diff --git a/CHANGELOG.md b/CHANGELOG.md
index 18d028c4d6..f1b33cbfb7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,8 @@ Important changes to data models, configuration, and migrations between each
 AppEngine version, listed here to ease deployment and troubleshooting.
 
 ## Next Release (replace with git tag when deployed)
+ * Bump runtimeVersion to `2025.10.21`.
+ * Upgraded stable Flutter analysis SDK to `3.35.6`.
 
 ## `20251017t101000-all`
  * Bump runtimeVersion to `2025.10.17`.
diff --git a/Dockerfile.worker b/Dockerfile.worker
index 101998ac3a..40bac4a8f2 100644
--- a/Dockerfile.worker
+++ b/Dockerfile.worker
@@ -26,7 +26,7 @@ RUN mkdir -p /home/worker/config/flutter-stable
 RUN XDG_CONFIG_HOME=/home/worker/config/dart-stable tool/setup-dart.sh /home/worker/dart/stable 3.9.4
 
 # Setup Flutter SDK into /home/worker/flutter/{stable,preview}/
-RUN XDG_CONFIG_HOME=/home/worker/config/flutter-stable tool/setup-flutter.sh /home/worker/flutter/stable 3.35.5
+RUN XDG_CONFIG_HOME=/home/worker/config/flutter-stable tool/setup-flutter.sh /home/worker/flutter/stable 3.35.6
 
 # Setup webp
 RUN tool/setup-webp.sh /home/worker/bin
diff --git a/app/lib/shared/versions.dart b/app/lib/shared/versions.dart
index 7cbd6469d0..2a38c1e89d 100644
--- a/app/lib/shared/versions.dart
+++ b/app/lib/shared/versions.dart
@@ -24,10 +24,10 @@ final RegExp runtimeVersionPattern = RegExp(r'^\d{4}\.\d{2}\.\d{2}$');
 /// when the version switch happens.
 const _acceptedRuntimeVersions = <String>[
   // The current [runtimeVersion].
-  '2025.10.17',
+  '2025.10.21',
   // Fallback runtime versions.
+  '2025.10.17',
   '2025.10.10',
-  '2025.09.30',
 ];
 
 /// Sets the current runtime versions.
@@ -63,7 +63,7 @@ bool shouldGCVersion(String version) =>
 // keep in-sync with SDK version in .github/workflows/all-test.yml and Dockerfile.app
 final String runtimeSdkVersion = '3.9.0';
 final String toolStableDartSdkVersion = '3.9.4';
-final String toolStableFlutterSdkVersion = '3.35.5';
+final String toolStableFlutterSdkVersion = '3.35.6';
 
 final semanticToolStableDartSdkVersion = Version.parse(
   toolStableDartSdkVersion,