Definition
Decompilation is Decompilation is the process of turning a compiled program back into human-readable source code, an approximation of what the original programmers wrote.
Decompilation runs the compiler in reverse. A compiler turns source code that people write into machine code that computers run. A decompiler takes that machine code and tries to produce readable source again.
Compiling throws information away. Variable names, comments, the structure of the original files and many of the programmer's choices disappear. So decompiled code is an approximation. It behaves like the original, but it often reads as a pile of generic names and tangled logic, and it may not be what the author actually typed.
A stricter goal is a matching decompilation: source code that, when built with the original compiler and settings, produces machine code identical to the shipped program, byte for byte. Hobbyists use this to rebuild old games and software. It is demanding, because you must find source that the compiler turns into exactly the same output.
Tools called disassemblers show the machine instructions, and decompilers turn those into code in a language such as C or C++. People then work through the program function by function, renaming things, restoring structure and checking the result against the original. It is slow, skilled work.
Reading unfamiliar code and proposing equivalent source is something AI agents can do at scale, and the task comes with a natural test: compile the candidate and compare it to the original. Builders who use agents for it generally rely on that automatic check instead of trusting the agent's own judgment of its work. That pattern, a pass-or-fail test the agent cannot change, is useful far beyond decompilation, and it sits inside the agent harness that runs the job.
Decompilation has legitimate uses: security research, recovering lost source, interoperability, preserving old software and studying malware. It also raises legal questions. Software licenses, copyright law and anti-circumvention rules vary by country and by situation, and many licenses forbid it. If you plan to decompile something, check the terms and get legal advice first.
If you ship compiled software, assume determined people can recover a rough version of your logic. Keep secrets such as keys and credentials out of the binary, and put sensitive logic on a server you control.
Decompilation differs from obfuscation, which deliberately scrambles code to make decompiling harder, and from reverse engineering in general, of which it is one technique.
Turning a compiled program back into readable source code. The result approximates the original, since names, comments and structure are lost in compiling.
It depends on the country, the purpose and the software license. Many licenses forbid it, so check the terms and ask a lawyer before you try.
A developer says Claude and Codex agents rebuilt a popular first-person shooter as C++ over about three months, with 83% of functions byte-exact. The game is the headline. The locked test harness that kept the agents honest is the part to copy.