Extends the cross-version harness, which covered the terminal stream only, with
the structured surface. The old side's advertised capabilities and registered
method names are read from the extracted release checkout rather than written by
hand: a build cannot advertise a string its own source never names, so "the old
side does not have it" stays a fact about a real release.
Mixed versions are the normal state of the remote-server feature: users update clients and servers independently. Until now nothing tested that. Every cross-version claim was made by code reading plus unit tests with hand-written old/new shapes — enough to catch design problems, not enough to catch a real skew regression.
This runs the REAL protocol implementations from two builds against each other in one process: the actual host methods and RPC dispatcher on one side, the actual renderer multiplexer on the other, with a transport that reproduces the production asymmetry — each side decodes with its OWN codec and drops frames whose opcode it does not know. A frame survives only if the RECEIVING build understands it, which is what makes this level sufficient without launching two apps. The old side is a genuine checkout extracted from the release tag; the extracted client was confirmed to lack a symbol that exists only on main.
Journey: subscribe, first snapshot, input reaching the process, live output, hide/reveal snapshot, transport drop, resubscribe, input landing again — across old->new, new->old, and a current/current control. Every step ends on an observed-state barrier; no sleeps. The oracle asserts the recorded step list, the exact 16-frame named sequence, negotiated capabilities, the exact input the host wrote to the PTY, rendered content, and zero decoder-rejected frames. A host method the stub lacks is recorded by name and asserted empty, so a harness gap cannot masquerade as a wire break.
Detection is proven per violation shape, and it attributes each to the correct side: an unnegotiated opcode goes red only where a decoder would reject it, a removed published field goes red only where an old client consumes it, and a legal additive field stays green in all three pairings so the harness will not cry wolf on safe changes.
It also documents the three compatibility rules in docs/reference/remote-wire-compatibility.md, linked from AGENTS.md, since they previously existed only as folklore — notably that "decoders reject unknown opcodes" is true for the desktop decoder but NOT for mobile, which silently drops them.
Deliberately scoped: terminal stream only. The session-tab sync channel is not covered, nor agent-session publications, file/Git RPCs, mobile E2EE framing, or the relay transport. Two version points, so a regression introduced and reverted between them is invisible.
CI selection was verified rather than assumed — `vitest list` confirms 0 matches under the shard's exclude and 4 under the dedicated job — because a lane silently running zero tests is precisely how a host-side defect escaped CI earlier in this series. Closes STA-3469.
Brennan Benson
Jinwoo Hong