diff --git a/nanobot/providers/openai_compat_provider.py b/nanobot/providers/openai_compat_provider.py
index ee44333a6..5b766edf6 100644
--- a/nanobot/providers/openai_compat_provider.py
+++ b/nanobot/providers/openai_compat_provider.py
@@ -93,6 +93,14 @@ def _model_slug(model_name: str) -> str:
     return model_name.lower().rsplit("/", 1)[-1]
 
 
+def _requires_max_completion_tokens(model_name: str) -> bool:
+    """Return True for models that reject ``max_tokens`` (GPT-5 family, o3/o4)."""
+    slug = _model_slug(model_name)
+    return "gpt-5" in slug or any(
+        slug == p or slug.startswith((p + "-", p + ".")) for p in ("o3", "o4")
+    )
+
+
 def _model_thinking_style(model_name: str) -> str:
     return _MODEL_THINKING_STYLES.get(_model_slug(model_name), "")
 
@@ -630,7 +638,9 @@ class OpenAICompatProvider(LLMProvider):
         if self._supports_temperature(model_name, reasoning_effort):
             kwargs["temperature"] = temperature
 
-        if spec and getattr(spec, "supports_max_completion_tokens", False):
+        if (
+            spec and getattr(spec, "supports_max_completion_tokens", False)
+        ) or _requires_max_completion_tokens(model_name):
             kwargs["max_completion_tokens"] = max(1, max_tokens)
         else:
             kwargs["max_tokens"] = max(1, max_tokens)
diff --git a/tests/providers/test_litellm_kwargs.py b/tests/providers/test_litellm_kwargs.py
index 0a1b85f70..81e5f5d0a 100644
--- a/tests/providers/test_litellm_kwargs.py
+++ b/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()