A2a Task Lifecycle

A2A Task Lifecycle

Before writing code

Fetch live docs:

1. Fetch https://a2a-protocol.org/latest/specification/ for the Task object schema and state machine
2. Web-search site:github.com a2aproject A2A task lifecycle states for state transition rules
3. Web-search site:github.com a2aproject a2a-samples task for task handling examples
4. Fetch SDK docs for task-related classes and state management utilities

Conceptual Architecture

What a Task Is

A Task is the central unit of work in A2A. It represents a request from a client agent to a server agent, tracks progress through well-defined states, accumulates messages and artifacts, and reaches a terminal state when complete.

Task Structure

Key fields of a Task object:

• id — Unique task identifier (set by client or server)
• status — Current state object containing state enum and optional message
• messages — Array of messages exchanged (if stateTransitionHistory enabled)
• artifacts — Array of output artifacts produced by the agent
• metadata — Optional key-value pairs for custom data

The 9 States

Valid State Transitions

submitted → working
submitted → rejected
submitted → canceled

working → completed
working → failed
working → canceled
working → input-required

input-required → working (when client provides more input)
input-required → canceled

auth-required → working (when auth is provided)
auth-required → canceled

Rules:

• Terminal states (completed, failed, canceled, rejected) are final — no transitions out
• Only the server transitions the task state (except canceled which client can request)
• input-required → working happens when the client sends a follow-up message

Task Creation

Tasks are created implicitly when a client sends a message without a taskId:

1. Client sends message/send or message/stream without taskId
2. Server creates a new task, assigns an ID
3. Task starts in submitted state
4. Server may immediately transition to working or return submitted

Task Continuation

When a client sends a message with an existing taskId:

1. The message is appended to the task's history
2. The server resumes processing
3. State typically transitions from input-required back to working

Artifacts

Artifacts are the outputs of a task:

• Produced during working state
• Each artifact has id, name, optional description, and parts
• Parts can be TextPart, FilePart, or DataPart
• In streaming mode, artifacts are delivered incrementally via TaskArtifactUpdateEvent
• Multiple artifacts can be produced per task

State Transition History

If the agent declares stateTransitionHistory: true in its Agent Card:

• The task object includes a complete history of all state transitions
• Each transition records the state, timestamp, and optional message
• Useful for auditing and debugging

Best Practices

• Always validate state transitions — reject invalid ones with appropriate errors
• Use task IDs that are globally unique (UUIDs recommended)
• Store task state durably for production (not just in-memory)
• Set timeouts for tasks stuck in non-terminal states
• Clean up old tasks to prevent unbounded storage growth
• Include meaningful messages in status updates (not just the state enum)
• Use artifacts for structured outputs, messages for conversational exchanges
• Implement idempotency — handle duplicate messages for the same task gracefully

Fetch the specification for the exact Task object schema, state enum values, and transition validation rules before implementing.