Add new a2a call task to the specification and schema

[emmanuelallen]

Please specify parts of this PR update:

• Specification
• Schema
• Examples
• Extensions
• Use Cases
• Community
• CTK
• Other

Discussion or Issue link:

#1102

What this PR does:

Adds A2A call task to the specification and schema

Additional information:

Push Notifications
A2A push notification currently only support webhooks as a way to send task update notifications. That said external services can convert those webhook invocations into cloud events that the workflow can listen for. Hopefully at some point A2A can support natively or through an extension sending out push notifications as cloud events directly.

Streaming
I am suggesting streaming be handled by collecting up all the objects until the stream is complete and returning them as an ordered array. In the future we can add a foreach argument similar to AsyncAPI that when set will run a set of tasks for each object received.

[yzhao244]

@ricardozanini @cdavernas Our first PR which covers some of our discussions about A2A lol , please review and if necessary, we can even discuss further on our Thursday meeting :)

[ricardozanini]

Many thanks!! Just a few comments so we can start the conversation.

[ricardozanini]

We have error definitions. I believe we should indicate which errors.

[cdavernas]

@ricardozanini Totally agree. I think a runtime error should be used, and should carry/document the A2A error code.

[emmanuelallen]

@cdavernas So you think that it should be an error of type https://serverlessworkflow.io/spec/1.0.0/errors/runtime. Where do you think the A2A error code should go because I don't think it should be the status code since they are in the range -32700 to -32000?

An example error conversion for a task not found a2a error could be:

type: https://serverlessworkflow.io/spec/1.0.0/errors/runtime
status: 500
title: Task not found, A2A Code: -32001
description: The specified task id does not correspond to an existing or active task. It might be invalid, expired, or already completed and purged.
instance: /do/0/callA2A

[cdavernas]

@emmanuelallen we could put it as status code, because even if it is logically related to http status codes, it is not restricted to them.
I however think your proposal is cleaner. The title could be the code and or a high level description like you proposed, that decision is purely esthetic and should therefore be up to runtimes.

[cdavernas]

@ricardozanini Totally agree. I think a runtime error should be used, and should carry/document the A2A error code.

[ricardozanini]

@emmanuelallen can you take a look on the red CI?

[emmanuelallen]

@emmanuelallen can you take a look on the red CI?

@ricardozanini

The failing CI check is because the a2a example I added to dsl-reference.md doesn't match the 1.0.0 schema (it's checking against https://serverlessworkflow.io/schemas/1.0.0/workflow.yaml). Possible solutions to fix this are:

• Remove the example
• Target a different branch with this PR (it currently targets main but maybe we should create a new branch for 1.1.0)
• Modify the CI check in some way

Do you have a suggestion for how I should resolve it?

[ricardozanini]

@emmanuelallen I think the CI must be fixed to match the schema accordingly to the current branch.

[ricardozanini]

I'll take a look later.

[emmanuelallen]

@ricardozanini

I was wrong yesterday the ci validation is working as it should. The problem was the with the example, I have now fixed it.
I thought it was pulling the schema from https://serverlessworkflow.io/schemas/1.0.0/workflow.yaml but it's actually just the id that is used.

If you approve the workflow it should succeed, I have run it locally and all the tests pass.

[yzhao244]

Speak of examples, lol — it would definitely be super helpful to include solid DSL examples later on to showcase A2A using our Serverless Workflow. This could highlight multi-agent use cases like:

Multi-agent customer service system:
User → Chat Agent → (A2A) → Billing Agent → (A2A) → Payment System → Response
Decision-making chain (e.g., fraud detection):
Payment Agent → (A2A) → Risk Agent → (A2A) → Compliance Agent → Approve/Reject
And so on lol

@emmanuelallen something you'd be up for helping with in a separate PR 😄? :)

New A2A examples:
https://github.com/serverlessworkflow/specification/tree/main/examples

[ricardozanini]

I'd say we can push these examples in a separate PR with a follow up issue linked to this one. Can you do it @emmanuelallen ?

[emmanuelallen]

@yzhao244 @ricardozanini

Yeah, I'm happy to add a few a2a examples as a separate PR tomorrow.

[cdavernas]

LGTM! Cheers ❤️

[yzhao244]

@ricardozanini @cdavernas @emmanuelallen Thanks guys for the review and valuable comments. Now we have A2A in our spec 👍

[ricardozanini]

Let's finish MCP and wrap up to release 1.1.0.