# AXL Rosetta v4.0.1 - Public Release Kernel
# Frozen at: v4.0.2-r6-freeze (commit 51e75de)
# Released: 2026-04-25
# Source: github.com/dcarranza-axl/axl-research/blob/v4.0.2-r6-freeze/spec/v4-kernel.md
# Mirror: github.com/axlprotocol/axl-research/blob/v4.0.2-r6-freeze/spec/v4-kernel.md
# License: Apache 2.0

# AXL Rosetta v4 Kernel

Version: 4.0.2-draft
Status: Research draft
Tier: NORMATIVE KERNEL
License: Apache 2.0

## Identity

Model-teachable compression protocol for machine-to-machine communication.
Kernel = deterministic syntax. Extensions = compression conventions.
Code layer = experimental lossy IR. Thesis = speculation.

## Packet Syntax

    PKT[VER|CLS|SUB|TAG|ARG1|ARG2|META]

Seven pipe-delimited fields inside `PKT[` and `]`.
Trailing empty fields may be omitted.

## Field Constraints

| Field | Width    | Required | Default |
|-------|----------|----------|---------|
| VER   | 1-3 char | yes      | `4`     |
| CLS   | 3 char   | yes      | none    |
| SUB   | 2-48 char| yes      | none    |
| TAG   | 2-16 char| yes      | none    |
| ARG1  | free     | no       | empty   |
| ARG2  | free     | no       | empty   |
| META  | free     | no       | empty   |

## Class Codes

INF (inform), ACT (act), QRY (query), RSP (respond), ERR (error).

## Parsing Contract

1. Extract all fields by pipe-split.
2. Parse META first (caret-prefixed ^key:value flags, space-separated).
3. META determines ARG2 dispatch: ^mode:code activates code-body grammar;
   VER >= 3.1 without ^mode:code activates math-arg grammar; else kernel-arg.
4. TAG dispatch follows the same META/VER logic.
5. Parse legality is separated from interpretation legality. A v3 parser
   legally extracts all fields from a v4 packet; it does not interpret
   extension semantics.

## Reserved Characters and Escaping

Reserved: `|` `[` `]` `\` `^` `;`
Escape with backslash: `\|` `\[` `\]` `\\` `\;` `\^`
In kernel-arg and math-arg: pipe, brackets, backslash must be escaped.
In code-body: semicolons are statement separators; literal semicolon: `\;`
Caret is literal in ARG fields; flag prefix only in META.

## Version Negotiation

VER values: 3, 3.1, 3.2, 4. Senders declare VER in every packet.
A receiver must accept any VER <= its own.
Unknown VER: PKT[4|ERR|sys|val|unsupported_ver|VER=X]

## Error Contract

Validation failure produces ERR packet with TAG indicating error class.
Error classes: syntax, version, alias, escape, mode, field.
See grammar.bnf ERROR TAXONOMY for full definitions and examples.

Mid-sequence behavior:
- Valid packets before the error are accepted (partial sequence is valid).
- Malformed packet triggers ERR identifying position via ^ref.
- Protocol is best-effort; retransmission is not required.

## Alias Scope

^alias:XX registers current SUB as alias XX for the remainder of
the current sequence. New sequence resets alias table.
Re-registration replaces previous binding.
Implementation guidance: alias tables should support at least 64 entries.

## Defaults

Missing VER defaults to 4. Missing optional fields default to empty.
Unknown ^meta keys are ignored. Verbose uncompressed content is always valid.

## Backward Compatibility

All v3 packets parse under v4 rules without modification.
Extension content in TAG/ARG2 is opaque text to older parsers.
Unknown ^meta flags are ignored by all versions.

## Grammar

See spec/grammar.bnf. This document and the grammar are co-authoritative.
Conflict: grammar governs syntax; this document governs semantics.