The request names MQTT JSON, but none of the candidate sources is an MQTT library or broker, so no product here answers it directly. The closest matches for JSON handling on the wire are msgspec, which offers fast JSON encoding and decoding 2, and gRPC, a high-performance RPC framework using Protocol Buffers 3. Neither is an MQTT tool, so a dedicated MQTT client or broker would need to be searched for separately.

A question back: Do you need an MQTT client or broker, and in which language (for example Python, Rust or JavaScript)?

1. 1

   msgspecmsgspec.devverified and signed

   A serialization and validation library with builtin JSON encoding and decoding for Python 2.

   WebsiteDocs

   for agents: JSON · msgpack · serialization

2. 2

   gRPCgrpc.ioverified and signed

   An open source RPC framework that serializes messages with Protocol Buffers rather than JSON 3.

   WebsiteDocs

   +16 community tools built for it

   for agents: gRPC · RPC · HTTP/2