diff --git a/src/dispatch/any.py b/src/dispatch/any.py
index d885709..84dbcb2 100644
--- a/src/dispatch/any.py
+++ b/src/dispatch/any.py
@@ -12,12 +12,15 @@
 
 
 def marshal_any(value: Any) -> google.protobuf.any_pb2.Any:
+    if not isinstance(value, google.protobuf.message.Message):
+        value = pickled_pb.Pickled(pickled_value=pickle.dumps(value))
+
     any = google.protobuf.any_pb2.Any()
-    if isinstance(value, google.protobuf.message.Message):
-        any.Pack(value)
+    if value.DESCRIPTOR.full_name.startswith("dispatch.sdk."):
+        any.Pack(value, type_url_prefix="buf.build/stealthrocket/dispatch-proto/")
     else:
-        p = pickled_pb.Pickled(pickled_value=pickle.dumps(value))
-        any.Pack(p, type_url_prefix="buf.build/stealthrocket/dispatch-proto/")
+        any.Pack(value)
+
     return any
 
 
diff --git a/tests/dispatch/test_any.py b/tests/dispatch/test_any.py
new file mode 100644
index 0000000..ee1d993
--- /dev/null
+++ b/tests/dispatch/test_any.py
@@ -0,0 +1,65 @@
+import pickle
+from datetime import datetime, timedelta
+
+from dispatch.any import marshal_any, unmarshal_any
+from dispatch.sdk.v1 import error_pb2 as error_pb
+
+
+def test_unmarshal_none():
+    boxed = marshal_any(None)
+    assert None == unmarshal_any(boxed)
+
+
+def test_unmarshal_bool():
+    boxed = marshal_any(True)
+    assert True == unmarshal_any(boxed)
+
+
+def test_unmarshal_integer():
+    boxed = marshal_any(1234)
+    assert 1234 == unmarshal_any(boxed)
+
+    boxed = marshal_any(-1234)
+    assert -1234 == unmarshal_any(boxed)
+
+
+def test_unmarshal_float():
+    boxed = marshal_any(3.14)
+    assert 3.14 == unmarshal_any(boxed)
+
+
+def test_unmarshal_string():
+    boxed = marshal_any("foo")
+    assert "foo" == unmarshal_any(boxed)
+
+
+def test_unmarshal_bytes():
+    boxed = marshal_any(b"bar")
+    assert b"bar" == unmarshal_any(boxed)
+
+
+def test_unmarshal_timestamp():
+    ts = datetime.fromtimestamp(
+        1719372909.641448
+    )  # datetime.datetime(2024, 6, 26, 13, 35, 9, 641448)
+    boxed = marshal_any(ts)
+    assert ts == unmarshal_any(boxed)
+
+
+def test_unmarshal_duration():
+    d = timedelta(seconds=1, microseconds=1234)
+    boxed = marshal_any(d)
+    assert d == unmarshal_any(boxed)
+
+
+def test_unmarshal_protobuf_message():
+    message = error_pb.Error(type="internal", message="oops")
+    boxed = marshal_any(message)
+
+    # Check the message isn't pickled (in which case the type_url would
+    # end with dispatch.sdk.python.v1.Pickled).
+    assert (
+        "buf.build/stealthrocket/dispatch-proto/dispatch.sdk.v1.Error" == boxed.type_url
+    )
+
+    assert message == unmarshal_any(boxed)