diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index ba5c7b4..e161a49 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -13,7 +13,12 @@ jobs:
       visibility: ${{ startsWith(github.ref, 'refs/tags/') && 'public' || '' }}
 name: CI
 "on":
-  pull_request: {}
+  pull_request:
+    types:
+      - opened
+      - synchronize
+      - reopened
+      - stacked
   push:
     branches:
       - master
diff --git a/dev/flake-module.nix b/dev/flake-module.nix
index a614e63..c7316d8 100644
--- a/dev/flake-module.nix
+++ b/dev/flake-module.nix
@@ -72,7 +72,7 @@
           name = "CI";
 
           on = {
-            pullRequest = {};
+            pullRequest.types = ["opened" "synchronize" "reopened" "stacked"];
             workflowDispatch = {};
             push = {
               branches = ["master"];
diff --git a/hk.pkl b/hk.pkl
index cf0d8cf..6a7fd2c 100644
--- a/hk.pkl
+++ b/hk.pkl
@@ -24,7 +24,11 @@ local linters = new Mapping<String, Step> {
   }
 
   // Infrastructure:
-  ["actionlint"] = Builtins.actionlint
+  ["actionlint"] = (Builtins.actionlint) {
+    // GitHub added this native-stack event before actionlint's event table.
+    // Ignore only this diagnostic until actionlint recognizes it.
+    check = "actionlint -ignore '^invalid activity type \"stacked\" for \"pull_request\" Webhook event\\.' {{files}}"
+  }
 
   // Configuration files:
   ["pkl"] = Builtins.pkl
diff --git a/modules/types/triggers.nix b/modules/types/triggers.nix
index 69b0649..974f8b5 100644
--- a/modules/types/triggers.nix
+++ b/modules/types/triggers.nix
@@ -88,6 +88,7 @@ in rec {
           "closed"
           "reopened"
           "synchronize"
+          "stacked"
           "converted_to_draft"
           "ready_for_review"
           "locked"