diff --git a/agent/auxiliary_client.py b/agent/auxiliary_client.py
index a04c347a9..49dea65f9 100644
--- a/agent/auxiliary_client.py
+++ b/agent/auxiliary_client.py
@@ -1223,8 +1223,12 @@ def _to_async_client(sync_client, model: str):
         return AsyncCodexAuxiliaryClient(sync_client), model
     if isinstance(sync_client, AnthropicAuxiliaryClient):
         return AsyncAnthropicAuxiliaryClient(sync_client), model
-    if sync_client.__class__.__name__ == "CopilotACPClient":
-        return sync_client, model
+    try:
+        from agent.copilot_acp_client import CopilotACPClient
+        if isinstance(sync_client, CopilotACPClient):
+            return sync_client, model
+    except ImportError:
+        pass
 
     async_kwargs = {
         "api_key": sync_client.api_key,
diff --git a/run_agent.py b/run_agent.py
index 3069a190a..5005153b3 100644
--- a/run_agent.py
+++ b/run_agent.py
@@ -705,10 +705,12 @@ class AIAgent:
         except Exception:
             pass
 
-        # GPT-5.x models often require the Responses API path — they are
-        # rejected on /v1/chat/completions by both OpenAI and OpenRouter.
-        # Also auto-upgrade for direct OpenAI URLs (api.openai.com) since
+        # GPT-5.x models require the Responses API path — they are rejected
+        # on /v1/chat/completions by both OpenAI and OpenRouter.  Also
+        # auto-upgrade for direct OpenAI URLs (api.openai.com) since all
         # newer tool-calling models prefer Responses there.
+        # ACP runtimes are excluded: CopilotACPClient handles its own
+        # routing and does not implement the Responses API surface.
         if (
             self.api_mode == "chat_completions"
             and self.provider != "copilot-acp"