Definition
Plain language
Mathematically proving that a piece of software does exactly what it's supposed to, for every possible input — not just testing it on examples.
As stated in the literature
The use of machine-checked mathematical proof to establish that a system satisfies a formal specification across all inputs, message orderings, and failures; contrasted with testing, which only samples behaviors. Underlies verified systems like seL4, CompCert, and Chapar.
Also called: formally verified
Why it matters: It provides certainty that ordinary testing cannot, catching bugs that only appear in rare input combinations that no test suite would ever happen to try.
For example, instead of testing a banking program on a few transactions, formal verification proves it never loses money for any sequence of transactions at all.
Heard on the show
“" A casual listener hears "formally verified agent" and imagines a correctness proof.”Episode 122 — When Your Coding Agent Lies About the Fix: Verifying the Plan Before the Model Runs