Every sidecar carries its own copy of the length-prefixed JSON-over-stdio wire protocol (_send, _recv, MAX_FRAME_BYTES), and only one of them was covered. This parametrizes the protocol invariants across all nine — send/recv roundtrip, EOF as an orderly shutdown, the oversized-frame cap that stops a corrupt length header allocating unbounded memory, and the truncated body that would otherwise hang the parent — so a bug in any single sidecar's copy is caught without a per-engine test file.
Modules are resolved through importlib inside a fixture rather than bound at collection, which keeps the suite honest under sys.modules pollution.
Thanks @paoloantinori!
