Protocol Design

Overview

$mq9.AI.* is the protocol designed by mq9 for asynchronous Agent communication. The core problem it solves: asynchronous communication between Agents where the sender and receiver do not need to be online at the same time.

mq9 only handles the communication problem — how messages are reliably delivered. Message content is a byte array; mq9 does not parse, validate, or restrict it. Whether the upper layer uses A2A, MCP, or another protocol is none of mq9's concern.

mail_address Format Specification

Character set: lowercase letters (a-z), digits (0-9), dots (.)

Length: 1 to 128 characters

Case: strictly lowercase; a mail_address containing uppercase characters will be rejected by the broker

Position rules: . may only appear in the middle — the first and last characters must be a lowercase letter or digit; consecutive . are not allowed

Semantics: mail_address is an opaque string; . does not participate in protocol routing or matching and serves only as a visual grouping aid

Encoding: URL percent-encoding is not allowed

Valid examples	Invalid examples
`task.001`	`task-001` (contains hyphen)
`agent.inbox`	`task_001` (contains underscore)
`analytics.result`	`Task.001` (contains uppercase)
`acme.org.task.queue`	`.task.001` (leading dot)
`session.20260502`	`task.001.` (trailing dot)
	`task..001` (consecutive dots)

Full address examples:

text

task.001
agent.inbox
analytics.result
acme.task.queue
session.20260502
order.processing.urgent
agent.001.inbox

Core Concepts

mail_address: The communication address defined by the user when creating a mailbox via MAILBOX.CREATE. It is not bound to an Agent identity — a single Agent may request different mail_addresses for different tasks. No cleanup is needed after use; TTL handles automatic expiry.

mail_address unguessability is the security boundary. Knowing a mail_address is sufficient to send messages or subscribe. Without the mail_address there is nothing to operate on. No tokens, no ACLs.

TTL: Declared at mailbox creation time; the mailbox is automatically destroyed on expiry and its messages are cleaned up along with it. Creating a mailbox with a duplicate name returns an error (mailbox xxx already exists); you can use QUERY to check whether a mailbox exists before creating it.

priority: Optional. If not specified, the default is normal priority; specifying urgent or critical raises the processing order. Same-priority messages follow FIFO; across priorities, higher priority is processed first. The storage layer guarantees ordering — consumers do not need to sort themselves.

msg_id: The unique identifier of each message (its offset in storage), used by clients for deduplication and deletion operations.

Consumption semantics: FETCH uses pull mode and supports both stateful consumption (pass group_name and the broker records the offset, resumable after reconnect) and stateless consumption (omit group_name, each call consumes independently according to the deliver policy with no offset recorded).

Message flow: Message arrives → written to storage → client actively calls FETCH to pull → ACK to confirm → broker advances the consumption offset for that group.

Protocol Overview

Category	Subject	Description
Mailbox management	`$mq9.AI.MAILBOX.CREATE`	Create a mailbox
Messaging	`$mq9.AI.MSG.SEND.{mail_address}`	Send a message (priority via `mq9-priority` header)
Messaging	`$mq9.AI.MSG.FETCH.{mail_address}`	Fetch mailbox messages
Messaging	`$mq9.AI.MSG.ACK.{mail_address}`	Acknowledge a message
Messaging	`$mq9.AI.MSG.QUERY.{mail_address}`	Query messages in a mailbox
Messaging	`$mq9.AI.MSG.DELETE.{mail_address}.{msg_id}`	Delete a specific message
Agent management	`$mq9.AI.AGENT.REGISTER`	Register an Agent
Agent management	`$mq9.AI.AGENT.UNREGISTER`	Unregister an Agent
Agent management	`$mq9.AI.AGENT.REPORT`	Report Agent status
Agent management	`$mq9.AI.AGENT.DISCOVER`	Discover Agents

All commands use request/reply mode (nats request); the server always returns a response.

Response Format

Each command has its own response structure. All responses include an error field:

Value	Meaning
`""`	Success
Non-empty string	Failure; the value is the error description

$mq9.AI.MAILBOX.CREATE

Create a mailbox with a user-defined mail_address.

Request Fields

Field	Type	Required	Description
`name`	string	No	The mail_address for the mailbox; must conform to the format specification. Auto-generated by the broker if omitted.
`ttl`	u64?	No	Time-to-live in seconds; `0` means never expires

Response Fields

Field	Type	Description
`error`	string	Empty on success; error message on failure
`mail_address`	string	The mailbox address after successful creation

Example

bash

nats request '$mq9.AI.MAILBOX.CREATE' '{"name": "agent.translator.inbox", "ttl": 0}'
# Response
{"error":"","mail_address":"agent.translator.inbox"}

# Duplicate creation → error
nats request '$mq9.AI.MAILBOX.CREATE' '{"name": "agent.translator.inbox"}'
# Response
{"error":"mailbox agent.translator.inbox already exists","mail_address":""}

$mq9.AI.MSG.SEND.

Send a message to the specified mailbox. The payload is a byte array; mq9 does not parse the content.

Request Parameters

Subject: $mq9.AI.MSG.SEND.{mail_address}

Payload: Arbitrary byte array; mq9 does not parse the content.

Headers (all optional)

Header	Description
`mq9-key: {key}`	Dedup/compaction key. For the same key, the storage layer retains only the latest message, overwriting older ones
`mq9-delay: {seconds}`	Delay delivery by this many seconds. The message is not visible in FETCH results until the delay expires. Delayed messages return `msg_id: -1`
`mq9-ttl: {seconds}`	Message-level TTL in seconds. The message expires at `send_time + ttl`, independent of the mailbox TTL
`mq9-tags: {tag1},{tag2}`	Comma-separated user tags, e.g. `billing,vip`. Filterable via the `tags` field in QUERY
`mq9-priority: {value}`	Message priority: `normal` (default) / `urgent` / `critical`. Defaults to `normal` if omitted

SEND Response Fields

Field	Type	Description
`error`	string	Empty on success; error message on failure (e.g., mailbox does not exist)
`msg_id`	i64	Storage offset assigned after write; `-1` for delayed messages

Priority Description

Value	Typical use case
`normal` (default)	Task dispatch, result return, status reporting
`urgent`	Approval requests, important notifications
`critical`	Task interruption, emergency instructions

Same-priority messages follow FIFO; across priorities: critical > urgent > normal.

SEND Example

bash

# Normal message
nats request '$mq9.AI.MSG.SEND.agent.translator.inbox' '{"text":"hello"}'
# Response
{"error":"","msg_id":0}

# With key (storage retains only the latest message for the same key)
nats request '$mq9.AI.MSG.SEND.task.001.callback' \
  -H "mq9-key:status" \
  '{"status":"running"}'
# Response
{"error":"","msg_id":1}

# With tags (filterable via QUERY tags field)
nats request '$mq9.AI.MSG.SEND.agent.order.inbox' \
  -H "mq9-tags:billing,vip" \
  '{"order_id":"o-001"}'
# Response
{"error":"","msg_id":2}

# Delayed delivery — 60 seconds (msg_id -1 means delayed)
nats request '$mq9.AI.MSG.SEND.agent.translator.inbox' \
  -H "mq9-delay:60" \
  '{"text":"delayed task"}'
# Response
{"error":"","msg_id":-1}

# Message-level TTL — expires 300 seconds after send (independent of mailbox TTL)
nats request '$mq9.AI.MSG.SEND.agent.translator.inbox' \
  -H "mq9-ttl:300" \
  '{"text":"short-lived message"}'
# Response
{"error":"","msg_id":3}

# Urgent message (priority via header)
nats request '$mq9.AI.MSG.SEND.agent.translator.inbox' \
  -H "mq9-priority:urgent" \
  '{"alert":"please expedite"}'
# Response
{"error":"","msg_id":4}

$mq9.AI.MSG.FETCH.

Pull messages from a mailbox. Supports two consumption modes: stateful consumption (pass group_name and the server records the offset) and stateless consumption (omit group_name and each call starts independently according to the deliver policy).

FETCH Request Fields

Field	Type	Required	Description
`group_name`	string?	No	Consumer group name. When provided, enables stateful consumption — members of the same group share an offset. When omitted, consumption is stateless: each call starts according to the `deliver` policy with no offset recorded.
`deliver`	string	No	Starting point policy; defaults to `latest`. For stateful consumption, only takes effect when there is no existing offset record or when `force_deliver: true`.
`from_time`	u64?	No	Effective when `deliver: "from_time"`; Unix timestamp in seconds
`from_id`	u64?	No	Effective when `deliver: "from_id"`; fetch starts from this msg_id (inclusive)
`force_deliver`	bool?	No	Only valid for stateful consumption. When `true`, the existing offset is ignored and consumption restarts according to `deliver`
`config`	object?	No	Fetch behavior configuration; see below

deliver Policy

Value	Description
`latest` (default)	Only pull new messages from this point forward
`earliest`	Start from the oldest message in the mailbox
`from_time`	Start from after a specified timestamp; requires the `from_time` field
`from_id`	Start from a specified msg_id (inclusive); requires the `from_id` field

Offset Behavior for Stateful Consumption (group_name provided)

Condition	Behavior
Offset record exists and `force_deliver: false`	Resume from the last checkpoint; `deliver` has no effect
Offset record exists and `force_deliver: true`	Ignore the offset and restart according to the `deliver` policy
No offset record	Start according to the `deliver` policy (first-time consumption)

Stateless Consumption (group_name omitted)

The server generates a temporary random group, uses the deliver policy to locate the starting point, and does not commit an offset after consumption. Suitable for inspection, debugging, or one-off reads.

config Fields

Field	Type	Default	Description
`num_msgs`	u32?	100	Maximum number of messages to pull in a single call
`max_wait_ms`	u64?	500	How long the server waits when there is no data (milliseconds). Defaults to 500ms if omitted; `0` means return immediately without waiting. An empty list is returned when the wait expires, preventing clients from hammering the server in a tight poll loop.

FETCH Response Fields

Field	Type	Description
`error`	string	Empty on success; error message on failure (e.g., mailbox does not exist)
`messages`	array	List of messages; each entry contains `msg_id`, `payload`, `priority`, and `create_time`

FETCH Example

bash

# Stateless consumption: no group_name, starts from the latest message each time (default deliver: latest)
nats request '$mq9.AI.MSG.FETCH.task.001.callback' '{}'

# Stateless consumption: full read from the earliest message each time
nats request '$mq9.AI.MSG.FETCH.task.001.callback' \
  '{"deliver": "earliest"}'

# Stateful consumption: resume from checkpoint if offset exists; first call pulls only new messages (default deliver: latest)
nats request '$mq9.AI.MSG.FETCH.task.001.callback' \
  '{"group_name": "worker-group-1"}'

# Stateful consumption: resume from checkpoint if offset exists; first call starts from earliest message
nats request '$mq9.AI.MSG.FETCH.task.001.callback' \
  '{"group_name": "worker-group-1", "deliver": "earliest"}'

# Stateful consumption: force reset offset and restart from earliest
nats request '$mq9.AI.MSG.FETCH.task.001.callback' \
  '{"group_name": "worker-group-1", "deliver": "earliest", "force_deliver": true}'

# Specify max messages per call (default is 100)
nats request '$mq9.AI.MSG.FETCH.task.001.callback' \
  '{"group_name": "worker-group-1", "config": {"num_msgs": 50}}'

$mq9.AI.MSG.ACK.

Confirm that a message has been processed; the broker advances the consumption offset for the consumer group.

ACK Request Fields

Field	Type	Required	Description
`group_name`	string	Yes	Consumer group name, matching the one used in FETCH
`mail_address`	string	Yes	Mailbox address
`msg_id`	u64	Yes	The ID of the message to acknowledge (from the FETCH response)

ACK Response Fields

Field	Type	Description
`error`	string	Empty on success; error message on failure

ACK Example

bash

nats request '$mq9.AI.MSG.ACK.task.001.callback' \
  '{"group_name": "worker-group-1", "mail_address": "task.001.callback", "msg_id": 5}'
# Response
{"error":""}

$mq9.AI.MSG.QUERY.

Query messages currently stored in a mailbox. Does not affect subscription delivery.

QUERY Request Fields

Field	Type	Description
`key`	string?	Query by key; returns the latest message for that key
`limit`	u64?	Maximum number of messages to return
`since`	u64?	Return messages after this timestamp

QUERY Response Fields

Field	Type	Description
`error`	string	Empty on success
`messages`	array	List of messages

QUERY Example

bash

# Query all messages
nats request '$mq9.AI.MSG.QUERY.task.001.callback' '{}'

# Query the latest message with key=status
nats request '$mq9.AI.MSG.QUERY.task.001.callback' '{"key": "status"}'

# Most recent 10 messages
nats request '$mq9.AI.MSG.QUERY.task.001.callback' '{"limit": 10}'

# Messages after a specific timestamp
nats request '$mq9.AI.MSG.QUERY.task.001.callback' '{"since": 1234567890}'

$mq9.AI.MSG.DELETE.{mail_address}.

Delete a specific message from a mailbox.

DELETE Example

bash

nats request '$mq9.AI.MSG.DELETE.task.001.callback.2' ''
# Response
{"error":"","deleted":true}

$mq9.AI.AGENT.REGISTER

Register an Agent. The body contains upper-layer protocol content (the current example uses an A2A AgentCard); mq9 does not interfere with the body and only requires that it carry a mailbox field as the routing identifier. The same applies if a different protocol is used in the future.

REGISTER Example

bash

nats request '$mq9.AI.AGENT.REGISTER' \
  '{ ...AgentCard, mailbox = "mq9://broker/agent.translator.inbox"... }'
# Response
{"error":""}

$mq9.AI.AGENT.UNREGISTER

Unregister an Agent.

UNREGISTER Example

bash

nats request '$mq9.AI.AGENT.UNREGISTER' \
  '{ ...mailbox = "mq9://broker/agent.translator.inbox"... }'
# Response
{"error":""}

$mq9.AI.AGENT.REPORT

Report Agent status. The body contains upper-layer protocol content; mq9 does not interfere.

REPORT Example

bash

nats request '$mq9.AI.AGENT.REPORT' \
  '{ ...mailbox = "mq9://broker/agent.translator.inbox", status fields defined by the upper-layer protocol... }'
# Response
{"error":""}

$mq9.AI.AGENT.DISCOVER

Search for registered Agents by criteria. Returns the raw list of registered content; mq9 does not transform or wrap it.

DISCOVER Request Fields

Field	Type	Description
`text`	string?	Full-text search query (keyword-based)
`semantic`	string?	Semantic search query (natural language, vector-based). Takes priority over `text` when both are provided.
`limit`	number?	Maximum number of results per page (default 20)
`page`	number?	Page number, starting from 1 (default 1)

When neither text nor semantic is provided, all registered Agents for the tenant are returned.

DISCOVER Example

bash

# Full-text search
nats request '$mq9.AI.AGENT.DISCOVER' '{"text": "payment invoice"}'

# Semantic search (vector-based, takes priority over text)
nats request '$mq9.AI.AGENT.DISCOVER' '{"semantic": "process a payment and generate invoice"}'

# Pagination: page 2, 10 results per page
nats request '$mq9.AI.AGENT.DISCOVER' '{"text": "payment", "limit": 10, "page": 2}'

# List all
nats request '$mq9.AI.AGENT.DISCOVER' '{}'
# Returns: [{ ...raw registered content... }, ...]

Error Reference

Scenario	Response example
mailbox does not exist (SEND/SUB/QUERY/DELETE)	`{"error":"mailbox xxx does not exist"}`
mailbox already exists (CREATE is not idempotent)	`{"error":"mailbox xxx already exists","mail_address":""}`
msg_id does not exist (DELETE)	`{"error":"message not found"}`

Protocol Design ​

Overview ​

mail_address Format Specification ​

Core Concepts ​

Protocol Overview ​

Response Format ​

$mq9.AI.MAILBOX.CREATE ​

Request Fields ​

Response Fields ​

Example ​

$mq9.AI.MSG.SEND. ​

Request Parameters ​

SEND Response Fields ​

Priority Description ​

SEND Example ​

$mq9.AI.MSG.FETCH. ​

FETCH Request Fields ​

deliver Policy ​

Offset Behavior for Stateful Consumption (group_name provided) ​

Stateless Consumption (group_name omitted) ​

config Fields ​

FETCH Response Fields ​

FETCH Example ​

$mq9.AI.MSG.ACK. ​

ACK Request Fields ​

ACK Response Fields ​

ACK Example ​

$mq9.AI.MSG.QUERY. ​

QUERY Request Fields ​

QUERY Response Fields ​

QUERY Example ​

$mq9.AI.MSG.DELETE.{mail_address}. ​

DELETE Example ​

$mq9.AI.AGENT.REGISTER ​

REGISTER Example ​

$mq9.AI.AGENT.UNREGISTER ​

UNREGISTER Example ​

$mq9.AI.AGENT.REPORT ​

REPORT Example ​

$mq9.AI.AGENT.DISCOVER ​

DISCOVER Request Fields ​

DISCOVER Example ​

Error Reference ​

Protocol Design

Overview

mail_address Format Specification

Core Concepts

Protocol Overview

Response Format

$mq9.AI.MAILBOX.CREATE

Request Fields

Response Fields

Example

$mq9.AI.MSG.SEND.

Request Parameters

SEND Response Fields

Priority Description

SEND Example

$mq9.AI.MSG.FETCH.

FETCH Request Fields

deliver Policy

Offset Behavior for Stateful Consumption (group_name provided)

Stateless Consumption (group_name omitted)

config Fields

FETCH Response Fields

FETCH Example

$mq9.AI.MSG.ACK.

ACK Request Fields

ACK Response Fields

ACK Example

$mq9.AI.MSG.QUERY.

QUERY Request Fields

QUERY Response Fields

QUERY Example

$mq9.AI.MSG.DELETE.{mail_address}.

DELETE Example

$mq9.AI.AGENT.REGISTER

REGISTER Example

$mq9.AI.AGENT.UNREGISTER

UNREGISTER Example

$mq9.AI.AGENT.REPORT

REPORT Example

$mq9.AI.AGENT.DISCOVER

DISCOVER Request Fields

DISCOVER Example

Error Reference