diff --git a/docs/CONFIGURATION.md b/docs/CONFIGURATION.md
index 7519bb1a..06b82c48 100644
--- a/docs/CONFIGURATION.md
+++ b/docs/CONFIGURATION.md
@@ -146,6 +146,13 @@ ANTHROPIC_API_KEY=sk-ant-...            # if set, clusters get LLM-quality label
 GOOGLE_CLIENT_ID=...                    # Google IdP client id
 GOOGLE_CLIENT_SECRET=...                # Google IdP client secret
 CODEOID_OAUTH_CLIENT_ID=codeoid          # optional: OAuth client id (default "codeoid")
+# The Google ID token is exchanged at ZeroID for a ZeroID token. ZeroID v1.12+
+# only accepts that exchange from an authenticated client the ID token was
+# issued to, so register a confidential client at ZeroID whose client_id is
+# GOOGLE_CLIENT_ID (token_endpoint_auth_method client_secret_post, with the
+# Google issuer configured for that audience) and set its secret here.
+CODEOID_ZEROID_CLIENT_ID=...             # optional: defaults to GOOGLE_CLIENT_ID
+CODEOID_ZEROID_CLIENT_SECRET=...         # env only; required by ZeroID v1.12+
 
 # Telegram frontend
 TELEGRAM_BOT_TOKEN=...
diff --git a/src/config.ts b/src/config.ts
index 86702d66..8dd7c2c8 100644
--- a/src/config.ts
+++ b/src/config.ts
@@ -564,6 +564,14 @@ const AuthSchemaFields = z
 const OAuthSchemaFields = z
   .object({
     clientId: z.string().optional(),
+    /**
+     * client_id codeoid authenticates as at ZeroID for the id_token exchange.
+     * ZeroID only lets the relying party an ID token was issued to redeem it
+     * (aud/azp), so this must be the client the IdP issued the ID token to.
+     * Defaults to GOOGLE_CLIENT_ID. Its secret comes from
+     * CODEOID_ZEROID_CLIENT_SECRET (env only, never config.json).
+     */
+    zeroidClientId: z.string().optional(),
   })
   .default({});
 
@@ -1542,6 +1550,7 @@ const ENV_OVERRIDES: readonly EnvOverride[] = [
   { env: "ZEROID_ISSUER", path: "auth.issuer", kind: "string" },
   { env: "ZEROID_AUDIENCE", path: "auth.audience", kind: "string" },
   { env: "CODEOID_OAUTH_CLIENT_ID", path: "oauth.clientId", kind: "string" },
+  { env: "CODEOID_ZEROID_CLIENT_ID", path: "oauth.zeroidClientId", kind: "string" },
   { env: "ZEROID_ACCOUNT_ID", path: "agentIdentity.accountId", kind: "string" },
   { env: "ZEROID_PROJECT_ID", path: "agentIdentity.projectId", kind: "string" },
   { env: "ZEROID_REGISTRAR_KEY", path: "agentIdentity.registrarKey", kind: "string" },
@@ -1731,6 +1740,8 @@ export function loadConfig(opts: LoadOptions = {}): CodeoidConfig {
     ? {
         zeroidTokenEndpoint: `${resolvedZeroidUrl}/oauth2/token`,
         clientId: parsed.oauth.clientId ?? "codeoid",
+        zeroidClientId: parsed.oauth.zeroidClientId ?? googleClientId,
+        zeroidClientSecret: env.CODEOID_ZEROID_CLIENT_SECRET || undefined,
         googleClientId,
         googleClientSecret,
         accountId: parsed.agentIdentity.accountId,
diff --git a/src/daemon/oauth.ts b/src/daemon/oauth.ts
index 5253c6a4..a8c17962 100644
--- a/src/daemon/oauth.ts
+++ b/src/daemon/oauth.ts
@@ -21,6 +21,15 @@ export interface OAuthConfig {
   zeroidTokenEndpoint: string;
   /** Registered OAuth client_id (used to identify this codeoid instance) */
   clientId: string;
+  /**
+   * client_id codeoid authenticates as at ZeroID's id_token exchange. ZeroID
+   * (v1.12+) requires an authenticated client and only accepts an ID token
+   * issued to that client (aud/azp), so this is the IdP client the ID token
+   * names: the Google OAuth client id by default.
+   */
+  zeroidClientId: string;
+  /** Secret for zeroidClientId (client_secret_post). Unset sends no secret. */
+  zeroidClientSecret?: string;
   /** Google OAuth client ID — passed to GoogleOAuthProvider */
   googleClientId: string;
   /** Google OAuth client secret — passed to GoogleOAuthProvider */
@@ -266,11 +275,14 @@ export class OAuthHandler {
       grant_type: "urn:ietf:params:oauth:grant-type:token-exchange",
       subject_token_type: "urn:ietf:params:oauth:token-type:id_token",
       subject_token: user.rawIdToken,
-      client_id: this.#config.clientId,
+      client_id: this.#config.zeroidClientId,
       account_id: this.#config.accountId,
       project_id: this.#config.projectId,
       scope,
     });
+    if (this.#config.zeroidClientSecret) {
+      body.set("client_secret", this.#config.zeroidClientSecret);
+    }
 
     const controller = new AbortController();
     const timeout = setTimeout(() => controller.abort(), 10_000);
@@ -288,7 +300,11 @@ export class OAuthHandler {
 
     if (!resp.ok) {
       const text = await resp.text().catch(() => "");
-      throw new Error(`ZeroID token exchange failed (${resp.status}): ${text.slice(0, 200)}`);
+      const hint =
+        resp.status === 401 && !this.#config.zeroidClientSecret
+          ? " — ZeroID requires client authentication for the id_token exchange; set CODEOID_ZEROID_CLIENT_SECRET"
+          : "";
+      throw new Error(`ZeroID token exchange failed (${resp.status}): ${text.slice(0, 200)}${hint}`);
     }
 
     const data = (await resp.json()) as { access_token?: string };
diff --git a/src/daemon/server.ts b/src/daemon/server.ts
index 3ca5148f..087aefa4 100644
--- a/src/daemon/server.ts
+++ b/src/daemon/server.ts
@@ -278,6 +278,14 @@ export class DaemonServer {
       });
       this.#oauthHandler = new OAuthHandler(config.oauth, idp);
       console.log(`[codeoid] auth provider: ${idp.name}`);
+      if (!config.oauth.zeroidClientSecret) {
+        // ZeroID v1.12+ refuses an id_token exchange from an unauthenticated
+        // client. Warn rather than fail so codeoid still starts against an
+        // older ZeroID that does not require it.
+        console.warn(
+          `[codeoid] CODEOID_ZEROID_CLIENT_SECRET is not set: ZeroID v1.12+ will refuse Google sign-in (register a confidential client "${config.oauth.zeroidClientId}" at ZeroID and set its secret)`,
+        );
+      }
     } else if (config.oauth) {
       console.warn("[codeoid] local mode: Google OAuth sign-in disabled (it needs ZeroID)");
     }
diff --git a/src/tests/config.test.ts b/src/tests/config.test.ts
index 3ff11013..9bccc8a0 100644
--- a/src/tests/config.test.ts
+++ b/src/tests/config.test.ts
@@ -151,6 +151,29 @@ describe("loadConfig — registrar key + tenant", () => {
   });
 });
 
+describe("loadConfig — ZeroID client for the id_token exchange", () => {
+  it("defaults to the Google client id, the client the ID token is issued to", () => {
+    const c = loadConfig({ configPath, env: { GOOGLE_CLIENT_ID: "gid", GOOGLE_CLIENT_SECRET: "gsec" } });
+    expect(c.oauth?.zeroidClientId).toBe("gid");
+    expect(c.oauth?.zeroidClientSecret).toBeUndefined();
+    expect(c.oauth?.clientId).toBe("codeoid");
+  });
+
+  it("takes an explicit client id and the secret from env", () => {
+    const c = loadConfig({
+      configPath,
+      env: {
+        GOOGLE_CLIENT_ID: "gid",
+        GOOGLE_CLIENT_SECRET: "gsec",
+        CODEOID_ZEROID_CLIENT_ID: "zeroid-client",
+        CODEOID_ZEROID_CLIENT_SECRET: "zsec",
+      },
+    });
+    expect(c.oauth?.zeroidClientId).toBe("zeroid-client");
+    expect(c.oauth?.zeroidClientSecret).toBe("zsec");
+  });
+});
+
 describe("loadConfig — env precedence", () => {
   it("env overrides file on every field type", () => {
     writeConfig({
diff --git a/src/tests/oauth-handler.test.ts b/src/tests/oauth-handler.test.ts
index 57d6630c..c2b22d03 100644
--- a/src/tests/oauth-handler.test.ts
+++ b/src/tests/oauth-handler.test.ts
@@ -24,6 +24,8 @@ import type { IdentityProvider, VerifiedUser } from "../daemon/identity-provider
 const BASE_CONFIG: OAuthConfig = {
   zeroidTokenEndpoint: "http://zeroid.test/oauth2/token",
   clientId: "codeoid",
+  zeroidClientId: "gid_xxx",
+  zeroidClientSecret: "zsecret_xxx",
   googleClientId: "gid_xxx",
   googleClientSecret: "gsecret_xxx",
   accountId: "acct_test",
@@ -218,13 +220,44 @@ describe("GET /auth/idp-callback — Google OAuth (ZeroID token exchange)", () =
     const body = new URLSearchParams(captured.body);
     expect(body.get("grant_type")).toBe("urn:ietf:params:oauth:grant-type:token-exchange");
     expect(body.get("subject_token_type")).toBe("urn:ietf:params:oauth:token-type:id_token");
-    expect(body.get("client_id")).toBe("codeoid");
+    // The exchange authenticates as the client the ID token was issued to
+    // (ZeroID D13), not as codeoid's own front-door client id.
+    expect(body.get("client_id")).toBe("gid_xxx");
+    expect(body.get("client_secret")).toBe("zsecret_xxx");
     expect(body.get("account_id")).toBe("acct_test");
     expect(body.get("project_id")).toBe("proj_test");
     expect(body.get("scope")).toBe("session:list"); // scope from /auth/authorize
     handler.stop();
   });
 
+  it("sends no client_secret when none is configured", async () => {
+    const handler = new OAuthHandler({ ...BASE_CONFIG, zeroidClientSecret: undefined }, new StubIdP());
+    const captured: { body?: string } = {};
+    (globalThis as { fetch: unknown }).fetch = async (_url: string, opts: RequestInit) => {
+      captured.body = opts.body as string;
+      return { ok: true, json: async () => ({ access_token: "tok" }), text: async () => "" };
+    };
+
+    const internalState = await authorize(handler);
+    await get(handler, `/auth/idp-callback?state=${internalState}&id_token=gid-tok`);
+
+    const body = new URLSearchParams(captured.body);
+    expect(body.get("client_id")).toBe("gid_xxx");
+    expect(body.has("client_secret")).toBe(false);
+    handler.stop();
+  });
+
+  it("names CODEOID_ZEROID_CLIENT_SECRET when ZeroID refuses an unauthenticated exchange", async () => {
+    const handler = new OAuthHandler({ ...BASE_CONFIG, zeroidClientSecret: undefined }, new StubIdP());
+    mockZeroIDFail(401, '{"error":"invalid_client"}');
+
+    const internalState = await authorize(handler);
+    const resp = await get(handler, `/auth/idp-callback?state=${internalState}&id_token=gid-tok`);
+    const location = decodeURIComponent(resp!.headers.get("Location")!);
+    expect(location).toContain("CODEOID_ZEROID_CLIENT_SECRET");
+    handler.stop();
+  });
+
   it("redirects to callback with ?error= when ZeroID rejects", async () => {
     const handler = makeHandler();
     mockZeroIDFail(401, "invalid id_token");