diff --git a/src/EncoderProvider.php b/src/EncoderProvider.php
index d6dfdcc..2ea1105 100644
--- a/src/EncoderProvider.php
+++ b/src/EncoderProvider.php
@@ -58,6 +58,7 @@ final class EncoderProvider implements ResetInterface
         'o4-mini-' => 'o200k_base',
         'chatgpt-4o-' => 'o200k_base',
         'gpt-5-' => 'o200k_base',
+        'gpt-5.1-' => 'o200k_base',
         'gpt-4-' => 'cl100k_base',
         'gpt-4.1-' => 'o200k_base',
         'gpt-4.5-' => 'o200k_base',
@@ -70,6 +71,7 @@ final class EncoderProvider implements ResetInterface
         'o3' => 'o200k_base',
         'o4-mini' => 'o200k_base',
         'gpt-5' => 'o200k_base',
+        'gpt-5.1' => 'o200k_base',
         'gpt-4' => 'cl100k_base',
         'gpt-4.1' => 'o200k_base',
         'gpt-4o' => 'o200k_base',