fix(providers): make max_tokens and max_completion_tokens mutually exclusive (#2491)

* fix(providers): make max_tokens and max_completion_tokens mutually exclusive

* docs: document supports_max_completion_tokens ProviderSpec option

nanobot/providers/openai_compat_provider.py

@@ -243,11 +243,14 @@ class OpenAICompatProvider(LLMProvider):
         kwargs: dict[str, Any] = {
             "model": model_name,
             "messages": self._sanitize_messages(self._sanitize_empty_content(messages)),
-            "max_tokens": max(1, max_tokens),
-            "max_completion_tokens": max(1, max_tokens),
             "temperature": temperature,
         }
 
+        if spec and getattr(spec, "supports_max_completion_tokens", False):
+            kwargs["max_completion_tokens"] = max(1, max_tokens)
+        else:
+            kwargs["max_tokens"] = max(1, max_tokens)
+
         if spec:
             model_lower = model_name.lower()
             for pattern, overrides in spec.model_overrides:

nanobot/providers/registry.py

@@ -49,6 +49,7 @@ class ProviderSpec:
 
     # gateway behavior
     strip_model_prefix: bool = False  # strip "provider/" before sending to gateway
+    supports_max_completion_tokens: bool = False
 
     # per-model param overrides, e.g. (("kimi-k2.5", {"temperature": 1.0}),)
     model_overrides: tuple[tuple[str, dict[str, Any]], ...] = ()