fix(providers): use max_completion_tokens for gpt-5/o-series on flagless specs (#4261)

tests/providers/test_litellm_kwargs.py

@@ -929,6 +929,43 @@ def test_openai_compat_build_kwargs_uses_gpt5_safe_parameters() -> None:
     assert "temperature" not in kwargs
 
 
+@pytest.mark.parametrize(
+    ("model_name", "expected_key"),
+    [
+        ("gpt-5.4", "max_completion_tokens"),
+        ("o3-mini", "max_completion_tokens"),
+        ("gpt-4", "max_tokens"),
+    ],
+)
+def test_openai_compat_build_kwargs_max_completion_tokens_by_model_name(
+    model_name: str,
+    expected_key: str,
+) -> None:
+    spec = find_by_name("custom")
+    with patch("nanobot.providers.openai_compat_provider.AsyncOpenAI"):
+        provider = OpenAICompatProvider(
+            api_key="sk-test-key",
+            default_model=model_name,
+            spec=spec,
+        )
+
+    kwargs = provider._build_kwargs(
+        messages=[{"role": "user", "content": "hello"}],
+        tools=None,
+        model=model_name,
+        max_tokens=2048,
+        temperature=0.7,
+        reasoning_effort=None,
+        tool_choice=None,
+    )
+
+    other_key = (
+        "max_tokens" if expected_key == "max_completion_tokens" else "max_completion_tokens"
+    )
+    assert kwargs[expected_key] == 2048
+    assert other_key not in kwargs
+
+
 def test_openai_compat_preserves_message_level_reasoning_fields() -> None:
     with patch("nanobot.providers.openai_compat_provider.AsyncOpenAI"):
         provider = OpenAICompatProvider()