Make UIMessage dumping match ai sdk better

src/ai/ui/ai_sdk/outbound_messages.py

@@ -118,7 +118,8 @@ def bundle_to_wire_output(bundle: messages_.MessageBundle) -> Any:
     The inbound side accepts either shape (see ``_build_result_part``).
     """
     dumped = [
-        m.model_dump(mode="json") for m in to_ui_messages(list(bundle.messages))
+        m.model_dump(mode="json", exclude_none=True)
+        for m in to_ui_messages(list(bundle.messages))
     ]
     if not dumped:
         return None

src/ai/ui/ai_sdk/ui_messages.py

@@ -14,7 +14,9 @@
 
 from ...types import messages as messages_
 
-_UI_MODEL_CONFIG = pydantic.ConfigDict(populate_by_name=True, extra="allow")
+_UI_MODEL_CONFIG = pydantic.ConfigDict(
+    populate_by_name=True, extra="allow", serialize_by_alias=True
+)
 
 
 class UITextPart(pydantic.BaseModel):
@@ -330,6 +332,10 @@ class UIMessage(pydantic.BaseModel):
     """Message in AI SDK v6 format.
 
     Reference: https://ai-sdk.dev/docs/reference/ai-sdk-core/ui-message
+
+    To send one to an AI SDK client, dump it with
+    ``model_dump(mode="json", exclude_none=True)``: the TS schemas mark
+    optional fields ``.optional()``, which rejects ``null``.
     """
 
     model_config = _UI_MODEL_CONFIG

tests/ui/ai_sdk/test_outbound_messages.py

@@ -666,3 +666,38 @@ def test_deferred_approval_hook_roundtrips_tool_call_id() -> None:
         assert hook.hook_id == hook_id
         assert hook.status == "pending"
         assert hook.tool_call_id == "tc-1"
+
+
+def test_bundle_to_wire_output_uses_wire_aliases() -> None:
+    bundle = messages_.MessageBundle(
+        messages=(
+            messages_.Message(
+                role="assistant",
+                parts=[
+                    messages_.ToolCallPart(
+                        tool_call_id="tc1", tool_name="bash", tool_args="{}"
+                    ),
+                    messages_.FilePart(data=b"png", media_type="image/png"),
+                ],
+            ),
+            messages_.Message(
+                role="tool",
+                parts=[
+                    messages_.ToolResultPart(
+                        tool_call_id="tc1",
+                        tool_name="bash",
+                        result="boom",
+                        result_kind="error",
+                    )
+                ],
+            ),
+        )
+    )
+
+    wire = outbound_messages.bundle_to_wire_output(bundle)
+
+    tool_part, file_part = wire["parts"]
+    assert tool_part["toolCallId"] == "tc1"
+    assert tool_part["errorText"] == "boom"
+    assert file_part["mediaType"] == "image/png"
+    assert "providerExecuted" not in tool_part