[BUG] Tokens in A2A response should be represented as Artifact Update events, not Status Updates

[brycewcole]

Checks

• I have updated to the lastest minor and patch version of Strands
• I have checked the documentation and this is not expected behavior
• I have searched ./issues and there are no duplicates of my issue

Strands Version

1.20

Python Version

3.12

Operating System

macOS 26.2

Installation Method

other

Steps to Reproduce

1. Follow official A2A tutorial at https://docs.aws.amazon.com/bedrock-agentcore/latest/devguide/runtime-a2a.html
2. Use A2A Inspector or similar product to send message.
3. Observe tokens streamed

Expected Behavior

Result messages should be streamed as artifact-update events with append set to true. Please see discussion on A2A's repo here.

Actual Behavior

Tokens were streamed as status-update events.

Additional Context

Possible Solution

Stream as artifact-update with append set to true

Related Issues

No response

[brycewcole]

I made a PR for this as it's causing unexpected behaviour when interacting with other a2a agents.
#1401

[zastrowm]

We should conform to the protocol - we should identify if there are any backwards compatibility concerns here (if folks wrote to our A2A implementation, do we have to preserve those changes) but otherwise it seems like the attached PR makes sense

[brycewcole]

Hey @zastrowm thanks for taking a look. I'm not sure how to preserve backwards compatibility in this case, if you have any ideas please let me know.

[brycewcole]

@Unshure anything I can assist with to get this change across the line?

[Unshure]

Hi @brycewcole, thanks for raising this issue. Did some digging myself and I agree with the conclusion you came to here, we should be emitting artifacts instead of status updates for these streamed events.

To build on @zastrowm 's point, we should make this fix, but we cant make it the default (for now). Anyone who has build something on top of expecting these "status-update" events will end up getting broken if we move ahead with your PR as-is. Instead, lets put it behind a feature flag, and log a warning if the flag isn't set. That way new users can enable this new code path, and old ones are notified of the issue.

I will be adding these comments to your pr as well, and will help shepherd this through for the next release. If you prefer, I can also make the changes and help get this in as well. Let me know what you prefer.

~Thanks!

[brycewcole]

Hey @Unshure, that makes sense to me. If possible I'd like to make the changes, I'll update the PR tomorrow. Thanks for helping me with this.

[Unshure]

Just merged the pull request here, ill close out this issue once this gets released.