Mercari's Merpay team built EGP Code, an in-house HTML-based LP editor that integrates an AI agent into the full production workflow. It replaces EGP Pages, a WYSIWYG JSON-based editor that proved costly and awkward for LLM editing due to its proprietary tree structure and high token usage. EGP Code uses plain HTML with a small set of custom Web Components for dynamic behavior, Tailwind CSS for styling, and Markdown-documented internal APIs that the AI loads on demand. The editor includes a built-in browser-side test runner with a Jest-like API and mock server, so AI-generated code can be verified without hitting real APIs. A dedicated Web Component handles environment-aware navigation between web and native app contexts. The result is a single editing experience covering UI generation, internal API integration, analytics logging, automated testing, and native app bridging — enabling PMs, backend engineers, and even marketers to ship production LPs with less engineering involvement.
