Introduction to TOON Format in Java

TOON (Token-Oriented Object Notation) is a compact alternative to JSON designed to reduce token consumption when sending structured data to LLMs. It uses indentation instead of braces, minimal quoting, and a tabular layout for uniform collections that states field names once rather than repeating them per object. Two Java libraries support TOON: JToon and json-io. Using json-io, you can serialize Java objects with JsonIo.toToon() and deserialize with JsonIo.fromToon(). Benchmarks using OpenAI's BPE tokenizer show 24-44% token savings for collections compared to JSON, with savings growing as collection size increases. For flat data, TOON is within 6% of CSV efficiency while also handling nested structures CSV cannot represent. A Spring AI starter (json-io-spring-ai-toon) enables automatic TOON conversion for tool call results via a ToonToolCallResultConverter annotation. TOON is best suited for LLM tool results, RAG payloads, and agentic workflows where token cost and context window size matter.