How I Made AI Stop Hallucinating on Our 3-Year-Old Fintech Codebase

Short summary

When AI tools hallucinate on your legacy codebase, the problem is usually incomplete context. A fintech team solved this by creating authoritative documentation: ADRs capturing design decisions, context.md defining domain terminology, and plot.md mapping the system. Combined with mandatory test coverage, this framework made AI predictable, caught cascading bugs automatically, and converted skeptical developers into advocates.

• •Structure is everything: AI needs explicit, authoritative documentation (ADRs, context.md, plot.md) to avoid hallucinations on legacy code
• •Mistakes become rules: Each AI error led to a new ADR, systematically closing knowledge gaps
• •Tests catch what docs can't: Mandatory test coverage on every API route caught cascading failures before commit

Generated with AI, which can make mistakes.