diff --git a/libc/src/__support/ctype_utils.h b/libc/src/__support/ctype_utils.h
index be0f25330af9e..61b7a0aeb5b67 100644
--- a/libc/src/__support/ctype_utils.h
+++ b/libc/src/__support/ctype_utils.h
@@ -27,7 +27,7 @@ namespace internal {
 // as well as a way to support non-ASCII character encodings.
 
 // Similarly, do not change these functions to use case ranges. e.g.
-//  bool islower(int ch) {
+//  bool islower(char ch) {
 //    switch(ch) {
 //    case 'a'...'z':
 //      return true;
@@ -37,7 +37,7 @@ namespace internal {
 // EBCDIC. Technically we could use some smaller ranges, but that's even harder
 // to read.
 
-LIBC_INLINE static constexpr bool islower(int ch) {
+LIBC_INLINE static constexpr bool islower(char ch) {
   switch (ch) {
   case 'a':
   case 'b':
@@ -71,7 +71,7 @@ LIBC_INLINE static constexpr bool islower(int ch) {
   }
 }
 
-LIBC_INLINE static constexpr bool isupper(int ch) {
+LIBC_INLINE static constexpr bool isupper(char ch) {
   switch (ch) {
   case 'A':
   case 'B':
@@ -105,7 +105,7 @@ LIBC_INLINE static constexpr bool isupper(int ch) {
   }
 }
 
-LIBC_INLINE static constexpr bool isdigit(int ch) {
+LIBC_INLINE static constexpr bool isdigit(char ch) {
   switch (ch) {
   case '0':
   case '1':
@@ -123,7 +123,7 @@ LIBC_INLINE static constexpr bool isdigit(int ch) {
   }
 }
 
-LIBC_INLINE static constexpr int tolower(int ch) {
+LIBC_INLINE static constexpr char tolower(char ch) {
   switch (ch) {
   case 'A':
     return 'a';
@@ -182,7 +182,7 @@ LIBC_INLINE static constexpr int tolower(int ch) {
   }
 }
 
-LIBC_INLINE static constexpr int toupper(int ch) {
+LIBC_INLINE static constexpr char toupper(char ch) {
   switch (ch) {
   case 'a':
     return 'A';
@@ -241,7 +241,7 @@ LIBC_INLINE static constexpr int toupper(int ch) {
   }
 }
 
-LIBC_INLINE static constexpr bool isalpha(int ch) {
+LIBC_INLINE static constexpr bool isalpha(char ch) {
   switch (ch) {
   case 'a':
   case 'b':
@@ -301,7 +301,7 @@ LIBC_INLINE static constexpr bool isalpha(int ch) {
   }
 }
 
-LIBC_INLINE static constexpr bool isalnum(int ch) {
+LIBC_INLINE static constexpr bool isalnum(char ch) {
   switch (ch) {
   case 'a':
   case 'b':
@@ -371,7 +371,7 @@ LIBC_INLINE static constexpr bool isalnum(int ch) {
   }
 }
 
-LIBC_INLINE static constexpr int b36_char_to_int(int ch) {
+LIBC_INLINE static constexpr int b36_char_to_int(char ch) {
   switch (ch) {
   case '0':
     return 0;
@@ -476,7 +476,7 @@ LIBC_INLINE static constexpr int b36_char_to_int(int ch) {
   }
 }
 
-LIBC_INLINE static constexpr int int_to_b36_char(int num) {
+LIBC_INLINE static constexpr char int_to_b36_char(int num) {
   // Can't actually use LIBC_ASSERT here because it depends on integer_to_string
   // which depends on this.
 
@@ -559,7 +559,7 @@ LIBC_INLINE static constexpr int int_to_b36_char(int num) {
   }
 }
 
-LIBC_INLINE static constexpr bool isspace(int ch) {
+LIBC_INLINE static constexpr bool isspace(char ch) {
   switch (ch) {
   case ' ':
   case '\t':
@@ -574,7 +574,7 @@ LIBC_INLINE static constexpr bool isspace(int ch) {
 }
 
 // not yet encoding independent.
-LIBC_INLINE static constexpr bool isgraph(int ch) {
+LIBC_INLINE static constexpr bool isgraph(char ch) {
   return 0x20 < ch && ch < 0x7f;
 }
 
diff --git a/libc/src/__support/integer_to_string.h b/libc/src/__support/integer_to_string.h
index 29449bd739730..5e7369de00962 100644
--- a/libc/src/__support/integer_to_string.h
+++ b/libc/src/__support/integer_to_string.h
@@ -378,9 +378,8 @@ template <typename T, typename Fmt = radix::Dec> class IntegerToString {
     using UNSIGNED_T = make_integral_or_big_int_unsigned_t<T>;
 
     LIBC_INLINE static char digit_char(uint8_t digit) {
-      const int result = internal::int_to_b36_char(digit);
-      return static_cast<char>(Fmt::IS_UPPERCASE ? internal::toupper(result)
-                                                 : result);
+      const char result = internal::int_to_b36_char(digit);
+      return Fmt::IS_UPPERCASE ? internal::toupper(result) : result;
     }
 
     LIBC_INLINE static void
diff --git a/libc/src/ctype/CMakeLists.txt b/libc/src/ctype/CMakeLists.txt
index 8830c1bccf9ea..68e982bd4529e 100644
--- a/libc/src/ctype/CMakeLists.txt
+++ b/libc/src/ctype/CMakeLists.txt
@@ -6,6 +6,7 @@ add_entrypoint_object(
     isalnum.h
   DEPENDS
     libc.include.ctype
+    libc.src.__support.CPP.limits
     libc.src.__support.ctype_utils
 )
 
@@ -16,6 +17,7 @@ add_entrypoint_object(
   HDRS
     isalpha.h
   DEPENDS
+    libc.src.__support.CPP.limits
     libc.src.__support.ctype_utils
 )
 
@@ -50,6 +52,7 @@ add_entrypoint_object(
   HDRS
     isdigit.h
   DEPENDS
+    libc.src.__support.CPP.limits
     libc.src.__support.ctype_utils
 )
 
@@ -60,6 +63,7 @@ add_entrypoint_object(
   HDRS
     isgraph.h
   DEPENDS
+    libc.src.__support.CPP.limits
     libc.src.__support.ctype_utils
 )
 
@@ -70,6 +74,7 @@ add_entrypoint_object(
   HDRS
     islower.h
   DEPENDS
+    libc.src.__support.CPP.limits
     libc.src.__support.ctype_utils
 )
 
@@ -88,6 +93,7 @@ add_entrypoint_object(
   HDRS
     ispunct.h
   DEPENDS
+    libc.src.__support.CPP.limits
     libc.src.__support.ctype_utils
 )
 
@@ -97,6 +103,9 @@ add_entrypoint_object(
     isspace.cpp
   HDRS
     isspace.h
+  DEPENDS
+    libc.src.__support.CPP.limits
+    libc.src.__support.ctype_utils
 )
 
 add_entrypoint_object(
@@ -106,6 +115,7 @@ add_entrypoint_object(
   HDRS
     isupper.h
   DEPENDS
+    libc.src.__support.CPP.limits  
     libc.src.__support.ctype_utils
 )
 
@@ -116,6 +126,7 @@ add_entrypoint_object(
   HDRS
     isxdigit.h
   DEPENDS
+    libc.src.__support.CPP.limits
     libc.src.__support.ctype_utils
 )
 
@@ -126,6 +137,7 @@ add_entrypoint_object(
   HDRS
     tolower.h
   DEPENDS
+    libc.src.__support.CPP.limits
     libc.src.__support.ctype_utils
 )
 
@@ -144,6 +156,7 @@ add_entrypoint_object(
   HDRS
     toupper.h
   DEPENDS
+    libc.src.__support.CPP.limits
     libc.src.__support.ctype_utils
 )
 
@@ -160,6 +173,7 @@ add_entrypoint_object(
     isalnum_l.h
   DEPENDS
     libc.include.ctype
+    libc.src.__support.CPP.limits
     libc.src.__support.ctype_utils
     libc.hdr.types.locale_t
 )
@@ -171,6 +185,7 @@ add_entrypoint_object(
   HDRS
     isalpha_l.h
   DEPENDS
+    libc.src.__support.CPP.limits
     libc.src.__support.ctype_utils
     libc.hdr.types.locale_t
 )
@@ -202,6 +217,7 @@ add_entrypoint_object(
   HDRS
     isdigit_l.h
   DEPENDS
+    libc.src.__support.CPP.limits
     libc.src.__support.ctype_utils
     libc.hdr.types.locale_t
 )
@@ -224,6 +240,7 @@ add_entrypoint_object(
   HDRS
     islower_l.h
   DEPENDS
+    libc.src.__support.CPP.limits
     libc.src.__support.ctype_utils
     libc.hdr.types.locale_t
 )
@@ -257,6 +274,8 @@ add_entrypoint_object(
     isspace_l.h
   DEPENDS
     libc.hdr.types.locale_t
+    libc.src.__support.CPP.limits
+    libc.src.__support.ctype_utils
 )
 
 add_entrypoint_object(
@@ -266,6 +285,7 @@ add_entrypoint_object(
   HDRS
     isupper_l.h
   DEPENDS
+    libc.src.__support.CPP.limits  
     libc.src.__support.ctype_utils
     libc.hdr.types.locale_t
 )
@@ -277,6 +297,7 @@ add_entrypoint_object(
   HDRS
     isxdigit_l.h
   DEPENDS
+    libc.src.__support.CPP.limits
     libc.src.__support.ctype_utils
     libc.hdr.types.locale_t
 )
@@ -288,6 +309,7 @@ add_entrypoint_object(
   HDRS
     tolower_l.h
   DEPENDS
+    libc.src.__support.CPP.limits
     libc.src.__support.ctype_utils
     libc.hdr.types.locale_t
 )
@@ -299,6 +321,7 @@ add_entrypoint_object(
   HDRS
     toupper_l.h
   DEPENDS
+    libc.src.__support.CPP.limits
     libc.src.__support.ctype_utils
     libc.hdr.types.locale_t
 )
diff --git a/libc/src/ctype/isalnum.cpp b/libc/src/ctype/isalnum.cpp
index 54a3e35748879..102b5e79e4a18 100644
--- a/libc/src/ctype/isalnum.cpp
+++ b/libc/src/ctype/isalnum.cpp
@@ -7,15 +7,18 @@
 //===----------------------------------------------------------------------===//
 
 #include "src/ctype/isalnum.h"
-#include "src/__support/ctype_utils.h"
 
+#include "src/__support/CPP/limits.h"
 #include "src/__support/common.h"
+#include "src/__support/ctype_utils.h"
 #include "src/__support/macros/config.h"
 
 namespace LIBC_NAMESPACE_DECL {
 
 LLVM_LIBC_FUNCTION(int, isalnum, (int c)) {
-  return static_cast<int>(internal::isalnum(static_cast<unsigned>(c)));
+  if (c < 0 || c > cpp::numeric_limits<unsigned char>::max())
+    return 0;
+  return static_cast<int>(internal::isalnum(static_cast<char>(c)));
 }
 
 } // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/ctype/isalnum_l.cpp b/libc/src/ctype/isalnum_l.cpp
index 671d9b75c4c33..173e1c174121e 100644
--- a/libc/src/ctype/isalnum_l.cpp
+++ b/libc/src/ctype/isalnum_l.cpp
@@ -7,15 +7,18 @@
 //===----------------------------------------------------------------------===//
 
 #include "src/ctype/isalnum_l.h"
-#include "src/__support/ctype_utils.h"
 
+#include "src/__support/CPP/limits.h"
 #include "src/__support/common.h"
+#include "src/__support/ctype_utils.h"
 #include "src/__support/macros/config.h"
 
 namespace LIBC_NAMESPACE_DECL {
 
 LLVM_LIBC_FUNCTION(int, isalnum_l, (int c, locale_t)) {
-  return static_cast<int>(internal::isalnum(static_cast<unsigned>(c)));
+  if (c < 0 || c > cpp::numeric_limits<unsigned char>::max())
+    return 0;
+  return static_cast<int>(internal::isalnum(static_cast<char>(c)));
 }
 
 } // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/ctype/isalpha.cpp b/libc/src/ctype/isalpha.cpp
index 78b26f6a486ea..7c874bf373866 100644
--- a/libc/src/ctype/isalpha.cpp
+++ b/libc/src/ctype/isalpha.cpp
@@ -8,6 +8,7 @@
 
 #include "src/ctype/isalpha.h"
 
+#include "src/__support/CPP/limits.h"
 #include "src/__support/common.h"
 #include "src/__support/ctype_utils.h"
 #include "src/__support/macros/config.h"
@@ -15,7 +16,9 @@
 namespace LIBC_NAMESPACE_DECL {
 
 LLVM_LIBC_FUNCTION(int, isalpha, (int c)) {
-  return static_cast<int>(internal::isalpha(static_cast<unsigned>(c)));
+  if (c < 0 || c > cpp::numeric_limits<unsigned char>::max())
+    return 0;
+  return static_cast<int>(internal::isalpha(static_cast<char>(c)));
 }
 
 } // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/ctype/isalpha_l.cpp b/libc/src/ctype/isalpha_l.cpp
index 0619d979bedf2..982bcc569faaf 100644
--- a/libc/src/ctype/isalpha_l.cpp
+++ b/libc/src/ctype/isalpha_l.cpp
@@ -8,6 +8,7 @@
 
 #include "src/ctype/isalpha_l.h"
 
+#include "src/__support/CPP/limits.h"
 #include "src/__support/common.h"
 #include "src/__support/ctype_utils.h"
 #include "src/__support/macros/config.h"
@@ -15,7 +16,9 @@
 namespace LIBC_NAMESPACE_DECL {
 
 LLVM_LIBC_FUNCTION(int, isalpha_l, (int c, locale_t)) {
-  return static_cast<int>(internal::isalpha(static_cast<unsigned>(c)));
+  if (c < 0 || c > cpp::numeric_limits<unsigned char>::max())
+    return 0;
+  return static_cast<int>(internal::isalpha(static_cast<char>(c)));
 }
 
 } // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/ctype/isdigit.cpp b/libc/src/ctype/isdigit.cpp
index 1f711943861f8..43553c794a2f3 100644
--- a/libc/src/ctype/isdigit.cpp
+++ b/libc/src/ctype/isdigit.cpp
@@ -7,6 +7,8 @@
 //===----------------------------------------------------------------------===//
 
 #include "src/ctype/isdigit.h"
+
+#include "src/__support/CPP/limits.h"
 #include "src/__support/common.h"
 #include "src/__support/ctype_utils.h"
 #include "src/__support/macros/config.h"
@@ -14,7 +16,9 @@
 namespace LIBC_NAMESPACE_DECL {
 
 LLVM_LIBC_FUNCTION(int, isdigit, (int c)) {
-  return static_cast<int>(internal::isdigit(static_cast<unsigned>(c)));
+  if (c < 0 || c > cpp::numeric_limits<unsigned char>::max())
+    return 0;
+  return static_cast<int>(internal::isdigit(static_cast<char>(c)));
 }
 
 } // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/ctype/isdigit_l.cpp b/libc/src/ctype/isdigit_l.cpp
index ca981362bfe83..40b5618906dac 100644
--- a/libc/src/ctype/isdigit_l.cpp
+++ b/libc/src/ctype/isdigit_l.cpp
@@ -7,6 +7,8 @@
 //===----------------------------------------------------------------------===//
 
 #include "src/ctype/isdigit_l.h"
+
+#include "src/__support/CPP/limits.h"
 #include "src/__support/common.h"
 #include "src/__support/ctype_utils.h"
 #include "src/__support/macros/config.h"
@@ -14,7 +16,9 @@
 namespace LIBC_NAMESPACE_DECL {
 
 LLVM_LIBC_FUNCTION(int, isdigit_l, (int c, locale_t)) {
-  return static_cast<int>(internal::isdigit(static_cast<unsigned>(c)));
+  if (c < 0 || c > cpp::numeric_limits<unsigned char>::max())
+    return 0;
+  return static_cast<int>(internal::isdigit(static_cast<char>(c)));
 }
 
 } // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/ctype/isgraph.cpp b/libc/src/ctype/isgraph.cpp
index 74bb2e75d138e..b9308ecb7367c 100644
--- a/libc/src/ctype/isgraph.cpp
+++ b/libc/src/ctype/isgraph.cpp
@@ -8,6 +8,7 @@
 
 #include "src/ctype/isgraph.h"
 
+#include "src/__support/CPP/limits.h"
 #include "src/__support/common.h"
 #include "src/__support/ctype_utils.h"
 #include "src/__support/macros/config.h"
@@ -15,7 +16,9 @@
 namespace LIBC_NAMESPACE_DECL {
 
 LLVM_LIBC_FUNCTION(int, isgraph, (int c)) {
-  return static_cast<int>(internal::isgraph(static_cast<unsigned>(c)));
+  if (c < 0 || c > cpp::numeric_limits<unsigned char>::max())
+    return 0;
+  return static_cast<int>(internal::isgraph(static_cast<char>(c)));
 }
 
 } // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/ctype/isgraph_l.cpp b/libc/src/ctype/isgraph_l.cpp
index cbef6df148aed..dddcb9be4f80c 100644
--- a/libc/src/ctype/isgraph_l.cpp
+++ b/libc/src/ctype/isgraph_l.cpp
@@ -8,6 +8,7 @@
 
 #include "src/ctype/isgraph_l.h"
 
+#include "src/__support/CPP/limits.h"
 #include "src/__support/common.h"
 #include "src/__support/ctype_utils.h"
 #include "src/__support/macros/config.h"
@@ -15,7 +16,9 @@
 namespace LIBC_NAMESPACE_DECL {
 
 LLVM_LIBC_FUNCTION(int, isgraph_l, (int c, locale_t)) {
-  return static_cast<int>(internal::isgraph(static_cast<unsigned>(c)));
+  if (c < 0 || c > cpp::numeric_limits<unsigned char>::max())
+    return 0;
+  return static_cast<int>(internal::isgraph(static_cast<char>(c)));
 }
 
 } // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/ctype/islower.cpp b/libc/src/ctype/islower.cpp
index 831aad32d3a22..920bfc1cc1a59 100644
--- a/libc/src/ctype/islower.cpp
+++ b/libc/src/ctype/islower.cpp
@@ -7,15 +7,18 @@
 //===----------------------------------------------------------------------===//
 
 #include "src/ctype/islower.h"
-#include "src/__support/ctype_utils.h"
 
+#include "src/__support/CPP/limits.h"
 #include "src/__support/common.h"
+#include "src/__support/ctype_utils.h"
 #include "src/__support/macros/config.h"
 
 namespace LIBC_NAMESPACE_DECL {
 
 LLVM_LIBC_FUNCTION(int, islower, (int c)) {
-  return static_cast<int>(internal::islower(static_cast<unsigned>(c)));
+  if (c < 0 || c > cpp::numeric_limits<unsigned char>::max())
+    return 0;
+  return static_cast<int>(internal::islower(static_cast<char>(c)));
 }
 
 } // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/ctype/islower_l.cpp b/libc/src/ctype/islower_l.cpp
index b9be6acc81c99..da97026dc59a7 100644
--- a/libc/src/ctype/islower_l.cpp
+++ b/libc/src/ctype/islower_l.cpp
@@ -7,15 +7,18 @@
 //===----------------------------------------------------------------------===//
 
 #include "src/ctype/islower_l.h"
-#include "src/__support/ctype_utils.h"
 
+#include "src/__support/CPP/limits.h"
 #include "src/__support/common.h"
+#include "src/__support/ctype_utils.h"
 #include "src/__support/macros/config.h"
 
 namespace LIBC_NAMESPACE_DECL {
 
 LLVM_LIBC_FUNCTION(int, islower_l, (int c, locale_t)) {
-  return static_cast<int>(internal::islower(static_cast<unsigned>(c)));
+  if (c < 0 || c > cpp::numeric_limits<unsigned char>::max())
+    return 0;
+  return static_cast<int>(internal::islower(static_cast<char>(c)));
 }
 
 } // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/ctype/ispunct.cpp b/libc/src/ctype/ispunct.cpp
index 0635294220b9c..4950036e9b81f 100644
--- a/libc/src/ctype/ispunct.cpp
+++ b/libc/src/ctype/ispunct.cpp
@@ -8,6 +8,7 @@
 
 #include "src/ctype/ispunct.h"
 
+#include "src/__support/CPP/limits.h"
 #include "src/__support/common.h"
 #include "src/__support/ctype_utils.h"
 #include "src/__support/macros/config.h"
@@ -15,7 +16,9 @@
 namespace LIBC_NAMESPACE_DECL {
 
 LLVM_LIBC_FUNCTION(int, ispunct, (int c)) {
-  const unsigned ch = static_cast<unsigned>(c);
+  if (c < 0 || c > cpp::numeric_limits<unsigned char>::max())
+    return 0;
+  const char ch = static_cast<char>(c);
   return static_cast<int>(!internal::isalnum(ch) && internal::isgraph(ch));
 }
 
diff --git a/libc/src/ctype/ispunct_l.cpp b/libc/src/ctype/ispunct_l.cpp
index e825fbe2001b0..79cd47b6a214d 100644
--- a/libc/src/ctype/ispunct_l.cpp
+++ b/libc/src/ctype/ispunct_l.cpp
@@ -8,6 +8,7 @@
 
 #include "src/ctype/ispunct_l.h"
 
+#include "src/__support/CPP/limits.h"
 #include "src/__support/common.h"
 #include "src/__support/ctype_utils.h"
 #include "src/__support/macros/config.h"
@@ -15,7 +16,9 @@
 namespace LIBC_NAMESPACE_DECL {
 
 LLVM_LIBC_FUNCTION(int, ispunct_l, (int c, locale_t)) {
-  const unsigned ch = static_cast<unsigned>(c);
+  if (c < 0 || c > cpp::numeric_limits<unsigned char>::max())
+    return 0;
+  const char ch = static_cast<char>(c);
   return static_cast<int>(!internal::isalnum(ch) && internal::isgraph(ch));
 }
 
diff --git a/libc/src/ctype/isspace.cpp b/libc/src/ctype/isspace.cpp
index 005bf460fc103..998dbf28f51d0 100644
--- a/libc/src/ctype/isspace.cpp
+++ b/libc/src/ctype/isspace.cpp
@@ -7,15 +7,18 @@
 //===----------------------------------------------------------------------===//
 
 #include "src/ctype/isspace.h"
-#include "src/__support/ctype_utils.h"
 
+#include "src/__support/CPP/limits.h"
 #include "src/__support/common.h"
+#include "src/__support/ctype_utils.h"
 #include "src/__support/macros/config.h"
 
 namespace LIBC_NAMESPACE_DECL {
 
 LLVM_LIBC_FUNCTION(int, isspace, (int c)) {
-  return static_cast<int>(internal::isspace(static_cast<unsigned>(c)));
+  if (c < 0 || c > cpp::numeric_limits<unsigned char>::max())
+    return 0;
+  return static_cast<int>(internal::isspace(static_cast<char>(c)));
 }
 
 } // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/ctype/isspace_l.cpp b/libc/src/ctype/isspace_l.cpp
index 5c46dd6805126..e40765326b35e 100644
--- a/libc/src/ctype/isspace_l.cpp
+++ b/libc/src/ctype/isspace_l.cpp
@@ -7,15 +7,18 @@
 //===----------------------------------------------------------------------===//
 
 #include "src/ctype/isspace_l.h"
-#include "src/__support/ctype_utils.h"
 
+#include "src/__support/CPP/limits.h"
 #include "src/__support/common.h"
+#include "src/__support/ctype_utils.h"
 #include "src/__support/macros/config.h"
 
 namespace LIBC_NAMESPACE_DECL {
 
 LLVM_LIBC_FUNCTION(int, isspace_l, (int c, locale_t)) {
-  return static_cast<int>(internal::isspace(static_cast<unsigned>(c)));
+  if (c < 0 || c > cpp::numeric_limits<unsigned char>::max())
+    return 0;
+  return static_cast<int>(internal::isspace(static_cast<char>(c)));
 }
 
 } // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/ctype/isupper.cpp b/libc/src/ctype/isupper.cpp
index 965fa336b28b4..c5c3dbd5d7d4a 100644
--- a/libc/src/ctype/isupper.cpp
+++ b/libc/src/ctype/isupper.cpp
@@ -7,15 +7,18 @@
 //===----------------------------------------------------------------------===//
 
 #include "src/ctype/isupper.h"
-#include "src/__support/ctype_utils.h"
 
+#include "src/__support/CPP/limits.h"
 #include "src/__support/common.h"
+#include "src/__support/ctype_utils.h"
 #include "src/__support/macros/config.h"
 
 namespace LIBC_NAMESPACE_DECL {
 
 LLVM_LIBC_FUNCTION(int, isupper, (int c)) {
-  return static_cast<int>(internal::isupper(static_cast<unsigned>(c)));
+  if (c < 0 || c > cpp::numeric_limits<unsigned char>::max())
+    return 0;
+  return static_cast<int>(internal::isupper(static_cast<char>(c)));
 }
 
 } // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/ctype/isupper_l.cpp b/libc/src/ctype/isupper_l.cpp
index 358990261d603..44ed9dab90a16 100644
--- a/libc/src/ctype/isupper_l.cpp
+++ b/libc/src/ctype/isupper_l.cpp
@@ -7,15 +7,18 @@
 //===----------------------------------------------------------------------===//
 
 #include "src/ctype/isupper_l.h"
-#include "src/__support/ctype_utils.h"
 
+#include "src/__support/CPP/limits.h"
 #include "src/__support/common.h"
+#include "src/__support/ctype_utils.h"
 #include "src/__support/macros/config.h"
 
 namespace LIBC_NAMESPACE_DECL {
 
 LLVM_LIBC_FUNCTION(int, isupper_l, (int c, locale_t)) {
-  return static_cast<int>(internal::isupper(static_cast<unsigned>(c)));
+  if (c < 0 || c > cpp::numeric_limits<unsigned char>::max())
+    return 0;
+  return static_cast<int>(internal::isupper(static_cast<char>(c)));
 }
 
 } // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/ctype/isxdigit.cpp b/libc/src/ctype/isxdigit.cpp
index 81f645c6f49fc..1b2e71769b3f8 100644
--- a/libc/src/ctype/isxdigit.cpp
+++ b/libc/src/ctype/isxdigit.cpp
@@ -7,15 +7,18 @@
 //===----------------------------------------------------------------------===//
 
 #include "src/ctype/isxdigit.h"
-#include "src/__support/ctype_utils.h"
 
+#include "src/__support/CPP/limits.h"
 #include "src/__support/common.h"
+#include "src/__support/ctype_utils.h"
 #include "src/__support/macros/config.h"
 
 namespace LIBC_NAMESPACE_DECL {
 
 LLVM_LIBC_FUNCTION(int, isxdigit, (int c)) {
-  const unsigned ch = static_cast<unsigned>(c);
+  if (c < 0 || c > cpp::numeric_limits<unsigned char>::max())
+    return 0;
+  const char ch = static_cast<char>(c);
   return static_cast<int>(internal::isalnum(ch) &&
                           internal::b36_char_to_int(ch) < 16);
 }
diff --git a/libc/src/ctype/isxdigit_l.cpp b/libc/src/ctype/isxdigit_l.cpp
index eddfd20a2da3b..e6150473b0043 100644
--- a/libc/src/ctype/isxdigit_l.cpp
+++ b/libc/src/ctype/isxdigit_l.cpp
@@ -7,15 +7,18 @@
 //===----------------------------------------------------------------------===//
 
 #include "src/ctype/isxdigit_l.h"
-#include "src/__support/ctype_utils.h"
 
+#include "src/__support/CPP/limits.h"
 #include "src/__support/common.h"
+#include "src/__support/ctype_utils.h"
 #include "src/__support/macros/config.h"
 
 namespace LIBC_NAMESPACE_DECL {
 
 LLVM_LIBC_FUNCTION(int, isxdigit_l, (int c, locale_t)) {
-  const unsigned ch = static_cast<unsigned>(c);
+  if (c < 0 || c > cpp::numeric_limits<unsigned char>::max())
+    return 0;
+  const char ch = static_cast<char>(c);
   return static_cast<int>(internal::isalnum(ch) &&
                           internal::b36_char_to_int(ch) < 16);
 }
diff --git a/libc/src/ctype/tolower.cpp b/libc/src/ctype/tolower.cpp
index 3ecad7bc5d5d5..b45c5f2688a61 100644
--- a/libc/src/ctype/tolower.cpp
+++ b/libc/src/ctype/tolower.cpp
@@ -7,13 +7,20 @@
 //===----------------------------------------------------------------------===//
 
 #include "src/ctype/tolower.h"
-#include "src/__support/ctype_utils.h"
 
+#include "src/__support/CPP/limits.h"
 #include "src/__support/common.h"
+#include "src/__support/ctype_utils.h"
 #include "src/__support/macros/config.h"
 
 namespace LIBC_NAMESPACE_DECL {
 
-LLVM_LIBC_FUNCTION(int, tolower, (int c)) { return internal::tolower(c); }
+LLVM_LIBC_FUNCTION(int, tolower, (int c)) {
+  if (c < cpp::numeric_limits<char>::min() ||
+      c > cpp::numeric_limits<char>::max()) {
+    return c;
+  }
+  return static_cast<int>(internal::tolower(static_cast<char>(c)));
+}
 
 } // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/ctype/tolower_l.cpp b/libc/src/ctype/tolower_l.cpp
index 7ccf31617e592..049e46aea13c0 100644
--- a/libc/src/ctype/tolower_l.cpp
+++ b/libc/src/ctype/tolower_l.cpp
@@ -7,15 +7,20 @@
 //===----------------------------------------------------------------------===//
 
 #include "src/ctype/tolower_l.h"
-#include "src/__support/ctype_utils.h"
 
+#include "src/__support/CPP/limits.h"
 #include "src/__support/common.h"
+#include "src/__support/ctype_utils.h"
 #include "src/__support/macros/config.h"
 
 namespace LIBC_NAMESPACE_DECL {
 
 LLVM_LIBC_FUNCTION(int, tolower_l, (int c, locale_t)) {
-  return internal::tolower(c);
+  if (c < cpp::numeric_limits<char>::min() ||
+      c > cpp::numeric_limits<char>::max()) {
+    return c;
+  }
+  return static_cast<int>(internal::tolower(static_cast<char>(c)));
 }
 
 } // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/ctype/toupper.cpp b/libc/src/ctype/toupper.cpp
index 1e1e8fc400711..0e387238ce3b6 100644
--- a/libc/src/ctype/toupper.cpp
+++ b/libc/src/ctype/toupper.cpp
@@ -7,13 +7,20 @@
 //===----------------------------------------------------------------------===//
 
 #include "src/ctype/toupper.h"
-#include "src/__support/ctype_utils.h"
 
+#include "src/__support/CPP/limits.h"
 #include "src/__support/common.h"
+#include "src/__support/ctype_utils.h"
 #include "src/__support/macros/config.h"
 
 namespace LIBC_NAMESPACE_DECL {
 
-LLVM_LIBC_FUNCTION(int, toupper, (int c)) { return internal::toupper(c); }
+LLVM_LIBC_FUNCTION(int, toupper, (int c)) {
+  if (c < cpp::numeric_limits<char>::min() ||
+      c > cpp::numeric_limits<char>::max()) {
+    return c;
+  }
+  return static_cast<int>(internal::toupper(static_cast<char>(c)));
+}
 
 } // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/ctype/toupper_l.cpp b/libc/src/ctype/toupper_l.cpp
index a435ca1ab5d41..d1dff262c9377 100644
--- a/libc/src/ctype/toupper_l.cpp
+++ b/libc/src/ctype/toupper_l.cpp
@@ -7,15 +7,20 @@
 //===----------------------------------------------------------------------===//
 
 #include "src/ctype/toupper_l.h"
-#include "src/__support/ctype_utils.h"
 
+#include "src/__support/CPP/limits.h"
 #include "src/__support/common.h"
+#include "src/__support/ctype_utils.h"
 #include "src/__support/macros/config.h"
 
 namespace LIBC_NAMESPACE_DECL {
 
 LLVM_LIBC_FUNCTION(int, toupper_l, (int c, locale_t)) {
-  return internal::toupper(c);
+  if (c < cpp::numeric_limits<char>::min() ||
+      c > cpp::numeric_limits<char>::max()) {
+    return c;
+  }
+  return static_cast<int>(internal::toupper(static_cast<char>(c)));
 }
 
 } // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/stdio/printf_core/float_dec_converter_limited.h b/libc/src/stdio/printf_core/float_dec_converter_limited.h
index 9cdc13573d320..0f85d0a8d26b4 100644
--- a/libc/src/stdio/printf_core/float_dec_converter_limited.h
+++ b/libc/src/stdio/printf_core/float_dec_converter_limited.h
@@ -363,8 +363,8 @@ DigitsOutput decimal_digits(DigitsInput input, int precision, bool e_mode) {
       // we made it from and doing the decimal conversion all over again.)
       for (size_t i = output.ndigits; i-- > 0;) {
         if (output.digits[i] != '9') {
-          output.digits[i] = static_cast<char>(internal::int_to_b36_char(
-              internal::b36_char_to_int(output.digits[i]) + 1));
+          output.digits[i] = internal::int_to_b36_char(
+              internal::b36_char_to_int(output.digits[i]) + 1);
           break;
         } else {
           output.digits[i] = '0';
diff --git a/libc/src/stdio/printf_core/float_hex_converter.h b/libc/src/stdio/printf_core/float_hex_converter.h
index 16592e7bac932..9b57f1d803e74 100644
--- a/libc/src/stdio/printf_core/float_hex_converter.h
+++ b/libc/src/stdio/printf_core/float_hex_converter.h
@@ -137,9 +137,9 @@ LIBC_INLINE int convert_float_hex_exp(Writer<write_mode> *writer,
   size_t first_non_zero = 1;
   for (; mant_cur > 0; --mant_cur, mantissa >>= 4) {
     char mant_mod_16 = static_cast<char>(mantissa % 16);
-    char new_digit = static_cast<char>(internal::int_to_b36_char(mant_mod_16));
+    char new_digit = internal::int_to_b36_char(mant_mod_16);
     if (internal::isupper(to_conv.conv_name))
-      new_digit = static_cast<char>(internal::toupper(new_digit));
+      new_digit = internal::toupper(new_digit);
     mant_buffer[mant_cur - 1] = new_digit;
     if (new_digit != '0' && first_non_zero < mant_cur)
       first_non_zero = mant_cur;
@@ -167,8 +167,7 @@ LIBC_INLINE int convert_float_hex_exp(Writer<write_mode> *writer,
 
   size_t exp_cur = EXP_LEN;
   for (; exponent > 0; --exp_cur, exponent /= 10) {
-    exp_buffer[exp_cur - 1] =
-        static_cast<char>(internal::int_to_b36_char(exponent % 10));
+    exp_buffer[exp_cur - 1] = internal::int_to_b36_char(exponent % 10);
   }
   if (exp_cur == EXP_LEN) { // if nothing else was written, write a 0.
     exp_buffer[EXP_LEN - 1] = '0';
diff --git a/libc/src/stdlib/l64a.cpp b/libc/src/stdlib/l64a.cpp
index d59e65e7dc4c2..d8fe8ef86bf7d 100644
--- a/libc/src/stdlib/l64a.cpp
+++ b/libc/src/stdlib/l64a.cpp
@@ -32,15 +32,13 @@ constexpr static char b64_int_to_char(uint32_t num) {
   if (num == 1)
     return '/';
   if (num < 38)
-    return static_cast<char>(
-        internal::toupper(internal::int_to_b36_char(num - 2)));
+    return internal::toupper(internal::int_to_b36_char(num - 2));
 
   // this tolower is technically unnecessary, but it provides safety if we
   // change the default behavior of int_to_b36_char. Also the compiler
   // completely elides it so there's no performance penalty, see:
   // https://godbolt.org/z/o5ennv7fc
-  return static_cast<char>(
-      internal::tolower(internal::int_to_b36_char(num - 2 - 26)));
+  return internal::tolower(internal::int_to_b36_char(num - 2 - 26));
 }
 
 // This function takes a long and converts the low 32 bits of it into at most 6
diff --git a/libc/src/string/strcasestr.cpp b/libc/src/string/strcasestr.cpp
index de8e4bec7fe0b..575d6bed16d11 100644
--- a/libc/src/string/strcasestr.cpp
+++ b/libc/src/string/strcasestr.cpp
@@ -21,8 +21,8 @@ namespace LIBC_NAMESPACE_DECL {
 LLVM_LIBC_FUNCTION(char *, strcasestr,
                    (const char *haystack, const char *needle)) {
   auto case_cmp = [](char a, char b) {
-    return LIBC_NAMESPACE::internal::tolower(a) -
-           LIBC_NAMESPACE::internal::tolower(b);
+    return static_cast<int>(LIBC_NAMESPACE::internal::tolower(a)) -
+           static_cast<int>(LIBC_NAMESPACE::internal::tolower(b));
   };
 
   LIBC_CRASH_ON_NULLPTR(haystack);
diff --git a/libc/src/strings/strcasecmp.cpp b/libc/src/strings/strcasecmp.cpp
index 4bbe2909df1e2..4518647deabe4 100644
--- a/libc/src/strings/strcasecmp.cpp
+++ b/libc/src/strings/strcasecmp.cpp
@@ -17,8 +17,8 @@ namespace LIBC_NAMESPACE_DECL {
 
 LLVM_LIBC_FUNCTION(int, strcasecmp, (const char *left, const char *right)) {
   auto case_cmp = [](char a, char b) {
-    return LIBC_NAMESPACE::internal::tolower(a) -
-           LIBC_NAMESPACE::internal::tolower(b);
+    return static_cast<int>(LIBC_NAMESPACE::internal::tolower(a)) -
+           static_cast<int>(LIBC_NAMESPACE::internal::tolower(b));
   };
   return inline_strcmp(left, right, case_cmp);
 }
diff --git a/libc/src/strings/strcasecmp_l.cpp b/libc/src/strings/strcasecmp_l.cpp
index 95117cb27a564..d77f95637a396 100644
--- a/libc/src/strings/strcasecmp_l.cpp
+++ b/libc/src/strings/strcasecmp_l.cpp
@@ -18,8 +18,8 @@ namespace LIBC_NAMESPACE_DECL {
 LLVM_LIBC_FUNCTION(int, strcasecmp_l,
                    (const char *left, const char *right, locale_t)) {
   auto case_cmp = [](char a, char b) {
-    return LIBC_NAMESPACE::internal::tolower(a) -
-           LIBC_NAMESPACE::internal::tolower(b);
+    return static_cast<int>(LIBC_NAMESPACE::internal::tolower(a)) -
+           static_cast<int>(LIBC_NAMESPACE::internal::tolower(b));
   };
   return inline_strcmp(left, right, case_cmp);
 }
diff --git a/libc/src/strings/strncasecmp.cpp b/libc/src/strings/strncasecmp.cpp
index 9c2f0ab131269..a5926495a3e22 100644
--- a/libc/src/strings/strncasecmp.cpp
+++ b/libc/src/strings/strncasecmp.cpp
@@ -18,8 +18,8 @@ namespace LIBC_NAMESPACE_DECL {
 LLVM_LIBC_FUNCTION(int, strncasecmp,
                    (const char *left, const char *right, size_t n)) {
   auto case_cmp = [](char a, char b) {
-    return LIBC_NAMESPACE::internal::tolower(a) -
-           LIBC_NAMESPACE::internal::tolower(b);
+    return static_cast<int>(LIBC_NAMESPACE::internal::tolower(a)) -
+           static_cast<int>(LIBC_NAMESPACE::internal::tolower(b));
   };
   return inline_strncmp(left, right, n, case_cmp);
 }
diff --git a/libc/src/strings/strncasecmp_l.cpp b/libc/src/strings/strncasecmp_l.cpp
index 91ac7e5e89107..a828f609fd9e8 100644
--- a/libc/src/strings/strncasecmp_l.cpp
+++ b/libc/src/strings/strncasecmp_l.cpp
@@ -18,8 +18,8 @@ namespace LIBC_NAMESPACE_DECL {
 LLVM_LIBC_FUNCTION(int, strncasecmp_l,
                    (const char *left, const char *right, size_t n, locale_t)) {
   auto case_cmp = [](char a, char b) {
-    return LIBC_NAMESPACE::internal::tolower(a) -
-           LIBC_NAMESPACE::internal::tolower(b);
+    return static_cast<int>(LIBC_NAMESPACE::internal::tolower(a)) -
+           static_cast<int>(LIBC_NAMESPACE::internal::tolower(b));
   };
   return inline_strncmp(left, right, n, case_cmp);
 }
diff --git a/libc/test/UnitTest/MemoryMatcher.cpp b/libc/test/UnitTest/MemoryMatcher.cpp
index 6e375768e9333..405f226798f7a 100644
--- a/libc/test/UnitTest/MemoryMatcher.cpp
+++ b/libc/test/UnitTest/MemoryMatcher.cpp
@@ -41,8 +41,8 @@ bool MemoryMatcher::match(MemoryView actualValue) {
 
 static void display(char C) {
   const auto print = [](unsigned char i) {
-    tlog << static_cast<char>(LIBC_NAMESPACE::internal::toupper(
-        LIBC_NAMESPACE::internal::int_to_b36_char(i)));
+    tlog << LIBC_NAMESPACE::internal::toupper(
+        LIBC_NAMESPACE::internal::int_to_b36_char(i));
   };
   print(static_cast<unsigned char>(C) / 16);
   print(static_cast<unsigned char>(C) & 15);
diff --git a/libc/test/src/ctype/islower_test.cpp b/libc/test/src/ctype/islower_test.cpp
index f877171abb9a3..e4e5f5cefd954 100644
--- a/libc/test/src/ctype/islower_test.cpp
+++ b/libc/test/src/ctype/islower_test.cpp
@@ -40,7 +40,7 @@ TEST(LlvmLibcIsLower, SimpleTest) {
 }
 
 TEST(LlvmLibcIsLower, DefaultLocale) {
-  // Loops through all characters, verifying that numbers and letters
+  // Loops through all characters, verifying that only lowercase letters
   // return non-zero integer and everything else returns a zero.
   for (int ch = -255; ch < 255; ++ch) {
     if (in_span(ch, LOWER_ARRAY))
diff --git a/libc/test/src/stdlib/StrtolTest.h b/libc/test/src/stdlib/StrtolTest.h
index 03f0a6539c785..3a7da1fa85ac7 100644
--- a/libc/test/src/stdlib/StrtolTest.h
+++ b/libc/test/src/stdlib/StrtolTest.h
@@ -177,8 +177,8 @@ struct StrtoTest : public LIBC_NAMESPACE::testing::ErrnoCheckingTest {
     char small_string[4] = {'\0', '\0', '\0', '\0'};
     for (int base = 2; base <= 36; ++base) {
       for (int first_digit = 0; first_digit <= 36; ++first_digit) {
-        small_string[0] = static_cast<char>(
-            LIBC_NAMESPACE::internal::int_to_b36_char(first_digit));
+        small_string[0] =
+            LIBC_NAMESPACE::internal::int_to_b36_char(first_digit);
         if (first_digit < base) {
           ASSERT_EQ(func(small_string, nullptr, base),
                     static_cast<ReturnT>(first_digit));
@@ -192,11 +192,11 @@ struct StrtoTest : public LIBC_NAMESPACE::testing::ErrnoCheckingTest {
 
     for (int base = 2; base <= 36; ++base) {
       for (int first_digit = 0; first_digit <= 36; ++first_digit) {
-        small_string[0] = static_cast<char>(
-            LIBC_NAMESPACE::internal::int_to_b36_char(first_digit));
+        small_string[0] =
+            LIBC_NAMESPACE::internal::int_to_b36_char(first_digit);
         for (int second_digit = 0; second_digit <= 36; ++second_digit) {
-          small_string[1] = static_cast<char>(
-              LIBC_NAMESPACE::internal::int_to_b36_char(second_digit));
+          small_string[1] =
+              LIBC_NAMESPACE::internal::int_to_b36_char(second_digit);
           if (first_digit < base && second_digit < base) {
             ASSERT_EQ(
                 func(small_string, nullptr, base),
@@ -216,14 +216,14 @@ struct StrtoTest : public LIBC_NAMESPACE::testing::ErrnoCheckingTest {
 
     for (int base = 2; base <= 36; ++base) {
       for (int first_digit = 0; first_digit <= 36; ++first_digit) {
-        small_string[0] = static_cast<char>(
-            LIBC_NAMESPACE::internal::int_to_b36_char(first_digit));
+        small_string[0] =
+            LIBC_NAMESPACE::internal::int_to_b36_char(first_digit);
         for (int second_digit = 0; second_digit <= 36; ++second_digit) {
-          small_string[1] = static_cast<char>(
-              LIBC_NAMESPACE::internal::int_to_b36_char(second_digit));
+          small_string[1] =
+              LIBC_NAMESPACE::internal::int_to_b36_char(second_digit);
           for (int third_digit = 0; third_digit <= limit; ++third_digit) {
-            small_string[2] = static_cast<char>(
-                LIBC_NAMESPACE::internal::int_to_b36_char(third_digit));
+            small_string[2] =
+                LIBC_NAMESPACE::internal::int_to_b36_char(third_digit);
 
             if (first_digit < base && second_digit < base &&
                 third_digit < base) {
diff --git a/utils/bazel/llvm-project-overlay/libc/BUILD.bazel b/utils/bazel/llvm-project-overlay/libc/BUILD.bazel
index 5a1e0b53b021c..fde68d1b910f4 100644
--- a/utils/bazel/llvm-project-overlay/libc/BUILD.bazel
+++ b/utils/bazel/llvm-project-overlay/libc/BUILD.bazel
@@ -1859,6 +1859,7 @@ libc_function(
     hdrs = ["src/ctype/isalnum.h"],
     deps = [
         ":__support_common",
+        ":__support_cpp_limits",
         ":__support_ctype_utils",
     ],
 )
@@ -1869,6 +1870,7 @@ libc_function(
     hdrs = ["src/ctype/isalpha.h"],
     deps = [
         ":__support_common",
+        ":__support_cpp_limits",
         ":__support_ctype_utils",
     ],
 )
@@ -1909,6 +1911,7 @@ libc_function(
     hdrs = ["src/ctype/isdigit.h"],
     deps = [
         ":__support_common",
+        ":__support_cpp_limits",
         ":__support_ctype_utils",
     ],
 )
@@ -1919,6 +1922,7 @@ libc_function(
     hdrs = ["src/ctype/isgraph.h"],
     deps = [
         ":__support_common",
+        ":__support_cpp_limits",
         ":__support_ctype_utils",
     ],
 )
@@ -1929,6 +1933,7 @@ libc_function(
     hdrs = ["src/ctype/islower.h"],
     deps = [
         ":__support_common",
+        ":__support_cpp_limits",
         ":__support_ctype_utils",
     ],
 )
@@ -1949,6 +1954,7 @@ libc_function(
     hdrs = ["src/ctype/ispunct.h"],
     deps = [
         ":__support_common",
+        ":__support_cpp_limits",
         ":__support_ctype_utils",
     ],
 )
@@ -1959,6 +1965,7 @@ libc_function(
     hdrs = ["src/ctype/isspace.h"],
     deps = [
         ":__support_common",
+        ":__support_cpp_limits",
         ":__support_ctype_utils",
     ],
 )
@@ -1969,6 +1976,7 @@ libc_function(
     hdrs = ["src/ctype/isupper.h"],
     deps = [
         ":__support_common",
+        ":__support_cpp_limits",
         ":__support_ctype_utils",
     ],
 )
@@ -1979,6 +1987,7 @@ libc_function(
     hdrs = ["src/ctype/isxdigit.h"],
     deps = [
         ":__support_common",
+        ":__support_cpp_limits",
         ":__support_ctype_utils",
     ],
 )
@@ -1999,6 +2008,7 @@ libc_function(
     hdrs = ["src/ctype/tolower.h"],
     deps = [
         ":__support_common",
+        ":__support_cpp_limits",
         ":__support_ctype_utils",
     ],
 )
@@ -2009,6 +2019,7 @@ libc_function(
     hdrs = ["src/ctype/toupper.h"],
     deps = [
         ":__support_common",
+        ":__support_cpp_limits",
         ":__support_ctype_utils",
     ],
 )