DEV Community

TOON vs JSON: The New Format Designed for AI

Akash Thakur on November 09, 2025

How a novel data format is saving developers 30-60% on LLM token costs If you've been working with Large Language Models, you've probably noticed ...

Read full post

Ali Farhat • Nov 12 • Edited

Thank you for the article, would you mind adding our JSON to TOON tool in your article?
scalevise.com/json-toon-converter

铁泪Tiě Lèi • Nov 10

Cobol is that you?

IDENTIFICATION DIVISION.
       PROGRAM-ID. PRODUCT-LIST.

       DATA DIVISION.
       WORKING-STORAGE SECTION.

       *> 1. Define the raw data for the table.
       *>    Each record is 18 bytes:
       *>    ID    (2) = 01
       *>    Name (10) = "Laptop    "
       *>    Price (6) = 399990  (for 9(4)V99)
       01 WS-PRODUCT-TABLE-DATA.
           05 FILLER PIC X(18) VALUE "01Laptop    399990".
           05 FILLER PIC X(18) VALUE "02Mouse     014990".
           05 FILLER PIC X(18) VALUE "03Headset   049900".

       *> 2. Redefine that block of memory as a structured COBOL table.
       *>    This maps the fields to the raw data above.
       01 WS-PRODUCT-TABLE REDEFINES WS-PRODUCT-TABLE-DATA.
           05 WS-PRODUCT-ENTRY OCCURS 3 TIMES.
               10 WS-PRODUCT-ID    PIC 9(02).
               10 WS-PRODUCT-NAME  PIC X(10).
               10 WS-PRODUCT-PRICE PIC 9(04)V99. *> Implied decimal

       *> 3. Define helper variables for looping and display
       01 WS-INDEX           PIC 9(01).
       01 WS-DISPLAY-PRICE   PIC Z,ZZ9.99. *> For formatting the output

       PROCEDURE DIVISION.
       MAIN-PROCEDURE.

           *> Loop through the table (from 1 to 3) and display each record
           PERFORM VARYING WS-INDEX FROM 1 BY 1 UNTIL WS-INDEX > 3

               DISPLAY "--------------------------"
               DISPLAY "Product Record: " WS-INDEX

               *> Access data using the index: WS-FIELD-NAME(WS-INDEX)
               DISPLAY "  ID:    " WS-PRODUCT-ID(WS-INDEX)
               DISPLAY "  Name:  " WS-PRODUCT-NAME(WS-INDEX)

               *> Move the computational price (9V9) to a display-ready
               *> field (Z,ZZ9.99) to add the decimal point and $
               MOVE WS-PRODUCT-PRICE(WS-INDEX) TO WS-DISPLAY-PRICE
               DISPLAY "  Price: " WS-DISPLAY-PRICE

           END-PERFORM.

           DISPLAY "--------------------------".
           STOP RUN.

david duymelinck • Nov 9

Why not use YAML if you want a more condensed format?

Shawn Bullock • Nov 17

Our system processes billions of tokens each month from the database alone, we can't use YAML or JSON because they are too token heavy. For flat results (like a database query) we simply return it as CSV (which is even 30% fewer tokens than TOON). We haven't adopted TOON yet (we have our own for structured objects) but its definitely more token friendly than YAML. At the scale we operate, YAML is expensive.

david duymelinck • Nov 17

I based my statement on the examples I have seen at that moment. And I agree if you use YAML for tabular data it is expensive. That is why in my other comment I mentioned a switch based on the shape of the data. CSV for tabular data and YAML for hierarchical data.
You can even have CSV in YAML.

people: |
  id,name,age
  1,Alice,30
  2,Bob,25

If that wasn't possible, I would go all in for TOON for those mixed cases.

TOON is YAML with hierarchical data. So it doesn't reduces tokens. And as you mention, in the case of tabular data CSV is better.

If you can show where TOON is saving tokens over the smart use of YAML and CSV, I'm glad to stand corrected.

Marcu Loreto • Nov 10

I got that TOON is one more compact data serialization format, similar in purpose to YAML

david duymelinck • Nov 10

True but why the need to invent a new format?
Most languages have mature YAML and CSV libraries if you need to condense the text that is send to an AI.

The main reason for TOON is probably that you can feed it content that is better compacted by YAML and better compacted by CSV. Instead of creating a function to switch the output between the two formats yourself.
Out of curiosity I asked an AI to create that function.

/**
 * Detects whether a JSON object represents a tree‑like structure
 * or a tabular (array‑of‑objects) structure, then converts it to
 * YAML or CSV accordingly.
 *
 * @param {object|array} data - Parsed JSON data.
 * @returns {string} YAML string for tree‑like data or CSV string for tabular data.
 */
function convertJson(data) {
  // Helper: check if value is a plain object (not array, not null)
  const isObject = v => v && typeof v === 'object' && !Array.isArray(v);

  // Detect tabular: an array where every element is an object
  // and each object has the same set of primitive keys.
  const isTabular = arr => {
    if (!Array.isArray(arr) || arr.length === 0) return false;
    // All elements must be objects (no nested arrays/objects as values)
    const firstKeys = Object.keys(arr[0]);
    if (firstKeys.length === 0) return false;

    return arr.every(item => {
      if (!isObject(item)) return false;
      const keys = Object.keys(item);
      // same keys as first row
      if (keys.length !== firstKeys.length) return false;
      for (let k of firstKeys) {
        if (!keys.includes(k)) return false;
        // values should be primitive (string, number, boolean, null)
        const v = item[k];
        if (v && typeof v === 'object') return false;
      }
      return true;
    });
  };

  // Convert tabular data to CSV
  const toCsv = arr => {
    const headers = Object.keys(arr[0]);
    const escape = v => {
      if (v == null) return '';
      const s = String(v);
      return s.includes(',') || s.includes('"') || s.includes('\n')
        ? `"${s.replace(/"/g, '""')}"`
        : s;
    };
    const rows = arr.map(row => headers.map(h => escape(row[h])).join(','));
    return [headers.join(','), ...rows].join('\n');
  };

  // Convert any object/array to YAML (simple implementation)
  const toYaml = obj => {
    const yaml = require('js-yaml'); // assumes js-yaml is available
    return yaml.dump(obj, { noRefs: true, indent: 2 });
  };

  // Main logic
  if (Array.isArray(data) && isTabular(data)) {
    return toCsv(data);
  } else {
    // For tree‑like structures we fall back to YAML
    // If js-yaml is not available, a minimal serializer could be used.
    try {
      return toYaml(data);
    } catch (e) {
      // Minimal fallback YAML serializer
      const serialize = (value, indent = 0) => {
        const pad = ' '.repeat(indent);
        if (Array.isArray(value)) {
          return value.map(v => `${pad}- ${serialize(v, indent + 2).trimStart()}`).join('\n');
        } else if (isObject(value)) {
          return Object.entries(value)
            .map(([k, v]) => `${pad}${k}: ${serialize(v, indent + 2).trimStart()}`)
            .join('\n');
        } else {
          return `${value}`;
        }
      };
      return serialize(data);
    }
  }
}

Algis • Nov 17

I’ve been experimenting with this new “code execution with MCP” concept and recently implemented it in my OSS mcpproxy project. The current version can generate a JSON → TOON converter on the fly for any MCP tool within an agent session, which makes data format conversion and reuse incredibly easy.

Prompt example: Read this post: dev.to/akki907/toon-vs-json-the-new-format-designed-for-ai-nk5. Then implement and run code that converts the tool's JSON output to TOON format, using the tool. Output ONLY TOON data in code execution response

David Burton • Nov 11

Surely XML would do better in peak accuracy because a close tag explicitly matches to an opening tag?
Surely Markdown and YAML offer similar compactness and readability to TOON with better support?
It would be good to see the article amended to test against the JSON, XML, YAML, Markdown and CSV (or perhaps better still another delimited format like tab-delimited) data, to get a better idea as to how it compares to a more complete range of options

richardevcom • Nov 17

I'll just leave this here for people still taking this seriously 👀

Gaurav Chandra • Nov 17

How it is different from TONL github.com/tonl-dev/tonl ?

Salient PR • Nov 13

Thank you for the article)

Ketan Gupta • Nov 10

Thanks for sharing this. Built a quick tool for anyone wanting to test TOON formatting: bestaitools.tech/tools/json-to-toon Runs client-side, no data sent to servers 🛠️