@@ -44,7 +44,7 @@
 
     # Reset this number to 0 on major V8 upgrades.
     # Increment by one for each non-official patch applied to deps/v8.
-    'v8_embedder_string': '-node.39',
+    'v8_embedder_string': '-node.40',
 
     ##### V8 defaults for Node.js #####
 
 
@@ -1067,7 +1067,12 @@ class JSDataObjectBuilder {
                   descriptor_index)),
           isolate_);
       target_map = expected_final_map_;
-    } else {
+    } else if (key_chars.data() != nullptr) {
+      // The byte-compare transition fast path is only valid when `key_chars`
+      // faithfully represent the decoded key. Callers that cannot provide valid
+      // chars (the JSON parser passes an empty vector for escaped keys, whose raw
+      // source bytes differ from the decoded key) fall through to the
+      // decode-then-FindTransitionToField path below.
       TransitionsAccessor transitions(isolate_, *map_);
       auto expected_transition = transitions.ExpectedTransition(key_chars);
       if (!expected_transition.first.is_null()) {
 
@@ -376,6 +376,12 @@ class JsonParser final {
   JsonString ScanJsonPropertyKey(JsonContinuation* cont);
   base::uc32 ScanUnicodeCharacter();
   base::Vector<const Char> GetKeyChars(JsonString key) {
+    // For escaped keys the source range starting at `key.start()` holds the raw
+    // (undecoded) characters while `key.length()` is the decoded length, so the
+    // bytes here do not represent the actual decoded key. Return an empty
+    // vector to signal that the byte-compare transition fast path must be
+    // skipped (see JSDataObjectBuilder::TryFastTransitionToPropertyKey).
+    if (key.has_escape()) return base::Vector<const Char>();
     return base::Vector<const Char>(chars_ + key.start(), key.length());
   }
   Handle<String> MakeString(const JsonString& string,
 
@@ -0,0 +1,76 @@
+// Copyright 2026 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Regression test: JSON.parse must not match a hidden-class transition for an
+// escaped property key using the raw (undecoded) source bytes. The builder's
+// fast transition path compared `chars_ + key.start()` over `key.length()`
+// (the *decoded* length), so escaped keys (e.g. "\t", "\uXXXX") collapsed to
+// the byte 0x5C and matched a previously-seen "\\" key, making JSON.parse adopt
+// the wrong key. Transitions are isolate-global, so a prior parse corrupted a
+// later, unrelated parse.
+
+// A single-char escaped key must keep its decoded value even after a sibling
+// "\\" (backslash) key has been parsed on the same prefix map.
+(function TestEscapedKeyNotRenamedByPriorBackslashKey() {
+  function canaryKeys() {
+    return Object.keys(JSON.parse('{"\\r":1,"\\f":2}'));
+  }
+  assertArrayEquals(['\r', '\f'], canaryKeys());
+  // Plants a {<prefix>} -> "\" transition in the global transition tree.
+  JSON.parse('{"\\r":3,"\\\\":4}');
+  // Must be unaffected: "\f" stays U+000C, not U+005C.
+  assertArrayEquals(['\r', '\f'], canaryKeys());
+})();
+
+// Multi-character escaped key: decoded length 2 must not be compared against
+// the first two raw bytes ("\" + "r").
+(function TestMultiCharEscapedKey() {
+  function keys() {
+    return Object.keys(JSON.parse('{"\\t":1,"\\r\\r":2}'));
+  }
+  assertArrayEquals(['\t', '\r\r'], keys());
+  JSON.parse('{"\\t":1,"\\\\r":2}');  // plants the literal "\r" (0x5C 0x72) key
+  assertArrayEquals(['\t', '\r\r'], keys());
+})();
+
+// \uXXXX-escaped single-character key (here decodes to plain 'A').
+(function TestUnicodeEscapedKey() {
+  function keys() {
+    return Object.keys(JSON.parse('{"x":1,"\\u0041":2}'));
+  }
+  assertArrayEquals(['x', 'A'], keys());
+  JSON.parse('{"x":1,"\\\\":2}');
+  assertArrayEquals(['x', 'A'], keys());
+})();
+
+// Override/collide: an escaped key must not be renamed onto a real "\" sibling
+// and silently destroy a property (key count must be preserved).
+(function TestNoPropertyDestruction() {
+  // Prime the transition state that previously triggered the collapse.
+  JSON.parse('{"p":1,"\\\\":0}');
+  JSON.parse('{"p":1,"\\\\":0,"\\t":0}');
+  const o = JSON.parse('{"p":1,"\\t":"a","\\\\":"b"}');
+  assertEquals(3, Object.keys(o).length);
+  assertEquals('a', o['\t']);
+  assertEquals('b', o['\\']);
+})();
+
+// Self-contained single document: a later sibling object value must not be
+// corrupted by an earlier one within the same parse.
+(function TestNestedSiblingNotCorrupted() {
+  const o = JSON.parse(
+      '{"a":{"p":1,"\\\\":9},"b":{"p":1,"\\\\":"A","\\t":7},' +
+      '"c":{"p":1,"\\t":7,"\\\\":"B"}}');
+  assertEquals(3, Object.keys(o.c).length);
+  assertEquals(7, o.c['\t']);
+  assertEquals('B', o.c['\\']);
+})();
+
+// A plain (unescaped) empty key must still work (regression guard for the
+// empty-key_chars path the fix relies on).
+(function TestEmptyKeyStillWorks() {
+  const o = JSON.parse('{"a":1,"":2,"b":3}');
+  assertArrayEquals(['a', '', 'b'], Object.keys(o));
+  assertEquals(2, o['']);
+})();