Add new a2a call task to the specification and schema

dsl-reference.md

@@ -14,6 +14,7 @@
         + [gRPC](#grpc-call)
         + [HTTP](#http-call)
         + [OpenAPI](#openapi-call)
+        + [A2A](#a2a-call)
     - [Do](#do)
     - [Emit](#emit)
     - [For](#for)
@@ -320,6 +321,7 @@ Serverless Workflow defines several default functions that **MUST** be supported
 - [gRPC](#grpc-call)
 - [HTTP](#http-call)
 - [OpenAPI](#openapi-call)
+- [A2A](#a2a-call)
 
 ##### AsyncAPI Call
 
@@ -483,6 +485,48 @@ do:
           status: available
 ```
 
+##### A2A Call
+
+The [A2A Call](#a2a-call) enables workflows to interact with AI agents described by [A2A](https://a2a-protocol.org/).
+
+###### Properties
+
+| Name | Type | Required | Description|
+|:--|:---:|:---:|:---|
+| method | `string` | `yes` | The A2A JSON-RPC method to send. |
+| agentCard | [`externalResource`](#external-resource) | `yes` | The AgentCard resource that describes the agent to call. |
+| parameters | `any` | `no` | The parameters for the A2A rpc method. For the `message/send` and `message/stream` methods the parameters `message.messageId` and `message.role` must be automatically set if missing. |
+
+> [!NOTE]
+> The `security` and `securitySchemes` fields of the AgentCard contain authentication requirements and schemes for when communicating with the agent.
+
+> [!NOTE]
+> On success the output is the JSON-RPC result. On failure runtimes must raise an error.
+
+> [!NOTE]
+> For `message/stream` and `tasks/resubscribe` methods the output is a sequentially ordered array of all the result objects.
+
+###### Examples
+
+```yaml
+document:
+  dsl: '1.0.0'
+  namespace: test
+  name: a2a-example
+  version: '0.1.0'
+do:
+  - GenerateReport:
+      call: a2a
+      with:
+        method: message/send
+        agentCard: https://example.com/.well-known/agent-card.json
+        parameters:
+          message:
+            parts:
+              - kind: text
+                text: Generate the Q1 sales report.
+```
+
 #### Do
 
 Serves as a fundamental building block within workflows, enabling the sequential execution of multiple subtasks. By defining a series of subtasks to perform in sequence, the Do task facilitates the efficient execution of complex operations, ensuring that each subtask is completed before the next one begins.

dsl.md

@@ -596,6 +596,7 @@ Serverless Workflow DSL is designed to seamlessly interact with a variety of ser
 - [**gRPC**](dsl-reference.md#grpc-call): Supports Remote Procedure Call (RPC) using gRPC, a high-performance, open-source universal RPC framework. This is suitable for connecting to services that require low-latency and high-throughput communication.
 - [**AsyncAPI**](dsl-reference.md#asyncapi-call): Facilitates interaction with asynchronous messaging protocols. AsyncAPI is designed for event-driven architectures, allowing workflows to publish and subscribe to events.
 - [**OpenAPI**](dsl-reference.md#openapi-call): Enables communication with services that provide OpenAPI specifications, which is useful for defining and consuming RESTful APIs.
+- [**A2A**](dsl-reference.md#a2a-call): Enables interaction with A2A servers (agents described by A2A).
 
 Runtimes **must** raise an error with type `https://serverlessworkflow.io/spec/1.0.0/errors/communication` if and when a problem occurs during a call.
 

schema/workflow.yaml

@@ -430,6 +430,36 @@ $defs:
                 description: Specifies whether redirection status codes (`300–399`) should be treated as errors.
             required: [ document, operationId ]
             unevaluatedProperties: false
+      - title: CallA2A
+        description: Defines the A2A call to perform.
+        $ref: '#/$defs/taskBase'
+        type: object
+        unevaluatedProperties: false
+        required: [ call, with ]
+        properties:
+          call:
+            type: string
+            const: a2a
+          with:
+            type: object
+            title: A2AArguments
+            description: The A2A call arguments.
+            properties:
+              agentCard:
+                $ref: '#/$defs/externalResource'
+                title: WithA2AAgentCard
+                description: The Agent Card that defines the agent to call.
+              method:
+                type: string
+                title: WithA2AMethod
+                description: The A2A method to send.
+              parameters:
+                type: object
+                title: WithA2AParameters
+                description: The parameters object to send with the A2A method.
+                additionalProperties: true
+            required: [ agentCard, method ]
+            unevaluatedProperties: false
       - title: CallFunction
         description: Defines the function call to perform.
         $ref: '#/$defs/taskBase'
@@ -440,7 +470,7 @@ $defs:
           call:
             type: string
             not:
-              enum: ["asyncapi", "grpc", "http", "openapi"]
+              enum: ["asyncapi", "grpc", "http", "openapi", "a2a"]
             description: The name of the function to call.
           with:
             type: object