diff --git a/README.md b/README.md
index 7f686b683..8929d3612 100644
--- a/README.md
+++ b/README.md
@@ -1157,6 +1157,7 @@ That's it! Environment variables, model routing, config matching, and `nanobot s
 | `detect_by_key_prefix` | Detect gateway by API key prefix | `"sk-or-"` |
 | `detect_by_base_keyword` | Detect gateway by API base URL | `"openrouter"` |
 | `strip_model_prefix` | Strip provider prefix before sending to gateway | `True` (for AiHubMix) |
+| `supports_max_completion_tokens` | Use `max_completion_tokens` instead of `max_tokens`; required for providers that reject both being set simultaneously (e.g. VolcEngine) | `True` |
 
 </details>
 
diff --git a/nanobot/providers/openai_compat_provider.py b/nanobot/providers/openai_compat_provider.py
index e9a6ad871..397b8e797 100644
--- a/nanobot/providers/openai_compat_provider.py
+++ b/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:
diff --git a/nanobot/providers/registry.py b/nanobot/providers/registry.py
index e42e1f95e..5644fc51d 100644
--- a/nanobot/providers/registry.py
+++ b/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]], ...] = ()