fix(gateway): carry request_overrides through /model session overrides

Follow-up to the previous commit (which fixed the default/fallback
provider path). A mid-session `/model` switch stores a per-session
override bundle in `_session_model_overrides` that omitted
`request_overrides`, and the two consumers
(`_resolve_session_agent_runtime` fast path and
`_apply_session_model_override`) only copied
provider/api_key/base_url/api_mode. So switching *to* a custom provider
via `/model` did not apply its `extra_body`.

- `ModelSwitchResult` gains a `request_overrides` field, derived for the
  switched provider via `_get_named_custom_provider` /
  `_custom_provider_request_overrides` (the same overrides
  `resolve_runtime_provider` surfaces for the default path).
- Both `/model` override-storage sites in slash_commands.py persist it.
- Both consumers apply it; `_apply_session_model_override` also clears a
  stale value when switching to a provider that has none.

Extends tests/gateway/test_turn_request_overrides.py (3 new cases).

gateway/run.py

@@ -8348,6 +8348,7 @@ class GatewayRunner(GatewayAuthorizationMixin, GatewayKanbanWatchersMixin, Gatew
                 "api_mode": override.get("api_mode"),
                 "max_tokens": override.get("max_tokens"),
                 "credential_pool": override.get("credential_pool"),
+                "request_overrides": override.get("request_overrides"),
             }
             if override_runtime.get("api_key"):
                 if override_runtime.get("credential_pool") is None:
@@ -27501,12 +27502,16 @@ class GatewayRunner(GatewayAuthorizationMixin, GatewayKanbanWatchersMixin, Gatew
             val = override.get(key)
             if val is not None:
                 runtime_kwargs[key] = val
-        override_request_overrides = override.get("request_overrides")
-        if isinstance(override_request_overrides, dict):
-            runtime_kwargs["request_overrides"] = _deep_merge_request_overrides(
-                runtime_kwargs.get("request_overrides"),
-                override_request_overrides,
-            )
+        # request_overrides reflects the switched-to provider; apply whenever
+        # the override recorded it (even as None) so switching to a provider
+        # without configured overrides clears a stale value left by the
+        # default provider's runtime resolution.
+        if "request_overrides" in override:
+            override_request_overrides = override.get("request_overrides")
+            if isinstance(override_request_overrides, dict) and override_request_overrides:
+                runtime_kwargs["request_overrides"] = dict(override_request_overrides)
+            else:
+                runtime_kwargs["request_overrides"] = override_request_overrides
         if (
             runtime_kwargs.get("api_key")
             and runtime_kwargs.get("credential_pool") is None

hermes_cli/model_switch.py

@@ -2185,6 +2185,22 @@ def switch_model(
     if hermes_warn:
         warnings.append(hermes_warn)
 
+    # Carry the switched provider's request_overrides (e.g. a custom_providers
+    # ``extra_body`` such as chat_template_kwargs) so a ``/model`` switch to a
+    # custom provider applies it on the gateway, matching the default-provider
+    # path. resolve_runtime_provider surfaces these for named custom providers.
+    request_overrides = None
+    try:
+        from hermes_cli.runtime_provider import (
+            _get_named_custom_provider,
+            _custom_provider_request_overrides,
+        )
+        _cp_for_ro = _get_named_custom_provider(target_provider)
+        if _cp_for_ro:
+            request_overrides = _custom_provider_request_overrides(_cp_for_ro) or None
+    except Exception:
+        request_overrides = None
+
     # --- Build result ---
     return ModelSwitchResult(
         success=True,

tests/gateway/test_turn_request_overrides.py

@@ -89,3 +89,52 @@ def test_resolve_runtime_agent_kwargs_carries_request_overrides(monkeypatch):
     )
     rk = gateway_run._resolve_runtime_agent_kwargs()
     assert rk["request_overrides"] == PROVIDER_OVERRIDES
+
+
+# --- /model session-override follow-up: request_overrides must survive a switch ---
+
+def test_session_override_applies_request_overrides():
+    """A /model switch to a custom provider carries its extra_body into runtime."""
+    runner = object.__new__(GatewayRunner)
+    runner._session_model_overrides = {
+        "sess1": {
+            "model": "thinkmodel",
+            "provider": "custom",
+            "api_key": "k",
+            "base_url": "http://10.0.0.1:8000/v1",
+            "api_mode": "chat_completions",
+            "request_overrides": PROVIDER_OVERRIDES,
+        }
+    }
+    rk = _runtime_kwargs()  # default resolution carried no overrides
+    model, out = runner._apply_session_model_override("sess1", "oldmodel", rk)
+    assert model == "thinkmodel"
+    assert out["request_overrides"] == PROVIDER_OVERRIDES
+
+
+def test_session_override_clears_stale_request_overrides():
+    """Switching to a provider with no overrides clears a stale value."""
+    runner = object.__new__(GatewayRunner)
+    runner._session_model_overrides = {
+        "sess1": {
+            "model": "plain",
+            "provider": "openrouter",
+            "api_key": "k",
+            "base_url": "https://openrouter.ai/api/v1",
+            "api_mode": "chat_completions",
+            "request_overrides": None,
+        }
+    }
+    rk = _runtime_kwargs(request_overrides=PROVIDER_OVERRIDES)  # stale, from default
+    _, out = runner._apply_session_model_override("sess1", "old", rk)
+    assert out.get("request_overrides") is None
+
+
+def test_session_override_absent_is_noop():
+    """No override for the session leaves runtime_kwargs untouched."""
+    runner = object.__new__(GatewayRunner)
+    runner._session_model_overrides = {}
+    rk = _runtime_kwargs(request_overrides=PROVIDER_OVERRIDES)
+    model, out = runner._apply_session_model_override("nope", "keepme", rk)
+    assert model == "keepme"
+    assert out["request_overrides"] == PROVIDER_OVERRIDES