fix(agent): separate preset snapshots from config reload

Co-authored-by: Cursor <cursoragent@cursor.com>

nanobot/agent/loop.py

@@ -293,6 +293,7 @@ class AgentLoop:
         provider_signature: tuple[object, ...] | None = None,
         model_presets: dict[str, ModelPresetConfig] | None = None,
         model_preset: str | None = None,
+        preset_snapshot_loader: Callable[[str], ProviderSnapshot] | None = None,
     ):
         from nanobot.config.schema import ToolsConfig
 
@@ -302,6 +303,7 @@ class AgentLoop:
         self.channels_config = channels_config
         self.provider = provider
         self._provider_snapshot_loader = provider_snapshot_loader
+        self._preset_snapshot_loader = preset_snapshot_loader
         self._provider_signature = provider_signature
         self._default_selection_signature = provider_signature[:2] if provider_signature else None
         self.workspace = workspace
@@ -431,7 +433,16 @@ class AgentLoop:
         model = extra.pop("model", None) or resolved.model
         context_window_tokens = extra.pop("context_window_tokens", None) or resolved.context_window_tokens
         provider_snapshot_loader = extra.pop("provider_snapshot_loader", None)
+        preset_snapshot_loader = extra.pop("preset_snapshot_loader", None)
         model_presets = {**config.model_presets, "default": config.resolve_default_preset()}
+        if preset_snapshot_loader is None:
+            if provider_snapshot_loader is not None:
+                preset_snapshot_loader = lambda name: provider_snapshot_loader(preset_name=name)
+            else:
+                preset_snapshot_loader = lambda name: build_provider_snapshot(
+                    config,
+                    preset_name=name,
+                )
         return cls(
             bus=bus,
             provider=provider,
@@ -455,9 +466,8 @@ class AgentLoop:
             tools_config=config.tools,
             model_presets=model_presets,
             model_preset=defaults.model_preset,
-            provider_snapshot_loader=provider_snapshot_loader or (
+            provider_snapshot_loader=provider_snapshot_loader,
-                lambda preset_name=None: build_provider_snapshot(config, preset_name=preset_name)
+            preset_snapshot_loader=preset_snapshot_loader,
-            ),
             **extra,
         )
 
@@ -468,8 +478,14 @@ class AgentLoop:
     def _publish_runtime_model_updated(self, model_preset: str | None = None) -> None:
         """Notify WebUI clients that the effective runtime model changed."""
         self.bus.outbound.put_nowait(OutboundMessage(
-            channel="websocket", chat_id="*", content="",
+            channel="websocket",
-            metadata={"_runtime_model_updated": True, "model": self.model, "model_preset": model_preset if model_preset is not None else self.model_preset},
+            chat_id="*",
+            content="",
+            metadata={
+                "_runtime_model_updated": True,
+                "model": self.model,
+                "model_preset": model_preset if model_preset is not None else self.model_preset,
+            },
         ))
 
     def _apply_provider_snapshot(
@@ -530,8 +546,8 @@ class AgentLoop:
 
     def _build_model_preset_snapshot(self, name: str) -> ProviderSnapshot:
         preset = self.model_presets[name]
-        if self._provider_snapshot_loader is not None:
+        if self._preset_snapshot_loader is not None:
-            return self._provider_snapshot_loader(preset_name=name)
+            return self._preset_snapshot_loader(name)
         self.provider.generation = preset.to_generation_settings()
         return ProviderSnapshot(
             provider=self.provider,

nanobot/command/builtin.py

@@ -213,6 +213,10 @@ def _active_model_preset_name(loop) -> str:
     return loop.model_preset or "default"
 
 
+def _command_error_message(exc: Exception) -> str:
+    return str(exc.args[0]) if isinstance(exc, KeyError) and exc.args else str(exc)
+
+
 def _model_command_status(loop) -> str:
     names = _model_preset_names(loop)
     active = _active_model_preset_name(loop)
@@ -256,7 +260,7 @@ async def cmd_model(ctx: CommandContext) -> OutboundMessage:
             channel=ctx.msg.channel,
             chat_id=ctx.msg.chat_id,
             content=(
-                f"Could not switch model preset: {exc}\n\n"
+                f"Could not switch model preset: {_command_error_message(exc)}\n\n"
                 f"Available presets: {_format_preset_names(names)}"
             ),
             metadata=metadata,

nanobot/config/schema.py

@@ -281,6 +281,8 @@ class Config(BaseSettings):
 
     @model_validator(mode="after")
     def _validate_model_preset(self) -> "Config":
+        if "default" in self.model_presets:
+            raise ValueError("model_preset name 'default' is reserved for agents.defaults")
         name = self.agents.defaults.model_preset
         if name and name != "default" and name not in self.model_presets:
             raise ValueError(f"model_preset {name!r} not found in model_presets")

tests/agent/test_self_model_preset.py

@@ -104,11 +104,11 @@ def test_model_preset_setter_replaces_provider_from_snapshot(tmp_path) -> None:
         model="base-model",
         context_window_tokens=1000,
         model_presets={"deep": preset},
-        provider_snapshot_loader=lambda preset_name=None: ProviderSnapshot(
+        preset_snapshot_loader=lambda name: ProviderSnapshot(
             provider=new_provider,
             model=preset.model,
             context_window_tokens=preset.context_window_tokens,
-            signature=(preset_name, preset.model),
+            signature=(name, preset.model),
         ),
     )
 
@@ -135,7 +135,7 @@ def test_model_preset_setter_failure_leaves_old_state(tmp_path) -> None:
         model="base-model",
         context_window_tokens=1000,
         model_presets={"fast": preset},
-        provider_snapshot_loader=lambda preset_name=None: (_ for _ in ()).throw(
+        preset_snapshot_loader=lambda _name: (_ for _ in ()).throw(
             RuntimeError("provider unavailable")
         ),
     )
@@ -175,9 +175,8 @@ def test_active_model_preset_survives_unchanged_config_refresh(tmp_path) -> None
         context_window_tokens=1000,
         provider_signature=default_snapshot.signature,
         model_presets={"fast": ModelPresetConfig(model="openai/gpt-4.1")},
-        provider_snapshot_loader=lambda preset_name=None: (
+        provider_snapshot_loader=lambda: default_snapshot,
-            fast_snapshot if preset_name == "fast" else default_snapshot
+        preset_snapshot_loader=lambda _name: fast_snapshot,
-        ),
     )
 
     loop.set_model_preset("fast")
@@ -210,11 +209,10 @@ def test_config_model_refresh_clears_active_model_preset(tmp_path) -> None:
         workspace=tmp_path,
         model="base-model",
         context_window_tokens=1000,
-        provider_snapshot_loader=lambda preset_name=None: (
+        provider_snapshot_loader=lambda: webui_snapshot,
-            fast_snapshot if preset_name == "fast" else webui_snapshot
-        ),
         provider_signature=("base-model", "auto", "openai", "sk-old"),
         model_presets={"fast": ModelPresetConfig(model="openai/gpt-4.1")},
+        preset_snapshot_loader=lambda _name: fast_snapshot,
     )
 
     loop.set_model_preset("fast")
@@ -286,17 +284,16 @@ def test_from_config_injects_default_preset(tmp_path) -> None:
     assert loop.model_presets["default"].model == "openai/gpt-4.1"
 
 
-def test_from_config_reserves_default_for_agent_defaults(tmp_path) -> None:
+def test_from_config_static_preset_loader_does_not_enable_hot_reload(tmp_path) -> None:
     from unittest.mock import patch
 
     from nanobot.config.schema import Config
     config = Config.model_validate({
         "agents": {"defaults": {"model": "openai/gpt-4.1", "workspace": str(tmp_path)}},
-        "model_presets": {
+        "model_presets": {"fast": {"model": "openai/gpt-4.1-mini"}},
-            "default": {"model": "custom-model"}
-        },
     })
     fake_provider = _provider("openai/gpt-4.1")
     with patch("nanobot.providers.factory.make_provider", return_value=fake_provider):
         loop = AgentLoop.from_config(config)
-    assert loop.model_presets["default"].model == "openai/gpt-4.1"
+    assert loop._provider_snapshot_loader is None
+    assert loop._preset_snapshot_loader is not None

tests/command/test_model_command.py

@@ -102,6 +102,7 @@ async def test_model_command_unknown_preset_keeps_old_state(tmp_path) -> None:
     out = await cmd_model(_ctx(loop, "/model missing", args="missing"))
 
     assert "Could not switch model preset" in out.content
+    assert "\"model_preset" not in out.content
     assert "Available presets: `default`, `fast`" in out.content
     assert loop.model_preset is None
     assert loop.model == "base-model"

tests/config/test_model_presets.py

@@ -110,6 +110,17 @@ def test_model_preset_accepts_explicit_default_name() -> None:
     assert config.resolve_preset().model == "openai/gpt-4.1"
 
 
+def test_model_presets_rejects_reserved_default_name() -> None:
+    import pytest
+
+    with pytest.raises(ValueError, match="model_preset name 'default' is reserved"):
+        Config.model_validate({
+            "modelPresets": {
+                "default": {"model": "custom-model"},
+            },
+        })
+
+
 def test_resolve_preset_rejects_unknown_named_preset() -> None:
     import pytest
     with pytest.raises(KeyError, match="model_preset 'missing' not found"):