Once you've built the A2A spec surface (Build an A2A Agent) your agent works as a plain A2A responder — workstacean discovers skills, dispatches, tracks tasks, reports to fleet health. But five additional capabilities give the dispatcher, observability, and HITL policy richer signal about each skill:

Extension	Purpose	Direction
cost-v1	Token + wall-time observations → fleet-health + dashboards	workstacean → stores
confidence-v1	Agent self-reported confidence → calibration metrics	your agent → stores
effect-domain-v1	Agent-reported world-state deltas → world.state.delta observability	your agent's response artifacts
blast-v1	Scope-of-effect declaration → HITL/policy routing	your agent's card
hitl-mode-v1	Per-skill approval policy → HITL flow selection	your agent's card

You don't have to implement any of them. You keep working as a plain A2A agent. But each extension unlocks behavior on the workstacean side that only fires when you opt in.

A sixth extension — tool-call-v1 — also exists, but it is not part of this interceptor pack. It is a streaming-only construct (live started → completed/failed tool frames emitted as artifact-update DataParts), not a card-declared before/after interceptor. See its own subsection below.

How workstacean picks them up

All five interceptors are registered unconditionally at workstacean startup (src/index.ts). They run on every outbound A2A call, but self-gate on whether you advertised the URI in your agent card. Workstacean never forces behavior on agents that don't opt in.

Two concrete consequences:

1. Card-only extensions (blast-v1, hitl-mode-v1, effect-domain-v1's declarations): the before hook reads defaultXxxRegistry — populated when SkillBrokerPlugin refreshes your card every 10 min. If you don't declare, the registry miss is a no-op; nothing is stamped on the outbound request.
2. Observation extensions (cost-v1, confidence-v1): the after hook reads the response's data field. If your agent doesn't include usage / confidence fields, the interceptor early-returns without recording.

This means extensions are a zero-risk gradient. Ship your agent, observe fleet health, decide one extension at a time whether it's worth declaring.

A complete example — Quinn's card

Here's what it looks like when an agent opts in to all five:

{
  "name": "quinn",
  "description": "QA engineer — PR review, bug triage, board audit",
  "url": "http://quinn:7870/a2a",
  "version": "1.0.0",
  "provider": { "organization": "protoLabsAI" },
  "capabilities": {
    "streaming": true,
    "pushNotifications": true,
    "stateTransitionHistory": false,
    "extensions": [
      { "uri": "https://proto-labs.ai/a2a/ext/cost-v1" },
      { "uri": "https://proto-labs.ai/a2a/ext/confidence-v1" },

      {
        "uri": "https://proto-labs.ai/a2a/ext/effect-domain-v1",
        "params": {
          "skills": {
            "pr_review":     { "effects": [{ "domain": "pr_pipeline", "path": "data.conflicting", "delta": -1, "confidence": 0.7 }] },
            "bug_triage":    { "effects": [{ "domain": "board", "path": "data.openBugs",   "delta": -1, "confidence": 0.8 }] },
            "board_audit":   { "effects": [] }
          }
        }
      },

      {
        "uri": "https://proto-labs.ai/a2a/ext/blast-v1",
        "params": {
          "skills": {
            "sitrep":          { "radius": "self" },
            "board_audit":     { "radius": "project" },
            "pr_review":       { "radius": "repo" },
            "bug_triage":      { "radius": "project" },
            "security_triage": { "radius": "fleet",  "note": "Can affect the entire fleet's security posture" }
          }
        }
      },

      {
        "uri": "https://proto-labs.ai/a2a/ext/hitl-mode-v1",
        "params": {
          "skills": {
            "sitrep":          { "mode": "autonomous" },
            "board_audit":     { "mode": "notification" },
            "pr_review":       { "mode": "veto", "vetoTtlMs": 300000 },
            "bug_triage":      { "mode": "notification" },
            "security_triage": { "mode": "gated", "reviewer": "operator" }
          }
        }
      }
    ]
  },

  "defaultInputModes": ["text/plain"],
  "defaultOutputModes": ["text/markdown"],
  "skills": [
    { "id": "sitrep",          "name": "Situation Report",  "description": "…" },
    { "id": "board_audit",     "name": "Board Audit",       "description": "…" },
    { "id": "pr_review",       "name": "PR Review",         "description": "…" },
    { "id": "bug_triage",      "name": "Bug Triage",        "description": "…" },
    { "id": "security_triage", "name": "Security Triage",   "description": "…" }
  ],
  "securitySchemes": {
    "apiKey": { "type": "apiKey", "in": "header", "name": "X-API-Key" }
  },
  "security": [{ "apiKey": [] }]
}

Notice:

• cost-v1 + confidence-v1 have no params — declaring the URI is enough to opt in. Your agent still has to include usage / confidence fields on terminal-task data for samples to be recorded.
• effect-domain-v1, blast-v1, hitl-mode-v1 carry params.skills — a map keyed by skill id.
• You don't declare every skill. Only the ones where the default (no declaration → no-op) isn't what you want.

Response-side requirements

For the observation extensions (cost-v1, confidence-v1), you also need to include the expected fields on your terminal Task.data. None of these are required by the A2A spec itself — they're extension-specific conventions:

{
  "status": { "state": "completed" },
  "artifacts": [{ "parts": [{ "kind": "text", "text": "…" }] }],
  "data": {
    "usage": {
      "input_tokens": 3421,
      "output_tokens": 890,
      "cache_read_input_tokens": 0
    },
    "durationMs": 4823,
    "costUsd": 0.0187,
    "confidence": 0.88,
    "confidenceExplanation": "Spec was unambiguous; all tests pass.",
    "success": true
  }
}

• usage.input_tokens / output_tokens — Anthropic-shaped token counts. Cached tokens are optional but tracked when provided.
• durationMs / costUsd — wall-time + dollar cost. costUsd is optional; the cost-v1 consumer can compute from tokens + MODEL_RATES if missing.
• confidence — float in [0, 1]. Required for confidence-v1 sample recording; the extension defensively clamps out-of-range values.
• confidenceExplanation — free-text, surfaced in calibration views.
• success — false explicitly marks a failure even when state: completed. Use this when your skill returns a textual failure message rather than throwing.

For effect-domain-v1 (the response side of it), attach a worldstate-delta DataPart to your terminal task's artifacts when you've mutated shared state:

{
  "artifacts": [
    {
      "parts": [
        { "kind": "text", "text": "Closed 3 stale PRs" },
        {
          "kind": "data",
          "data": {
            "deltas": [
              { "domain": "pr_pipeline", "path": "data.staleOpen", "op": "inc", "value": -3 }
            ]
          },
          "metadata": { "mimeType": "application/vnd.protolabs.worldstate-delta-v1+json" }
        }
      ]
    }
  ]
}

Workstacean's effect-domain-v1 after-hook extracts this and publishes world.state.delta so dashboards and fleet-health observe the mutation in near-real-time.

Registration at workstacean side

For reference — you don't need to do anything here; this is what workstacean runs at startup:

// src/index.ts
registerCostExtension(bus);
registerConfidenceExtension(bus);
registerEffectDomainExtension(bus);
registerBlastExtension();
registerHitlModeExtension();

All five interceptors live in defaultExtensionRegistry and fire on every A2AExecutor.execute() call. Self-gating keeps them safe for non-opt-in agents.

Adoption order — recommended

1. cost-v1 — zero card changes, just emit usage + durationMs in your terminal data. Gets you dashboard + fleet-health visibility for your agent.
2. confidence-v1 — add confidence to your terminal data. Gets your agent into calibration views and unblocks the "high-confidence failure" signal for OutcomeAnalysis.
3. blast-v1 — declare scope per skill on the card. Unlocks policy-driven HITL gating without code changes.
4. hitl-mode-v1 — declare per-skill approval policy. Now your skills route through the right HITL flow by default (autonomous / notification / veto / gated / compound).
5. effect-domain-v1 — include observed worldstate-delta artifacts in your response. Lets workstacean publish world.state.delta so the mutation shows up in observability in ~1s instead of one poll cycle.

Each step is independently useful. The pack composes — e.g. blast-v1 + hitl-mode-v1 together let you say "anything fleet or public radius is gated, reviewer: operator."

tool-call-v1 — a streaming-only extension, not an interceptor

tool-call-v1 is the odd one out: it is not one of the five interceptors above and not something you declare on your card with before / after hooks. It is a live streaming construct — per-tool lifecycle frames (started → completed/failed) emitted on the SSE stream as artifact-update DataParts (MIME application/vnd.protolabs.tool-call-v1+json) while the task is still working.

Key differences from the interceptor pack:

• No card declaration, no self-gating. The five interceptors register on the A2AExecutor dispatch pipeline and fire (then no-op) based on whether you advertised the URI. tool-call-v1 has no dispatch-pipeline registration at all — the frames originate inside the in-process runtime and ride the live stream.
• Streaming, not terminal telemetry. cost-v1 / confidence-v1 read the terminal task's data; tool-call-v1 frames arrive mid-task, one artifact-update per tool turn, keyed by toolCallId so a client can stitch a started to its later completed / failed.
• It's the structured sibling of status.message narration. Workstacean's in-process agents emit both a humanized text line (on status.message) and these structured frames in parallel. Simple clients read the text; rich clients read the frames to draw a tool timeline. See A2A Streaming → Structured tool-call frames.

You don't opt into it from your card. Workstacean's own /a2a endpoint emits it for its in-process agents; a remote A2A agent that wants to offer the same rich timeline streams equivalent artifact-update DataParts (built with emitToolCall) on its own SSE response.

Related

• Build an A2A Agent — the spec-side recipe (task store, webhooks, health)
• Extension reference pages — cost-v1, confidence-v1, effect-domain-v1, blast-v1, hitl-mode-v1, tool-call-v1
• A2A Streaming (SSE) — the streaming pipeline tool-call-v1 frames ride on