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

agent/auxiliary_client.py

@@ -1223,6 +1223,8 @@ 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
 
     async_kwargs = {
         "api_key": sync_client.api_key,
@@ -1467,7 +1469,11 @@ def resolve_provider_client(
 
     # ── API-key providers from PROVIDER_REGISTRY ─────────────────────
     try:
-        from hermes_cli.auth import PROVIDER_REGISTRY, resolve_api_key_provider_credentials
+        from hermes_cli.auth import (
+            PROVIDER_REGISTRY,
+            resolve_api_key_provider_credentials,
+            resolve_external_process_provider_credentials,
+        )
     except ImportError:
         logger.debug("hermes_cli.auth not available for provider %s", provider)
         return None, None
@@ -1541,6 +1547,41 @@ def resolve_provider_client(
         return (_to_async_client(client, final_model) if async_mode
                 else (client, final_model))
 
+    if pconfig.auth_type == "external_process":
+        creds = resolve_external_process_provider_credentials(provider)
+        final_model = _normalize_resolved_model(model or _read_main_model(), provider)
+        if provider == "copilot-acp":
+            api_key = str(creds.get("api_key", "")).strip()
+            base_url = str(creds.get("base_url", "")).strip()
+            command = str(creds.get("command", "")).strip() or None
+            args = list(creds.get("args") or [])
+            if not final_model:
+                logger.warning(
+                    "resolve_provider_client: copilot-acp requested but no model "
+                    "was provided or configured"
+                )
+                return None, None
+            if not api_key or not base_url:
+                logger.warning(
+                    "resolve_provider_client: copilot-acp requested but external "
+                    "process credentials are incomplete"
+                )
+                return None, None
+            from agent.copilot_acp_client import CopilotACPClient
+
+            client = CopilotACPClient(
+                api_key=api_key,
+                base_url=base_url,
+                command=command,
+                args=args,
+            )
+            logger.debug("resolve_provider_client: %s (%s)", provider, final_model)
+            return (_to_async_client(client, final_model) if async_mode
+                    else (client, final_model))
+        logger.warning("resolve_provider_client: external-process provider %s not "
+                       "directly supported", provider)
+        return None, None
+
     elif pconfig.auth_type in ("oauth_device_code", "oauth_external"):
         # OAuth providers — route through their specific try functions
         if provider == "nous":

run_agent.py

@@ -705,13 +705,19 @@ class AIAgent:
         except Exception:
             pass
 
-        # GPT-5.x models require the Responses API path — they are rejected
+        # GPT-5.x models often require the Responses API path — they are
-        # on /v1/chat/completions by both OpenAI and OpenRouter.  Also
+        # rejected on /v1/chat/completions by both OpenAI and OpenRouter.
-        # auto-upgrade for direct OpenAI URLs (api.openai.com) since all
+        # Also auto-upgrade for direct OpenAI URLs (api.openai.com) since
         # newer tool-calling models prefer Responses there.
-        if self.api_mode == "chat_completions" and (
+        if (
-            self._is_direct_openai_url()
+            self.api_mode == "chat_completions"
-            or self._model_requires_responses_api(self.model)
+            and self.provider != "copilot-acp"
+            and not str(self.base_url or "").lower().startswith("acp://copilot")
+            and not str(self.base_url or "").lower().startswith("acp+tcp://")
+            and (
+                self._is_direct_openai_url()
+                or self._model_requires_responses_api(self.model)
+            )
         ):
             self.api_mode = "codex_responses"
 

tests/agent/test_auxiliary_client.py

@@ -944,6 +944,46 @@ model:
         }
 
 
+def test_resolve_provider_client_supports_copilot_acp_external_process():
+    fake_client = MagicMock()
+
+    with patch("agent.auxiliary_client._read_main_model", return_value="gpt-5.4-mini"), \
+         patch("agent.auxiliary_client.CodexAuxiliaryClient", MagicMock()), \
+         patch("agent.copilot_acp_client.CopilotACPClient", return_value=fake_client) as mock_acp, \
+         patch("hermes_cli.auth.resolve_external_process_provider_credentials", return_value={
+             "provider": "copilot-acp",
+             "api_key": "copilot-acp",
+             "base_url": "acp://copilot",
+             "command": "/usr/bin/copilot",
+             "args": ["--acp", "--stdio"],
+         }):
+        client, model = resolve_provider_client("copilot-acp")
+
+    assert client is fake_client
+    assert model == "gpt-5.4-mini"
+    assert mock_acp.call_args.kwargs["api_key"] == "copilot-acp"
+    assert mock_acp.call_args.kwargs["base_url"] == "acp://copilot"
+    assert mock_acp.call_args.kwargs["command"] == "/usr/bin/copilot"
+    assert mock_acp.call_args.kwargs["args"] == ["--acp", "--stdio"]
+
+
+def test_resolve_provider_client_copilot_acp_requires_explicit_or_configured_model():
+    with patch("agent.auxiliary_client._read_main_model", return_value=""), \
+         patch("agent.copilot_acp_client.CopilotACPClient") as mock_acp, \
+         patch("hermes_cli.auth.resolve_external_process_provider_credentials", return_value={
+             "provider": "copilot-acp",
+             "api_key": "copilot-acp",
+             "base_url": "acp://copilot",
+             "command": "/usr/bin/copilot",
+             "args": ["--acp", "--stdio"],
+         }):
+        client, model = resolve_provider_client("copilot-acp")
+
+    assert client is None
+    assert model is None
+    mock_acp.assert_not_called()
+
+
 class TestAuxiliaryMaxTokensParam:
     def test_codex_fallback_uses_max_tokens(self, monkeypatch):
         """Codex adapter translates max_tokens internally, so we return max_tokens."""

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(