diff --git a/hokusai/builder.py b/hokusai/builder.py
index 0a908b3..8a0f43b 100644
--- a/hokusai/builder.py
+++ b/hokusai/builder.py
@@ -252,9 +252,6 @@ def _should_skip_failed_dep(
     return any(d in result.failed for d in _collect_all_deps(target_name, config))
 
 
-_DOWNLOAD_MODEL_SENTINEL = "__download__"
-
-
 def _is_dirty(
     target_name: str,
     config: ProjectConfig,
@@ -267,10 +264,7 @@ def _is_dirty(
     if isinstance(target_cfg, DownloadTargetConfig):
         return is_target_dirty(
             target_name,
-            resolved_prompt=target_cfg.download,
-            model=_DOWNLOAD_MODEL_SENTINEL,
-            dep_files=[],
-            extra_params={},
+            download=target_cfg.download,
             state=state,
             project_dir=project_dir,
         )
@@ -346,24 +340,27 @@ def _process_outcomes(
             target_cfg = config.targets[name]
 
             if isinstance(target_cfg, DownloadTargetConfig):
-                resolved_prompt = target_cfg.download
-                model_name = _DOWNLOAD_MODEL_SENTINEL
+                record_target_state(
+                    name,
+                    download=target_cfg.download,
+                    state=state,
+                    project_dir=project_dir,
+                )
             else:
                 model_info = resolve_model(name, target_cfg, config.defaults)
                 resolved_prompt = resolve_prompt(target_cfg.prompt, project_dir)
-                model_name = model_info.name
+                dep_files = _collect_dep_files(name, config, project_dir)
+                extra = _collect_extra_params(name, config)
 
-            dep_files = _collect_dep_files(name, config, project_dir)
-            extra = _collect_extra_params(name, config)
+                record_target_state(
+                    name,
+                    resolved_prompt=resolved_prompt,
+                    model=model_info.name,
+                    dep_files=dep_files,
+                    extra_params=extra,
+                    state=state,
+                    project_dir=project_dir,
+                )
 
-            record_target_state(
-                name,
-                resolved_prompt=resolved_prompt,
-                model=model_name,
-                dep_files=dep_files,
-                extra_params=extra,
-                state=state,
-                project_dir=project_dir,
-            )
             result.built.append(name)
             on_progress(BuildEvent.TARGET_OK, name, "")
diff --git a/hokusai/state.py b/hokusai/state.py
index d10b955..8aee172 100644
--- a/hokusai/state.py
+++ b/hokusai/state.py
@@ -21,10 +21,11 @@ def state_filename(project_name: str) -> str:
 class TargetState(BaseModel):
     """Recorded state of a single target from its last successful build."""
 
-    input_hashes: dict[str, str]
-    prompt: str
-    model: str
+    input_hashes: dict[str, str] = {}
+    prompt: str | None = None
+    model: str | None = None
     extra_params: dict[str, object] = {}
+    download: str | None = None
 
 
 class BuildState(BaseModel):
@@ -58,16 +59,22 @@ def save_state(state: BuildState, project_dir: Path, project_name: str) -> None:
     """Persist build state to disk."""
     state_path = project_dir / state_filename(project_name)
     with state_path.open("w") as f:
-        yaml.dump(state.model_dump(), f, default_flow_style=False, sort_keys=False)
+        yaml.dump(
+            state.model_dump(exclude_defaults=True),
+            f,
+            default_flow_style=False,
+            sort_keys=False,
+        )
 
 
 def is_target_dirty(
     target_name: str,
     *,
-    resolved_prompt: str,
-    model: str,
-    dep_files: list[Path],
-    extra_params: dict[str, object],
+    resolved_prompt: str | None = None,
+    model: str | None = None,
+    dep_files: list[Path] | None = None,
+    extra_params: dict[str, object] | None = None,
+    download: str | None = None,
     state: BuildState,
     project_dir: Path,
 ) -> bool:
@@ -76,10 +83,8 @@ def is_target_dirty(
     A target is dirty if:
     - Its output file does not exist
     - It has never been built (not recorded in state)
-    - Any dependency file hash has changed
-    - The resolved prompt text has changed
-    - The model has changed
-    - Extra parameters (width, height, etc.) have changed
+    - For download targets: the download URL has changed
+    - For generate targets: any dependency file hash, prompt, model, or extra params changed
     """
     output_path = project_dir / target_name
     if not output_path.exists():
@@ -90,16 +95,21 @@ def is_target_dirty(
 
     prev = state.targets[target_name]
 
+    # Download targets only compare the URL.
+    if download is not None:
+        return prev.download != download
+
+    # Generate targets compare prompt, model, extra params, and input hashes.
     if prev.model != model:
         return True
 
     if prev.prompt != resolved_prompt:
         return True
 
-    if prev.extra_params != extra_params:
+    if prev.extra_params != (extra_params or {}):
         return True
 
-    for dep_path in dep_files:
+    for dep_path in dep_files or []:
         dep_key = str(dep_path.relative_to(project_dir))
         current_hash = hash_file(dep_path)
         if prev.input_hashes.get(dep_key) != current_hash:
@@ -111,16 +121,21 @@ def is_target_dirty(
 def record_target_state(
     target_name: str,
     *,
-    resolved_prompt: str,
-    model: str,
-    dep_files: list[Path],
-    extra_params: dict[str, object],
+    resolved_prompt: str | None = None,
+    model: str | None = None,
+    dep_files: list[Path] | None = None,
+    extra_params: dict[str, object] | None = None,
+    download: str | None = None,
     state: BuildState,
     project_dir: Path,
 ) -> None:
     """Record the state of a successfully built target."""
+    if download is not None:
+        state.targets[target_name] = TargetState(download=download)
+        return
+
     input_hashes: dict[str, str] = {}
-    for dep_path in dep_files:
+    for dep_path in dep_files or []:
         dep_key = str(dep_path.relative_to(project_dir))
         input_hashes[dep_key] = hash_file(dep_path)
 
@@ -128,5 +143,5 @@ def record_target_state(
         input_hashes=input_hashes,
         prompt=resolved_prompt,
         model=model,
-        extra_params=extra_params,
+        extra_params=extra_params or {},
     )