[BUG] Browser tool fails when used with stream_async

[Hweinstock]

Checks

• I have updated to the lastest minor and patch version of Strands
• I have checked the documentation and this is not expected behavior
• I have searched ./issues and there are no duplicates of my issue

Strands Version

1.35.0

Tools Package Version

0.4.0

Tools used

AgentCoreBrowser

Python Version

3.10

Operating System

Amazon Linux 2

Installation Method

pip

Steps to Reproduce

1. Create an AgentCoreBrowser instance (this sets the event loop on the current thread)
2. Pass browser_instance.browser as a tool to a strands Agent
3. Call agent.stream_async() from within an already-running async event loop (e.g. uvicorn, asyncio.run(main()))
4. Send a message that causes the model to invoke the browser tool

The tool call fails with RuntimeError: There is no current event loop in thread 'asyncio_0'.

This does not reproduce with the sync agent() call — only stream_async() / invoke_async().

Expected Behavior

tool executes successfully.

Actual Behavior

blocking error.

Additional Context

My understanding of why this is happening:

Browser.init creates an event loop and registers it on the current thread.

_execute_async is defined as a synchronous method.

Therefore calling _execute_async causes strands to dispatch it to another thread based on the logic in _stream.

However, that thread is different from the one where the browser was initiated, meaning its missing the event loop leading to the error.

Possible Solution

Add asyncio.set_event_loop(self._loop) in Browser._execute_async before nest_asyncio.apply(), so the
browser's loop is registered on whatever thread runs the tool.

Related Issues

No response