orca/docs/reference/remote-wire-compatibility.md

Remote wire compatibility

Orca's remote-server feature pairs a desktop client to a remote Orca runtime, and users update the two independently. Mixed versions are the normal state, not an edge case. This page is the contract for changing anything a paired client and host exchange: the runtime RPC envelope, the terminal binary stream, and the content either side publishes over them.

src/shared/protocol-version.ts says when to bump RUNTIME_PROTOCOL_VERSION. This page covers the changes that do not bump it and are therefore easy to get wrong.

Rule 1 — a new optional JSON field on an existing frame is safe

Every JSON payload is parsed with a decoder that ignores unknown keys (zod .strip() on RPC params, JSON.parse on stream frames). An older peer that has never heard of the field simply does not read it.

Safe:

// host adds a field; older clients ignore it
encodeTerminalStreamJson({ kind, cols, rows, hiddenOutputReason })

The field is safe only for as long as every reader treats it as optional. The moment a newer client requires it, that client is broken against every host that predates the field — which is the same defect as removing a field, just discovered later. If new behavior depends on the field being present, that is Rule 2: negotiate it, or make the reader fall back.

Rule 2 — a new stream opcode is NOT safe; negotiate it

decodeTerminalStreamFrame returns null for an opcode it does not know, and runtime-rpc.ts drops that frame without an error:

const frame = decodeTerminalStreamFrame(bytes)
if (!frame) {
  return // silently dropped — the sender never learns
}

So a new opcode sent to an older peer does not fail loudly. It vanishes, and the feature behind it appears to hang. Input sent under a new opcode is swallowed.

A new opcode must be announced in the subscribe handshake and sent only after the peer confirms it. The existing pattern is SetOutputPaused (opcode 16):

• the client advertises support in the Subscribe frame's capabilities;
• the host echoes capabilities: { outputPause: 1 } on the subscribed event;
• the client sends opcode 16 only after that echo (stream.supportsOutputPause);
• the host only acts on opcode 16 when it negotiated it (stream.supportsOutputPause).

Reuse an existing opcode with a new optional payload field (Rule 1) whenever that expresses the change; reach for a new opcode only when framing genuinely differs.

Opcode numbers are permanent. See the Ack = 13 and ClaimViewport = 14 comments in src/shared/terminal-stream-protocol.ts for why a shipped number cannot be reused even if the feature behind it is removed.

Rule 3 — changing what the host publishes breaks old clients with no wire change

The frame shape can be untouched and the skew still real, because clients react to frame content. PR #12641 is the worked example: the host stopped synthesizing a finished agent status, and clients running older code saw different content in an identical frame.

Treat these as wire changes even though nothing in the codec moves:

• a field the host stops populating (an old client reading it now sees undefined);
• a value whose meaning, units, or nullability changes;
• content the host stops synthesizing, trims, or starts deriving from a new source;
• a frame the host stops sending, or starts sending, on an existing path.

If old clients cannot interpret the new projection correctly, gate it behind a runtime capability the same way Rule 2 gates an opcode.

Enforcement

tests/e2e/cross-version-wire/cross-version-terminal-wire.unit.test.ts runs the real host RPC methods and the real renderer multiplexer from two builds against each other — current working tree against the newest release tag, in both skew directions — over one scripted terminal journey (subscribe, input, hide/reveal snapshot, drop, reconnect).

Run it with:

pnpm exec vitest run --config config/vitest.config.ts tests/e2e/cross-version-wire/cross-version-terminal-wire.unit.test.ts

It fails when a frame is refused by the receiving build's decoder (Rule 2), when the observed frame sequence changes (Rule 3), or when published snapshot content or negotiated capabilities differ from the contract. Adding an optional field keeps it green (Rule 1); making a client depend on that field turns the new-client/old-host pairing red.

tests/e2e/cross-version-wire/cross-version-agent-session-wire.unit.test.ts pairs the same two builds over the structured agentSession.* surface. Because a released build cannot name a capability string its own source never contains, the old side's advertised list and registered method names are read from the extracted checkout rather than hand-written. It covers the three skews that surface can fail on:

• an old client — advertising only what the baseline build defines — is told the whole surface does not exist and reaches no host method;
• a new client against the old dispatcher gets method_not_found on every method, and can see the absence during negotiation instead of by calling;
• a cursor survives a host restart: the client's fence is refused as stale with the live one attached, and resuming from the held cursor replays only what it missed.

Run it with:

pnpm exec vitest run --config config/vitest.config.ts tests/e2e/cross-version-wire/cross-version-agent-session-wire.unit.test.ts

The harness covers the terminal stream and the structured agent-session surface. It does not cover the session-tab sync channel, legacy agent-session publications, file or Git RPCs, mobile/E2EE framing, or the relay transport. A change on those paths still needs its own reasoning against the three rules above.

Worked example: agentWait on terminal and worker reads

terminal.show, orchestration.workerShow and orchestration.federationShow carry an optional agentWait naming a pane parked on a prompt only a human can answer. It is Rule 1 — a new optional field — but it has a second state that Rule 1 alone does not describe, and getting that wrong turns a skew into a false "nothing is blocked".

• present object — this pane is waiting, with the evidence that proved it.
• present null — the host evaluated this pane and nothing proves a wait.
• absent — the host never evaluated it: it predates the field, the worker identity was unverifiable, the pane was unreadable, or the agent probe did not answer in time.

A new client against an old host sees the field absent, which is why absence must read as unknown and never as not waiting. Collapsing absent into null at any hop — including a convenience ?? null in an RPC handler — makes an old or unreachable peer indistinguishable from a healthy idle worker, which is the exact failure the field exists to remove.

An old client against a new host ignores the key, as Rule 1 allows. New members added to RuntimeTerminalWaitBlockedReason are also Rule 1: no consumer switches exhaustively on it, and both the CLI and worker-start interpolate it as an opaque string.

Known debt: JSON-RPC errors drop Node's string code

An error raised on an SSH host crosses the relay as JSON-RPC, and ssh-channel-multiplexer rebuilds it with the TRANSPORT's numeric code. Node's string code — 'ENOENT', 'EACCES' — does not survive, so a caller on this side cannot ask what kind of failure it was.

isENOENT in src/main/ipc/filesystem-path-containment.ts pays for that by also matching Node's canonical message text, which is what makes remote worktree creation work. The cost is that a host can make an unrelated failure read as "absent" by putting that sentence in a message.

The exit is Rule 1: carry the original string code in a new optional field on the error payload and read that instead. An old host omits it and the message match still covers them; once hosts that send it are the floor, the message match can be deleted rather than lived with at its ~10 call sites. Narrowing isENOENT back to .code without doing this reinstates the bug — the transport has already overwritten it.

Known hazard: clients ignore host-published failure fields on client-placed pages

RuntimeMobileSessionBrowserTab — the browser tab a host publishes on the session-tab sync channel — permits placement, loadError and certificateFailure together. But for a tab whose placement.kind is 'client' the engine runs in the client's own app: the failure is raised by the local guest webview, and the host has no view of it (RuntimeBrowserClientPage, what the registry actually publishes from, carries neither field). Clients from this version on therefore refuse host ownership of both records for client-placed pages (web-session-tabs-sync.ts, the placement?.kind !== 'client' carve-outs) — without that, each metadata snapshot deletes the locally recorded failure and the page's failure overlay disappears mid-navigation.

The hazard is forward-facing and Rule 3 shaped. A host that later starts publishing loadError or certificateFailure for a client-placed page reaches these clients as content they silently drop, so the host would see no error and no effect. Publishing it has to be capability-gated, with the carve-out narrowed to clients that did not negotiate the capability. Note the cross-version harness does not exercise the session-tab sync channel, so nothing fails if this is forgotten — this note is the only record.

A related carve-out covers title, url, loading, canGoBack and canGoForward (resolveMirroredBrowserPageContent), and for those the hazard is already live rather than forward-facing: the host does publish them, from a RuntimeBrowserClientPage it can only learn about second-hand through the client's own browser.clientHost.pageMetadata calls. Its copy therefore starts at the registry defaults ('Browser', the create-time url), and while those publishes are failing it never leaves them.

That copy is not simply behind, though, and a client must not treat it as such. When a lease reattaches, the host refreshes the page from the client host's own inventory (runtime-browser-client-page-recovery.ts), which reads the live guest — so it can be strictly fresher than a local row whose pane is unmounted and whose metadata publisher was disposed with it. A client that ignores the host url is relying on its own guest to re-answer on remount, which ClientHostedBrowserPagePane's mount-time syncNavigation is what makes true.

These five are therefore refused only by the client whose guest actually runs the page: placement.browserHostClientId is compared against this client's own host id (readBrowserClientHostId). Main stamps that id into the guest-hosting window's additionalArguments at creation, and the preload reads it back out of its own argv — the answer has to be there before the first snapshot is interpreted, which is earlier than any IPC handler a renderer could wait on. Every other viewer — a second desktop, the web client, which installs no page renderer at all, the dashboard pop-out, which is deliberately left unstamped — keeps tracking the host, which is the only reason a mirrored viewer shows anything but its first snapshot forever. Improving what a second client sees still means fixing the publish, not the carve-out; the carve-out no longer stands in the way of it.

The two failure fields above are deliberately left on the looser placement?.kind !== 'client' predicate. It is unobservable today — the host publishes neither field for a client-placed page at all, so a mirror has nothing to take either way. If the capability-gated publish this section anticipates ever lands, narrow them the same way rather than by placement kind: a mirror should take a failure it cannot otherwise see, and only the hosting client should refuse it.