fix: route openai-codex through models.dev so GPT-5.5 exposes xhigh

- Remove openai-codex special case that called github_model_reasoning_efforts()
- Codex now falls through to _models_dev_reasoning_efforts() (full efforts)
- GitHub/Copilot still use the GitHub helper (caps at high)
- Added regression tests for both behaviors

api/config.py

@@ -2202,10 +2202,6 @@ def resolve_model_reasoning_efforts(
         if provider in {"copilot", "github-copilot"}:
             return github_model_reasoning_efforts(hinted_model)
 
-        if provider == "openai-codex":
-            bare = hinted_model.rsplit("/", 1)[-1]
-            return github_model_reasoning_efforts(bare)
-
         if provider == "lmstudio":
             probe_base = resolved_base_url or _get_provider_base_url(provider)
             opts = lmstudio_model_reasoning_options(hinted_model, probe_base)

tests/test_models_dev_reasoning.py

@@ -2,6 +2,8 @@
 
 import sys
 import types
+
+import pytest
 from types import SimpleNamespace
 
 
@@ -78,6 +80,35 @@ def test_models_dev_false_suppresses_prefix_heuristic(monkeypatch):
     ) == []
 
 
+def test_codex_gpt55_uses_models_dev_including_xhigh(monkeypatch):
+    _install_fake_models_dev(
+        monkeypatch,
+        lambda provider, model: SimpleNamespace(supports_reasoning=True),
+    )
+
+    import api.config as cfg
+
+    result = cfg.resolve_model_reasoning_efforts(
+        "gpt-5.5", provider_id="openai-codex"
+    )
+    assert result == list(cfg.VALID_REASONING_EFFORTS)
+    assert "xhigh" in result
+
+
+def test_copilot_gpt55_caps_at_high(monkeypatch):
+    import api.config as cfg
+
+    try:
+        from hermes_cli.models import github_model_reasoning_efforts
+    except ImportError:
+        pytest.skip("hermes_cli not available")
+
+    result = cfg.resolve_model_reasoning_efforts(
+        "gpt-5.5", provider_id="copilot"
+    )
+    assert "xhigh" not in result
+
+
 def test_get_reasoning_status_uses_config_default_model(monkeypatch, tmp_path):
     _install_fake_models_dev(
         monkeypatch,