diff --git a/core/metadata_utils.py b/core/metadata_utils.py
index e8b82c9..56debf0 100644
--- a/core/metadata_utils.py
+++ b/core/metadata_utils.py
@@ -11,6 +11,43 @@ from .path_utils import (
 )
 
 
+def build_metadata_source_payload(
+    *,
+    source: str,
+    details_source: str,
+    filename: str,
+    category: str,
+    model_name: str,
+    model_type: str,
+    sha256: str,
+    size: Any,
+    result: Dict[str, Any],
+    description: str,
+    version_description: str,
+    extra_fields: Optional[Dict[str, Any]] = None,
+) -> Dict[str, Any]:
+    """Build the normalized source payload passed to the sidecar writer."""
+    payload = {
+        "source": source,
+        "details_source": details_source,
+        "filename": filename,
+        "category": category,
+        "model_name": model_name,
+        "name": model_name,
+        "model_type": model_type,
+        "sha256": sha256,
+        "size": size,
+        "base_model": result.get("base_model"),
+        "base_model_source": result.get("base_model_source"),
+        "base_model_inferred": bool(result.get("base_model_inferred")),
+        "description": description,
+        "model_description": description,
+        "version_description": version_description,
+    }
+    payload.update(extra_fields or {})
+    return payload
+
+
 def build_sidecar_file_identity(
     model_path: str,
     *,
diff --git a/core/services/civitai_search_service.py b/core/services/civitai_search_service.py
index 973b7f4..d4b7286 100644
--- a/core/services/civitai_search_service.py
+++ b/core/services/civitai_search_service.py
@@ -1,48 +1,13 @@
 """CivitAI model search service."""
 
 from .. import path_utils
+from ..metadata_utils import build_metadata_source_payload
 from ..request_utils import extract_request_sha256
 from ..routes.context import RouteContext
 from ..sources.common import collect_download_urls
 from .model_utils import CivitAISearchDependencies, ModelServiceDependencies
 
 
-def _build_metadata_sidecar_payload(
-    *,
-    source,
-    details_source,
-    filename,
-    category,
-    model_name,
-    model_type,
-    sha256,
-    size,
-    result,
-    description,
-    version_description,
-    extra_fields=None,
-):
-    payload = {
-        "source": source,
-        "details_source": details_source,
-        "filename": filename,
-        "category": category,
-        "model_name": model_name,
-        "name": model_name,
-        "model_type": model_type,
-        "sha256": sha256,
-        "size": size,
-        "base_model": result.get("base_model"),
-        "base_model_source": result.get("base_model_source"),
-        "base_model_inferred": bool(result.get("base_model_inferred")),
-        "description": description,
-        "model_description": description,
-        "version_description": version_description,
-    }
-    payload.update(extra_fields or {})
-    return payload
-
-
 class CivitAISearchService(ModelServiceDependencies):
     """Execute exact-match CivitAI and model metadata searches."""
 
@@ -489,7 +454,7 @@ class CivitAISearchService(ModelServiceDependencies):
                 model_type = result.get("model_type", "") or result.get(
                     "type", ""
                 )
-                metadata_payload = _build_metadata_sidecar_payload(
+                metadata_payload = build_metadata_source_payload(
                     source=source_name,
                     details_source=result.get("details_source") or source_name,
                     filename=filename,
@@ -591,7 +556,7 @@ class CivitAISearchService(ModelServiceDependencies):
                             split_result_descriptions(result)
                         )
                         model_type = result.get("model_type", "")
-                        metadata_payload = _build_metadata_sidecar_payload(
+                        metadata_payload = build_metadata_source_payload(
                             source="metadata_import",
                             details_source=result.get("source") or "metadata",
                             filename=filename,
@@ -858,7 +823,7 @@ class CivitAISearchService(ModelServiceDependencies):
                 model_description = response.get("model_description") or response.get(
                     "description"
                 ) or ""
-                metadata_payload = _build_metadata_sidecar_payload(
+                metadata_payload = build_metadata_source_payload(
                     source="local",
                     details_source=response.get("details_source") or "",
                     filename=filename,
diff --git a/tests/test_metadata_builder.py b/tests/test_metadata_builder.py
index 37db30a..2aa3b7a 100644
--- a/tests/test_metadata_builder.py
+++ b/tests/test_metadata_builder.py
@@ -13,11 +13,57 @@ from core.metadata_builder import (
     build_missing_local_metadata,
     normalize_metadata_build_mode,
 )
-from core.metadata_utils import build_sidecar_file_identity, merge_counted_payload
+from core.metadata_utils import (
+    build_metadata_source_payload,
+    build_sidecar_file_identity,
+    merge_counted_payload,
+)
 from core.path_utils import get_model_resolver_sidecar_path
 
 
 class MetadataBuilderTests(unittest.TestCase):
+    def test_build_metadata_source_payload_preserves_common_source_fields(self):
+        payload = build_metadata_source_payload(
+            source="civitai",
+            details_source="civitai",
+            filename="model.safetensors",
+            category="checkpoints",
+            model_name="Example Model",
+            model_type="Checkpoint",
+            sha256="a" * 64,
+            size=1234,
+            result={
+                "base_model": "SDXL",
+                "base_model_source": "civitai",
+                "base_model_inferred": 1,
+            },
+            description="Model description",
+            version_description="Version description",
+            extra_fields={"author": "Author", "size": 5678},
+        )
+
+        self.assertEqual(
+            {
+                "source": "civitai",
+                "details_source": "civitai",
+                "filename": "model.safetensors",
+                "category": "checkpoints",
+                "model_name": "Example Model",
+                "name": "Example Model",
+                "model_type": "Checkpoint",
+                "sha256": "a" * 64,
+                "size": 5678,
+                "base_model": "SDXL",
+                "base_model_source": "civitai",
+                "base_model_inferred": True,
+                "description": "Model description",
+                "model_description": "Model description",
+                "version_description": "Version description",
+                "author": "Author",
+            },
+            payload,
+        )
+
     def test_build_sidecar_file_identity_returns_common_schema_fields(self):
         identity = build_sidecar_file_identity(
             r"models\example.safetensors",