Message Format

Specification Version

A2E Protocol Version 1.0 — a strict superset of SCP (Skill Call Protocol) 1.0.

Framing

Messages are transmitted as NDJSON (newline-delimited JSON) over a reliable byte stream. Each message is a single JSON object on one line, terminated by \n.

<JSON object>\n
<JSON object>\n
...

Compact JSON encoding is used — no extraneous whitespace.

Base Message Schema

Every A2E message conforms to this base schema:

{
  "a2e": "1.0",
  "type": "<namespace>/<verb>",
  "id": "<uuid-hex>",
  "ts": <unix-epoch-float>
}

Field	Type	Required	Description
a2e	string	Yes	Protocol version, always "1.0"
type	string	Yes	Message type identifier (e.g. "tool/call/req")
id	string	Yes	UUID hex, unique per message
ts	number	Yes	Unix epoch timestamp (float, seconds)

Capability-specific messages add additional fields.

Type String Convention

Type strings follow the pattern: <namespace>/<verb> or <namespace>/<verb>/<suffix>

Pattern	Example	Meaning
<ns>/<verb>/req	tool/call/req	Request from agent
<ns>/<verb>/resp	tool/call/resp	Response from host
<ns>/<verb>	chain/event	Event or notification
<ns>/<verb>/<action>	mcp/server/register/req	Nested namespace

Encoding Rules

1. UTF-8 encoding for all text
2. Compact JSON — no extra whitespace
3. No binary framing — newline delimits messages
4. Pydantic v2 validation on both sides — invalid messages produce A2EError with schema_violation code

Decoding

The receiver maintains a type registry mapping type strings to Pydantic model classes:

# Base types (always registered)
A2E_BASE_TYPE_MAP = {
    "handshake/req":  HandshakeRequest,
    "handshake/resp": HandshakeResponse,
    "invoke/event":   A2EEvent,
    "ping":           Ping,
    "pong":           Pong,
    "shutdown":       Shutdown,
    "error":          A2EError,
}

# Plugin types (registered at startup)
TOOL_TYPE_MAP = {
    "tool/list/req":  ToolListRequest,
    "tool/list/resp": ToolListResponse,
    "tool/call/req":  ToolCallRequest,
    "tool/call/resp": ToolCallResponse,
    "tool/event":     ToolEvent,
}

If a type string is not found in the registry, the message is decoded as a generic A2EMessage (graceful degradation).

Message Ordering

• Within a session: Messages are processed in order of arrival
• Across sessions: No ordering guarantee
• Events: Each A2EEvent has a monotonic seq number within its req_id scope
• Correlation: Responses reference their request via req_id

Size Limits

No explicit message size limit is defined in the protocol. Implementations may impose limits via global_limits configuration.

Extensibility

New capability namespaces can be added by:

1. Defining new MessageType enum values
2. Creating Pydantic request/response/event models
3. Building a TYPE_MAP dict
4. Implementing an A2EPlugin subclass

Unknown message types degrade gracefully — they are decoded as A2EMessage and can be routed to a catch-all handler.