Client's LitJson serializer emits the C# property name verbatim — the
SetParameter param classes in Wizard/GenerateDeckCodeTask.cs use cardID /
phantomCardID, and the matching Parse() reads jsonData["cardID"]. Snake-case
keys bound to empty in msgpack deserialize, the controller saw 0 cards, and
returned INVALID_DECK — surfaced as a blank deck code in the in-game UI.
Repro lived in data_dumps/traffic.ndjson #19-20. Existing tests pass through
the same JsonPropertyName on both serialize and deserialize, so they happily
round-tripped any consistent key — adding a wire-shape regression test that
posts the literal client JSON would be the right way to catch this class of
bug in the future (out of scope here).
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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