How I Fixed Claude's Math Problem with 100 Lines...

LLMs are great at explaining math. They're inconsistent at doing math. Ask Claude to calculate a 30-year mortgage on $400,000 at 6.5% APR twice. You might get $2,528 and $2,533 in the same session. The correct answer is $2,528.27. This is not a hallucination bug you can prompt your way out of. It's an architecture problem. LLMs are probabilistic text generators — arithmetic is deterministic. The fix is MCP: give the LLM a structured tool that always returns the right number. Here's how to do it