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Documentation Index

Fetch the complete documentation index at: https://docs.adcontextprotocol.org/llms.txt

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This guide shows how task responses are wrapped by different protocol layers. The same task response payload appears in different envelope formats depending on the protocol.

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The Separation Principle

Task Response (what you implement): 
{
  "products": [
    {
      "product_id": "ctv_premium",
      "name": "CTV Premium",
      "pricing": { "model": "cpm", "amount": 45.00, "currency": "USD" }
    }
  ]
}
Protocol Envelope (what the protocol layer adds):
  • Session tracking (context_id)
  • Async operation tracking (task_id, status)
  • Human-readable message
  • Webhook configuration

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MCP (Model Context Protocol)

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Successful Response

{
  "content": [
    {
      "type": "text",
      "text": "Found 3 products matching your criteria for CTV inventory"
    },
    {
      "type": "resource",
      "resource": {
        "uri": "adcp://response/get_products",
        "mimeType": "application/json",
        "text": "{\"products\": [...]}"
      }
    }
  ],
  "metadata": {
    "context_id": "ctx_abc123",
    "status": "completed"
  }
}
Key Points:
  • Human message in content[].text
  • Task payload in content[].resource.text (includes application-level context when present)
  • Protocol metadata in metadata
  • MCP doesn’t expose task_id - async is handled via MCP’s progress notifications

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Async Response (Long-Running Operation)

{
  "content": [
    {
      "type": "text",
      "text": "Creating media buy. This will take 5-10 minutes."
    }
  ],
  "metadata": {
    "context_id": "ctx_def456",
    "status": "working"
  },
  "isError": false,
  "_meta": {
    "progressToken": "prog_789"
  }
}
MCP sends progress notifications separately: 
{
  "method": "notifications/progress",
  "params": {
    "progressToken": "prog_789",
    "progress": 50,
    "total": 100
  }
}

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A2A (Agent-to-Agent Protocol)

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Successful Response

{
  "task": {
    "task_id": "task_123",
    "state": "completed",
    "artifacts": [
      {
        "artifactId": "artifact-get-products-abc123",
        "name": "get_products_result",
        "content_type": "application/json",
        "content": "{\"products\": [...]}"
      }
    ]
  },
  "messages": [
    {
      "role": "assistant",
      "content": "Found 3 products matching your criteria for CTV inventory"
    }
  ],
  "context_id": "ctx_abc123"
}
Key Points:
  • Human message in messages[] array
  • Task payload in task.artifacts[]
  • Explicit task_id for tracking
  • state field for task status
  • A2A native support for async via task state machine

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Async Response (Submitted)

{
  "task": {
    "task_id": "task_456",
    "state": "working",
    "estimated_duration_seconds": 600,
    "webhook_url": "https://buyer.example.com/webhooks/a2a"
  },
  "messages": [
    {
      "role": "assistant",
      "content": "Creating media buy. This will take 5-10 minutes. I'll notify you via webhook when complete."
    }
  ],
  "context_id": "ctx_def456"
}
Later, webhook notification: 
{
  "task_id": "task_456",
  "state": "completed",
  "artifacts": [
    {
      "artifactId": "artifact-media-buy-webhook-def456",
      "name": "create_media_buy_result",
      "content_type": "application/json",
      "content": "{\"media_buy_id\": \"mb_789\", ...}"
    }
  ]
}

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Protocol Compliance Testing

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Validating Your Implementation

Here’s how to test that your protocol adapter is compliant: 
import { validateTaskResponse } from '@adcp/schemas';
import { mcpAdapter, a2aAdapter } from './protocol-adapters';

// 1. Test task response schema (domain-specific)
const taskResponse = getProductsHandler(request);
const isValid = validateTaskResponse('get_products', taskResponse);
assert(isValid, 'Task response must match schema');

// 2. Test protocol envelope construction
const mcpEnvelope = mcpAdapter.wrap({
  contextId: 'ctx_123',
  status: 'completed',
  message: 'Found 3 products',
  payload: taskResponse
});

// Verify MCP structure
assert(mcpEnvelope.content, 'MCP must have content array');
assert(mcpEnvelope.content[0].type === 'text', 'First content must be text');
assert(mcpEnvelope.metadata.context_id, 'Must include context_id');

// 3. Test A2A envelope
const a2aEnvelope = a2aAdapter.wrap({
  contextId: 'ctx_123',
  taskId: 'task_456',
  status: 'completed',
  message: 'Found 3 products',
  payload: taskResponse
});

assert(a2aEnvelope.task.task_id, 'A2A must have task_id');
assert(a2aEnvelope.messages[0].role === 'assistant', 'Must have assistant message');

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Testing Cross-Protocol Compatibility

// The same task response should work in all protocols
const taskResponse = { products: [...] };

test('task response works across all protocols', () => {
  // All adapters should successfully wrap the response
  const mcp = mcpAdapter.wrap({ payload: taskResponse, status: 'completed' });
  const a2a = a2aAdapter.wrap({ payload: taskResponse, status: 'completed' });

  // All should extract the same payload
  assert.deepEqual(
    mcpAdapter.unwrap(mcp),
    a2aAdapter.unwrap(a2a)
  );
});

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Implementation Checklist

When implementing AdCP support:
  • Task handlers return ONLY domain data (no message, context_id, status, task_id)
  • Protocol adapter adds envelope fields based on protocol requirements
  • Message generation creates human-readable text from task results
  • Context tracking maintains conversation state across operations
  • Async support handles long-running operations appropriately
  • Error mapping translates domain errors to protocol-specific formats
  • Schema validation validates task responses against AdCP schemas
  • Cross-protocol tests verify same task response works everywhere

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Common Pitfalls

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❌ Don’t Do This

// Task handler includes protocol fields - WRONG!
function getProducts(request) {
  return {
    status: 'completed',  // ❌ Protocol concern
    message: 'Found 3',   // ❌ Protocol concern
    products: [...]       // ✅ Domain data
  };
}

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✅ Do This Instead

// Task handler returns only domain data
function getProducts(request) {
  return {
    products: [...]  // ✅ Domain data only
  };
}

// Protocol adapter adds envelope
function wrapResponse(taskResponse, metadata) {
  return {
    status: metadata.status,
    message: generateMessage(taskResponse),
    context_id: metadata.contextId,
    data: taskResponse  // Task response becomes payload
  };
}

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Summary

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Webhook

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Task Status Webhook Payload

{
  "operation_id": "op_456",
  "task_id": "task_456",
  "task_type": "create_media_buy",
  "status": "completed",
  "message": "Media buy created successfully",
  "timestamp": "2025-01-22T10:30:00Z",
  "result": {
    "media_buy_id": "mb_123",
    "buyer_ref": "campaign_2024_q1",
    "packages": [
      { "package_id": "pkg_001" }
    ]
  }
}
ProtocolMessage LocationPayload LocationAsync Mechanism
MCPcontent[].textcontent[].resource.textProgress notifications
A2Amessages[].contenttask.artifacts[].contentTask state + webhooks
The envelope schema (/schemas/v2/core/protocol-envelope.json) provides the conceptual model. This document shows how each protocol actually serializes that model on the wire.