diff --git a/libc/test/config/linux/x86_64/syscall_test.cpp b/libc/test/config/linux/x86_64/syscall_test.cpp
--- a/libc/test/config/linux/x86_64/syscall_test.cpp
+++ b/libc/test/config/linux/x86_64/syscall_test.cpp
@@ -11,6 +11,8 @@
 
 #include "utils/CPP/Functional.h"
 
+namespace __llvm_libc {
+
 TEST(X86_64_SyscallTest, APITest) {
   // We only do a signature test here. Actual functionality tests are
   // done by the unit tests of the syscall wrappers like mmap.
@@ -42,3 +44,5 @@
   function<long(long, void *)> notLongType(
       [](long n, void *a1) { return __llvm_libc::syscall(n, a1); });
 }
+
+} // namespace __llvm_libc
diff --git a/libc/test/loader/linux/args_test.cpp b/libc/test/loader/linux/args_test.cpp
--- a/libc/test/loader/linux/args_test.cpp
+++ b/libc/test/loader/linux/args_test.cpp
@@ -9,6 +9,8 @@
 #undef NDEBUG
 #include "src/assert/assert.h"
 
+namespace __llvm_libc {
+
 static bool my_streq(const char *lhs, const char *rhs) {
   const char *l, *r;
   for (l = lhs, r = rhs; *l != '\0' && *r != '\0'; ++l, ++r)
@@ -38,3 +40,5 @@
 
   return 0;
 }
+
+} // namespace __llvm_libc
diff --git a/libc/test/src/assert/assert_test.cpp b/libc/test/src/assert/assert_test.cpp
--- a/libc/test/src/assert/assert_test.cpp
+++ b/libc/test/src/assert/assert_test.cpp
@@ -12,6 +12,8 @@
 
 extern "C" int close(int);
 
+namespace __llvm_libc {
+
 TEST(Assert, Enabled) {
   // -1 matches against any signal, which is necessary for now until
   // __llvm_libc::abort() unblocks SIGABRT. Close standard error for the
@@ -30,3 +32,5 @@
 TEST(Assert, Disabled) {
   EXPECT_EXITS([] { assert(0); }, 0);
 }
+
+} // namespace __llvm_libc
diff --git a/libc/test/src/errno/errno_test.cpp b/libc/test/src/errno/errno_test.cpp
--- a/libc/test/src/errno/errno_test.cpp
+++ b/libc/test/src/errno/errno_test.cpp
@@ -9,8 +9,12 @@
 #include "src/errno/llvmlibc_errno.h"
 #include "utils/UnitTest/Test.h"
 
+namespace __llvm_libc {
+
 TEST(ErrnoTest, Basic) {
   int test_val = 123;
   llvmlibc_errno = test_val;
   ASSERT_EQ(test_val, llvmlibc_errno);
 }
+
+} // namespace __llvm_libc
diff --git a/libc/test/src/signal/raise_test.cpp b/libc/test/src/signal/raise_test.cpp
--- a/libc/test/src/signal/raise_test.cpp
+++ b/libc/test/src/signal/raise_test.cpp
@@ -11,6 +11,8 @@
 
 #include "utils/UnitTest/Test.h"
 
+namespace __llvm_libc {
+
 TEST(SignalTest, Raise) {
   // SIGCONT is ingored unless stopped, so we can use it to check the return
   // value of raise without needing to block.
@@ -20,3 +22,5 @@
   // for example and incorrectly report test failure.
   EXPECT_DEATH([] { __llvm_libc::raise(SIGKILL); }, SIGKILL);
 }
+
+} // namespace __llvm_libc
diff --git a/libc/test/src/signal/sigaction_test.cpp b/libc/test/src/signal/sigaction_test.cpp
--- a/libc/test/src/signal/sigaction_test.cpp
+++ b/libc/test/src/signal/sigaction_test.cpp
@@ -18,6 +18,8 @@
 using __llvm_libc::testing::ErrnoSetterMatcher::Fails;
 using __llvm_libc::testing::ErrnoSetterMatcher::Succeeds;
 
+namespace __llvm_libc {
+
 TEST(Sigaction, Invalid) {
   // -1 is a much larger signal that NSIG, so this should fail.
   EXPECT_THAT(__llvm_libc::sigaction(-1, nullptr, nullptr), Fails(EINVAL));
@@ -64,3 +66,5 @@
 
   EXPECT_EXITS([] { __llvm_libc::raise(SIGUSR1); }, 0);
 }
+
+} // namespace __llvm_libc
diff --git a/libc/test/src/signal/sigaddset_test.cpp b/libc/test/src/signal/sigaddset_test.cpp
--- a/libc/test/src/signal/sigaddset_test.cpp
+++ b/libc/test/src/signal/sigaddset_test.cpp
@@ -13,6 +13,8 @@
 #include "utils/UnitTest/ErrnoSetterMatcher.h"
 #include "utils/UnitTest/Test.h"
 
+namespace __llvm_libc {
+
 // This tests invalid inputs and ensures errno is properly set.
 TEST(SignalTest, SigaddsetInvalid) {
   using __llvm_libc::testing::ErrnoSetterMatcher::Fails;
@@ -31,3 +33,5 @@
   EXPECT_THAT(__llvm_libc::sigaddset(&sigset, 0), Fails(EINVAL));
   EXPECT_THAT(__llvm_libc::sigaddset(&sigset, bitsInSigsetT), Succeeds());
 }
+
+} // namespace __llvm_libc
diff --git a/libc/test/src/signal/sigdelset_test.cpp b/libc/test/src/signal/sigdelset_test.cpp
--- a/libc/test/src/signal/sigdelset_test.cpp
+++ b/libc/test/src/signal/sigdelset_test.cpp
@@ -16,6 +16,8 @@
 #include "utils/UnitTest/ErrnoSetterMatcher.h"
 #include "utils/UnitTest/Test.h"
 
+namespace __llvm_libc {
+
 TEST(Sigdelset, Invalid) {
   using __llvm_libc::testing::ErrnoSetterMatcher::Fails;
   // Invalid set.
@@ -34,3 +36,5 @@
   EXPECT_THAT(__llvm_libc::sigprocmask(SIG_SETMASK, &set, nullptr), Succeeds());
   EXPECT_DEATH([] { __llvm_libc::raise(SIGUSR1); }, SIGUSR1);
 }
+
+} // namespace __llvm_libc
diff --git a/libc/test/src/signal/sigfillset_test.cpp b/libc/test/src/signal/sigfillset_test.cpp
--- a/libc/test/src/signal/sigfillset_test.cpp
+++ b/libc/test/src/signal/sigfillset_test.cpp
@@ -15,6 +15,8 @@
 #include "utils/UnitTest/ErrnoSetterMatcher.h"
 #include "utils/UnitTest/Test.h"
 
+namespace __llvm_libc {
+
 TEST(Sigfillset, Invalid) {
   using __llvm_libc::testing::ErrnoSetterMatcher::Fails;
   EXPECT_THAT(__llvm_libc::sigfillset(nullptr), Fails(EINVAL));
@@ -27,3 +29,5 @@
   EXPECT_THAT(__llvm_libc::sigprocmask(SIG_SETMASK, &set, nullptr), Succeeds());
   EXPECT_EXITS([] { __llvm_libc::raise(SIGUSR1); }, 0);
 }
+
+} // namespace __llvm_libc
diff --git a/libc/test/src/signal/signal_test.cpp b/libc/test/src/signal/signal_test.cpp
--- a/libc/test/src/signal/signal_test.cpp
+++ b/libc/test/src/signal/signal_test.cpp
@@ -18,6 +18,8 @@
 using __llvm_libc::testing::ErrnoSetterMatcher::Fails;
 using __llvm_libc::testing::ErrnoSetterMatcher::Succeeds;
 
+namespace __llvm_libc {
+
 TEST(Signal, Invalid) {
   llvmlibc_errno = 0;
   __llvm_libc::sighandler_t valid = +[](int) {};
@@ -39,3 +41,5 @@
     ASSERT_THAT(__llvm_libc::raise(SIGUSR1), Succeeds());
   EXPECT_EQ(sum, 11);
 }
+
+} // namespace __llvm_libc
diff --git a/libc/test/src/signal/sigprocmask_test.cpp b/libc/test/src/signal/sigprocmask_test.cpp
--- a/libc/test/src/signal/sigprocmask_test.cpp
+++ b/libc/test/src/signal/sigprocmask_test.cpp
@@ -17,6 +17,8 @@
 #include "utils/UnitTest/ErrnoSetterMatcher.h"
 #include "utils/UnitTest/Test.h"
 
+namespace __llvm_libc {
+
 class SignalTest : public __llvm_libc::testing::Test {
   sigset_t oldSet;
 
@@ -58,3 +60,5 @@
   EXPECT_EQ(__llvm_libc::sigprocmask(SIG_SETMASK, &sigset, nullptr), 0);
   EXPECT_EXITS([] { __llvm_libc::raise(SIGUSR1); }, 0);
 }
+
+} // namespace __llvm_libc
diff --git a/libc/test/src/stdio/fwrite_test.cpp b/libc/test/src/stdio/fwrite_test.cpp
--- a/libc/test/src/stdio/fwrite_test.cpp
+++ b/libc/test/src/stdio/fwrite_test.cpp
@@ -11,6 +11,8 @@
 #include "utils/CPP/Array.h"
 #include "utils/UnitTest/Test.h"
 
+namespace __llvm_libc {
+
 TEST(Stdio, FWriteBasic) {
   struct StrcpyFile : __llvm_libc::FILE {
     char *buf;
@@ -26,3 +28,5 @@
   EXPECT_EQ(fwrite("hello", 1, 6, &f), 6UL);
   EXPECT_STREQ(array, "hello");
 }
+
+} // namespace __llvm_libc
diff --git a/libc/test/src/stdlib/_Exit_test.cpp b/libc/test/src/stdlib/_Exit_test.cpp
--- a/libc/test/src/stdlib/_Exit_test.cpp
+++ b/libc/test/src/stdlib/_Exit_test.cpp
@@ -10,7 +10,11 @@
 #include "src/stdlib/_Exit.h"
 #include "utils/UnitTest/Test.h"
 
+namespace __llvm_libc {
+
 TEST(Stdlib, _Exit) {
   EXPECT_EXITS([] { __llvm_libc::_Exit(1); }, 1);
   EXPECT_EXITS([] { __llvm_libc::_Exit(65); }, 65);
 }
+
+} // namespace __llvm_libc
diff --git a/libc/test/src/stdlib/abort_test.cpp b/libc/test/src/stdlib/abort_test.cpp
--- a/libc/test/src/stdlib/abort_test.cpp
+++ b/libc/test/src/stdlib/abort_test.cpp
@@ -11,8 +11,12 @@
 #include "src/stdlib/abort.h"
 #include "utils/UnitTest/Test.h"
 
+namespace __llvm_libc {
+
 TEST(Stdlib, abort) {
   // -1 matches against any signal, which is necessary for now until
   // __llvm_libc::abort() unblocks SIGABRT.
   EXPECT_DEATH([] { __llvm_libc::abort(); }, -1);
 }
+
+} // namespace __llvm_libc
diff --git a/libc/test/src/string/memcpy_test.cpp b/libc/test/src/string/memcpy_test.cpp
--- a/libc/test/src/string/memcpy_test.cpp
+++ b/libc/test/src/string/memcpy_test.cpp
@@ -15,6 +15,8 @@
 using __llvm_libc::cpp::MutableArrayRef;
 using Data = Array<char, 2048>;
 
+namespace __llvm_libc {
+
 static const ArrayRef<char> kNumbers("0123456789", 10);
 static const ArrayRef<char> kDeadcode("DEADC0DE", 8);
 
@@ -51,3 +53,5 @@
 // FIXME: Add tests with reads and writes on the boundary of a read/write
 // protected page to check we're not reading nor writing prior/past the allowed
 // regions.
+
+} // namespace __llvm_libc
diff --git a/libc/test/src/string/strcat_test.cpp b/libc/test/src/string/strcat_test.cpp
--- a/libc/test/src/string/strcat_test.cpp
+++ b/libc/test/src/string/strcat_test.cpp
@@ -9,6 +9,8 @@
 #include "src/string/strcat.h"
 #include "utils/UnitTest/Test.h"
 
+namespace __llvm_libc {
+
 TEST(StrCatTest, EmptyDest) {
   const char *abc = "abc";
   char dest[4];
@@ -35,3 +37,5 @@
   ASSERT_STREQ(dest, result);
   ASSERT_STREQ(dest, "xyzabc");
 }
+
+} // namespace __llvm_libc
diff --git a/libc/test/src/string/strcpy_test.cpp b/libc/test/src/string/strcpy_test.cpp
--- a/libc/test/src/string/strcpy_test.cpp
+++ b/libc/test/src/string/strcpy_test.cpp
@@ -9,6 +9,8 @@
 #include "src/string/strcpy.h"
 #include "utils/UnitTest/Test.h"
 
+namespace __llvm_libc {
+
 TEST(StrCpyTest, EmptyDest) {
   const char *abc = "abc";
   char dest[4];
@@ -32,3 +34,5 @@
   ASSERT_STREQ(dest + 3, result);
   ASSERT_STREQ(dest, "xyzabc");
 }
+
+} // namespace __llvm_libc
diff --git a/libc/test/src/string/strlen_test.cpp b/libc/test/src/string/strlen_test.cpp
--- a/libc/test/src/string/strlen_test.cpp
+++ b/libc/test/src/string/strlen_test.cpp
@@ -9,6 +9,8 @@
 #include "src/string/strlen.h"
 #include "utils/UnitTest/Test.h"
 
+namespace __llvm_libc {
+
 TEST(StrLenTest, EmptyString) {
   const char *empty = "";
 
@@ -22,3 +24,5 @@
   size_t result = __llvm_libc::strlen(any);
   ASSERT_EQ((size_t)12, result);
 }
+
+} // namespace __llvm_libc
diff --git a/libc/test/src/sys/mman/linux/mmap_test.cpp b/libc/test/src/sys/mman/linux/mmap_test.cpp
--- a/libc/test/src/sys/mman/linux/mmap_test.cpp
+++ b/libc/test/src/sys/mman/linux/mmap_test.cpp
@@ -17,6 +17,8 @@
 using __llvm_libc::testing::ErrnoSetterMatcher::Fails;
 using __llvm_libc::testing::ErrnoSetterMatcher::Succeeds;
 
+namespace __llvm_libc {
+
 TEST(MMapTest, NoError) {
   size_t alloc_size = 128;
   llvmlibc_errno = 0;
@@ -41,3 +43,5 @@
 
   EXPECT_THAT(__llvm_libc::munmap(0, 0), Fails(EINVAL));
 }
+
+} // namespace __llvm_libc
diff --git a/libc/test/src/threads/mtx_test.cpp b/libc/test/src/threads/mtx_test.cpp
--- a/libc/test/src/threads/mtx_test.cpp
+++ b/libc/test/src/threads/mtx_test.cpp
@@ -14,6 +14,8 @@
 #include "src/threads/thrd_join.h"
 #include "utils/UnitTest/Test.h"
 
+namespace __llvm_libc {
+
 constexpr int START = 0;
 constexpr int MAX = 10000;
 
@@ -114,3 +116,5 @@
   __llvm_libc::thrd_join(&thread, &retval);
   ASSERT_EQ(retval, 0);
 }
+
+} // namespace __llvm_libc
diff --git a/libc/test/src/threads/thrd_test.cpp b/libc/test/src/threads/thrd_test.cpp
--- a/libc/test/src/threads/thrd_test.cpp
+++ b/libc/test/src/threads/thrd_test.cpp
@@ -11,6 +11,8 @@
 #include "src/threads/thrd_join.h"
 #include "utils/UnitTest/Test.h"
 
+namespace __llvm_libc {
+
 static constexpr int thread_count = 1000;
 static int counter = 0;
 static int thread_func(void *) {
@@ -50,3 +52,5 @@
     ASSERT_EQ(retval, i);
   }
 }
+
+} // namespace __llvm_libc
diff --git a/libc/test/src/unistd/write_test.cpp b/libc/test/src/unistd/write_test.cpp
--- a/libc/test/src/unistd/write_test.cpp
+++ b/libc/test/src/unistd/write_test.cpp
@@ -12,6 +12,8 @@
 #include "utils/UnitTest/Test.h"
 #include "utils/testutils/FDReader.h"
 
+namespace __llvm_libc {
+
 TEST(UniStd, WriteBasic) {
   using __llvm_libc::testing::ErrnoSetterMatcher::Succeeds;
   constexpr const char *hello = "hello";
@@ -27,3 +29,5 @@
   EXPECT_THAT(__llvm_libc::write(1, reinterpret_cast<const void *>(-1), 1),
               Fails(EFAULT));
 }
+
+} // namespace __llvm_libc