I have been wiring TOON support with toon-token-diff into this MCP server to understand whether converting JSON payloads to TOON meaningfully reduces prompt costs. The short answer: TOON is elegant, but in my test harness it delivered microscopic savings for real-world workloads.

Environment

  • Project mode: toon-token-diff in libraryMode via npm install toon-token-diff
  • Models monitored: openai (tiktoken GPT-5 profile) and claude
  • Integration strategy: lightweight instrumentation that appends token stats into a JSONL ledger for later analysis
import { estimateAndLog } from "toon-token-diff/libraryMode";

// inside my MCP tool handler
estimateAndLog(JSON.stringify(result), {
  models: ["openai", "claude"],
  file: "./token-logs.jsonl",
  format: "json",
  label: "mcp_tool_call",
});

This snippet runs after the MCP tool produces a JSON response. It serializes the payload, estimates TOON vs JSON tokens, and emits a structured record to token-logs.jsonl. The rest of the MCP server stays untouched—no need to change transport or business logic.

Observations

Timestamp (UTC) openai JSON openai TOON openai Δ (%) claude JSON claude TOON claude Δ (%)
2025-11-19T14:16:54.296Z 127 126 0.79 130 129 0.77
2025-11-19T14:17:15.720Z 53,703 53,702 0.0019 54,977 54,976 0.0018
2025-11-19T14:17:34.988Z 14 12 14.29 14 12 14.29
2025-11-19T14:17:39.246Z 53,703 53,702 0.0019 54,977 54,976 0.0018
2025-11-19T14:17:48.333Z 29 29 0.00 28 28 0.00
2025-11-19T14:18:13.725Z 91,729 91,728 0.0011 98,607 98,606 0.0010
2025-11-19T14:21:19.174Z 127 126 0.79 130 129 0.77
2025-11-19T14:21:23.370Z 91,729 91,728 0.0011 98,607 98,606 0.0010
2025-11-19T14:21:30.314Z 53,703 53,702 0.0019 54,977 54,976 0.0018

Nine consecutive tool runs told the same story: production payloads barely moved. Only the intentionally tiny sample showed double-digit savings, which is irrelevant for backlog-scale prompts.

Why the Reduction Rate Is Flat

  1. Content dominates token volume – The payload body itself accounts for nearly every token, so TOON’s structural tweaks barely register in the total.

Practical Guidance

  • Keep TOON handy as a normalization format, but don’t promise cost savings without benchmarking your actual payloads.
  • Instrument with the libraryMode snippet above before ship time; it gives you historical evidence of whether TOON helps.
  • If savings are negligible, redirect effort toward higher-impact tactics: pruning unused fields, batching small tool calls, or applying semantic compression upstream.

Next Experiments

  • Compare with alternative tokenizers (Gemini, Llama) to see whether non-GPT vocabularies respond differently.
  • Add diff tooling that highlights specific fields TOON shrinks, so we can manually prune them if needed.
  • Explore policy-driven trimming (e.g., dropping debug blobs) prior to TOON conversion.

TOON remains a clever serialization trick, but as my MCP experiment showed, it is not an automatic token economy lever. Measure, log, and decide based on real numbers.

Resources

Also republished on dev.to.