[FEATURE] Migrate Steering to Use Plugin Protocol

[github-actions]

Overview

Migrate the experimental SteeringHandler from HookProvider to the new Plugin protocol, serving as the first official Plugin implementation in the SDK.

Parent Issue: #1636

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Problem Statement

The SteeringHandler currently implements HookProvider, but as an experimental feature, it should be migrated to the new Plugin protocol to demonstrate the pattern and benefit from the cleaner API.

Proposed Solution

Update SteeringHandler to implement the Plugin protocol instead of HookProvider.

Implementation Requirements

Current Implementation

File: src/strands/experimental/steering/core/handler.py

class SteeringHandler(HookProvider, ABC):
    def __init__(self, context_providers: list[SteeringContextProvider] | None = None):
        super().__init__()
        self.steering_context = SteeringContext()
        self._context_callbacks = []
        for provider in context_providers or []:
            self._context_callbacks.extend(provider.context_providers())

    def register_hooks(self, registry: HookRegistry, **kwargs: Any) -> None:
        # Register callbacks...
        pass

New Implementation

from strands.hooks import Plugin

class SteeringHandler(ABC):
    """Base class for steering handlers that provide contextual guidance to agents.
    
    Steering handlers maintain local context and register hook callbacks
    to populate context data as needed for guidance decisions.
    """
    
    name: str = "steering"
    
    def __init__(self, context_providers: list[SteeringContextProvider] | None = None):
        self.steering_context = SteeringContext()
        self._context_callbacks = []
        for provider in context_providers or []:
            self._context_callbacks.extend(provider.context_providers())

    def init_plugin(self, agent: Agent) -> None:
        """Initialize the steering handler with an agent.
        
        Args:
            agent: The agent instance to attach steering to.
        """
        # Use agent.add_hook() instead of registry.add_callback()
        agent.add_hook(BeforeModelCallEvent, self._on_before_model_call)
        # ... other hook registrations

Migration Changes

1. Remove HookProvider from class inheritance
2. Add name class attribute
3. Replace register_hooks(registry) with init_plugin(agent)
4. Use agent.add_hook() instead of registry.add_callback()

Files to Modify

• src/strands/experimental/steering/core/handler.py - Migrate to Plugin
• src/strands/experimental/steering/__init__.py - Update exports if needed
• tests/strands/experimental/steering/ - Update tests
• Any example code using SteeringHandler

Example Usage After Migration

from strands import Agent
from strands.experimental.steering import MySteeringHandler

# New way with plugins
agent = Agent(
    plugins=[MySteeringHandler(context_providers=[...])]
)

# Old way still works during deprecation period
agent = Agent(
    hooks=[MySteeringHandler(context_providers=[...])]  # Will show deprecation warning
)

Acceptance Criteria

• SteeringHandler implements Plugin protocol (has name and init_plugin)
• SteeringHandler no longer inherits from HookProvider
• All existing steering functionality is preserved
• Unit tests pass with updated implementation
• Works correctly with new plugins parameter
• Subclasses of SteeringHandler continue to work

Technical Notes

• This serves as the reference implementation for migrating HookProviders to Plugins
• Since steering is experimental, this is a safe place to demonstrate the pattern
• Document the migration in the PR as a guide for others

Dependencies

• Depends on: Plugin Protocol (1636.1), plugins parameter (1636.2), add_hook method (1636.3)