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@jayy-77 jayy-77 commented Feb 9, 2026

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Summary of Changes

Hello @jayy-77, I'm Gemini Code Assist1! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the system's ability to track and report token usage at a granular level for individual tools and sub-agents. By introducing dedicated fields for tool usage metadata in event objects, providing utility for aggregating this data, and integrating it into the analytics and tracing infrastructure, the changes enable more precise cost analysis and performance monitoring for complex AI applications, especially those involving nested agent architectures and Vertex AI features.

Highlights

  • Event Class Enhancement: The Event class now includes a tool_usage_metadata field to store token usage details for individual tools and sub-agents, enabling granular cost tracking.
  • Tool Usage Aggregation Utility: A new static method merge_usage_metadata has been added to LlmResponse to combine multiple token usage metadata objects into a single aggregate.
  • Contextual Tool Usage Management: The ToolContext class has been extended with methods (set_tool_usage, get_tool_usage, get_all_tool_usage) to manage and retrieve usage metadata specific to tools within a given context.
  • Flow Integration for Usage Data: The __build_response_event function now collects and attaches tool usage metadata from the tool_context to the generated Event objects, ensuring data propagation.
  • Enhanced Analytics and Tracing: The BigQueryAgentAnalyticsPlugin and tracing module have been updated to capture and log the new tool-level usage metadata, providing deeper insights into agent performance and costs in analytics and telemetry.
  • Sub-Agent Usage Collection: The AgentTool's run_async method now actively collects and aggregates usage metadata from nested sub-agent executions, ensuring comprehensive tracking across complex agent hierarchies.

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Changelog
  • src/google/adk/events/event.py
    • Added tool_usage_metadata: Optional[dict[str, types.GenerateContentResponseUsageMetadata]] to the Event class, along with a detailed docstring and example.
  • src/google/adk/flows/llm_flows/functions.py
    • Modified __build_response_event to retrieve tool_usage from tool_context.get_all_tool_usage() and pass it to the Event constructor as tool_usage_metadata.
  • src/google/adk/models/llm_response.py
    • Implemented merge_usage_metadata static method to sum prompt_token_count, candidates_token_count, total_token_count, and cached_content_token_count from a list of GenerateContentResponseUsageMetadata objects.
  • src/google/adk/plugins/bigquery_agent_analytics_plugin.py
    • Updated after_model_callback to extract and serialize tool_usage_metadata from llm_response into the content_dict under the key "tool_usage", formatting token counts into 'prompt', 'completion', and 'total' fields.
  • src/google/adk/telemetry/tracing.py
    • Enhanced trace_tool_call to aggregate prompt_token_count, candidates_token_count, and total_token_count from tool_usage_metadata and set them as GEN_AI_USAGE_INPUT_TOKENS, GEN_AI_USAGE_OUTPUT_TOKENS, and gcp.vertex.agent.tool_usage_total_tokens span attributes.
    • Added a detailed breakdown of tool usage per tool as a JSON-serialized attribute gcp.vertex.agent.tool_usage_breakdown.
  • src/google/adk/tools/agent_tool.py
    • Introduced logic within run_async to collect usage_metadata and tool_usage_metadata from events generated by sub-agents into a usage_metadata_list.
    • Aggregated collected usage metadata using LlmResponse.merge_usage_metadata and recorded it in the tool_context under the sub-agent's name using set_tool_usage.
  • src/google/adk/tools/tool_context.py
    • Added an internal dictionary _tool_usage: dict[str, Any] to store tool-specific usage metadata.
    • Exposed set_tool_usage, get_tool_usage, and get_all_tool_usage methods for managing tool usage data within the context.
Activity
  • The pull request was created by jayy-77 to enhance tool usage tracking with metadata aggregation.
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@adk-bot adk-bot added the tracing [Component] This issue is related to OpenTelemetry tracing label Feb 9, 2026
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adk-bot commented Feb 9, 2026

Response from ADK Triaging Agent

Hello @jayy-77, thank you for creating this PR!

To help reviewers understand and verify your changes, could you please fill out the "Testing Plan" section in your PR description? As per our contribution guidelines, this is required for all PRs that are not small documentation or typo fixes.

Additionally, could you please clarify the relationship with issue #4419 or, if a dedicated issue doesn't exist for this change, describe the problem and solution in the PR description?

This information will help the review process go more smoothly. Thanks!

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Code Review

This pull request introduces a valuable enhancement for tracking tool and sub-agent token usage by adding tool_usage_metadata. The implementation is well-integrated across the data models, telemetry, and agent logic. My review focuses on improving efficiency in the telemetry code, enhancing type safety for better maintainability, and adhering to Python's standard coding style.

Comment on lines +222 to +248
total_prompt_tokens = 0
total_completion_tokens = 0
total_tokens = 0

for tool_name, usage_metadata in function_response_event.tool_usage_metadata.items():
if usage_metadata:
total_prompt_tokens += getattr(usage_metadata, 'prompt_token_count', 0) or 0
total_completion_tokens += getattr(usage_metadata, 'candidates_token_count', 0) or 0
total_tokens += getattr(usage_metadata, 'total_token_count', 0) or 0

if total_tokens > 0:
span.set_attribute(GEN_AI_USAGE_INPUT_TOKENS, total_prompt_tokens)
span.set_attribute(GEN_AI_USAGE_OUTPUT_TOKENS, total_completion_tokens)
span.set_attribute('gcp.vertex.agent.tool_usage_total_tokens', total_tokens)

# Add detailed breakdown as custom attribute
span.set_attribute(
'gcp.vertex.agent.tool_usage_breakdown',
_safe_json_serialize({
name: {
'prompt_tokens': getattr(usage, 'prompt_token_count', 0) or 0,
'completion_tokens': getattr(usage, 'candidates_token_count', 0) or 0,
'total_tokens': getattr(usage, 'total_token_count', 0) or 0,
}
for name, usage in function_response_event.tool_usage_metadata.items()
})
)
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medium

The current implementation iterates over function_response_event.tool_usage_metadata twice: once to calculate the total token counts and again to create the breakdown dictionary. This can be optimized by performing both operations in a single loop. This refactoring will improve efficiency and make the code more concise.

    total_prompt_tokens = 0
    total_completion_tokens = 0
    total_tokens = 0
    breakdown = {}

    for name, usage in function_response_event.tool_usage_metadata.items():
      if not usage:
        continue

      prompt_tokens = getattr(usage, 'prompt_token_count', 0) or 0
      completion_tokens = getattr(usage, 'candidates_token_count', 0) or 0
      usage_total_tokens = getattr(usage, 'total_token_count', 0) or 0

      total_prompt_tokens += prompt_tokens
      total_completion_tokens += completion_tokens
      total_tokens += usage_total_tokens

      breakdown[name] = {
          'prompt_tokens': prompt_tokens,
          'completion_tokens': completion_tokens,
          'total_tokens': usage_total_tokens,
      }

    if total_tokens > 0:
      span.set_attribute(GEN_AI_USAGE_INPUT_TOKENS, total_prompt_tokens)
      span.set_attribute(GEN_AI_USAGE_OUTPUT_TOKENS, total_completion_tokens)
      span.set_attribute('gcp.vertex.agent.tool_usage_total_tokens', total_tokens)

      # Add detailed breakdown as custom attribute
      span.set_attribute(
          'gcp.vertex.agent.tool_usage_breakdown',
          _safe_json_serialize(breakdown),
      )


# Aggregate and record usage for this sub-agent
if usage_metadata_list:
from ..models.llm_response import LlmResponse
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medium

This local import should be moved to the top of the file (e.g., with other from .. imports) to adhere to PEP 8 style guidelines and improve code readability. Top-level imports make dependencies explicit and easier to manage.

super().__init__(invocation_context, event_actions=event_actions)
self.function_call_id = function_call_id
self.tool_confirmation = tool_confirmation
self._tool_usage: dict[str, Any] = {}
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medium

For better type safety, consider using a more specific type hint instead of Any. You can use a forward reference string (e.g., 'types.GenerateContentResponseUsageMetadata') to avoid circular import issues. This would also require adding from google.genai import types to the TYPE_CHECKING block at the top of the file.

Suggested change
self._tool_usage: dict[str, Any] = {}
self._tool_usage: dict[str, "types.GenerateContentResponseUsageMetadata"] = {}

Comment on lines +68 to +99
def set_tool_usage(
self,
tool_name: str,
usage_metadata: Any,
) -> None:
"""Records usage metadata for a tool or sub-agent invocation.

Args:
tool_name: Name of the tool or agent that generated usage.
usage_metadata: Usage metadata object (GenerateContentResponseUsageMetadata
or dict with token counts).
"""
self._tool_usage[tool_name] = usage_metadata

def get_tool_usage(self, tool_name: str) -> Optional[Any]:
"""Retrieves usage metadata for a specific tool.

Args:
tool_name: Name of the tool to retrieve usage for.

Returns:
Usage metadata if recorded, None otherwise.
"""
return self._tool_usage.get(tool_name)

def get_all_tool_usage(self) -> dict[str, Any]:
"""Returns all tool usage metadata recorded in this context.

Returns:
Dictionary mapping tool names to their usage metadata.
"""
return self._tool_usage.copy()
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medium

To improve type safety and code clarity, the type hints for usage_metadata and the return types of these new methods should be made more specific than Any. Using 'types.GenerateContentResponseUsageMetadata' with a forward reference string is recommended. This change should be applied consistently with the update to _tool_usage.

  def set_tool_usage(
      self,
      tool_name: str,
      usage_metadata: "types.GenerateContentResponseUsageMetadata",
  ) -> None:
    """Records usage metadata for a tool or sub-agent invocation.

    Args:
      tool_name: Name of the tool or agent that generated usage.
      usage_metadata: Usage metadata object (GenerateContentResponseUsageMetadata
        or dict with token counts).
    """
    self._tool_usage[tool_name] = usage_metadata

  def get_tool_usage(self, tool_name: str) -> Optional["types.GenerateContentResponseUsageMetadata"]:
    """Retrieves usage metadata for a specific tool.

    Args:
      tool_name: Name of the tool to retrieve usage for.

    Returns:
      Usage metadata if recorded, None otherwise.
    """
    return self._tool_usage.get(tool_name)

  def get_all_tool_usage(self) -> dict[str, "types.GenerateContentResponseUsageMetadata"]:
    """Returns all tool usage metadata recorded in this context.

    Returns:
      Dictionary mapping tool names to their usage metadata.
    """
    return self._tool_usage.copy()

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