diff --git a/docs/runbook.md b/docs/runbook.md
index b23c2b2..8adb0f4 100644
--- a/docs/runbook.md
+++ b/docs/runbook.md
@@ -7,8 +7,10 @@
   `SIMPLE_UPDATE_KEY` unset, this route is ungated — which is production's
   state until the key is enforced.
 * Composer routes (vnext sites): `/packages.json`, `/p2/reason-dev/<slug>.json`,
-  `/dist/reason-dev/<slug>/<slug>-<version>.zip`. Key via `Authorization: Bearer`
-  only. Same rule as above: ungated while `SIMPLE_UPDATE_KEY` is unset, so a
+  `/dist/reason-dev/<slug>/<slug>-<version>.zip`. Key via `Authorization: Bearer`,
+  or as the HTTP Basic password with any username (for Dependabot, whose
+  `composer-repository` registry can only send a username and password).
+  Never from the query string. Same rule as above: ungated while `SIMPLE_UPDATE_KEY` is unset, so a
   Composer site can build against production before the legacy fleet sends the
   key. Setting the key gates both routes in the same deploy.
 
@@ -43,7 +45,9 @@ One key serves the whole fleet. Rotation touches, in this order:
    the old key until their constant changes.
 2. Every vnext site repo's `COMPOSER_AUTH_JSON` secret
    (`{"bearer":{"packages.reason.com":"<key>"}}`).
-3. The org secret `REASON_PACKAGES_UPDATES_KEY` used by plugin CI.
+3. The org secret `REASON_PACKAGES_UPDATES_KEY` used by plugin CI, in both
+   the Actions and the Dependabot secret stores (Dependabot runs read only the
+   latter, including `dependabot.yml` registries).
 4. SSM `/wp-update-server/prod/SIMPLE_UPDATE_KEY`, then redeploy.
 
 Step 1 only applies once legacy sites are actually sending the key; until
@@ -52,6 +56,28 @@ window where either legacy sites or Composer builds fail against the key
 in place at that moment. Accepting two keys at once during rotation would
 remove that window; that's possible future work, not implemented here.
 
+## Dependabot in plugin and theme repos
+
+A repo whose Dependabot `composer` job resolves `reason-dev/*` packages needs a
+registry entry, because Dependabot can't send a bearer token:
+
+```yaml
+registries:
+  packages-reason:
+    type: composer-repository
+    url: https://packages.reason.com
+    username: dependabot
+    password: ${{ secrets.REASON_PACKAGES_UPDATES_KEY }}
+
+updates:
+  - package-ecosystem: composer
+    directory: /
+    registries: [packages-reason]
+```
+
+The username is ignored. The secret must exist in the org's **Dependabot**
+secret store; the Actions copy is not visible to Dependabot.
+
 ## Test stage
 
 `dev` stage: SSM `/wp-update-server/dev/SIMPLE_UPDATE_KEY`, stack
diff --git a/includes/Wpup/ComposerRepository.php b/includes/Wpup/ComposerRepository.php
index ade10f7..db632f6 100644
--- a/includes/Wpup/ComposerRepository.php
+++ b/includes/Wpup/ComposerRepository.php
@@ -12,8 +12,10 @@
  *
  * The key rule is the update-checker route's: while SIMPLE_UPDATE_KEY is unset
  * the routes are open, and once it is set every route requires it as
- * "Authorization: Bearer <key>". The key is never accepted from the query
- * string here, because Composer records download URLs in composer.lock.
+ * "Authorization: Bearer <key>", or as the password of HTTP Basic auth (any
+ * username) for clients that cannot send a bearer token. The key is never
+ * accepted from the query string here, because Composer records download URLs
+ * in composer.lock.
  */
 class Wpup_ComposerRepository {
 	const VENDOR = 'reason-dev';
@@ -84,12 +86,28 @@ public function handle($path, Wpup_Headers $headers) {
 		return Wpup_ComposerResponse::error(404, 'Not found.');
 	}
 
+	/**
+	 * "Bearer <key>", or "Basic base64(<any user>:<key>)". Basic exists for
+	 * clients that can only send a username and password: Dependabot's
+	 * composer-repository registry has no token option.
+	 */
 	protected function isAuthorized(Wpup_Headers $headers) {
 		$header = $headers->get('Authorization', '');
-		if ( !is_string($header) || stripos($header, 'Bearer ') !== 0 ) {
+		if ( !is_string($header) ) {
 			return false;
 		}
-		return hash_equals($this->key, trim(substr($header, 7)));
+		if ( stripos($header, 'Bearer ') === 0 ) {
+			return hash_equals($this->key, trim(substr($header, 7)));
+		}
+		if ( stripos($header, 'Basic ') === 0 ) {
+			$decoded = base64_decode(trim(substr($header, 6)), true);
+			if ( !is_string($decoded) || strpos($decoded, ':') === false ) {
+				return false;
+			}
+			list(, $password) = explode(':', $decoded, 2);
+			return hash_equals($this->key, $password);
+		}
+		return false;
 	}
 
 	protected function serveP2($slug) {
diff --git a/tests/composer-repository-test.php b/tests/composer-repository-test.php
index 59c12a5..46bebc9 100644
--- a/tests/composer-repository-test.php
+++ b/tests/composer-repository-test.php
@@ -66,7 +66,19 @@ function s3_error($status, $code) {
 check('no header: 401', $res->status === 401);
 check('no header: WWW-Authenticate set', isset($res->headers['WWW-Authenticate']));
 check('wrong key: 401', $repo->handle('/packages.json', bearer('nope'))->status === 401);
-check('basic scheme: 401', $repo->handle('/packages.json', new Wpup_Headers(array('Authorization' => 'Basic ' . base64_encode('x:' . KEY))))->status === 401);
+function basic($credentials) {
+	return new Wpup_Headers(array('Authorization' => 'Basic ' . base64_encode($credentials)));
+}
+check('basic, key as password: 200', $repo->handle('/packages.json', basic('x:' . KEY))->status === 200);
+check('basic, empty username: 200', $repo->handle('/packages.json', basic(':' . KEY))->status === 200);
+check('basic, extra text after the key: 401', $repo->handle('/packages.json', basic('x:' . KEY . ':extra'))->status === 401);
+check('lowercase basic scheme: 200', $repo->handle('/packages.json', new Wpup_Headers(array('Authorization' => 'basic ' . base64_encode('x:' . KEY))))->status === 200);
+check('basic, wrong password: 401', $repo->handle('/packages.json', basic('x:nope'))->status === 401);
+check('basic, key as username: 401', $repo->handle('/packages.json', basic(KEY . ':x'))->status === 401);
+check('basic, no colon: 401', $repo->handle('/packages.json', basic(KEY))->status === 401);
+check('basic, not base64: 401', $repo->handle('/packages.json', new Wpup_Headers(array('Authorization' => 'Basic !!!')))->status === 401);
+check('basic, guards the dist route: 401', $repo->handle('/dist/reason-dev/acme-widget/acme-widget-1.4.2.zip', basic('x:nope'))->status === 401);
+check('unknown scheme: 401', $repo->handle('/packages.json', new Wpup_Headers(array('Authorization' => 'Token ' . KEY)))->status === 401);
 check('lowercase bearer scheme: 200', $repo->handle('/packages.json', new Wpup_Headers(array('Authorization' => 'bearer ' . KEY)))->status === 200);
 
 // --- packages.json ---