diff --git a/document-parser/infra/serve_converter.py b/document-parser/infra/serve_converter.py
index 2edbb91..d85e4a5 100644
--- a/document-parser/infra/serve_converter.py
+++ b/document-parser/infra/serve_converter.py
@@ -108,14 +108,14 @@ class ServeConverter:
             return False
 
 
-def _build_form_data(options: ConversionOptions) -> dict[str, str]:
-    """Build individual form fields matching Docling Serve's FormDepends pattern.
+def _build_form_data(options: ConversionOptions) -> dict[str, str | list[str]]:
+    """Build form fields matching Docling Serve's multipart form contract.
 
-    Docling Serve uses FormDepends to flatten ConvertDocumentsRequestOptions
-    into individual form fields (not a JSON blob).
+    Array fields (to_formats) are sent as lists — httpx encodes them as
+    repeated form keys (to_formats=md&to_formats=html&to_formats=json).
     """
     return {
-        "to_formats": '["md","html","json"]',
+        "to_formats": ["md", "html", "json"],
         "do_ocr": str(options.do_ocr).lower(),
         "do_table_structure": str(options.do_table_structure).lower(),
         "table_mode": options.table_mode,
diff --git a/document-parser/tests/test_serve_converter.py b/document-parser/tests/test_serve_converter.py
index 09de6b3..89490b3 100644
--- a/document-parser/tests/test_serve_converter.py
+++ b/document-parser/tests/test_serve_converter.py
@@ -32,7 +32,7 @@ class TestBuildFormData:
         assert data["do_picture_description"] == "false"
         assert data["include_images"] == "false"
         assert data["images_scale"] == "1.0"
-        assert '"json"' in data["to_formats"]
+        assert set(data["to_formats"]) == {"md", "html", "json"}
 
     def test_custom_options(self):
         opts = ConversionOptions(
@@ -299,11 +299,11 @@ class TestServeConverterConvert:
         assert len(result.pages[0].elements) == 1
         assert result.pages[0].elements[0].type == "title"
 
-        # Verify form fields sent individually (not as JSON blob)
+        # Verify form fields sent as dict with list for repeated keys
         call_kwargs = mock_client.post.call_args
         sent_data = call_kwargs.kwargs.get("data", {})
-        assert "do_ocr" in sent_data
         assert sent_data["do_ocr"] == "true"
+        assert set(sent_data["to_formats"]) == {"md", "html", "json"}
 
     @pytest.mark.asyncio
     async def test_http_error_raises(self, tmp_path):