deepseek-harness/.agents/notes/implemented/architecture/2026-09-19-remote-duplex-stream.md

Agent Note: Remote duplex stream: one kind of stream, an uplink channel, and an invocation context

Status: implemented

English | 中文

Problem

Current state

dsh-api-gateway multiplexes Typert Remote streams over one WebSocket at /api/remote.mux. A Host method declares both directions with @Remote({ mode: 'stream' }) and RemoteStream<Out, In>; the generated Client method returns RemoteStreamHandle<Out, In>. Existing Iterable and AsyncIterable return types declare downlink-only methods. The optional final signal: AbortSignal is supplied by the Gateway and does not enter the wire arguments.

Both directions share one streamId:

Direction	Frame	Definition
Client to Host	{ type: 'open', streamId, endpoint, payload }	stream-protocol.ts
Client to Host	{ type: 'item', streamId, value? }	stream-protocol.ts
Client to Host	{ type: 'end', streamId }	stream-protocol.ts
Client to Host	{ type: 'cancel', streamId }	stream-protocol.ts
Host to Client	{ type: 'item', streamId, value? }	stream-protocol.ts
Host to Client	{ type: 'error', streamId, error: { code, message, details } }	stream-protocol.ts
Host to Client	{ type: 'end', streamId }	stream-protocol.ts

The Client handle's send() supplies uplink items, end() half-closes the uplink, and dispose() cancels the logical stream. The Host reads decoded items through this.ctx.invocation.uplink<In>(); the invocation also carries the admitted Peer and cancellation signal.

The transport owns validation, cancellation, and its bounded inbox. Consumer protocols own input authority, acknowledgements, and recovery after disconnection.

Uplink consumers

Feature	Uplink path	Location	Cost
Web terminal input and resize	terminal.attach carries TerminalInput uplink items and TerminalFrame downlink items; the Client pipelines commands and releases its raw-input byte budget on ack	packages/api/terminal-controller/src/client/model.ts; Host src/terminal.ts	The current attachment owns input; inputFull when the Client budget is exhausted; input is never replayed after reconnection
File upload	A Blob or ReadableStream goes to the native POST /api/session/uploadFileBinary outside the Remote layer, with duplex: 'half'	packages/client/file-upload/src/client/runtime.ts; route src/index.ts	Bypasses Typert descriptors; no type projection
Answers to approvals and questions	The Host sends waterfall frames down the $events stream, the Client answers through a separate unary RPC $events/result, correlated by clientId plus eventId	packages/api/gateway/src/client/remote-events.ts; Host index.ts	A second correlation key; index.ts records the race this creates

These consumers need Client-to-Host data while a related operation remains active. Web terminals use the duplex stream; file upload and event answers retain their own transports.

Consumer responsibilities

• Input authority. A terminal retains its controlling attachment as a Host-local follower. Snapshot and state frames report each attachment's writable state; older attachments keep receiving output after control transfers.
• Ordering and acknowledgement. Terminal writes and resizes share a consecutive command sequence and the terminal's operation queue. An ack reports provider acceptance or a control refusal; it never authorizes replay after disconnection.
• Independent lifetimes. Cancelling attach stops its input and output delivery, drops unstarted commands, and joins an in-flight input operation. The PTY lifetime is owned separately by retention and explicit close.
• Coordinated byte limits. The Gateway exposes its complete-frame inbox capacity and this stream's JSON envelope cost through invocation.uplinkWindow. The terminal publishes that window in its snapshot and uses provider acknowledgements to release in-flight capacity. Writes and resizes wait in a bounded Client queue when the window is full. maxInputBytes limits raw UTF-8 input; maxBufferedBytes separately limits the complete encoded input retained by the Client and output retained by each follower.
• Remaining separate carriers. Approval answers still correlate $events with the $events/result RPC through clientId and eventId. Moving this protocol onto the duplex stream remains deferred.

Relation to background jobs

The process-shaped duplex stream face of background jobs (archived as a draft PR on branch worktree/job-stdio-stream-pair-archive) models a job as a pair of mirrored stream faces; for the browser to attach to it, one Remote stream must carry both a downlink and an uplink. It is one motivation for this Note but not in its scope: whether jobs need a dedicated read-write stream is an architecture question for the jobs line itself. This Note covers only the transport layer.

Goals

1. The transport layer has one kind of stream, duplex by nature: downlink item / end / error, uplink item / end, one streamId, one generation.
2. The uplink type and the downlink type are written in the same place: the method's return type.
3. A Host method takes its uplink from the invocation context; the invocation context also carries the caller (the Peer).
4. The Client receives a handle that can read, write, and close, not a bare AsyncIterable.
5. Upper-layer protocols (snapshot, journal, any business-defined stream) stay out of the transport layer.

Decision

In one sentence

Every Remote stream accepts a Client uplink. The method's return type RemoteStream<Out, In = never> declares the downlink item type and the uplink item type together; the generator derives two codecs from it. A Host method takes the uplink iterator through this.ctx.invocation.uplink<In>() and learns the caller through this.ctx.invocation.peer. The generated Client method returns a RemoteStreamHandle<Out, In>: for await reads the downlink, send / end write the uplink, dispose closes. Reopening after disconnection, cursor resumption, and baseline validation all belong to upper-layer protocols.

                 Client                                             Host
   ─────────────────────────────────              ───────────────────────────────────────────
   const s = remote.job.attach(req, signal)
        │  open { streamId, endpoint, payload } ───────▶  opener(endpoint, payload, uplink, peer, control, window)
        │                                                     │ prepareInvocation
        │  s.send(v1) → item { streamId, v1 }    ───────▶  inbox.push(v1)   │ invocation = { request, service, peer, signal, uplink() }
        │  s.send(v2) → item { streamId, v2 }    ───────▶  inbox.push(v2)   │ receiverContext.extend({ invocation }).get(service)
        │  s.end()    → end  { streamId }        ───────▶  inbox.end()      │ method(...args, signal)
        │                                        ◀───────  item { streamId, o1 }   │   for await (v of this.ctx.invocation.uplink()) …
        │  for await (o of s) …                  ◀───────  item { streamId, o2 }   │   yield o …
        │  s.dispose() → cancel { streamId }     ───────▶  control.abort()         │ signal aborts; uplink.return()
        │                                        ◀───────  end | error

Terms

Term	Meaning
Logical stream	One call identified by one streamId, spanning open to the terminal frame
Generation	One physical lifetime of a logical stream. The transport layer knows exactly one generation; disconnection is failure, and reopening belongs to the upper layer
Downlink	The Host-to-Client item sequence, terminated by end or error
Uplink	The Client-to-Host item sequence, terminated by end (half-close) or ended together with cancel
Half-close	One direction has ended while the other remains open
Item	One business value in one direction; every item is validated by the codec independently
inbox	The bounded uplink queue the Host keeps per logical stream
Peer	The party one connection speaks for, one PeerScope carrying its principal; every browser connection shares connection.browserPeer, and in-process code creates its own through connection.createPeer() (Peer principals)
Invocation context	RemoteInvocation: the request, receiving service, Peer, cancellation signal, and uplink entry of this call

Type signatures

// @deepseek-ai/dsh-typert-protocol
/**
 * One Remote stream. Host face: the method returns it, and at runtime it is AsyncIterable<Out>.
 * On the Client face the generated method returns RemoteStreamHandle<Out, In>; each name has exactly one meaning.
 * In is the item type the Client may send uplink; the default never means the method reads no uplink.
 */
export type RemoteStream<Out, In = never> = AsyncIterable<Out> & { readonly [STREAM_UPLINK]?: In }

// Host
@Remote({ mode: 'stream' })
async *attach(request: JobAttachRequest, signal: AbortSignal): RemoteStream<JobFollowFrame, JobInputFrame> {
  const uplink = this.ctx.invocation.uplink<JobInputFrame>()
  void (async () => { for await (const frame of uplink) … })()
  yield …
}

// Generated Client signature: same parameters, returns the handle
attach(request: JobAttachRequest, signal?: AbortSignal): RemoteStreamHandle<JobFollowFrame, JobInputFrame>

// Client usage
const stream = remote.job.attach(req, signal)
for await (const frame of stream) …
stream.send(frame)     // typed as JobInputFrame
stream.end()
stream.dispose()

RemoteStream is an alias rather than an interface, deliberately: if the transport layer later takes a Node stream form, only the alias definition and the adapters at both ends change; method signatures, descriptors, and upper-layer protocols stay untouched.

Host face

/** The context of this Remote call. A method reads it through this.ctx.invocation. */
export interface RemoteInvocation {
  readonly request: {
    readonly namespace: string
    readonly method: string
    readonly args: Readonly<Record<string, unknown>>
  }
  /** Cordis service key of the receiving service. */
  readonly service: string
  /** The Peer that initiated the call; every carrier names it explicitly. */
  readonly peer: PeerScope
  /** Carrier cancellation: Client cancel, socket closure, uplink failure. */
  readonly signal: AbortSignal
  readonly uplinkWindow?: RemoteUplinkWindow | undefined
  /**
   * The uplink items of this call. Can be taken once; the second call throws. With an uplink codec
   * in the descriptor each item is decoded to In; without one, items are delivered as unknown with
   * only a JSON-safety check.
   * The iteration ends after the Client's end; when the method ends the downlink, the Gateway calls
   * its return() and drops unconsumed items.
   * The generic In is only the caller's type assertion; runtime decodes per the descriptor and does not cross-check.
   */
  uplink<In = unknown>(): AsyncIterable<In>
}

declare module '@deepseek-ai/cordis' {
  interface Context {
    /** The Remote call this Context was derived for; undefined on a Context not derived from a Remote call. */
    readonly invocation: RemoteInvocation | undefined
  }
}

Rules:

1. Any mode: 'stream' method may call this.ctx.invocation.uplink(); a unary method may too, but its stream ends when the method returns, so uplink items are readable only while the method runs.
2. While uplink() has not been called, uplink items accumulate in the inbox until the limit is exceeded (see Backpressure) or the stream ends and they are dropped. This is deliberate: a method that does not read the uplink need not know the uplink exists.
3. When In defaults to never, the descriptor has no uplink codec; the Client handle's send has type never, so nothing can be sent at compile time. If frames still arrive at runtime (from a hand-built client), they are delivered as unknown.
4. this.ctx.invocation is undefined on a Context not derived from a Remote call; a method that reads it handles the optional as TypeScript requires, or asserts it on the method's first line. The accessor for that property is registered once when the TypertRemoteService base class is constructed, so any composition that installs a Remote service can read it.

How the invocation context reaches the method

The mechanism relies on two existing Cordis behaviors:

• ctx.extend(meta) is Object.create(parent) plus own properties (vendor/cordis/src/context.ts:99-107): no registration, no fiber, nothing to release, so deriving one per call costs nothing.
• ctx.get(service) returns the createTraceable(ctx, service) proxy (vendor/cordis/src/reflect.ts:233-234, utils.ts:117-125). Reading the ctx property returns the accessing Context directly (utils.ts:175); reading a method wraps it in createShadowMethod, whose apply replaces this with a shadow receiver whose ctx is ctx.extend({ [symbols.shadow]: origin }) (utils.ts:149-163).
• The Gateway therefore writes receiverContext.extend({ invocation }).get(descriptor.service) to obtain a per-call view and calls Reflect.apply(method, callReceiver, args); inside the method body, this.ctx.invocation reads this call through the prototype chain. The deferred execution of an async generator body is unaffected, because this was bound to the shadow object at call time.

PeerScope

/** The opaque identity of one Peer. */
export type PeerId = Branded<'PeerId'>

/** One Peer's session on this Host: opened and released by its creator; ctx owns connection-lifetime registrations. */
export interface PeerScope {
  readonly id: PeerId
  readonly ctx: Context
  /** Caller identity fixed at creation. */
  readonly principal: Principal
  /** Aborted once the Peer is disposed. */
  readonly signal: AbortSignal
  /** Participation in forwarded interactions; independent of RBAC. */
  readonly interactions: 'answer' | 'none'
  dispose(): Promise<void>
}

@deepseek-ai/dsh-client-connection implements the one Peer class, Peer(owner, { principal, interactions? }), with createScope(owner, peer), the same mechanism an Agent uses to create its own scope: the Peer object is the ScopeKey, peer.ctx carries connection-lifetime effects, peer.signal aborts when the scope ends, and dispose() quiets the fiber. Connection creates connection.browserPeer with the owner principal when the service applies; Peer principals and method-level Remote access control owns principals, createPeer(), and access checks.

Member	Semantics
connection.browserPeer: PeerScope	The Peer every authenticated browser connection shares; owner principal; lives and dies with the Connection
connection.admit(request): PeerAdmission	Checks Host/Origin and signed browser or API Cookies; returns { rejection: 401 | 403 } or the credential's live Peer; connect publishes the API Peer and its lifecycle socket releases it

The Gateway's handleUpgrade(req, socket, head, peer) binds an opener with a fixed Peer to that socket and registers socket closure in peer.ctx.effect; when the scope is already inactive, it closes the socket directly with 1001 instead. InvokeRemoteRequest.peer and the peer parameter of RemoteStreamOpener and wireStream.open are required, so every in-process caller names its Peer, and the Gateway creates none. Looking up a Peer by id and Peer open and close events wait until the first consumer that needs them appears.

The Peer carries its principal; what that principal may call is decided by the roles, bindings, and policies that Peer principals and method-level Remote access control defines.

Client face

/** The handle a generated method returns on the Client face. A different name from the Host's RemoteStream, exported from the same entry point. */
export interface RemoteStreamHandle<Out, In> extends AsyncIterable<Out> {
  /** Send one uplink item. Throws on a value that is not lossless JSON, after termination, or after end. */
  send(item: In): void
  /** Half-close the uplink: sends the end frame. Idempotent. */
  end(): void
  /** Cancel the whole logical stream: sends the cancel frame (when no terminal frame has arrived); the downlink iterator ends quietly. */
  dispose(): void
}

• The handle represents one generation. When the carrier is lost, for await fails with RemoteStreamCarrierError and the handle ends there; whether to reopen, and with which arguments, belongs to the upper-layer protocol.
• send is synchronous: it runs the isRemoteJsonValue check before enqueuing and throws synchronously on a value that is not lossless JSON. The browser's WebSocket.send has no write callback, so throttling sends would be meaningless; the Host-side inbox limit is the only backpressure point.
• When a terminal frame (end / error) arrives, the mux client stops the pump and closes the uplink queue immediately on the receive path, so later send / end throw without waiting for the consumer's next read.
• dispose() calls return() on the carrier iterator, so the cancel frame is sent even when nobody is reading the downlink; afterwards the handle's next() ends immediately and buffered items are dropped. A consumer that breaks out of for await early is equivalent to dispose().
• The $stream supervisor, RemoteSnapshotStream, and RemoteJournalStream are upper-layer tools outside the transport layer. The object their open factories receive is the handle itself, with unchanged for await behavior; exposing send to these upper-layer protocols is deferred.

Wire frames

export type RemoteStreamClientMessage =
  | { readonly type: 'open'; readonly streamId: string; readonly endpoint: string; readonly payload: unknown }
  | { readonly type: 'item'; readonly streamId: string; readonly value?: unknown }   // new: uplink item
  | { readonly type: 'end'; readonly streamId: string }                              // new: uplink half-close
  | { readonly type: 'cancel'; readonly streamId: string }

/** Frames sent by the Host are unchanged. */
export type RemoteStreamServerMessage =
  | { readonly type: 'item'; readonly streamId: string; readonly value?: unknown }
  | { readonly type: 'error'; readonly streamId: string; readonly error: RemoteStreamFailure }
  | { readonly type: 'end'; readonly streamId: string }

Parsing rules: item has exactly the keys type and streamId plus an optional value, and value must be a lossless JSON value (isRemoteJsonValue); end has exactly the keys type and streamId. Any other frame is a frame-format violation, and the socket closes with 1008. No binary frames are introduced.

Logical stream state machine

Host side

                open frame
   (none) ───────────────────▶ opening ──── opener resolves ────▶ running
                                  │                                  │
                                  │ item frame: inbox.push           │ item frame: inbox.push
                                  │ end frame: inbox.end             │ end frame: inbox.end (uplink half-close)
                                  │                                  │ method yield: sends item frame
                                  │ cancel frame / socket close ────▶│ cancel frame / socket close: control.abort
                                  ▼                                  ▼
                               aborted ◀───────────────────────── finished (sends end or error frame)

item frames that arrive during opening enter the inbox: receive() is synchronous and creates the ActiveStream and its inbox while handling the open frame, whereas the opener is asynchronous. Items the Client sends immediately after open are not lost.

Client side

   call generated method ──▶ await socket ──▶ send open ──▶ handle usable
                                                            │ send(v): sends item; end(): sends end
                                                            │ downlink: inbox.next() → yield; end → ends; error → throws
                                                            │ downlink terminates first: send/end throw
                                                            │ dispose or caller signal abort: sends cancel (when no terminal frame has arrived)

Half-close and termination

Uplink	Downlink	Result
Client end	Still open	The Host's uplink() iteration ends; the method keeps producing. This is the stdin EOF form
Still open	The Host method finishes and sends end	The Client handle terminates, and later send / end throw; the Host calls return() on the uplink iterator and drops unconsumed items
Still open	The Host method throws and sends error	Same as above; the Client downlink fails with RemoteError
Client dispose or signal abort	Any	Sends cancel; the Host runs control.abort(), cancellableStream closes uplink first and then calls return() on the method iterator; no terminal frame is sent
Socket closes	Any	The Host aborts every stream and awaits done; on the Client every stream fails with RemoteStreamCarrierError
item received after end	Any	The stream fails with a gateway/protocol error frame and aborts; the socket stays open
item / end with an unknown streamId	Any	Ignored, like cancel: after the Host ends a stream and deletes its id, uplink frames still in flight from the Client are normal and must not take down the other streams on the same socket

Cancellation wait lifetime

cancellableStream, UplinkDecoder.next(), and the Client uplink pump each create a cancellation or stop promise per read. Promise.race does not remove the losing input's reaction when a read completes: reusing a stream-wide pending promise retains completed races and their item results until that promise settles or becomes unreachable. Read-local promises keep retained cancellation state independent of the number of delivered items.

The Host downlink clears its current reject callback after each read and checks cancellation before reading again. The uplink decoder retains only pending reads: abort rejects them, while normal close resolves all of them with done, including overlapping reads. The Client pump drops its stop promise after each read and does not start another read after stopping. Source iterators retain their existing release ownership; closing the Host uplink still precedes returning the method iterator.

Ordering guarantees

Frames on one socket arrive in total order: a logical stream's open, item…, end reach the Host in send order, and uplink items enter uplink() in the order of send. There is no cross-direction ordering guarantee between uplink and downlink; a protocol that needs request-response pairing carries its own sequence numbers.

Backpressure

WebSocket has no per-stream window like HTTP/2: one connection is one serial write chain, and a slow stream holds up the other streams on the same connection. This property of the downlink is unchanged. For the uplink:

A bounded inbox plus whole-stream failure. The Host meters each logical stream's inbox in UTF-8 bytes of its frames, with the limit set by the Gateway config streamInboxBytes (default 262144) and applied to every stream. On overflow the whole logical stream fails with gateway/uplink-overflow: the downlink breaks with it, an error frame goes to the Client, and the uplink() iterator ends with the same error. No data is discarded; a method that does not read its uplink while the Client keeps sending is a usage error, and the failure exposes it explicitly.

Credit frames wait until a consumer that needs sustained high-volume uplink appears; the inbox limit is the initial credit, and adding them is incremental.

Security and validation

Item	Rule
Authentication	Same as today: connection.requestRejection(req) at upgrade; descriptor and lookup-parameter validation at open. Uplink items belong to an already authenticated stream and are not authenticated again
Peer	Admitted at upgrade as one PeerScope; invocation.peer of every stream on that socket is that Peer
Item validation	With an uplink codec, every item is decoded strictly, and a failure fails the whole stream with the existing gateway/input-invalid (details { endpoint, field: 'uplink' }); without a codec, only JSON-safety validation applies
Frame size	A single frame keeps the maxPayload of ws; the inbox limit bounds the accumulated total
Frame compression	Each connection negotiates permessage-deflate when the Gateway config websocketCompression is 'deflate' (the default); frames below 1,024 characters are sent uncompressed. See the compression decision

In-process and worker carriers

Carrier	Change
ClientConnectionRpc.open (packages/client/connection/src/rpc.ts:245-250) and RpcStreamOpen (client/rpc.ts:21-25)	Append an optional uplink?: AsyncIterable<unknown>; in process there are no frames and no inbox, the caller's iterator is the source of uplink() directly, and backpressure is the pace of the iterator itself
Worker tunnel (packages/experimental/webworker-runtime)	The page adds stream-uplink-item / stream-uplink-end frames; serveStream assembles them into an uplink handed to seams.openStream. Same-origin trusted boundary: no inbox limit, and items after end are dropped
remote-mock (packages/test-support/remote-mock)	StreamHandle.uplink: AsyncIterable<unknown>; rpc.open passes it through

An in-process carrier passes its Peer explicitly; the in-page Host of the worker tunnel passes connection.browserPeer.

Relation to $events

$events is a stream on a reserved endpoint; with uplink frames, waterfall answers can become uplink items on the same streamId, clientId becomes redundant because streamId already identifies the generation, and $events/result disappears together with the race at index.ts:527-529. Out of scope for this Note; see Deferred.

Type and wire definitions

typert protocol (packages/typert/protocol/src)

// types.ts
export type RemoteStream<Out, In = never> = AsyncIterable<Out> & { readonly [STREAM_UPLINK]?: In }
export type PeerId = Branded<'PeerId'>
export interface PeerScope { readonly id: PeerId; readonly ctx: Context; readonly principal: Principal; readonly signal: AbortSignal; dispose(): Promise<void> }
export interface RemoteInvocation { … }              // see Host face
declare module '@deepseek-ai/cordis' { interface Context { readonly invocation: RemoteInvocation | undefined } }

export interface InvocationDescriptor {
  // existing fields unchanged; mode still has only 'stream'
  readonly mode?: 'stream'
  /** Uplink item codec, generated from In of the return type RemoteStream<Out, In>; absent when In is never. */
  readonly uplink?: { readonly codec: TypertCodec }
  readonly cancellation?: { readonly parameter: 'signal' }
  readonly result: TypertCodec
}

The Remote decorator, RemoteMethodOptions, and RemoteMethodMarker recognize only mode: 'stream'.

typert generator (packages/typert/generator/src)

• model.ts: InvocationModel.uplink?: { boundary: RemoteBoundaryModel }.
• analyzer.ts remoteResultType: for mode: 'stream', the accepted return-type wrappers are Iterable<Out>, AsyncIterable<Out>, and RemoteStream<Out, In?>. RemoteStream is recognized the same way as the standard library's AsyncIterable: by symbol name plus declaring file (types.ts of @deepseek-ai/dsh-typert-protocol). The first type argument is the downlink item; when the second is present and is not never, an uplink boundary is generated under the key ${endpoint}:uplink.
• emitter.ts: the descriptor literal emits uplink: { codec }; the generated Client signature returns RemoteStreamHandle<Out, In>.
• The parameter loop recognizes no parameter named uplink.

Two names, one entry point

A Host method declares a stream with RemoteStream<Out, In>; the Client holds a RemoteStreamHandle<Out, In>. Both are exported from the main entry point of @deepseek-ai/dsh-typert-protocol, and the handle interface has no Host dependency. The generated Client contract writes the return type as RemoteStreamHandle<Out, In>. One name has one meaning: the RemoteStream that Client code obtains from the main entry point is always the declaration type and is never confused with the handle.

Gateway Host (packages/api/gateway/src)

// types.ts
export interface InvokeRemoteRequest {
  readonly namespace: string
  readonly method: string
  readonly args: Readonly<Record<string, unknown>>
  /** Uplink items; absent is equivalent to an iterator that ends immediately. */
  readonly uplink?: AsyncIterable<unknown>
  /** The Peer that initiated the call; required on every path. */
  readonly peer: PeerScope
  readonly signal?: AbortSignal
}

export interface TypertGatewayWireStream {
  open: (endpoint: string, payload: unknown, uplink: AsyncIterable<unknown>, peer: PeerScope, signal: AbortSignal) => Promise<AsyncIterable<unknown>>
  failure: (error: unknown) => RemoteStreamFailure
}

// stream-server.ts
export type RemoteStreamOpener = (
  endpoint: string,
  payload: unknown,
  uplink: AsyncIterable<unknown>,
  peer: PeerScope,
  control: AbortController,
  window: RemoteUplinkWindow,
) => Promise<AsyncIterable<unknown>>

// index.ts Config addition
/** Uplink inbox limit per logical stream, in UTF-8 bytes of frames (default 262144). */
readonly streamInboxBytes?: number
/** Per-message WebSocket compression; 'none' disables the extension (default 'deflate'). */
readonly websocketCompression?: 'none' | 'deflate'

prepareInvocation:

const args = await Promise.all(descriptor.parameters.map(parameter => this.resolveParameter(parameter, request.args, endpoint)))
const signal = request.signal ?? NEVER_ABORTED_SIGNAL
const invocation = new GatewayInvocation(
  { namespace: request.namespace, method: request.method, args: request.args },
  descriptor.service,
  request.peer,
  signal,
  new UplinkDecoder(request.uplink ?? EMPTY_ASYNC_ITERABLE, descriptor.uplink?.codec, endpoint, control),
)
const callReceiver = receiverContext.extend({ invocation }).get(descriptor.service) as object
if (descriptor.cancellation !== undefined) args.push(signal)

GatewayInvocation.uplink() returns the decoder on the first call and throws on the second. UplinkDecoder is a hand-written iterator: with a codec it runs decode(codec, value, endpoint, 'uplink') per item and, on failure, calls control.abort(failure) and throws; without a codec it applies only the isRemoteJsonValue check. The finally of cancellableStream closes the decoder first (invocation.close()) and then awaits the method iterator's return(), so a method blocked on uplink.next() can exit. The mode branches are the existing two, unary and stream. SRC fallback: the descriptor has no uplink codec, and uplink() delivers unknown.

Gateway Host mux (stream-server.ts)

interface ActiveStream { readonly control: AbortController; readonly inbox: UplinkInbox; done: Promise<void> }

/** Bounded uplink queue; iterated as the source of uplink(). */
class UplinkInbox implements AsyncIterable<unknown>, AsyncIterator<unknown> {
  constructor(maxBytes: number, onOverflow: (error: RemoteError) => void)
  push(value: unknown, frameBytes: number): void   // push after ended fails the stream with gateway/protocol; overflow calls onOverflow
  end(): void                                        // idempotent
  fail(error: unknown): void                         // ends the iterator with an error on cancel or carrier closure
  next() / return()                                  // hand-written: next races failure, return releases immediately
}

receive(text) dispatch:

open   : exists → throw (socket 1008); otherwise create { control, inbox, done } and start the pump
item   : missing → ignore; otherwise inbox.push(value, bytes)
end    : missing → ignore; otherwise inbox.end()
cancel : missing → ignore; otherwise control.abort(new Error('Remote stream cancelled'))

pump passes the inbox and the socket's Peer to the opener and calls inbox.fail(...) in its finally. An abort whose reason is a RemoteError is sent by the pump as an error frame (a failure raised by the Gateway or the mux); an abort whose reason is a plain Error (cancel, socket closure) sends no terminal frame.

Gateway Client (packages/api/gateway/src/client)

• prepareInvocation strips no uplink parameter; the positional parameters are the business parameters plus the optional signal.
• invokeStream returns a RemoteStreamHandle: it holds an uplink queue internally, send enqueues and a pump sends the item frames, end sends the end frame, and dispose aborts the generation signal. The downlink is still the iterator that openRemoteStream returns.
• RemoteStreamMuxClient.open(endpoint, payload, signal, uplink?): after sending the open frame it starts the uplink pump, which sends one item per item and end when the source ends; when the downlink terminates or the signal aborts, it stops the pump and calls the source's return() (without awaiting, so a keyboard-style generator cannot hang it); when the source throws, it calls inbox.fail(error), the downlink fails with that error, and finally sends cancel under the existing rules. The pump checks this.socket === socket, so an old generation never sends frames on a new socket.
• In-process carrier: connection.rpc.open(channel, endpoint, payload, signal, uplink).

Placement

Package	Carries
dsh-typert-protocol (types.ts, index.ts)	RemoteStream, RemoteStreamHandle, PeerId, PeerScope, RemoteInvocation, the ctx.invocation declaration merge, InvocationDescriptor.uplink; the decorator recognizes only mode: 'stream'; TypertRemoteService registers the invocation accessor when constructed
dsh-typert-registry	Load-time validation that uplink.codec is a valid strict codec
dsh-typert-generator (model.ts, analyzer.ts, emitter.ts)	remoteResultType recognizes RemoteStream<Out, In> and generates the uplink boundary; the descriptor emits uplink: { codec }; the generated Client signature returns RemoteStreamHandle<Out, In>; the parameter loop accepts only business parameters and a final signal
dsh-client-connection (peer.ts, rpc.ts, rpc-host.ts, index.ts)	The Peer class; connection.browserPeer and admit; the /api and upgrade routes obtain the Peer through admit; the optional uplink of rpc.open
dsh-api-gateway Host (stream-protocol.ts, stream-server.ts, types.ts, index.ts)	item / end frames; UplinkInbox; Config.streamInboxBytes and Config.websocketCompression; GatewayInvocation and extend({ invocation }); UplinkDecoder; handleUpgrade(req, socket, head, peer)
dsh-api-gateway Client (client/index.ts, client/stream-client.ts)	invokeStream returns the handle; the uplink pump is driven by the handle's queue
dsh-webworker-runtime, dsh-remote-mock	The worker tunnel's two uplink frames and the serveStream assembly; StreamHandle.uplink; the direct proxy returns the real handle
Documentation	The READMEs of gateway, the api group, connection, protocol, and generator; docs/api-gateway; the type-equiv blocks of docs/subsystems/typert; the config and Cordis catalogs

Key sequences

Open, uplink, half-close, downlink end

Client caller      mux client              mux server              gateway                 Host method
   │ attach(req)        │                       │                     │                         │
   │───────────────────▶│ open { id, ep, payload } ──────────────────▶│ create inbox            │
   │ s.send(v1)         │ item { id, v1 } ───────────────────────────▶│ inbox.push(v1)          │
   │                    │                       │                     │ prepareInvocation       │
   │                    │                       │                     │ extend({ invocation })  │
   │                    │                       │                     │── method(req, signal) ─▶│
   │                    │                       │                     │                         │ uplink() → v1
   │ s.send(v2)         │ item { id, v2 } ───────────────────────────▶│ inbox.push(v2)          │ → v2
   │ s.end()            │ end { id } ────────────────────────────────▶│ inbox.end()             │ iteration ends
   │◀── yield o1 ───────│◀────────────────────── item { id, o1 } ─────│◀── yield o1 ────────────│
   │◀── ends ───────────│◀────────────────────── end { id } ──────────│◀── method returns ──────│ gateway calls uplink.return()

Client dispose

Client caller      mux client              mux server              gateway                 Host method
   │ s.dispose()        │                       │                     │                         │
   │───────────────────▶│ stop pump; source return() (not awaited)    │                         │
   │◀── iterator ends ──│ cancel { id } ─────────────────────────────▶│ control.abort()         │
   │                    │                       │                     │ cancellableStream: invocation.close() closes uplink
   │                    │                       │                     │ iterator.return() ─────▶│ finally cleanup
   │                    │                       │ no terminal frame   │                         │

inbox overflow

Client caller      mux client              mux server                        gateway / Host method
   │ s.send(vN)         │ item { id, vN } ─────▶│ inbox.push: bytes > streamInboxBytes │
   │                    │                       │ onOverflow → control.abort(RemoteError uplink-overflow)
   │                    │                       │                                      │ uplink() ends with that error; method iterator return()
   │◀── throw ──────────│◀──────────────────────│ error { id, uplink-overflow }: the pump sends an error frame because the reason is a RemoteError

Boundary matrix

Event	opening	running	Uplink ended	Downlink terminated
item received	Buffered in the inbox	Into the inbox	The stream fails with gateway/protocol	Ignored
end received	inbox.end	inbox.end	Idempotent	Ignored
cancel received	abort	abort	abort	Ignored
inbox overflow	The stream fails with uplink-overflow	Same	Impossible	Impossible
Decode failure	Impossible	The stream fails with input-invalid	Same	Impossible
Host method finishes	Impossible	Sends end; uplink.return()	Sends end	Already sent
Host method throws	Sends error	Sends error; uplink.return()	Sends error	Already sent
Socket closes	abort, await done	Same	Same	Nothing
Client send	Queued, sent after open	Sent	Throws	Throws
Client end	Recorded, sent after open	Sent	Idempotent	Ignored
Client-side downlink ends first	Impossible	Stop the pump; source return(); no end sent	Nothing	Nothing

Error codes

Code	When
gateway/input-invalid	Existing: a parameter codec fails to decode; new: an uplink item fails to decode
gateway/uplink-overflow	New: the inbox exceeds streamInboxBytes
gateway/protocol	New: an item after end
gateway/cancelled	Existing: signal aborted

New codes are registered in packages/api/gateway/src/remote-error-codes.ts and in the README's error-code section.

Minimal echo method

packages/api/gateway/tests/echo-stream.host.spec.ts runs the same method through the WebSocket carrier and through the in-process carrier:

class EchoService extends TypertRemoteService {
  constructor(ctx: Context) { super(ctx, 'echo', { namespace: 'echo' }) }

  @Remote({ mode: 'stream' })
  async *echo(prefix: string, signal: AbortSignal): RemoteStream<string, string> {
    for await (const item of this.ctx.invocation!.uplink<string>()) {
      signal.throwIfAborted()
      yield `${prefix}${item}`
    }
  }
}

// Client
const stream = remote.echo.echo('> ')
stream.send('a'); stream.send('b'); stream.end()
const replies: string[] = []
for await (const reply of stream) replies.push(reply)   // ['> a', '> b']

Alternatives considered

A shared cancellation controller for all three reading loops. Each loop already has an owner with distinct close behavior. Read-local promises bound retained state without moving those responsibilities into a new abstraction.

Requiring source iterators to cancel their own next(). Gateway accepts arbitrary iterables, whose pending reads need not respond to cancellation or return(). Gateway must still wake its own waiters independently of source cleanup.

A separate mode: 'duplex' plus a reserved parameter uplink: AsyncIterable<In> alongside signal. Special logic everywhere: the analyzer must recognize a second reserved parameter and its position rules, the generated signature gains a parameter, load time must validate duplex ⟺ uplink, and because the mux does not know the method mode, the Gateway must reject uplink frames on behalf of 'stream' methods. Duplex is an inherent property of the transport-layer stream, not a mode of the method: the carrier already has streamId, open, and cancel; the uplink adds just two more frames, which any stream can receive and which are simply not sent when unused.

Declaring the uplink type in a decorator option, @Remote({ mode: 'stream', uplink: 'JobInputFrame' }). A string reference to a type name is not checked by the compiler and breaks on rename. The return-type alias puts both types in one compiler-visible place.

Passing a schema at runtime: uplink(schema). Bypasses the generator, the Client loses type projection, and it splits from the "strict codec per item" system. The codec still comes from the generator; the generic in the method is only an assertion.

Having the analyzer cross-check the hand-written generic of uplink<In>() against the return type. A business-specific check whose mistakes the caller bears, consistent with the other hand-written assertions in the repository; not added.

A transport-layer handle with reopening, cursors, and a reopen hook. Reopening after disconnection belongs to the upper-layer protocol: a job resumes through an opened frame carrying from and a client that remembers next and calls again; snapshot and journal each have their own anchors and classification. The transport layer knows one generation.

Putting the Client's send on the AsyncIterable the generated method returns. Hanging methods on an AsyncIterable is unconventional; the handle type RemoteStreamHandle extends it explicitly.

Copying the Access mechanism. Access control is business semantics, served by vouch and the remote/invoke waterfall; this Note only needs "whose stream is this", and PeerScope suffices. Method-level access control later moved to Peer principals and method-level Remote access control, which supersedes this part.

Keeping unary RPC for the uplink. One HTTP round trip, one authentication, and one lookup per uplink; two carriers with no ordering relation that only Client-side serialization can order; no EOF; cancellation that does not cover both halves; a hand-made correlation key per feature.

A native HTTP request body stream. A one-shot request body with no association to the downlink stream, bypassing Typert descriptors, and browser support for full-duplex fetch cannot be relied on.

One dedicated WebSocket per duplex stream. Loses the authentication, heartbeat, and multiplexing shared with the existing downlink streams, and every feature manages its own socket lifecycle.

WebTransport or HTTP/2 bidirectional streams. They have per-stream flow control, but support on the browser and Node sides, proxy traversal, and the Electron carrier are all immature; adding two frames to the existing mux is orders of magnitude cheaper than replacing the transport layer.

Consequences

• Bought: one stream with two directions, so interactive scenarios such as terminal keystrokes, approval answers, and background-job stdin no longer each build their own "one stream plus one unary"; a Host method knows its caller; the uplink and downlink types are declared in one place, the return type; uplink items come from the browser and are validated strictly per item at the Host, while downlink items are Host-produced typed values that pass straight through; the generated output and wire frames of existing AsyncIterable<Out> methods and unary methods are completely unchanged.
• this.ctx.invocation is undefined outside a Remote call, so a method that reads it must handle the optional; a service method invoked only directly in process correctly reads undefined.
• Web terminals use attach for screen output and input commands, with provider acknowledgements and per-attachment writable state; their Client retains the current generation's handle and never replays input.
• The inbox limit applies to every stream: a method that does not read its uplink fails as a whole when it receives many uplink frames. This is a deliberate explicit failure, recorded in the README.
• Every browser connection shares browserPeer, and admit is the only network admission point; in-process code creates further Peers with connection.createPeer().
• No cross-direction ordering guarantee between uplink and downlink; a protocol that needs request-response pairing carries its own sequence numbers.
• Upper-layer supervisors expose only the downlink: $stream, snapshot and journal do not expose send; a domain that writes uplink retains its generated physical handle for that generation.
• The worker tunnel behaves differently from the WebSocket carrier: no inbox limit, and items after end are dropped, because page to worker is a same-origin trusted boundary.

Testing

Retention cases in gateway-stream.host.spec.ts and gateway.client.spec.ts count live iterator results after a full GC while the streams remain open. Lifecycle cases cover normal close waking multiple pending uplink reads, cancellation between reads, synchronous abort followed by a source exception, and termination during a Client send.

• packages/api/gateway/tests/echo-stream.host.spec.ts: the same echo method passes through the WebSocket carrier and through the in-process carrier.
• Gateway Host: exact-key validation in frame parsing; UplinkInbox buffering during opening, item after end, overflow, ignored unknown streamId, and cleanup on socket closure; UplinkDecoder delivery with and without a codec and abort on decode failure; uplink() taken only once; the extend({ invocation }) view and the origin of peer; cancellableStream closing the uplink before return().
• Gateway Client: the handle's send (JSON validation, queued before open, throws after termination), idempotent end, dispose driving the cancel frame and ending later reads; the pump's three exits; the pump stopping as soon as a terminal frame arrives.
• typert: the analyzer's handling of RemoteStream<Out, In>, RemoteStream<Out>, never, and the existing AsyncIterable<Out>; decorator options; registry load-time validation.
• connection: the three outcomes of admit, 401, 403, and browserPeer; browserPeer reaching handlers through the bridge and the shared handler; release with the Connection.
• remote-mock and the worker tunnel: uplink pass-through; the direct proxy returning the real handle.
• Existing stream and unary method tests pass without changed expectations.

Deferred

• $stream / snapshot / journal expose the handle's send to upper-layer protocols.
• Duplex $events: waterfall answers become uplink items on the same streamId, and $events/result and clientId retire.
• Credit frames: wait for a consumer that needs sustained high-volume uplink; the inbox limit is the initial credit.
• A Host-initiated half-close frame (the Host stops reading the uplink but keeps writing the downlink): no consumer; not added.
• uplink() on unary methods: it can read items that arrive while the method runs, semantically valid but of doubtful use; allowed, noted in the README.

Related

• Remote event delivery: the unary $events/result answer path; the duplex $events item under Deferred partially supersedes it once it lands.
• Session history and event transport: the $stream() supervisor and the journal / snapshot protocols above it, which this note leaves unchanged.
• Web sidebar terminal: the attach stream carries input commands, provider acknowledgements and per-attachment writable state; reconnect never replays input.