Agent skill

An agent skill is an AgentSkill object in an Agent Card’s skills array that describes one distinct capability the agent offers. Clients read skills to decide whether an agent fits a job and what to send it.

Fields. Per a2a.proto, each skill must have an id, a name, a description and a list of tags. It may add examples (sample prompts or inputs), inputModes and outputModes (media types that override the card’s defaultInputModes and defaultOutputModes for this skill), and securityRequirements naming the security schemes the skill needs. The card’s skills field is required, and section 5.7 says required arrays must hold at least one element, so a valid card lists at least one skill.

Illustrative:

{
  "id": "shipment-status",
  "name": "Shipment status",
  "description": "Returns the current status and last scan location for a tracking number.",
  "tags": ["shipping", "tracking"],
  "examples": ["Where is shipment 4471?", "{\"trackingNumber\": \"4471\"}"],
  "inputModes": ["text/plain", "application/json"],
  "outputModes": ["application/json"]
}

How skills are used. A2A has no request field for calling a skill by its id. The client sends an ordinary message, and the remote agent maps it to one of its capabilities (Life of a Task guide). The proto describes skills as largely descriptive: a focused set of behaviours the agent is likely to succeed at. Clear description and examples text therefore matters, because a client agent or a person often chooses an agent by reading it, and the examples show which inputs the agent expects.

Visibility and access. An extended Agent Card may list skills that the public card does not show, for authenticated clients only (section 13.3). Skill-level securityRequirements let one skill require different authentication from the rest of the card.

Sources

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