A2A

The client pins A2A 1.0 and discovers an Agent Card. It selects a same-origin JSON-RPC interface advertising version 1.0, then supports SendMessage, bounded GetTask polling, SubscribeToTask SSE updates, CancelTask, terminal states, messages, structured parts, and artifacts.

Card and RPC authentication headers are secret references resolved at dispatch and refreshed once after 401. The card URL is subject to network policy, redirects are disabled, and the selected interface must have the same scheme, host, and effective port as that reviewed card URL. Cards, skills, messages, parts, and artifacts are untrusted data. An A2A delegation is an at_most_once remote_agent effect and may require approval.

Configure observation bounds explicitly when the defaults are unsuitable:

a2aPeers:
  worker:
    cardUrl: https://agents.example.test/card.json
    protocolVersion: "1.0"
    timeoutSeconds: 10
    maxPolls: 100
    pollIntervalMs: 100

The runtime persists the call identity and remote task ID before polling. A lost polling or streaming connection can refresh the same-origin Agent Card once and resume observation of that task. It never sends another SendMessage. If the submission response itself was ambiguous and no task ID was received, automatic continuation is refused.

Continue a known uncertain task with:

agentctl effects --db .agentctl/runtime.db continue-remote EFFECT_ID \
  --actor operator --reason "resume persisted task" --approved

The command observes the existing remote task, ingests completed inline or same-origin URL artifacts into the local CAS, and records an applied effect reconciliation. A following failed-only retry materializes that completed boundary and executes only its descendants. Repair, retry, and replay retain source-linked protocol evidence without submitting the task again.

Each artifact part must contain exactly one of text, raw, data, or url. Retrieval is bounded to 16 MiB per part and same-origin URL policy. Mock peers cover known-task continuation, ambiguous-send refusal, task polling, artifacts, streaming fallback, cancellation, protocol mismatch, origin enforcement, timeout, and zero-resubmission retry.

Canonical source: docs/A2A.md. Verified against agentctl commit 2aeaa88fba71162206b5f08f5bda4f0150247e4f.