diff --git a/src/lib/auth/idpLogout.test.ts b/src/lib/auth/idpLogout.test.ts
new file mode 100644
index 0000000..74ac8cc
--- /dev/null
+++ b/src/lib/auth/idpLogout.test.ts
@@ -0,0 +1,47 @@
+import { describe, expect, it } from "bun:test";
+
+import { buildIdpLogoutUrl } from "./idpLogout";
+
+const full = {
+  authBaseUrl: "https://identity.example.com",
+  clientId: "client-123",
+  redirectUri: "https://app.example.com",
+  idTokenHint: "the-id-token",
+};
+
+describe("buildIdpLogoutUrl", () => {
+  it("builds the end-session url with all params when everything is present", () => {
+    const url = new URL(buildIdpLogoutUrl(full) as string);
+    expect(url.origin + url.pathname).toBe(
+      "https://identity.example.com/oauth2/end-session",
+    );
+    expect(url.searchParams.get("client_id")).toBe("client-123");
+    expect(url.searchParams.get("post_logout_redirect_uri")).toBe(
+      "https://app.example.com",
+    );
+    expect(url.searchParams.get("id_token_hint")).toBe("the-id-token");
+  });
+
+  it("returns null without an id token (the endpoint requires it)", () => {
+    expect(buildIdpLogoutUrl({ ...full, idTokenHint: undefined })).toBeNull();
+    expect(buildIdpLogoutUrl({ ...full, idTokenHint: "" })).toBeNull();
+  });
+
+  it("returns null when any IDP config part is missing", () => {
+    expect(buildIdpLogoutUrl({ ...full, authBaseUrl: undefined })).toBeNull();
+    expect(buildIdpLogoutUrl({ ...full, clientId: undefined })).toBeNull();
+    expect(buildIdpLogoutUrl({ ...full, redirectUri: undefined })).toBeNull();
+  });
+
+  it("url-encodes the id token and redirect uri", () => {
+    const url = buildIdpLogoutUrl({
+      ...full,
+      idTokenHint: "a b+c/d=",
+      redirectUri: "https://app.example.com/back?x=1",
+    }) as string;
+    expect(url).toContain("id_token_hint=a+b%2Bc%2Fd%3D");
+    expect(url).toContain(
+      "post_logout_redirect_uri=https%3A%2F%2Fapp.example.com%2Fback%3Fx%3D1",
+    );
+  });
+});
diff --git a/src/lib/auth/idpLogout.ts b/src/lib/auth/idpLogout.ts
new file mode 100644
index 0000000..4caf740
--- /dev/null
+++ b/src/lib/auth/idpLogout.ts
@@ -0,0 +1,35 @@
+/**
+ * Build the IDP (Gatekeeper) OIDC end-session URL for federated logout.
+ *
+ * Pure and config-injected so it is testable without env. Returns null unless
+ * every part is present, INCLUDING `idTokenHint`: Gatekeeper's end-session
+ * endpoint requires `id_token_hint` and rejects a request without it, so a
+ * caller that lacks the id token must fall back to a local-only sign-out rather
+ * than redirect the browser into a validation error.
+ */
+export interface IdpLogoutConfig {
+  /** IDP base URL, e.g. https://identity.omni.dev */
+  authBaseUrl?: string;
+  /** OAuth client id */
+  clientId?: string;
+  /** Post-logout redirect target (the app's own base URL) */
+  redirectUri?: string;
+  /** The user's id token; required by the end-session endpoint */
+  idTokenHint?: string;
+}
+
+export const buildIdpLogoutUrl = ({
+  authBaseUrl,
+  clientId,
+  redirectUri,
+  idTokenHint,
+}: IdpLogoutConfig): string | null => {
+  if (!authBaseUrl || !clientId || !redirectUri || !idTokenHint) return null;
+
+  const url = new URL(`${authBaseUrl}/oauth2/end-session`);
+  url.searchParams.set("client_id", clientId);
+  url.searchParams.set("post_logout_redirect_uri", redirectUri);
+  url.searchParams.set("id_token_hint", idTokenHint);
+
+  return url.toString();
+};
diff --git a/src/server/functions/auth.ts b/src/server/functions/auth.ts
index d4bccac..41de60e 100644
--- a/src/server/functions/auth.ts
+++ b/src/server/functions/auth.ts
@@ -9,6 +9,7 @@ import { getRequest, getRequestHeaders } from "@tanstack/react-start/server";
 import { decodeJwt } from "jose";
 
 import auth from "@/lib/auth/auth";
+import { buildIdpLogoutUrl } from "@/lib/auth/idpLogout";
 import {
   AUTH_BASE_URL,
   AUTH_CLIENT_ID,
@@ -95,16 +96,16 @@ export const signOutAndRedirect = createServerFn({ method: "POST" }).handler(
  * Build the IDP end_session URL for federated logout
  */
 export function getIdpLogoutUrl(idTokenHint?: string): string | null {
-  if (!AUTH_BASE_URL || !AUTH_CLIENT_ID || !BASE_URL) return null;
-
-  const endSessionUrl = new URL(`${AUTH_BASE_URL}/oauth2/end-session`);
-  endSessionUrl.searchParams.set("client_id", AUTH_CLIENT_ID);
-  endSessionUrl.searchParams.set("post_logout_redirect_uri", BASE_URL);
-  if (idTokenHint) {
-    endSessionUrl.searchParams.set("id_token_hint", idTokenHint);
-  }
-
-  return endSessionUrl.toString();
+  // Returns null (falling back to a local-only sign-out) unless every part is
+  // present, including the id token: Gatekeeper's end-session endpoint requires
+  // id_token_hint, so redirecting without it 400s and breaks sign-out. The id
+  // token may be absent after a token refresh, which does not re-issue one.
+  return buildIdpLogoutUrl({
+    authBaseUrl: AUTH_BASE_URL,
+    clientId: AUTH_CLIENT_ID,
+    redirectUri: BASE_URL,
+    idTokenHint,
+  });
 }
 
 /**