Shopify rewrites theme code for AI—and humans get cleaner HTML

Shopify is launching a new unnamed storefront theme designed as a successor to Horizon, its current JSON-heavy base theme used by merchants to customize their Shopify storefronts. The new theme is built primarily in HTML interpolated with Shopify's Liquid templating language, cutting code volume by 93% compared to Horizon. Ben Sehl, Shopify's product director for storefronts, explained that the shift was motivated by AI: "Every leading model already understands HTML. It is expressive, local, and token-efficient."

The rearchitecture was driven by the success of Shopify Sidekick, an AI assistant launched earlier that lets merchants declare design changes in natural language rather than manually editing templates. This year, 20% of Shopify merchants are using Sidekick, which has generated 25 million edits. Sehl recalled that a decade earlier, when he was himself a novice merchant, Shopify offered only basic templates that were easy to modify. Over time, the company enriched templating with the Online Store Editor, giving merchants fine-grained control over every button and component—but the JSON format that enabled that flexibility made templates opaque. "The moment templates became JSON, they stopped being a great developer surface. They became an auto-saved output," Sehl wrote.

The new theme introduces several technical improvements. It features a new composable block tag containing nested parameters for page components, similar to React props but without requiring a virtual DOM. A new partial primitive allows developers to update specific page regions without re-rendering the entire page—a pattern familiar to React developers. Shopify added standard actions, a collection of event triggers like shopping-cart updates, which Sehl noted "will let us remove thousands of lines of reactivity code from Horizon." Liquid gained new syntax including Boolean expressions, infix operators with precedence, and literal arrays and objects. The company developed 20 new rules to validate generated themes against policies around contracts, structure, and file-size limits. Tailwind CSS support is planned.

The practical benefit is immediate: in the old declarative architecture, developers had to anticipate how many buttons they needed before starting layout; in HTML, they simply wrap additional buttons in a div tag. Sehl's post on X noted that AI systems and humans want the same things from code: readability, explicit contracts, and constructive feedback. While Shopify maintained Liquid themes in a "forever API" for backward compatibility, the new architecture suggests that code optimized for AI agents may paradoxically be better for humans too.

Shopify's decision to rebuild its theme architecture around HTML reflects a broader shift in how companies design systems for AI consumption. The company initially moved to JSON to give merchants more granular control through the Online Store Editor, but that flexibility introduced complexity—templates became auto-generated outputs rather than readable code. The breakthrough came when Shopify deployed Sidekick, its AI assistant, and discovered that simplifying code for the machine also made it simpler for humans. This mirrors findings elsewhere in the industry: researchers have found that LLMs process semantic-rich formats like XML more effectively than dense symbolic languages, and that plain SQL outperforms domain-specific languages for AI tasks. Shopify's 93% reduction in code lines suggests that the JSON-heavy approach was unnecessarily verbose once you accept that both merchants and AI agents benefit from explicit, readable contracts. The company maintained backward compatibility by keeping Liquid themes in a "forever API," so existing customizations continue to work while new themes can adopt the cleaner architecture.