseDeps, fetchImpl: mockFetch(200, { active: false }) });
+    const r = await introspect("tok");
+    expect(r.active).toBe(false);
+    expect(r.reason).toBe("token_inactive");
+  });
+
+  it("fails closed on non-200 (Zitadel returns 400 on bad client auth)", async () => {
+    const introspect = createIntrospector({ ...baseDeps, fetchImpl: mockFetch(400, {}) });
+    const r = await introspect("tok");
+    expect(r.active).toBe(false);
+    expect(r.reason).toBe("introspection_http_400");
+  });
+
+  it("fails closed when introspection is unreachable", async () => {
+    const fetchImpl = vi.fn().mockRejectedValue(new Error("fetch failed"));
+    const introspect = createIntrospector({ ...baseDeps, fetchImpl });
+    const r = await introspect("tok");
+    expect(r.active).toBe(false);
+    expect(r.reason).toBe("introspection_unreachable");
+  });
+
+  it("fails closed on invalid JSON", async () => {
+    const fetchImpl = vi.fn().mockResolvedValue(
+      new Response("<html>not json</html>", { status: 200 }),
+    );
+    const introspect = createIntrospector({ ...baseDeps, fetchImpl });
+    const r = await introspect("tok");
+    expect(r.active).toBe(false);
+    expect(r.reason).toBe("introspection_invalid_response");
+  });
+
+  it("accepts a token without aud (RFC 7662)", async () => {
+    const introspect = createIntrospector({
+      ...baseDeps,
+      expectedAudience: "proj-1",
+      fetchImpl: mockFetch(200, { active: true, sub: "u1" }),
+    });
+    const r = await introspect("tok");
+    expect(r.active).toBe(true);
+  });
+
+  it("enforces asserted aud against expected audience", async () => {
+    const mismatch = createIntrospector({
+      ...baseDeps,
+      expectedAudience: "proj-1",
+      fetchImpl: mockFetch(200, { active: true, aud: ["other", "cid"] }),
+    });
+    const r = await mismatch("tok");
+    expect(r.active).toBe(false);
+    expect(r.reason).toBe("audience_mismatch");
+
+    const match = createIntrospector({
+      ...baseDeps,
+      expectedAudience: "proj-1",
+      fetchImpl: mockFetch(200, { active: true, aud: ["cid", "proj-1"] }),
+    });
+    const r2 = await match("tok");
+    expect(r2.active).toBe(true);
+  });
+});
diff --git a/packages/config/src/index.ts b/packages/config/src/index.ts
index d3f563d..71ea29f 100644
--- a/packages/config/src/index.ts
+++ b/packages/config/src/index.ts
@@ -7,6 +7,9 @@ export const envSchema = z.object({
   ZITADEL_ISSUER: z.string().url(),
   ZITADEL_AUDIENCE: z.string(),
   DATABASE_URL: z.string(),
+  ZITADEL_INTROSPECTION_CLIENT_ID: z.string().optional(),
+  ZITADEL_INTROSPECTION_CLIENT_SECRET: z.string().optional(),
+  ZITADEL_EXPECTED_AUDIENCE: z.string().optional(),
   AUD0_BASE_URL: z.string().url().optional(),
   AUD0_API_KEY: z.string().optional(),
   ST0RE_BASE_URL: z.string().url().optional(),