[bugfix] no shield_call when there's no shields configured (#642)

# What does this PR do?

**Why**
- When AgentConfig has no `input_shields` / `output_shields` defined, we
still outputs a shield_call step with violation=None. This is impossible
to distinguish the case b/w (1) no violation from running shields v.s.
(2) no shields call

**What**
- We should not have a shield_call step when no `input_shields` /
`output_shields` are defined.

- Also removes a never reached try/catch code block in agent loop.
`run_multiple_shields` is never called in the try block (verified by
stacktrace print)

**Side Note**
- pre-commit fix

## Test Plan

Tested w/ DirectClient via:
https://gist.github.com/yanxi0830/b48f2a53b6f5391b9ff1e39992bc05b3

**No Shields**
<img width="858" alt="image"
src="https://github.com/user-attachments/assets/67319370-329f-4954-bd16-d21ce54c6ebf"
/>

**With Input + Output Shields**
<img width="854" alt="image"
src="https://github.com/user-attachments/assets/75ab1bee-3ba9-4549-ab51-23210be83da7"
/>

**Input Shields Only**
<img width="858" alt="image"
src="https://github.com/user-attachments/assets/1897206b-13dd-4ea5-92c2-b39bf68e9286"
/>


E2E pytest
```
LLAMA_STACK_BASE_URL=http://localhost:5000 pytest -v ./tests/client-sdk/agents/test_agents.py
```

## Sources

Please link relevant resources if necessary.


## Before submitting

- [ ] This PR fixes a typo or improves the docs (you can dismiss the
other checks if that's the case).
- [ ] Ran pre-commit to handle lint / formatting issues.
- [ ] Read the [contributor
guideline](https://github.com/meta-llama/llama-stack/blob/main/CONTRIBUTING.md),
      Pull Request section?
- [ ] Updated relevant documentation.
- [ ] Wrote necessary unit or integration tests.

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

@@ -239,13 +239,14 @@ class ChatAgent(ShieldRunnerMixin):
         # return a "final value" for the `yield from` statement. we simulate that by yielding a
         # final boolean (to see whether an exception happened) and then explicitly testing for it.
 
-        async for res in self.run_multiple_shields_wrapper(
+        if len(self.input_shields) > 0:
-            turn_id, input_messages, self.input_shields, "user-input"
+            async for res in self.run_multiple_shields_wrapper(
-        ):
+                turn_id, input_messages, self.input_shields, "user-input"
-            if isinstance(res, bool):
+            ):
-                return
+                if isinstance(res, bool):
-            else:
+                    return
-                yield res
+                else:
+                    yield res
 
         async for res in self._run(
             session_id, turn_id, input_messages, attachments, sampling_params, stream
@@ -262,13 +263,14 @@ class ChatAgent(ShieldRunnerMixin):
         # for output shields run on the full input and output combination
         messages = input_messages + [final_response]
 
-        async for res in self.run_multiple_shields_wrapper(
+        if len(self.output_shields) > 0:
-            turn_id, messages, self.output_shields, "assistant-output"
+            async for res in self.run_multiple_shields_wrapper(
-        ):
+                turn_id, messages, self.output_shields, "assistant-output"
-            if isinstance(res, bool):
+            ):
-                return
+                if isinstance(res, bool):
-            else:
+                    return
-                yield res
+                else:
+                    yield res
 
         yield final_response
 
@@ -531,106 +533,72 @@ class ChatAgent(ShieldRunnerMixin):
                     input_messages = input_messages + [message]
             else:
                 log.info(f"{str(message)}")
-                try:
+                tool_call = message.tool_calls[0]
-                    tool_call = message.tool_calls[0]
 
-                    name = tool_call.tool_name
+                name = tool_call.tool_name
-                    if not isinstance(name, BuiltinTool):
+                if not isinstance(name, BuiltinTool):
-                        yield message
+                    yield message
-                        return
-
-                    step_id = str(uuid.uuid4())
-                    yield AgentTurnResponseStreamChunk(
-                        event=AgentTurnResponseEvent(
-                            payload=AgentTurnResponseStepStartPayload(
-                                step_type=StepType.tool_execution.value,
-                                step_id=step_id,
-                            )
-                        )
-                    )
-                    yield AgentTurnResponseStreamChunk(
-                        event=AgentTurnResponseEvent(
-                            payload=AgentTurnResponseStepProgressPayload(
-                                step_type=StepType.tool_execution.value,
-                                step_id=step_id,
-                                tool_call=tool_call,
-                            )
-                        )
-                    )
-
-                    with tracing.span(
-                        "tool_execution",
-                        {
-                            "tool_name": tool_call.tool_name,
-                            "input": message.model_dump_json(),
-                        },
-                    ) as span:
-                        result_messages = await execute_tool_call_maybe(
-                            self.tools_dict,
-                            [message],
-                        )
-                        assert (
-                            len(result_messages) == 1
-                        ), "Currently not supporting multiple messages"
-                        result_message = result_messages[0]
-                        span.set_attribute("output", result_message.model_dump_json())
-
-                    yield AgentTurnResponseStreamChunk(
-                        event=AgentTurnResponseEvent(
-                            payload=AgentTurnResponseStepCompletePayload(
-                                step_type=StepType.tool_execution.value,
-                                step_details=ToolExecutionStep(
-                                    step_id=step_id,
-                                    turn_id=turn_id,
-                                    tool_calls=[tool_call],
-                                    tool_responses=[
-                                        ToolResponse(
-                                            call_id=result_message.call_id,
-                                            tool_name=result_message.tool_name,
-                                            content=result_message.content,
-                                        )
-                                    ],
-                                ),
-                            )
-                        )
-                    )
-
-                    # TODO: add tool-input touchpoint and a "start" event for this step also
-                    # but that needs a lot more refactoring of Tool code potentially
-                    yield AgentTurnResponseStreamChunk(
-                        event=AgentTurnResponseEvent(
-                            payload=AgentTurnResponseStepCompletePayload(
-                                step_type=StepType.shield_call.value,
-                                step_details=ShieldCallStep(
-                                    step_id=str(uuid.uuid4()),
-                                    turn_id=turn_id,
-                                    violation=None,
-                                ),
-                            )
-                        )
-                    )
-
-                except SafetyException as e:
-                    yield AgentTurnResponseStreamChunk(
-                        event=AgentTurnResponseEvent(
-                            payload=AgentTurnResponseStepCompletePayload(
-                                step_type=StepType.shield_call.value,
-                                step_details=ShieldCallStep(
-                                    step_id=str(uuid.uuid4()),
-                                    turn_id=turn_id,
-                                    violation=e.violation,
-                                ),
-                            )
-                        )
-                    )
-
-                    yield CompletionMessage(
-                        content=str(e),
-                        stop_reason=StopReason.end_of_turn,
-                    )
-                    yield False
                     return
 
+                step_id = str(uuid.uuid4())
+                yield AgentTurnResponseStreamChunk(
+                    event=AgentTurnResponseEvent(
+                        payload=AgentTurnResponseStepStartPayload(
+                            step_type=StepType.tool_execution.value,
+                            step_id=step_id,
+                        )
+                    )
+                )
+                yield AgentTurnResponseStreamChunk(
+                    event=AgentTurnResponseEvent(
+                        payload=AgentTurnResponseStepProgressPayload(
+                            step_type=StepType.tool_execution.value,
+                            step_id=step_id,
+                            tool_call=tool_call,
+                        )
+                    )
+                )
+
+                with tracing.span(
+                    "tool_execution",
+                    {
+                        "tool_name": tool_call.tool_name,
+                        "input": message.model_dump_json(),
+                    },
+                ) as span:
+                    result_messages = await execute_tool_call_maybe(
+                        self.tools_dict,
+                        [message],
+                    )
+                    assert (
+                        len(result_messages) == 1
+                    ), "Currently not supporting multiple messages"
+                    result_message = result_messages[0]
+                    span.set_attribute("output", result_message.model_dump_json())
+
+                yield AgentTurnResponseStreamChunk(
+                    event=AgentTurnResponseEvent(
+                        payload=AgentTurnResponseStepCompletePayload(
+                            step_type=StepType.tool_execution.value,
+                            step_details=ToolExecutionStep(
+                                step_id=step_id,
+                                turn_id=turn_id,
+                                tool_calls=[tool_call],
+                                tool_responses=[
+                                    ToolResponse(
+                                        call_id=result_message.call_id,
+                                        tool_name=result_message.tool_name,
+                                        content=result_message.content,
+                                    )
+                                ],
+                            ),
+                        )
+                    )
+                )
+
+                # TODO: add tool-input touchpoint and a "start" event for this step also
+                # but that needs a lot more refactoring of Tool code potentially
+
                 if out_attachment := interpret_content_as_attachment(
                     result_message.content
                 ):

llama_stack/providers/remote/inference/bedrock/bedrock.py

@@ -7,6 +7,7 @@
 from typing import *  # noqa: F403
 import json
 import uuid
+
 from botocore.client import BaseClient
 from llama_models.datatypes import CoreModelId
 

llama_stack/templates/bedrock/bedrock.py

@@ -7,12 +7,14 @@
 from pathlib import Path
 
 from llama_models.sku_list import all_registered_models
+
+from llama_stack.apis.models import ModelInput
 from llama_stack.distribution.datatypes import Provider
 
 from llama_stack.providers.inline.memory.faiss.config import FaissImplConfig
-from llama_stack.templates.template import DistributionTemplate, RunConfigSettings
 from llama_stack.providers.remote.inference.bedrock.bedrock import MODEL_ALIASES
-from llama_stack.apis.models import ModelInput
+from llama_stack.templates.template import DistributionTemplate, RunConfigSettings
+
 
 def get_distribution_template() -> DistributionTemplate:
     providers = {