diff --git a/Source/WTF/wtf/bun/BannedIncludes/iostream b/Source/WTF/wtf/bun/BannedIncludes/iostream
new file mode 100644
index 0000000000000..ddfe8423927e9
--- /dev/null
+++ b/Source/WTF/wtf/bun/BannedIncludes/iostream
@@ -0,0 +1,17 @@
+// This directory is placed first on the include search path for non-Debug
+// builds when USE_BUN_JSC_ADDITIONS is on, so `#include <iostream>` resolves
+// here instead of the toolchain header.
+//
+// <iostream> is unlike <ostream>/<istream>/<sstream>: on libstdc++ it emits a
+// reference to std::ios_base_library_init in every translation unit that
+// includes it. One such reference anywhere in the link pulls globals_io.o from
+// libstdc++, whose static initializer constructs cin/cout/cerr/clog and their
+// wchar_t siblings before main, dragging the full std::locale facet set in
+// with it. Bun never touches C++ iostreams at runtime.
+//
+// Use WTF::dataLog()/dataLogLn() or fputs/fprintf for diagnostics.
+#ifndef BUN_ALLOW_IOSTREAM
+#error "<iostream> is banned in Bun WebKit release builds: it forces std::ios_base::Init and the full std::locale facet set to run before main in every bun process. Use WTF::dataLog() or <cstdio>. See Source/WTF/wtf/bun/BannedIncludes/iostream."
+#else
+#include_next <iostream>
+#endif
diff --git a/Source/WTF/wtf/simdutf/simdutf_impl.h b/Source/WTF/wtf/simdutf/simdutf_impl.h
index 10110d1a703af..da723062681e8 100644
--- a/Source/WTF/wtf/simdutf/simdutf_impl.h
+++ b/Source/WTF/wtf/simdutf/simdutf_impl.h
@@ -9942,11 +9942,9 @@ static_assert(to_base64_url_value[uint8_t('_')] == 63,
 #ifndef SIMDUTF_BASE64_H
 #define SIMDUTF_BASE64_H
 
-#include <algorithm>
 #include <cstddef>
 #include <cstdint>
 #include <cstring>
-#include <iostream>
 
 namespace simdutf {
 namespace scalar {
diff --git a/Source/cmake/OptionsJSCOnly.cmake b/Source/cmake/OptionsJSCOnly.cmake
index dc25e8945abbd..988efb83e1533 100644
--- a/Source/cmake/OptionsJSCOnly.cmake
+++ b/Source/cmake/OptionsJSCOnly.cmake
@@ -46,6 +46,16 @@ if(USE_BUN_JSC_ADDITIONS)
     # Causing test/cli/test/bun-test.test.ts to fail.
     SET_AND_EXPOSE_TO_BUILD(BUSE_TZONE 0)
     SET_AND_EXPOSE_TO_BUILD(USE_TZONE_MALLOC 0)
+
+    # Shadow <iostream> with a #error shim in non-Debug builds. On libstdc++ a
+    # single <iostream> include anywhere emits a reference to
+    # std::ios_base_library_init, which pulls globals_io.o (the
+    # cin/cout/cerr/clog + full std::locale facet static initializer) into the
+    # final Bun link. <ostream>/<istream>/<sstream> are unaffected. Debug
+    # builds keep the real header for ad-hoc dataLog-style debugging.
+    if(NOT CMAKE_BUILD_TYPE STREQUAL "Debug")
+        include_directories(BEFORE "${WTF_DIR}/wtf/bun/BannedIncludes")
+    endif()
 endif()
 
 # Only works on macOS for now.