# ADR-007: TTY Inside Channels — Sub-Streams, Not Wire Format

## Status

Accepted (**reversed 2026-07-18 by alknet ADR-093, ported here as
[ADR-008](008-channels-pure-channel-multiplexing.md): TTY always uses
its 5-byte format; the channels layer carries it transparently in the
payload — see "Reversal (ADR-008, 2026-07-18)" below.** Ported from
alknet ADR-077 2026-08-17; cross-references renumbered to alktty's
ADR range — ADR-052→001, ADR-053→002, ADR-054→003, ADR-055→004,
ADR-056→005, ADR-057→006, ADR-077→007, ADR-093→008. The alknet ADRs
referenced by alknet number (052, 053, 055, 057, 061, 071, 073, 074,
092) are not ported into alktty's ADR range — they are alknet-core /
alknet-channels ADRs. The alknet originals at
`/workspace/@alkdev/alknet/docs/architecture/decisions/` remain
authoritative.)

## Reversal (ADR-008, 2026-07-18)

The two-mode TTY design (direct vs inside-channels, with different
sub-stream access paths) is **reversed**. TTY's 5-byte format
(`[stream_type:u8][length:u32][payload]`, ADR-001) is TTY's internal
format, used in **both** direct mode and inside-channels mode. The two
modes differ only in *where the `BiStream` comes from* (a top-level
`alk/tty` connection vs a `channel/open` with ALPN `alk/tty`), not in
*how TTY parses it*. The same `wire.rs` code runs in both modes.

When TTY is inside channels, the channels layer strips its 8-byte header
(alknet ADR-093 / ADR-008) and hands TTY the payload bytes. TTY parses
its 5-byte header from the payload. The channels layer carries TTY's
5-byte chunks transparently in its payload — no shared fields, no
leaked abstraction, no double-chunking concern (the 13-byte total
header is 8 channels + 5 TTY, not 8 + 9; the channels `length` is
always `tty_len + 5`).

The `channels` feature on alktty (if such a feature were added) becomes
"run TTY's sub-demux on a channels-backed `BiStream`" — the same code
as direct mode, different `BiStream` source. The control channel split
(`STREAM_CTRL_IN` / `STREAM_CTRL_OUT`, Phase 7) is TTY-internal; the
channels layer doesn't know about it. alknet ADR-074's
`into_sub_streams()` (the accessor this ADR's two-mode design relied
on) is removed by alknet ADR-093 / ADR-008; TTY sub-demuxes its
`BiStream` via its own 5-byte format instead.

The body below describes the **original** (two-mode) shape; the
reversal above is the operative decision. The two-mode description is
kept as the historical context for the reversal. See ADR-008 for the
resolution rationale (the channels layer has no `stream_type` concept;
the handler owns its sub-stream multiplexing) and the cross-ADR
impacts.

## Context

ADR-001 defines the alktty wire format: `[stream_type: u8][length: u32
be][payload]`, a 5-byte chunk header for five sub-streams
(stdin/stdout/stderr/ctrl-in/ctrl-out) within one bidi stream. This
format is stable, implemented (`src/wire.rs`), and used for direct
`alk/tty` connections.

The phase-0 research recommended that the TTY chunk format be "absorbed
into channels" and that `alk/tty` remain as a "direct-connect
shortcut." But the research did not pin what changes in the TTY crate
when a TTY session runs *inside* a channels connection. This is a real
integration question the research hand-waved.

The problem: the `TtyAdapter`'s current `handle()` loops `accept_bi()`,
spawning a `drive_session` task per bidi stream that parses 5-byte TTY
chunks off the stream. Inside a channels connection, the stream is
*already de-chunked* by the channels layer's 9-byte format — the
handler sees an `AsyncRead + AsyncWrite` pair, not a chunk-encoded
stream. If the TTY adapter tries to parse 5-byte chunks off an
already-de-chunked stream, it breaks.

## Decision

### Two modes for TTY, one adapter

The `TtyAdapter` operates in two modes, determined by how it receives
its `Connection`:

| Mode | When | Wire format | How the adapter gets sub-streams |
|------|------|-------------|---------------------------------|
| **Direct (`alk/tty` ALPN)** | Top-level QUIC/TCP connection with ALPN `alk/tty` | TTY's 5-byte format (ADR-001, amended — see below) | `accept_bi()` → parse 5-byte chunks → split into stream_types 0-4 |
| **Inside channels** | `channel/open` with ALPN `alk/tty` on a channels connection | Channels' 9-byte format (alknet ADR-071) — the channels layer de-chunks | `into_sub_streams()` (alknet ADR-074) → five named handles for stream_types 0-4 |

In both modes, the `TtyBackend` trait and `TtyHandle` are unchanged
(ADR-002). The backend allocates a PTY and returns a `TtyHandle`; the
adapter pumps data between the handle and the sub-streams. The
difference is only in how the adapter gets the sub-streams — 5-byte
chunk parsing (direct) vs. `into_sub_streams()` (channels).

### The control channel is now properly bidirectional

The phase-0 findings flagged that the TTY control channel "isn't
actually bidirectional… the adapter ignores Exit from the client." The
root cause: `stream_type 3` was "bidirectional" — one stream both sides
wrote to, which is not properly multiplexed.

alknet ADR-071's stream_type decomposition fixes this: **every
stream_type is unidirectional.** Control is now two halves:

| stream_type | direction | purpose |
|-------------|-----------|---------|
| 3 | write (client→server) | control in: resize, signal, eof |
| 4 | read (server→client) | control out: exit, keepalive response |

The TTY adapter writes resize/signal/eof to `ctrl_in` (stream_type 3)
and reads exit/keepalive from `ctrl_out` (stream_type 4). Each has its
own reassembly buffer, its own flow control, its own EOF. The control
channel is *actually* bidirectional — two unidirectional streams, not
one shared stream both sides write to.

This amends ADR-001's stream_type assignments for direct mode too:
direct `alk/tty` connections now use stream_types [0, 1, 2, 3, 4] (data
in/out/err + control in/out), not [0, 1, 2, 3]. The 5-byte format's
`stream_type` field gains value 4; the `ControlMessage` enum is
unchanged (the JSON shape is the same; the stream_type it rides on
splits from 3 into 3+4).

### What changes in alktty

1. **The adapter's session-driving code splits into two entry
   points:**
   - `drive_session_direct(send, recv, backends, ...)` — the existing
     path: parse 5-byte chunks, split into stream_types 0-4, pump. Used
     for direct `alk/tty` connections.
   - `drive_session_channels(sub_streams, backends, ...)` — the new
     path: receive `ChannelSubStreams` (five named handles: stdin=
     SendStream, stdout=RecvStream, stderr=Option<RecvStream>,
     ctrl_in=SendStream, ctrl_out=RecvStream), pump directly without
     chunk parsing. Used when the channel's `Connection` is backed by
     `ChannelBidiStreamSource`.

2. **The `TtyAdapter::handle()` branches on the `Connection`'s source
   type.** The channels crate's `ChannelBidiStreamSource` is a
   `BidiStreamSource` (alknet ADR-070); the `Connection` wraps it. The
   adapter detects whether the `Connection` is channels-backed (via a
   downcast or a channels-crate extension trait — exact ergonomics per
   alknet ADR-074's implementation detail) and calls
   `drive_session_channels` instead of `drive_session_direct`.

   **This is the one place alktty knows about channels.** It is a
   branch on the connection source, not a dependency on channels' wire
   format. The branch can be feature-gated (`channels` feature on
   alktty) so the direct-only path has no channels dependency.

3. **The 5-byte wire format (ADR-001) is unchanged for direct
   connections.** ADR-001's scope is now "the wire format for direct
   `alk/tty` connections." The channels path does not use it. This
   amends ADR-001's scope — the format is not replaced, it's scoped.

4. **The control channel works the same in both modes, now properly
   bidirectional.** In direct mode, control-in JSON rides in 5-byte
   chunks with `stream_type=3` and control-out rides with
   `stream_type=4`. In channels mode, control-in rides in 9-byte chunks
   with `stream_type=3` (write) and control-out with `stream_type=4`
   (read) — but the channels layer de-chunks them, so the adapter
   reads raw JSON bytes from `ctrl_in`/`ctrl_out` in both cases. The
   `ControlMessage` enum (resize, signal, eof, exit) is unchanged —
   the JSON shape is the same; only the stream_type assignments change
   (3 splits into 3+4).

5. **The exit-chunk-is-last invariant (ADR-004) generalizes.** In
   direct mode, the exit chunk is the last 5-byte chunk on
   `stream_type=4` (read, server→client) before stream close (ADR-004,
   amended). In channels mode, the exit control message is the last
   data on `stream_type=4` before `channel/close` is sent on channel 0
   (alknet ADR-073 §channel/close). The ordering invariant is the same
   — exit before close — but the mechanism differs: 5-byte chunk
   ordering on stream_type 4 (direct) vs. `stream_type 4` ordering +
   `channel/close` after pump completion (channels).

### What does NOT change

- **`TtyBackend` trait, `TtyHandle`, `TtyControl`** (ADR-002) —
  unchanged. Backends don't know about channels or direct mode.
- **`DockerTtyBackend`, `LocalTtyBackend`** — unchanged. They implement
  `TtyBackend::allocate()` and return a `TtyHandle`.
- **`ControlMessage` enum** — unchanged. The JSON shape is the same in
  both modes.
- **The `alk/tty` ALPN string** — unchanged. Direct connections use it;
  channels `channel/open` requests it.

### Crate dependency

`alktty` does **not** depend on `alknet-channels` unconditionally. The
channels-integration code is behind a `channels` feature on `alktty`.
When the feature is off, `TtyAdapter` only supports direct mode (the
existing behavior). When the feature is on, the adapter branches into
channels mode for channels-backed connections. This preserves alknet
ADR-003's no-handler-depends-on-another-handler rule for the default
build; the feature-gated dependency is opt-in, same as `alknet-docker`'s
`tty` feature (alknet ADR-061).

## Consequences

**Positive:**
- The TTY crate's direct mode is unchanged — existing `alk/tty`
  deployments (browser terminals over WebSocket, direct QUIC TTY) keep
  working with the 5-byte format.
- The channels path uses the channels layer's de-chunking — no
  double-chunking (5-byte inside 9-byte). The TTY adapter sees clean
  sub-streams.
- The `TtyBackend` trait is insulated — backends don't know which mode
  the adapter is in. Docker, SSH, and local backends work in both
  modes without changes.
- The control channel and exit-chunk invariant carry forward cleanly —
  the `ControlMessage` enum and ordering semantics are
  mode-independent.

**Negative:**
- `alktty` has two session-driving entry points
  (`drive_session_direct` vs `drive_session_channels`). This is the
  necessary cost of supporting both direct and channels modes without
  double-chunking. The alternative (always use channels format, even
  for direct) would break existing direct deployments and add 4 bytes
  of overhead per chunk for no benefit.
- The `channels` feature on `alktty` adds a dependency edge (`alktty`
  → `alknet-channels`, feature-gated). This is the same pattern as
  `alknet-docker`'s `tty` feature (alknet ADR-061) and is opt-in.
- ADR-001's scope is amended (from "the TTY wire format" to "the TTY
  wire format for direct connections"). This is a scope clarification,
  not a format change — the 5-byte format itself is unchanged.

## Door type

**One-way (scope amendment) + two-way (feature gate).** ADR-001's scope
amendment (direct-only) is one-way — once the channels path exists,
re-merging the formats would require unifying 5-byte and 9-byte chunk
handling, which is a rewrite. The `channels` feature gate is two-way —
it can be removed if channels integration is no longer needed.

**Reversed by ADR-008 (2026-07-18):** the two-mode design is reversed —
TTY always uses its 5-byte format, carried transparently in the
channels payload. The one-way door is re-cast (the channels crate is
not yet implemented, so this is the right time). See ADR-008 for the
amended door-type discussion.

## References

- **[ADR-008](008-channels-pure-channel-multiplexing.md)**: channels
  pure channel multiplexing (reverses this ADR — TTY always uses its
  5-byte format; the channels layer carries it transparently; the
  two-mode design is preserved but differs only in `BiStream` source,
  not in parsing)
- [ADR-001](001-wire-format-and-two-carriage.md): alktty wire format
  (amended — scoped to direct connections by this ADR; **re-amended by
  ADR-008 — TTY always uses its 5-byte format, in both direct and
  inside-channels modes**)
- [ADR-002](002-ttybackend-trait-and-ttyhandle.md): TtyBackend trait
  and TtyHandle (unchanged by this ADR)
- [ADR-004](004-exit-code-on-control-chunk.md): exit-chunk-is-last
  (generalized by this ADR + alknet ADR-073)
- [ADR-006](006-negotiation-framing-self-contained.md): alktty does not
  depend on alkcall's internal wire types (preserved — the channels
  feature is on alknet-channels, not alkcall's internal wire types)
- alknet ADR-071: channels wire format (the 9-byte format the channels
  path uses; **amended by alknet ADR-093 / ADR-008 — 8-byte format, no
  `stream_type`**)
- alknet ADR-074: ChannelBidiStreamSource / `into_sub_streams` (the
  accessor the channels path uses; **amended by alknet ADR-093 /
  ADR-008 — `into_sub_streams()` removed**)
- alknet ADR-092: `BiStream` as the handler leaf (the transport-leaf
  decision that enables the reversal — `accept_bi` returns `BiStream`)
- alknet ADR-061: DockerTtyBackend in alknet-docker (the feature-gated
  dependency pattern this ADR mirrors)
- alknet channels phase-0 findings §DP-3, §OQ-CH-02, §Relationship to
  Existing Crates / alktty
- Port origin: alknet ADR-077 at
  `/workspace/@alkdev/alknet/docs/architecture/decisions/077-tty-inside-channels.md`