Agent Card inaccessible via legacy path after a2a-sdk upgrade

[ftnext]

Describe the bug
When using get_fast_api_app(a2a=True)1 to serve a remote agent, the Agent Card becomes inaccessible via the legacy path /.well-known/agent.json after upgrading a2a-sdk from v0.2.x to v0.3.x. This breaks backward compatibility for existing clients that expect the old Agent Card path.

To Reproduce
Steps to reproduce the behavior:

1. Install a2a-sdk v0.2.x and google-adk
2. Create a remote agent using get_fast_api_app(a2a=True)
3. Verify Agent Card is accessible at /.well-known/agent.json
4. Upgrade a2a-sdk to v0.3.x
5. Restart the agent server
6. Access /.well-known/agent.json - returns 404 error
7. Access /.well-known/agent-card.json - works correctly

Expected behavior
The Agent Card should be accessible via both the new path (/.well-known/agent-card.json) and the legacy path (/.well-known/agent.json) to maintain backward compatibility, similar to how a2a-sdk itself handles this transition.

Screenshots
N/A

Desktop (please complete the following information):

• OS: macOS
• Python version: 3.13.0
• ADK version: 1.10.0
• a2a-sdk version: v0.3.1

Model Information:
Any model can be used to reproduce this issue.

Additional context

• The issue stems from ADK's dependency on AGENT_CARD_WELL_KNOWN_PATH constant from a2a-sdk, which changed from /.well-known/agent.json to /.well-known/agent-card.json in v0.3.0
  • https://github.com/a2aproject/a2a-python/pull/352/files#diff-92e9501133b553aecce30b3cb1c9175690e6bb589e1959518bf7880ff246c7fd
• a2a-sdk v0.3.x maintains backward compatibility by serving both paths and shows a deprecation warning for the old path
• ADK currently only serves the path defined by the current AGENT_CARD_WELL_KNOWN_PATH constant
• This breaks existing RemoteA2aAgent implementations that hardcode the old Agent Card URL
• Relevant code: fast_api.py#L375-L378
• Related blog post: https://nikkie-ftnext.hatenablog.com/entry/a2a-spec-0.3.0-breaking-change-agent-card-path (in Japanese)

Proposed solution
Add support for PREV_AGENT_CARD_WELL_KNOWN_PATH from a2a-sdk to serve the Agent Card on both the current and legacy paths, maintaining backward compatibility similar to a2a-sdk's approach.

[weimeilin79]

Same issue + 1. I have multiple agents, some run on legacy, some has new lib, it should support backward compatibility