diff --git a/run_agent.py b/run_agent.py
index cce83f6b6..21ffbd9f3 100644
--- a/run_agent.py
+++ b/run_agent.py
@@ -2530,6 +2530,31 @@ class AIAgent:
 
         return msg
 
+    @staticmethod
+    def _sanitize_tool_calls_for_strict_api(api_msg: dict) -> dict:
+        """Strip Codex Responses API fields from tool_calls for strict providers.
+
+        Providers like Mistral strictly validate the Chat Completions schema
+        and reject unknown fields (call_id, response_item_id) with 422.
+        These fields are preserved in the internal message history — this
+        method only modifies the outgoing API copy.
+
+        Creates new tool_call dicts rather than mutating in-place, so the
+        original messages list retains call_id/response_item_id for Codex
+        Responses API compatibility (e.g. if the session falls back to a
+        Codex provider later).
+        """
+        tool_calls = api_msg.get("tool_calls")
+        if not isinstance(tool_calls, list):
+            return api_msg
+        _STRIP_KEYS = {"call_id", "response_item_id"}
+        api_msg["tool_calls"] = [
+            {k: v for k, v in tc.items() if k not in _STRIP_KEYS}
+            if isinstance(tc, dict) else tc
+            for tc in tool_calls
+        ]
+        return api_msg
+
     def flush_memories(self, messages: list = None, min_turns: int = None):
         """Give the model one turn to persist memories before context is lost.
 
@@ -2567,6 +2592,7 @@ class AIAgent:
 
         try:
             # Build API messages for the flush call
+            _is_strict_api = "api.mistral.ai" in self.base_url.lower()
             api_messages = []
             for msg in messages:
                 api_msg = msg.copy()
@@ -2577,6 +2603,8 @@ class AIAgent:
                 api_msg.pop("reasoning", None)
                 api_msg.pop("finish_reason", None)
                 api_msg.pop("_flush_sentinel", None)
+                if _is_strict_api:
+                    self._sanitize_tool_calls_for_strict_api(api_msg)
                 api_messages.append(api_msg)
 
             if self._cached_system_prompt:
@@ -3042,11 +3070,14 @@ class AIAgent:
         try:
             # Build API messages, stripping internal-only fields
             # (finish_reason, reasoning) that strict APIs like Mistral reject with 422
+            _is_strict_api = "api.mistral.ai" in self.base_url.lower()
             api_messages = []
             for msg in messages:
                 api_msg = msg.copy()
                 for internal_field in ("reasoning", "finish_reason"):
                     api_msg.pop(internal_field, None)
+                if _is_strict_api:
+                    self._sanitize_tool_calls_for_strict_api(api_msg)
                 api_messages.append(api_msg)
 
             effective_system = self._cached_system_prompt or ""
@@ -3425,6 +3456,12 @@ class AIAgent:
                 # Remove finish_reason - not accepted by strict APIs (e.g. Mistral)
                 if "finish_reason" in api_msg:
                     api_msg.pop("finish_reason")
+                # Strip Codex Responses API fields (call_id, response_item_id) for
+                # strict providers like Mistral that reject unknown fields with 422.
+                # Uses new dicts so the internal messages list retains the fields
+                # for Codex Responses compatibility.
+                if "api.mistral.ai" in self.base_url.lower():
+                    self._sanitize_tool_calls_for_strict_api(api_msg)
                 # Keep 'reasoning_details' - OpenRouter uses this for multi-turn reasoning context
                 # The signature field helps maintain reasoning continuity
                 api_messages.append(api_msg)