diff --git a/src/graphql.ts b/src/graphql.ts
index defb53b..5d10d4c 100644
--- a/src/graphql.ts
+++ b/src/graphql.ts
@@ -4,7 +4,7 @@ import {
   RouterContext,
   graphql,
   makeExecutableSchema,
-} from "./mods";
+} from "./mods.js";
 
 export interface ResolversProps {
   Query?: any;
diff --git a/src/graphql/index.ts b/src/graphql/index.ts
index 9135422..0b02cd6 100644
--- a/src/graphql/index.ts
+++ b/src/graphql/index.ts
@@ -1,6 +1,7 @@
-import { mergeTypeDefs, mergeResolvers } from "../mods";
+import { mergeTypeDefs, mergeResolvers } from "../mods.js";
+import * as sudoku from "./sudoku.js";
 
-const modules = [require("./sudoku")];
+const modules = [sudoku];
 
 export const typeDefs = mergeTypeDefs(modules.map((mod) => mod.typeDefs));
 export const resolvers = mergeResolvers(modules.map((mod) => mod.resolvers));
diff --git a/src/graphql/sudoku.ts b/src/graphql/sudoku.ts
index add567d..8cecf9a 100644
--- a/src/graphql/sudoku.ts
+++ b/src/graphql/sudoku.ts
@@ -1,5 +1,5 @@
-import { gql } from "../mods";
-import { generate, GenerateArguments } from "../sudoku/index";
+import { gql } from "../mods.js";
+import { generate, GenerateArguments } from "../sudoku/index.js";
 
 export const typeDefs = gql`
   """
diff --git a/src/main.ts b/src/main.ts
index 5dc18e6..7d6ebc1 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -1,6 +1,6 @@
-import { Application, bodyParser } from "./mods";
-import { applyGraphQL } from "./graphql";
-import { typeDefs, resolvers } from "./graphql/index";
+import { Application, bodyParser } from "./mods.js";
+import { applyGraphQL } from "./graphql.js";
+import { typeDefs, resolvers } from "./graphql/index.js";
 import stoppable from "stoppable";
 
 import cors from "@koa/cors";
diff --git a/src/sudoku/index.ts b/src/sudoku/index.ts
index 00e0e20..d7549af 100644
--- a/src/sudoku/index.ts
+++ b/src/sudoku/index.ts
@@ -1,7 +1,8 @@
-import { StaticPool, isTimeoutError } from "node-worker-threads-pool";
+import { spawn, Thread, Worker } from "threads";
 
 import WORKERS from "physical-cpu-count";
-import { prettyPrint } from "./util";
+import { prettyPrint } from "./util.js";
+
 const TIMEOUT = 20000;
 
 export type Cell = number;
@@ -19,46 +20,64 @@ export type Sudoku = {
   cells: Cell[];
 };
 
-const pool = new StaticPool<GenerateArguments, Cell[]>({
-  size: WORKERS,
-  task: "./src/sudoku/worker.js",
-});
+function getWorker() {
+  return spawn(new Worker("./worker"));
+}
+
+const available: any = [];
+
+function initialize() {
+  console.log(`Starting ${WORKERS} worker threads`);
+  for (let n = 0; n < WORKERS; n++) {
+    getWorker().then((worker) => available.push(worker));
+  }
+}
+initialize();
+
+/**
+ * Awaits a promise with a timeout.
+ *
+ * @param promise the promise to await
+ * @param ms the timeout in milliseconds
+ * @param cb a callback to call when the timeout is reached. The promise is
+ *           rejected with whatever gets returned here.
+ */
+function withTimeout<T>(promise: Promise<T>, ms: number, cb: () => any) {
+  let timeout: NodeJS.Timeout;
+  return new Promise<T>((resolve, reject) => {
+    timeout = setTimeout(() => {
+      reject(cb());
+    }, ms);
+    promise.then(resolve).catch(reject);
+  }).finally(() => clearTimeout(timeout!));
+}
 
-let activeWorkers = 0;
 export async function generate(
   regionWidth: number,
   regionHeight: number,
   clues: number
 ): Promise<Sudoku> {
-  if (activeWorkers >= WORKERS) {
-    throw new Error("No workers available. Please try again in a moment.");
+  const proxy = available.pop();
+  if (!proxy) {
+    throw new Error("No workers available right now. Please try again.");
   }
 
-  try {
-    activeWorkers++;
-    const puzzle = await pool.exec(
-      {
-        regionWidth,
-        regionHeight,
-        clues,
-      },
-      TIMEOUT
-    );
-
-    prettyPrint(regionWidth, regionHeight, puzzle);
-
-    return {
-      regionWidth,
-      regionHeight,
-      size: (regionWidth * regionHeight) ** 2,
-      cells: puzzle,
-    };
-  } catch (err) {
-    if (isTimeoutError(err)) {
-      throw new Error("Timed out. Try increasing the number of clues.");
+  const puzzle = await withTimeout<number[]>(
+    proxy.generate(regionWidth, regionHeight, clues),
+    TIMEOUT,
+    () => {
+      Thread.terminate(proxy);
+      getWorker().then((worker) => available.push(worker));
+      return new Error("Timed out. Try reducing the number of clues.");
     }
-    throw err;
-  } finally {
-    activeWorkers--;
-  }
+  );
+
+  available.push(proxy);
+  prettyPrint(regionWidth, regionHeight, puzzle);
+  return {
+    regionWidth,
+    regionHeight,
+    size: (regionWidth * regionHeight) ** 2,
+    cells: puzzle,
+  };
 }
diff --git a/src/sudoku/math.ts b/src/sudoku/math.ts
index 0e08db3..a083e74 100644
--- a/src/sudoku/math.ts
+++ b/src/sudoku/math.ts
@@ -6,9 +6,9 @@ import {
   addNodeToColumn,
   maskRow,
   unmaskRow,
-} from "./dlx";
-import { shuffle, range } from "./util";
-import { Cell } from "./index";
+} from "./dlx.js";
+import { shuffle, range } from "./util.js";
+import { Cell } from "./index.js";
 
 type NodeMeta = {
   index: number;
diff --git a/src/sudoku/util.ts b/src/sudoku/util.ts
index 0920b48..bea6aff 100644
--- a/src/sudoku/util.ts
+++ b/src/sudoku/util.ts
@@ -1,4 +1,4 @@
-import { Cell } from "./index";
+import { Cell } from "./index.js";
 
 export function randInt(lower: number, upper: number) {
   return Math.floor(Math.random() * (upper - lower)) + lower;
diff --git a/src/sudoku/worker.js b/src/sudoku/worker.js
index 61a0d50..a9db9b7 100644
--- a/src/sudoku/worker.js
+++ b/src/sudoku/worker.js
@@ -1,15 +1,13 @@
-const { SudokuMath } = require("./math");
-const { parentPort } = require("worker_threads");
+import { SudokuMath } from "./math.js";
+import { expose } from "threads/worker";
 
 const maths = {};
 
-parentPort.on("message", ({ regionWidth, regionHeight, clues }) => {
-  const math =
-    maths[`${regionWidth}:${regionHeight}`] ||
-    (maths[`${regionWidth}:${regionHeight}`] = new SudokuMath(
-      regionWidth,
-      regionHeight
-    ));
-  const puzzle = math.generate(clues);
-  parentPort.postMessage(puzzle);
+expose({
+  generate(regionWidth, regionHeight, clues) {
+    const key = `${regionWidth}:${regionHeight}`;
+    const math =
+      maths[key] ?? (maths[key] = new SudokuMath(regionWidth, regionHeight));
+    return math.generate(clues);
+  },
 });