feat(hooks): add resume flag to AfterInvocationEvent

[mkmeral]

Description

Motivation

Hooks that run after an agent invocation sometimes need to trigger a follow-up invocation automatically. For example, a coding agent hook could run tests and linting after each response, and if checks fail, resume the agent with the error output so it can fix the issues autonomously. Similarly, a validation hook could inspect the agent's output against acceptance criteria and loop until the result passes. Today there's no way to do this from a hook; callers have to build the retry loop externally, which duplicates lifecycle logic and bypasses the hook system entirely.

This PR adds a resume field to AfterInvocationEvent. When a hook sets resume to any valid AgentInput, the agent re-invokes itself with that input, firing a full new invocation cycle (including BeforeInvocationEvent). When resume is None (the default), behavior is unchanged.

Public API Changes

AfterInvocationEvent gains a writable resume attribute:

from strands import Agent
from strands.hooks.events import AfterInvocationEvent
import subprocess

async def validate_code(event: AfterInvocationEvent):
    result = subprocess.run(["hatch", "run", "prepare"], capture_output=True, text=True)
    if result.returncode != 0:
        event.resume = f"Tests/linting failed. Fix these errors:\n{result.stdout}\n{result.stderr}"

agent = Agent()
agent.add_hook(validate_code)
agent("implement a fibonacci function in utils.py")

Each resume triggers a complete invocation cycle: BeforeInvocationEvent → event loop → AfterInvocationEvent, so all existing hooks participate in every iteration. The resume input is converted to messages via the same _convert_prompt_to_messages path as normal agent calls.

Use Cases

• Code quality loops: A hook runs tests and static analysis after each agent response, resuming with failure output until all checks pass
• Output validation: Inspect the agent's result against acceptance criteria or schema constraints and loop until the output is correct
• Multi-step workflows: Chain invocations where each step's output determines the next step's input, all within the hook lifecycle

Related Issues

Resolves: #1181

Documentation PR

TBD

Type of Change

New feature

Testing

• 4 unit tests covering resume triggering, no-resume default, BeforeInvocationEvent firing on resume, and multi-step chaining

• 3 unit tests for the resume field on AfterInvocationEvent (default value, writability, input type acceptance)

• Manually tested with a Jupyter notebook demo

[codecov]

Codecov Report

❌ Patch coverage is 96.42857% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/strands/agent/agent.py	95.83%	0 Missing and 1 partial ⚠️

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