Call Task

Enables the execution of a specified function within a workflow, allowing seamless integration with custom business logic or external services.

Properties

call

string

required

The name of the function to call.

with

map

A name/value mapping of the parameters to call the function with.

Built-in Functions

Serverless Workflow defines several default functions that MUST be supported by all implementations and runtimes:

AsyncAPI - Interact with AsyncAPI-described services
gRPC - Communicate via gRPC protocol
HTTP - Make HTTP requests
OpenAPI - Call OpenAPI-described operations
A2A - Interact with AI agents
MCP - Interact with Model Context Protocol servers

AsyncAPI Call

The AsyncAPI Call enables workflows to interact with external services described by AsyncAPI.

Properties

document

externalResource

required

The AsyncAPI document that defines the operation to call.

channel

string

required

The name of the channel on which to perform the operation. Used only in case the referenced document uses AsyncAPI v2.6.0.

operation

string

required

A reference to the AsyncAPI operation to call. Used only in case the referenced document uses AsyncAPI v3.0.0.

server

asyncApiServer

An object used to configure the server to call the specified AsyncAPI operation on. If not set, defaults to the first server matching the operation’s channel.

protocol

string

The protocol to use to select the target server. Ignored if server has been set.Supported values: amqp, amqp1, anypointmq, googlepubsub, http, ibmmq, jms, kafka, mercure, mqtt, mqtt5, nats, pulsar, redis, sns, solace, sqs, stomp, ws

message

asyncApiMessage

An object used to configure the message to publish using the target operation. Required if subscription has not been set.

subscription

asyncApiSubscription

An object used to configure the subscription to messages consumed using the target operation. Required if message has not been set.

authentication

string | authentication

The authentication policy, or the name of the authentication policy, to use when calling the AsyncAPI operation.

Example

document:
  dsl: '1.0.3'
  namespace: test
  name: asyncapi-example
  version: '0.1.0'
do:
  - publishGreetings:
      call: asyncapi
      with:
        document:
          endpoint: https://fake.com/docs/asyncapi.json
        operation: greet
        server:
          name: greetingsServer
          variables:
            environment: dev
        message:
          payload:
            greetings: Hello, World!
          headers:
            foo: bar
            bar: baz
  - subscribeToChatInbox:
      call: asyncapi
      with:
        document:
          endpoint: https://fake.com/docs/asyncapi.json
        operation: chat-inbox
        protocol: http
        subscription:
          filter: ${ . == $workflow.input.chat.roomId }
          consume:
            amount: 5
            for:
              seconds: 10

gRPC Call

The gRPC Call enables communication with external systems via the gRPC protocol.

Properties

proto

externalResource

required

The proto resource that describes the gRPC service to call.

service.name

string

required

The name of the gRPC service to call.

service.host

string

required

The hostname of the gRPC service to call.

service.port

integer

The port number of the gRPC service to call.

service.authentication

authentication

The authentication policy to use when calling the gRPC service.

method

string

required

The name of the gRPC service method to call.

arguments

map

A name/value mapping of the method call’s arguments, if any.

Example

document:
  dsl: '1.0.3'
  namespace: test
  name: grpc-example
  version: '0.1.0'
do:
  - greet:
      call: grpc
      with:
        proto:
          endpoint: file://app/greet.proto
        service:
          name: GreeterApi.Greeter
          host: localhost
          port: 5011
        method: SayHello
        arguments:
          name: ${ .user.preferredDisplayName }

HTTP Call

The HTTP Call enables workflows to interact with external services over HTTP.

Properties

method

string

required

The HTTP request method.

endpoint

string | endpoint

required

An URI or an object that describes the HTTP endpoint to call.

headers

map

A name/value mapping of the HTTP headers to use, if any.

body

any

The HTTP request body, if any.

query

map[string, any]

A name/value mapping of the query parameters to use, if any.

output

string

default:"content"

The HTTP call’s output format.Supported values:

raw - Outputs the base-64 encoded HTTP response content, if any
content - Outputs the content of HTTP response, possibly deserialized
response - Outputs the HTTP response

redirect

boolean

default:false

Specifies whether redirection status codes (300-399) should be treated as errors.

If false, runtimes must raise an error for response status codes outside the 200-299 range
If true, runtimes must raise an error for status codes outside the 200-399 range

Example

document:
  dsl: '1.0.3'
  namespace: test
  name: http-example
  version: '0.1.0'
do:
  - getPet:
      call: http
      with:
        method: get
        endpoint: https://petstore.swagger.io/v2/pet/{petId}

OpenAPI Call

The OpenAPI Call enables workflows to interact with external services described by OpenAPI.

Properties

document

externalResource

required

The OpenAPI document that defines the operation to call.

operationId

string

required

The id of the OpenAPI operation to call.

parameters

map

A name/value mapping of the parameters, if any, of the OpenAPI operation to call.

authentication

The authentication policy to use when calling the OpenAPI operation.

output

string

default:"content"

The OpenAPI call’s output format.Supported values:

raw - Outputs the base-64 encoded HTTP response content, if any
content - Outputs the content of HTTP response, possibly deserialized
response - Outputs the HTTP response

redirect

boolean

default:false

Specifies whether redirection status codes (300-399) should be treated as errors.

Example

document:
  dsl: '1.0.3'
  namespace: test
  name: openapi-example
  version: '0.1.0'
do:
  - findPet:
      call: openapi
      with:
        document:
          endpoint: https://petstore.swagger.io/v2/swagger.json
        operationId: findPetsByStatus
        parameters:
          status: available

A2A Call

The A2A Call enables workflows to interact with AI agents described by A2A.

Properties

method

string

required

The A2A JSON-RPC method to send.Supported values: message/send, message/stream, tasks/get, tasks/list, tasks/cancel, tasks/resubscribe, tasks/pushNotificationConfig/set, tasks/pushNotificationConfig/get, tasks/pushNotificationConfig/list, tasks/pushNotificationConfig/delete, agent/getAuthenticatedExtendedCard

agentCard

externalResource

The AgentCard resource that describes the agent to call. Required if server has not been set.

server

string | endpoint

An URI or an object that describes the A2A server to call. Required if agentCard has not been set.

parameters

map | string

The parameters for the A2A RPC method. For message/send and message/stream methods, runtimes must default message.messageId to a uuid and message.role to user. Supports runtime expressions.

The security and securitySchemes fields of the AgentCard contain authentication requirements and schemes for communicating with the agent.On success, the output is the JSON-RPC result. On failure, runtimes must raise an error with type https://serverlessworkflow.io/spec/1.0.0/errors/runtime.For message/stream and tasks/resubscribe methods, the output is a sequentially ordered array of all result objects.

Example

document:
  dsl: '1.0.3'
  namespace: test
  name: a2a-example
  version: '0.1.0'
do:
  - GenerateReport:
      call: a2a
      with:
        method: message/send
        agentCard:
          endpoint: https://example.com/.well-known/agent-card.json
        parameters:
          message:
            parts:
              - kind: text
                text: Generate the Q1 sales report.

MCP Call

The MCP Call enables workflows to interact with Model Context Protocol (MCP) servers.

Properties

protocolVersion

string

default:"2025-06-18"

required

The version of the MCP protocol to use.

method

string

required

The MCP method to call.Supported values:

tools/list - Lists available tools
tools/call - Calls a specific tool
prompts/list - Lists available prompts
prompts/get - Gets a specific prompt
resources/list - Lists available resources
resources/read - Reads a specific resource
resources/templates/list - Lists available resource templates

parameters

map | string

The MCP method parameters. Supports runtime expressions.

timeout

string | duration

The duration after which the MCP call times out.

transport

required

The transport to use to perform the MCP call.

client

Describes the client used to perform the MCP call.

Before making any MCP requests, runtimes must first send an initialize call to establish the connection. In most cases, client libraries handle this initialization automatically.

On success, the output of the call is the JSON-RPC result. On failure, runtimes must raise an error with type https://serverlessworkflow.io/spec/1.0.0/errors/runtime.

Example

document:
  dsl: '1.0.3'
  namespace: test
  name: mcp-example
  version: '0.1.0'
do:
  - publishMessageToSlack:
      call: mcp
      with:
        method: tools/call
        parameters:
          name: conversations_add_message
          arguments:
            channel_id: 'C1234567890'
            thread_ts: '1623456789.123456'
            payload: 'Hello, world! :wave:'
            content_type: text/markdown
        transport:
          stdio:
            command: npx
            arguments: [ slack-mcp-server@latest, --transport, stdio ]
            environment:
              SLACK_MCP_XOXP_TOKEN: xoxp-xv6Cv3jKqNW8esm5YnsftKwIzoQHUzAP

Simple Example

document:
  dsl: '1.0.3'
  namespace: test
  name: call-example
  version: '0.1.0'
do:
  - getPet:
      call: http
      with:
        method: get
        endpoint: https://petstore.swagger.io/v2/pet/{petId}

Workflow Definition

Tasks

Integration & Protocols

Configuration

Properties

Built-in Functions

AsyncAPI Call

Properties

Example

gRPC Call

Properties

Example

HTTP Call

Properties

Example

OpenAPI Call

Properties

Example

A2A Call

Properties

Example

MCP Call

Properties

Example

Simple Example

Build docs developers (and LLMs) love

Workflow Definition

Tasks

Integration & Protocols

Configuration

​Properties

​Built-in Functions

​AsyncAPI Call

​Properties

​Example

​gRPC Call

​Properties

​Example

​HTTP Call

​Properties

​Example

​OpenAPI Call

​Properties

​Example

​A2A Call

​Properties

​Example

​MCP Call

​Properties

​Example

​Simple Example

Build docs developers (and LLMs) love

Properties

Built-in Functions

AsyncAPI Call

Properties

Example

gRPC Call

Properties

Example

HTTP Call

Properties

Example

OpenAPI Call

Properties

Example

A2A Call

Properties

Example

MCP Call

Properties

Example

Simple Example