diff --git a/run_agent.py b/run_agent.py
index 40c3eae12..6cfcb1b8f 100644
--- a/run_agent.py
+++ b/run_agent.py
@@ -2126,7 +2126,8 @@ class AIAgent:
         _is_openrouter = "openrouter" in self.base_url.lower()
         _is_nous = "nousresearch" in self.base_url.lower()
 
-        if _is_openrouter or _is_nous:
+        _is_mistral = "api.mistral.ai" in self.base_url.lower()
+        if (_is_openrouter or _is_nous) and not _is_mistral:
             if self.reasoning_config is not None:
                 extra_body["reasoning"] = self.reasoning_config
             else:
@@ -2271,6 +2272,7 @@ class AIAgent:
                     if reasoning:
                         api_msg["reasoning_content"] = reasoning
                 api_msg.pop("reasoning", None)
+                api_msg.pop("finish_reason", None)
                 api_messages.append(api_msg)
 
             if self._cached_system_prompt:
@@ -2912,6 +2914,9 @@ class AIAgent:
                 # We've copied it to 'reasoning_content' for the API above
                 if "reasoning" in api_msg:
                     api_msg.pop("reasoning")
+                # Remove finish_reason - not accepted by strict APIs (e.g. Mistral)
+                if "finish_reason" in api_msg:
+                    api_msg.pop("finish_reason")
                 # Keep 'reasoning_details' - OpenRouter uses this for multi-turn reasoning context
                 # The signature field helps maintain reasoning continuity
                 api_messages.append(api_msg)