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.

Rule 4 — an enum arm set is a wire surface; unknown arms must degrade, never reject

A closed z.enum in a client-side reply schema is a version claim: it asserts the host will never send an arm this build has not heard of. A newer host that adds one arm then has its whole reply refused, or has the row carrying it silently dropped, even though every member the client actually reads is present and well-formed.

Declare the arm set open instead, with openEnum in src/shared/zod-salvage.ts:

// unknown arm degrades to a member the reader already handles; a non-string stays fatal
status: openEnum(GIT_BRANCH_COMPARE_STATUS, 'error')

Do not reach for .catch(). It swallows absence and the wrong type as well, which turns a member the reader depends on into a silent default.

A fallback does not have to be an arm. git.status's area is the worked example: staged, unstaged and untracked each grant an affordance, so coercing an unknown area to one of them offers stage, unstage or commit against a row the client cannot place. It degrades to absent instead, which withholds all three — every reader is an equality check against a known arm, so the row lands in no section — while keeping the row itself. That last part is the point. Dropping the row would also drop it from the unresolved-conflict gate, and a conflicted worktree that looks clean is granted a hosted-review create it should not have. Withholding an affordance is a degrade; removing the evidence a gate reads is not.

A fallback is only ever allowed to shape a reading. If the member is sent back to the host — a token the client echoes into a later call's params — pass it through as z.string() and let the send site keep it verbatim. hostedReview's provider is the case: the eligibility reply names it and the create call returns it, so an openEnum(..., 'unsupported') there does not soften how the client reads a newer host's provider, it puts unsupported on the wire and makes that host refuse its own. A reply-schema fallback must never shape a param. Gate on the token instead, where the client decides what it is willing to do with an arm it does not know.

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. Repeated frame shapes are compared by corresponding journey occurrence (initial, reveal, reconnect), so a field removed from one occurrence cannot hide behind a sibling that still publishes it. Adding an optional field keeps the suite green (Rule 1); making a client depend on that field turns the new-client/old-host pairing red.

Never write down what the old side has

The baseline is whichever release tag is newest, so it moves on every cut. An expectation of the form "the old side does not have X" — a not.toHaveProperty, a not.toContain, a hard-coded field list — stops being true the first time a release ships X. The suite then reddens on whatever pull request is in flight, with no code change anywhere, and the job trains people to ignore it. That is worse than no test, because a rolling baseline eventually contains every additive field the wire has, and adding one is the sanctioned way to evolve it.

Derive the expectation from the baseline that was actually checked out:

• for a published frame, pair each build against a client of its own version and compare the skewed pairing against that same-version reference, so the expectation is whatever that build publishes today. Compare repeated frames by corresponding occurrence with comparePublishedFieldOccurrences in tests/e2e/cross-version-wire/published-field-shape.ts; never union keys across initial, reveal, and reconnect frames, because a sibling can mask one occurrence's removed field;
• for a negotiated surface, read the old build's advertised capabilities and registered method names from its checkout, and assert they agree with each other rather than asserting the old build lacks them;
• for a "client too old to know X", derive that client's advertised list by removing X from the baseline's own list, so the gate stays exercised after X ships.

Name the direction in the assertion. new client against old server and old client against new server fail for different reasons, and the host is the only side that authors a published frame — the terminal terminalOwner false positive on 2026-08-29 was misread as a new client sending an unknown field when the old server was publishing it. Two things are still safe to state literally: the current build's own contract, and an invariant that holds for every version.

Pinning a legacy ref is the fallback when a contract genuinely needs a release from before a feature shipped, as cross-version-browser-placement.unit.test.ts does with LEGACY_BROWSER_PLACEMENT_RELEASE_REF. It does not rot on a cut, but it is hand-maintained, so prefer deriving.

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 the baseline's list minus this capability — is told the whole surface does not exist and reaches no host method;
• a new client against the old dispatcher always gets an answer rather than silence, and method_not_found for every method that release does not register, so the absence is visible 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.

Worked example: the turn journal item and its transitional downgrade

The structured chat journal records a turn as a first-class item, { kind: 'turn', turnId, state, userItemId?, startedAt?, completedAt?, durationMs? }, where it used to write { kind: 'status', text, turnLifecycle }. Nothing in the codec moves, but it is Rule 3: a client that predates the item does not know the kind and renders it as a text bubble with no text. So the item is gated on a client capability, agent-session.turn-item.v1.

The gate lives at the RPC boundary only, in src/main/runtime/rpc/methods/structured-agent-session-turn-item-capability.ts, composed around agentSession.history and agentSession.subscribe next to the background-task projection. A client that does not advertise the capability receives every turn item rewritten to the legacy status form with the full lifecycle under turnLifecycle; a client that advertises it, and any in-process caller, receives the canonical body. The journal, the status feed, and every host-side reader keep the turn item; readAgentJournalTurn in src/shared/agent-session-turn-record.ts reads either form, so a new client against an old host that still writes the status row also works.

An old client against a new host sees the status row it always did. A new client against an old host advertises a capability the host ignores and reads the status row through the shared reader. The downgrade is transitional: once no supported release lacks the capability, delete the projection module and the capability check, and leave the reader.

The cross-version suite derives the old client's list by removing this capability from the baseline's own list, per the rule above, so the downgrade stays exercised after a release ships it.

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.

Known hazard: on the mobile surface a scope refusal is not a missing method

The agent-session harness above asserts that a peer probing an unknown method is told method_not_found. That holds for the runtime-scoped surface and not for the mobile one.

runtime-rpc-websocket-dispatch.ts checks MOBILE_RPC_METHOD_ALLOWLIST and answers forbidden before it calls the dispatcher. A method a desktop predates is on neither the allowlist nor the registry, and the gate answers first, so a phone never sees method_not_found for it. method_not_found would reach a mobile-scoped device only for a method that is allowlisted but unregistered, and src/main/runtime/mobile-rpc-allowlist.test.ts requires every method the phone calls to be both, so that combination cannot ship. The reverse — a registered method that no release has allowlisted yet — is the skew that does occur.

So a phone-side "is this host new enough to serve X?" probe must read both codes as absence. Keying it on method_not_found alone compiles and passes every same-version test while never firing. The Files and Git fallbacks have read both since they shipped (isMobileMethodUnavailableError, isMobileGitUnavailable); the Relay pairing probes were the outlier, and the cost was a write-once direct-relay upgrade journal, holding a pending resume secret, that an old desktop could never retire (mobile/src/transport/pairing-relay-rpc-unavailable.ts, with the gate pinned desktop-side by src/main/runtime/runtime-rpc-mobile-unknown-method-scope-refusal.test.ts).

Widening is safe only where forbidden cannot also mean a real authorization failure. Prove that per probe rather than globally: the pairing handlers cannot emit it (an unwired provider answers runtime_error), a bad or revoked token answers unauthorized, and the gate is one of only two places in src/main that emits the code at all. A probe whose handler can refuse by authorization must not be widened.

The cross-version harness dispatches as a mobile client but calls the dispatcher directly, so it never runs this gate and nothing reddens if any of the above is forgotten.