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.
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