diff --git a/ext/json/fbuffer/fbuffer.h b/ext/json/fbuffer/fbuffer.h
index 4c42e14b6f..b56955e8f4 100644
--- a/ext/json/fbuffer/fbuffer.h
+++ b/ext/json/fbuffer/fbuffer.h
@@ -3,6 +3,7 @@
 
 #include "ruby.h"
 #include "ruby/encoding.h"
+#include "../vendor/jeaiii-ltoa.h"
 
 /* shims */
 /* This is the fallback definition from Ruby 3.4 */
@@ -150,6 +151,13 @@ static void fbuffer_append(FBuffer *fb, const char *newstr, unsigned long len)
     }
 }
 
+/* Appends a character into a buffer. The buffer needs to have sufficient capacity, via fbuffer_inc_capa(...). */
+static inline void fbuffer_append_reserved_char(FBuffer *fb, char chr)
+{
+    fb->ptr[fb->len] = chr;
+    fb->len += 1;
+}
+
 static void fbuffer_append_str(FBuffer *fb, VALUE str)
 {
     const char *newstr = StringValuePtr(str);
@@ -167,25 +175,39 @@ static inline void fbuffer_append_char(FBuffer *fb, char newchr)
     fb->len++;
 }
 
-static long fltoa(long number, char *buf)
-{
-    static const char digits[] = "0123456789";
-    long sign = number;
-    char* tmp = buf;
-
-    if (sign < 0) number = -number;
-    do *tmp-- = digits[number % 10]; while (number /= 10);
-    if (sign < 0) *tmp-- = '-';
-    return buf - tmp;
-}
-
-#define LONG_BUFFER_SIZE 20
+/*
+ * Appends the decimal string representation of \a number into the buffer.
+ */
 static void fbuffer_append_long(FBuffer *fb, long number)
 {
-    char buf[LONG_BUFFER_SIZE];
-    char *buffer_end = buf + LONG_BUFFER_SIZE;
-    long len = fltoa(number, buffer_end - 1);
-    fbuffer_append(fb, buffer_end - len, len);
+    /*
+     * The to_text_from_ulong() function produces digits left-to-right,
+     * allowing us to write directly into the buffer, but we don't know
+     * the number of resulting characters.
+     *
+     * We do know, however, that the `number` argument is always in the
+     * range 0xc000000000000000 to 0x3fffffffffffffff, or, in decimal, 
+     * -4611686018427387904 to 4611686018427387903. The max number of chars
+     * generated is therefore 20 (including a potential sign character).
+     */
+
+    static const int MAX_CHARS_FOR_LONG = 20;
+
+    fbuffer_inc_capa(fb, MAX_CHARS_FOR_LONG);
+
+    if (number < 0) {
+        fbuffer_append_reserved_char(fb, '-');
+
+        /* 
+         * Since number is always > LONG_MIN, `-number` will not overflow
+         * and is always the positive abs() value.
+         */
+        number = -number;
+    }
+
+    char* d = fb->ptr + fb->len;
+    char* end = to_text_from_ulong(d, number);
+    fb->len += end - d;
 }
 
 static VALUE fbuffer_finalize(FBuffer *fb)
diff --git a/ext/json/generator/generator.c b/ext/json/generator/generator.c
index 428f5e21ff..e0d7d0ca4f 100644
--- a/ext/json/generator/generator.c
+++ b/ext/json/generator/generator.c
@@ -1710,6 +1710,7 @@ void Init_generator(void)
     cFragment = rb_const_get(mJSON, rb_intern("Fragment"));
 
     VALUE mExt = rb_define_module_under(mJSON, "Ext");
+
     VALUE mGenerator = rb_define_module_under(mExt, "Generator");
 
     rb_global_variable(&eGeneratorError);
diff --git a/ext/json/vendor/jeaiii-ltoa.h b/ext/json/vendor/jeaiii-ltoa.h
new file mode 100644
index 0000000000..f83e036e78
--- /dev/null
+++ b/ext/json/vendor/jeaiii-ltoa.h
@@ -0,0 +1,257 @@
+/*
+
+This file is released under the terms of the MIT License. It is based on the
+work of James Edward Anhalt III, with the original license listed below.
+
+MIT License
+
+Copyright (c) 2024,2025 Enrico Thierbach - https://github.com/radiospiel
+Copyright (c) 2022 James Edward Anhalt III - https://github.com/jeaiii/itoa
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+*/
+
+#ifndef JEAIII_TO_TEXT_H_
+#define JEAIII_TO_TEXT_H_
+
+#include <stdint.h>
+
+typedef uint_fast32_t u32_t;
+typedef uint_fast64_t u64_t;
+
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wshorten-64-to-32"
+
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wmissing-braces"
+
+#define u32(x) ((u32_t)(x))
+#define u64(x) ((u64_t)(x))
+
+struct pair
+{
+    char dd[2];
+};
+
+#define cast_to_pair_ptr(b) ((struct pair*)(void*)(b))
+
+static struct pair digits_dd[100] =
+{
+    { '0', '0' }, { '0', '1' }, { '0', '2' }, { '0', '3' }, { '0', '4' }, { '0', '5' }, { '0', '6' }, { '0', '7' }, { '0', '8' }, { '0', '9' },
+    { '1', '0' }, { '1', '1' }, { '1', '2' }, { '1', '3' }, { '1', '4' }, { '1', '5' }, { '1', '6' }, { '1', '7' }, { '1', '8' }, { '1', '9' },
+    { '2', '0' }, { '2', '1' }, { '2', '2' }, { '2', '3' }, { '2', '4' }, { '2', '5' }, { '2', '6' }, { '2', '7' }, { '2', '8' }, { '2', '9' },
+    { '3', '0' }, { '3', '1' }, { '3', '2' }, { '3', '3' }, { '3', '4' }, { '3', '5' }, { '3', '6' }, { '3', '7' }, { '3', '8' }, { '3', '9' },
+    { '4', '0' }, { '4', '1' }, { '4', '2' }, { '4', '3' }, { '4', '4' }, { '4', '5' }, { '4', '6' }, { '4', '7' }, { '4', '8' }, { '4', '9' },
+    { '5', '0' }, { '5', '1' }, { '5', '2' }, { '5', '3' }, { '5', '4' }, { '5', '5' }, { '5', '6' }, { '5', '7' }, { '5', '8' }, { '5', '9' },
+    { '6', '0' }, { '6', '1' }, { '6', '2' }, { '6', '3' }, { '6', '4' }, { '6', '5' }, { '6', '6' }, { '6', '7' }, { '6', '8' }, { '6', '9' },
+    { '7', '0' }, { '7', '1' }, { '7', '2' }, { '7', '3' }, { '7', '4' }, { '7', '5' }, { '7', '6' }, { '7', '7' }, { '7', '8' }, { '7', '9' },
+    { '8', '0' }, { '8', '1' }, { '8', '2' }, { '8', '3' }, { '8', '4' }, { '8', '5' }, { '8', '6' }, { '8', '7' }, { '8', '8' }, { '8', '9' },
+    { '9', '0' }, { '9', '1' }, { '9', '2' }, { '9', '3' }, { '9', '4' }, { '9', '5' }, { '9', '6' }, { '9', '7' }, { '9', '8' }, { '9', '9' },
+};
+
+#define NUL 'x'
+
+static struct pair digits_fd[100] =
+{
+    { '0', NUL }, { '1', NUL }, { '2', NUL }, { '3', NUL }, { '4', NUL }, { '5', NUL }, { '6', NUL }, { '7', NUL }, { '8', NUL }, { '9', NUL },
+    { '1', '0' }, { '1', '1' }, { '1', '2' }, { '1', '3' }, { '1', '4' }, { '1', '5' }, { '1', '6' }, { '1', '7' }, { '1', '8' }, { '1', '9' },
+    { '2', '0' }, { '2', '1' }, { '2', '2' }, { '2', '3' }, { '2', '4' }, { '2', '5' }, { '2', '6' }, { '2', '7' }, { '2', '8' }, { '2', '9' },
+    { '3', '0' }, { '3', '1' }, { '3', '2' }, { '3', '3' }, { '3', '4' }, { '3', '5' }, { '3', '6' }, { '3', '7' }, { '3', '8' }, { '3', '9' },
+    { '4', '0' }, { '4', '1' }, { '4', '2' }, { '4', '3' }, { '4', '4' }, { '4', '5' }, { '4', '6' }, { '4', '7' }, { '4', '8' }, { '4', '9' },
+    { '5', '0' }, { '5', '1' }, { '5', '2' }, { '5', '3' }, { '5', '4' }, { '5', '5' }, { '5', '6' }, { '5', '7' }, { '5', '8' }, { '5', '9' },
+    { '6', '0' }, { '6', '1' }, { '6', '2' }, { '6', '3' }, { '6', '4' }, { '6', '5' }, { '6', '6' }, { '6', '7' }, { '6', '8' }, { '6', '9' },
+    { '7', '0' }, { '7', '1' }, { '7', '2' }, { '7', '3' }, { '7', '4' }, { '7', '5' }, { '7', '6' }, { '7', '7' }, { '7', '8' }, { '7', '9' },
+    { '8', '0' }, { '8', '1' }, { '8', '2' }, { '8', '3' }, { '8', '4' }, { '8', '5' }, { '8', '6' }, { '8', '7' }, { '8', '8' }, { '8', '9' },
+    { '9', '0' }, { '9', '1' }, { '9', '2' }, { '9', '3' }, { '9', '4' }, { '9', '5' }, { '9', '6' }, { '9', '7' }, { '9', '8' }, { '9', '9' },
+};
+
+#undef NUL
+
+static u64_t mask24 = (u64(1) << 24) - 1;
+static u64_t mask32 = (u64(1) << 32) - 1;
+static u64_t mask57 = (u64(1) << 57) - 1;
+
+static 
+char* to_text_from_ulong(char* b, u64_t n) {
+    if (n < u32(1e2))
+    {
+        *cast_to_pair_ptr(b) = digits_fd[n];
+        return n < 10 ? b + 1 : b + 2;
+    }
+    if (n < u32(1e6))
+    {
+        if (n < u32(1e4))
+        {
+            u32_t f0 = u32(10 * (1 << 24) / 1e3 + 1) * n;
+            *cast_to_pair_ptr(b) = digits_fd[f0 >> 24];
+            b -= n < u32(1e3);
+            u32_t f2 = (f0 & mask24) * 100;
+            *cast_to_pair_ptr(b + 2) = digits_dd[f2 >> 24];
+            return b + 4;
+        }
+        u64_t f0 = u64(10 * (1ull << 32ull)/ 1e5 + 1) * n;
+        *cast_to_pair_ptr(b) = digits_fd[f0 >> 32];
+        b -= n < u32(1e5);
+        u64_t f2 = (f0 & mask32) * 100;
+        *cast_to_pair_ptr(b + 2) = digits_dd[f2 >> 32];
+        u64_t f4 = (f2 & mask32) * 100;
+        *cast_to_pair_ptr(b + 4) = digits_dd[f4 >> 32];
+        return b + 6;
+    }
+    if (n < u64(1ull << 32ull))
+    {
+        if (n < u32(1e8))
+        {
+            u64_t f0 = u64(10 * (1ull << 48ull) / 1e7 + 1) * n >> 16;
+            *cast_to_pair_ptr(b) = digits_fd[f0 >> 32];
+            b -= n < u32(1e7);
+            u64_t f2 = (f0 & mask32) * 100;
+            *cast_to_pair_ptr(b + 2) = digits_dd[f2 >> 32];
+            u64_t f4 = (f2 & mask32) * 100;
+            *cast_to_pair_ptr(b + 4) = digits_dd[f4 >> 32];
+            u64_t f6 = (f4 & mask32) * 100;
+            *cast_to_pair_ptr(b + 6) = digits_dd[f6 >> 32];
+            return b + 8;
+        }
+        u64_t f0 = u64(10 * (1ull << 57ull) / 1e9 + 1) * n;
+        *cast_to_pair_ptr(b) = digits_fd[f0 >> 57];
+        b -= n < u32(1e9);
+        u64_t f2 = (f0 & mask57) * 100;
+        *cast_to_pair_ptr(b + 2) = digits_dd[f2 >> 57];
+        u64_t f4 = (f2 & mask57) * 100;
+        *cast_to_pair_ptr(b + 4) = digits_dd[f4 >> 57];
+        u64_t f6 = (f4 & mask57) * 100;
+        *cast_to_pair_ptr(b + 6) = digits_dd[f6 >> 57];
+        u64_t f8 = (f6 & mask57) * 100;
+        *cast_to_pair_ptr(b + 8) = digits_dd[f8 >> 57];
+        return b + 10;
+    }
+
+    // if we get here U must be u64 but some compilers don't know that, so reassign n to a u64 to avoid warnings
+    u32_t z = n % u32(1e8);
+    u64_t u = n / u32(1e8);
+
+    if (u < u32(1e2))
+    {
+        // u can't be 1 digit (if u < 10 it would have been handled above as a 9 digit 32bit number)
+        *cast_to_pair_ptr(b) = digits_dd[u];
+        b += 2;
+    }
+    else if (u < u32(1e6))
+    {
+        if (u < u32(1e4))
+        {
+            u32_t f0 = u32(10 * (1 << 24) / 1e3 + 1) * u;
+            *cast_to_pair_ptr(b) = digits_fd[f0 >> 24];
+            b -= u < u32(1e3);
+            u32_t f2 = (f0 & mask24) * 100;
+            *cast_to_pair_ptr(b + 2) = digits_dd[f2 >> 24];
+            b += 4;
+        }
+        else
+        {
+            u64_t f0 = u64(10 * (1ull << 32ull) / 1e5 + 1) * u;
+            *cast_to_pair_ptr(b) = digits_fd[f0 >> 32];
+            b -= u < u32(1e5);
+            u64_t f2 = (f0 & mask32) * 100;
+            *cast_to_pair_ptr(b + 2) = digits_dd[f2 >> 32];
+            u64_t f4 = (f2 & mask32) * 100;
+            *cast_to_pair_ptr(b + 4) = digits_dd[f4 >> 32];
+            b += 6;
+        }
+    }
+    else if (u < u32(1e8))
+    {
+        u64_t f0 = u64(10 * (1ull << 48ull) / 1e7 + 1) * u >> 16;
+        *cast_to_pair_ptr(b) = digits_fd[f0 >> 32];
+        b -= u < u32(1e7);
+        u64_t f2 = (f0 & mask32) * 100;
+        *cast_to_pair_ptr(b + 2) = digits_dd[f2 >> 32];
+        u64_t f4 = (f2 & mask32) * 100;
+        *cast_to_pair_ptr(b + 4) = digits_dd[f4 >> 32];
+        u64_t f6 = (f4 & mask32) * 100;
+        *cast_to_pair_ptr(b + 6) = digits_dd[f6 >> 32];
+        b += 8;
+    }
+    else if (u < u64(1ull << 32ull))
+    {
+        u64_t f0 = u64(10 * (1ull << 57ull) / 1e9 + 1) * u;
+        *cast_to_pair_ptr(b) = digits_fd[f0 >> 57];
+        b -= u < u32(1e9);
+        u64_t f2 = (f0 & mask57) * 100;
+        *cast_to_pair_ptr(b + 2) = digits_dd[f2 >> 57];
+        u64_t f4 = (f2 & mask57) * 100;
+        *cast_to_pair_ptr(b + 4) = digits_dd[f4 >> 57];
+        u64_t f6 = (f4 & mask57) * 100;
+        *cast_to_pair_ptr(b + 6) = digits_dd[f6 >> 57];
+        u64_t f8 = (f6 & mask57) * 100;
+        *cast_to_pair_ptr(b + 8) = digits_dd[f8 >> 57];
+        b += 10;
+    }
+    else
+    {
+        u32_t y = u % u32(1e8);
+        u /= u32(1e8);
+
+        // u is 2, 3, or 4 digits (if u < 10 it would have been handled above)
+        if (u < u32(1e2))
+        {
+            *cast_to_pair_ptr(b) = digits_dd[u];
+            b += 2;
+        }
+        else
+        {
+            u32_t f0 = u32(10 * (1 << 24) / 1e3 + 1) * u;
+            *cast_to_pair_ptr(b) = digits_fd[f0 >> 24];
+            b -= u < u32(1e3);
+            u32_t f2 = (f0 & mask24) * 100;
+            *cast_to_pair_ptr(b + 2) = digits_dd[f2 >> 24];
+            b += 4;
+        }
+        // do 8 digits
+        u64_t f0 = (u64((1ull << 48ull) / 1e6 + 1) * y >> 16) + 1;
+        *cast_to_pair_ptr(b) = digits_dd[f0 >> 32];
+        u64_t f2 = (f0 & mask32) * 100;
+        *cast_to_pair_ptr(b + 2) = digits_dd[f2 >> 32];
+        u64_t f4 = (f2 & mask32) * 100;
+        *cast_to_pair_ptr(b + 4) = digits_dd[f4 >> 32];
+        u64_t f6 = (f4 & mask32) * 100;
+        *cast_to_pair_ptr(b + 6) = digits_dd[f6 >> 32];
+        b += 8;
+    }
+    // do 8 digits
+    u64_t f0 = (u64((1ull << 48ull) / 1e6 + 1) * z >> 16) + 1;
+    *cast_to_pair_ptr(b) = digits_dd[f0 >> 32];
+    u64_t f2 = (f0 & mask32) * 100;
+    *cast_to_pair_ptr(b + 2) = digits_dd[f2 >> 32];
+    u64_t f4 = (f2 & mask32) * 100;
+    *cast_to_pair_ptr(b + 4) = digits_dd[f4 >> 32];
+    u64_t f6 = (f4 & mask32) * 100;
+    *cast_to_pair_ptr(b + 6) = digits_dd[f6 >> 32];
+    return b + 8;
+}
+
+#undef u32
+#undef u64
+
+#pragma clang diagnostic pop
+#pragma GCC diagnostic pop
+
+#endif // JEAIII_TO_TEXT_H_
+
diff --git a/test/json/json_generator_test.rb b/test/json/json_generator_test.rb
index bf4796a6bf..9cca447e2a 100755
--- a/test/json/json_generator_test.rb
+++ b/test/json/json_generator_test.rb
@@ -707,4 +707,16 @@ class JSONGeneratorTest < Test::Unit::TestCase
         assert_equal expected, value.to_json
       end
   end
+  
+  def test_numbers_of_various_sizes
+    numbers = [
+      0, 1, -1, 9, -9, 13, -13, 91, -91, 513, -513, 7513, -7513,
+      17591, -17591, -4611686018427387904, 4611686018427387903,
+      2**62, 2**63, 2**64, -(2**62), -(2**63), -(2**64)
+    ]
+
+    numbers.each do |number|
+      assert_equal "[#{number}]", JSON.generate([number])
+    end
+  end
 end