diff --git a/mypy/typeshed/stubs/librt/librt/strings.pyi b/mypy/typeshed/stubs/librt/librt/strings.pyi
index 94e3b69abf243..af9ae60936d8e 100644
--- a/mypy/typeshed/stubs/librt/librt/strings.pyi
+++ b/mypy/typeshed/stubs/librt/librt/strings.pyi
@@ -42,7 +42,8 @@ def read_f64_le(b: bytes, index: i64, /) -> float: ...
 def read_f64_be(b: bytes, index: i64, /) -> float: ...
 
 # Codepoint classification helpers operating on i32 codepoints (typically
-# obtained via ord(s[i])). Negative inputs return False.
+# obtained via ord(s[i])). Out-of-range inputs (negative, or past the maximum
+# Unicode code point 0x10FFFF) return False.
 def isspace(c: i32, /) -> bool: ...
 def isdigit(c: i32, /) -> bool: ...
 def isalnum(c: i32, /) -> bool: ...
@@ -53,7 +54,8 @@ def isidentifier(c: i32, /) -> bool: ...
 # uppercase / lowercase expands to multiple codepoints (e.g. U+00DF
 # uppercases to "SS", U+FB01 to "FI"), returns the input unchanged so
 # the signature stays i32 -> i32. Use str.upper() / str.lower() for full
-# Unicode case conversion when those cases matter. Negative inputs are
-# returned unchanged.
+# Unicode case conversion when those cases matter. Out-of-range inputs
+# (negative, or past the maximum Unicode code point 0x10FFFF) are returned
+# unchanged.
 def toupper(c: i32, /) -> i32: ...
 def tolower(c: i32, /) -> i32: ...
diff --git a/mypyc/build.py b/mypyc/build.py
index 13bd50fef3b1a..57438c7d5f52b 100644
--- a/mypyc/build.py
+++ b/mypyc/build.py
@@ -54,7 +54,12 @@ class ModDesc(NamedTuple):
 
 LIBRT_MODULES = [
     ModDesc("librt.internal", ["internal/librt_internal.c"], [], ["internal"]),
-    ModDesc("librt.strings", ["strings/librt_strings.c"], [], ["strings"]),
+    ModDesc(
+        "librt.strings",
+        ["strings/librt_strings.c"],
+        ["strings/librt_strings.h", "strings/librt_strings_common.h"],
+        ["strings"],
+    ),
     ModDesc(
         "librt.base64",
         [
diff --git a/mypyc/lib-rt/strings/librt_strings.h b/mypyc/lib-rt/strings/librt_strings.h
index 6c1942667ba44..33af57dc2a2a8 100644
--- a/mypyc/lib-rt/strings/librt_strings.h
+++ b/mypyc/lib-rt/strings/librt_strings.h
@@ -31,8 +31,9 @@ typedef struct {
 } StringWriterObject;
 
 // Codepoint classification helpers. Inputs are signed i32 for compatibility
-// with mypyc's int32_rprimitive; negative values are non-codepoints and
-// return false. Defined `static inline` so they compile statically into
+// with mypyc's int32_rprimitive; out-of-range values (negative, or past the
+// maximum Unicode code point 0x10FFFF) are non-codepoints and return false.
+// Defined `static inline` so they compile statically into
 // both the librt.strings module and any mypyc-compiled extension that
 // includes this header, avoiding the capsule indirection that would dwarf
 // the work of a single Py_UNICODE_IS* macro call.
@@ -58,12 +59,14 @@ static inline bool LibRTStrings_IsAlpha(int32_t c) {
 // PyUnicode_IsIdentifier on a 1-character string. Aborts via
 // CPyError_OutOfMemory on allocation failure to keep this ERR_NEVER.
 static inline bool LibRTStrings_IsIdentifier(int32_t c) {
-    if (c < 0) return false;
-    if (c < 128) {
+    // Unsigned compare: negatives wrap to large values and skip the fast path.
+    if ((uint32_t)c < 128) {
         return (c >= 'a' && c <= 'z')
             || (c >= 'A' && c <= 'Z')
             || c == '_';
     }
+    // Reject negatives and code points past the Unicode maximum.
+    if ((uint32_t)c > 0x10FFFF) return false;
     PyObject *s = PyUnicode_FromOrdinal((int)c);
     if (s == NULL) {
         CPyError_OutOfMemory();
@@ -101,9 +104,13 @@ static inline int32_t LibRTStrings_ChangeCase_slow(int32_t c, const char *method
 // non-ASCII delegates to str.upper on a 1-character string. Returns the
 // input unchanged when uppercasing expands to multiple codepoints.
 static inline int32_t LibRTStrings_ToUpper(int32_t c) {
-    if (c < 0) return c;
-    if (c >= 'a' && c <= 'z') return c - 32;
-    if (c < 128) return c;
+    // Unsigned compare: negatives wrap to large values and skip the fast path.
+    if ((uint32_t)c < 128) {
+        if (c >= 'a' && c <= 'z') return c - 32;
+        return c;
+    }
+    // Negatives and code points past the Unicode maximum are returned unchanged.
+    if ((uint32_t)c > 0x10FFFF) return c;
     return LibRTStrings_ChangeCase_slow(c, "upper");
 }
 
@@ -111,9 +118,13 @@ static inline int32_t LibRTStrings_ToUpper(int32_t c) {
 // non-ASCII delegates to str.lower on a 1-character string. Returns the
 // input unchanged when lowercasing expands to multiple codepoints.
 static inline int32_t LibRTStrings_ToLower(int32_t c) {
-    if (c < 0) return c;
-    if (c >= 'A' && c <= 'Z') return c + 32;
-    if (c < 128) return c;
+    // Unsigned compare: negatives wrap to large values and skip the fast path.
+    if ((uint32_t)c < 128) {
+        if (c >= 'A' && c <= 'Z') return c + 32;
+        return c;
+    }
+    // Negatives and code points past the Unicode maximum are returned unchanged.
+    if ((uint32_t)c > 0x10FFFF) return c;
     return LibRTStrings_ChangeCase_slow(c, "lower");
 }
 
diff --git a/mypyc/test-data/run-librt-strings.test b/mypyc/test-data/run-librt-strings.test
index 7efff12667d87..333b5962c4c99 100644
--- a/mypyc/test-data/run-librt-strings.test
+++ b/mypyc/test-data/run-librt-strings.test
@@ -1449,8 +1449,9 @@ from testutil import assertRaises
 
 
 def test_codepoint_classifiers() -> None:
-    # Negative values are not codepoints.
-    for bad in (i32(-1), i32(-113)):
+    # Out-of-range values are not codepoints: negative, just past the maximum
+    # valid code point (0x10FFFF), and the largest i32.
+    for bad in (i32(-1), i32(-113), i32(0x110000), i32(0x7FFFFFFF)):
         assert not isspace(bad)
         assert not isdigit(bad)
         assert not isalnum(bad)
@@ -1485,6 +1486,10 @@ def test_codepoint_classifiers_via_any() -> None:
         assert f(ord(false_input)) is False
         # Negative values are valid i32, just not codepoints.
         assert f(-1) is False
+        # Values within i32 range but past the maximum code point (0x10FFFF)
+        # are not codepoints either.
+        assert f(0x110000) is False
+        assert f(0x7FFFFFFF) is False
         # Inputs outside i32 range raise OverflowError through the wrapper.
         with assertRaises(OverflowError, "codepoint out of i32 range"):
             f(1 << 40)
@@ -1509,8 +1514,9 @@ def _expect(c: str, method: str) -> int:
 
 
 def test_codepoint_case_conversion() -> None:
-    # Negative inputs return unchanged.
-    for bad in (i32(-1), i32(-113)):
+    # Out-of-range inputs return unchanged: negative, just past the maximum
+    # valid code point (0x10FFFF), and the largest i32.
+    for bad in (i32(-1), i32(-113), i32(0x110000), i32(0x7FFFFFFF)):
         assert toupper(bad) == bad
         assert tolower(bad) == bad
     # Agree with str.upper / str.lower across the full Unicode range
@@ -1534,6 +1540,10 @@ def test_codepoint_case_conversion_via_any() -> None:
         assert f(in_cp) == out_cp
         # Negative values are valid i32, returned unchanged.
         assert f(-1) == -1
+        # Values within i32 range but past the maximum code point (0x10FFFF)
+        # are returned unchanged.
+        assert f(0x110000) == 0x110000
+        assert f(0x7FFFFFFF) == 0x7FFFFFFF
         # Inputs outside i32 range raise OverflowError through the wrapper.
         with assertRaises(OverflowError, "codepoint out of i32 range"):
             f(1 << 40)