diff --git a/.github/workflows/pr-checks.yml b/.github/workflows/pr-checks.yml
index b2ce0a614e..2ae0594407 100644
--- a/.github/workflows/pr-checks.yml
+++ b/.github/workflows/pr-checks.yml
@@ -128,9 +128,9 @@ jobs:
         working-directory: pr-checks
         run: npx tsx --test
 
-      - name: Run `pr-checks/changenotes.mts` to ensure that all unreleased change notes are valid
+      - name: Run `pr-checks/changenotes.ts` to ensure that all unreleased change notes are valid
         if: ${{ !cancelled() && steps.install-deps.outcome == 'success' }}
-        run: npx tsx pr-checks/changenotes.mts validate
+        run: npx tsx pr-checks/changenotes.ts validate
 
       - name: Verify all Actions use the same Node version
         id: head-version
diff --git a/eslint.config.mjs b/eslint.config.mjs
index 115da235c7..34fe49a9df 100644
--- a/eslint.config.mjs
+++ b/eslint.config.mjs
@@ -158,7 +158,7 @@ export default [
     },
   },
   {
-    files: ["**/*.ts", "**/*.js", "**/*.mts"],
+    files: ["**/*.ts", "**/*.js"],
 
     rules: {
       "@typescript-eslint/no-explicit-any": "off",
@@ -180,7 +180,7 @@ export default [
     },
   },
   {
-    files: ["pr-checks/**/*.ts", "pr-checks/**/*.mts"],
+    files: ["pr-checks/**/*.ts"],
 
     languageOptions: {
       parserOptions: {
diff --git a/package-lock.json b/package-lock.json
index 27bf393bfd..167f0801b3 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -10508,9 +10508,26 @@
         "yaml": "^2.9.1"
       },
       "devDependencies": {
-        "@types/node": "^20.19.43",
+        "@types/node": "^24.19.0",
         "tsx": "^4.23.15"
       }
+    },
+    "pr-checks/node_modules/@types/node": {
+      "version": "24.19.0",
+      "resolved": "https://registry.npmjs.org/@types/node/-/node-24.19.0.tgz",
+      "integrity": "sha512-zY+5tKxXdhGh1PYI0ac+7juvEu4OI6vWtVVoj5i2m42jxAY1U+zHGt6QCyOFwykdP62sM3MJ9stoYYUw5aCWew==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "undici-types": ">=7.24.0 <7.24.7"
+      }
+    },
+    "pr-checks/node_modules/undici-types": {
+      "version": "7.24.6",
+      "resolved": "https://registry.npmjs.org/undici-types/-/undici-types-7.24.6.tgz",
+      "integrity": "sha512-WRNW+sJgj5OBN4/0JpHFqtqzhpbnV0GuB+OozA9gCL7a993SmU+1JBZCzLNxYsbMfIeDL+lTsphD5jN5N+n0zg==",
+      "dev": true,
+      "license": "MIT"
     }
   }
 }
diff --git a/pr-checks/.nvmrc b/pr-checks/.nvmrc
new file mode 100644
index 0000000000..a45fd52cc5
--- /dev/null
+++ b/pr-checks/.nvmrc
@@ -0,0 +1 @@
+24
diff --git a/pr-checks/bundle-changelog.ts b/pr-checks/bundle-changelog.ts
index 557a8556c1..230b80fb86 100755
--- a/pr-checks/bundle-changelog.ts
+++ b/pr-checks/bundle-changelog.ts
@@ -122,6 +122,6 @@ function main() {
 }
 
 // Only call `main` if this script was run directly.
-if (require.main === module) {
+if (import.meta.main) {
   process.exit(main());
 }
diff --git a/pr-checks/bundle-metadata.ts b/pr-checks/bundle-metadata.ts
index 25c282e9ae..f06b7104d3 100755
--- a/pr-checks/bundle-metadata.ts
+++ b/pr-checks/bundle-metadata.ts
@@ -43,6 +43,6 @@ async function main() {
 }
 
 // Only call `main` if this script was run directly.
-if (require.main === module) {
+if (import.meta.main) {
   void main();
 }
diff --git a/pr-checks/changelog/validate.test.mts b/pr-checks/changelog/validate.test.ts
similarity index 99%
rename from pr-checks/changelog/validate.test.mts
rename to pr-checks/changelog/validate.test.ts
index b8b1e33bb0..379b5737a8 100644
--- a/pr-checks/changelog/validate.test.mts
+++ b/pr-checks/changelog/validate.test.ts
@@ -9,7 +9,7 @@ import {
   isValidChangenoteFile,
   isValidChangenoteFilename,
   VALID_CHANGE_NOTE_CATEGORIES,
-} from "./validate.mjs";
+} from "./validate";
 
 await describe("isValidChangenoteContent", async () => {
   await it("recognizes an unordered Markdown list", () => {
diff --git a/pr-checks/changelog/validate.mts b/pr-checks/changelog/validate.ts
similarity index 100%
rename from pr-checks/changelog/validate.mts
rename to pr-checks/changelog/validate.ts
diff --git a/pr-checks/changenotes.mts b/pr-checks/changenotes.ts
similarity index 94%
rename from pr-checks/changenotes.mts
rename to pr-checks/changenotes.ts
index 980d103a28..933d255cd5 100755
--- a/pr-checks/changenotes.mts
+++ b/pr-checks/changenotes.ts
@@ -14,7 +14,7 @@ import {
   renderChangelog,
   withChangelog,
 } from "./changelog";
-import { isValidChangenoteFile } from "./changelog/validate.mjs";
+import { isValidChangenoteFile } from "./changelog/validate";
 import { CHANGENOTES_DIR } from "./config";
 
 /**
@@ -82,9 +82,9 @@ function main(): ExitCode {
 
 function usage(): ExitCode {
   const message =
-    "Usage: changenotes.mts assemble\n" +
-    "       changenotes.mts validate\n" +
-    "       changenotes.mts help";
+    "Usage: changenotes.ts assemble\n" +
+    "       changenotes.ts validate\n" +
+    "       changenotes.ts help";
   console.log(message);
   return ExitCode.Success;
 }
diff --git a/pr-checks/check-repo-size.ts b/pr-checks/check-repo-size.ts
index e38b19cce0..b15a85c1b3 100644
--- a/pr-checks/check-repo-size.ts
+++ b/pr-checks/check-repo-size.ts
@@ -218,6 +218,6 @@ async function run(): Promise<void> {
   }
 }
 
-if (require.main === module) {
+if (import.meta.main) {
   void run();
 }
diff --git a/pr-checks/config.ts b/pr-checks/config.ts
index 05bcbfe3ce..876654a1cc 100644
--- a/pr-checks/config.ts
+++ b/pr-checks/config.ts
@@ -1,4 +1,9 @@
 import path from "path";
+import { fileURLToPath } from "url";
+
+// For backwards-compatibility.
+const __filename = fileURLToPath(import.meta.url);
+const __dirname = path.dirname(__filename);
 
 /** The oldest supported major version of the CodeQL Action. */
 export const OLDEST_SUPPORTED_MAJOR_VERSION = 3;
diff --git a/pr-checks/package.json b/pr-checks/package.json
index d758d3200d..3d6b3d07e6 100644
--- a/pr-checks/package.json
+++ b/pr-checks/package.json
@@ -1,6 +1,7 @@
 {
   "private": true,
   "description": "Dependencies for codeql-action scripts",
+  "type": "module",
   "dependencies": {
     "@actions/core": "^2.0.3",
     "@actions/github": "^8.0.1",
@@ -13,7 +14,7 @@
     "yaml": "^2.9.1"
   },
   "devDependencies": {
-    "@types/node": "^20.19.43",
+    "@types/node": "^24.19.0",
     "tsx": "^4.23.15"
   }
 }
diff --git a/pr-checks/prepare-changelog.ts b/pr-checks/prepare-changelog.ts
index 0c89699fc8..7ddf185202 100755
--- a/pr-checks/prepare-changelog.ts
+++ b/pr-checks/prepare-changelog.ts
@@ -77,6 +77,6 @@ function main() {
 }
 
 // Only call `main` if this script was run directly.
-if (require.main === module) {
+if (import.meta.main) {
   process.exit(main());
 }
diff --git a/pr-checks/release-branches.ts b/pr-checks/release-branches.ts
index d0b4702018..04ea087334 100755
--- a/pr-checks/release-branches.ts
+++ b/pr-checks/release-branches.ts
@@ -116,6 +116,6 @@ async function main() {
 }
 
 // Only call `main` if this script was run directly.
-if (require.main === module) {
+if (import.meta.main) {
   void main();
 }
diff --git a/pr-checks/rollback-changelog.ts b/pr-checks/rollback-changelog.ts
index 15a37b1b7c..9a744dc9fc 100755
--- a/pr-checks/rollback-changelog.ts
+++ b/pr-checks/rollback-changelog.ts
@@ -79,6 +79,6 @@ function main() {
 }
 
 // Only call `main` if this script was run directly.
-if (require.main === module) {
+if (import.meta.main) {
   process.exit(main());
 }
diff --git a/pr-checks/sync-back.ts b/pr-checks/sync-back.ts
index bb442b2fe1..233621ea74 100755
--- a/pr-checks/sync-back.ts
+++ b/pr-checks/sync-back.ts
@@ -21,10 +21,11 @@ import * as fs from "fs";
 import { parseArgs } from "node:util";
 import * as path from "path";
 
-const THIS_DIR = __dirname;
-const CHECKS_DIR = path.join(THIS_DIR, "checks");
-const WORKFLOW_DIR = path.join(THIS_DIR, "..", ".github", "workflows");
-const SYNC_TS_PATH = path.join(THIS_DIR, "sync.ts");
+import { PR_CHECKS_DIR, REPO_ROOT } from "./config";
+
+const CHECKS_DIR = path.join(PR_CHECKS_DIR, "checks");
+const WORKFLOW_DIR = path.join(REPO_ROOT, ".github", "workflows");
+const SYNC_TS_PATH = path.join(PR_CHECKS_DIR, "sync.ts");
 
 /**
  * Scan generated workflow files to extract the latest action versions.
@@ -232,6 +233,6 @@ function main(): number {
 }
 
 // Only call `main` if this script was run directly.
-if (require.main === module) {
+if (import.meta.main) {
   process.exit(main());
 }
diff --git a/pr-checks/sync-checks.ts b/pr-checks/sync-checks.ts
index afebc5831e..435b3a29e8 100755
--- a/pr-checks/sync-checks.ts
+++ b/pr-checks/sync-checks.ts
@@ -342,6 +342,6 @@ async function main(): Promise<void> {
 }
 
 // Only call `main` if this script was run directly.
-if (require.main === module) {
+if (import.meta.main) {
   void main();
 }
diff --git a/pr-checks/sync.ts b/pr-checks/sync.ts
index 23ff6fc234..9b40c2e411 100755
--- a/pr-checks/sync.ts
+++ b/pr-checks/sync.ts
@@ -7,6 +7,8 @@ import * as yaml from "yaml";
 
 import { BuiltInLanguage } from "../src/languages";
 
+import { PR_CHECKS_DIR, REPO_ROOT } from "./config";
+
 /**
  * Returns a `uses` value for `action` pinned to a commit SHA, with the
  * human-readable version recorded in a trailing comment.
@@ -312,9 +314,8 @@ const languageSetups: LanguageSetups = {
 // See https://github.com/github/codeql-action/pull/3423
 const YQ_VERSION = "v4.50.1";
 
-const THIS_DIR = __dirname;
-const CHECKS_DIR = path.join(THIS_DIR, "checks");
-const OUTPUT_DIR = path.join(THIS_DIR, "..", ".github", "workflows");
+const CHECKS_DIR = path.join(PR_CHECKS_DIR, "checks");
+const OUTPUT_DIR = path.join(REPO_ROOT, ".github", "workflows");
 
 /**
  * Loads and parses a YAML file.
diff --git a/pr-checks/tsconfig.json b/pr-checks/tsconfig.json
index 67f31d2cdb..e0a6c856c9 100644
--- a/pr-checks/tsconfig.json
+++ b/pr-checks/tsconfig.json
@@ -8,6 +8,6 @@
     "sourceMap": false,
     "noEmit": true
   },
-  "include": ["./**/*.ts", "./**/*.mts", "../src/**/*.ts"],
+  "include": ["./**/*.ts", "../src/**/*.ts"],
   "exclude": ["node_modules"]
 }
diff --git a/pr-checks/update-ghes-versions.ts b/pr-checks/update-ghes-versions.ts
index 055424dff1..03b53c5dee 100755
--- a/pr-checks/update-ghes-versions.ts
+++ b/pr-checks/update-ghes-versions.ts
@@ -238,6 +238,6 @@ function main() {
 }
 
 // Only call `main` if this script was run directly.
-if (require.main === module) {
+if (import.meta.main) {
   main();
 }
diff --git a/pr-checks/update-release-branch.ts b/pr-checks/update-release-branch.ts
index 088da59281..93e50f7ad2 100755
--- a/pr-checks/update-release-branch.ts
+++ b/pr-checks/update-release-branch.ts
@@ -835,6 +835,6 @@ async function main(): Promise<void> {
 }
 
 // Only call `main` if this script was run directly.
-if (require.main === module) {
+if (import.meta.main) {
   void main();
 }