diff --git a/libc/test/src/__support/File/CMakeLists.txt b/libc/test/src/__support/File/CMakeLists.txt
@@ -21,18 +21,34 @@ add_libc_test(
     libc.src.__support.File.file
 )
 
-if (TARGET libc.src.__support.File.platform_file)
-  add_libc_test(
-    platform_file_test
-    SUITE
-      libc-support-tests
-    SRCS
-      platform_file_test.cpp
-    DEPENDS
-      libc.src.__support.File.file
-      libc.src.__support.File.platform_file
-      libc.include.stdio
-  )
-endif()
+add_libc_test(
+  platform_file_test
+  SUITE
+    libc-support-tests
+  SRCS
+    platform_file_test.cpp
+  DEPENDS
+    libc.src.__support.File.file
+    libc.src.__support.File.platform_file
+    libc.include.stdio
+)
+
+set(platform_stream_targets "platform_stdout;platform_stdin;platform_stderr")
+foreach(target IN LISTS platform_file_targets)
+  if(TARGET libc.src.__support.File.${target})
+    add_libc_test(
+      ${target}_test
+      HERMETIC_TEST_ONLY
+      SUITE
+        libc-support-tests
+      SRCS
+        ${target}_test.cpp
+      DEPENDS
+        libc.src.__support.File.file
+        libc.src.__support.File.${target}
+        libc.include.stdio
+    )
+  endif()
+endforeach()
 
 add_subdirectory(testdata)
diff --git a/libc/test/src/__support/File/platform_file_test.cpp b/libc/test/src/__support/File/platform_file_test.cpp
@@ -201,8 +201,3 @@ TEST(LlvmLibcPlatformFileTest, IncorrectOperation) {
   ASSERT_TRUE(file->error());
   ASSERT_EQ(file->close(), 0);
 }
-
-TEST(LlvmLibcPlatformFileTest, StdOutStdErrSmokeTest) {
-  EXPECT_FALSE(__llvm_libc::stdout == nullptr);
-  EXPECT_FALSE(__llvm_libc::stderr == nullptr);
-}
diff --git a/libc/test/src/__support/File/platform_stderr_test.cpp b/libc/test/src/__support/File/platform_stderr_test.cpp
@@ -0,0 +1,12 @@
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/__support/File/file.h"
+#include "test/UnitTest/Test.h"
+
+TEST(LlvmLibcPlatformStreamTest, StdErrSmokeTest) {
+  EXPECT_FALSE(__llvm_libc::stderr == nullptr);
+}
diff --git a/libc/test/src/__support/File/platform_stdin_test.cpp b/libc/test/src/__support/File/platform_stdin_test.cpp
@@ -0,0 +1,12 @@
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/__support/File/file.h"
+#include "test/UnitTest/Test.h"
+
+TEST(LlvmLibcPlatformStreamTest, StdInSmokeTest) {
+  EXPECT_FALSE(__llvm_libc::stdin == nullptr);
+}
diff --git a/libc/test/src/__support/File/platform_stdout_test.cpp b/libc/test/src/__support/File/platform_stdout_test.cpp
@@ -0,0 +1,12 @@
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/__support/File/file.h"
+#include "test/UnitTest/Test.h"
+
+TEST(LlvmLibcPlatformStreamTest, StdOutSmokeTest) {
+  EXPECT_FALSE(__llvm_libc::stdout == nullptr);
+}
diff --git a/libc/test/src/stdio/CMakeLists.txt b/libc/test/src/stdio/CMakeLists.txt
@@ -136,13 +136,16 @@ add_libc_unittest(
 )
 
 if(LLVM_LIBC_FULL_BUILD)
-# In fullbuild mode, fprintf's tests use the internal FILE for other functions.
- list(APPEND fprintf_test_deps
-      libc.src.stdio.fclose
-      libc.src.stdio.ferror
-      libc.src.stdio.fopen
-      libc.src.stdio.fread
+  # In fullbuild mode, fprintf's tests use the internal FILE for other functions.
+  list(APPEND fprintf_test_deps
+       libc.src.stdio.fclose
+       libc.src.stdio.ferror
+       libc.src.stdio.fopen
+       libc.src.stdio.fread
   )
+  # This is to be used for tests which write to libc's platform streams
+  # under full build but write to system-lib's streams otherwise.
+  set(hermetic_test_only HERMETIC_TEST_ONLY)
 else()
 # Else in overlay mode they use the system's FILE.
  set(fprintf_test_copts "-DLIBC_COPT_PRINTF_USE_SYSTEM_FILE")
@@ -161,8 +164,9 @@ add_libc_unittest(
     ${fprintf_test_copts}
 )
 
-add_libc_unittest(
+add_libc_test(
   printf_test
+  ${hermetic_test_only}
   SUITE
     libc_stdio_unittests
   SRCS
@@ -205,8 +209,9 @@ add_libc_unittest(
     ${fprintf_test_copts}
 )
 
-add_libc_unittest(
+add_libc_test(
   vprintf_test
+  ${hermetic_test_only}
   SUITE
     libc_stdio_unittests
   SRCS
@@ -244,6 +249,7 @@ add_libc_unittest(
 
 add_libc_test(
   puts_test
+  HERMETIC_TEST_ONLY # writes to libc's stdout
   SUITE
     libc_stdio_unittests
   SRCS
@@ -254,6 +260,7 @@ add_libc_test(
 
 add_libc_test(
   fputs_test
+  HERMETIC_TEST_ONLY # writes to libc's stdout and stderr
   SUITE
     libc_stdio_unittests
   SRCS

diff --git a/libc/test/src/unistd/CMakeLists.txt b/libc/test/src/unistd/CMakeLists.txt
@@ -430,8 +430,9 @@ add_libc_unittest(
     libc.src.unistd.sysconf
 )
 
-add_libc_unittest(
+add_libc_test(
   getopt_test
+  HERMETIC_TEST_ONLY # Uses libc's own stderr
   SUITE
     libc_unistd_unittests
   SRCS

diff --git a/libc/test/src/unistd/getopt_test.cpp b/libc/test/src/unistd/getopt_test.cpp
@@ -34,27 +34,25 @@ void set_state(FILE *errstream) {
       &test_globals::optpos, &test_globals::opterr, errstream);
 }
 
+static void my_memcpy(char *dest, const char *src, size_t size) {
+  for (size_t i = 0; i < size; i++)
+    dest[i] = src[i];
+}
+
+ssize_t cookie_write(void *cookie, const char *buf, size_t size) {
+  char **pos = static_cast<char **>(cookie);
+  my_memcpy(*pos, buf, size);
+  *pos += size;
+  return size;
+}
+
+static cookie_io_functions_t cookie{nullptr, &cookie_write, nullptr, nullptr};
+
 // TODO: <stdio> could be either llvm-libc's or the system libc's. The former
 // doesn't currently support fmemopen but does have fopencookie. In the future
 // just use that instead. This memopen does no error checking for the size
 // of the buffer, etc.
-FILE *memopen(char **pos) {
-  static auto memcpy = [](char *dest, const char *src, size_t size) {
-    for (size_t i = 0; i < size; i++)
-      dest[i] = src[i];
-  };
-
-  static auto *write =
-      +[](void *cookie, const char *buf, size_t size) -> ssize_t {
-    char **pos = static_cast<char **>(cookie);
-    memcpy(*pos, buf, size);
-    *pos += size;
-    return size;
-  };
-
-  static cookie_io_functions_t cookie{nullptr, write, nullptr, nullptr};
-  return __llvm_libc::fopencookie(pos, "w", cookie);
-}
+FILE *memopen(char **pos) { return __llvm_libc::fopencookie(pos, "w", cookie); }
 
 struct LlvmLibcGetoptTest : public __llvm_libc::testing::Test {
   FILE *errstream;