fix(server): accept empty bytes data in Audio like Image

Audio.__init__ validated with `not bool(path) ^ bool(data)`, treating
`data=b""` as missing and raising ValueError, while Image accepts an empty
byte payload (`Image(data=b"")` works). Both helpers should treat only
None as absent. Align Audio's validation with Image's (None-based checks,
same error messages).

Fixes #3656

Author: KaiyiQuan

src/mcp/server/mcpserver/utilities/types.py

@@ -63,8 +63,10 @@ def __init__(
         data: bytes | None = None,
         format: str | None = None,
     ):
-        if not bool(path) ^ bool(data):  # pragma: no cover
-            raise ValueError("Either path or data can be provided")
+        if path is None and data is None:  # pragma: no cover
+            raise ValueError("Either path or data must be provided")
+        if path is not None and data is not None:  # pragma: no cover
+            raise ValueError("Only one of path or data can be provided")
 
         self.path = Path(path) if path else None
         self.data = data

tests/server/mcpserver/utilities/test_media_types.py

@@ -0,0 +1,36 @@
+"""Audio/Image media helper validation (#3656)."""
+
+import pytest
+
+from mcp.server.mcpserver.utilities.types import Audio, Image
+
+
+class TestAudioDataValidation:
+    def test_audio_accepts_empty_bytes_payload(self):
+        audio = Audio(data=b"")
+        assert audio.data == b""
+        assert audio.path is None
+
+    def test_audio_accepts_nonempty_bytes_payload(self):
+        audio = Audio(data=b"\x00\x01")
+        assert audio.data == b"\x00\x01"
+
+    def test_audio_accepts_path_only(self):
+        audio = Audio(path="/tmp/x.wav")
+        assert audio.path is not None
+        assert audio.data is None
+
+    def test_audio_raises_when_nothing_provided(self):
+        with pytest.raises(ValueError):
+            Audio()
+
+    def test_audio_raises_when_both_provided(self):
+        with pytest.raises(ValueError):
+            Audio(path="/tmp/x.wav", data=b"")
+
+
+class TestImageDataValidation:
+    def test_image_accepts_empty_bytes_payload(self):
+        image = Image(data=b"")
+        assert image.data == b""
+        assert image.path is None