diff --git a/package.json b/package.json
index a855732d5..7182ab59d 100644
--- a/package.json
+++ b/package.json
@@ -20,7 +20,8 @@
         "lint:fix": "eslint \"**/*.{js,mjs,jsx,ts,tsx}\" --fix",
         "pretty:check": "prettier --check ./",
         "pretty:fix": "prettier --write ./",
-        "format": "npm run pretty:fix && npm run lint:fix",
+        "format": "node tests/format.mjs",
+        "format:all": "node tests/format.mjs --all",
         "test:chrome": "node tests/run-unittests.mjs --browser chrome",
         "test:firefox": "node tests/run-unittests.mjs --browser firefox",
         "test:safari": "node tests/run-unittests.mjs --browser safari",
@@ -46,16 +47,16 @@
         "eslint-plugin-react": "^7.32.2",
         "eslint-plugin-vue": "^9.10.0",
         "expect.js": "^0.3.1",
-        "mocha": "^10.2.0",
-        "prettier": "^2.8.3",
-        "selenium-webdriver": "^4.27.0",
-        "sinon": "^17.0.1",
-        "typescript": "^5.0.4",
         "lws": "^4.2.0",
         "lws-cors": "^4.2.1",
         "lws-index": "^3.1.1",
         "lws-log": "^3.0.0",
         "lws-static": "^3.1.1",
-        "prettier-plugin-tailwindcss": "^0.4.1"
+        "mocha": "^10.2.0",
+        "prettier": "^2.8.3",
+        "prettier-plugin-tailwindcss": "^0.4.1",
+        "selenium-webdriver": "^4.27.0",
+        "sinon": "^17.0.1",
+        "typescript": "^5.0.4"
     }
 }
diff --git a/tests/format.mjs b/tests/format.mjs
new file mode 100644
index 000000000..82b0b1004
--- /dev/null
+++ b/tests/format.mjs
@@ -0,0 +1,97 @@
+import { execFileSync } from "child_process";
+import { parseArgs } from "util";
+import fs from "fs";
+
+function formatAll() {
+    console.log("Formatting all files...");
+    execFileSync("npm", ["run", "pretty:fix"], { stdio: "inherit" });
+    execFileSync("npm", ["run", "lint:fix"], { stdio: "inherit" });
+}
+
+// Chunk files into batches to avoid "Argument list too long" errors on large changelists.
+function execFileSyncInBatches(command, args, files, batchSize = 100) {
+    for (let i = 0; i < files.length; i += batchSize) {
+        const batch = files.slice(i, i + batchSize);
+        execFileSync("npx", [command, ...args, ...batch], { stdio: "inherit" });
+    }
+}
+
+function runPrettier(files) {
+    try {
+        console.log(`Running prettier on ${files.length} file(s)`);
+        execFileSyncInBatches("prettier", ["--write", "--ignore-unknown"], files);
+    } catch (e) {
+        console.error("Prettier formatting failed");
+        process.exit(1);
+    }
+}
+
+function runEslint(files) {
+    const jsTsFiles = files.filter((f) => /\.(js|mjs|jsx|ts|tsx)$/.test(f));
+    if (jsTsFiles.length === 0)
+        return;
+    try {
+        console.log(`Running eslint on ${jsTsFiles.length} file(s)`);
+        execFileSyncInBatches("eslint", ["--fix"], jsTsFiles);
+    } catch (e) {
+        console.error("ESLint formatting failed");
+        process.exit(1);
+    }
+}
+
+function getChangedFiles() {
+    // "--diff-filter=ACMR" => ignore deleted files.
+    const diffOut = execFileSync("git", ["diff", "--name-only", "--diff-filter=ACMR", "@{upstream}"], { encoding: "utf8" });
+    const files = diffOut
+        .split("\n")
+        .map((f) => f.trim())
+        .filter((f) => f.length > 0 && fs.existsSync(f));
+    return [...new Set(files)];
+}
+
+let values;
+try {
+    ({ values } = parseArgs({
+        options: {
+            all: { type: "boolean" },
+            help: { type: "boolean", short: "h" },
+        },
+        strict: true,
+    }));
+} catch (err) {
+    console.error(err.message);
+    console.error("Run 'node tests/format.mjs --help' for usage.");
+    process.exit(1);
+}
+
+if (values.help) {
+    console.log(`Usage: node tests/format.mjs [options]
+
+Options:
+  --all          Format all files across the repository instead of just changed files
+  -h, --help     Show this help message`);
+    process.exit(0);
+}
+
+if (values.all) {
+    formatAll();
+    process.exit(0);
+}
+
+let changedFiles = [];
+try {
+    changedFiles = getChangedFiles();
+} catch (e) {
+    console.error("Failed to get changed files from git. Falling back to formatting all files.");
+    formatAll();
+    process.exit(0);
+}
+
+if (changedFiles.length === 0) {
+    console.log("No files changed compared to upstream.");
+    process.exit(0);
+}
+console.log(`Formatting ${changedFiles.length} changed files compared to upstream`);
+
+runPrettier(changedFiles);
+runEslint(changedFiles);