diff --git a/include/ruby/internal/intern/bignum.h b/include/ruby/internal/intern/bignum.h
index c27f77a1fb6eb9..6ab26fef715d74 100644
--- a/include/ruby/internal/intern/bignum.h
+++ b/include/ruby/internal/intern/bignum.h
@@ -26,6 +26,10 @@
 # include <stddef.h>
 #endif
 
+#ifdef HAVE_SYS_TYPES_H
+# include <sys/types.h>         /* for ssize_t (note: on Windows ssize_t is */
+#endif                          /* `#define`d in ruby/config.h) */
+
 #include "ruby/internal/attr/nonnull.h"
 #include "ruby/internal/dllexport.h"
 #include "ruby/internal/value.h"
@@ -841,6 +845,47 @@ size_t rb_absint_numwords(VALUE val, size_t word_numbits, size_t *nlz_bits_ret);
  */
 int rb_absint_singlebit_p(VALUE val);
 
+/**
+ * @name Flags for rb_int_parse_cstr()
+ * @{
+ */
+
+/** Allows a leading sign (`+` or `-`). */
+#define RB_INT_PARSE_SIGN       0x01
+
+/** Allows underscores between digits. */
+#define RB_INT_PARSE_UNDERSCORE 0x02
+
+/** Allows a base prefix (`0x`, `0b`, `0o`, `0d`). */
+#define RB_INT_PARSE_PREFIX     0x04
+
+/** Shorthand for all of the above flags combined. */
+#define RB_INT_PARSE_ALL        0x07
+
+/** Default flags (all features enabled). */
+#define RB_INT_PARSE_DEFAULT    0x07
+
+/** @} */
+
+/**
+ * Parses a C string to convert into a Ruby integer, with detailed control over
+ * parsing behaviour.  Unlike rb_cstr_to_inum(), this function can report the
+ * end position of the parsed integer and the number of parsed digits.
+ *
+ * @param[in]   str       Pointer to the string to parse.
+ * @param[in]   len       Length of `str` in bytes, or `-1` to use `strlen(str)`.
+ * @param[out]  endp      If not `NULL`, the pointer to the first unparsed
+ *                        character is stored here on return.
+ * @param[out]  ndigits   If not `NULL`, the number of parsed digits (excluding
+ *                        prefix, sign, and underscores) is stored here.
+ * @param[in]   base      Base of conversion.  `0` means auto-detect from
+ *                        prefix.  `2..36` for explicit base.
+ * @param[in]   flags     Bitwise or of `RB_INT_PARSE_*` macros.
+ * @return      A Ruby Integer (Fixnum or Bignum), or `Qnil` on parse failure
+ *              (when no digits are found).
+ */
+VALUE rb_int_parse_cstr(const char *str, ssize_t len, char **endp, size_t *ndigits, int base, int flags);
+
 RBIMPL_SYMBOL_EXPORT_END()
 
 #endif /* RBIMPL_INTERN_BIGNUM_H */
diff --git a/internal/bignum.h b/internal/bignum.h
index 0692bafed30d73..f11fbd3a4d096a 100644
--- a/internal/bignum.h
+++ b/internal/bignum.h
@@ -89,14 +89,6 @@
     (FL_USHIFT+3) /* bit offset of BIGNUM_EMBED_LEN_MASK */
 #define BIGNUM_EMBED_LEN_MAX (BIGNUM_EMBED_LEN_MASK >> BIGNUM_EMBED_LEN_SHIFT)
 
-enum rb_int_parse_flags {
-    RB_INT_PARSE_SIGN       = 0x01,
-    RB_INT_PARSE_UNDERSCORE = 0x02,
-    RB_INT_PARSE_PREFIX     = 0x04,
-    RB_INT_PARSE_ALL        = 0x07,
-    RB_INT_PARSE_DEFAULT    = 0x07,
-};
-
 struct RBignum {
     struct RBasic basic;
     union {
@@ -166,7 +158,6 @@ VALUE rb_big_divrem_gmp(VALUE x, VALUE y);
 VALUE rb_big2str_gmp(VALUE x, int base);
 VALUE rb_str2big_gmp(VALUE arg, int base, int badcheck);
 #endif
-VALUE rb_int_parse_cstr(const char *str, ssize_t len, char **endp, size_t *ndigits, int base, int flags);
 RUBY_SYMBOL_EXPORT_END
 
 #if HAVE_LONG_LONG