diff --git a/src/batdetect2/api_v2.py b/src/batdetect2/api_v2.py
index 53230c7..ec068b1 100644
--- a/src/batdetect2/api_v2.py
+++ b/src/batdetect2/api_v2.py
@@ -19,7 +19,8 @@ if TYPE_CHECKING:
         LoggerConfig,
         LoggingCallback,
     )
-    from batdetect2.models import Model, ModelConfig
+    from batdetect2.models import ModelConfig
+    from batdetect2.models.types import ModelProtocol
     from batdetect2.outputs import (
         OutputFormatConfig,
         OutputFormatterProtocol,
@@ -88,7 +89,7 @@ class BatDetect2API:
         evaluator: EvaluatorProtocol,
         formatter: OutputFormatterProtocol,
         output_transform: OutputTransformProtocol,
-        model: Model,
+        model: ModelProtocol,
     ):
         """Create a fully configured API instance.
 
@@ -128,7 +129,7 @@ class BatDetect2API:
             Default formatter used to save predictions.
         output_transform : OutputTransformProtocol
             Transform that converts model outputs into detections.
-        model : Model
+        model : ModelProtocol
             Model instance.
         """
         self.model_config = model_config
@@ -1177,5 +1178,5 @@ class BatDetect2API:
                 parameter.requires_grad = True
 
         if trainable in {"heads", "bbox_head"}:
-            for parameter in detector.bbox_head.parameters():
+            for parameter in detector.size_head.parameters():
                 parameter.requires_grad = True
diff --git a/src/batdetect2/cli/finetune.py b/src/batdetect2/cli/finetune.py
index 33b1c8b..f0f4791 100644
--- a/src/batdetect2/cli/finetune.py
+++ b/src/batdetect2/cli/finetune.py
@@ -47,24 +47,24 @@ __all__ = ["finetune_command"]
 @click.option(
     "--training-config",
     type=click.Path(exists=True),
-    help="Path to a training config file.",
+    help="Path to training config file.",
 )
 @click.option(
     "--audio-config",
     type=click.Path(exists=True),
-    help="Path to an audio config file.",
+    help="Path to audio config file.",
 )
 @click.option(
     "--logging-config",
     type=click.Path(exists=True),
-    help="Path to a logging config file.",
+    help="Path to logging config file.",
 )
 @click.option(
     "--trainable",
     type=click.Choice(["all", "heads", "classifier_head", "bbox_head"]),
     default="heads",
     show_default=True,
-    help="Which model parameters stay trainable during fine-tuning.",
+    help="Which model parameters remain trainable during fine-tuning.",
 )
 @click.option(
     "--ckpt-dir",
@@ -127,11 +127,7 @@ def finetune_command(
     experiment_name: str | None = None,
     run_name: str | None = None,
 ):
-    """Fine-tune a checkpoint on a new target definition.
-
-    Use this command when you want to adapt an existing model to a new class
-    list or ROI mapping.
-    """
+    """Fine-tune a BatDetect2 checkpoint on a new target definition."""
     from batdetect2.api_v2 import BatDetect2API
     from batdetect2.audio import AudioConfig
     from batdetect2.data import load_dataset, load_dataset_config
diff --git a/src/batdetect2/models/__init__.py b/src/batdetect2/models/__init__.py
index cc3ab69..ee96d93 100644
--- a/src/batdetect2/models/__init__.py
+++ b/src/batdetect2/models/__init__.py
@@ -62,7 +62,7 @@ from batdetect2.models.encoder import (
     build_encoder,
 )
 from batdetect2.models.heads import BBoxHead, ClassifierHead, DetectorHead
-from batdetect2.models.types import DetectionModel
+from batdetect2.models.types import DetectorProtocol, ModelProtocol
 from batdetect2.postprocess.config import PostprocessConfig
 from batdetect2.postprocess.types import (
     ClipDetectionsTensor,
@@ -149,7 +149,7 @@ class Model(torch.nn.Module):
 
     Attributes
     ----------
-    detector : DetectionModel
+    detector : DetectorProtocol
         The neural network that processes spectrograms and produces raw
         detection, classification, and bounding-box outputs.
     preprocessor : PreprocessorProtocol
@@ -164,7 +164,7 @@ class Model(torch.nn.Module):
         Size-dimension names corresponding to the model size outputs.
     """
 
-    detector: DetectionModel
+    detector: DetectorProtocol
     preprocessor: PreprocessorProtocol
     postprocessor: PostprocessorProtocol
     class_names: list[str]
@@ -173,7 +173,7 @@ class Model(torch.nn.Module):
 
     def __init__(
         self,
-        detector: DetectionModel,
+        detector: DetectorProtocol,
         preprocessor: PreprocessorProtocol,
         postprocessor: PostprocessorProtocol,
         class_names: list[str],
@@ -224,7 +224,7 @@ def build_model(
     dimension_names: list[str] | None = None,
     preprocessor: PreprocessorProtocol | None = None,
     postprocessor: PostprocessorProtocol | None = None,
-) -> Model:
+) -> ModelProtocol:
     """Build a complete, ready-to-use BatDetect2 model.
 
     Assembles a ``Model`` instance from a ``ModelConfig`` and optional
@@ -256,7 +256,7 @@ def build_model(
 
     Returns
     -------
-    Model
+    ModelProtocol
         A fully assembled ``Model`` instance ready for inference or
         training.
     """
@@ -285,8 +285,8 @@ def build_model(
         config=config.postprocess,
     )
     detector = build_detector(
-        num_classes=len(class_names),
-        num_sizes=len(dimension_names),
+        class_names=class_names,
+        dimension_names=dimension_names,
         config=config.architecture,
     )
     return Model(
@@ -300,14 +300,14 @@ def build_model(
 
 
 def build_model_with_new_targets(
-    model: Model,
+    model: ModelProtocol,
     targets: TargetProtocol,
     roi_mapper: ROIMapperProtocol,
-) -> Model:
+) -> ModelProtocol:
     """Build a new model with a different target set."""
     detector = build_detector(
-        num_classes=len(targets.class_names),
-        num_sizes=len(roi_mapper.dimension_names),
+        class_names=targets.class_names,
+        dimension_names=roi_mapper.dimension_names,
         backbone=model.detector.backbone,
     )
 
diff --git a/src/batdetect2/models/checkpoints/batdetect2_uk_same.ckpt b/src/batdetect2/models/checkpoints/batdetect2_uk_same.ckpt
index 49b64a4..b849167 100644
Binary files a/src/batdetect2/models/checkpoints/batdetect2_uk_same.ckpt and b/src/batdetect2/models/checkpoints/batdetect2_uk_same.ckpt differ
diff --git a/src/batdetect2/models/detectors.py b/src/batdetect2/models/detectors.py
index a3894ce..586beca 100644
--- a/src/batdetect2/models/detectors.py
+++ b/src/batdetect2/models/detectors.py
@@ -6,8 +6,8 @@ bounding-box size regression.
 
 Components
 ----------
-- ``Detector`` – the ``torch.nn.Module`` that wires together a backbone
-  (``BackboneModel``) with a ``ClassifierHead`` and a ``BBoxHead`` to
+- ``Detector`` - the ``torch.nn.Module`` that wires together a backbone
+  (``BackboneProtocol``) with a ``ClassifierHead`` and a ``BBoxHead`` to
   produce a ``ModelOutput`` tuple from an input spectrogram.
 - ``build_detector`` – factory function that builds a ready-to-use
   ``Detector`` from a backbone configuration and a target class count.
@@ -18,15 +18,16 @@ preprocessing and output postprocessing are handled by
 """
 
 import torch
-from loguru import logger
 
-from batdetect2.models.backbones import (
-    BackboneConfig,
-    UNetBackboneConfig,
-    build_backbone,
-)
+from batdetect2.models.backbones import BackboneConfig, build_backbone
 from batdetect2.models.heads import BBoxHead, ClassifierHead
-from batdetect2.models.types import BackboneModel, DetectionModel, ModelOutput
+from batdetect2.models.types import (
+    BackboneProtocol,
+    ClassifierHeadProtocol,
+    DetectorProtocol,
+    ModelOutput,
+    SizeHeadProtocol,
+)
 
 __all__ = [
     "Detector",
@@ -34,7 +35,7 @@ __all__ = [
 ]
 
 
-class Detector(DetectionModel):
+class Detector(torch.nn.Module):
     """Complete BatDetect2 detection and classification model.
 
     Combines a backbone feature extractor with two prediction heads:
@@ -51,7 +52,7 @@ class Detector(DetectionModel):
 
     Attributes
     ----------
-    backbone : BackboneModel
+    backbone : BackboneProtocol
         The feature extraction backbone.
     num_classes : int
         Number of target classes (inferred from the classifier head).
@@ -61,13 +62,13 @@ class Detector(DetectionModel):
         Produces duration and bandwidth predictions from backbone features.
     """
 
-    backbone: BackboneModel
+    backbone: BackboneProtocol
 
     def __init__(
         self,
-        backbone: BackboneModel,
-        classifier_head: ClassifierHead,
-        bbox_head: BBoxHead,
+        backbone: BackboneProtocol,
+        classifier_head: ClassifierHeadProtocol,
+        size_head: SizeHeadProtocol,
     ):
         """Initialise the Detector model.
 
@@ -76,7 +77,7 @@ class Detector(DetectionModel):
 
         Parameters
         ----------
-        backbone : BackboneModel
+        backbone : BackboneProtocol
             An initialised backbone module (e.g. built by
             ``build_backbone``).
         classifier_head : ClassifierHead
@@ -90,7 +91,7 @@ class Detector(DetectionModel):
         self.backbone = backbone
         self.num_classes = classifier_head.num_classes
         self.classifier_head = classifier_head
-        self.bbox_head = bbox_head
+        self.size_head = size_head
 
     def forward(self, spec: torch.Tensor) -> ModelOutput:
         """Run the complete detection model on an input spectrogram.
@@ -125,7 +126,7 @@ class Detector(DetectionModel):
         features = self.backbone(spec)
         classification = self.classifier_head(features)
         detection = classification.sum(dim=1, keepdim=True)
-        size_preds = self.bbox_head(features)
+        size_preds = self.size_head(features)
         return ModelOutput(
             detection_probs=detection,
             size_preds=size_preds,
@@ -135,11 +136,11 @@ class Detector(DetectionModel):
 
 
 def build_detector(
-    num_classes: int,
-    num_sizes: int = 2,
+    class_names: list[str],
+    dimension_names: list[str],
     config: BackboneConfig | None = None,
-    backbone: BackboneModel | None = None,
-) -> DetectionModel:
+    backbone: BackboneProtocol | None = None,
+) -> DetectorProtocol:
     """Build a complete BatDetect2 detection model.
 
     Constructs a backbone from ``config``, attaches a ``ClassifierHead``
@@ -158,7 +159,7 @@ def build_detector(
 
     Returns
     -------
-    DetectionModel
+    DetectorProtocol
         An initialised ``Detector`` instance ready for training or
         inference.
 
@@ -168,24 +169,18 @@ def build_detector(
         If ``num_classes`` is not positive, or if the backbone
         configuration is invalid.
     """
-    if backbone is None:
-        config = config or UNetBackboneConfig()
-        logger.opt(lazy=True).debug(
-            "Building model with config: \n{}",
-            lambda: config.to_yaml_string(),  # type: ignore
-        )
-        backbone = build_backbone(config=config)
+    backbone = backbone or build_backbone(config=config)
 
     classifier_head = ClassifierHead(
-        num_classes=num_classes,
+        class_names=class_names,
         in_channels=backbone.out_channels,
     )
     bbox_head = BBoxHead(
         in_channels=backbone.out_channels,
-        num_sizes=num_sizes,
+        dimension_names=dimension_names,
     )
     return Detector(
         backbone=backbone,
         classifier_head=classifier_head,
-        bbox_head=bbox_head,
+        size_head=bbox_head,
     )
diff --git a/src/batdetect2/models/heads.py b/src/batdetect2/models/heads.py
index ba7b437..250ddb6 100644
--- a/src/batdetect2/models/heads.py
+++ b/src/batdetect2/models/heads.py
@@ -54,12 +54,14 @@ class ClassifierHead(nn.Module):
         1×1 convolution with ``num_classes + 1`` output channels.
     """
 
-    def __init__(self, num_classes: int, in_channels: int):
+    def __init__(self, class_names: list[str], in_channels: int):
         """Initialise the ClassifierHead."""
         super().__init__()
 
-        self.num_classes = num_classes
+        self.class_names = class_names
+        self.num_classes = len(class_names)
         self.in_channels = in_channels
+
         self.classifier = nn.Conv2d(
             self.in_channels,
             self.num_classes + 1,
@@ -165,11 +167,12 @@ class BBoxHead(nn.Module):
         1×1 convolution with 2 output channels (duration, bandwidth).
     """
 
-    def __init__(self, in_channels: int, num_sizes: int = 2):
+    def __init__(self, dimension_names: list[str], in_channels: int):
         """Initialise the BBoxHead."""
         super().__init__()
         self.in_channels = in_channels
-        self.num_sizes = num_sizes
+        self.dimension_names = dimension_names
+        self.num_sizes = len(dimension_names)
 
         self.bbox = nn.Conv2d(
             in_channels=self.in_channels,
diff --git a/src/batdetect2/train/checkpoints.py b/src/batdetect2/train/checkpoints.py
index a443743..be1c165 100644
--- a/src/batdetect2/train/checkpoints.py
+++ b/src/batdetect2/train/checkpoints.py
@@ -34,6 +34,7 @@ class CheckpointConfig(BaseConfig):
     monitor: str | None = None
     mode: str = "max"
     save_top_k: int = 1
+    # Save distributable inference checkpoints by default.
     save_weights_only: bool = True
     filename: str | None = None
     save_last: bool | Literal["link"] = "link"
diff --git a/tests/test_api_v2/test_api_v2.py b/tests/test_api_v2/test_api_v2.py
index a4b2758..9f7a109 100644
--- a/tests/test_api_v2/test_api_v2.py
+++ b/tests/test_api_v2/test_api_v2.py
@@ -299,10 +299,10 @@ def test_checkpoint_with_same_targets_config_keeps_heads_unchanged(
             value,
         )
 
-    for key, value in source_detector.bbox_head.state_dict().items():
-        assert key in detector.bbox_head.state_dict()
+    for key, value in source_detector.size_head.state_dict().items():
+        assert key in detector.size_head.state_dict()
         torch.testing.assert_close(
-            detector.bbox_head.state_dict()[key],
+            detector.size_head.state_dict()[key],
             value,
         )
 
diff --git a/tests/test_api_v2/test_finetune.py b/tests/test_api_v2/test_finetune.py
index 8d8c6a2..5d8b223 100644
--- a/tests/test_api_v2/test_finetune.py
+++ b/tests/test_api_v2/test_finetune.py
@@ -18,7 +18,7 @@ def test_user_can_finetune_only_heads(
 
     api = BatDetect2API.from_config()
     source_classifier_head = api.model.detector.classifier_head
-    source_bbox_head = api.model.detector.bbox_head
+    source_size_head = api.model.detector.size_head
     source_backbone = api.model.detector.backbone
     finetune_dir = tmp_path / "heads_only"
 
@@ -39,7 +39,7 @@ def test_user_can_finetune_only_heads(
 
     backbone_params = list(detector.backbone.parameters())
     classifier_params = list(detector.classifier_head.parameters())
-    bbox_params = list(detector.bbox_head.parameters())
+    bbox_params = list(detector.size_head.parameters())
 
     assert backbone_params
     assert classifier_params
@@ -50,7 +50,7 @@ def test_user_can_finetune_only_heads(
     assert finetuned_api is not api
     assert detector.backbone is source_backbone
     assert detector.classifier_head is not source_classifier_head
-    assert detector.bbox_head is not source_bbox_head
+    assert detector.size_head is not source_size_head
     assert list(finetune_dir.rglob("*.ckpt"))
 
 
diff --git a/tests/test_models/test_detectors.py b/tests/test_models/test_detectors.py
index f5ce769..35d39a9 100644
--- a/tests/test_models/test_detectors.py
+++ b/tests/test_models/test_detectors.py
@@ -1,6 +1,7 @@
 import numpy as np
 import pytest
 import torch
+from typing import cast
 
 from batdetect2.models import UNetBackbone
 from batdetect2.models.backbones import UNetBackboneConfig
@@ -19,12 +20,15 @@ def dummy_spectrogram() -> torch.Tensor:
 def test_build_detector_default():
     """Test building the default detector without a config."""
     num_classes = 5
-    model = build_detector(num_classes=num_classes)
+    model = build_detector(
+        class_names=[f"class_{i}" for i in range(num_classes)],
+        dimension_names=["width", "height"],
+    )
 
     assert isinstance(model, Detector)
     assert model.num_classes == num_classes
     assert isinstance(model.classifier_head, ClassifierHead)
-    assert isinstance(model.bbox_head, BBoxHead)
+    assert isinstance(model.size_head, BBoxHead)
 
 
 def test_build_detector_custom_config():
@@ -32,13 +36,19 @@ def test_build_detector_custom_config():
     num_classes = 3
     config = UNetBackboneConfig(in_channels=2, input_height=128)
 
-    model = build_detector(num_classes=num_classes, config=config)
+    model = build_detector(
+        class_names=[f"class_{i}" for i in range(num_classes)],
+        dimension_names=["width", "height"],
+        config=config,
+    )
 
     assert isinstance(model, Detector)
     assert model.backbone.input_height == 128
 
-    assert isinstance(model.backbone.encoder, Encoder)
-    assert model.backbone.encoder.in_channels == 2
+    backbone = cast(UNetBackbone, model.backbone)
+
+    assert isinstance(backbone.encoder, Encoder)
+    assert backbone.encoder.in_channels == 2
 
 
 def test_build_detector_custom_size_channels():
@@ -47,8 +57,8 @@ def test_build_detector_custom_size_channels():
     config = UNetBackboneConfig(in_channels=1, input_height=128)
 
     model = build_detector(
-        num_classes=num_classes,
-        num_sizes=num_sizes,
+        class_names=[f"class_{i}" for i in range(num_classes)],
+        dimension_names=[f"size_{i}" for i in range(num_sizes)],
         config=config,
     )
 
@@ -62,7 +72,11 @@ def test_detector_forward_pass_shapes(dummy_spectrogram):
     num_classes = 4
     # Build model matching the dummy input shape
     config = UNetBackboneConfig(in_channels=1, input_height=256)
-    model = build_detector(num_classes=num_classes, config=config)
+    model = build_detector(
+        class_names=[f"class_{i}" for i in range(num_classes)],
+        dimension_names=["width", "height"],
+        config=config,
+    )
 
     # Process the spectrogram through the model
     # PyTorch expects shape (Batch, Channels, Height, Width)
@@ -132,7 +146,11 @@ def test_detector_forward_pass_with_preprocessor(sample_preprocessor):
     config = UNetBackboneConfig(
         in_channels=spec.shape[1], input_height=spec.shape[2]
     )
-    model = build_detector(num_classes=3, config=config)
+    model = build_detector(
+        class_names=["class_0", "class_1", "class_2"],
+        dimension_names=["width", "height"],
+        config=config,
+    )
 
     # Process
     output = model(spec)
diff --git a/tests/test_train/test_checkpoints.py b/tests/test_train/test_checkpoints.py
index 77a2856..4cffa48 100644
--- a/tests/test_train/test_checkpoints.py
+++ b/tests/test_train/test_checkpoints.py
@@ -8,7 +8,7 @@ from soundevent import data
 
 from batdetect2.train import TrainingConfig, run_train
 from batdetect2.train.checkpoints import (
-    DEFAULT_BUNDLED_CHECKPOINT,
+    DEFAULT_CHECKPOINT,
     get_bundled_checkpoint_names,
     resolve_checkpoint_path,
 )
@@ -145,7 +145,7 @@ def test_resolve_checkpoint_path_returns_local_path_unchanged(
 
 def test_get_bundled_checkpoint_names_lists_supported_aliases() -> None:
     assert get_bundled_checkpoint_names() == (
-        DEFAULT_BUNDLED_CHECKPOINT,
+        DEFAULT_CHECKPOINT,
         "batdetect2_uk_same",
     )
 
@@ -153,11 +153,11 @@ def test_get_bundled_checkpoint_names_lists_supported_aliases() -> None:
 def test_resolve_checkpoint_path_uses_default_bundled_alias() -> None:
     resolved = resolve_checkpoint_path()
 
-    assert resolved == resolve_checkpoint_path(DEFAULT_BUNDLED_CHECKPOINT)
+    assert resolved == resolve_checkpoint_path(DEFAULT_CHECKPOINT)
 
 
 def test_resolve_checkpoint_path_accepts_bundled_alias() -> None:
-    resolved = resolve_checkpoint_path(DEFAULT_BUNDLED_CHECKPOINT)
+    resolved = resolve_checkpoint_path(DEFAULT_CHECKPOINT)
 
     assert resolved.name == "batdetect2_uk_same.ckpt"
     assert resolved.exists()
@@ -227,6 +227,6 @@ def test_resolve_checkpoint_path_rejects_incomplete_huggingface_uri() -> None:
 def test_resolve_checkpoint_path_rejects_missing_local_path() -> None:
     with pytest.raises(
         FileNotFoundError,
-        match="bundled checkpoint alias",
+        match="checkpoint alias",
     ):
         resolve_checkpoint_path("missing.ckpt")
diff --git a/tests/test_train/test_lightning.py b/tests/test_train/test_lightning.py
index 9ab9e02..756329d 100644
--- a/tests/test_train/test_lightning.py
+++ b/tests/test_train/test_lightning.py
@@ -368,7 +368,7 @@ def test_build_model_with_new_targets_reuses_backbone_and_rebuilds_heads() -> (
     assert (
         rebuilt_detector.classifier_head is not source_detector.classifier_head
     )
-    assert rebuilt_detector.bbox_head is not source_detector.bbox_head
+    assert rebuilt_detector.size_head is not source_detector.size_head
     assert rebuilt_model.class_names == ["single_class"]
     assert rebuilt_model.dimension_names == ["width", "height"]