A2A Key Concepts

( A2A Key Concepts)

• Publicly accessible JSON document (“digital business card”) typically hosted at /.well-known/agent-card
• Details the agent’s identity, operational scope, specific functions, endpoint URL, authentication methods, skill list, and supported A2A capabilities
• Enables discovery: clients retrieve Agent Cards to find remote agents whose capabilities match task requirements

• Stateful unit of work with a unique ID and a defined lifecycle
• Status is continuously updated over multiple rounds of interaction to reflect real-time progress
• Each task is explicitly linked to a ContextId, establishing a traceable association between the task and its execution environment ( Agentic Web 2507.21206)
• Supports long-running operations — agents can subscribe to receive progress updates as the task evolves

• Single communication turn with a user or agent role and a unique messageId
• Contains one or more Parts supporting multimodal interaction
• Each Message includes the current task ID and a list of related task IDs, establishing a bidirectional, structured link between messages and tasks ( Agentic Web 2507.21206)
• Typically conveys dialogue, instructions, or status updates — not the final deliverable

• Fundamental content container within Messages and Artifacts
• Must contain exactly one of the following content types:
  • text — plain textual content
  • raw — binary file data (inline)
  • url — external file reference
  • data — structured JSON values
• Optional fields: mediaType (MIME type), filename, metadata (key-value map)

• Finalized, tangible output generated by an agent during task execution
• Has a unique artifactId and a human-readable name; may contain multiple Part objects
• Represents a deliverable result (document, image, structured data) — distinct from Message which conveys dialogue
• Can be incrementally streamed to clients as it is produced

• Server-generated identifier that logically groups multiple related Task objects
• Creates explicit, traceable links between tasks and their associated execution environments
• Enables cross-task, multi-turn context consistency tracking in complex multi-agent workflows ( Agentic Web 2507.21206)

• HTTP(S) with JSON-RPC 2.0 payload format
• Authentication via standard web security practices (HTTP headers); schemes declared in the Agent Card