fix(copilot-acp): keep acp runtime off responses path

tests/run_agent/test_run_agent_codex_responses.py

@@ -243,6 +243,22 @@ def test_api_mode_respects_explicit_openrouter_provider_over_codex_url(monkeypat
     assert agent.provider == "openrouter"
 
 
+def test_copilot_acp_stays_on_chat_completions_for_gpt_5_models(monkeypatch):
+    _patch_agent_bootstrap(monkeypatch)
+    agent = run_agent.AIAgent(
+        model="gpt-5.4-mini",
+        base_url="acp://copilot",
+        provider="copilot-acp",
+        api_key="copilot-acp",
+        quiet_mode=True,
+        max_iterations=1,
+        skip_context_files=True,
+        skip_memory=True,
+    )
+    assert agent.provider == "copilot-acp"
+    assert agent.api_mode == "chat_completions"
+
+
 def test_build_api_kwargs_codex(monkeypatch):
     agent = _build_agent(monkeypatch)
     kwargs = agent._build_api_kwargs(