diff --git a/deploy/nginx/README.md b/deploy/nginx/README.md
index 6283d53b..acb05c6d 100644
--- a/deploy/nginx/README.md
+++ b/deploy/nginx/README.md
@@ -90,11 +90,15 @@ The shared routes name `site`, `content`, `analysis`, `content_node`,
 absorbs both: node in this repository taking over most of what Tomcat serves, and
 the site being startable locally, where they become compose service names.
 
-`/api/` for DeltaSignal exists in none of the Apache configuration, which is why
-DeltaSignal cannot work on beta today — the Angular side calls it as a bare
-relative path, routed only by the dev server's proxy, which does not exist in a
-built artifact. Note the port on the dev box: **8090**, not the 8080 DeltaSignal's
-own compose binds, because 8080 there is Tomcat.
+`/api/` is DeltaSignal's: the Angular side calls `/api/pathways`, `/api/parse` and
+`/api/solve` as bare relative paths, which `common/routes.conf` sends to the
+`deltasignal` upstream (the dev server's proxy does the same under `ng serve`).
+Beta's site profile turns the Perturb button on because of this route; a deployment
+without it must leave `deltaSignal` off. The DeltaSignal service is not part of
+this repository's compose: it runs from DeltaSignal's own repository on the same
+host, published on loopback only, at **8090**, not the 8080 DeltaSignal's own
+compose binds, because 8080 there is Tomcat. If it stops, the button's calls fail,
+so it is checked with the rest of beta (`/api/pathways` answers a list).
 
 ## Connections, which is a reason this is worth doing
 
diff --git a/e2e/deltasignal-toggle.spec.ts b/e2e/deltasignal-toggle.spec.ts
index b26c0514..2ccde966 100644
--- a/e2e/deltasignal-toggle.spec.ts
+++ b/e2e/deltasignal-toggle.spec.ts
@@ -1,10 +1,10 @@
 /**
  * DeltaSignal is offered only where a deployment asks for it.
  *
- * It is a research feature that depends on a DeltaSignal backend, and `/api` is
- * routed only by proxy.conf.js -- the dev server's proxy, which does not exist in
- * a built artifact. A deployment without that backend must not offer a button
- * whose every call fails, so the profile decides, and absent means off.
+ * It is a research feature that depends on a DeltaSignal backend, reached at
+ * `/api` through proxy.conf.js under the dev server and through nginx on beta. A
+ * deployment without that route must not offer a button whose every call fails,
+ * so the profile decides, and absent means off.
  *
  * Both states are checked against one build by setting `window.__APP_ENV` before
  * the app's scripts run, which is the runtime profile override the config already
@@ -54,8 +54,10 @@ test.describe('The DeltaSignal toggle', () => {
     await expect(page.locator('cr-deltasignal-panel')).toHaveCount(0);
   });
 
-  test('offers it on a deployment that does', async ({ page }) => {
-    await openWithProfile(page, 'development');
-    await expect(perturb(page)).toHaveCount(1);
-  });
+  for (const profile of ['development', 'beta']) {
+    test(`offers it on a deployment that does: ${profile}`, async ({ page }) => {
+      await openWithProfile(page, profile);
+      await expect(perturb(page)).toHaveCount(1);
+    });
+  }
 });
diff --git a/projects/website-angular/src/config/environments.ts b/projects/website-angular/src/config/environments.ts
index 4476a00a..1186eea0 100644
--- a/projects/website-angular/src/config/environments.ts
+++ b/projects/website-angular/src/config/environments.ts
@@ -52,9 +52,9 @@ export interface SiteProfile {
    * `specs/005-search-page-answers/research.md`, D1 and D3.
    *
    * Absent means off, like `deltaSignal`, so a deployment opts in rather than
-   * inherits. Only `development` has it today: the endpoint is not deployed on
-   * beta yet, and no keypair has been exchanged, so every other profile would
-   * offer a button whose calls fail.
+   * inherits. `development` and `beta` have it: beta's route is served by
+   * serve-prod.js, which holds the signing key (see the beta profile); any other
+   * profile would offer a button whose calls fail.
    */
   searchAnswerEndpoint?: string;
   /**