diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 4f783c2..e78f0f1 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -17,6 +17,7 @@ jobs:
           - 22.x
           - 24.x
           - 25.x
+          - 26.x
     runs-on: ${{ matrix.runner }}
     steps:
     - name: Harden Runner
diff --git a/implementors/node/assert.js b/implementors/node/assert.js
index 8320281..3a6be80 100644
--- a/implementors/node/assert.js
+++ b/implementors/node/assert.js
@@ -7,16 +7,17 @@ import {
   match,
 } from 'node:assert/strict';
 
-const assert = Object.assign((value, message) => ok(value, message), {
-  ok: (value, message) => ok(value, message),
-  strictEqual: (actual, expected, message) =>
-    strictEqual(actual, expected, message),
-  notStrictEqual: (actual, expected, message) =>
-    notStrictEqual(actual, expected, message),
-  deepStrictEqual: (actual, expected, message) =>
-    deepStrictEqual(actual, expected, message),
-  throws: (fn, error, message) => throws(fn, error, message),
-  match: (string, regex, message) => match(string, regex, message),
+// Forward with rest arguments rather than named parameters: an omitted trailing
+// message has to stay omitted. Node.js 26 reads the message as a variadic tuple,
+// so an explicitly passed `undefined` is a message of the wrong type there and
+// the assertion fails with ERR_INVALID_ARG_TYPE instead of the value comparison.
+const assert = Object.assign((...args) => ok(...args), {
+  ok: (...args) => ok(...args),
+  strictEqual: (...args) => strictEqual(...args),
+  notStrictEqual: (...args) => notStrictEqual(...args),
+  deepStrictEqual: (...args) => deepStrictEqual(...args),
+  throws: (...args) => throws(...args),
+  match: (...args) => match(...args),
 });
 
 Object.assign(globalThis, { assert });
diff --git a/tests/harness/assert.js b/tests/harness/assert.js
index 65bf43c..a1848ae 100644
--- a/tests/harness/assert.js
+++ b/tests/harness/assert.js
@@ -105,3 +105,49 @@ if (!threw) throw new Error('assert.match must throw when input is not a string'
 threw = false;
 try { assert.match('hello', 'hello'); } catch { threw = true; }
 if (!threw) throw new Error('assert.match must throw when pattern is not a RegExp');
+
+// The message is optional on every method. A failure without one must still
+// report the comparison, and a failure with one must report that message.
+function failureOf(label, fn) {
+  try {
+    fn();
+  } catch (error) {
+    return error;
+  }
+  throw new Error(`${label} was expected to throw`);
+}
+
+const optionalMessage = [
+  ['assert', () => assert(false), (m) => assert(false, m)],
+  ['assert.ok', () => assert.ok(false), (m) => assert.ok(false, m)],
+  ['assert.strictEqual', () => assert.strictEqual(1, 2), (m) => assert.strictEqual(1, 2, m)],
+  ['assert.notStrictEqual', () => assert.notStrictEqual(1, 1), (m) => assert.notStrictEqual(1, 1, m)],
+  [
+    'assert.deepStrictEqual',
+    () => assert.deepStrictEqual({ a: 1 }, { a: 2 }),
+    (m) => assert.deepStrictEqual({ a: 1 }, { a: 2 }, m),
+  ],
+  ['assert.match', () => assert.match('hello', /world/), (m) => assert.match('hello', /world/, m)],
+  ['assert.throws', () => assert.throws(() => {}, /oops/), (m) => assert.throws(() => {}, /oops/, m)],
+];
+
+const customMessage = 'a message the caller chose';
+
+for (const [label, withoutMessage, withMessage] of optionalMessage) {
+  // Only a substring check: the strict assert methods append their own value
+  // comparison to a caller supplied message.
+  const described = failureOf(label, () => withMessage(customMessage));
+  if (!described.message.includes(customMessage)) {
+    throw new Error(
+      `${label} must fail with the message it was given but failed with "${described.message}"`,
+    );
+  }
+
+  const bare = failureOf(label, withoutMessage);
+  if (bare.name !== described.name) {
+    throw new Error(
+      `${label} must fail the same way with and without a message, but without one it failed ` +
+      `with "${bare.name}: ${bare.message}" instead of ${described.name}`,
+    );
+  }
+}