Developer Used Claude AI to Build Load Tests, Uncovered 3 Critical API Bottlenecks Pre-Launch

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A developer with no prior load-testing experience used Claude Code, an AI coding agent, to build a k6 load test suite for a Node.js API just two weeks before launch. The API, built on Fastify, Postgres 16, and Redis 7, had 38 endpoints but zero performance testing despite having hundreds of unit and integration tests. Rather than testing endpoints in isolation, the developer prompted Claude to model three realistic user journeys — including browse-to-checkout, order history review, and admin pagination — with weighted traffic splits up to 300 virtual users. The resulting test suite identified three bottlenecks that, left unaddressed, could have caused service failures on launch day. The exercise was completed within a single working day, and the developer outlined five key lessons for using AI agents in performance testing workflows.

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