diff --git a/jest.config.js b/jest.config.js
new file mode 100644
index 0000000..9a75e12
--- /dev/null
+++ b/jest.config.js
@@ -0,0 +1,23 @@
+import { createDefaultPreset } from "ts-jest";
+
+const presetConfig = createDefaultPreset();
+
+const jestConfig = {
+  ...presetConfig,
+  resetMocks: true,
+  testEnvironment: "node",
+  extensionsToTreatAsEsm: [".ts"],
+  moduleNameMapper: {
+    "^(\\.{1,2}/.*)\\.js$": "$1",
+  },
+  transform: {
+    "^.+\\.ts$": [
+      "ts-jest",
+      {
+        useESM: true,
+      },
+    ],
+  },
+};
+
+export default jestConfig;
diff --git a/jest.config.ts b/jest.config.ts
deleted file mode 100644
index b0ac37e..0000000
--- a/jest.config.ts
+++ /dev/null
@@ -1,13 +0,0 @@
-import { createDefaultPreset, type JestConfigWithTsJest } from "ts-jest";
-
-const presetConfig = createDefaultPreset({
-  isolatedModules: true,
-});
-
-const jestConfig: JestConfigWithTsJest = {
-  ...presetConfig,
-  resetMocks: true,
-  testEnvironment: "node",
-};
-
-export default jestConfig;
diff --git a/package.json b/package.json
index 25d905d..d380268 100644
--- a/package.json
+++ b/package.json
@@ -1,13 +1,14 @@
 {
   "name": "stack-action",
   "version": "0.0.0",
+  "type": "module",
   "description": "Build and test stack-based Haskell projects",
   "main": "lib/main.js",
   "scripts": {
     "build": "tsc && ncc build lib/main.js && sed -i 's/\\x0D$//' ./dist/index.js",
     "format": "prettier --write \"**/*.ts\"",
     "format-check": "prettier --check \"**/*.ts\"",
-    "test": "jest",
+    "test": "NODE_OPTIONS='--experimental-vm-modules' jest",
     "readme": "npx action-docs -u && prettier --write README.md"
   },
   "repository": {
diff --git a/src/dirty-files.test.ts b/src/dirty-files.test.ts
index fad91fb..32a67fb 100644
--- a/src/dirty-files.test.ts
+++ b/src/dirty-files.test.ts
@@ -1,4 +1,6 @@
-import { parseGitStatus, isInterestingFile } from "./dirty-files";
+import { jest } from "@jest/globals";
+
+import { parseGitStatus, isInterestingFile } from "./dirty-files.js";
 
 describe("parseGitStatus", () => {
   test("parse file name, and filters untracked", () => {
diff --git a/src/envsubst.test.ts b/src/envsubst.test.ts
index 3f842d4..472ebf1 100644
--- a/src/envsubst.test.ts
+++ b/src/envsubst.test.ts
@@ -1,4 +1,6 @@
-import { envsubst } from "./envsubst";
+import { jest } from "@jest/globals";
+
+import { envsubst } from "./envsubst.js";
 
 const HOME = process.env.HOME;
 
diff --git a/src/get-cache-keys.test.ts b/src/get-cache-keys.test.ts
index 24731c6..cb6d2b1 100644
--- a/src/get-cache-keys.test.ts
+++ b/src/get-cache-keys.test.ts
@@ -1,4 +1,6 @@
-import { getCacheKeys } from "./get-cache-keys";
+import { jest } from "@jest/globals";
+
+import { getCacheKeys } from "./get-cache-keys.js";
 
 test("getCacheKeys", () => {
   const keys = getCacheKeys(["prefix-os-compiler", "package", "source"]);
diff --git a/src/hie.ts b/src/hie.ts
index 170b8ea..ad45d74 100644
--- a/src/hie.ts
+++ b/src/hie.ts
@@ -1,7 +1,7 @@
 import * as fs from "fs";
 import * as core from "@actions/core";
 
-import { StackCLI } from "./stack-cli";
+import { StackCLI } from "./stack-cli.js"
 
 export const HIE_YAML: string = "hie.yaml";
 
diff --git a/src/inputs.ts b/src/inputs.ts
index c48fcea..a2ed41a 100644
--- a/src/inputs.ts
+++ b/src/inputs.ts
@@ -1,7 +1,7 @@
 import * as core from "@actions/core";
 import * as Shellwords from "shellwords-ts";
-import { envsubst } from "./envsubst";
-import { type OnDirtyFiles, parseOnDirtyFiles } from "./dirty-files";
+import { envsubst } from "./envsubst.js"
+import { type OnDirtyFiles, parseOnDirtyFiles } from "./dirty-files.js"
 
 export type Inputs = {
   workingDirectory: string | null;
diff --git a/src/main.ts b/src/main.ts
index 4c5df21..3a6f792 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -1,13 +1,13 @@
 import * as core from "@actions/core";
 
-import { checkDirtyFiles } from "./dirty-files";
-import { StackCLI } from "./stack-cli";
-import { getCacheKeys } from "./get-cache-keys";
-import { hashProject } from "./hash-project";
-import { getInputs } from "./inputs";
-import { readStackYamlSync, getStackDirectories } from "./stack-yaml";
-import { DEFAULT_CACHE_OPTIONS, withCache } from "./with-cache";
-import { GenHIE } from "./hie";
+import { checkDirtyFiles } from "./dirty-files.js"
+import { StackCLI } from "./stack-cli.js"
+import { getCacheKeys } from "./get-cache-keys.js"
+import { hashProject } from "./hash-project.js"
+import { getInputs } from "./inputs.js"
+import { readStackYamlSync, getStackDirectories } from "./stack-yaml.js"
+import { DEFAULT_CACHE_OPTIONS, withCache } from "./with-cache.js"
+import { GenHIE } from "./hie.js"
 
 async function run() {
   try {
diff --git a/src/parse-stack-path.test.ts b/src/parse-stack-path.test.ts
index 557ff0c..b9d5c35 100644
--- a/src/parse-stack-path.test.ts
+++ b/src/parse-stack-path.test.ts
@@ -1,4 +1,6 @@
-import { parseStackPath } from "./parse-stack-path";
+import { jest } from "@jest/globals";
+
+import { parseStackPath } from "./parse-stack-path.js";
 
 const EXAMPLE = [
   "snapshot-doc-root: /home/patrick/.stack/snapshots/x86_64-linux-tinfo6/0dd02c1d8a380045321779c4567c2aa8873910743eac342f72d56f3d26881028/9.2.7/doc",
diff --git a/src/parse-stack-query.test.ts b/src/parse-stack-query.test.ts
index 5dde394..24fdf64 100644
--- a/src/parse-stack-query.test.ts
+++ b/src/parse-stack-query.test.ts
@@ -1,4 +1,6 @@
-import { parseStackQuery } from "./parse-stack-query";
+import { jest } from "@jest/globals";
+
+import { parseStackQuery } from "./parse-stack-query.js";
 
 const EXAMPLE = [
   "compiler:",
diff --git a/src/stack-cli.test.ts b/src/stack-cli.test.ts
index f142206..b382c26 100644
--- a/src/stack-cli.test.ts
+++ b/src/stack-cli.test.ts
@@ -1,12 +1,17 @@
-import * as exec from "@actions/exec";
+import { ExecOptions } from "@actions/exec";
+import { jest } from "@jest/globals";
 
-import { StackCLI } from "./stack-cli";
+import { ExecDelegate, StackCLI } from "./stack-cli.js";
 
-jest.spyOn(exec, "exec");
+const exec: ExecDelegate = {
+  exec: jest.fn((command: string, args: string[], options?: ExecOptions) =>
+    Promise.resolve(0),
+  ),
+};
 
 describe("StackCLI", () => {
   test("Respects --resolver given", async () => {
-    const stackCLI = new StackCLI(["--resolver", "lts"], false);
+    const stackCLI = new StackCLI(["--resolver", "lts"], false, exec);
 
     await stackCLI.setup([]);
 
@@ -21,6 +26,7 @@ describe("StackCLI", () => {
     const stackCLI = new StackCLI(
       ["--stack-yaml", "sub/stack-nightly.yaml"],
       false,
+      exec,
     );
 
     await stackCLI.setup([]);
@@ -47,6 +53,7 @@ describe("StackCLI", () => {
         "nightly-20240201",
       ],
       false,
+      exec,
     );
 
     await stackCLI.setup([]);
@@ -65,7 +72,7 @@ describe("StackCLI", () => {
   });
 
   test("installCompilerTools", async () => {
-    const stackCLI = new StackCLI([], false);
+    const stackCLI = new StackCLI([], false, exec);
     await stackCLI.installCompilerTools(["hlint", "weeder"]);
 
     expect(exec.exec).toHaveBeenCalledWith(
@@ -76,14 +83,14 @@ describe("StackCLI", () => {
   });
 
   test("installCompilerTools with empty arguments", async () => {
-    const stackCLI = new StackCLI([], false);
+    const stackCLI = new StackCLI([], false, exec);
     await stackCLI.installCompilerTools([]);
 
     expect(exec.exec).not.toHaveBeenCalled();
   });
 
   test("buildDependencies", async () => {
-    const stackCLI = new StackCLI([], false);
+    const stackCLI = new StackCLI([], false, exec);
 
     await stackCLI.buildDependencies(["--coverage"]);
 
@@ -101,7 +108,7 @@ describe("StackCLI", () => {
   });
 
   test("buildNoTest", async () => {
-    const stackCLI = new StackCLI([], false);
+    const stackCLI = new StackCLI([], false, exec);
 
     await stackCLI.buildNoTest(["--coverage"]);
 
@@ -113,7 +120,7 @@ describe("StackCLI", () => {
   });
 
   test("buildTest", async () => {
-    const stackCLI = new StackCLI([], false);
+    const stackCLI = new StackCLI([], false, exec);
 
     await stackCLI.buildTest(["--coverage"]);
 
@@ -125,7 +132,7 @@ describe("StackCLI", () => {
   });
 
   test("build", async () => {
-    const stackCLI = new StackCLI([], false);
+    const stackCLI = new StackCLI([], false, exec);
 
     await stackCLI.build(["--coverage"]);
 
diff --git a/src/stack-cli.ts b/src/stack-cli.ts
index 7d7b0c6..cce736f 100644
--- a/src/stack-cli.ts
+++ b/src/stack-cli.ts
@@ -2,12 +2,12 @@ import * as fs from "fs";
 import * as path from "path";
 import { devNull } from "os";
 import type { ExecOptions } from "@actions/exec";
-import * as exec from "@actions/exec";
+import * as realExec from "@actions/exec";
 
-import type { StackPath } from "./parse-stack-path";
-import { parseStackPath } from "./parse-stack-path";
-import type { StackQuery } from "./parse-stack-query";
-import { parseStackQuery } from "./parse-stack-query";
+import type { StackPath } from "./parse-stack-path.js";
+import { parseStackPath } from "./parse-stack-path.js";
+import type { StackQuery } from "./parse-stack-query.js";
+import { parseStackQuery } from "./parse-stack-query.js";
 
 export interface ExecDelegate {
   exec: (
@@ -23,10 +23,12 @@ export class StackCLI {
 
   private debug: boolean;
   private globalArgs: string[];
+  private execImpl: ExecDelegate;
 
-  constructor(args: string[], debug?: boolean) {
+  constructor(args: string[], debug?: boolean, exec?: ExecDelegate) {
     this.debug = debug ?? false;
     this.globalArgs = args;
+    this.execImpl = exec ?? realExec;
 
     // Capture --stack-yaml if given
     const stackYamlIdx = args.indexOf("--stack-yaml");
@@ -49,7 +51,7 @@ export class StackCLI {
   }
 
   async installed(): Promise<boolean> {
-    const ec = await exec.exec("which", ["stack"], {
+    const ec = await this.execImpl.exec("which", ["stack"], {
       silent: true,
       ignoreReturnCode: true,
     });
@@ -59,14 +61,14 @@ export class StackCLI {
   async install(): Promise<void> {
     const url = "https://get.haskellstack.org";
     const tmp = "install-stack.sh";
-    await exec.exec("curl", ["-sSL", "-o", tmp, url]);
-    await exec.exec("sh", [tmp]);
+    await this.execImpl.exec("curl", ["-sSL", "-o", tmp, url]);
+    await this.execImpl.exec("sh", [tmp]);
     fs.rmSync(tmp);
   }
 
   async upgrade(): Promise<number> {
     // Avoid this.exec because we don't need/want globalArgs
-    return await exec.exec("stack", ["upgrade"]);
+    return await this.execImpl.exec("stack", ["upgrade"]);
   }
 
   async setup(args: string[]): Promise<number> {
@@ -139,6 +141,10 @@ export class StackCLI {
   }
 
   private async exec(args: string[], options?: ExecOptions): Promise<number> {
-    return await exec.exec("stack", this.globalArgs.concat(args), options);
+    return await this.execImpl.exec(
+      "stack",
+      this.globalArgs.concat(args),
+      options,
+    );
   }
 }
diff --git a/src/stack-yaml.test.ts b/src/stack-yaml.test.ts
index 56779c6..600e61c 100644
--- a/src/stack-yaml.test.ts
+++ b/src/stack-yaml.test.ts
@@ -1,5 +1,7 @@
-import { parseStackYaml, getStackDirectories } from "./stack-yaml";
-import { StackCLI } from "./stack-cli";
+import { jest } from "@jest/globals";
+
+import { parseStackYaml, getStackDirectories } from "./stack-yaml.js";
+import { StackCLI } from "./stack-cli.js";
 
 const testStackRoot = "/home/me/.stack";
 const testPrograms = `${testStackRoot}/programs/x86_64-linux`;
diff --git a/src/stack-yaml.ts b/src/stack-yaml.ts
index 708c68c..b1e8f8a 100644
--- a/src/stack-yaml.ts
+++ b/src/stack-yaml.ts
@@ -2,7 +2,7 @@ import * as fs from "fs";
 import { join as pathJoin } from "path";
 import * as yaml from "js-yaml";
 
-import { StackCLI } from "./stack-cli";
+import { StackCLI } from "./stack-cli.js"
 
 export type StackYaml = {
   resolver: string;
diff --git a/src/with-cache.test.ts b/src/with-cache.test.ts
index ac9fa1a..f19438e 100644
--- a/src/with-cache.test.ts
+++ b/src/with-cache.test.ts
@@ -1,12 +1,19 @@
 import * as core from "@actions/core";
-import * as cache from "@actions/cache";
+import { jest } from "@jest/globals";
 
-import { getCacheKeys } from "./get-cache-keys";
-import { DEFAULT_CACHE_OPTIONS, withCache } from "./with-cache";
+import { getCacheKeys } from "./get-cache-keys.js";
+import {
+  CacheDelegate,
+  DEFAULT_CACHE_OPTIONS,
+  withCache,
+} from "./with-cache.js";
+
+const cache: CacheDelegate = {
+  restoreCache: jest.fn(() => Promise.resolve("")),
+  saveCache: jest.fn(() => Promise.resolve(0)),
+};
 
 const restoreCacheMock = jest.spyOn(cache, "restoreCache");
-jest.spyOn(cache, "saveCache");
-jest.spyOn(core, "info");
 
 async function testFunction(): Promise<number> {
   return 42;
@@ -41,7 +48,8 @@ test("withCache skips on primary-key hit", async () => {
     cachePaths,
     cacheKeys,
     testFunction,
-    DEFAULT_CACHE_OPTIONS,
+    { ...DEFAULT_CACHE_OPTIONS, silent: true },
+    cache,
   );
 
   expect(result).toBeUndefined();
@@ -62,7 +70,8 @@ test("withCache acts and saves if no primary-key hit", async () => {
     cachePaths,
     cacheKeys,
     testFunction,
-    DEFAULT_CACHE_OPTIONS,
+    { ...DEFAULT_CACHE_OPTIONS, silent: true },
+    cache,
   );
 
   expect(result).toEqual(42);
@@ -82,10 +91,17 @@ test("withCache can be configured to act and save anyway", async () => {
   const cacheKeys = getCacheKeys(["a-b", "c", "d"]);
   restoreCacheMock.mockImplementation(simulateCacheHit);
 
-  const result = await withCache(cachePaths, cacheKeys, testFunction, {
-    ...DEFAULT_CACHE_OPTIONS,
-    skipOnHit: false,
-  });
+  const result = await withCache(
+    cachePaths,
+    cacheKeys,
+    testFunction,
+    {
+      ...DEFAULT_CACHE_OPTIONS,
+      skipOnHit: false,
+      silent: true,
+    },
+    cache,
+  );
 
   expect(result).toEqual(42);
   expect(cache.restoreCache).toHaveBeenCalledWith(
@@ -108,7 +124,8 @@ test("withCache does not save on error", async () => {
       cachePaths,
       cacheKeys,
       testFunctionThrows,
-      DEFAULT_CACHE_OPTIONS,
+      { ...DEFAULT_CACHE_OPTIONS, silent: true },
+      cache,
     );
   }).rejects.toThrow();
 
@@ -128,10 +145,17 @@ test("withCache can be configured to save on error", async () => {
   restoreCacheMock.mockImplementation(simulateCacheMiss);
 
   await expect(async () => {
-    await withCache(cachePaths, cacheKeys, testFunctionThrows, {
-      ...DEFAULT_CACHE_OPTIONS,
-      saveOnError: true,
-    });
+    await withCache(
+      cachePaths,
+      cacheKeys,
+      testFunctionThrows,
+      {
+        ...DEFAULT_CACHE_OPTIONS,
+        saveOnError: true,
+        silent: true,
+      },
+      cache,
+    );
   }).rejects.toThrow();
 
   expect(cache.restoreCache).toHaveBeenCalledWith(
diff --git a/src/with-cache.ts b/src/with-cache.ts
index 590225e..92e3a6b 100644
--- a/src/with-cache.ts
+++ b/src/with-cache.ts
@@ -1,30 +1,45 @@
 import * as core from "@actions/core";
-import * as cache from "@actions/cache";
-import type { CacheKeys } from "./get-cache-keys";
+import * as realCache from "@actions/cache";
+import type { CacheKeys } from "./get-cache-keys.js";
 
 export type CacheOptions = {
   skipOnHit: boolean;
   saveOnError: boolean;
+  silent: boolean;
 };
 
 export const DEFAULT_CACHE_OPTIONS = {
   skipOnHit: true,
   saveOnError: false,
+  silent: false,
 };
 
+export interface CacheDelegate {
+  restoreCache: (
+    paths: string[],
+    primaryKey: string,
+    restoreKeys?: string[],
+  ) => Promise<string>;
+  saveCache: (paths: string[], key: string) => Promise<number>;
+}
+
 export async function withCache<T>(
   paths: string[],
   keys: CacheKeys,
   fn: () => Promise<T>,
   options: CacheOptions = DEFAULT_CACHE_OPTIONS,
+  cache?: CacheDelegate,
 ): Promise<T | undefined> {
-  const { skipOnHit, saveOnError } = options;
+  const cacheImpl = cache ?? realCache;
+  const { skipOnHit, saveOnError, silent } = options;
 
-  core.info(`Cached paths:\n - ${paths.join("\n - ")}`);
-  core.info(`Cache key: ${keys.primaryKey}`);
-  core.info(`Cache restore keys:\n - ${keys.restoreKeys.join("\n - ")}`);
+  if (!silent) {
+    core.info(`Cached paths:\n - ${paths.join("\n - ")}`);
+    core.info(`Cache key: ${keys.primaryKey}`);
+    core.info(`Cache restore keys:\n - ${keys.restoreKeys.join("\n - ")}`);
+  }
 
-  const restoredKey = await cache.restoreCache(
+  const restoredKey = await cacheImpl.restoreCache(
     paths,
     keys.primaryKey,
     keys.restoreKeys,
@@ -33,13 +48,17 @@ export async function withCache<T>(
   const primaryKeyHit = restoredKey == keys.primaryKey;
 
   if (restoredKey) {
-    core.info(`Cache restored from key: ${restoredKey}`);
+    if (!silent) {
+      core.info(`Cache restored from key: ${restoredKey}`);
+    }
   } else {
     core.warning("No cache found");
   }
 
   if (primaryKeyHit && skipOnHit && !saveOnError) {
-    core.info("Skipping due to primary key hit");
+    if (!silent) {
+      core.info("Skipping due to primary key hit");
+    }
     return;
   }
 
@@ -49,11 +68,11 @@ export async function withCache<T>(
     result = await fn();
 
     if (!primaryKeyHit) {
-      await cache.saveCache(paths, keys.primaryKey);
+      await cacheImpl.saveCache(paths, keys.primaryKey);
     }
   } catch (ex) {
     if (saveOnError && !primaryKeyHit) {
-      await cache.saveCache(paths, keys.primaryKey);
+      await cacheImpl.saveCache(paths, keys.primaryKey);
     }
 
     throw ex;
diff --git a/tsconfig.json b/tsconfig.json
index 3dfd4fe..4ed1658 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -14,8 +14,10 @@
     "noEmitOnError": true,
     "removeComments": true,
     "module": "NodeNext",
+    "moduleResolution": "NodeNext",
     "esModuleInterop": true,
-    "strict": true
+    "strict": true,
+    "isolatedModules": true
   },
   "include": ["./src/**/*"],
   "exclude": ["./src/**/*.test.ts"]