diff --git a/tests/test_tui_gateway_server.py b/tests/test_tui_gateway_server.py
index 03d44c057a..a76cc17911 100644
--- a/tests/test_tui_gateway_server.py
+++ b/tests/test_tui_gateway_server.py
@@ -747,6 +747,40 @@ def test_config_set_fast_status_is_non_mutating(monkeypatch):
         server._sessions.pop("sid", None)
 
 
+def test_config_set_fast_rejects_unsupported_model(monkeypatch):
+    writes = []
+    agent = types.SimpleNamespace(
+        model="unsupported-model",
+        request_overrides={},
+        service_tier=None,
+    )
+    server._sessions["sid"] = _session(agent=agent)
+
+    monkeypatch.setattr(
+        server, "_write_config_key", lambda path, value: writes.append((path, value))
+    )
+    monkeypatch.setattr(
+        "hermes_cli.models.resolve_fast_mode_overrides",
+        lambda _model_id: None,
+    )
+
+    try:
+        resp = server.handle_request(
+            {
+                "id": "1",
+                "method": "config.set",
+                "params": {"session_id": "sid", "key": "fast", "value": "fast"},
+            }
+        )
+        assert resp["error"]["code"] == 4002
+        assert "not available" in resp["error"]["message"]
+        assert agent.service_tier is None
+        assert agent.request_overrides == {}
+        assert writes == []
+    finally:
+        server._sessions.pop("sid", None)
+
+
 def test_config_busy_get_and_set(monkeypatch):
     writes = []
 
diff --git a/tui_gateway/server.py b/tui_gateway/server.py
index a48058674b..604a8d5e4b 100644
--- a/tui_gateway/server.py
+++ b/tui_gateway/server.py
@@ -2880,8 +2880,9 @@ def _(rid, params: dict) -> dict:
 
     if key == "fast":
         raw = str(value or "").strip().lower()
-        if session and session.get("agent") is not None:
-            current_fast = getattr(session["agent"], "service_tier", None) == "priority"
+        agent = session.get("agent") if session else None
+        if agent is not None:
+            current_fast = getattr(agent, "service_tier", None) == "priority"
         else:
             current_fast = _load_service_tier() == "priority"
 
@@ -2900,19 +2901,27 @@ def _(rid, params: dict) -> dict:
         else:
             return _err(rid, 4002, f"unknown fast mode: {value}")
 
+        overrides = None
+        if nv == "fast":
+            from hermes_cli.models import resolve_fast_mode_overrides
+
+            target_model = getattr(agent, "model", None) if agent is not None else _resolve_model()
+            overrides = resolve_fast_mode_overrides(target_model)
+            if not overrides:
+                return _err(
+                    rid,
+                    4002,
+                    "fast mode is not available for this model",
+                )
+
         _write_config_key("agent.service_tier", nv)
-        if session and session.get("agent") is not None:
-            agent = session["agent"]
+        if agent is not None:
             agent.service_tier = "priority" if nv == "fast" else None
             current_overrides = dict(getattr(agent, "request_overrides", {}) or {})
             current_overrides.pop("service_tier", None)
             current_overrides.pop("speed", None)
             if nv == "fast":
-                from hermes_cli.models import resolve_fast_mode_overrides
-
-                current_overrides.update(
-                    resolve_fast_mode_overrides(getattr(agent, "model", None)) or {}
-                )
+                current_overrides.update(overrides)
             agent.request_overrides = current_overrides
             _emit(
                 "session.info",