femtoasm is a tiny (291 bytes) self-hosting x86-64 assembler inspired by stage0.
In fact, the assembler itself takes only 171 bytes - the remaining 120 are needed for the ELF header.
femtoasm has 3 implementations: C++23, Flat Assembler and femtoasm. Only the C++23 implementation performs error handling, while the others assume the code is correct. Otherwise, as long as a program is valid and compiles on the C++23 implementation, all three implementations should generate the exact same binary.
For an example, see femtoasm itself.