diff --git a/apps/api/src/auth/permission.guard.spec.ts b/apps/api/src/auth/permission.guard.spec.ts
index 3bf4c6ed6a..46ac2ec522 100644
--- a/apps/api/src/auth/permission.guard.spec.ts
+++ b/apps/api/src/auth/permission.guard.spec.ts
@@ -197,10 +197,17 @@ describe('PermissionGuard', () => {
 
       const result = await guard.canActivate(context);
       expect(result).toBe(true);
+      // The body MUST include `permission: undefined` explicitly — zod 4 in
+      // better-auth's hasPermission schema requires both discriminated-union
+      // keys to be present, and treats omitted-vs-undefined as different.
+      // Without the explicit undefined, every cookie-authenticated request
+      // 403s with "Unable to verify permissions". Pin this in the test so
+      // a future refactor that drops the field gets caught.
       expect(mockHasPermission).toHaveBeenCalledWith({
         headers: expect.any(Headers),
         body: {
           permissions: { control: ['delete'] },
+          permission: undefined,
         },
       });
     });
diff --git a/apps/api/src/auth/permission.guard.ts b/apps/api/src/auth/permission.guard.ts
index 0f19f79498..fee0efac3b 100644
--- a/apps/api/src/auth/permission.guard.ts
+++ b/apps/api/src/auth/permission.guard.ts
@@ -179,9 +179,19 @@ export class PermissionGuard implements CanActivate {
       return false;
     }
 
+    // better-auth 1.4.x's `hasPermission` body schema is a discriminated
+    // union that requires BOTH keys present, with the unused side set to
+    // an explicit `undefined`. zod 4 (which better-auth's plugin uses
+    // internally) rejects "key is absent" — only "key is undefined" passes
+    // the `z.undefined()` branch. Without the explicit `permission: undefined`,
+    // the schema rejects every request with `[body] Invalid input`, the
+    // catch in canActivate turns that into a generic "Unable to verify
+    // permissions" 403, and EVERY cookie-authenticated request returns 403.
+    // Reproduced repo-side via `bun run zod-repro.mjs`. Discovered on
+    // ENG-221 and the same fix applies here.
     const result = await auth.api.hasPermission({
       headers,
-      body: { permissions },
+      body: { permissions, permission: undefined },
     });
 
     return result.success === true;