fix: correct Copilot API mode selection to match opencode

The previous copilot_model_api_mode() checked the catalog's
supported_endpoints first and picked /chat/completions when a model
supported both endpoints. This is wrong — GPT-5+ models should use
the Responses API even when the catalog lists both.

Replicate opencode's shouldUseCopilotResponsesApi() logic:
- GPT-5+ models (gpt-5.4, gpt-5.3-codex, etc.) → Responses API
- gpt-5-mini → Chat Completions (explicit exception)
- Everything else (gpt-4o, claude, gemini, etc.) → Chat Completions
- Model ID pattern is the primary signal, catalog is secondary

The catalog fallback now only matters for non-GPT-5 models that might
exclusively support /v1/messages (e.g. Claude via Copilot).

Models are auto-detected from the live catalog at
api.githubcopilot.com/models — no hardcoded list required for
supported models, only a static fallback for when the API is
unreachable.

tests/hermes_cli/test_copilot_auth.py

@@ -160,6 +160,36 @@ class TestCopilotDefaultHeaders:
         assert "x-initiator" in headers
 
 
+class TestApiModeSelection:
+    """API mode selection matching opencode's shouldUseCopilotResponsesApi."""
+
+    def test_gpt5_uses_responses(self):
+        from hermes_cli.models import _should_use_copilot_responses_api
+        assert _should_use_copilot_responses_api("gpt-5.4") is True
+        assert _should_use_copilot_responses_api("gpt-5.4-mini") is True
+        assert _should_use_copilot_responses_api("gpt-5.3-codex") is True
+        assert _should_use_copilot_responses_api("gpt-5.2-codex") is True
+        assert _should_use_copilot_responses_api("gpt-5.2") is True
+        assert _should_use_copilot_responses_api("gpt-5.1-codex-max") is True
+
+    def test_gpt5_mini_excluded(self):
+        from hermes_cli.models import _should_use_copilot_responses_api
+        assert _should_use_copilot_responses_api("gpt-5-mini") is False
+
+    def test_gpt4_uses_chat(self):
+        from hermes_cli.models import _should_use_copilot_responses_api
+        assert _should_use_copilot_responses_api("gpt-4.1") is False
+        assert _should_use_copilot_responses_api("gpt-4o") is False
+        assert _should_use_copilot_responses_api("gpt-4o-mini") is False
+
+    def test_non_gpt_uses_chat(self):
+        from hermes_cli.models import _should_use_copilot_responses_api
+        assert _should_use_copilot_responses_api("claude-sonnet-4.6") is False
+        assert _should_use_copilot_responses_api("claude-opus-4.6") is False
+        assert _should_use_copilot_responses_api("gemini-2.5-pro") is False
+        assert _should_use_copilot_responses_api("grok-code-fast-1") is False
+
+
 class TestEnvVarOrder:
     """PROVIDER_REGISTRY has correct env var order."""
 

tests/hermes_cli/test_model_validation.py

@@ -276,7 +276,37 @@ class TestCopilotNormalization:
         catalog = [{"id": "gpt-4.1"}, {"id": "gpt-5.4"}]
         assert normalize_copilot_model_id("openai/gpt-4.1-mini", catalog=catalog) == "gpt-4.1"
 
-    def test_copilot_api_mode_prefers_responses(self):
+    def test_copilot_api_mode_gpt5_uses_responses(self):
+        """GPT-5+ models should use Responses API (matching opencode)."""
+        assert copilot_model_api_mode("gpt-5.4") == "codex_responses"
+        assert copilot_model_api_mode("gpt-5.4-mini") == "codex_responses"
+        assert copilot_model_api_mode("gpt-5.3-codex") == "codex_responses"
+        assert copilot_model_api_mode("gpt-5.2-codex") == "codex_responses"
+        assert copilot_model_api_mode("gpt-5.2") == "codex_responses"
+
+    def test_copilot_api_mode_gpt5_mini_uses_chat(self):
+        """gpt-5-mini is the exception — uses Chat Completions."""
+        assert copilot_model_api_mode("gpt-5-mini") == "chat_completions"
+
+    def test_copilot_api_mode_non_gpt5_uses_chat(self):
+        """Non-GPT-5 models use Chat Completions."""
+        assert copilot_model_api_mode("gpt-4.1") == "chat_completions"
+        assert copilot_model_api_mode("gpt-4o") == "chat_completions"
+        assert copilot_model_api_mode("gpt-4o-mini") == "chat_completions"
+        assert copilot_model_api_mode("claude-sonnet-4.6") == "chat_completions"
+        assert copilot_model_api_mode("claude-opus-4.6") == "chat_completions"
+        assert copilot_model_api_mode("gemini-2.5-pro") == "chat_completions"
+
+    def test_copilot_api_mode_with_catalog_both_endpoints(self):
+        """When catalog shows both endpoints, model ID pattern wins."""
+        catalog = [{
+            "id": "gpt-5.4",
+            "supported_endpoints": ["/chat/completions", "/responses"],
+        }]
+        # GPT-5.4 should use responses even though chat/completions is listed
+        assert copilot_model_api_mode("gpt-5.4", catalog=catalog) == "codex_responses"
+
+    def test_copilot_api_mode_with_catalog_only_responses(self):
         catalog = [{
             "id": "gpt-5.4",
             "supported_endpoints": ["/responses"],