The Apache Thrift JSON protocol: every value is a JSON document whose shape carries the Thrift types. A message is the array [1,"name",type,seqid,payload]; a struct is an object keyed by field id where every value is a one-key object naming the type ("i32", "str", "rec", ...); a list or set is ["i32",2,1,2], a map ["i32","i32",2,{"1":10}]. Wire-compatible with the vendored Apache thrift_json_protocol — the golden suite runs the official JSON client against a JSON server of ours.

One deliberate bend to fit this codebase's pure-codec shape: the envelope's closing ] is folded into the TOP-LEVEL struct decode (decode/2 on a struct consumes one optional trailing ]), and decode_message/1 returns the payload including it. Nested values are never touched by this — a struct inside a list keeps the ] that closes the list — but it makes the payload self-contained: the completeness probe of the buffered transport (an empty descriptor over the message tail) accounts for every byte exactly, like it does for the binary and compact protocols.

Struct keys of maps are limited to what a JSON object key can carry: booleans, integers, doubles and strings. A map keyed by a struct, list, set or map compiles to {j_unsupported, ...} and both encode and decode refuse it (the reference implementation would produce invalid JSON). uuid is likewise uncarryable. Doubles are written with 16 fractional digits, mirroring the reference round-trip.

Descriptors compile once (see thrift_runtime_binary, the type vocabulary is shared): inline {struct, {Kind, Fields}} descriptors are memoized per process under exact equality, {struct, {ref, M, N}} schemas resolve through M:struct_info/1 into a persistent_term cache. Generated types modules also probe their specialized json_codec/1 pair — the encode side everywhere, the decode side carries the non-folding struct body and the bracket fold is applied by whichever top-level entry calls it (decode/2, never decode_nested/2). Decoding is strict — declared fields whose type tag disagrees with the descriptor are rejected, unknown fields are skipped by their wire tag, required fields must be present, trailing bytes answer {error, trailing}.