MessagePack (msgpack) is a compact binary serialization format that is roughly a drop-in faster, smaller alternative to JSON. It is common in Redis tooling, RPC systems, game networking and high-throughput APIs.

How are binary and extension types shown?

Raw binary (bin) values are rendered as bin:0x followed by their hex bytes, and extension types appear as ext(type=n):0x… so you can see both the extension code and its payload without losing data.

Does it handle all MessagePack types?

Yes. It decodes fixint, fixstr, fixmap and fixarray short forms plus str/bin/array/map 8/16/32, the full uint and int range, float 32/64, nil, booleans, and fixext and ext blocks.

Is my data uploaded to a server?

No. The decoder is pure JavaScript that runs in your browser using the built-in DataView and TextDecoder, so nothing you paste or upload ever leaves your device.

Why are some big integers slightly off?

JavaScript numbers are 64-bit floats, so integers beyond 2 to the 53 lose precision. The decoder reads 64-bit ints correctly but converts them to numbers for JSON display, which can round very large values.

MessagePack Viewer — Gera Tools

MessagePack is a binary serialization format that encodes the same data model as JSON — maps, arrays, strings, numbers, booleans and null — but in a far more compact byte stream. It is widely used in RPC systems, Redis-based tooling, real-time game networking and high-throughput APIs where parsing speed and payload size matter. This viewer decodes raw MessagePack back into readable JSON so you can debug those payloads.

How it works

MessagePack uses a single leading byte to select a format family, and the value or length follows:

Small integers, strings, arrays and maps use compact “fix” forms: positive fixint 0x00–0x7f, negative fixint 0xe0–0xff, fixstr 0xa0–0xbf, fixarray 0x90–0x9f and fixmap 0x80–0x8f.
Longer strings, binary blobs, arrays and maps use str, bin, array and map markers with 8, 16 or 32-bit length prefixes.
nil, false and true are the single bytes 0xc0, 0xc2 and 0xc3, while floats use 0xca (float 32) and 0xcb (float 64).
Extension types (fixext and ext) carry an application-defined type code plus a payload — these are shown without losing data.

The decoder reads the leading byte, dispatches to the matching family, and recurses into nested arrays and maps until the whole stream is consumed.

Example

The hex 82 a1 61 01 a1 62 92 02 03 decodes to:

{
  "a": 1,
  "b": [2, 3]
}

82 is a 2-entry fixmap, a1 61 is the fixstr "a", 01 is the integer 1, a1 62 is "b", and 92 02 03 is a 2-element fixarray [2, 3]. Binary values appear as bin:0x… and extensions as ext(type=n):0x…. All decoding happens locally in your browser.