feat: add plugin for returning GenAI Parts from tools into the model request

Added to mitigate https://github.com/google/adk-python/issues/3064

Co-authored-by: Max Ind <maxind@google.com>
PiperOrigin-RevId: 830135940

src/google/adk/plugins/multimodal_tool_results_plugin.py

@@ -0,0 +1,90 @@
+# 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 typing import Any
+from typing import Optional
+
+from google.genai import types
+
+from ..agents.callback_context import CallbackContext
+from ..models.llm_request import LlmRequest
+from ..models.llm_response import LlmResponse
+from ..tools.base_tool import BaseTool
+from ..tools.tool_context import ToolContext
+from .base_plugin import BasePlugin
+
+PARTS_RETURNED_BY_TOOLS_ID = "temp:PARTS_RETURNED_BY_TOOLS_ID"
+
+
+class MultimodalToolResultsPlugin(BasePlugin):
+  """A plugin that modifies function tool responses to support returning list of parts directly.
+
+  Should be removed in favor of directly supporting FunctionResponsePart when these
+  are supported outside of computer use tool.
+  For context see: https://github.com/google/adk-python/issues/3064#issuecomment-3463067459
+  """
+
+  def __init__(self, name: str = "multimodal_tool_results_plugin"):
+    """Initialize the multimodal tool results plugin.
+
+    Args:
+      name: The name of the plugin instance.
+    """
+    super().__init__(name)
+
+  async def after_tool_callback(
+      self,
+      *,
+      tool: BaseTool,
+      tool_args: dict[str, Any],
+      tool_context: ToolContext,
+      result: dict,
+  ) -> Optional[dict]:
+    """Saves parts returned by the tool in ToolContext.
+
+    Later these are passed to LLM's context as-is.
+    No-op if tool doesn't return list[google.genai.types.Part] or google.genai.types.Part.
+    """
+
+    if not (
+        isinstance(result, types.Part)
+        or isinstance(result, list)
+        and result
+        and isinstance(result[0], types.Part)
+    ):
+      return result
+
+    parts = [result] if isinstance(result, types.Part) else result[:]
+
+    if PARTS_RETURNED_BY_TOOLS_ID in tool_context.state:
+      tool_context.state[PARTS_RETURNED_BY_TOOLS_ID] += parts
+    else:
+      tool_context.state[PARTS_RETURNED_BY_TOOLS_ID] = parts
+
+    return None
+
+  async def before_model_callback(
+      self, *, callback_context: CallbackContext, llm_request: LlmRequest
+  ) -> Optional[LlmResponse]:
+    """Attach saved list[google.genai.types.Part] returned by the tool to llm_request."""
+
+    if saved_parts := callback_context.state.get(
+        PARTS_RETURNED_BY_TOOLS_ID, None
+    ):
+      llm_request.contents[-1].parts += saved_parts
+      callback_context.state.update({PARTS_RETURNED_BY_TOOLS_ID: []})
+
+    return None