Toon

TOON solves one problem narrowly and well: the tokens you burn shipping structured data into a prompt. The measured claim is specific — 42.6% fewer tokens than JSON with retrieval accuracy of 72.2% versus 71.4% — and the mechanism is not a trick. Where JSON repeats every field name on every row, TOON declares the field list once in a {fields} header and streams one row per line. Where JSON wraps everything in braces and quotes, TOON uses indentation and drops quotes it does not need. The v4 release is the one that makes the format hold up under real nesting. In v3, a single object-valued column was enough to kick an array out of tabular form and back into indented lines: the vendor's own worked example shows a three-row forecast array at 102 tokens in v3 and 49 tokens in v4, because the header can now carry a nested group like temp{min,max}. The keyed tabular form, [N:], does the same job for an object whose values are uniform objects — the vendor's stations lookup drops from 76 tokens to 53. Nested groups nest arbitrarily deep, and where no group applies, v4 output matches v3 byte for byte, so the migration risk on existing documents is bounded. The guardrails are the second reason this is credible rather than a novelty. Because [N] declares the row count and {fields} declares the width, a model that truncates a table or drops a column produces something you can detect cheaply, instead of a silently short answer. The # comment form, stripped in a lexical pre-pass and never counted toward a declared length, is a small but real ergonomic win for hand-maintained prompt fixtures. The costs are equally specific. The syntax is indentation-sensitive, which is easy to break by hand in large nested structures — though encoders never emit comments and always quote strings that begin with #, so encoder output cannot be misread as one. The v4 spec removes keyFolding, flattenDepth and expandPaths from the encoder and decoder APIs and the --key-folding, --flatten-depth and --expand-paths CLI flags, so code using those must migrate. The keyed tabular form needs at least two entries and cannot be used inside a column. The ecosystem, while it covers TypeScript, Python, Go, Rust and .NET with one conformance suite, is a rounding error next to JSON's. Where it fits: prompt engineers cutting cost on tool results and retrieved records, developers building structured data pipelines for LLMs, and teams re-sending the same table-shaped payload on every request. Where it does not: storing data, exchanging data over a wire, or any workload where you need the widest possible library support. Round-trips to and from JSON are deterministic and lossless, which means the safe architecture is to keep JSON at rest and encode TOON only at the prompt boundary.