NEF Format

Back to Manifest Specification

Overview

The Neo Executable Format (NEF) is the binary container for NeoVM bytecode. The compiler produces a .nef file alongside the manifest for every compiled contract.

Field	Size	Description
Magic	4 bytes	NEF3 — identifies the file as a Neo N3 executable
Compiler ID	64 bytes	UTF-8 string, zero-padded (e.g. neo-devpack-solidity-0.18.0)
Source URL	variable	Length-prefixed string, max 256 bytes. Set via --nef-source
Reserved	1 byte	Must be 0x00
Method Tokens	variable	Varint count + up to 128 token entries for CALLT optimization
Reserved	2 bytes	Must be 0x0000
Script	variable	Length-prefixed NeoVM bytecode (max 512 KB)
Checksum	4 bytes	First 4 bytes of SHA256(SHA256(header + script))

TIP

The --nef-source flag embeds a URL or identifier into the NEF header. This is useful for linking deployed contracts back to their source repository. If the value exceeds 256 bytes it is silently truncated to the nearest valid UTF-8 boundary and a NEF_SOURCE_TRUNCATED warning is emitted.