fix(dspy): exclude gpt-5-chat from reasoning model classification (#9033)

The regex pattern for identifying reasoning models was incorrectly
matching gpt-5-chat, causing it to be subject to reasoning model
validation requirements and attempting to pass reasoning parameters
that Azure's API doesn't support for this model.

Updated the regex to use negative lookahead (?!-chat) to exclude
gpt-5-chat while still allowing other gpt-5 variants like gpt-5-mini,
gpt-5-nano, and gpt-5-pro to be recognized as reasoning models.

- Updated regex pattern in dspy/clients/lm.py
- Added test case for gpt-5-chat in test_reasoning_model_token_parameter
- Added dedicated test test_gpt_5_chat_not_reasoning_model

Fixes issue where gpt-5-chat (Azure AI Foundry) was incorrectly
classified as a reasoning model, preventing normal usage with
standard temperature and max_tokens values.

Author: mindful-time

dspy/clients/lm.py

@@ -85,7 +85,13 @@ def __init__(
         model_family = model.split("/")[-1].lower() if "/" in model else model.lower()
 
         # Recognize OpenAI reasoning models (o1, o3, o4, gpt-5 family)
-        model_pattern = re.match(r"^(?:o[1345]|gpt-5)(?:-(?:mini|nano))?", model_family)
+        # Exclude non-reasoning variants like gpt-5-chat this is in azure ai foundry
+        # Allow date suffixes like -2023-01-01 after model name or mini/nano/pro
+        # For gpt-5, use negative lookahead to exclude -chat and allow other suffixes
+        model_pattern = re.match(
+            r"^(?:o[1345](?:-(?:mini|nano|pro))?(?:-\d{4}-\d{2}-\d{2})?|gpt-5(?!-chat)(?:-.*)?)$",
+            model_family,
+        )
 
         if model_pattern:
             if (temperature and temperature != 1.0) or (max_tokens and max_tokens < 16000):

tests/clients/test_lm.py

@@ -298,6 +298,7 @@ def test_reasoning_model_token_parameter():
         ("openai/gpt-5", True),
         ("openai/gpt-5-mini", True),
         ("openai/gpt-5-nano", True),
+        ("azure/gpt-5-chat", False),  # gpt-5-chat is NOT a reasoning model
         ("openai/gpt-4", False),
         ("anthropic/claude-2", False),
     ]
@@ -318,7 +319,7 @@ def test_reasoning_model_token_parameter():
             assert lm.kwargs["max_tokens"] == 1000
 
 
-@pytest.mark.parametrize("model_name", ["openai/o1", "openai/gpt-5-nano"])
+@pytest.mark.parametrize("model_name", ["openai/o1", "openai/gpt-5-nano", "openai/gpt-5-mini"])
 def test_reasoning_model_requirements(model_name):
     # Should raise assertion error if temperature or max_tokens requirements not met
     with pytest.raises(
@@ -347,6 +348,21 @@ def test_reasoning_model_requirements(model_name):
     assert lm.kwargs["max_completion_tokens"] is None
 
 
+def test_gpt_5_chat_not_reasoning_model():
+    """Test that gpt-5-chat is NOT treated as a reasoning model."""
+    # Should NOT raise validation error - gpt-5-chat is not a reasoning model
+    lm = dspy.LM(
+        model="openai/gpt-5-chat",
+        temperature=0.7,  # Can be any value
+        max_tokens=1000,  # Can be any value
+    )
+    # Should use max_tokens, not max_completion_tokens
+    assert "max_completion_tokens" not in lm.kwargs
+    assert "max_tokens" in lm.kwargs
+    assert lm.kwargs["max_tokens"] == 1000
+    assert lm.kwargs["temperature"] == 0.7
+
+
 def test_dump_state():
     lm = dspy.LM(
         model="openai/gpt-4o-mini",