fix: sanitize lone Unicode surrogates in YAML serialization

packages/injected/src/yaml.ts

@@ -21,9 +21,12 @@ export function yamlEscapeKeyIfNeeded(str: string): string {
 }
 
 export function yamlEscapeValueIfNeeded(str: string): string {
-  if (!yamlStringNeedsQuotes(str))
-    return str;
-  return '"' + str.replace(/[\\"\x00-\x1f\x7f-\x9f]/g, c => {
+  // Sanitize lone surrogates (valid in JS, invalid in JSON per RFC 8259 §8.2).
+  // Same approach as cli/driver.ts for non-JS language bindings.
+  const sanitized = (str as any).toWellFormed ? (str as any).toWellFormed() : str;
+  if (!yamlStringNeedsQuotes(sanitized))
+    return sanitized;
+  return '"' + sanitized.replace(/[\\"\x00-\x1f\x7f-\x9f]/g, c => {
     switch (c) {
       case '\\':
         return '\\\\';

packages/playwright-core/src/utils/isomorphic/yaml.ts

@@ -21,9 +21,12 @@ export function yamlEscapeKeyIfNeeded(str: string): string {
 }
 
 export function yamlEscapeValueIfNeeded(str: string): string {
-  if (!yamlStringNeedsQuotes(str))
-    return str;
-  return '"' + str.replace(/[\\"\x00-\x1f\x7f-\x9f]/g, c => {
+  // Sanitize lone surrogates (valid in JS, invalid in JSON per RFC 8259 §8.2).
+  // Same approach as cli/driver.ts for non-JS language bindings.
+  const sanitized = (str as any).toWellFormed ? (str as any).toWellFormed() : str;
+  if (!yamlStringNeedsQuotes(sanitized))
+    return sanitized;
+  return '"' + sanitized.replace(/[\\"\x00-\x1f\x7f-\x9f]/g, c => {
     switch (c) {
       case '\\':
         return '\\\\';

tests/mcp/snapshot-unicode.spec.ts

@@ -0,0 +1,67 @@
+/**
+ * Copyright (c) Microsoft Corporation.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { test, expect } from './fixtures';
+
+test('should handle lone high surrogate in snapshot', async ({ client, server }) => {
+  server.setContent('/', `
+    <div id="container"></div>
+    <script>
+      const container = document.getElementById('container');
+      // Insert lone high surrogate (U+D800)
+      container.textContent = 'before' + String.fromCharCode(0xD800) + 'after';
+    </script>
+  `, 'text/html');
+
+  const response = await client.callTool({
+    name: 'browser_navigate',
+    arguments: {
+      url: server.PREFIX,
+    },
+  });
+
+  expect(response).toHaveResponse({
+    snapshot: expect.any(String),
+  });
+
+  // Lone surrogates should be replaced with U+FFFD (replacement character)
+  expect(response.content[0].text).toContain('\uFFFD');
+});
+
+test('should handle lone low surrogate in snapshot', async ({ client, server }) => {
+  server.setContent('/', `
+    <div id="container"></div>
+    <script>
+      const container = document.getElementById('container');
+      // Insert lone low surrogate (U+DC00)
+      container.textContent = 'before' + String.fromCharCode(0xDC00) + 'after';
+    </script>
+  `, 'text/html');
+
+  const response = await client.callTool({
+    name: 'browser_navigate',
+    arguments: {
+      url: server.PREFIX,
+    },
+  });
+
+  expect(response).toHaveResponse({
+    snapshot: expect.any(String),
+  });
+
+  // Lone surrogates should be replaced with U+FFFD
+  expect(response.content[0].text).toContain('\uFFFD');
+});