diff --git a/settings.json.template b/settings.json.template
index 7e6ab93aa..072e6a74d 100644
--- a/settings.json.template
+++ b/settings.json.template
@@ -230,6 +230,19 @@
     "requireAdminForStatus": false
   },
 
+  /*
+   * Admin OpenAPI document at /admin/openapi.json.
+   *
+   * Disabled by default per Etherpad's "new features behind a flag, off by
+   * default" policy. The admin UI's typed client embeds the spec at build
+   * time, so the runtime route is only useful for third-party tooling
+   * (Postman, swagger-ui, downstream clients). Set `enabled: true` to mount
+   * the route; the SPA at /admin/ continues to serve normally either way.
+   */
+  "adminOpenAPI": {
+    "enabled": false
+  },
+
   /*
    * Contact address for admin notifications (updates, security advisories, future features).
    * Set to null to disable outbound mail from the updater.
diff --git a/src/node/hooks/express/openapi-admin.ts b/src/node/hooks/express/openapi-admin.ts
index c01cec34a..1b4b52769 100644
--- a/src/node/hooks/express/openapi-admin.ts
+++ b/src/node/hooks/express/openapi-admin.ts
@@ -1,7 +1,7 @@
 'use strict';
 
 import {ArgsExpressType} from '../../types/ArgsExpressType';
-import {getEpVersion} from '../../utils/Settings';
+import settings, {getEpVersion} from '../../utils/Settings';
 
 const OPENAPI_VERSION = '3.0.2';
 
@@ -11,8 +11,8 @@ const OPENAPI_VERSION = '3.0.2';
  * Distinct from the public versioned API document built by openapi.ts —
  * admin routes are plain Express handlers (not APIHandler-driven), so this
  * spec is hand-authored. The shape is consumed by admin/scripts/dump-spec.ts
- * for client-side codegen and exposed at GET /admin/openapi.json for
- * downstream tooling.
+ * for client-side codegen, and (when settings.adminOpenAPI.enabled) exposed
+ * at GET /admin/openapi.json for downstream tooling.
  */
 export const generateAdminDefinition = (): any => ({
   openapi: OPENAPI_VERSION,
@@ -159,6 +159,11 @@ export const expressPreSession = async (
   _hookName: string,
   {app}: ArgsExpressType,
 ): Promise<void> => {
+  // Behind a feature flag, default off. Etherpad policy
+  // (CONTRIBUTING.md, AGENTS.MD) requires new features to ship disabled by
+  // default. The route is only useful for third-party tooling — codegen
+  // imports generateAdminDefinition() in-process and does not depend on it.
+  if (!settings.adminOpenAPI?.enabled) return;
   app.get('/admin/openapi.json', (_req: any, res: any) => {
     res.header('Access-Control-Allow-Origin', '*');
     res.json(generateAdminDefinition());
diff --git a/src/node/utils/Settings.ts b/src/node/utils/Settings.ts
index ff2e93ec8..34cf966c7 100644
--- a/src/node/utils/Settings.ts
+++ b/src/node/utils/Settings.ts
@@ -331,6 +331,9 @@ export type SettingsType = {
     githubRepo: string,
     requireAdminForStatus: boolean,
   },
+  adminOpenAPI: {
+    enabled: boolean,
+  },
   adminEmail: string | null,
   getPublicSettings: () => Pick<SettingsType, "title" | "skinVariants"|"randomVersionString"|"skinName"|"toolbar"| "exposeVersion"| "gitVersion" | "enablePadWideSettings" | "privacyBanner">,
 }
@@ -510,6 +513,18 @@ const settings: SettingsType = {
     // disabling the updater itself.
     requireAdminForStatus: false,
   },
+  /**
+   * Admin OpenAPI document endpoint at /admin/openapi.json.
+   *
+   * Disabled by default per Etherpad's "new features behind a flag, off by
+   * default" policy (see CONTRIBUTING.md). The codegen pipeline imports
+   * generateAdminDefinition() in-process and does not depend on the route;
+   * enable this only if you want third-party tooling (Postman, swagger-ui,
+   * downstream clients) to consume the spec at runtime.
+   */
+  adminOpenAPI: {
+    enabled: false,
+  },
   /**
    * Contact address for admin notifications (updates, future security advisories).
    * Null disables outbound mail from the updater.
diff --git a/src/tests/backend/specs/openapi-admin.ts b/src/tests/backend/specs/openapi-admin.ts
index c48afca7c..0f07fb20d 100644
--- a/src/tests/backend/specs/openapi-admin.ts
+++ b/src/tests/backend/specs/openapi-admin.ts
@@ -171,8 +171,16 @@ describe('admin OpenAPI document', function () {
   });
 
   describe('GET /admin/openapi.json', function () {
+    // The route is feature-flagged (settings.adminOpenAPI.enabled, default
+    // false). expressPreSession reads the flag once at registration time, so
+    // we set it before common.init() boots Express. Mocha runs this `before`
+    // hook prior to any inner `it`, and it runs before the default-off
+    // describe below sees common.init().
     let agent: any;
     before(async function () {
+      const settings = require('../../../node/utils/Settings').default;
+      settings.adminOpenAPI = settings.adminOpenAPI || {enabled: true};
+      settings.adminOpenAPI.enabled = true;
       const common = require('../common');
       agent = await common.init();
     });
@@ -191,4 +199,32 @@ describe('admin OpenAPI document', function () {
       assert.equal(res.headers['access-control-allow-origin'], '*');
     });
   });
+
+  describe('feature-flag default-off behavior', function () {
+    it('expressPreSession is a no-op when settings.adminOpenAPI.enabled is false', async function () {
+      // Boot a stub express app, run expressPreSession with the flag off,
+      // and assert no GET /admin/openapi.json route was registered. We
+      // assert on the spy directly because the live server in the previous
+      // describe has the flag forced on.
+      const registered: string[] = [];
+      const stubApp: any = {
+        get: (path: string, _h: any) => {
+          registered.push(path);
+        },
+      };
+      const settingsModule = require('../../../node/utils/Settings').default;
+      const prev = settingsModule.adminOpenAPI?.enabled;
+      try {
+        settingsModule.adminOpenAPI = {enabled: false};
+        await openapiAdmin.expressPreSession('expressPreSession', {app: stubApp});
+        assert.equal(
+          registered.includes('/admin/openapi.json'),
+          false,
+          'route should not be registered when flag is off',
+        );
+      } finally {
+        if (settingsModule.adminOpenAPI) settingsModule.adminOpenAPI.enabled = !!prev;
+      }
+    });
+  });
 });