diff --git a/src/runloop_api_client/_files.py b/src/runloop_api_client/_files.py
index 1a013e434..6afd86ad1 100644
--- a/src/runloop_api_client/_files.py
+++ b/src/runloop_api_client/_files.py
@@ -66,7 +66,11 @@ def _transform_file(file: FileTypes) -> HttpxFileTypes:
     if is_file_content(file):
         if isinstance(file, os.PathLike):
             path = pathlib.Path(file)
-            return (path.name, path.read_bytes())
+            # Hand httpx an open file handle so its multipart encoder reads the
+            # file lazily in chunks. read_bytes() would buffer the entire file
+            # in memory up front, which is wasteful for the large files this
+            # endpoint targets.
+            return (path.name, path.open("rb"))
 
         return file
 
@@ -107,8 +111,11 @@ async def async_to_httpx_files(files: RequestFiles | None) -> HttpxRequestFiles
 async def _async_transform_file(file: FileTypes) -> HttpxFileTypes:
     if is_file_content(file):
         if isinstance(file, os.PathLike):
-            path = anyio.Path(file)
-            return (path.name, await path.read_bytes())
+            path = pathlib.Path(file)
+            # Same rationale as the sync path: stream from an open handle rather
+            # than buffering the whole file. httpx's multipart encoder reads from
+            # this handle in chunks as it serializes the request body.
+            return (path.name, path.open("rb"))
 
         return file
 
diff --git a/tests/test_files.py b/tests/test_files.py
index b5ee73ae2..84b3c41c8 100644
--- a/tests/test_files.py
+++ b/tests/test_files.py
@@ -1,8 +1,9 @@
+import io
 from pathlib import Path
 
 import anyio
 import pytest
-from dirty_equals import IsDict, IsList, IsBytes, IsTuple
+from dirty_equals import IsDict, IsList, IsTuple, IsInstance
 
 from runloop_api_client._files import to_httpx_files, deepcopy_with_paths, async_to_httpx_files
 from runloop_api_client._utils import extract_files
@@ -10,37 +11,63 @@
 readme_path = Path(__file__).parent.parent.joinpath("README.md")
 
 
+def _close_file_handles(value: object) -> None:
+    """Recursively close any open file handles in to_httpx_files output.
+
+    The transform returns streaming file handles, so tests that don't actually
+    issue a request must close them to avoid resource warnings.
+    """
+    if isinstance(value, io.IOBase):
+        value.close()
+    elif isinstance(value, dict):
+        for v in value.values():
+            _close_file_handles(v)
+    elif isinstance(value, (list, tuple)):
+        for v in value:
+            _close_file_handles(v)
+
+
 def test_pathlib_includes_file_name() -> None:
     result = to_httpx_files({"file": readme_path})
-    print(result)
-    assert result == IsDict({"file": IsTuple("README.md", IsBytes())})
+    try:
+        assert result == IsDict({"file": IsTuple("README.md", IsInstance(io.IOBase))})
+    finally:
+        _close_file_handles(result)
 
 
 def test_tuple_input() -> None:
     result = to_httpx_files([("file", readme_path)])
-    print(result)
-    assert result == IsList(IsTuple("file", IsTuple("README.md", IsBytes())))
+    try:
+        assert result == IsList(IsTuple("file", IsTuple("README.md", IsInstance(io.IOBase))))
+    finally:
+        _close_file_handles(result)
 
 
 @pytest.mark.asyncio
 async def test_async_pathlib_includes_file_name() -> None:
     result = await async_to_httpx_files({"file": readme_path})
-    print(result)
-    assert result == IsDict({"file": IsTuple("README.md", IsBytes())})
+    try:
+        assert result == IsDict({"file": IsTuple("README.md", IsInstance(io.IOBase))})
+    finally:
+        _close_file_handles(result)
 
 
 @pytest.mark.asyncio
 async def test_async_supports_anyio_path() -> None:
     result = await async_to_httpx_files({"file": anyio.Path(readme_path)})
-    print(result)
-    assert result == IsDict({"file": IsTuple("README.md", IsBytes())})
+    try:
+        assert result == IsDict({"file": IsTuple("README.md", IsInstance(io.IOBase))})
+    finally:
+        _close_file_handles(result)
 
 
 @pytest.mark.asyncio
 async def test_async_tuple_input() -> None:
     result = await async_to_httpx_files([("file", readme_path)])
-    print(result)
-    assert result == IsList(IsTuple("file", IsTuple("README.md", IsBytes())))
+    try:
+        assert result == IsList(IsTuple("file", IsTuple("README.md", IsInstance(io.IOBase))))
+    finally:
+        _close_file_handles(result)
 
 
 def test_string_not_allowed() -> None: