diff --git a/Source/WTF/wtf/Vector.h b/Source/WTF/wtf/Vector.h
index ccd6c489f82d5..81139961d7221 100644
--- a/Source/WTF/wtf/Vector.h
+++ b/Source/WTF/wtf/Vector.h
@@ -208,8 +208,17 @@ struct VectorTypeOperations
     }
 };
 
+#if USE(BUN_JSC_ADDITIONS)
+// m_capacity counts elements in 31 bits, next to the borrow bit. 310668@main halved the limit on the size in bytes
+// instead, which halved the capacity of every Vector whose T is wider than a byte. This keeps the limit on the size
+// in bytes from before that change and adds the limit on the count.
+template<typename T>
+constexpr inline bool isValidCapacityForVector(size_t capacity) { return capacity <= std::min<size_t>(std::numeric_limits<unsigned>::max() / sizeof(T), std::numeric_limits<unsigned>::max() >> 1); }
+static_assert(isValidCapacityForVector<uint8_t>(std::numeric_limits<unsigned>::max() >> 1) && !isValidCapacityForVector<uint8_t>((std::numeric_limits<unsigned>::max() >> 1) + 1), "a valid capacity fits in the 31 bits of m_capacity");
+#else
 template<typename T>
 constexpr inline bool isValidCapacityForVector(size_t capacity) { return capacity <= (std::numeric_limits<unsigned>::max() >> 1) / sizeof(T); }
+#endif
 
 template<typename Collection> struct CopyOrMoveToVectorResult;
 
diff --git a/Source/WTF/wtf/text/StringImpl.h b/Source/WTF/wtf/text/StringImpl.h
index cf4a02dac2ec3..6739dfd64ce50 100644
--- a/Source/WTF/wtf/text/StringImpl.h
+++ b/Source/WTF/wtf/text/StringImpl.h
@@ -1268,6 +1268,12 @@ inline constexpr bool StringImpl::isValidLength(size_t length)
     return length <= max;
 }
 
+#if USE(BUN_JSC_ADDITIONS)
+// The UTF-8 converters size a Vector<char16_t> by the length of their input. This fails to compile if a merge
+// takes the halved limit of 310668@main for isValidCapacityForVector again.
+static_assert(isValidCapacityForVector<char16_t>(StringImpl::MaxLength));
+#endif
+
 template<typename T> constexpr size_t StringImpl::tailOffset()
 {
     return roundUpToMultipleOf<alignof(T)>(offsetof(StringImpl, m_hashAndFlags) + sizeof(StringImpl::m_hashAndFlags));