{
  "id": "wire-format/messagepack",
  "family": "wire-format",
  "slug": "messagepack",
  "title": "MessagePack encoding (format byte + data)",
  "summary": "MessagePack encodes each value with a leading format byte that selects a type and, for many small values, packs the value into the same byte (positive fixint 0x00-0x7F, fixstr 0xA0-0xBF, fixarray 0x90-0x9F, fixmap 0x80-0x8F). Larger values use a format byte followed by a big-endian length/value. It is a compact, self-describing tag-then-value scheme similar to CBOR.",
  "kind": "wire-format",
  "aliases": [
    "MessagePack",
    "msgpack",
    "fixint",
    "fixstr"
  ],
  "status": "de-facto",
  "verification": "verified",
  "tier": "C",
  "source_url": "https://github.com/msgpack/msgpack/blob/master/spec.md",
  "source_version": "MessagePack specification (msgpack/msgpack spec.md), retrieved 2026-05-29",
  "retrieved_date": "2026-05-29",
  "attribution": [
    {
      "claim_ref": "ext.structure",
      "source_url": "https://github.com/msgpack/msgpack/blob/master/spec.md",
      "source_version": "retrieved 2026-05-29",
      "note": "Defines the format-byte ranges: positive fixint 0x00-0x7f, fixmap 0x80-0x8f, fixarray 0x90-0x9f, fixstr 0xa0-0xbf, nil 0xc0, etc."
    },
    {
      "claim_ref": "ext.notes",
      "source_url": "https://www.rfc-editor.org/rfc/rfc8949#section-3",
      "source_version": "RFC 8949",
      "note": "CBOR (RFC 8949) is the standardized format with a very similar major-type design; used here only to corroborate the self-describing tag-then-value pattern."
    }
  ],
  "see_also": [
    "wire-format/cbor-encoding",
    "wire-format/protobuf-wire"
  ],
  "ext_type": "wire-format@1",
  "ext": {
    "spec": "MessagePack specification",
    "summary": "A leading format byte names the type and, for compact 'fix' forms, embeds small values directly. Strings/arrays/maps then carry a length and (for str/bin) the bytes — a self-describing tag-then-length-then-value form. MessagePack predates and heavily influenced CBOR.",
    "structure": [
      {
        "field": "Format byte",
        "size": "1 byte",
        "meaning": "Type selector; some ranges embed the value: positive fixint 0x00-0x7F, negative fixint 0xE0-0xFF, fixmap 0x80-0x8F (low nibble = entry count), fixarray 0x90-0x9F, fixstr 0xA0-0xBF (low 5 bits = byte length)."
      },
      {
        "field": "Length / value",
        "size": "0/1/2/4/8 bytes",
        "meaning": "For typed forms: nil 0xC0, false 0xC2, true 0xC3, uint8 0xCC, uint16 0xCD, uint32 0xCE, uint64 0xCF, str8/16/32 0xD9/0xDA/0xDB, bin8/16/32 0xC4-0xC6, array16/32 0xDC/0xDD, map16/32 0xDE/0xDF — each followed by its big-endian length or value."
      },
      {
        "field": "Content",
        "size": "= length",
        "meaning": "For str/bin: that many data bytes. For array: that many elements. For map: that many key/value pairs."
      }
    ],
    "example_hex": "A3 66 6F 6F",
    "example_decoded": "String \"foo\": 0xA3 = fixstr with length 3 (0xA0 | 3); 66 6F 6F = ASCII 'foo'.",
    "see": [
      "wire-format/cbor-encoding"
    ],
    "notes": [
      "Small integers and short strings cost a single byte because the value/length is packed into the format byte (fixint/fixstr).",
      "The ext type family (0xC7-0xC9, fixext 0xD4-0xD8) carries an application-defined type byte plus payload, akin to a tag.",
      "CBOR (RFC 8949) standardized the same self-describing idea with cleaner major-type math and IANA-registered tags."
    ]
  },
  "updated": "2026-05-29T00:00:00Z"
}