diff --git a/v8/src/maglev/maglev-graph-builder.cc b/v8/src/maglev/maglev-graph-builder.cc
index 81afa73b592..bandaidfix000 100644
--- a/v8/src/maglev/maglev-graph-builder.cc
+++ b/v8/src/maglev/maglev-graph-builder.cc
@@ -17652,6 +17652,15 @@ std::optional<Float64> MaglevGraphBuilder::TryGetFloat64OrHoleyFloat64Constant(
 MaybeHandle<String> MaglevGraphBuilder::TryGetStringConstant(ValueNode* value) {
   if (Constant* constant = value->TryCast<Constant>()) {
     if (constant->object().IsString()) {
+      // The caller (BuildStringConcat) reads the string's characters via
+      // ConcatenateStrings -> String::WriteToFlat on a background compiler
+      // thread. Gate on IsContentAccessible() so that uncached, non-cacheable
+      // ExternalStrings (e.g. SlicedString over a Blink ParkableString-backed
+      // Script::source()) are not read concurrently without resource->Lock().
+      // Mirrors JSNativeContextSpecialization::StringCanSafelyBeRead.
+      if (!constant->object().AsString().IsContentAccessible()) {
+        return {};
+      }
       return handle(Cast<String>(*constant->object().object()),
                     local_isolate());
     }