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MessagePack to JSON Converter

Decode MessagePack binary data (hex or Base64) to readable JSON, or encode JSON to compact MessagePack bytes. Shows the size saving over JSON. Runs entirely in your browser.

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Convert between MessagePack and JSON in your browser. Paste MessagePack bytes as hex or Base64 to decode them into readable, pretty-printed JSON — or paste JSON to encode it into compact MessagePack bytes as hex or Base64. The converter also reports how much smaller the MessagePack payload is than the equivalent JSON.

What is MessagePack?

MessagePack (often shortened to msgpack) is a binary serialization format. Like JSON, it represents the same handful of data types — objects (maps), arrays, strings, numbers, booleans and null — but instead of writing them as human-readable text, it packs them into a tight sequence of bytes. Its tagline says it well: "It's like JSON, but fast and small."

MessagePack is widely used where JSON's verbosity becomes a cost: real-time messaging, IoT devices with limited bandwidth, Redis modules, game networking, and inter-service RPC (it is the wire format behind protocols such as msgpack-RPC). Libraries exist for virtually every programming language, so a payload packed in one language decodes cleanly in another.

Why is MessagePack smaller than JSON?

JSON spends a lot of bytes on formatting that a machine does not need. Consider {"name":"Alice","age":30}:

  • Structural characters cost bytes. Every {, }, ", : and , is a full byte of overhead. MessagePack replaces them with compact type prefixes — a whole small map opens with a single byte (0x82 means "a map with two entries").
  • Short strings carry their length in one byte. The string "name" in MessagePack is 0xa4 (fixstr, length 4) followed by the four raw characters — no surrounding quotes.
  • Numbers are stored as numbers, not digits. JSON writes 30 as the two text characters 3 and 0. MessagePack stores it as the single byte 0x1e. Larger integers, floats and 64-bit values each get the smallest binary form that fits.

The result for that example: JSON needs 25 bytes, MessagePack needs 17 — roughly a third smaller. The saving grows with deeply nested structures and repeated keys, which is why bandwidth-sensitive systems reach for it.

How to use this tool

  1. Decode (MessagePack → JSON): choose whether your bytes are pasted as hex (e.g. 82a46e616d65…) or Base64, paste them in, and the decoded value appears as formatted JSON.
  2. Encode (JSON → MessagePack): paste valid JSON, pick hex or Base64 for the output, and the packed bytes appear. Byte-string (bin) values inside decoded data are shown as an h'…' hex marker so the JSON stays readable.

The size comparison table shows the MessagePack size, the JSON size, and the percentage reduction for your specific input.

Is my data uploaded anywhere?

No. Both encoding and decoding run entirely in your browser — nothing you paste is sent to a server.

What happens with invalid input?

Nothing breaks. Malformed hex or Base64, truncated or invalid MessagePack bytes, and invalid JSON each produce a clear inline Error: … message instead of crashing the page.

messagepackmsgpackjsonbinaryencoderdecoderserialization

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