diff --git a/ext/node/lib.rs b/ext/node/lib.rs
index b4eeb71c25..2b31f704f6 100644
--- a/ext/node/lib.rs
+++ b/ext/node/lib.rs
@@ -186,6 +186,8 @@ deno_core::extension!(deno_node,
   deps = [ deno_io, deno_fs ],
   parameters = [P: NodePermissions],
   ops = [
+    ops::buffer::op_is_ascii,
+    ops::buffer::op_is_utf8,
     ops::crypto::op_node_create_decipheriv,
     ops::crypto::op_node_cipheriv_encrypt,
     ops::crypto::op_node_cipheriv_final,
diff --git a/ext/node/ops/buffer.rs b/ext/node/ops/buffer.rs
new file mode 100644
index 0000000000..74a011ab80
--- /dev/null
+++ b/ext/node/ops/buffer.rs
@@ -0,0 +1,13 @@
+// Copyright 2018-2024 the Deno authors. All rights reserved. MIT license.
+
+use deno_core::op2;
+
+#[op2(fast)]
+pub fn op_is_ascii(#[buffer] buf: &[u8]) -> bool {
+  buf.is_ascii()
+}
+
+#[op2(fast)]
+pub fn op_is_utf8(#[buffer] buf: &[u8]) -> bool {
+  std::str::from_utf8(buf).is_ok()
+}
diff --git a/ext/node/ops/mod.rs b/ext/node/ops/mod.rs
index 6381530dd1..ae703e3f36 100644
--- a/ext/node/ops/mod.rs
+++ b/ext/node/ops/mod.rs
@@ -1,5 +1,6 @@
 // Copyright 2018-2024 the Deno authors. All rights reserved. MIT license.
 
+pub mod buffer;
 pub mod crypto;
 pub mod fs;
 pub mod http;
diff --git a/ext/node/polyfills/buffer.ts b/ext/node/polyfills/buffer.ts
index 5925475c49..c5a910cb40 100644
--- a/ext/node/polyfills/buffer.ts
+++ b/ext/node/polyfills/buffer.ts
@@ -7,6 +7,8 @@ export {
   Buffer,
   constants,
   default,
+  isAscii,
+  isUtf8,
   kMaxLength,
   kStringMaxLength,
   SlowBuffer,
diff --git a/ext/node/polyfills/internal/buffer.mjs b/ext/node/polyfills/internal/buffer.mjs
index 5c76a21a58..0521c56aaa 100644
--- a/ext/node/polyfills/internal/buffer.mjs
+++ b/ext/node/polyfills/internal/buffer.mjs
@@ -6,6 +6,7 @@
 // deno-lint-ignore-file prefer-primordials
 
 import { core } from "ext:core/mod.js";
+import { op_is_ascii, op_is_utf8 } from "ext:core/ops";
 
 import { TextDecoder, TextEncoder } from "ext:deno_web/08_text_encoding.js";
 import { codes } from "ext:deno_node/internal/error_codes.ts";
@@ -26,10 +27,12 @@ import {
 import {
   isAnyArrayBuffer,
   isArrayBufferView,
+  isTypedArray,
 } from "ext:deno_node/internal/util/types.ts";
 import { normalizeEncoding } from "ext:deno_node/internal/util.mjs";
 import { validateBuffer } from "ext:deno_node/internal/validators.mjs";
 import { isUint8Array } from "ext:deno_node/internal/util/types.ts";
+import { ERR_INVALID_STATE } from "ext:deno_node/internal/errors.ts";
 import {
   forgivingBase64Encode,
   forgivingBase64UrlEncode,
@@ -2536,12 +2539,58 @@ export function writeU_Int24LE(buf, value, offset, min, max) {
   return offset;
 }
 
+export function isUtf8(input) {
+  if (isTypedArray(input)) {
+    if (input.buffer.detached) {
+      throw new ERR_INVALID_STATE("Cannot validate on a detached buffer");
+    }
+    return op_is_utf8(input);
+  }
+
+  if (isAnyArrayBuffer(input)) {
+    if (input.detached) {
+      throw new ERR_INVALID_STATE("Cannot validate on a detached buffer");
+    }
+    return op_is_utf8(new Uint8Array(input));
+  }
+
+  throw new codes.ERR_INVALID_ARG_TYPE("input", [
+    "ArrayBuffer",
+    "Buffer",
+    "TypedArray",
+  ], input);
+}
+
+export function isAscii(input) {
+  if (isTypedArray(input)) {
+    if (input.buffer.detached) {
+      throw new ERR_INVALID_STATE("Cannot validate on a detached buffer");
+    }
+    return op_is_ascii(input);
+  }
+
+  if (isAnyArrayBuffer(input)) {
+    if (input.detached) {
+      throw new ERR_INVALID_STATE("Cannot validate on a detached buffer");
+    }
+    return op_is_ascii(new Uint8Array(input));
+  }
+
+  throw new codes.ERR_INVALID_ARG_TYPE("input", [
+    "ArrayBuffer",
+    "Buffer",
+    "TypedArray",
+  ], input);
+}
+
 export default {
   atob,
   btoa,
   Blob,
   Buffer,
   constants,
+  isAscii,
+  isUtf8,
   kMaxLength,
   kStringMaxLength,
   SlowBuffer,
diff --git a/ext/node/polyfills/internal/errors.ts b/ext/node/polyfills/internal/errors.ts
index c3aeff8b20..a16656087e 100644
--- a/ext/node/polyfills/internal/errors.ts
+++ b/ext/node/polyfills/internal/errors.ts
@@ -2564,6 +2564,12 @@ export class ERR_HTTP_SOCKET_ASSIGNED extends NodeError {
   }
 }
 
+export class ERR_INVALID_STATE extends NodeError {
+  constructor(message: string) {
+    super("ERR_INVALID_STATE", `Invalid state: ${message}`);
+  }
+}
+
 interface UvExceptionContext {
   syscall: string;
   path?: string;
@@ -2824,6 +2830,7 @@ export default {
   ERR_INVALID_RETURN_PROPERTY,
   ERR_INVALID_RETURN_PROPERTY_VALUE,
   ERR_INVALID_RETURN_VALUE,
+  ERR_INVALID_STATE,
   ERR_INVALID_SYNC_FORK_INPUT,
   ERR_INVALID_THIS,
   ERR_INVALID_TUPLE,
diff --git a/tests/node_compat/config.jsonc b/tests/node_compat/config.jsonc
index af45cc6631..56f43b444f 100644
--- a/tests/node_compat/config.jsonc
+++ b/tests/node_compat/config.jsonc
@@ -179,7 +179,9 @@
       "test-buffer-includes.js",
       "test-buffer-indexof.js",
       "test-buffer-inheritance.js",
+      "test-buffer-isascii.js",
       "test-buffer-isencoding.js",
+      "test-buffer-isutf8.js",
       "test-buffer-iterator.js",
       "test-buffer-new.js",
       "test-buffer-no-negative-allocation.js",
diff --git a/tests/node_compat/runner/TODO.md b/tests/node_compat/runner/TODO.md
index b812981e5f..0bc19a0766 100644
--- a/tests/node_compat/runner/TODO.md
+++ b/tests/node_compat/runner/TODO.md
@@ -227,8 +227,6 @@ NOTE: This file should not be manually edited. Please edit `tests/node_compat/co
 - [parallel/test-buffer-constructor-outside-node-modules.js](https://github.com/nodejs/node/tree/v18.12.1/test/parallel/test-buffer-constructor-outside-node-modules.js)
 - [parallel/test-buffer-fill.js](https://github.com/nodejs/node/tree/v18.12.1/test/parallel/test-buffer-fill.js)
 - [parallel/test-buffer-inspect.js](https://github.com/nodejs/node/tree/v18.12.1/test/parallel/test-buffer-inspect.js)
-- [parallel/test-buffer-isascii.js](https://github.com/nodejs/node/tree/v18.12.1/test/parallel/test-buffer-isascii.js)
-- [parallel/test-buffer-isutf8.js](https://github.com/nodejs/node/tree/v18.12.1/test/parallel/test-buffer-isutf8.js)
 - [parallel/test-buffer-pending-deprecation.js](https://github.com/nodejs/node/tree/v18.12.1/test/parallel/test-buffer-pending-deprecation.js)
 - [parallel/test-buffer-pool-untransferable.js](https://github.com/nodejs/node/tree/v18.12.1/test/parallel/test-buffer-pool-untransferable.js)
 - [parallel/test-buffer-prototype-inspect.js](https://github.com/nodejs/node/tree/v18.12.1/test/parallel/test-buffer-prototype-inspect.js)
diff --git a/tests/node_compat/test/parallel/test-buffer-isascii.js b/tests/node_compat/test/parallel/test-buffer-isascii.js
new file mode 100644
index 0000000000..59612bb9d9
--- /dev/null
+++ b/tests/node_compat/test/parallel/test-buffer-isascii.js
@@ -0,0 +1,49 @@
+// deno-fmt-ignore-file
+// deno-lint-ignore-file
+
+// Copyright Joyent and Node contributors. All rights reserved. MIT license.
+// Taken from Node 18.12.1
+// This file is automatically generated by `tests/node_compat/runner/setup.ts`. Do not modify this file manually.
+
+'use strict';
+
+require('../common');
+const assert = require('assert');
+const { isAscii, Buffer } = require('buffer');
+const { TextEncoder } = require('util');
+
+const encoder = new TextEncoder();
+
+assert.strictEqual(isAscii(encoder.encode('hello')), true);
+assert.strictEqual(isAscii(encoder.encode('ğ')), false);
+assert.strictEqual(isAscii(Buffer.from([])), true);
+
+[
+  undefined,
+  '', 'hello',
+  false, true,
+  0, 1,
+  0n, 1n,
+  Symbol(),
+  () => {},
+  {}, [], null,
+].forEach((input) => {
+  assert.throws(
+    () => { isAscii(input); },
+    {
+      code: 'ERR_INVALID_ARG_TYPE',
+    },
+  );
+});
+
+{
+  // Test with detached array buffers
+  const arrayBuffer = new ArrayBuffer(1024);
+  structuredClone(arrayBuffer, { transfer: [arrayBuffer] });
+  assert.throws(
+    () => { isAscii(arrayBuffer); },
+    {
+      code: 'ERR_INVALID_STATE'
+    }
+  );
+}
diff --git a/tests/node_compat/test/parallel/test-buffer-isutf8.js b/tests/node_compat/test/parallel/test-buffer-isutf8.js
new file mode 100644
index 0000000000..02f142db89
--- /dev/null
+++ b/tests/node_compat/test/parallel/test-buffer-isutf8.js
@@ -0,0 +1,93 @@
+// deno-fmt-ignore-file
+// deno-lint-ignore-file
+
+// Copyright Joyent and Node contributors. All rights reserved. MIT license.
+// Taken from Node 18.12.1
+// This file is automatically generated by `tests/node_compat/runner/setup.ts`. Do not modify this file manually.
+
+'use strict';
+
+require('../common');
+const assert = require('assert');
+const { isUtf8, Buffer } = require('buffer');
+const { TextEncoder } = require('util');
+
+const encoder = new TextEncoder();
+
+assert.strictEqual(isUtf8(encoder.encode('hello')), true);
+assert.strictEqual(isUtf8(encoder.encode('ğ')), true);
+assert.strictEqual(isUtf8(Buffer.from([])), true);
+
+// Taken from test/fixtures/wpt/encoding/textdecoder-fatal.any.js
+[
+  [0xFF], // 'invalid code'
+  [0xC0], // 'ends early'
+  [0xE0], // 'ends early 2'
+  [0xC0, 0x00], // 'invalid trail'
+  [0xC0, 0xC0], // 'invalid trail 2'
+  [0xE0, 0x00], // 'invalid trail 3'
+  [0xE0, 0xC0], // 'invalid trail 4'
+  [0xE0, 0x80, 0x00], // 'invalid trail 5'
+  [0xE0, 0x80, 0xC0], // 'invalid trail 6'
+  [0xFC, 0x80, 0x80, 0x80, 0x80, 0x80], // '> 0x10FFFF'
+  [0xFE, 0x80, 0x80, 0x80, 0x80, 0x80], // 'obsolete lead byte'
+
+  // Overlong encodings
+  [0xC0, 0x80], // 'overlong U+0000 - 2 bytes'
+  [0xE0, 0x80, 0x80], // 'overlong U+0000 - 3 bytes'
+  [0xF0, 0x80, 0x80, 0x80], // 'overlong U+0000 - 4 bytes'
+  [0xF8, 0x80, 0x80, 0x80, 0x80], // 'overlong U+0000 - 5 bytes'
+  [0xFC, 0x80, 0x80, 0x80, 0x80, 0x80], // 'overlong U+0000 - 6 bytes'
+
+  [0xC1, 0xBF], // 'overlong U+007F - 2 bytes'
+  [0xE0, 0x81, 0xBF], // 'overlong U+007F - 3 bytes'
+  [0xF0, 0x80, 0x81, 0xBF], // 'overlong U+007F - 4 bytes'
+  [0xF8, 0x80, 0x80, 0x81, 0xBF], // 'overlong U+007F - 5 bytes'
+  [0xFC, 0x80, 0x80, 0x80, 0x81, 0xBF], // 'overlong U+007F - 6 bytes'
+
+  [0xE0, 0x9F, 0xBF], // 'overlong U+07FF - 3 bytes'
+  [0xF0, 0x80, 0x9F, 0xBF], // 'overlong U+07FF - 4 bytes'
+  [0xF8, 0x80, 0x80, 0x9F, 0xBF], // 'overlong U+07FF - 5 bytes'
+  [0xFC, 0x80, 0x80, 0x80, 0x9F, 0xBF], // 'overlong U+07FF - 6 bytes'
+
+  [0xF0, 0x8F, 0xBF, 0xBF], // 'overlong U+FFFF - 4 bytes'
+  [0xF8, 0x80, 0x8F, 0xBF, 0xBF], // 'overlong U+FFFF - 5 bytes'
+  [0xFC, 0x80, 0x80, 0x8F, 0xBF, 0xBF], // 'overlong U+FFFF - 6 bytes'
+
+  [0xF8, 0x84, 0x8F, 0xBF, 0xBF], // 'overlong U+10FFFF - 5 bytes'
+  [0xFC, 0x80, 0x84, 0x8F, 0xBF, 0xBF], // 'overlong U+10FFFF - 6 bytes'
+
+  // UTF-16 surrogates encoded as code points in UTF-8
+  [0xED, 0xA0, 0x80], // 'lead surrogate'
+  [0xED, 0xB0, 0x80], // 'trail surrogate'
+  [0xED, 0xA0, 0x80, 0xED, 0xB0, 0x80], // 'surrogate pair'
+].forEach((input) => {
+  assert.strictEqual(isUtf8(Buffer.from(input)), false);
+});
+
+[
+  null,
+  undefined,
+  'hello',
+  true,
+  false,
+].forEach((input) => {
+  assert.throws(
+    () => { isUtf8(input); },
+    {
+      code: 'ERR_INVALID_ARG_TYPE',
+    },
+  );
+});
+
+{
+  // Test with detached array buffers
+  const arrayBuffer = new ArrayBuffer(1024);
+  structuredClone(arrayBuffer, { transfer: [arrayBuffer] });
+  assert.throws(
+    () => { isUtf8(arrayBuffer); },
+    {
+      code: 'ERR_INVALID_STATE'
+    }
+  );
+}
diff --git a/tools/copyright_checker.js b/tools/copyright_checker.js
index 2cabab6fac..4ffde77c1b 100755
--- a/tools/copyright_checker.js
+++ b/tools/copyright_checker.js
@@ -103,6 +103,7 @@ export async function checkCopyright() {
     // show all the errors at the same time to prevent overlap with
     // other running scripts that may be outputting
     console.error(errors.join("\n"));
+    console.error(`Expected copyright:\n\`\`\`\n${COPYRIGHT_LINE}\n\`\`\``);
     throw new Error(`Copyright checker had ${errors.length} errors.`);
   }
 }