fix: Change handling of empty state objects for AG-UI (#3462)

Co-authored-by: Douwe Maan <[email protected]>

Author: MikeRyanDev

pydantic_ai_slim/pydantic_ai/ui/ag_ui/_adapter.py

@@ -2,11 +2,12 @@
 
 from __future__ import annotations
 
-from collections.abc import Sequence
+from collections.abc import Mapping, Sequence
 from functools import cached_property
 from typing import (
     TYPE_CHECKING,
     Any,
+    cast,
 )
 
 from ... import ExternalToolset, ToolDefinition
@@ -107,7 +108,14 @@ def toolset(self) -> AbstractToolset[AgentDepsT] | None:
     @cached_property
     def state(self) -> dict[str, Any] | None:
         """Frontend state from the AG-UI run input."""
-        return self.run_input.state
+        state = self.run_input.state
+        if state is None:
+            return None
+
+        if isinstance(state, Mapping) and not state:
+            return None
+
+        return cast('dict[str, Any]', state)
 
     @classmethod
     def load_messages(cls, messages: Sequence[Message]) -> list[ModelMessage]:

tests/test_ag_ui.py

@@ -226,6 +226,27 @@ async def simple_stream(messages: list[ModelMessage], agent_info: AgentInfo) ->
     yield '(no tool calls)'
 
 
+async def test_agui_adapter_state_none() -> None:
+    """Ensure adapter exposes `None` state when no frontend state provided."""
+    agent = Agent(
+        model=FunctionModel(stream_function=simple_stream),
+    )
+
+    run_input = RunAgentInput(
+        thread_id=uuid_str(),
+        run_id=uuid_str(),
+        messages=[],
+        state=None,
+        context=[],
+        tools=[],
+        forwarded_props=None,
+    )
+
+    adapter = AGUIAdapter(agent=agent, run_input=run_input, accept=None)
+
+    assert adapter.state is None
+
+
 async def test_basic_user_message() -> None:
     """Test basic user message with text response."""
     agent = Agent(
@@ -1193,6 +1214,24 @@ async def test_request_with_state_without_handler() -> None:
             pass
 
 
+async def test_request_with_empty_state_without_handler() -> None:
+    agent = Agent(model=FunctionModel(stream_function=simple_stream))
+
+    run_input = create_input(
+        UserMessage(
+            id='msg_1',
+            content='Hello, how are you?',
+        ),
+        state={},
+    )
+
+    events = list[dict[str, Any]]()
+    async for event in run_ag_ui(agent, run_input):
+        events.append(json.loads(event.removeprefix('data: ')))
+
+    assert events == simple_result()
+
+
 async def test_request_with_state_with_custom_handler() -> None:
     @dataclass
     class CustomStateDeps: