Fix A2A metadata key prefix for cross-language compatibility

[fsonntag]

Fix A2A metadata key prefix for cross-language compatibility

Problem

When evaluating Java agents from Python via A2A protocol, tool calls were not being tracked because metadata keys didn't match Python's expectations.

Root cause: Python expects metadata keys with adk_ prefix (e.g., adk_type), but Java was sending unprefixed keys (e.g., type).

Solution

Added ADK_METADATA_KEY_PREFIX = "adk_" constant and prefixed metadata keys to align with Python's convention:

• A2A_DATA_PART_METADATA_TYPE_KEY → "adk_type"
• A2A_DATA_PART_METADATA_IS_LONG_RUNNING_KEY → "adk_is_long_running"

Metadata values (like "function_call") remain unprefixed as expected.

Changes

File: a2a/src/main/java/com/google/adk/a2a/converters/PartConverter.java

• Added ADK_METADATA_KEY_PREFIX constant at line 33
• Updated metadata key constants at lines 36-38 to use prefix
• Value constants (lines 39-43) correctly remain unprefixed

Python Reference

Python's implementation using _get_adk_metadata_key() helper:
https://github.com/google/adk/blob/main/adk-python/src/google/adk/a2a/converters/part_converter.py#L36-L41

Python adds prefix when reading metadata:
https://github.com/google/adk/blob/main/adk-python/src/google/adk/a2a/converters/part_converter.py#L93-L98

Testing

Verified with cross-language A2A evaluation showing tool calls now properly tracked.

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[gemini-code-assist]

Summary of Changes

Hello @fsonntag, 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 addresses a critical cross-language compatibility issue within the A2A protocol, specifically when Java agents are evaluated by Python. By standardizing the metadata key prefixes used by Java to match Python's expectations, it ensures that tool calls are correctly tracked across the two languages, thereby improving the robustness and interoperability of the A2A system.

Highlights

• Cross-language A2A compatibility fix: Resolves an issue where Java agents evaluated from Python failed to track tool calls due to a mismatch in A2A metadata key prefixes.
• Metadata key prefixing: Introduces an "adk_" prefix for specific A2A metadata keys ("type" and "is_long_running") in Java to align with Python's expected format.
• New constant introduced: A new constant, "ADK_METADATA_KEY_PREFIX", was added to centralize the "adk_" prefix for metadata keys.

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[gemini-code-assist]

Code Review

This pull request correctly addresses a cross-language compatibility issue between the Java and Python A2A implementations by adding the expected adk_ prefix to metadata keys. The use of a dedicated constant for the prefix is a good choice for maintainability. The change is focused, well-explained, and effectively resolves the issue. I have one suggestion to improve the clarity of a new comment.