fix(browser): runtime fallback to local Chromium when cloud provider fails

Wraps provider.create_session() in _get_session_info() with try/except
to catch cloud provider runtime failures (timeouts, auth errors, rate
limits, invalid responses). Falls back to _create_local_session() so
browser automation continues working when cloud APIs are down.

Marks fallback sessions with fallback_from_cloud, fallback_reason, and
fallback_provider metadata for observability. If both cloud and local
fail, raises RuntimeError with chained context from both errors.

Closes #10883
Co-authored-by: konsisumer <konsisumer@users.noreply.github.com>

tests/tools/test_browser_cloud_fallback.py

@@ -0,0 +1,166 @@
+"""Tests for cloud browser provider runtime fallback to local Chromium.
+
+Covers the fallback logic in _get_session_info() when a cloud provider
+is configured but fails at runtime (issue #10883).
+"""
+import logging
+from unittest.mock import Mock, patch
+
+import pytest
+
+import tools.browser_tool as browser_tool
+
+
+def _reset_session_state(monkeypatch):
+    """Clear caches so each test starts fresh."""
+    monkeypatch.setattr(browser_tool, "_active_sessions", {})
+    monkeypatch.setattr(browser_tool, "_cached_cloud_provider", None)
+    monkeypatch.setattr(browser_tool, "_cloud_provider_resolved", False)
+    monkeypatch.setattr(browser_tool, "_start_browser_cleanup_thread", lambda: None)
+    monkeypatch.setattr(browser_tool, "_update_session_activity", lambda t: None)
+
+
+class TestCloudProviderRuntimeFallback:
+    """Tests for _get_session_info cloud → local fallback."""
+
+    def test_cloud_failure_falls_back_to_local(self, monkeypatch):
+        """When cloud provider.create_session raises, fall back to local."""
+        _reset_session_state(monkeypatch)
+
+        provider = Mock()
+        provider.create_session.side_effect = RuntimeError("401 Unauthorized")
+        monkeypatch.setattr(browser_tool, "_get_cloud_provider", lambda: provider)
+        monkeypatch.setattr(browser_tool, "_get_cdp_override", lambda: None)
+
+        session = browser_tool._get_session_info("task-1")
+
+        assert session["fallback_from_cloud"] is True
+        assert "401 Unauthorized" in session["fallback_reason"]
+        assert session["fallback_provider"] == "Mock"
+        assert session["features"]["local"] is True
+        assert session["cdp_url"] is None
+
+    def test_cloud_success_no_fallback(self, monkeypatch):
+        """When cloud succeeds, no fallback markers are present."""
+        _reset_session_state(monkeypatch)
+
+        provider = Mock()
+        provider.create_session.return_value = {
+            "session_name": "cloud-sess",
+            "bb_session_id": "bb_123",
+            "cdp_url": None,
+            "features": {"browser_use": True},
+        }
+        monkeypatch.setattr(browser_tool, "_get_cloud_provider", lambda: provider)
+        monkeypatch.setattr(browser_tool, "_get_cdp_override", lambda: None)
+
+        session = browser_tool._get_session_info("task-2")
+
+        assert session["session_name"] == "cloud-sess"
+        assert "fallback_from_cloud" not in session
+        assert "fallback_reason" not in session
+
+    def test_cloud_and_local_both_fail(self, monkeypatch):
+        """When both cloud and local fail, raise RuntimeError with both contexts."""
+        _reset_session_state(monkeypatch)
+
+        provider = Mock()
+        provider.create_session.side_effect = RuntimeError("cloud boom")
+        monkeypatch.setattr(browser_tool, "_get_cloud_provider", lambda: provider)
+        monkeypatch.setattr(browser_tool, "_get_cdp_override", lambda: None)
+        monkeypatch.setattr(
+            browser_tool, "_create_local_session",
+            Mock(side_effect=OSError("no chromium")),
+        )
+
+        with pytest.raises(RuntimeError, match="cloud boom.*local.*no chromium"):
+            browser_tool._get_session_info("task-3")
+
+    def test_no_provider_uses_local_directly(self, monkeypatch):
+        """When no cloud provider is configured, local mode is used with no fallback markers."""
+        _reset_session_state(monkeypatch)
+
+        monkeypatch.setattr(browser_tool, "_get_cloud_provider", lambda: None)
+        monkeypatch.setattr(browser_tool, "_get_cdp_override", lambda: None)
+
+        session = browser_tool._get_session_info("task-4")
+
+        assert session["features"]["local"] is True
+        assert "fallback_from_cloud" not in session
+
+    def test_cdp_override_bypasses_provider(self, monkeypatch):
+        """CDP override takes priority — cloud provider is never consulted."""
+        _reset_session_state(monkeypatch)
+
+        provider = Mock()
+        monkeypatch.setattr(browser_tool, "_get_cloud_provider", lambda: provider)
+        monkeypatch.setattr(browser_tool, "_get_cdp_override", lambda: "ws://host:9222/devtools/browser/abc")
+
+        session = browser_tool._get_session_info("task-5")
+
+        provider.create_session.assert_not_called()
+        assert session["cdp_url"] == "ws://host:9222/devtools/browser/abc"
+
+    def test_fallback_logs_warning_with_provider_name(self, monkeypatch, caplog):
+        """Fallback emits a warning log with the provider class name and error."""
+        _reset_session_state(monkeypatch)
+
+        BrowserUseProviderFake = type("BrowserUseProvider", (), {
+            "create_session": Mock(side_effect=ConnectionError("timeout")),
+        })
+        provider = BrowserUseProviderFake()
+        monkeypatch.setattr(browser_tool, "_get_cloud_provider", lambda: provider)
+        monkeypatch.setattr(browser_tool, "_get_cdp_override", lambda: None)
+
+        with caplog.at_level(logging.WARNING, logger="tools.browser_tool"):
+            session = browser_tool._get_session_info("task-6")
+
+        assert session["fallback_from_cloud"] is True
+        assert any("BrowserUseProvider" in r.message and "timeout" in r.message
+                    for r in caplog.records)
+
+    def test_cloud_failure_does_not_poison_next_task(self, monkeypatch):
+        """A fallback for one task_id doesn't affect a new task_id when cloud recovers."""
+        _reset_session_state(monkeypatch)
+
+        call_count = 0
+
+        def create_session_flaky(task_id):
+            nonlocal call_count
+            call_count += 1
+            if call_count == 1:
+                raise RuntimeError("transient failure")
+            return {
+                "session_name": "cloud-ok",
+                "bb_session_id": "bb_999",
+                "cdp_url": None,
+                "features": {"browser_use": True},
+            }
+
+        provider = Mock()
+        provider.create_session.side_effect = create_session_flaky
+        monkeypatch.setattr(browser_tool, "_get_cloud_provider", lambda: provider)
+        monkeypatch.setattr(browser_tool, "_get_cdp_override", lambda: None)
+
+        # First call fails → fallback
+        s1 = browser_tool._get_session_info("task-a")
+        assert s1["fallback_from_cloud"] is True
+
+        # Second call (different task) → cloud succeeds
+        s2 = browser_tool._get_session_info("task-b")
+        assert "fallback_from_cloud" not in s2
+        assert s2["session_name"] == "cloud-ok"
+
+    def test_cloud_returns_invalid_session_triggers_fallback(self, monkeypatch):
+        """Cloud provider returning None or empty dict triggers fallback."""
+        _reset_session_state(monkeypatch)
+
+        provider = Mock()
+        provider.create_session.return_value = None
+        monkeypatch.setattr(browser_tool, "_get_cloud_provider", lambda: provider)
+        monkeypatch.setattr(browser_tool, "_get_cdp_override", lambda: None)
+
+        session = browser_tool._get_session_info("task-7")
+
+        assert session["fallback_from_cloud"] is True
+        assert "invalid session" in session["fallback_reason"]