diff --git a/tests/test_misc.py b/tests/test_misc.py
index 0832c8ee..e239a186 100644
--- a/tests/test_misc.py
+++ b/tests/test_misc.py
@@ -19,6 +19,9 @@ def test_encoding_for_model():
     assert enc.name == "o200k_base"
     enc = tiktoken.encoding_for_model("gpt-oss-120b")
     assert enc.name == "o200k_harmony"
+    # gpt-5.1 uses a dotted name and must not require a hyphenated prefix match
+    assert tiktoken.model.encoding_name_for_model("gpt-5.1") == "o200k_base"
+    assert tiktoken.model.encoding_name_for_model("gpt-5.1-2025-11-13") == "o200k_base"
 
 
 def test_optional_blobfile_dependency():
diff --git a/tiktoken/model.py b/tiktoken/model.py
index 5c669af4..3ddaa63c 100644
--- a/tiktoken/model.py
+++ b/tiktoken/model.py
@@ -9,6 +9,9 @@
     "o3-": "o200k_base",
     "o4-mini-": "o200k_base",
     # chat
+    # gpt-5.1 / gpt-5.2 style names use a dot, not a hyphen, so they do not
+    # match the "gpt-5-" prefix used for dated gpt-5-* snapshots.
+    "gpt-5.": "o200k_base",
     "gpt-5-": "o200k_base",
     "gpt-4.5-": "o200k_base",
     "gpt-4.1-": "o200k_base",
@@ -33,6 +36,7 @@
     "o4-mini": "o200k_base",
     # chat
     "gpt-5": "o200k_base",
+    "gpt-5.1": "o200k_base",
     "gpt-4.1": "o200k_base",
     "gpt-4o": "o200k_base",
     "gpt-4": "cl100k_base",