diff --git a/nanobot/agent/loop.py b/nanobot/agent/loop.py
index adb797bd3..e7753df51 100644
--- a/nanobot/agent/loop.py
+++ b/nanobot/agent/loop.py
@@ -437,8 +437,7 @@ class AgentLoop:
         context_window_tokens = extra.pop("context_window_tokens", None) or resolved.context_window_tokens
         model_preset_snapshot_builder = extra.pop("model_preset_snapshot_builder", None)
         model_presets = dict(config.model_presets)
-        if "default" not in model_presets:
-            model_presets["default"] = resolved
+        model_presets["default"] = config.resolve_default_preset()
         return cls(
             bus=bus,
             provider=provider,
diff --git a/nanobot/config/schema.py b/nanobot/config/schema.py
index 43936597b..c2fceff22 100644
--- a/nanobot/config/schema.py
+++ b/nanobot/config/schema.py
@@ -282,17 +282,12 @@ class Config(BaseSettings):
     @model_validator(mode="after")
     def _validate_model_preset(self) -> "Config":
         name = self.agents.defaults.model_preset
-        if name and name not in self.model_presets:
+        if name and name != "default" and name not in self.model_presets:
             raise ValueError(f"model_preset {name!r} not found in model_presets")
         return self
 
-    def resolve_preset(self, name: str | None = None) -> ModelPresetConfig:
-        """Return effective model params: from active preset, or individual defaults."""
-        name = self.agents.defaults.model_preset if name is None else name
-        if name:
-            if name not in self.model_presets:
-                raise KeyError(f"model_preset {name!r} not found in model_presets")
-            return self.model_presets[name]
+    def resolve_default_preset(self) -> ModelPresetConfig:
+        """Return the implicit `default` preset from agents.defaults fields."""
         d = self.agents.defaults
         return ModelPresetConfig(
             model=d.model, provider=d.provider, max_tokens=d.max_tokens,
@@ -300,6 +295,15 @@ class Config(BaseSettings):
             temperature=d.temperature, reasoning_effort=d.reasoning_effort,
         )
 
+    def resolve_preset(self, name: str | None = None) -> ModelPresetConfig:
+        """Return effective model params from a named preset or the implicit default."""
+        name = self.agents.defaults.model_preset if name is None else name
+        if not name or name == "default":
+            return self.resolve_default_preset()
+        if name not in self.model_presets:
+            raise KeyError(f"model_preset {name!r} not found in model_presets")
+        return self.model_presets[name]
+
     @property
     def workspace_path(self) -> Path:
         """Get expanded workspace path."""
diff --git a/tests/agent/test_self_model_preset.py b/tests/agent/test_self_model_preset.py
index cbde23672..a996d75f2 100644
--- a/tests/agent/test_self_model_preset.py
+++ b/tests/agent/test_self_model_preset.py
@@ -284,7 +284,7 @@ def test_from_config_injects_default_preset(tmp_path) -> None:
     assert loop.model_presets["default"].model == "openai/gpt-4.1"
 
 
-def test_from_config_preserves_existing_default_preset(tmp_path) -> None:
+def test_from_config_reserves_default_for_agent_defaults(tmp_path) -> None:
     from unittest.mock import patch
 
     from nanobot.config.schema import Config
@@ -297,4 +297,4 @@ def test_from_config_preserves_existing_default_preset(tmp_path) -> None:
     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 == "custom-model"
+    assert loop.model_presets["default"].model == "openai/gpt-4.1"
diff --git a/tests/config/test_model_presets.py b/tests/config/test_model_presets.py
index b243d6e27..171f9834e 100644
--- a/tests/config/test_model_presets.py
+++ b/tests/config/test_model_presets.py
@@ -39,6 +39,24 @@ def test_resolve_preset_returns_active_preset() -> None:
     assert resolved.reasoning_effort == "low"
 
 
+def test_default_preset_is_agents_defaults_even_when_named_preset_is_active() -> None:
+    config = Config.model_validate({
+        "agents": {
+            "defaults": {
+                "model": "openai/gpt-4.1",
+                "provider": "openai",
+                "modelPreset": "fast",
+            }
+        },
+        "modelPresets": {
+            "fast": {"model": "openai/gpt-4.1-mini", "provider": "openai"},
+        },
+    })
+
+    assert config.resolve_preset().model == "openai/gpt-4.1-mini"
+    assert config.resolve_preset("default").model == "openai/gpt-4.1"
+
+
 def test_model_presets_accepts_camel_case_root_key() -> None:
     config = Config.model_validate({
         "modelPresets": {
@@ -79,6 +97,19 @@ def test_validator_rejects_unknown_preset() -> None:
         })
 
 
+def test_model_preset_accepts_explicit_default_name() -> None:
+    config = Config.model_validate({
+        "agents": {
+            "defaults": {
+                "model": "openai/gpt-4.1",
+                "modelPreset": "default",
+            }
+        }
+    })
+
+    assert config.resolve_preset().model == "openai/gpt-4.1"
+
+
 def test_resolve_preset_rejects_unknown_named_preset() -> None:
     import pytest
     with pytest.raises(KeyError, match="model_preset 'missing' not found"):