From 9379ff6ca47c60f51d9f7fad7b22920aa6ad9558 Mon Sep 17 00:00:00 2001
From: Harrison Weinstock <hkobew@amazon.com>
Date: Thu, 16 Apr 2026 18:07:10 +0000
Subject: [PATCH] fix(browser): set event loop on calling thread before
 nest_asyncio.apply()

---
 src/strands_tools/browser/browser.py |  2 ++
 tests/browser/test_browser.py        | 20 ++++++++++++++++++++
 2 files changed, 22 insertions(+)

diff --git a/src/strands_tools/browser/browser.py b/src/strands_tools/browser/browser.py
index b00f1b8e..12a01a7b 100644
--- a/src/strands_tools/browser/browser.py
+++ b/src/strands_tools/browser/browser.py
@@ -938,6 +938,8 @@ def close(self, action: CloseAction) -> Dict[str, Any]:
     def _execute_async(self, action_coro) -> Any:
         # Apply nest_asyncio if not already applied
         if not self._nest_asyncio_applied:
+            # Ensure the event loop is correctly referenced on the active thread.
+            asyncio.set_event_loop(self._loop)
             nest_asyncio.apply()
             self._nest_asyncio_applied = True
 
diff --git a/tests/browser/test_browser.py b/tests/browser/test_browser.py
index ce3f9149..732dc5d6 100644
--- a/tests/browser/test_browser.py
+++ b/tests/browser/test_browser.py
@@ -2,6 +2,8 @@
 Unit tests for the Browser base class using MockBrowser.
 """
 
+import asyncio
+import concurrent.futures
 from unittest.mock import AsyncMock, Mock, patch
 
 from playwright.async_api import Browser as PlaywrightBrowser
@@ -81,3 +83,21 @@ def test_browser_tool_integration(mock_async_playwright):
     tool_func = browser.browser
     assert hasattr(tool_func, "__name__")
     assert tool_func.__name__ == "browser"
+
+
+def test_execute_async_works_from_foreign_thread():
+    """_execute_async must work when called from a thread other than the one that ran __init__.
+
+    The strands SDK dispatches sync tools via asyncio.to_thread, which runs the tool
+    on a worker thread. Without setting the event loop on that thread,
+    nest_asyncio.apply() fails with 'There is no current event loop in thread'.
+    """
+    browser = MockBrowser()
+
+    async def dummy():
+        return "ok"
+
+    with concurrent.futures.ThreadPoolExecutor(max_workers=1) as pool:
+        result = pool.submit(browser._execute_async, dummy()).result(timeout=5)
+
+    assert result == "ok"