SMELT

Custom A2A

Custom A2A lets a workflow delegate a task to an external agent that speaks the A2A (Agent2Agent) protocol. Connect an agent, write the task in plain language, and call it from a Custom A2A block.

What it is

A2A is an open protocol for one agent to hand work to another. Unlike Custom MCP, which calls a named tool with structured parameters, a Custom A2A connection sends the target agent a natural-language task and lets the agent decide how to carry it out. Studio validates the agent address, lets you optionally discover the skills it advertises, and passes your task to it when the block runs.

Custom A2A reaches agents outside SMELT. To have your own agents delegate to each other inside a run, attach them as sub-agents in the agent editor instead; see Agent Blocks.

Create a connection

Open a Custom A2A block’s configuration panel and choose Saved connection, then New connection. Enter:

  • The agent URL
  • An optional auth header, for agents that require one
  • An optional display name

The agent URL is checked before the connection is saved. The auth header, if you provide one, is stored encrypted with the connection and is never written into the workflow definition. A saved connection can be reused across any number of blocks and workflows.

For an agent that does not need an auth header, choose Public URL instead and enter the address directly in the block. This is quicker for a quick test but is not saved for reuse.

Add the block

Press the + handle on the block you want it to follow and choose Custom A2A from the palette. Pick its source: a saved connection or a public URL, as above.

Write the message

In the Message field, describe the task to delegate to the agent in plain language, for example “Summarise this document and list the open questions”. The field supports {{ref}} references to earlier blocks, so you can build the task from data already in the workflow. When the block runs, this message is sent to the agent as its task; the agent decides how to complete it.

Discover skills

With a source selected, click Discover skills. Studio queries the agent and lists the skills it advertises as clickable chips. Clicking a skill sets it as a hint alongside your message; clicking it again clears the hint. Discovery is advisory only: it is never required, and if no skill hint is set, the agent decides which of its skills to use for the task.

Pro and Business only

Custom A2A is available on the Pro and Business tiers. A Free workspace gets a 402 response with code TIER_LIMIT_CUSTOM_AGENT_TO_AGENT when it tries to save a connection. See Billing & Limits to upgrade.

Limits

  • The agent URL must be a public address; Studio blocks addresses that resolve to private or internal networks.
  • A block uses exactly one source: a saved connection or a public URL, never both at once.
  • Skill discovery and the task call run within your workflow’s usual execution limits. See Billing & Limits.

Next: Custom MCP or Integrations