---
status: draft
last_updated: 2026-08-17
---

# alktty — BAST Document for the `alk/tty` Wire Format

This document is the **BAST (Binary Abstract Syntax Tree)** specification
for the binary portions of the `alk/tty` wire format. It is a normative
JSON document conforming to the BAST meta-schema at
`https://alk.dev/bast/v1/schema` (a standard JSON Schema Draft 2020-12
document); any JSON Schema Draft 2020-12 validator can check whether the
BAST below is well-formed.

**alktty does not depend on alktype.** The hand-rolled `ChunkReader` /
`ChunkWriter` in `src/wire.rs` is the runtime codec; this BAST document
is the human-readable contract that describes what those types
round-trip. If runtime validation against the BAST becomes desirable
later, alktype becomes an optional dep and this document is already
there to feed it. See [ADR-001](decisions/001-wire-format-and-two-carriage.md)
and [ADR-006](decisions/006-negotiation-framing-self-contained.md).

## Scope

The `alk/tty` wire format mixes a small binary framing layer with
UTF-8 JSON payloads for the structured parts. Per the BAST format
spec, a BAST document describes **binary data layouts** — it is not a
format for validating JSON payloads ([`validate_bytes` vs
`validate_json`](https://alk.dev/bast/v1/spec)). This document
therefore specifies only the binary framing; the JSON shapes are
specified in prose in [tty-wire.md](tty-wire.md) and implemented by the
Rust source (`src/negotiation.rs`, `src/control.rs`), which are the
source of truth for those payloads.

The binary portions this BAST covers (both one-way doors per
[ADR-001](decisions/001-wire-format-and-two-carriage.md)):

1. **The 5-byte chunk header** (`[stream_type: u8][length: u32 BE]
   [payload]`) — the raw chunk codec in `src/wire.rs`. Five stream
   types: `STREAM_STDIN=0`, `STREAM_STDOUT=1`, `STREAM_STDERR=2`,
   `STREAM_CTRL_IN=3`, `STREAM_CTRL_OUT=4`.
2. **The negotiation frame length prefix** (4-byte BE `u32` followed
   by `length` bytes of UTF-8 JSON `NegotiateRequest` body) —
   self-contained per
   [ADR-006](decisions/006-negotiation-framing-self-contained.md).

What this BAST deliberately does **not** cover:

- **`NegotiateRequest` JSON shape** — a UTF-8 JSON object, not a
  binary layout. Specified in [tty-wire.md](tty-wire.md) §"Phase 1"
  and implemented by `NegotiateRequest` in `src/negotiation.rs`.
- **`ControlMessage` JSON shape** — the UTF-8 JSON payloads of
  `STREAM_CTRL_IN`/`STREAM_CTRL_OUT` chunks (the `resize`/`signal`/
  `eof`/`exit` variants). Specified in [tty-wire.md](tty-wire.md)
  §"Control Channel" and implemented by `ControlMessage` in
  `src/control.rs`.
- **`TerminalParams` JSON shape** — the `tty` field of
  `NegotiateRequest`. Specified in [tty-wire.md](tty-wire.md) §"Phase
  1" and implemented by `TerminalParamsWire` in
  `src/negotiation.rs`.

A previous revision of this document modeled the JSON payloads as
BAST `struct`/`union` definitions with `uint16`/`int32` fields. That
was a category error: BAST describes binary layouts, and the control
and negotiation payloads on the wire are UTF-8 JSON text, not
struct-encoded binary. The JSON field types (`u16`, `i32`, `String`)
are JSON value types produced and consumed by `serde_json`, not binary
field widths. Describing them as BAST structs implied a binary
encoding that does not exist on the wire and would have misled any
generated validator. The simpler model — BAST for the binary framing,
prose + Rust source for the JSON payloads — matches what the wire
format actually is.

## The BAST document

```json
{
  "$schema": "https://alk.dev/bast/v1/schema",
  "$defs": {
    "ChunkHeader": {
      "kind": "struct",
      "endian": "big",
      "fields": [
        { "name": "stream_type", "kind": "uint8" },
        { "name": "length",      "kind": "uint32" }
      ]
    },

    "StreamType": {
      "kind": "enum",
      "values": ["Stdin", "Stdout", "Stderr", "CtrlIn", "CtrlOut"]
    },

    "Chunk": {
      "kind": "struct",
      "endian": "big",
      "fields": [
        { "name": "header",  "kind": { "$ref": "#/$defs/ChunkHeader" } },
        { "name": "payload", "kind": "bytes", "encoding": "length-prefixed", "maxLength": 16777216 }
      ]
    },

    "NegotiationFrame": {
      "kind": "struct",
      "endian": "big",
      "fields": [
        { "name": "length", "kind": "uint32" },
        { "name": "body",   "kind": "bytes", "encoding": "length-prefixed", "maxLength": 16777216 }
      ]
    }
  }
}
```

## Annotations

### `ChunkHeader` — binary, exact

Maps 1:1 to the on-wire bytes. 5 bytes: `stream_type` as `uint8` (1
byte), `length` as `uint32` big-endian (4 bytes). This is the 5-byte
chunk header committed by [ADR-001](decisions/001-wire-format-and-two-carriage.md).
The Rust types are `STREAM_STDIN`/`STREAM_STDOUT`/`STREAM_STDERR`/
`STREAM_CTRL_IN`/`STREAM_CTRL_OUT` and `CHUNK_HEADER_LEN = 5` in
`src/wire.rs`.

### `StreamType` — enum, documented deviation: on-wire encoding is `uint8`, not BAST's standard `u32` enum index

The BAST meta-schema specifies that an `enum`'s binary representation
is a `u32` index into `values` (0-based). The `alk/tty` wire format
encodes `stream_type` as a **`uint8`** (1 byte), not a `u32` (4
bytes) — the chunk header is 5 bytes, not 8. This is a deliberate
deviation: the chunk header needs a 1-byte discriminator, and 4 bytes
of padding would double the per-chunk overhead.

The `StreamType` enum is therefore **normative for the name→value
mapping** (the index of a name in `values` is the integer that appears
on the wire as the `uint8` `stream_type`), but a generated validator
must NOT emit a `u32` read for `stream_type`. The on-wire encoding is
the `uint8` declared in `ChunkHeader`; the enum provides the
integer→name table for validation and diagnostics.

| index | name      | on-wire byte | direction      | payload             |
|-------|-----------|--------------|----------------|---------------------|
| 0     | `Stdin`   | `0x00`       | client→server  | raw bytes           |
| 1     | `Stdout`  | `0x01`       | server→client  | raw bytes           |
| 2     | `Stderr`  | `0x02`       | server→client  | raw bytes           |
| 3     | `CtrlIn`  | `0x03`       | client→server  | UTF-8 JSON control  |
| 4     | `CtrlOut` | `0x04`       | server→client  | UTF-8 JSON control  |

`stream_type > 4` is a protocol error (`InvalidStreamType`); there is
no extension escape hatch in the byte (a 6th channel is a wire-format
change requiring a new ALPN — [ADR-001](decisions/001-wire-format-and-two-carriage.md)).

### `Chunk` — binary, exact

A chunk is the 5-byte `ChunkHeader` followed by `length` bytes of
payload. The `payload` field is `bytes` with `encoding:
"length-prefixed"` and `maxLength: 16777216` (16 MiB = `MAX_CHUNK_LEN`
in `src/wire.rs`). The `payload`'s interpretation depends on
`stream_type`:

- `Stdin`/`Stdout`/`Stderr` (0/1/2): raw bytes. Zero-length payloads on
  these channels are sentinels (zero-length stdin = EOF from client;
  zero-length stdout = "drained" from server); see
  [tty-wire.md](tty-wire.md) §"Sentinels".
- `CtrlIn`/`CtrlOut` (3/4): UTF-8 JSON text (a serialized
  `ControlMessage`). Control chunks are never zero-length (the JSON
  payload is at least `{}`). The JSON shape is **not** specified by
  this BAST — see [tty-wire.md](tty-wire.md) §"Control Channel" and
  `src/control.rs`.

### `NegotiationFrame` — binary, exact (out-of-band for the chunk codec)

The negotiation frame is a 4-byte big-endian length prefix + a UTF-8
JSON body. This maps 1:1 to the on-wire bytes: `length` as `uint32`
big-endian (4 bytes), `body` as a length-prefixed byte string
(`length` bytes of UTF-8 JSON). The `maxLength: 16777216` (16 MiB)
constraint is the same as `MAX_CHUNK_LEN` — error frames MUST be under
16 MiB so the 4-byte length prefix's high byte is `0x00`, which is
what makes the framing-disambiguation trick sound (first byte `0x00`
= error/negotiation frame; first byte `1`/`2`/`4` = raw chunk; see
[tty-wire.md](tty-wire.md) §"Constraints").

The `body` is modeled as `bytes` (not `string`) because the BAST
describes the binary framing layer: the body is `length` bytes whose
interpretation (UTF-8 JSON, parseable as `NegotiateRequest`) is a
contract above this BAST layer, specified in
[tty-wire.md](tty-wire.md) §"Phase 1" and implemented in
`src/negotiation.rs`. The `NegotiateRequest` JSON shape is **not**
specified by this BAST.

The negotiation frame is **out-of-band for the chunk codec**: it is
read once at session start (Phase 1, JSON carriage), then the stream
switches to raw chunks (Phase 2). The `ChunkReader`/`ChunkWriter` in
`src/wire.rs` does not read or write negotiation frames; the
`NegotiationReader`/`NegotiationWriter` in `src/negotiation.rs` does.
See [ADR-006](decisions/006-negotiation-framing-self-contained.md).

## Validation

The BAST document above is validatable in two ways:

1. **Structural validation** — run any JSON Schema Draft 2020-12
   validator against the BAST meta-schema at
   `https://alk.dev/bast/v1/schema`. This checks whether the BAST is
   well-formed (correct `kind` strings, required fields present,
   `$ref` targets exist). It does NOT check whether a binary payload
   conforms to the layout — that is the BAST-native validator's job
   (see the alktype BAST format spec,
   `/workspace/@alkdev/alktype/docs/architecture/bast-format.md`).
2. **Drift detection** — a cheap test that parses the BAST document
   with `serde_json` and asserts the `StreamType` enum values match
   `wire.rs`'s `STREAM_STDIN`/`STREAM_STDOUT`/`STREAM_STDERR`/
   `STREAM_CTRL_IN`/`STREAM_CTRL_OUT` constants (the index of each
   name in `values` is the integer constant). This catches the common
   drift case (a new stream_type added to `wire.rs` but not the BAST,
   or vice versa) without requiring alktype as a dep — just
   `serde_json`, which alktty already has. See the project setup plan's
   "Risk: BAST schema drift" mitigation.

## Design Decisions

| Decision | ADR | Summary |
|----------|-----|---------|
| Wire format and two-carriage model | [ADR-001](decisions/001-wire-format-and-two-carriage.md) | The 5-byte chunk header + negotiation frame this BAST describes |
| Self-contained negotiation framing | [ADR-006](decisions/006-negotiation-framing-self-contained.md) | The `NegotiationFrame` is self-contained in alktty (not reused from alkcall's `EventEnvelope` framing) |
| Exit code on a control chunk | [ADR-004](decisions/004-exit-code-on-control-chunk.md) | The `exit` JSON variant carried on `CtrlOut` (specified in `tty-wire.md`, not this BAST) |

## References

- [ADR-001](decisions/001-wire-format-and-two-carriage.md) — the wire
  format decision this BAST specifies
- [ADR-006](decisions/006-negotiation-framing-self-contained.md) — the
  negotiation framing is self-contained (the `NegotiationFrame` is not
  reused from alkcall)
- [tty-wire.md](tty-wire.md) — the prose wire format spec this BAST
  formalizes (covers both the binary framing and the JSON payloads)
- `src/wire.rs` — the runtime chunk codec (`ChunkReader`/`ChunkWriter`,
  `STREAM_*` constants, `MAX_CHUNK_LEN`) this BAST describes
- `src/negotiation.rs` — the `NegotiationReader`/`NegotiationWriter`
  for the `NegotiationFrame`, and `NegotiateRequest` (JSON shape, out
  of BAST scope)
- `src/control.rs` — `ControlMessage` (JSON shape, out of BAST scope)
- [alktype BAST format spec](https://alk.dev/bast/v1/schema) — the
  normative format spec for BAST documents (the meta-schema this
  document conforms to); see also
  `/workspace/@alkdev/alktype/docs/architecture/bast-format.md`
- Project setup plan, "Risk: BAST schema drift" — the drift-detection
  test mitigation (`docs/plans/project-setup.md`)