Poison

Poison is a fast and lightweight JSON library for Elixir focused on performance and idiomatic APIs. It provides straightforward encode and decode functions, along with a protocol-based encoder that lets you customize how your structs become JSON. Developers can derive or implement Poison.Encoder for domain types, control which fields are included, and map complex values into JSON-friendly forms. On the decoding side, it supports options for key handling and flexible parsing of JSON into Elixir maps, lists, and primitive values. Internally it uses optimized binary processing to keep allocations low and throughput high, which is why it became a popular choice in early Elixir ecosystems. The API is intentionally small and unsurprising, making it easy to drop into controllers, background jobs, or data pipelines. Many codebases still rely on Poison for its speed and simplicity, even as alternative JSON libraries exist in the community.