A2A

The A2A (Agent-to-Agent) protocol lets Sim call external AI agents that expose an A2A-compatible endpoint. Use it to connect your workflows to remote agents — LLM-powered bots, microservices, and other AI systems — through a standardized message format.

With the A2A block you can:

Send messages to external agents: Pass prompts, structured data, or files to a remote agent and get its response.
Track and cancel tasks: Poll the state of a long-running task or request its cancellation.
Discover capabilities: Fetch an agent's Agent Card to inspect its skills, capabilities, and supported modes.

You need the external agent's endpoint URL and, if it requires authentication, an API key.

Usage Instructions

Use the A2A (Agent-to-Agent) protocol to call external AI agents over the latest A2A specification.

Tools

`a2a_send_message`

Send a message to an external A2A agent and return its response.

Input

Parameter	Type	Required	Description
`agentUrl`	string	Yes	The A2A agent endpoint URL
`message`	string	Yes	The message text to send
`data`	string	No	Optional structured JSON data to attach (JSON string or object)
`files`	array	No	Files to attach, uploaded or referenced from a previous block
`taskId`	string	No	Existing task ID to continue
`contextId`	string	No	Conversation context ID to continue
`apiKey`	string	No	API key for authentication (if required)

Output

Parameter	Type	Description
`content`	string	Agent response text
`taskId`	string	Task identifier
`contextId`	string	Conversation/context identifier
`state`	string	Task lifecycle state: `submitted`, `working`, `input-required`, `auth-required`, `completed`, `failed`, `canceled`, or `rejected`
`artifacts`	array	Structured task output artifacts

`a2a_get_task`

Retrieve the current state and result of an A2A task.

Input

Parameter	Type	Required	Description
`agentUrl`	string	Yes	The A2A agent endpoint URL
`taskId`	string	Yes	The task ID to retrieve
`historyLength`	number	No	Maximum number of history messages to include
`apiKey`	string	No	API key for authentication (if required)

Output

Parameter	Type	Description
`content`	string	Agent response text
`taskId`	string	Task identifier
`contextId`	string	Conversation/context identifier
`state`	string	Task lifecycle state
`artifacts`	array	Structured task output artifacts

`a2a_cancel_task`

Request cancellation of an in-progress A2A task.

Input

Parameter	Type	Required	Description
`agentUrl`	string	Yes	The A2A agent endpoint URL
`taskId`	string	Yes	The task ID to cancel
`apiKey`	string	No	API key for authentication (if required)

Output

Parameter	Type	Description
`taskId`	string	Task identifier
`state`	string	Task lifecycle state after cancellation
`canceled`	boolean	Whether the task reached the canceled state

`a2a_get_agent_card`

Fetch the Agent Card (discovery document) for an external A2A agent.

Input

Parameter	Type	Required	Description
`agentUrl`	string	Yes	The A2A agent endpoint URL
`apiKey`	string	No	API key for authentication (if required)

Output

Parameter	Type	Description
`name`	string	Agent display name
`description`	string	Agent description
`url`	string	Agent endpoint URL
`version`	string	The agent's own version
`protocolVersion`	string	A2A protocol version the agent exposes
`capabilities`	json	Agent capability flags
`skills`	array	Skills the agent can perform
`defaultInputModes`	array	Default accepted input media types
`defaultOutputModes`	array	Default produced output media types

Notes

Category: blocks
Type: a2a
Send Message blocks until the agent reaches a terminal (completed, failed, canceled, rejected) or interrupted (input-required, auth-required) state. Use Get Task to poll a task you continue later, and branch on state.
Task IDs are scoped to the external agent, not to Sim. Anyone who knows an agent URL and task ID can read or cancel that task unless the agent enforces its own authentication — set an API key for agents that require one.

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