What is TOON?

TOON is a compact text format for structured data. You can think of it as JSON with the noise removed: the same information, written so it takes up less space and, importantly, fewer tokens when you hand it to a large language model.

If you have ever pasted a big JSON blob into a prompt and watched it eat your context window, TOON is aimed squarely at that problem.

The problem TOON solves

JSON is everywhere and easy to read, but it is wordy. Every object repeats its keys, every string is wrapped in quotes, and braces and commas pile up. A human skims past all that. A language model does not — it pays for every one of those characters in tokens, and tokens cost money, time, and context space.

When you feed the same dataset to a model dozens of times across a workflow, that overhead adds up fast. TOON keeps the structure but strips the repetition, so the model sees the data, not the punctuation.

What TOON looks like

Here is a small dataset in JSON:

{
  "users": [
    { "id": 1, "name": "Ada", "role": "writer" },
    { "id": 2, "name": "Lin", "role": "developer" }
  ]
}

Readable, but every key is repeated for every row.

The same data in TOON collapses the repeated keys into a single header and lists the rows underneath, much like a lightweight table:

users[2]{id,name,role}:
  1,Ada,writer
  2,Lin,developer

Same information, far fewer characters and tokens.

The keys appear once. The rows carry only the values. Nothing about the data is lost, but a lot of the syntactic weight is gone.

When to use TOON

• Feeding tables of data to an LLM. Lists of records — users, products, rows from a spreadsheet — benefit most, because that is where JSON repeats keys the hardest.
• Long or repeated prompts. If the same data is sent many times, the token savings compound.
• Tight context windows. When you are close to a model's limit, trimming format overhead buys you room for actual content.

For a single small object, the difference is negligible — JSON is fine. TOON earns its keep on arrays of structured records.

Converting between JSON and TOON

You do not have to write TOON by hand. MarkDone's JSON to TOON converter turns existing JSON into TOON in your browser, and TOON to JSON converts it back when you need standard JSON again. Both run locally — your data is never uploaded.

Want the practical angle on token savings? Read how converting JSON to TOON saves tokens.

For a more comprehensive explanation visit the TOON format deep-dive on openapi.com.