fix(mypy): resolve agent_instance type issues (part 1 of 2)

Fixed 35 errors, 46 remaining:
- Add isinstance() checks for union type discrimination
- Fix list type annotations for Message types
- Convert strings to datetime/StepType where needed
- Use assert to narrow AgentTurnCreateRequest vs AgentTurnResumeRequest
- Add explicit type annotations to avoid inference issues

Still to fix:
- Remaining str to datetime/StepType conversions
- Optional list handling for shields
- Type annotations for tool maps
- List variance issues for input_messages
- Fix turn_id variable redefinition

src/llama_stack/providers/inline/agents/meta_reference/agent_instance.py

@@ -11,6 +11,7 @@ import uuid
 import warnings
 from collections.abc import AsyncGenerator
 from datetime import UTC, datetime
+from typing import Any
 
 import httpx
 
@@ -125,12 +126,12 @@ class ChatAgent(ShieldRunnerMixin):
         )
 
     def turn_to_messages(self, turn: Turn) -> list[Message]:
-        messages = []
+        messages: list[Message] = []
 
         # NOTE: if a toolcall response is in a step, we do not add it when processing the input messages
         tool_call_ids = set()
         for step in turn.steps:
-            if step.step_type == StepType.tool_execution.value:
+            if step.step_type == StepType.tool_execution.value and isinstance(step, ToolExecutionStep):
                 for response in step.tool_responses:
                     tool_call_ids.add(response.call_id)
 
@@ -149,9 +150,9 @@ class ChatAgent(ShieldRunnerMixin):
             messages.append(msg)
 
         for step in turn.steps:
-            if step.step_type == StepType.inference.value:
+            if step.step_type == StepType.inference.value and isinstance(step, InferenceStep):
                 messages.append(step.model_response)
-            elif step.step_type == StepType.tool_execution.value:
+            elif step.step_type == StepType.tool_execution.value and isinstance(step, ToolExecutionStep):
                 for response in step.tool_responses:
                     messages.append(
                         ToolResponseMessage(
@@ -159,7 +160,7 @@ class ChatAgent(ShieldRunnerMixin):
                             content=response.content,
                         )
                     )
-            elif step.step_type == StepType.shield_call.value:
+            elif step.step_type == StepType.shield_call.value and isinstance(step, ShieldCallStep):
                 if step.violation:
                     # CompletionMessage itself in the ShieldResponse
                     messages.append(
@@ -174,7 +175,7 @@ class ChatAgent(ShieldRunnerMixin):
         return await self.storage.create_session(name)
 
     async def get_messages_from_turns(self, turns: list[Turn]) -> list[Message]:
-        messages = []
+        messages: list[Message] = []
         if self.agent_config.instructions != "":
             messages.append(SystemMessage(content=self.agent_config.instructions))
 
@@ -231,7 +232,13 @@ class ChatAgent(ShieldRunnerMixin):
 
         steps = []
         messages = await self.get_messages_from_turns(turns)
+
+        turn_id: str
+        start_time: datetime
+        input_messages: list[Message]
+
         if is_resume:
+            assert isinstance(request, AgentTurnResumeRequest), "Expected AgentTurnResumeRequest for resume"
             tool_response_messages = [
                 ToolResponseMessage(call_id=x.call_id, content=x.content) for x in request.tool_responses
             ]
@@ -252,20 +259,21 @@ class ChatAgent(ShieldRunnerMixin):
             in_progress_tool_call_step = await self.storage.get_in_progress_tool_call_step(
                 request.session_id, request.turn_id
             )
-            now = datetime.now(UTC).isoformat()
+            now_iso = datetime.now(UTC).isoformat()
+            now_dt = datetime.now(UTC)
             tool_execution_step = ToolExecutionStep(
                 step_id=(in_progress_tool_call_step.step_id if in_progress_tool_call_step else str(uuid.uuid4())),
                 turn_id=request.turn_id,
                 tool_calls=(in_progress_tool_call_step.tool_calls if in_progress_tool_call_step else []),
                 tool_responses=request.tool_responses,
-                completed_at=now,
-                started_at=(in_progress_tool_call_step.started_at if in_progress_tool_call_step else now),
+                completed_at=now_dt,
+                started_at=(in_progress_tool_call_step.started_at if in_progress_tool_call_step else now_dt),
             )
             steps.append(tool_execution_step)
             yield AgentTurnResponseStreamChunk(
                 event=AgentTurnResponseEvent(
                     payload=AgentTurnResponseStepCompletePayload(
-                        step_type=StepType.tool_execution.value,
+                        step_type=StepType.tool_execution,
                         step_id=tool_execution_step.step_id,
                         step_details=tool_execution_step,
                     )
@@ -276,18 +284,22 @@ class ChatAgent(ShieldRunnerMixin):
             turn_id = request.turn_id
             start_time = last_turn.started_at
         else:
+            assert isinstance(request, AgentTurnCreateRequest), "Expected AgentTurnCreateRequest for create"
             messages.extend(request.messages)
-            start_time = datetime.now(UTC).isoformat()
+            start_time = datetime.now(UTC)
             input_messages = request.messages
 
         output_message = None
+        req_documents = request.documents if isinstance(request, AgentTurnCreateRequest) and not is_resume else None
+        req_sampling = self.agent_config.sampling_params if self.agent_config.sampling_params is not None else SamplingParams()
+
         async for chunk in self.run(
             session_id=request.session_id,
             turn_id=turn_id,
             input_messages=messages,
-            sampling_params=self.agent_config.sampling_params,
+            sampling_params=req_sampling,
             stream=request.stream,
-            documents=request.documents if not is_resume else None,
+            documents=req_documents,
         ):
             if isinstance(chunk, CompletionMessage):
                 output_message = chunk
@@ -295,8 +307,12 @@ class ChatAgent(ShieldRunnerMixin):
 
             assert isinstance(chunk, AgentTurnResponseStreamChunk), f"Unexpected type {type(chunk)}"
             event = chunk.event
-            if event.payload.event_type == AgentTurnResponseEventType.step_complete.value:
-                steps.append(event.payload.step_details)
+            if (
+                event.payload.event_type == AgentTurnResponseEventType.step_complete.value
+                and hasattr(event.payload, "step_details")
+            ):
+                step_details = getattr(event.payload, "step_details")
+                steps.append(step_details)
 
             yield chunk
 
@@ -308,7 +324,7 @@ class ChatAgent(ShieldRunnerMixin):
             input_messages=input_messages,
             output_message=output_message,
             started_at=start_time,
-            completed_at=datetime.now(UTC).isoformat(),
+            completed_at=datetime.now(UTC),
             steps=steps,
         )
         await self.storage.add_turn_to_session(request.session_id, turn)