diff --git a/src/split_markdown4gpt/splitter.py b/src/split_markdown4gpt/splitter.py
index 68bd484..91913cb 100644
--- a/src/split_markdown4gpt/splitter.py
+++ b/src/split_markdown4gpt/splitter.py
@@ -1,4 +1,5 @@
 #!/usr/bin/env python3
+import sys
 from collections import defaultdict
 from functools import lru_cache
 from io import TextIOWrapper
@@ -35,6 +36,12 @@ def meta_data(md: str) -> tuple:
 
 
 OPENAI_MODELS = {
+    "gpt-5": 400000,
+    "gpt-5-mini": 400000,
+    "gpt-5-nano": 400000,
+    "gpt-4.1": 1000000,
+    "gpt-4o": 128000,
+    "gpt-4-turbo": 128000,
     "gpt-4": 8192,
     "gpt-4-32k": 32768,
     "gpt-4-32k-0613": 32768,
@@ -70,7 +77,16 @@ class MarkdownLLMSplitter:
     def __init__(
         self, gptok_model: str = "gpt-3.5-turbo", gptok_limit: int = None
     ) -> None:
-        self.gptoker = tiktoken.encoding_for_model(gptok_model)
+        try:
+            self.gptoker = tiktoken.encoding_for_model(gptok_model)
+        except KeyError:
+            self.gptoker = tiktoken.get_encoding("cl100k_base")
+        if gptok_model not in OPENAI_MODELS:
+            print(
+                f"Warning: Model '{gptok_model}' not found in the list of known models. "
+                f"Token limits may be inaccurate.",
+                file=sys.stderr,
+            )
         self.gptok_limit = gptok_limit or OPENAI_MODELS.get(gptok_model, 2048)
         self.md_meta = {}
         self.md_str = ""
diff --git a/tests/test_splitter.py b/tests/test_splitter.py
index 2d96678..13e9b33 100644
--- a/tests/test_splitter.py
+++ b/tests/test_splitter.py
@@ -1,5 +1,7 @@
 import pytest
 from pathlib import Path
+import sys
+from io import StringIO
 from split_markdown4gpt.splitter import MarkdownLLMSplitter, split
 
 def test_split():
@@ -9,3 +11,25 @@ def test_split():
     assert isinstance(sections, list)
     assert all(isinstance(section, str) for section in sections)
 
+def test_new_openai_models():
+    """Test that new OpenAI models are recognized and their limits are set correctly."""
+    splitter_gpt5 = MarkdownLLMSplitter(gptok_model="gpt-5")
+    assert splitter_gpt5.gptok_limit == 400000
+    splitter_gpt5_mini = MarkdownLLMSplitter(gptok_model="gpt-5-mini")
+    assert splitter_gpt5_mini.gptok_limit == 400000
+    splitter_4_1 = MarkdownLLMSplitter(gptok_model="gpt-4.1")
+    assert splitter_4_1.gptok_limit == 1000000
+
+def test_unknown_model_warning():
+    """Test that a warning is printed for unknown models."""
+    # Redirect stderr to capture the warning message
+    old_stderr = sys.stderr
+    sys.stderr = captured_stderr = StringIO()
+
+    MarkdownLLMSplitter(gptok_model="claude-3-opus-20240229")
+
+    # Restore stderr
+    sys.stderr = old_stderr
+
+    warning_message = captured_stderr.getvalue()
+    assert "Warning: Model 'claude-3-opus-20240229' not found" in warning_message