diff --git a/benchmark.js b/benchmark.js
new file mode 100644
index 00000000..c89da155
--- /dev/null
+++ b/benchmark.js
@@ -0,0 +1,400 @@
+// benchmark.js
+const {
+  Worker,
+  isMainThread,
+  parentPort,
+  workerData,
+} = require('worker_threads');
+
+const { fork } = require('child_process');
+
+const net = require('net');
+const fs = require('fs');
+const os = require('os');
+const path = require('path');
+const readline = require('readline');
+
+const SMALL_ITERATIONS = 10_000;
+const BIG_ITERATIONS = 1_000;
+
+const smallPayload = {
+  id: 123,
+  name: 'hello',
+  ok: true,
+};
+
+const bigPayload = {
+  items: Array.from({ length: 10_000 }, (_, i) => ({
+    id: i,
+    name: `item-${i}`,
+    value: Math.random(),
+    tags: ['a', 'b', 'c'],
+  })),
+};
+
+function hrMs(start) {
+  return Number(process.hrtime.bigint() - start) / 1e6;
+}
+
+async function benchmarkWorker(generator) {
+  return new Promise((resolve, reject) => {
+    const worker = new Worker(__filename);
+
+    let iterations = 0;
+    const start = process.hrtime.bigint();
+
+    worker.on('message', () => {
+      iterations++;
+      const r = generator.next();
+
+      if (r.done) {
+        const totalMs = hrMs(start);
+
+        worker.terminate().then(() => {
+          resolve({
+            totalMs,
+            avgMs: totalMs / iterations,
+          });
+        });
+      } else {
+        worker.postMessage(r.value);
+      }
+    });
+
+    worker.on('error', reject);
+
+    worker.postMessage(generator.next().value);
+  });
+}
+
+async function benchmarkWorkerJson(generator) {
+  return new Promise((resolve, reject) => {
+    const worker = new Worker(__filename, {
+      workerData: { mode: 'worker-json' },
+    });
+
+    let iterations = 0;
+    const start = process.hrtime.bigint();
+
+    worker.on('message', (msg) => {
+      JSON.parse(msg);
+
+      iterations++;
+      const r = generator.next();
+
+      if (r.done) {
+        const totalMs = hrMs(start);
+
+        worker.terminate().then(() => {
+          resolve({
+            totalMs,
+            avgMs: totalMs / iterations,
+          });
+        });
+      } else {
+        worker.postMessage(JSON.stringify(r.value));
+      }
+    });
+
+    worker.on('error', reject);
+
+    worker.postMessage(JSON.stringify(generator.next().value));
+  });
+}
+
+async function benchmarkForkIPC(generator) {
+  return new Promise((resolve, reject) => {
+    const child = fork(__filename, ['fork-ipc'], {
+      stdio: ['inherit', 'inherit', 'inherit', 'ipc'],
+    });
+
+    let iterations = 0;
+    const start = process.hrtime.bigint();
+
+    child.on('message', () => {
+      iterations++;
+      const r = generator.next();
+
+      if (r.done) {
+        const totalMs = hrMs(start);
+
+        child.kill();
+
+        resolve({
+          totalMs,
+          avgMs: totalMs / iterations,
+        });
+      } else {
+        child.send(r.value);
+      }
+    });
+
+    child.on('error', reject);
+
+    child.send(generator.next().value);
+  });
+}
+
+async function benchmarkForkIPCJson(generator) {
+  return new Promise((resolve, reject) => {
+    const child = fork(__filename, ['fork-ipc-json'], {
+      stdio: ['inherit', 'inherit', 'inherit', 'ipc'],
+    });
+
+    let iterations = 0;
+    const start = process.hrtime.bigint();
+
+    child.on('message', (msg) => {
+      JSON.parse(msg);
+
+      iterations++;
+      const r = generator.next();
+
+      if (r.done) {
+        const totalMs = hrMs(start);
+
+        child.kill();
+
+        resolve({
+          totalMs,
+          avgMs: totalMs / iterations,
+        });
+      } else {
+        child.send(JSON.stringify(r.value));
+      }
+    });
+
+    child.on('error', reject);
+
+    child.send(JSON.stringify(generator.next().value));
+  });
+}
+
+async function benchmarkUnixSocket(generator) {
+  const pipeName =
+    process.platform === 'win32'
+      ? '\\\\.\\pipe\\node-benchmark-' + process.pid
+      : path.join(os.tmpdir(), `node-benchmark-${process.pid}.sock`);
+
+  try {
+    fs.unlinkSync(pipeName);
+  } catch {
+    // Ignore errors.
+  }
+
+  return new Promise((resolve, reject) => {
+    const child = fork(__filename, ['fork-socket', pipeName], {
+      stdio: ['inherit', 'inherit', 'inherit', 'ipc'],
+    });
+
+    child.once('message', () => {
+      const socket = net.createConnection(pipeName);
+
+      let iterations = 0;
+
+      socket.setEncoding('utf8');
+      socket.setNoDelay(true);
+
+      // https://nodejs.org/api/readline.html#example-read-file-stream-line-by-line
+      const rl = readline.createInterface({
+        input: socket,
+        crlfDelay: Infinity,
+      });
+
+      rl.on('line', (line) => {
+        JSON.parse(line);
+
+        iterations++;
+        const r = generator.next();
+
+        if (r.done) {
+          const totalMs = hrMs(start);
+
+          socket.destroy();
+          child.kill();
+
+          resolve({
+            totalMs,
+            avgMs: totalMs / iterations,
+          });
+
+          return;
+        }
+
+        socket.write(JSON.stringify(r.value) + '\n');
+      });
+
+      socket.on('error', reject);
+
+      const start = process.hrtime.bigint();
+
+      socket.write(JSON.stringify(generator.next().value) + '\n');
+    });
+
+    child.on('error', reject);
+  });
+}
+
+async function runBenchmarks() {
+  const cases = [
+    {
+      name: 'real',
+      generator: function* () {
+        const messages = fs
+          .readFileSync('messages.txt', 'utf8')
+          .trim()
+          .split('\n')
+          .map(JSON.parse);
+        for (var j = 0; j < 10; j++) {
+          for (var i = 0; i < messages.length; i++) {
+            yield messages[i];
+          }
+        }
+      },
+    },
+    {
+      name: 'small JSON',
+      generator: function* () {
+        for (var i = 0; i < SMALL_ITERATIONS; i++) {
+          yield smallPayload;
+        }
+      },
+    },
+    {
+      name: 'big JSON',
+      generator: function* () {
+        for (var i = 0; i < BIG_ITERATIONS; i++) {
+          yield bigPayload;
+        }
+      },
+    },
+  ];
+
+  for (const testCase of cases) {
+    console.log(`\n=== ${testCase.name} ===`);
+
+    console.log('worker');
+    const worker = await benchmarkWorker(testCase.generator());
+
+    console.log('workerJson');
+    const workerJson = await benchmarkWorkerJson(testCase.generator());
+
+    console.log('forkIPC');
+    const forkIPC = await benchmarkForkIPC(testCase.generator());
+
+    console.log('forkIPCJson');
+    const forkIPCJson = await benchmarkForkIPCJson(testCase.generator());
+
+    console.log('socket');
+    const socket = await benchmarkUnixSocket(testCase.generator());
+
+    console.table([
+      {
+        transport: 'worker_threads',
+        avgMs: worker.avgMs.toFixed(6),
+        totalMs: worker.totalMs.toFixed(2),
+      },
+      {
+        transport: 'worker_threads + JSON',
+        avgMs: workerJson.avgMs.toFixed(6),
+        totalMs: workerJson.totalMs.toFixed(2),
+      },
+      {
+        transport: 'fork + process.send',
+        avgMs: forkIPC.avgMs.toFixed(6),
+        totalMs: forkIPC.totalMs.toFixed(2),
+      },
+      {
+        transport: 'fork + process.send + JSON',
+        avgMs: forkIPCJson.avgMs.toFixed(6),
+        totalMs: forkIPCJson.totalMs.toFixed(2),
+      },
+      {
+        transport: 'fork + UNIX socket',
+        avgMs: socket.avgMs.toFixed(6),
+        totalMs: socket.totalMs.toFixed(2),
+      },
+    ]);
+  }
+}
+
+//
+// Worker implementation (regular and JSON)
+//
+if (!isMainThread) {
+  if (workerData?.mode === 'worker-json') {
+    parentPort.on('message', (str) => {
+      const obj = JSON.parse(str);
+
+      parentPort.postMessage(JSON.stringify(obj));
+    });
+  } else {
+    parentPort.on('message', (msg) => {
+      parentPort.postMessage(msg);
+    });
+  }
+}
+
+//
+// Fork IPC implementation
+//
+else if (process.argv[2] === 'fork-ipc') {
+  process.on('message', (msg) => {
+    process.send(msg);
+  });
+}
+
+//
+// Fork IPC JSON implementation
+//
+else if (process.argv[2] === 'fork-ipc-json') {
+  process.on('message', (str) => {
+    const obj = JSON.parse(str);
+
+    process.send(JSON.stringify(obj));
+  });
+}
+
+//
+// Fork socket implementation
+//
+else if (process.argv[2] === 'fork-socket') {
+  const pipeName = process.argv[3];
+
+  try {
+    fs.unlinkSync(pipeName);
+  } catch {
+    // Ignore errors.
+  }
+
+  const server = net.createServer((socket) => {
+    socket.setEncoding('utf8');
+    socket.setNoDelay(true);
+
+    // https://nodejs.org/api/readline.html#example-read-file-stream-line-by-line
+    const rl = readline.createInterface({
+      input: socket,
+      crlfDelay: Infinity,
+    });
+
+    rl.on('line', (line) => {
+      const obj = JSON.parse(line);
+
+      socket.write(JSON.stringify(obj) + '\n');
+    });
+  });
+
+  server.listen(pipeName, () => {
+    process.send('ready');
+  });
+}
+
+//
+// Main
+//
+else {
+  runBenchmarks().catch((err) => {
+    console.error(err);
+    process.exit(1);
+  });
+}
diff --git a/elm/src/Test/Runner/Node.elm b/elm/src/Test/Runner/Node.elm
index 75235773..679d7e35 100644
--- a/elm/src/Test/Runner/Node.elm
+++ b/elm/src/Test/Runner/Node.elm
@@ -81,7 +81,7 @@ type Msg
 {-| The port names are prefixed to reduce the likelihood of the project
 having a port with the same name, which is a compile error.
 -}
-port elmTestPort__send : String -> Cmd msg
+port elmTestPort__send : Decode.Value -> Cmd msg
 
 
 port elmTestPort__receive : (Decode.Value -> msg) -> Sub msg
@@ -140,7 +140,6 @@ update msg ({ testReporter } as model) =
                                 , ( "exitCode", Encode.int exitCode )
                                 , ( "message", summary )
                                 ]
-                                |> Encode.encode 0
                                 |> elmTestPort__send
                     in
                     ( model, cmd )
@@ -152,7 +151,6 @@ update msg ({ testReporter } as model) =
                                 [ ( "type", Encode.string "ERROR" )
                                 , ( "message", Encode.string (Decode.errorToString err) )
                                 ]
-                                |> Encode.encode 0
                                 |> elmTestPort__send
                     in
                     ( model, cmd )
@@ -224,7 +222,6 @@ sendResults isFinished testReporter results =
                 |> Encode.object
           )
         ]
-        |> Encode.encode 0
         |> elmTestPort__send
 
 
@@ -245,7 +242,6 @@ sendBegin model =
                     []
     in
     Encode.object (baseFields ++ extraFields)
-        |> Encode.encode 0
         |> elmTestPort__send
 
 
@@ -336,7 +332,6 @@ failInit message report _ =
                 , ( "exitCode", Encode.int 1 )
                 , ( "message", Encode.string message )
                 ]
-                |> Encode.encode 0
                 |> elmTestPort__send
     in
     ( model, cmd )
diff --git a/lib/Generate.js b/lib/Generate.js
index f38e5b0e..c781bce9 100644
--- a/lib/Generate.js
+++ b/lib/Generate.js
@@ -15,11 +15,10 @@ const after = fs.readFileSync(
 );
 
 /**
- * @param { string } pipeFilename
  * @param { string } dest
  * @returns { void }
  */
-function prepareCompiledJsFile(pipeFilename, dest) {
+function prepareCompiledJsFile(dest) {
   const content = fs.readFileSync(dest, 'utf8');
   const finalContent = `
 ${before}
@@ -27,7 +26,6 @@ var Elm = (function(module) {
 ${addKernelTestChecking(content)}
 return this.Elm;
 })({});
-var pipeFilename = ${JSON.stringify(pipeFilename)};
 ${after}
   `.trim();
   fs.writeFileSync(dest, finalContent);
diff --git a/lib/RunTests.js b/lib/RunTests.js
index 7da7be85..123ecf42 100644
--- a/lib/RunTests.js
+++ b/lib/RunTests.js
@@ -10,26 +10,6 @@ const Project = require('./Project');
 const Report = require('./Report');
 const Supervisor = require('./Supervisor');
 
-/**
- * Incorporate the process PID into the socket name, so elm-test processes can
- * be run parallel without accidentally sharing each others' sockets.
- *
- * See https://github.com/rtfeldman/node-test-runner/pull/231
- * Also incorporate a salt number into it on Windows, to avoid EADDRINUSE -
- * see https://github.com/rtfeldman/node-test-runner/issues/275 - because the
- * alternative approach of deleting the file before creating a new one doesn't
- * work on Windows. We have to let Windows clean up the named pipe. This is
- * essentially a band-aid fix. The alternative is to rewrite a ton of stuff.
- *
- * @param { number } runsExecuted
- * @returns { string }
- */
-function getPipeFilename(runsExecuted) {
-  return process.platform === 'win32'
-    ? `\\\\.\\pipe\\elm_test-${process.pid}-${runsExecuted}`
-    : `/tmp/elm_test-${process.pid}.sock`;
-}
-
 /**
  * This lets you log something like `elm.json changed > Compiling > Starting tests`
  * where each segment appears over time.
@@ -166,8 +146,6 @@ function runTests(
   let watcher = undefined;
   /** @type { Array<string> } */
   let watchedPaths = [];
-  /** @type { number } */
-  let runsExecuted = 0;
   /** @type { Promise<void> | void } */
   let currentRun = undefined;
   /** @type { Queue } */
@@ -236,8 +214,6 @@ function runTests(
         watcher.unwatch(diff.removed);
       }
 
-      runsExecuted++;
-      const pipeFilename = getPipeFilename(runsExecuted);
       const testModules = FindTests.findTests(testFilePaths, project);
       const mainModule = Generate.getMainModule(project.generatedCodeDir);
       const dest = path.join(project.generatedCodeDir, 'elmTestOutput.js');
@@ -265,19 +241,12 @@ function runTests(
         report
       );
 
-      Generate.prepareCompiledJsFile(pipeFilename, dest);
+      Generate.prepareCompiledJsFile(dest);
 
       progressLogger.log('Starting tests');
       progressLogger.newLine();
 
-      return await Supervisor.run(
-        packageInfo.version,
-        pipeFilename,
-        report,
-        processes,
-        dest,
-        watch
-      );
+      return await Supervisor.run(packageInfo.version, report, processes, dest);
     } catch (err) {
       progressLogger.newLine();
       console.error(err.message);
diff --git a/lib/Supervisor.js b/lib/Supervisor.js
index 3bd85fda..8f2230dd 100644
--- a/lib/Supervisor.js
+++ b/lib/Supervisor.js
@@ -1,34 +1,25 @@
-const chalk = require('./chalk');
-const child_process = require('child_process');
-const fs = require('fs');
-const net = require('net');
-const split = require('split');
+const { Worker } = require('worker_threads');
 const Report = require('./Report');
 const XMLBuilder = require('./XMLBuilder');
 
 /**
  * @param { string } elmTestVersion
- * @param { string } pipeFilename
  * @param { import('./Report').Report } report
  * @param { number } processes
  * @param { string } dest
- * @param { boolean } watch
  * @returns { Promise<number> }
  */
-function run(elmTestVersion, pipeFilename, report, processes, dest, watch) {
+function run(elmTestVersion, report, processes, dest) {
   return new Promise(function (resolve) {
     /** @type { number | null } */
     var nextResultToPrint = null;
     var finishedWorkers = 0;
-    var closedWorkers = 0;
     var results = new Map();
     var failures = 0;
     /** @type { Array<{ labels: Array<string>, todo: string }> } */
     var todos = [];
     var testsToRun = -1;
     var startingTime = Date.now();
-    /** @type { Array<import('child_process').ChildProcess> } */
-    var workers = [];
 
     /**
      * @param { any } result This `any` became explicit instead of implicit when migrating from Flow to TypeScript.
@@ -95,13 +86,6 @@ function run(elmTestVersion, pipeFilename, report, processes, dest, watch) {
         }
       }
     }
-    function reportRuntimeException() {
-      console.error(
-        chalk.red(
-          '\n\nThere was an unexpected runtime exception while running tests\n\n'
-        )
-      );
-    }
 
     /**
      * @param { any } response This `any` became explicit instead of implicit when migrating from Flow to TypeScript.
@@ -155,27 +139,11 @@ function run(elmTestVersion, pipeFilename, report, processes, dest, watch) {
       flushResults();
     }
 
-    /**
-     * @param { import('net').Socket } socket
-     * @returns { void }
-     */
-    function initWorker(socket) {
-      socket.setEncoding('utf8');
-      socket.setNoDelay(true);
-
-      // See the long note near client.write() in worker.js for why we do this.
-      // It fixes a nasty bug!
-      var stream = socket.pipe(split());
-
-      stream.on('data', function (data) {
-        // In watch mode, the socket is drained which results in an extraneous
-        // message being sent. If we receive no data, ignore it.
-        if (!data) {
-          return;
-        }
-
-        var response = JSON.parse(data);
+    /** @type { Array<import('worker_threads').Worker> } */
+    var workers = Array.from({ length: processes }, (_, index) => {
+      var worker = new Worker(dest, { workerData: index });
 
+      worker.on('message', function (response) {
         switch (response.type) {
           case 'FINISHED':
             handleResults(response);
@@ -183,16 +151,14 @@ function run(elmTestVersion, pipeFilename, report, processes, dest, watch) {
             // This worker found no tests remaining to run; it's finished!
             finishedWorkers++;
 
-            // If all the workers have finished, print the summmary.
+            // If all the workers have finished, print the summary.
             if (finishedWorkers === workers.length) {
-              socket.write(
-                JSON.stringify({
-                  type: 'SUMMARY',
-                  duration: Date.now() - startingTime,
-                  failures: failures,
-                  todos: todos,
-                })
-              );
+              worker.postMessage({
+                type: 'SUMMARY',
+                duration: Date.now() - startingTime,
+                failures: failures,
+                todos: todos,
+              });
             }
             break;
           case 'SUMMARY':
@@ -219,7 +185,7 @@ function run(elmTestVersion, pipeFilename, report, processes, dest, watch) {
 
             // Close all the workers.
             workers.forEach(function (worker) {
-              worker.kill();
+              worker.terminate();
             });
             resolve(response.exitCode);
             break;
@@ -253,58 +219,9 @@ function run(elmTestVersion, pipeFilename, report, processes, dest, watch) {
             );
         }
       });
-    }
 
-    var pendingException = false,
-      server = net.createServer(initWorker);
-
-    server.on('error', function (err) {
-      console.error(err.stack);
-      server.close();
+      return worker;
     });
-
-    server.on('listening', function () {
-      workers = Array.from({ length: processes }, (_, index) => {
-        var worker = child_process.fork(dest, [index.toString()]);
-
-        worker.on('close', function (code) {
-          // code can be null.
-          var hasNonZeroExitCode = typeof code === 'number' && code !== 0;
-
-          if (watch && !Report.isMachineReadable(report)) {
-            if (hasNonZeroExitCode) {
-              // Queue up complaining about an exception.
-              // Don't print it immediately, or else it might print N times
-              // where N is the number of cores.
-              pendingException = true;
-            }
-            closedWorkers++;
-            // If all the workers have closed, we're done! Continue watching.
-            if (closedWorkers === workers.length) {
-              if (pendingException) {
-                // If we had an exception pending, print it and clear pending flag.
-                reportRuntimeException();
-                pendingException = false;
-              }
-              resolve(1);
-            }
-          } else if (hasNonZeroExitCode) {
-            reportRuntimeException();
-            resolve(1);
-          }
-        });
-
-        return worker;
-      });
-    });
-
-    if (fs.existsSync(pipeFilename) && process.platform !== 'win32') {
-      // Never remove named pipes on Windows. The OS will clean them up when
-      // nothing has a handle to them anymore.
-      fs.unlinkSync(pipeFilename);
-    }
-
-    server.listen(pipeFilename);
   });
 }
 
diff --git a/messages.txt b/messages.txt
new file mode 100644
index 00000000..a6d86f37
--- /dev/null
+++ b/messages.txt
@@ -0,0 +1,15 @@
+{"type":"FINISHED","results":{"5":{"type":"complete","status":"pass"},"17":{"type":"complete","status":"pass"},"29":{"type":"complete","status":"pass"},"41":{"type":"complete","status":"pass"},"53":{"type":"complete","status":"pass"},"65":{"type":"complete","status":"pass"},"77":{"type":"complete","status":"pass"},"89":{"type":"complete","status":"pass"},"101":{"type":"complete","status":"pass"},"113":{"type":"complete","status":"pass"},"125":{"type":"complete","status":"pass"},"137":{"type":"complete","status":"pass"},"149":{"type":"complete","status":"pass"},"161":{"type":"complete","status":"pass"},"173":{"type":"complete","status":"pass"},"185":{"type":"complete","status":"pass"},"197":{"type":"complete","status":"pass"},"209":{"type":"complete","status":"pass"},"221":{"type":"complete","status":"pass"},"233":{"type":"complete","status":"pass"},"245":{"type":"complete","status":"pass"},"257":{"type":"complete","status":"pass"},"269":{"type":"complete","status":"pass"},"281":{"type":"complete","status":"pass"},"293":{"type":"complete","status":"pass"},"305":{"type":"complete","status":"pass"},"317":{"type":"complete","status":"pass"},"329":{"type":"complete","status":"pass"},"341":{"type":"complete","status":"pass"},"353":{"type":"complete","status":"pass"},"365":{"type":"complete","status":"pass"},"377":{"type":"complete","status":"pass"},"389":{"type":"complete","status":"pass"},"401":{"type":"complete","status":"pass"},"413":{"type":"complete","status":"pass"},"425":{"type":"complete","status":"pass"},"437":{"type":"complete","status":"pass"},"449":{"type":"complete","status":"pass"},"461":{"type":"complete","status":"pass"},"473":{"type":"complete","status":"pass"},"485":{"type":"complete","status":"pass"},"497":{"type":"complete","status":"pass"},"509":{"type":"complete","status":"pass"},"521":{"type":"complete","status":"pass"},"533":{"type":"complete","status":"pass"},"545":{"type":"complete","status":"pass"},"557":{"type":"complete","status":"pass"},"569":{"type":"complete","status":"pass"},"581":{"type":"complete","status":"pass"}}}
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+{"type":"FINISHED","results":{"7":{"type":"complete","status":"pass"},"19":{"type":"complete","status":"pass"},"31":{"type":"complete","status":"pass"},"43":{"type":"complete","status":"pass"},"55":{"type":"complete","status":"pass"},"67":{"type":"complete","status":"pass"},"79":{"type":"complete","status":"pass"},"91":{"type":"complete","status":"pass"},"103":{"type":"complete","status":"pass"},"115":{"type":"complete","status":"pass"},"127":{"type":"complete","status":"pass"},"139":{"type":"complete","status":"pass"},"151":{"type":"complete","status":"pass"},"163":{"type":"complete","status":"pass"},"175":{"type":"complete","status":"pass"},"187":{"type":"complete","status":"pass"},"199":{"type":"complete","status":"pass"},"211":{"type":"complete","status":"pass"},"223":{"type":"complete","status":"pass"},"235":{"type":"complete","status":"pass"},"247":{"type":"complete","status":"pass"},"259":{"type":"complete","status":"pass"},"271":{"type":"complete","status":"pass"},"283":{"type":"complete","status":"pass"},"295":{"type":"complete","status":"pass"},"307":{"type":"complete","status":"pass"},"319":{"type":"complete","status":"pass"},"331":{"type":"complete","status":"pass"},"343":{"type":"complete","status":"pass"},"355":{"type":"complete","status":"pass"},"367":{"type":"complete","status":"pass"},"379":{"type":"complete","status":"pass"},"391":{"type":"complete","status":"pass"},"403":{"type":"complete","status":"pass"},"415":{"type":"complete","status":"pass"},"427":{"type":"complete","status":"pass"},"439":{"type":"complete","status":"pass"},"451":{"type":"complete","status":"pass"},"463":{"type":"complete","status":"pass"},"475":{"type":"complete","status":"pass"},"487":{"type":"complete","status":"pass"},"499":{"type":"complete","status":"pass"},"511":{"type":"complete","status":"pass"},"523":{"type":"complete","status":"pass"},"535":{"type":"complete","status":"pass"},"547":{"type":"complete","status":"pass"},"559":{"type":"complete","status":"pass"},"571":{"type":"complete","status":"pass"},"583":{"type":"complete","status":"pass"}}}
+{"type":"SUMMARY","exitCode":2,"message":{"type":"summary","summary":"\u001b[4m\u001b[31m\nTEST RUN FAILED\u001b[39m\u001b[24m\u001b[31m\n\n\u001b[39m\u001b[2mDuration: \u001b[22m2274 ms\n\u001b[2mPassed:   \u001b[22m591\n\u001b[2mFailed:   \u001b[22m1\n"}}
diff --git a/package-lock.json b/package-lock.json
index 785e200d..6ddb8ca0 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -12,7 +12,6 @@
         "chokidar": "^3.5.3",
         "commander": "^9.4.1",
         "elm-solve-deps-wasm": "^1.0.2 || ^2.0.0",
-        "split": "^1.0.1",
         "tinyglobby": "^0.2.10",
         "which": "^2.0.2"
       },
@@ -23,7 +22,6 @@
         "@eslint/js": "9.20.0",
         "@types/mocha": "10.0.10",
         "@types/node": "26.0.1",
-        "@types/split": "1.0.5",
         "@types/which": "3.0.4",
         "elm-review": "2.13.1",
         "elm-tooling": "1.18.0",
@@ -447,27 +445,6 @@
         "@types/node": "*"
       }
     },
-    "node_modules/@types/split": {
-      "version": "1.0.5",
-      "resolved": "https://registry.npmjs.org/@types/split/-/split-1.0.5.tgz",
-      "integrity": "sha512-gMiDr4vA6YofTpAkPQtP+5pvStIf3CMYphf32YAG/3RwogNL8ii1CQKDc+sxN62KuxPoRaJXcf2zDCDkEBH4FA==",
-      "dev": true,
-      "license": "MIT",
-      "dependencies": {
-        "@types/node": "*",
-        "@types/through": "*"
-      }
-    },
-    "node_modules/@types/through": {
-      "version": "0.0.33",
-      "resolved": "https://registry.npmjs.org/@types/through/-/through-0.0.33.tgz",
-      "integrity": "sha512-HsJ+z3QuETzP3cswwtzt2vEIiHBk/dCcHGhbmG5X3ecnwFD/lPrMpliGXxSCg03L9AhrdwA4Oz/qfspkDW+xGQ==",
-      "dev": true,
-      "license": "MIT",
-      "dependencies": {
-        "@types/node": "*"
-      }
-    },
     "node_modules/@types/which": {
       "version": "3.0.4",
       "resolved": "https://registry.npmjs.org/@types/which/-/which-3.0.4.tgz",
@@ -2845,17 +2822,6 @@
       "integrity": "sha512-bLGGlR1QxBcynn2d5YmDX4MGjlZvy2MRBDRNHLJ8VI6l6+9FUiyTFNJ0IveOSP0bcXgVDPRcfGqA0pjaqUpfVg==",
       "dev": true
     },
-    "node_modules/split": {
-      "version": "1.0.1",
-      "resolved": "https://registry.npmjs.org/split/-/split-1.0.1.tgz",
-      "integrity": "sha512-mTyOoPbrivtXnwnIxZRFYRrPNtEFKlpB2fvjSnCQUiAA6qAZzqwna5envK4uk6OIeP17CsdF3rSBGYVBsU0Tkg==",
-      "dependencies": {
-        "through": "2"
-      },
-      "engines": {
-        "node": "*"
-      }
-    },
     "node_modules/string_decoder": {
       "version": "1.3.0",
       "resolved": "https://registry.npmjs.org/string_decoder/-/string_decoder-1.3.0.tgz",
@@ -2999,11 +2965,6 @@
         "url": "https://github.com/sponsors/sindresorhus"
       }
     },
-    "node_modules/through": {
-      "version": "2.3.8",
-      "resolved": "https://registry.npmjs.org/through/-/through-2.3.8.tgz",
-      "integrity": "sha1-DdTJ/6q8NXlgsbckEV1+Doai4fU="
-    },
     "node_modules/tinyglobby": {
       "version": "0.2.10",
       "resolved": "https://registry.npmjs.org/tinyglobby/-/tinyglobby-0.2.10.tgz",
@@ -3540,25 +3501,6 @@
         "@types/node": "*"
       }
     },
-    "@types/split": {
-      "version": "1.0.5",
-      "resolved": "https://registry.npmjs.org/@types/split/-/split-1.0.5.tgz",
-      "integrity": "sha512-gMiDr4vA6YofTpAkPQtP+5pvStIf3CMYphf32YAG/3RwogNL8ii1CQKDc+sxN62KuxPoRaJXcf2zDCDkEBH4FA==",
-      "dev": true,
-      "requires": {
-        "@types/node": "*",
-        "@types/through": "*"
-      }
-    },
-    "@types/through": {
-      "version": "0.0.33",
-      "resolved": "https://registry.npmjs.org/@types/through/-/through-0.0.33.tgz",
-      "integrity": "sha512-HsJ+z3QuETzP3cswwtzt2vEIiHBk/dCcHGhbmG5X3ecnwFD/lPrMpliGXxSCg03L9AhrdwA4Oz/qfspkDW+xGQ==",
-      "dev": true,
-      "requires": {
-        "@types/node": "*"
-      }
-    },
     "@types/which": {
       "version": "3.0.4",
       "resolved": "https://registry.npmjs.org/@types/which/-/which-3.0.4.tgz",
@@ -5110,14 +5052,6 @@
       "integrity": "sha512-bLGGlR1QxBcynn2d5YmDX4MGjlZvy2MRBDRNHLJ8VI6l6+9FUiyTFNJ0IveOSP0bcXgVDPRcfGqA0pjaqUpfVg==",
       "dev": true
     },
-    "split": {
-      "version": "1.0.1",
-      "resolved": "https://registry.npmjs.org/split/-/split-1.0.1.tgz",
-      "integrity": "sha512-mTyOoPbrivtXnwnIxZRFYRrPNtEFKlpB2fvjSnCQUiAA6qAZzqwna5envK4uk6OIeP17CsdF3rSBGYVBsU0Tkg==",
-      "requires": {
-        "through": "2"
-      }
-    },
     "string_decoder": {
       "version": "1.3.0",
       "resolved": "https://registry.npmjs.org/string_decoder/-/string_decoder-1.3.0.tgz",
@@ -5224,11 +5158,6 @@
         "supports-hyperlinks": "^2.0.0"
       }
     },
-    "through": {
-      "version": "2.3.8",
-      "resolved": "https://registry.npmjs.org/through/-/through-2.3.8.tgz",
-      "integrity": "sha1-DdTJ/6q8NXlgsbckEV1+Doai4fU="
-    },
     "tinyglobby": {
       "version": "0.2.10",
       "resolved": "https://registry.npmjs.org/tinyglobby/-/tinyglobby-0.2.10.tgz",
diff --git a/package.json b/package.json
index f759c00c..1ce5dc03 100644
--- a/package.json
+++ b/package.json
@@ -45,7 +45,6 @@
     "chokidar": "^3.5.3",
     "commander": "^9.4.1",
     "elm-solve-deps-wasm": "^1.0.2 || ^2.0.0",
-    "split": "^1.0.1",
     "tinyglobby": "^0.2.10",
     "which": "^2.0.2"
   },
@@ -53,7 +52,6 @@
     "@eslint/js": "9.20.0",
     "@types/mocha": "10.0.10",
     "@types/node": "26.0.1",
-    "@types/split": "1.0.5",
     "@types/which": "3.0.4",
     "elm-review": "2.13.1",
     "elm-tooling": "1.18.0",
diff --git a/templates/after.js b/templates/after.js
index 3ea350fc..c6a4cda5 100644
--- a/templates/after.js
+++ b/templates/after.js
@@ -1,29 +1,13 @@
-var net = require('net'),
-  client = net.createConnection(pipeFilename);
-
-client.on('error', function (error) {
-  console.error(error);
-  client.end();
-  process.exit(1);
-});
-
-client.setEncoding('utf8');
-client.setNoDelay(true);
+var { parentPort, workerData } = require('worker_threads');
 
 // Run the Elm app.
-var app = Elm.Test.Generated.Main.init({ flags: Number(process.argv[2]) });
+var app = Elm.Test.Generated.Main.init({ flags: workerData });
 
-client.on('data', function (msg) {
-  app.ports.elmTestPort__receive.send(JSON.parse(msg));
+parentPort.on('message', function (msg) {
+  app.ports.elmTestPort__receive.send(msg);
 });
 
 // Use ports for inter-process communication.
 app.ports.elmTestPort__send.subscribe(function (msg) {
-  // We split incoming messages on the socket on newlines. The gist is that node
-  // is rather unpredictable in whether or not a single `write` will result in a
-  // single `on('data')` callback. Sometimes it does, sometimes multiple writes
-  // result in a single callback and - worst of all - sometimes a single read
-  // results in multiple callbacks, each receiving a piece of the data. The
-  // horror.
-  client.write(msg + '\n');
+  parentPort.postMessage(msg);
 });