AMTP Protocol Specification v1.0 (Draft)

Introduction
Protocol Architecture
Addressing and Discovery
Message Format
Transport Layer
Schema Framework Integration
Multi-Agent Coordination
Reliability and Delivery Guarantees
Security Model
Gateway Implementation
Protocol Negotiation and Bridging
API Specifications
Error Handling
Compliance and Conformance

1. Introduction

1.1 Protocol Overview

The Agent Message Transfer Protocol (AMTP) is a federated, asynchronous communication protocol designed for reliable agent-to-agent communication across organizational boundaries. AMTP extends the email addressing model with native support for structured data, multi-agent coordination, and guaranteed delivery semantics.

1.2 Key Features

Universal addressing using agent@domain format
Transparent protocol upgrade with SMTP bridging
At-least-once delivery with idempotency guarantees
Standard schema integration via AGNTCY framework
Multi-agent workflow coordination
Federated architecture with DNS-based discovery

1.3 Protocol Versions

This document specifies AMTP Protocol Version 1.0. Version negotiation is handled through DNS TXT records and HTTP headers.

2. Protocol Architecture

2.1 System Components

┌─────────────────┐     ┌─────────────────┐     ┌─────────────────┐
│   Agent A       │────▶│  AMTP Gateway   │────▶│  AMTP Gateway   │
│ (sender.com)    │     │  (sender.com)   │     │ (receiver.com)  │
└─────────────────┘     └─────────────────┘     └─────────────────┘
                                 │                        │
                                 ▼                        ▼
                        ┌─────────────────┐     ┌─────────────────┐
                        │ Message Queue   │     │   Agent B       │
                        │ (Optional)      │     │ (receiver.com)  │
                        └─────────────────┘     └─────────────────┘

Messages MAY include a logical schema identifier (e.g., schema:"agntcy:commerce.order.v2"), but AMTP Core makes no assumptions about how that identifier is minted, stored, versioned, or resolved. Gateways and participants MAY validate payloads against schemas using any mechanism (or none).

AMTP does not mandate any particular schema or agent registry. Each implementation is free to choose their own registry architecture: centralized, federated, file-based, database-driven, or any other approach that meets their organizational needs.

2.2 Protocol Stack

┌─────────────────────────────────────┐
│        Application Layer            │ ← Agent Logic
├─────────────────────────────────────┤
│         AMTP Protocol               │ ← Message Format & Coordination
├─────────────────────────────────────┤
│      Transport Layer (HTTPS)       │ ← Reliable Delivery
├─────────────────────────────────────┤
│       Discovery Layer (DNS)        │ ← Addressing & Capabilities
├─────────────────────────────────────┤
│         Network Layer (IP)          │ ← Standard Internet
└─────────────────────────────────────┘

3. Addressing and Discovery

3.1 Address Format

AMTP uses the standard email address format: local-part@domain

Examples:

3.2 DNS Discovery

3.2.1 AMTP Capability Advertisement

Domain operators MUST publish a DNS TXT record at _amtp.{domain} to advertise AMTP capabilities:

_amtp.example.com. IN TXT "v=amtp1;gateway=https://amtp.example.com:443"

Parameters:

v: Protocol version (required)
gateway: AMTP gateway endpoint URL (required)
auth: Authentication methods (cert, oauth, apikey) (optional)
max-size: Maximum message size in bytes (optional, default: 10MB)

3.2.2 Discovery Algorithm

Query DNS TXT record for _amtp.{domain}
If AMTP record exists and is valid, use AMTP protocol
Otherwise, fall back to standard MX record lookup for SMTP
Cache discovery results with TTL from DNS record

3.3 Gateway Endpoint Resolution

The gateway parameter in the DNS TXT record specifies the HTTPS endpoint for AMTP message delivery. The endpoint MUST:

Use HTTPS with valid TLS certificate for the domain
Accept HTTP POST requests to /v1/messages and GET requests to /v1/discovery/agents, /v1/inbox/*
Support HTTP/2 for improved performance

3.4 Agent Discovery (Optional)

Agent discovery provides flexibility for different messaging scenarios. Gateways MAY implement agent discovery endpoints to support various use cases.

3.4.1 Use Cases

Direct Addressing: When senders already know the destination address (e.g., [email protected]), agent discovery is not required. Messages can be sent directly to known addresses.

True Discovery: When senders need to find appropriate agents for a domain (e.g., "who handles purchase orders at company.com"), discovery enables programmatic agent selection based on capabilities, roles, or supported schemas.

3.4.2 Discovery Endpoint

Endpoint: GET https://{gateway}/v1/discovery/agents

Query Parameters:

address (optional): Query specific agent address for validation (e.g., [email protected])
role (optional): Filter by business role
delivery_mode (optional): Filter agents by delivery mode (pull or push)
active_only (optional): Only return agents active within the last 30 days (true or false)
capability (optional): Filter by semantic capabilities
schema (optional): Filter by supported schema patterns (e.g., agntcy:commerce.*)

Response Examples:

Full agent listing:

{
  "agents": [
    {
      "address": "[email protected]",
      "delivery_mode": "pull",
      "capabilities": ["sales_inquiries", "product_demos"],
      "supported_schemas": ["agntcy:commerce.inquiry.*"],
      "role": "sales",
      "description": "Handles sales inquiries and product demonstrations",
      "last_active": "2024-01-15T10:30:00Z"
    },
    {
      "address": "[email protected]", 
      "delivery_mode": "push",
      "webhook_url": "https://example.com/webhooks/amtp",
      "capabilities": ["technical_support", "issue_resolution"],
      "supported_schemas": ["agntcy:support.ticket.*"],
      "role": "support",
      "description": "Technical support and issue resolution",
      "last_active": "2024-01-15T14:22:00Z"
    }
  ],
  "agent_count": 2,
  "domain": "example.com",
  "timestamp": "2024-01-15T15:00:00Z"
}

Address validation (using [email protected]):

{
  "agents": [
    {
      "address": "[email protected]",
      "delivery_mode": "pull",
      "active": true,
      "last_active": "2024-01-15T10:30:00Z"
    }
  ],
  "agent_count": 1,
  "domain": "example.com",
  "timestamp": "2024-01-15T15:00:00Z"
}

3.4.3 Implementation Flexibility

Gateways MAY implement discovery with different levels of detail:

Minimal: Address validation only
Basic: Simple agent listing without metadata
Enhanced: Rich metadata with semantic search capabilities

Clients SHOULD cache discovery responses appropriately to reduce load on gateways. Discovery is entirely optional - agents can communicate effectively using only direct addressing.

4. Message Format

4.1 Message Structure

AMTP messages use JSON format with the following structure:

{
  "version": "1.0",
  "message_id": "01H8X9Z2K3M4N5P6Q7R8S9T0",
  "idempotency_key": "uuid-v4-string",
  "timestamp": "2025-08-14T10:30:00.000Z",
  "sender": "[email protected]",
  "recipients": ["[email protected]", "[email protected]"],
  "subject": "Purchase Order Confirmation",
  "schema": "agntcy:commerce.order.v2",
  "coordination": {
    "type": "parallel",
    "timeout": 3600,
    "required_responses": ["[email protected]"]
  },
  "headers": {
    "priority": "normal",
    "reply-to": "[email protected]",
    "custom-header": "custom-value"
  },
  "payload": {
    // Schema-validated content
  },
  "attachments": [
    {
      "filename": "invoice.pdf",
      "content_type": "application/pdf",
      "size": 1024000,
      "hash": "sha256:abc123...",
      "url": "https://attachments.sender.com/xyz789"
    }
  ],
  "signature": {
    "algorithm": "RS256",
    "value": "base64-encoded-signature"
  }
}

4.2 Field Specifications

4.2.1 Core Required Fields

The following fields are MANDATORY for all AMTP implementations and ensure basic interoperability:

version: AMTP protocol version (string, e.g., "1.0")
sender: Sender address in agent@domain format
recipients: Array of recipient addresses
payload: Message content (any valid JSON object or primitive)

4.2.2 Implementation-Defined Fields (Optional)

The following fields MAY be implemented based on specific requirements and use cases:

Reliability & Tracking:

message_id: UUIDv7 for time-ordered unique identification (recommended for message tracking)
idempotency_key: UUIDv4 for duplicate detection (required for at-least-once delivery semantics)
timestamp: ISO 8601 UTC timestamp (may be added by gateways)

Message Metadata:

subject: Human-readable message summary
headers: Additional custom metadata
in_reply_to: Reference to original message for responses

Schema & Validation:

schema: Schema identifier for payload validation (e.g., "agntcy:commerce.order.v2")

Advanced Features:

coordination: Multi-agent workflow configuration
attachments: External file references
signature: Digital signature for non-repudiation

4.3 Message Size Limits

Maximum message size: 10MB (configurable via DNS)
Attachments: Referenced by URL, not embedded
Large payloads: Use chunking or external storage

5. Transport Layer

5.1 Delivery Paths

AMTP Core defines delivery semantics independent of storage mechanisms. Implementations MAY choose between two paths per deployment or per message policy:

Immediate Path (Optional): The gateway processes and forwards messages directly without requiring durable store-and-forward prior to acknowledging the sender. This path targets low-latency, real-time scenarios and is best-effort with documented durability limits.
Durable Path (Recommended): The gateway persists messages (e.g., WAL/disk or equivalent) before acknowledging acceptance to the sender, providing at-least-once delivery across restarts and transient failures.

Gateways MAY route messages to either path based on local policy. No new headers are required by AMTP Core to use either path.

5.2 HTTP Transport

AMTP uses HTTPS as the primary transport protocol.

5.2.1 Message Delivery Endpoint

Endpoint: POST /v1/messages

Request Headers:

Content-Type: application/json
Content-Length: {size}
User-Agent: AMTP/1.0 ({implementation})
X-AMTP-Version: 1.0
Authorization: Bearer {token} (if required)

Response Codes:

200 OK: Immediate Path only. The gateway has obtained a correlated reply within a short processing window and returns the reply payload in the response body. AMTP remains asynchronous at the protocol level; this is an implementation optimization.
202 Accepted: Message accepted for delivery (either Immediate or Durable Path). Sender MAY poll status or await a reply message referencing in_reply_to.
400 Bad Request: Invalid message format
401 Unauthorized: Authentication required
413 Payload Too Large: Message exceeds size limit
429 Too Many Requests: Rate limit exceeded
503 Service Unavailable: Gateway temporarily unavailable

5.2.2 Status Query Endpoint

Endpoint: GET /v1/messages/{message_id}/status

Response:

{
  "message_id": "01H8X9Z2K3M4N5P6Q7R8S9T0",
  "status": "delivered",
  "recipients": [
    {
      "address": "[email protected]",
      "status": "delivered",
      "timestamp": "2025-08-14T10:31:45.000Z"
    }
  ],
  "attempts": 1,
  "next_retry": null
}

5.2.3 Inbox Management (Pull Mode)

For agents using pull-based delivery, gateways provide inbox endpoints for message retrieval and acknowledgment.

Inbox Retrieval: GET /v1/inbox/{recipient}

Agents can retrieve pending messages from their inbox with optional filtering and pagination. Authentication is required via API key to ensure agents can only access their own messages.

Message Acknowledgment: DELETE /v1/inbox/{recipient}/{message_id}

After processing a message, agents acknowledge receipt to remove it from their inbox. This ensures at-least-once delivery semantics while preventing message duplication.

Security Model:

Each agent requires a unique API key for inbox access
Agents can only access messages addressed to their own local-part
Acknowledged messages are permanently removed from the inbox
Unacknowledged messages remain available for retrieval

See section 12.1.5 for detailed API specifications.

5.3 Store-and-Forward Architecture (Optional)

5.3.1 Message Queue (optional)

Each AMTP gateway maintains optional persistent message queues for:

Outbound messages awaiting delivery
Inbound messages for local delivery
Dead letter queue for failed messages

5.3.2 Retry Logic

Retry Schedule:

Initial delay: 1 second
Exponential backoff: 2^n seconds (capped at 1 hour)
Maximum retries: 168 (7 days with hourly retries)
Jitter: ±25% randomization to prevent thundering herd

6. Schema Framework Integration

6.1 AGNTCY Schema Support

AMTP integrates with the AGNTCY standard schema framework for payload validation and semantic interoperability.

6.1.1 Schema Identifier Format

agntcy:{domain}.{entity}.{version}

Examples:

agntcy:commerce.order.v2
agntcy:finance.payment.v1
agntcy:logistics.shipment.v3

6.1.2 Schema Resolution

Schema resolution is implementation-defined. A typical flow might include:

Parse schema identifier from message
Resolve schema definition via the receiving gateway's Schema Engine from a trusted local cache; the gateway MAY fetch from a configured registry (AGNTCY, internal, or other) to populate/refresh the cache; fetched schemas MUST be signature-verified before use
Validate payload against schema
Reject invalid messages with detailed error response

The protocol does not specify registry endpoints, discovery mechanisms, or storage formats. Each implementation chooses their own schema management approach.

6.2 Schema Negotiation

6.2.1 Schema Discovery

Schema discovery mechanisms are implementation-defined. The protocol does not mandate how agents or senders discover which schemas are supported by recipients.

Common implementation approaches may include:

Agent-level advertisement through discovery endpoints
Out-of-band negotiation and configuration
Schema registries (centralized or federated)
Static configuration files
Runtime schema probing and error handling

6.2.2 Version Compatibility

Minor version differences: Backward compatible
Major version differences: Explicit negotiation required
Unsupported schema: Return error with supported alternatives

7. Multi-Agent Coordination

7.1 Coordination Patterns

7.1.1 Parallel Execution

All recipients process the message simultaneously:

{
  "coordination": {
    "type": "parallel",
    "timeout": 3600,
    "required_responses": ["[email protected]"],
    "optional_responses": ["[email protected]"]
  }
}

7.1.2 Sequential Execution

Recipients process in specified order:

{
  "coordination": {
    "type": "sequential",
    "timeout": 7200,
    "sequence": ["[email protected]", "[email protected]", "[email protected]"],
    "stop_on_failure": true
  }
}

7.1.3 Conditional Execution

Execution based on response conditions:

{
  "coordination": {
    "type": "conditional",
    "timeout": 1800,
    "conditions": [
      {
        "if": "[email protected] responds with approve=true",
        "then": ["[email protected]"],
        "else": ["[email protected]"]
      }
    ]
  }
}

7.2 Workflow State Management

7.2.1 State Tracking

The originating gateway maintains workflow state:

{
  "workflow_id": "01H8X9Z2K3M4N5P6Q7R8S9T0",
  "status": "in_progress",
  "created_at": "2025-08-14T10:30:00.000Z",
  "updated_at": "2025-08-14T10:35:15.000Z",
  "participants": [
    {
      "address": "[email protected]",
      "status": "completed",
      "response": {...}
    },
    {
      "address": "[email protected]", 
      "status": "pending",
      "deadline": "2025-08-14T11:30:00.000Z"
    }
  ]
}

7.2.2 Response Handling

Responses reference the original workflow:

{
  "version": "1.0",
  "message_id": "01H8X9Z2K3M4N5P6Q7R8S9T1",
  "in_reply_to": "01H8X9Z2K3M4N5P6Q7R8S9T0",
  "sender": "[email protected]",
  "recipients": ["[email protected]"],
  "response_type": "workflow_response",
  "payload": {
    "status": "approved",
    "data": {...}
  }
}

8. Reliability and Delivery Guarantees

8.1 Delivery Semantics

AMTP provides at-least-once delivery guarantees across federated gateways.

8.1.1 Idempotency

Every message includes a unique idempotency_key
Receiving gateways MUST deduplicate based on this key
Idempotency window: 7 days minimum

8.1.2 Acknowledgment Protocol

Gateway receives message → Returns 202 Accepted
Gateway attempts delivery → Updates internal status
Successful delivery → Mark as delivered
Failed delivery → Schedule retry with exponential backoff

8.2 Failure Handling

8.2.1 Dead Letter Queue

Messages that fail after maximum retries are moved to DLQ:

{
  "message_id": "01H8X9Z2K3M4N5P6Q7R8S9T0",
  "original_message": {...},
  "failure_reason": "recipient_unavailable",
  "attempts": 168,
  "first_attempt": "2025-08-14T10:30:00.000Z",
  "last_attempt": "2025-08-21T10:30:00.000Z",
  "dlq_timestamp": "2025-08-21T11:00:00.000Z"
}

8.2.2 Error Reporting

Failed deliveries generate error reports to sender:

{
  "version": "1.0",
  "message_type": "delivery_failure",
  "original_message_id": "01H8X9Z2K3M4N5P6Q7R8S9T0",
  "failed_recipients": ["[email protected]"],
  "error_code": "recipient_unavailable",
  "error_message": "Domain gateway not responding",
  "retry_count": 168,
  "final_attempt": "2025-08-21T10:30:00.000Z"
}

9. Security Model

9.1 Transport Security

TLS 1.3: Mandatory for all HTTPS connections
Certificate Validation: Gateway certificates MUST be valid for the domain
HTTP Strict Transport Security (HSTS): Recommended

9.2 Authentication

9.2.1 Domain-Based Authentication

Default authentication uses domain ownership verification through TLS certificates.

9.2.2 Additional Authentication Methods

API Keys:

Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...

OAuth 2.0:

Authorization: Bearer oauth-access-token

Mutual TLS: Client certificate authentication for high-security environments.

9.3 Message Integrity

9.3.1 Digital Signatures

Optional message signing using RS256:

{
  "signature": {
    "algorithm": "RS256",
    "keyid": "sender.com:key1",
    "value": "base64-encoded-signature"
  }
}

9.3.2 End-to-End Encryption

Optional payload encryption:

{
  "payload": {
    "encrypted": true,
    "algorithm": "AES-256-GCM",
    "encrypted_data": "base64-encoded-data",
    "recipients": [
      {
        "keyid": "receiver.com:pubkey1",
        "encrypted_key": "base64-encoded-key"
      }
    ]
  }
}

9.4 Access Control

9.4.1 Domain Policies

Configure allowed senders and schemas:

# amtp-policy.yaml
version: "1.0"
default_policy: "deny"
rules:
  - sender_pattern: "*.trusted-partner.com"
    action: "allow"
    schemas: ["agntcy:commerce.*"]
  - sender_pattern: "[email protected]"
    action: "allow"
    schemas: ["agntcy:public.*"]
    rate_limit: "100/hour"

10. Gateway Implementation

10.1 Core Components

┌─────────────────────────────────────────────────────────────┐
│                    AMTP Gateway                             │
├─────────────────┬─────────────────┬─────────────────────────┤
│  HTTP Server    │  Message Queue  │    Protocol Bridge     │
│  - Receive      │  - Persistence  │    - AMTP ↔ SMTP       │
│  - Send         │  - Retry Logic  │    - Schema Conversion  │
│  - Status API   │  - DLQ          │    - Format Translation │
├─────────────────┼─────────────────┼─────────────────────────┤
│  DNS Resolver   │  Schema Engine  │    Coordination Engine  │
│  - Discovery    │  - Validation   │    - Workflow State     │
│  - Caching      │  - AGNTCY API   │    - Multi-Agent Logic  │
├─────────────────┼─────────────────┼─────────────────────────┤
│  Auth Manager   │  Policy Engine  │    Monitoring          │
│  - TLS Certs    │  - Access Rules │    - Metrics           │
│  - API Keys     │  - Rate Limits  │    - Logging           │
└─────────────────┴─────────────────┴─────────────────────────┘

10.2 Delivery to Local Agents

When an AMTP gateway receives a message for an address in its own domain (e.g., [email protected]), it is responsible for delivery to the local agent identified by the local-part. AMTP Core does not prescribe how this delivery is implemented; each domain is free to choose.

Gateways MAY deliver messages using one or both of the following models:

Push Model: The gateway invokes a registered delivery target for the local-part, such as an HTTPS webhook, gRPC service, or internal worker queue. If the target is temporarily unavailable, the gateway MAY retry delivery according to its durability policy.
Pull Model: The gateway buffers messages internally and exposes them through a retrieval API (e.g., GET /v1/inbox/{recipient}/...). Agents fetch messages at their convenience and acknowledge receipt. This model is useful for agents that cannot expose inbound endpoints.

Normative requirement: Regardless of delivery model, the gateway MUST either (a) deliver the message to a local agent, or (b) return a structured error such as AGENT_UNKNOWN if no mapping exists for the local-part.

11. Protocol Negotiation and Bridging

11.1 Discovery Process

flowchart TD
    A[Agent sends to [email protected]] --> B[Query DNS for _amtp.domain.com]
    B --> C{AMTP record found?}
    C -->|Yes| D[Use AMTP protocol]
    C -->|No| E[Query MX records]
    E --> F[Use SMTP Bridging]
    D --> G[Send AMTP message]
    G --> H{Delivery successful?}
    H -->|Yes| I[Done]
    H -->|No| J[Retry with exponential backoff]
    J --> K{Max retries exceeded?}
    K -->|No| G
    K -->|Yes| F

11.2 SMTP Bridge

11.2.1 AMTP to Email Conversion

When falling back to SMTP, convert AMTP message to email format:

Email Headers:

From: [email protected]
To: [email protected]
Subject: [AMTP] Original Subject
X-AMTP-Message-ID: 01H8X9Z2K3M4N5P6Q7R8S9T0
X-AMTP-Schema: agntcy:commerce.order.v2
X-AMTP-Sender: [email protected]
Content-Type: multipart/mixed

Email Body:

This message was sent via AMTP protocol but delivered via SMTP bridging.

Original Message:
Subject: Purchase Order Confirmation
From: [email protected]
Schema: agntcy:commerce.order.v2

Human-readable summary:
A purchase order for 100 widgets has been confirmed.

Structured Data (JSON):
{
  "order_id": "12345",
  "items": [...]
}

To upgrade to native AMTP support, add DNS TXT record:
_amtp.receiver.com. IN TXT "v=amtp1;gateway=https://amtp.receiver.com"

11.2.2 Email to AMTP Conversion

Process incoming emails and attempt AMTP delivery:

Parse email headers for AMTP metadata
Extract structured data from email body
Reconstruct AMTP message format
Deliver via AMTP if recipient supports it
Otherwise, deliver as standard email

12. API Specifications

12.1 Gateway REST API

12.1.1 Send Message

Endpoint: POST /v1/messages

Request Body:

{
  "version": "1.0",
  "sender": "[email protected]",
  "recipients": ["[email protected]"],
  "subject": "Test Message",
  "schema": "agntcy:test.message.v1",
  "payload": {
    "text": "Hello, World!"
  }
}

Response:

{
  "message_id": "01H8X9Z2K3M4N5P6Q7R8S9T0",
  "status": "accepted",
  "recipients": [
    {
      "address": "[email protected]",
      "status": "queued"
    }
  ]
}

12.1.2 Query Message Status

Endpoint: GET /v1/messages/{message_id}

Response:

{
  "message_id": "01H8X9Z2K3M4N5P6Q7R8S9T0",
  "status": "delivered",
  "created_at": "2025-08-14T10:30:00.000Z",
  "delivered_at": "2025-08-14T10:30:15.000Z",
  "recipients": [
    {
      "address": "[email protected]",
      "status": "delivered",
      "delivered_at": "2025-08-14T10:30:15.000Z",
      "attempts": 1
    }
  ]
}

12.1.3 List Messages

Endpoint: GET /v1/messages

Query Parameters:

status: Filter by status (pending, delivered, failed)
sender: Filter by sender address
recipient: Filter by recipient address
since: Messages since timestamp
limit: Number of results (default: 100, max: 1000)
offset: Pagination offset

12.1.4 Webhook Configuration

Endpoint: POST /v1/webhooks

Configure webhooks for delivery notifications:

{
  "url": "https://my-app.com/amtp-webhook",
  "events": ["message.delivered", "message.failed"],
  "secret": "webhook-secret-key"
}

12.1.5 Inbox Management (Pull Mode)

For agents using pull-based message delivery, gateways provide inbox management endpoints.

Get Inbox Messages

Endpoint: GET /v1/inbox/{recipient}

Headers:

Authorization: Bearer {agent_api_key}

Query Parameters:

limit (optional): Number of messages to retrieve (default: 100, max: 1000)
offset (optional): Pagination offset
since (optional): Only return messages since timestamp
status (optional): Filter by message status (unread, read)

Response:

{
  "messages": [
    {
      "message_id": "01H8X9Z2K3M4N5P6Q7R8S9T0",
      "sender": "[email protected]",
      "subject": "Process Order",
      "schema": "agntcy:commerce.order.v2",
      "timestamp": "2025-08-14T10:30:00.000Z",
      "status": "unread",
      "payload": {
        "order_id": "12345",
        "customer": {...},
        "items": [...]
      }
    }
  ],
  "message_count": 1,
  "recipient": "[email protected]",
  "has_more": false
}

Security: Requires the agent's API key. Each agent can only access their own inbox.

Acknowledge Message

Endpoint: DELETE /v1/inbox/{recipient}/{message_id}

Headers:

Authorization: Bearer {agent_api_key}

Response:

{
  "message_id": "01H8X9Z2K3M4N5P6Q7R8S9T0",
  "status": "acknowledged",
  "timestamp": "2025-08-14T10:35:00.000Z"
}

Security: Requires the agent's API key. Agents can only acknowledge messages in their own inbox.

12.2 Agent Integration API

12.2.1 Receive Message Callback

Agents register endpoints to receive AMTP messages:

Endpoint: POST /amtp/receive (agent-defined)

Request:

{
  "message_id": "01H8X9Z2K3M4N5P6Q7R8S9T0",
  "sender": "[email protected]",
  "subject": "Process Order",
  "schema": "agntcy:commerce.order.v2",
  "payload": {
    "order_id": "12345",
    "customer": {...},
    "items": [...]
  },
  "coordination": {
    "workflow_id": "01H8X9Z2K3M4N5P6Q7R8S9T0",
    "type": "parallel",
    "requires_response": true
  }
}

Response:

{
  "status": "accepted",
  "response": {
    "order_status": "confirmed",
    "estimated_delivery": "2025-08-21"
  }
}

13. Error Handling

13.1 Error Codes

13.1.1 Protocol Errors

INVALID_MESSAGE_FORMAT: Message doesn't conform to AMTP format
UNSUPPORTED_VERSION: Protocol version not supported
SCHEMA_VALIDATION_FAILED: Payload doesn't match specified schema
MESSAGE_TOO_LARGE: Message exceeds size limits
INVALID_RECIPIENT: Recipient address format invalid

13.1.2 Delivery Errors

RECIPIENT_UNAVAILABLE: Target gateway not responding
RECIPIENT_NOT_FOUND: No route to recipient domain
AUTHENTICATION_FAILED: Authentication rejected by recipient
RATE_LIMIT_EXCEEDED: Too many messages sent
TEMPORARY_FAILURE: Temporary issue, will retry

13.1.3 Coordination Errors

WORKFLOW_TIMEOUT: Coordination timeout exceeded
REQUIRED_RESPONSE_MISSING: Required participant didn't respond
COORDINATION_FAILED: Multi-agent coordination failed

13.2 Error Response Format

{
  "error": {
    "code": "SCHEMA_VALIDATION_FAILED",
    "message": "Payload validation failed for schema agntcy:commerce.order.v2",
    "details": {
      "schema": "agntcy:commerce.order.v2",
      "validation_errors": [
        {
          "field": "order.items[0].price",
          "error": "must be a positive number"
        }
      ]
    },
    "timestamp": "2025-08-14T10:30:00.000Z",
    "request_id": "req_01H8X9Z2K3M4N5P6Q7R8S9T0"
  }
}

14. Compliance and Conformance

14.1 Conformance Levels

14.1.1 Core Conformance

Implementations MUST support:

DNS discovery mechanism (_amtp.{domain} TXT records)
Core message format with mandatory fields: version, sender, recipients, payload
HTTPS transport with TLS 1.3
Basic message delivery (Immediate Path)
JSON message format validation

14.1.2 Extended Conformance

Implementations MAY support:

Reliability Features:

Message tracking (message_id field)
At-least-once delivery semantics (idempotency_key field)
Durable Path with persistent storage
Message status tracking and retry logic

Integration Features:

SMTP bridging capability
Schema validation (AGNTCY or custom frameworks)
Agent discovery endpoints

Advanced Features:

Multi-agent coordination workflows
Digital signatures and message integrity
End-to-end encryption
Webhook notifications
Custom headers and metadata

14.2 Testing and Validation

14.2.1 Interoperability Tests

Standard test suite for validating AMTP implementations:

Basic Delivery Test: Send message between different implementations
SMTP Bridging Test: Verify SMTP bridging with AMTP
Schema Validation Test: Test schema-based message validation
Coordination Test: Multi-agent workflow execution
Error Handling Test: Proper error reporting and retry logic

14.2.2 Performance Benchmarks

Message throughput: >1000 messages/second
Delivery latency: <100ms for same-datacenter delivery
Storage efficiency: <1KB overhead per message
Memory usage: <10MB per 10,000 queued messages

14.3 Security Considerations

14.3.1 Threat Model

Addressed Threats:

Message tampering (TLS, digital signatures)
Unauthorized access (domain authentication, access policies)
Denial of service (rate limiting, resource controls)
Man-in-the-middle attacks (certificate validation)

Out of Scope:

Advanced persistent threats
Side-channel attacks
Social engineering attacks
Endpoint compromise

14.3.2 Security Recommendations

Use TLS 1.3 with valid certificates
Implement rate limiting to prevent abuse
Validate all inputs including message payloads
Monitor and log all activities
Regular security updates for gateway software
Network segmentation for gateway infrastructure

Appendix A: Message Examples

A.1 Minimal Core Message

{
  "version": "1.0",
  "sender": "[email protected]",
  "recipients": ["[email protected]"],
  "payload": {
    "message": "Hello, this is a minimal AMTP message"
  }
}

A.2 Simple Message with Optional Fields

{
  "version": "1.0",
  "message_id": "01H8X9Z2K3M4N5P6Q7R8S9T0",
  "idempotency_key": "550e8400-e29b-41d4-a716-446655440000",
  "timestamp": "2025-08-14T10:30:00.000Z",
  "sender": "[email protected]",
  "recipients": ["[email protected]"],
  "subject": "Simple Notification",
  "payload": {
    "message": "Hello, this is a simple AMTP message",
    "timestamp": "2025-08-14T10:30:00.000Z"
  }
}

A.3 Schema-Validated Commerce Message

{
  "version": "1.0",
  "message_id": "01H8X9Z2K3M4N5P6Q7R8S9T1",
  "idempotency_key": "550e8400-e29b-41d4-a716-446655440001",
  "timestamp": "2025-08-14T11:00:00.000Z",
  "sender": "[email protected]",
  "recipients": ["[email protected]"],
  "subject": "Purchase Order PO-2025-001234",
  "schema": "agntcy:commerce.order.v2",
  "headers": {
    "priority": "high",
    "department": "procurement"
  },
  "payload": {
    "order_id": "PO-2025-001234",
    "order_date": "2025-08-14",
    "customer": {
      "company_name": "Retailer Corp",
      "contact_email": "[email protected]",
      "billing_address": {
        "street": "123 Business Ave",
        "city": "Commerce City",
        "state": "CA",
        "zip": "90210",
        "country": "US"
      }
    },
    "items": [
      {
        "sku": "WIDGET-001",
        "description": "Premium Widget",
        "quantity": 100,
        "unit_price": 29.99,
        "total_price": 2999.00
      },
      {
        "sku": "GADGET-002", 
        "description": "Smart Gadget",
        "quantity": 50,
        "unit_price": 149.99,
        "total_price": 7499.50
      }
    ],
    "total_amount": 10498.50,
    "currency": "USD",
    "payment_terms": "NET30",
    "delivery_date": "2025-08-28"
  }
}

A.4 Multi-Agent Coordination Message

{
  "version": "1.0",
  "message_id": "01H8X9Z2K3M4N5P6Q7R8S9T2",
  "idempotency_key": "550e8400-e29b-41d4-a716-446655440002",
  "timestamp": "2025-08-14T12:00:00.000Z",
  "sender": "[email protected]",
  "recipients": [
    "[email protected]",
    "[email protected]", 
    "[email protected]"
  ],
  "subject": "Coordinate Shipment SHIP-789",
  "schema": "agntcy:logistics.shipment.v1",
  "coordination": {
    "type": "sequential",
    "timeout": 7200,
    "sequence": [
      "[email protected]",
      "[email protected]",
      "[email protected]"
    ],
    "stop_on_failure": true,
    "required_responses": [
      "[email protected]",
      "[email protected]"
    ]
  },
  "payload": {
    "shipment_id": "SHIP-789",
    "order_reference": "PO-2025-001234",
    "pickup_location": {
      "address": "456 Warehouse Blvd, Industrial City, CA 90123",
      "contact": "[email protected]",
      "hours": "08:00-17:00 PST"
    },
    "delivery_location": {
      "address": "123 Business Ave, Commerce City, CA 90210",
      "contact": "[email protected]",
      "hours": "09:00-16:00 PST"
    },
    "items": [
      {
        "sku": "WIDGET-001",
        "quantity": 100,
        "weight_kg": 50.0,
        "dimensions_cm": {
          "length": 60,
          "width": 40,
          "height": 30
        }
      }
    ],
    "service_level": "standard",
    "requested_pickup_date": "2025-08-15",
    "requested_delivery_date": "2025-08-17"
  }
}

A.5 Workflow Response Message

{
  "version": "1.0",
  "message_id": "01H8X9Z2K3M4N5P6Q7R8S9T3",
  "idempotency_key": "550e8400-e29b-41d4-a716-446655440003",
  "timestamp": "2025-08-14T12:15:00.000Z",
  "sender": "[email protected]",
  "recipients": ["[email protected]"],
  "subject": "Re: Coordinate Shipment SHIP-789",
  "in_reply_to": "01H8X9Z2K3M4N5P6Q7R8S9T2",
  "response_type": "workflow_response",
  "schema": "agntcy:logistics.pickup_confirmation.v1",
  "payload": {
    "shipment_id": "SHIP-789",
    "status": "confirmed",
    "pickup_scheduled": "2025-08-15T10:00:00.000Z",
    "packages": [
      {
        "tracking_number": "TRK123456789",
        "weight_kg": 50.0,
        "dimensions_verified": true
      }
    ],
    "warehouse_notes": "Items ready for pickup at dock 3",
    "contact_person": "John Smith",
    "contact_phone": "+1-555-0123"
  }
}

Appendix B: DNS Configuration Examples

B.1 Basic AMTP Capability Advertisement

; Basic AMTP support
_amtp.example.com. 300 IN TXT "v=amtp1;gateway=https://amtp.example.com:443"

; With authentication and size limits
_amtp.retailer.com. 300 IN TXT "v=amtp1;gateway=https://amtp.retailer.com;auth=cert,oauth"

; Enterprise configuration with custom limits
_amtp.enterprise.com. 300 IN TXT "v=amtp1;gateway=https://amtp-gw.enterprise.com;auth=cert,oauth;max-size=50000000"

B.2 Bridge MX Records

; AMTP with SMTP bridging
_amtp.hybrid.com. 300 IN TXT "v=amtp1;gateway=https://amtp.hybrid.com"
hybrid.com. 300 IN MX 10 mail.hybrid.com.

Appendix C: Implementation Guidelines

C.1 Gateway Deployment Checklist

C.1.1 Infrastructure Requirements

Load Balancer: SSL termination, health checks
Application Server: AMTP gateway implementation
Message Queue: Redis/PostgreSQL for persistence
Database: Message storage and workflow state
Monitoring: Metrics, logging, alerting
DNS: TXT record configuration

C.1.2 Security Hardening

Network: Firewall rules, VPC isolation
TLS: Certificate management, HSTS headers
Authentication: API key rotation, OAuth integration
Authorization: Role-based access control
Audit: Comprehensive logging and monitoring

C.2 Agent Integration Patterns

C.2.1 Simple Agent

import requests
import json
from datetime import datetime

class FluxAgent:
    def __init__(self, gateway_url, agent_address):
        self.gateway_url = gateway_url
        self.agent_address = agent_address
    
    def send_message(self, recipients, subject, payload, schema=None):
        message = {
            "sender": self.agent_address,
            "recipients": recipients,
            "subject": subject,
            "payload": payload,
            "timestamp": datetime.utcnow().isoformat() + "Z"
        }
        
        if schema:
            message["schema"] = schema
        
        response = requests.post(
            f"{self.gateway_url}/v1/messages",
            json=message,
            headers={"Content-Type": "application/json"}
        )
        
        return response.json()
    
    def handle_incoming_message(self, message):
        # Process incoming AMTP message
        print(f"Received: {message['subject']}")
        
        # Return response if coordination requires it
        if message.get("coordination", {}).get("requires_response"):
            return {
                "status": "processed",
                "timestamp": datetime.utcnow().isoformat() + "Z"
            }

# Usage example
agent = FluxAgent("https://amtp.mycompany.com", "[email protected]")

# Send a message
result = agent.send_message(
    recipients=["[email protected]"],
    subject="System Alert",
    payload={"alert_type": "warning", "message": "Database CPU at 85%"},
    schema="agntcy:monitoring.alert.v1"
)

print(f"Message sent: {result['message_id']}")

C.2.2 Workflow Coordinator

class WorkflowCoordinator:
    def __init__(self, amtp_agent):
        self.amtp_agent = amtp_agent
        self.active_workflows = {}
    
    def start_parallel_workflow(self, recipients, payload, timeout=3600):
        message_id = self.amtp_agent.send_message(
            recipients=recipients,
            subject="Parallel Workflow",
            payload=payload,
            coordination={
                "type": "parallel",
                "timeout": timeout,
                "required_responses": recipients
            }
        )["message_id"]
        
        self.active_workflows[message_id] = {
            "type": "parallel",
            "recipients": recipients,
            "responses": {},
            "status": "pending"
        }
        
        return message_id
    
    def handle_response(self, response_message):
        workflow_id = response_message["in_reply_to"]
        sender = response_message["sender"]
        
        if workflow_id in self.active_workflows:
            workflow = self.active_workflows[workflow_id]
            workflow["responses"][sender] = response_message["payload"]
            
            # Check if workflow is complete
            if len(workflow["responses"]) == len(workflow["recipients"]):
                workflow["status"] = "completed"
                self.on_workflow_complete(workflow_id, workflow)
    
    def on_workflow_complete(self, workflow_id, workflow):
        print(f"Workflow {workflow_id} completed with {len(workflow['responses'])} responses")

This completes the comprehensive AMTP Protocol Specification v1.0. The document provides detailed technical specifications for implementing a federated, asynchronous communication protocol that enhances email with structured data support, multi-agent coordination, and reliable delivery guarantees while maintaining universal addressing and backward compatibility.

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AMTP Protocol Specification v1.0 (Draft)

Table of Contents

1. Introduction

1.1 Protocol Overview

1.2 Key Features

1.3 Protocol Versions

2. Protocol Architecture

2.1 System Components

2.2 Protocol Stack

3. Addressing and Discovery

3.1 Address Format

3.2 DNS Discovery

3.2.1 AMTP Capability Advertisement

3.2.2 Discovery Algorithm

3.3 Gateway Endpoint Resolution

3.4 Agent Discovery (Optional)

3.4.1 Use Cases

3.4.2 Discovery Endpoint

3.4.3 Implementation Flexibility

4. Message Format

4.1 Message Structure

4.2 Field Specifications

4.2.1 Core Required Fields

4.2.2 Implementation-Defined Fields (Optional)

4.3 Message Size Limits

5. Transport Layer

5.1 Delivery Paths

5.2 HTTP Transport

5.2.1 Message Delivery Endpoint

5.2.2 Status Query Endpoint

5.2.3 Inbox Management (Pull Mode)

5.3 Store-and-Forward Architecture (Optional)

5.3.1 Message Queue (optional)

5.3.2 Retry Logic

6. Schema Framework Integration

6.1 AGNTCY Schema Support

6.1.1 Schema Identifier Format

6.1.2 Schema Resolution

6.2 Schema Negotiation

6.2.1 Schema Discovery

6.2.2 Version Compatibility

7. Multi-Agent Coordination

7.1 Coordination Patterns

7.1.1 Parallel Execution

7.1.2 Sequential Execution

7.1.3 Conditional Execution

7.2 Workflow State Management

7.2.1 State Tracking

7.2.2 Response Handling

8. Reliability and Delivery Guarantees

8.1 Delivery Semantics

8.1.1 Idempotency

8.1.2 Acknowledgment Protocol

8.2 Failure Handling

8.2.1 Dead Letter Queue

8.2.2 Error Reporting

9. Security Model

9.1 Transport Security

9.2 Authentication

9.2.1 Domain-Based Authentication

9.2.2 Additional Authentication Methods

9.3 Message Integrity

9.3.1 Digital Signatures

9.3.2 End-to-End Encryption

9.4 Access Control

9.4.1 Domain Policies

10. Gateway Implementation

10.1 Core Components

10.2 Delivery to Local Agents

11. Protocol Negotiation and Bridging

11.1 Discovery Process

11.2 SMTP Bridge

11.2.1 AMTP to Email Conversion

11.2.2 Email to AMTP Conversion

Packages