--- title: Four Mechanisms Behind EGP Code, an AI-Powered Landing... description: A deep dive into four internal mechanisms powering EGP Code, Mercari's AI-driven landing page editor. The post covers: (1) a recursive agent loop where the AI... canonical: https://daily.dev/posts/four-mechanisms-behind-egp-code-an-ai-powered-landing-page-editor-p0exjkqxt twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: Four Mechanisms Behind EGP Code, an AI-Powered Landing Page Editor | daily.dev og:description: A deep dive into four internal mechanisms powering EGP Code, Mercari's AI-driven landing page editor. The post covers: (1) a recursive agent loop where the AI... og:url: https://daily.dev/posts/four-mechanisms-behind-egp-code-an-ai-powered-landing-page-editor-p0exjkqxt og:image: https://api.daily.dev/og/posts/p0ExjkQxT.png og:image:alt: Four Mechanisms Behind EGP Code, an AI-Powered Landing Page Editor og:image:width: 1200 og:image:height: 630 og:locale: en --- [Mercari Engineering](https://daily.dev/sources/mercari) [Read post](https://api.daily.dev/r/p0ExjkQxT) # [Four Mechanisms Behind EGP Code, an AI-Powered Landing Page Editor](https://api.daily.dev/r/p0ExjkQxT "Go to post") A deep dive into four internal mechanisms powering EGP Code, Mercari's AI-driven landing page editor. The post covers: (1) a recursive agent loop where the AI reasons, calls tools, and self-corrects on errors; (2) real-time sync using Firestore so users see live progress without WebSockets; (3) a browser-native test runner built on @vitest/expect and Testing Library that runs inside an iframe; and (4) a mapping table using injected data-egp-src-id attributes to link rendered preview elements back to their positions in the HTML source, enabling Monaco editor jumps and precise AI instructions. The article also details real-time steering — injecting mid-flight instructions into an already-running agent loop — and how Zero Data Retention requirements led to self-managed conversation history. [#webdev](/tags/webdev "Check all #webdev posts")[#ai-agents](/tags/ai-agents "Check all #ai-agents posts")[#web-components](/tags/web-components "Check all #web-components posts")[#firestore](/tags/firestore "Check all #firestore posts") Aug 06•12m read time•From [engineering.mercari.com](https://api.daily.dev/r/p0ExjkQxT "engineering.mercari.com") [](https://api.daily.dev/r/p0ExjkQxT "Go to post") Table of contents [Mechanism 1: A recursive agent loop that bundles context and tools](https://api.daily.dev/r/p0ExjkQxT "Mechanism 1: A recursive agent loop that bundles context and tools")[Mechanism 2: Real-time sync using Firestore as the handoff point for instructions](https://api.daily.dev/r/p0ExjkQxT "Mechanism 2: Real-time sync using Firestore as the handoff point for instructions")[Mechanism 3: A test runner that runs entirely in the browser](https://api.daily.dev/r/p0ExjkQxT "Mechanism 3: A test runner that runs entirely in the browser")[Mechanism 4: A mapping table linking preview elements to positions in the HTML](https://api.daily.dev/r/p0ExjkQxT "Mechanism 4: A mapping table linking preview elements to positions in the HTML")[Conclusion](https://api.daily.dev/r/p0ExjkQxT "Conclusion") 880 Impressions1 Upvote Comment Bookmark Copy Share your thoughtsPost [](https://daily.dev/sources/mercari) [Mercari Engineering](https://daily.dev/sources/mercari "https://daily.dev/sources/mercari") Mercari's platform is a resource for developers and technology enthusiasts, offering insights into e... 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