diff --git a/.chronus/changes/fix_body_root-2024-4-31-12-40-38.md b/.chronus/changes/fix_body_root-2024-4-31-12-40-38.md
new file mode 100644
index 00000000000..fec8c3a4516
--- /dev/null
+++ b/.chronus/changes/fix_body_root-2024-4-31-12-40-38.md
@@ -0,0 +1,8 @@
+---
+changeKind: fix
+packages:
+  - "@autorest/python"
+  - "@azure-tools/typespec-python"
+---
+
+don't lowercase serialized names when building a body from splatted arguments
\ No newline at end of file
diff --git a/packages/autorest.python/autorest/preprocess/__init__.py b/packages/autorest.python/autorest/preprocess/__init__.py
index 70c18c7f566..c9cf86e9ec3 100644
--- a/packages/autorest.python/autorest/preprocess/__init__.py
+++ b/packages/autorest.python/autorest/preprocess/__init__.py
@@ -324,9 +324,9 @@ def update_parameter(self, yaml_data: Dict[str, Any]) -> None:
         if yaml_data.get("propertyToParameterName"):
             # need to create a new one with padded keys and values
             yaml_data["propertyToParameterName"] = {
-                self.pad_reserved_words(prop, PadType.PROPERTY)
-                .lower(): self.pad_reserved_words(param_name, PadType.PARAMETER)
-                .lower()
+                self.pad_reserved_words(prop, PadType.PROPERTY): self.pad_reserved_words(
+                    param_name, PadType.PARAMETER
+                ).lower()
                 for prop, param_name in yaml_data["propertyToParameterName"].items()
             }
         wire_name_lower = (yaml_data.get("wireName") or "").lower()
diff --git a/packages/typespec-python/test/azure/generated/payload-multipart/payload/multipart/aio/operations/_operations.py b/packages/typespec-python/test/azure/generated/payload-multipart/payload/multipart/aio/operations/_operations.py
index ebe54e6b439..b1ff0c1506a 100644
--- a/packages/typespec-python/test/azure/generated/payload-multipart/payload/multipart/aio/operations/_operations.py
+++ b/packages/typespec-python/test/azure/generated/payload-multipart/payload/multipart/aio/operations/_operations.py
@@ -843,7 +843,7 @@ async def anonymous_model(  # pylint: disable=inconsistent-return-statements
         if body is _Unset:
             if profile_image is _Unset:
                 raise TypeError("missing required argument: profile_image")
-            body = {"profileimage": profile_image}
+            body = {"profileImage": profile_image}
             body = {k: v for k, v in body.items() if v is not None}
         _body = body.as_dict() if isinstance(body, _model_base.Model) else body
         _file_fields: List[str] = ["profileImage"]
diff --git a/packages/typespec-python/test/azure/generated/payload-multipart/payload/multipart/operations/_operations.py b/packages/typespec-python/test/azure/generated/payload-multipart/payload/multipart/operations/_operations.py
index f2d0406b16a..2077c9f2d3f 100644
--- a/packages/typespec-python/test/azure/generated/payload-multipart/payload/multipart/operations/_operations.py
+++ b/packages/typespec-python/test/azure/generated/payload-multipart/payload/multipart/operations/_operations.py
@@ -904,7 +904,7 @@ def anonymous_model(  # pylint: disable=inconsistent-return-statements
         if body is _Unset:
             if profile_image is _Unset:
                 raise TypeError("missing required argument: profile_image")
-            body = {"profileimage": profile_image}
+            body = {"profileImage": profile_image}
             body = {k: v for k, v in body.items() if v is not None}
         _body = body.as_dict() if isinstance(body, _model_base.Model) else body
         _file_fields: List[str] = ["profileImage"]
diff --git a/packages/typespec-python/test/unbranded/generated/payload-multipart/payload/multipart/aio/operations/_operations.py b/packages/typespec-python/test/unbranded/generated/payload-multipart/payload/multipart/aio/operations/_operations.py
index dfa40939b62..4d78d105071 100644
--- a/packages/typespec-python/test/unbranded/generated/payload-multipart/payload/multipart/aio/operations/_operations.py
+++ b/packages/typespec-python/test/unbranded/generated/payload-multipart/payload/multipart/aio/operations/_operations.py
@@ -834,7 +834,7 @@ async def anonymous_model(  # pylint: disable=inconsistent-return-statements
         if body is _Unset:
             if profile_image is _Unset:
                 raise TypeError("missing required argument: profile_image")
-            body = {"profileimage": profile_image}
+            body = {"profileImage": profile_image}
             body = {k: v for k, v in body.items() if v is not None}
         _body = body.as_dict() if isinstance(body, _model_base.Model) else body
         _file_fields: List[str] = ["profileImage"]
diff --git a/packages/typespec-python/test/unbranded/generated/payload-multipart/payload/multipart/operations/_operations.py b/packages/typespec-python/test/unbranded/generated/payload-multipart/payload/multipart/operations/_operations.py
index 2699fc916a3..1b34506d237 100644
--- a/packages/typespec-python/test/unbranded/generated/payload-multipart/payload/multipart/operations/_operations.py
+++ b/packages/typespec-python/test/unbranded/generated/payload-multipart/payload/multipart/operations/_operations.py
@@ -895,7 +895,7 @@ def anonymous_model(  # pylint: disable=inconsistent-return-statements
         if body is _Unset:
             if profile_image is _Unset:
                 raise TypeError("missing required argument: profile_image")
-            body = {"profileimage": profile_image}
+            body = {"profileImage": profile_image}
             body = {k: v for k, v in body.items() if v is not None}
         _body = body.as_dict() if isinstance(body, _model_base.Model) else body
         _file_fields: List[str] = ["profileImage"]