diff --git a/src/jsc/bindings/bindings.cpp b/src/jsc/bindings/bindings.cpp
index ae3d8986c710..b3021c959b3a 100644
--- a/src/jsc/bindings/bindings.cpp
+++ b/src/jsc/bindings/bindings.cpp
@@ -3459,7 +3459,12 @@ JSC::EncodedJSValue JSC__JSGlobalObject__createAggregateError(JSC::JSGlobalObjec
     auto& vm = JSC::getVM(globalObject);
     auto scope = DECLARE_THROW_SCOPE(vm);
 
-    WTF::String message = Zig::toString(*arg3);
+    // External-tagged messages transfer ownership of their heap buffer to the
+    // JS string (VirtualMachine.rs marks the joined build-failure message
+    // global and relies on C++ freeing it); everything else is copied so an
+    // untagged ZigString can't leave the AggregateError aliasing the caller's
+    // buffer. See Zig::getErrorInstance.
+    WTF::String message = Zig::isTaggedExternalPtr(arg3->ptr) ? Zig::toString(*arg3) : Zig::toStringCopy(*arg3);
     JSC::JSValue cause = JSC::jsUndefined();
     JSC::JSArray* array = nullptr;
     {
diff --git a/src/jsc/bindings/helpers.h b/src/jsc/bindings/helpers.h
index cb9bc987755c..996920fc149b 100644
--- a/src/jsc/bindings/helpers.h
+++ b/src/jsc/bindings/helpers.h
@@ -393,7 +393,16 @@ static const WTF::String toStringStatic(ZigString str)
 
 static JSC::JSValue getErrorInstance(const ZigString* str, JSC::JSGlobalObject* globalObject)
 {
-    WTF::String message = toString(*str);
+    // External-tagged strings transfer ownership: the caller heap-allocated
+    // the bytes and marked them global so the JS string adopts them (and
+    // frees them via free_global_string) — keep that path, like toIdentifier
+    // does. Everything else is copied: an untagged ZigString would otherwise
+    // become a JSString that aliases the caller's bytes with no ownership,
+    // and several callers (AsyncModule's resolve/download error paths) pass
+    // stack-local buffers that are freed before JS can ever read
+    // error.message. The TypeError/SyntaxError/RangeError siblings below
+    // already copy.
+    WTF::String message = isTaggedExternalPtr(str->ptr) ? toString(*str) : toStringCopy(*str);
     if (message.isNull() && str->len > 0) [[unlikely]] {
         // pending exception while creating an error.
         return {};