Build-Fix-Validate Loop

Anyone who creates something complicated will rarely get it right on the first try. That’s why work happens in rounds. In the first round, you build a version that works roughly. Afterward, you look for the spots that aren’t right and improve them. Finally, you check whether the improvement really helped and nothing else broke. These three steps are called, in English, build, fix, and validate — that is, building, repairing, and checking. Because you start over again after checking, it’s called a loop.

Why no one can do without these rounds

A modern program consists of hundreds of thousands of lines of instructions. No human can keep so much in mind that they foresee all the interactions. Errors are therefore not the exception but the normal case. The loop is the organized answer to this: instead of planning the perfect throw, you plan many small corrections.

What matters here is the length of a round. If a developer waits twenty minutes for the result after every change, they might manage twenty rounds a day. If checking only takes ten seconds, it’s hundreds. That’s exactly why companies invest a lot of money in fast test systems. Short loops lead to better software without people having to become smarter.

Also important is the third step, checking. Without it, you would never know whether a repair in one place caused damage somewhere else. Such side effects are called regressions. A loop without checking is therefore not an improvement, just motion.

What happens in the three steps

In the build step, a runnable version is created. For software, that means: the written text is translated into a form the computer can execute. Often the first error already shows up here, such as a forgotten character. The build is thus also a first, very rough check.

In the fix step, you look for the cause of a problem and change the instructions. This is the part that requires experience, because the visible symptom is rarely where the error actually lies. In the validate step, automated tests then run: small programs that check whether the main program produces the expected outputs for known inputs. If a test fails, the next round begins.

For AI systems, the loop looks similar, but checking is harder. A chat program doesn’t have a single correct answer you could match against. Instead, you work with evaluation runs, so-called evaluations: you send hundreds of example questions through the system and measure how often the result is usable. The loop should not be confused with training a model. During training, the model itself changes; in the Build-Fix-Validate loop, humans change what surrounds it.

From school assignments to AI agents

You encounter this pattern far beyond computer science by now. Anyone writing an essay for German class makes a rough draft, cuts out the weak parts, and proofreads it once more at the end. That’s the same structure. When building a prototype in a technology class, too, you build, improve, and test in rounds.

In the tech industry, the term comes up especially in connection with AI agents these days. These are programs that work through a task independently in sub-steps. Such an agent writes program code, runs it, reads the error message, and rewrites it. It runs the loop by itself, without a human sitting in between.

In company reports and stock market news, this is often used as an argument. Whoever turns the loop faster brings products to market sooner. Providers of development tools and cloud computing time therefore sell exactly this promise. When you read in the news about shorter development cycles, it’s usually this loop that’s meant.