diff --git a/constraints.pro b/constraints.pro
index bfe1280c9..48aa60a24 100644
--- a/constraints.pro
+++ b/constraints.pro
@@ -309,7 +309,6 @@ gen_enforced_field(WorkspaceCwd, 'scripts.changelog:update', CorrectChangelogUpd
 
 % All non-root packages must have the same "test" script.
 gen_enforced_field(WorkspaceCwd, 'scripts.test', 'vitest run --config vitest.config.ts') :-
-  WorkspaceCwd \= 'packages/shims',
   WorkspaceCwd \= 'packages/test-utils',
   WorkspaceCwd \= '.'.
 
diff --git a/packages/extension/src/background.ts b/packages/extension/src/background.ts
index b899e06a7..eff0c6b8e 100644
--- a/packages/extension/src/background.ts
+++ b/packages/extension/src/background.ts
@@ -1,6 +1,6 @@
 /* eslint-disable import-x/no-unassigned-import */
 import './dev-console.js';
-import './endoify.mjs';
+import './endoify.js';
 /* eslint-enable import-x/no-unassigned-import */
 
 import type { ExtensionMessage } from './shared.js';
diff --git a/packages/extension/src/endoify.test.ts b/packages/extension/src/endoify.test.ts
new file mode 100644
index 000000000..645882923
--- /dev/null
+++ b/packages/extension/src/endoify.test.ts
@@ -0,0 +1,47 @@
+import './endoify.js';
+// eslint-disable-next-line n/no-extraneous-import
+import type { HandledPromiseConstructor } from '@endo/eventual-send';
+import { describe, expect, it } from 'vitest';
+
+describe('endoified', () => {
+  it('calls lockdown', () => {
+    expect(Object.isFrozen(Array.prototype)).toBe(true); // Due to `lockdown()`, and therefore `ses`
+  });
+
+  it('loads eventual-send', () => {
+    expect(typeof HandledPromise).not.toBe('undefined'); // Due to eventual send
+  });
+});
+
+describe(`endoify`, () => {
+  const assertions = [
+    (): boolean => typeof globalThis === 'object',
+    (): boolean => typeof lockdown === 'function',
+    (): boolean => typeof repairIntrinsics === 'function',
+    (): boolean => typeof Compartment === 'function',
+    (): boolean => typeof assert === 'function',
+    (): boolean => typeof HandledPromise === 'function',
+    (): boolean => typeof harden === 'function',
+    (): boolean => typeof getStackString === 'function',
+    (): boolean => {
+      try {
+        return !Object.assign(harden({ a: 1 }), { b: 2 });
+      } catch {
+        return true;
+      }
+    },
+  ];
+
+  for (const assertion of assertions) {
+    it(`asserts ${String(assertion).replace(/^.*?=>\s*/u, '')}`, () => {
+      expect(assertion()).toBe(true);
+    });
+  }
+});
+
+declare global {
+  // eslint-disable-next-line no-var
+  var getStackString: (error: Error) => string;
+  // eslint-disable-next-line no-var
+  var HandledPromise: HandledPromiseConstructor;
+}
diff --git a/packages/extension/src/endoify.ts b/packages/extension/src/endoify.ts
new file mode 100644
index 000000000..11479b6c9
--- /dev/null
+++ b/packages/extension/src/endoify.ts
@@ -0,0 +1,3 @@
+/* eslint-disable import-x/extensions */
+/* eslint-disable import-x/no-unassigned-import */
+import '@ocap/shims/endoify';
diff --git a/packages/extension/src/iframe-manager.test.ts b/packages/extension/src/iframe-manager.test.ts
index 6c3d4c9a1..b8dd6ef16 100644
--- a/packages/extension/src/iframe-manager.test.ts
+++ b/packages/extension/src/iframe-manager.test.ts
@@ -1,3 +1,4 @@
+import './endoify.js';
 import * as snapsUtils from '@metamask/snaps-utils';
 import { delay, makePromiseKitMock } from '@ocap/test-utils';
 import { vi, describe, it, expect } from 'vitest';
diff --git a/packages/extension/src/shared.test.ts b/packages/extension/src/shared.test.ts
index 2412a8716..d10870d46 100644
--- a/packages/extension/src/shared.test.ts
+++ b/packages/extension/src/shared.test.ts
@@ -1,3 +1,4 @@
+import './endoify.js';
 import { delay } from '@ocap/test-utils';
 import { vi, describe, it, expect } from 'vitest';
 
diff --git a/packages/extension/vite.config.ts b/packages/extension/vite.config.ts
index 7d5e5d364..36a591a40 100644
--- a/packages/extension/vite.config.ts
+++ b/packages/extension/vite.config.ts
@@ -14,10 +14,7 @@ const projectRoot = './src';
  * Module specifiers that will be ignored by Rollup if imported, and therefore
  * not transformed. **Only applies to JavaScript and TypeScript files.**
  */
-const externalModules: readonly string[] = [
-  './dev-console.js',
-  './endoify.mjs',
-];
+const externalModules: readonly string[] = ['./dev-console.js', './endoify.js'];
 
 /**
  * Files that need to be statically copied to the destination directory.
@@ -28,10 +25,7 @@ const staticCopyTargets: readonly string[] = [
   'manifest.json',
   // External modules
   'dev-console.js',
-  '../../shims/dist/endoify.mjs',
-  // Dependencies of external modules
-  '../../shims/dist/eventual-send.mjs',
-  '../../../node_modules/ses/dist/ses.mjs',
+  '../../shims/dist/endoify.js',
 ];
 
 // https://vitejs.dev/config/
@@ -56,6 +50,15 @@ export default defineConfig({
     },
   },
 
+  resolve: {
+    alias: [
+      {
+        find: '@ocap/shims/endoify',
+        replacement: './endoify.js',
+      },
+    ],
+  },
+
   plugins: [
     endoifyHtmlFilesPlugin(),
     viteStaticCopy({
@@ -73,25 +76,33 @@ export default defineConfig({
  * @returns The Vite plugin.
  */
 function endoifyHtmlFilesPlugin(): Plugin {
-  const endoifyElement = '<script src="endoify.mjs" type="module"></script>';
+  const endoifyElement = '<script src="endoify.js" type="module"></script>';
 
   return {
     name: 'externalize-plugin',
     async transformIndexHtml(htmlString): Promise<string> {
-      if (htmlString.includes('endoify.mjs')) {
+      const htmlDoc = loadHtml(htmlString);
+
+      if (htmlDoc('script[src="endoify.ts"]').length > 0) {
+        throw new Error(
+          `HTML document should not reference "endoify.ts" directly:\n${htmlString}`,
+        );
+      }
+
+      if (htmlDoc('script[src="endoify.js"]').length > 0) {
         throw new Error(
           `HTML document already references endoify script:\n${htmlString}`,
         );
       }
 
-      const htmlDoc = loadHtml(htmlString);
-      if (htmlDoc('head').length !== 1 || htmlDoc('head script').length < 1) {
+      if (htmlDoc('head').length !== 1 || htmlDoc('head > script').length < 1) {
         throw new Error(
           `Expected HTML document with a single <head> containing at least one <script>. Received:\n${htmlString}`,
         );
       }
 
       htmlDoc(endoifyElement).insertBefore('head:first script:first');
+
       return await prettierFormat(htmlDoc.html(), {
         parser: 'html',
         tabWidth: 2,
diff --git a/packages/extension/vitest.config.ts b/packages/extension/vitest.config.ts
index 3a99ca98e..88a0aef15 100644
--- a/packages/extension/vitest.config.ts
+++ b/packages/extension/vitest.config.ts
@@ -1,23 +1,33 @@
 // eslint-disable-next-line spaced-comment
 /// <reference types="vitest" />
-
+import path from 'path';
 import { defineConfig, mergeConfig } from 'vitest/config';
 
 import viteConfig from './vite.config.js';
 import { getDefaultConfig } from '../../vitest.config.packages.js';
 
 const defaultConfig = getDefaultConfig();
-// @ts-expect-error We can and will delete this.
-delete defaultConfig.test.coverage.thresholds;
 
-export default mergeConfig(
+const config = mergeConfig(
   viteConfig,
   mergeConfig(
     defaultConfig,
     defineConfig({
       test: {
-        setupFiles: '../test-utils/src/env/mock-endo.ts',
+        pool: 'vmThreads',
+        alias: [
+          {
+            find: '@ocap/shims/endoify',
+            replacement: path.resolve('../shims/src/endoify.js'),
+            // eslint-disable-next-line @typescript-eslint/explicit-function-return-type
+            customResolver: (id) => ({ external: true, id }),
+          },
+        ],
       },
     }),
   ),
 );
+
+delete config.test.coverage.thresholds;
+
+export default config;
diff --git a/packages/shims/.eslintrc.cjs b/packages/shims/.eslintrc.cjs
index 9c6c9b4df..6f33f2ef4 100644
--- a/packages/shims/.eslintrc.cjs
+++ b/packages/shims/.eslintrc.cjs
@@ -5,7 +5,7 @@ module.exports = {
 
   overrides: [
     {
-      files: ['src/**/*.mjs'],
+      files: ['src/**/*.js'],
       globals: { lockdown: 'readonly' },
       rules: {
         'import-x/extensions': 'off',
diff --git a/packages/shims/package.json b/packages/shims/package.json
index 120ae5f53..8ea2ac779 100644
--- a/packages/shims/package.json
+++ b/packages/shims/package.json
@@ -10,9 +10,7 @@
   "sideEffects": false,
   "type": "module",
   "exports": {
-    "./apply-lockdown": "./dist/apply-lockdown.mjs",
-    "./endoify": "./dist/endoify.mjs",
-    "./eventual-send": "./dist/eventual-send.mjs",
+    "./endoify": "./dist/endoify.js",
     "./package.json": "./package.json"
   },
   "main": "./dist/index.cjs",
@@ -26,7 +24,7 @@
     "changelog:validate": "../../scripts/validate-changelog.sh @ocap/shims",
     "clean": "rimraf --glob ./dist './*.tsbuildinfo'",
     "publish:preview": "yarn npm publish --tag preview",
-    "test": "yarn build && vitest run --config vitest.config.ts --passWithNoTests",
+    "test": "vitest run --config vitest.config.ts",
     "test:clean": "yarn test --no-cache --coverage.clean",
     "test:dev": "yarn test --coverage false",
     "test:verbose": "yarn test --reporter verbose",
@@ -42,6 +40,7 @@
     "@metamask/auto-changelog": "^3.4.4",
     "deepmerge": "^4.3.1",
     "rimraf": "^6.0.1",
+    "typescript": "~5.5.4",
     "vite": "^5.3.5",
     "vitest": "^2.0.5"
   },
diff --git a/packages/shims/scripts/bundle.js b/packages/shims/scripts/bundle.js
index b27a9f6d7..f822cc1ec 100644
--- a/packages/shims/scripts/bundle.js
+++ b/packages/shims/scripts/bundle.js
@@ -2,47 +2,37 @@ import 'ses';
 import '@endo/lockdown/commit.js';
 
 import bundleSource from '@endo/bundle-source';
-import { createReadStream, createWriteStream } from 'fs';
-import { mkdir } from 'fs/promises';
-import path from 'path';
+import { mkdir, writeFile } from 'node:fs/promises';
+import path from 'node:path';
+import { fileURLToPath } from 'node:url';
 import { rimraf } from 'rimraf';
-import { Readable } from 'stream';
-import { fileURLToPath } from 'url';
+
+import endoScriptIdentifierTransformPlugin from './helpers/rollup-plugin-endo-script-identifier-transform.js';
 
 console.log('Bundling shims...');
 
 const rootDir = fileURLToPath(new URL('..', import.meta.url));
-const src = path.resolve(rootDir, 'src');
-const dist = path.resolve(rootDir, 'dist');
-
-await mkdir(dist, { recursive: true });
-await rimraf(`${dist}/*`, { glob: true });
-
-/**
- * Bundles the target file as endoScript and returns the content as a readable stream.
- *
- * @param {string} specifier - Import path to the file to bundle, e.g. `'@endo/eventual-send/shim.js'`.
- * @returns {Promise<Readable>} The bundled file contents as a Readable stream.
- */
-const createEndoBundleReadStream = async (specifier) => {
-  const filePath = fileURLToPath(import.meta.resolve(specifier));
-  const { source: bundle } = await bundleSource(filePath, {
-    format: 'endoScript',
-  });
-  return Readable.from(bundle);
-};
-
-const sources = {
-  ses: createReadStream(
-    path.resolve(rootDir, '../../node_modules/ses/dist/ses.mjs'),
-  ),
-  eventualSend: await createEndoBundleReadStream('@endo/eventual-send/shim.js'),
-  shim: createReadStream(path.resolve(src, 'endoify.mjs')),
-};
-
-const target = createWriteStream(path.resolve(dist, 'endoify.mjs'));
-
-sources.ses.pipe(target, { end: false });
-sources.ses.on('end', () => sources.eventualSend.pipe(target, { end: false }));
-sources.eventualSend.on('end', () => sources.shim.pipe(target, { end: true }));
-sources.shim.on('end', () => console.log('Success!'));
+const srcDir = path.resolve(rootDir, 'src');
+const distDir = path.resolve(rootDir, 'dist');
+
+await mkdir(distDir, { recursive: true });
+await rimraf(`${distDir}/*`, { glob: true });
+
+for (const [name, specifier] of Object.entries({
+  endoify: path.resolve(srcDir, 'endoify.js'),
+})) {
+  const outputPath = path.resolve(distDir, `${name}.js`);
+  const sourcePath = fileURLToPath(import.meta.resolve(specifier));
+
+  let { source } = await bundleSource(sourcePath, { format: 'endoScript' });
+
+  if (!process.argv.includes('--without-rollup-transform')) {
+    source = endoScriptIdentifierTransformPlugin({
+      scopedRoot: path.resolve(rootDir, '../..'),
+    }).transform(source, specifier).code;
+  }
+
+  await writeFile(outputPath, source);
+}
+
+console.log('Success!');
diff --git a/packages/shims/scripts/helpers/rollup-plugin-endo-script-identifier-transform.js b/packages/shims/scripts/helpers/rollup-plugin-endo-script-identifier-transform.js
new file mode 100644
index 000000000..68056e122
--- /dev/null
+++ b/packages/shims/scripts/helpers/rollup-plugin-endo-script-identifier-transform.js
@@ -0,0 +1,123 @@
+/* eslint-disable jsdoc/require-returns-type */
+/* eslint-disable spaced-comment */
+/* eslint-disable jsdoc/valid-types */
+/* eslint-disable no-plusplus */
+
+/**
+ * Quickly removes the normalized scope prefix from a normalized pathname.
+ *
+ * @param {string} normalizedPathname - The normalized pathname.
+ * @param {string} [normalizedScope] - The normalized scope.
+ * @returns {string} The scoped path.
+ */
+const scopedPath = (normalizedPathname, normalizedScope = '') =>
+  normalizedPathname.slice(
+    normalizedPathname.indexOf(normalizedScope) + normalizedScope.length + 1 ||
+      0,
+  );
+
+/**
+ * Rollup plugin to transform endoScript identifiers.
+ *
+ * @param {object} [options] - The plugin options.
+ * @param {(id: string) => boolean} [options.maybeEndoScript] - A function to determine if the script should be transformed.
+ * @param {string} [options.scopedRoot] - The root directory to scope the transformed script.
+ * @param {boolean} [options.timing] - Whether to log the transform time.
+ * @param {boolean} [options.debugging] - Whether to log the transform details.
+ * @param {boolean} [options.validation] - Whether to validate the transform.
+ * @returns The Rollup plugin.
+ */
+export default function endoScriptIdentifierTransformPlugin({
+  maybeEndoScript,
+  scopedRoot,
+  timing = false,
+  debugging = false,
+  validation = false,
+}) {
+  const zwjIdentifierMatcher =
+    /(?<!\w)\$h\u200d(_{1,4})(\w+\b(?:\$*))+?(?!\w)/gu;
+  const cgjIdentifierMatcher =
+    /(?<!\w)\$\u034f+(\w+\b(?:\$*))+?\u034f\$(?!\w)/gu;
+  return {
+    name: 'endo-script-identifier-transform',
+    transform(code, id) {
+      if (
+        !((maybeEndoScript?.(id) ?? true) && zwjIdentifierMatcher.test(code))
+      ) {
+        return null;
+      }
+
+      const scopedId = scopedPath(id, scopedRoot);
+
+      if (cgjIdentifierMatcher.test(code)) {
+        throw new Error(
+          `Endoify script contains both U+200D and U+034F identifier characters: ${scopedId}`,
+        );
+      }
+
+      if (timing) {
+        console.time(`transform ${scopedId}`);
+      }
+
+      const records = validation ? {} : undefined;
+      let replacements = 0;
+
+      const replacedCode = code.replace(
+        zwjIdentifierMatcher,
+        (match, underscores, identifier, index) => {
+          const replacement = `$${'\u034f'.repeat(
+            underscores.length,
+          )}${identifier}\u034f$`;
+
+          if (validation) {
+            records[index] = { match, replacement, identifier };
+          }
+
+          replacements++;
+          return replacement;
+        },
+      );
+
+      if (validation) {
+        for (const match of replacedCode.matchAll(cgjIdentifierMatcher)) {
+          if (match[0] !== records[match.index ?? -1]?.replacement) {
+            throw new Error(
+              `Mismatched replacement: ${match[0]} !== ${
+                records[match.index ?? -1]?.replacement
+              }`,
+            );
+          }
+          if (match[1] !== records[match.index ?? -1]?.identifier) {
+            throw new Error(
+              `Mismatched replacement: ${match[1]} !== ${
+                records[match.index ?? -1]?.identifier
+              }`,
+            );
+          }
+        }
+      }
+
+      if (timing) {
+        console.timeEnd(`transform ${scopedId}`);
+      }
+
+      if (debugging) {
+        console.dir(
+          {
+            transform: {
+              id: scopedId,
+              replacements,
+              delta: replacedCode.length - code.length,
+            },
+          },
+          { depth: 1, maxStringLength: 100, compact: true },
+        );
+      }
+
+      return {
+        code: replacedCode,
+        moduleSideEffects: 'no-treeshake',
+      };
+    },
+  };
+}
diff --git a/packages/shims/src/apply-lockdown.mjs b/packages/shims/src/endoify.js
similarity index 76%
rename from packages/shims/src/apply-lockdown.mjs
rename to packages/shims/src/endoify.js
index e01804d3a..2208eeac8 100644
--- a/packages/shims/src/apply-lockdown.mjs
+++ b/packages/shims/src/endoify.js
@@ -1,11 +1,11 @@
-import './ses.mjs';
+import 'ses';
+import '@endo/eventual-send/shim.js';
 
 lockdown({
   consoleTaming: 'unsafe',
-  dateTaming: 'unsafe',
-  domainTaming: 'unsafe',
   errorTaming: 'unsafe',
   mathTaming: 'unsafe',
+  dateTaming: 'unsafe',
+  domainTaming: 'unsafe',
   overrideTaming: 'severe',
-  stackFiltering: 'verbose',
 });
diff --git a/packages/shims/src/endoify.mjs b/packages/shims/src/endoify.mjs
deleted file mode 100644
index 40124f6f0..000000000
--- a/packages/shims/src/endoify.mjs
+++ /dev/null
@@ -1,14 +0,0 @@
-/* eslint-disable import-x/unambiguous */
-// @inline './ses.mjs';
-// @inline './eventual-send.mjs';
-
-lockdown({
-  consoleTaming: 'unsafe',
-  errorTaming: 'unsafe',
-  mathTaming: 'unsafe',
-  dateTaming: 'unsafe',
-  domainTaming: 'unsafe',
-  overrideTaming: 'severe',
-});
-
-/* eslint-enable import-x/unambiguous */
diff --git a/packages/shims/src/endoify.test.ts b/packages/shims/src/endoify.test.ts
index de8618c6c..e2069c343 100644
--- a/packages/shims/src/endoify.test.ts
+++ b/packages/shims/src/endoify.test.ts
@@ -1,4 +1,5 @@
-import { HandledPromise } from '@endo/eventual-send';
+import './endoify.js';
+import type { HandledPromiseConstructor } from '@endo/eventual-send';
 import { describe, expect, it } from 'vitest';
 
 describe('endoified', () => {
@@ -10,3 +11,8 @@ describe('endoified', () => {
     expect(typeof HandledPromise).not.toBe('undefined'); // Due to eventual send
   });
 });
+
+declare global {
+  // eslint-disable-next-line no-var
+  var HandledPromise: HandledPromiseConstructor;
+}
diff --git a/packages/shims/src/vitest-environment-endoified.ts b/packages/shims/src/vitest-environment-endoified.ts
deleted file mode 100644
index 63a817387..000000000
--- a/packages/shims/src/vitest-environment-endoified.ts
+++ /dev/null
@@ -1,25 +0,0 @@
-import type { Context } from 'node:vm';
-import type { Environment } from 'vitest/environments';
-
-export default {
-  name: 'endoified',
-  transformMode: 'ssr',
-  async setupVM() {
-    const vm = await import('node:vm');
-    return {
-      getVmContext(): Context {
-        return vm.createContext({
-          setTimeout,
-          clearTimeout,
-        });
-      },
-      // eslint-disable-next-line no-empty-function
-      teardown(): void {},
-    };
-  },
-  async setup() {
-    throw new Error(
-      'endoified environment requires vitest option --pool=vmThreads or --pool=vmForks',
-    );
-  },
-} as Environment;
diff --git a/packages/shims/tsconfig.json b/packages/shims/tsconfig.json
index 6f1d89de4..43b2f489c 100644
--- a/packages/shims/tsconfig.json
+++ b/packages/shims/tsconfig.json
@@ -1,7 +1,9 @@
 {
   "extends": "../../tsconfig.packages.json",
   "compilerOptions": {
-    "baseUrl": "./"
+    "baseUrl": "./",
+    "allowJs": true,
+    "checkJs": true
   },
   "references": [],
   "include": ["./src"]
diff --git a/packages/shims/vitest.config.ts b/packages/shims/vitest.config.ts
index 4d7480d27..6b2328fbd 100644
--- a/packages/shims/vitest.config.ts
+++ b/packages/shims/vitest.config.ts
@@ -12,12 +12,9 @@ const config = mergeConfig(
   defineConfig({
     test: {
       pool: 'vmThreads',
-      environment: './vitest-environment-endoified.ts',
-      setupFiles: '../dist/endoify.mjs',
     },
   }),
 );
 
-// @ts-expect-error We can and will delete this.
 delete config.test.coverage.thresholds;
 export default config;
diff --git a/yarn.lock b/yarn.lock
index c60081c49..3f8df0378 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -1324,6 +1324,7 @@ __metadata:
     deepmerge: "npm:^4.3.1"
     rimraf: "npm:^6.0.1"
     ses: "npm:^1.7.0"
+    typescript: "npm:~5.5.4"
     vite: "npm:^5.3.5"
     vitest: "npm:^2.0.5"
   languageName: unknown