Understanding the Format

This section explains how Dota 2 replay files are structured at the binary level. Read in order — each page builds on the previous.

A .dem file is a 200–300 MB binary blob. Nothing inside it is plain text or JSON. To extract a single field value — say, a hero's HP at a specific tick — a parser must work through six nested layers of encoding before arriving at a number. These pages explain each layer.

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Page	What you will learn
1 · Protocol Buffers	The binary serialisation format used for nearly every payload
2 · The .dem Format	The outer message framing — magic bytes, command+tick+size+payload
3 · Snappy Compression	When and how payloads are compressed
4 · Send Tables & Schema	The entity class schema: serialisers, fields, types, decoders
5 · Field Paths & Huffman	How field addresses inside entities are Huffman-coded
6 · Field Decoders	How individual field values are read from the bit stream
7 · String Tables	Key→value dictionaries built incrementally through the replay
8 · The Entity System	Entity lifecycle: create, update, delete, and the delta loop
9 · The Combat Log	Damage, kills, abilities, gold, and items — two ingestion paths
10 · Game Events	Rune pickups, aegis, chat messages, and typed event dispatch