feat: implement early exit mechanism for SequentialAgent using escalate action

Author: tommyhutcheson

src/google/adk/tools/exit_sequence_tool.py

@@ -0,0 +1,48 @@
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from __future__ import annotations
+
+from .tool_context import ToolContext
+
+
+def exit_sequence(tool_context: ToolContext):
+    """Exits the sequential execution of agents immediately.
+
+    Call this function when you encounter a terminal condition and want to
+    prevent subsequent agents in the sequence from executing. This will also
+    stop any remaining events from the current agent.
+
+    This tool is specifically designed for use within SequentialAgent contexts.
+    When called, it sets the escalate flag, which causes the SequentialAgent
+    to terminate the sequence immediately, preventing both:
+    - Subsequent events from the current sub-agent
+    - All remaining sub-agents in the sequence
+
+    Use cases:
+    - A blocking error is encountered that makes further processing impossible
+    - A definitive answer is found early, making subsequent agents unnecessary
+    - A security or validation check fails and the workflow must stop
+    - Resource limits are reached and safe termination is required
+
+    Example:
+      If you're in a sequence of [validator, processor, finalizer] agents,
+      and the validator finds invalid data, it can call exit_sequence() to
+      prevent the processor and finalizer from running on bad data.
+
+    Args:
+      tool_context: The context of the current tool invocation.
+    """
+    tool_context.actions.escalate = True
+    tool_context.actions.skip_summarization = True