feat: support multiple reference images with model-aware API mapping

Replace singular reference_image field with reference_images list to
support an arbitrary number of reference images. Map them to the correct
BFL API parameter names based on model family:
- flux-2-*: input_image, input_image_2, ..., input_image_8
- flux-kontext-*: input_image, input_image_2, ..., input_image_4
- flux 1.x: image_prompt (single)

BREAKING CHANGE: reference_image config field renamed to reference_images (list).

bulkgen/builder.py

@@ -51,8 +51,7 @@ def _collect_dep_files(
     """Collect all dependency file paths for a target."""
     target_cfg = config.targets[target_name]
     deps: list[str] = list(target_cfg.inputs)
-    if target_cfg.reference_image is not None:
-        deps.append(target_cfg.reference_image)
+    deps.extend(target_cfg.reference_images)
     deps.extend(target_cfg.control_images)
     return [project_dir / d for d in deps]
 
@@ -65,19 +64,18 @@ def _collect_extra_params(target_name: str, config: ProjectConfig) -> dict[str,
         params["width"] = target_cfg.width
     if target_cfg.height is not None:
         params["height"] = target_cfg.height
-    if target_cfg.reference_image is not None:
-        params["reference_image"] = target_cfg.reference_image
+    if target_cfg.reference_images:
+        params["reference_images"] = tuple(target_cfg.reference_images)
     if target_cfg.control_images:
         params["control_images"] = tuple(target_cfg.control_images)
     return params
 
 
 def _collect_all_deps(target_name: str, config: ProjectConfig) -> list[str]:
-    """Collect all dependency names (inputs + reference_image + control_images)."""
+    """Collect all dependency names (inputs + reference_images + control_images)."""
     target_cfg = config.targets[target_name]
     deps: list[str] = list(target_cfg.inputs)
-    if target_cfg.reference_image is not None:
-        deps.append(target_cfg.reference_image)
+    deps.extend(target_cfg.reference_images)
     deps.extend(target_cfg.control_images)
     return deps
 

bulkgen/config.py

@@ -34,7 +34,7 @@ class TargetConfig(BaseModel):
     prompt: str
     model: str | None = None
     inputs: list[str] = []
-    reference_image: str | None = None
+    reference_images: list[str] = []
     control_images: list[str] = []
     width: int | None = None
     height: int | None = None

bulkgen/graph.py

@@ -26,8 +26,7 @@ def build_graph(config: ProjectConfig, project_dir: Path) -> nx.DiGraph[str]:
         graph.add_node(target_name)
 
         deps: list[str] = list(target_cfg.inputs)
-        if target_cfg.reference_image is not None:
-            deps.append(target_cfg.reference_image)
+        deps.extend(target_cfg.reference_images)
         deps.extend(target_cfg.control_images)
 
         for dep in deps:

bulkgen/providers/image.py

@@ -18,6 +18,32 @@ def _encode_image_b64(path: Path) -> str:
     return base64.b64encode(path.read_bytes()).decode("ascii")
 
 
+# Parameter names for reference images, keyed by model prefix.
+_INPUT_IMAGE_KEYS = ["input_image"] + [f"input_image_{i}" for i in range(2, 9)]
+_IMAGE_PROMPT_KEYS = ["image_prompt"]
+
+
+def _ref_image_keys(model: str) -> list[str]:
+    """Return the ordered API parameter names for reference images."""
+    if model.startswith("flux-2-"):
+        return _INPUT_IMAGE_KEYS  # up to 8
+    if model.startswith("flux-kontext-"):
+        return _INPUT_IMAGE_KEYS[:4]  # up to 4
+    return _IMAGE_PROMPT_KEYS  # flux 1.x: single image_prompt
+
+
+def _add_reference_images(
+    inputs: dict[str, object],
+    reference_images: list[str],
+    model: str,
+    project_dir: Path,
+) -> None:
+    """Encode reference images and add them under the correct API keys."""
+    keys = _ref_image_keys(model)
+    for key, ref_name in zip(keys, reference_images, strict=False):
+        inputs[key] = _encode_image_b64(project_dir / ref_name)
+
+
 class ImageProvider(Provider):
     """Generates images via the BlackForestLabs API."""
 
@@ -44,9 +70,10 @@ class ImageProvider(Provider):
         if target_config.height is not None:
             inputs["height"] = target_config.height
 
-        if target_config.reference_image is not None:
-            ref_path = project_dir / target_config.reference_image
-            inputs["image_prompt"] = _encode_image_b64(ref_path)
+        if target_config.reference_images:
+            _add_reference_images(
+                inputs, target_config.reference_images, resolved_model, project_dir
+            )
 
         for control_name in target_config.control_images:
             ctrl_path = project_dir / control_name