Shopify cut theme code by 93% to make AI agents (and humans) happier

Shopify has a blunt problem: its storefront themes got too hard to read. The company is launching a new base theme that it says is “completely understandable by anyone with even a smidgen of HTML knowledge,” and it backs that claim with a hard metric. This new theme has 93% fewer lines of code than Horizon, Shopify’s current JSON-heavy base theme.

The reason is not aesthetics. Shopify’s increasingly popular AI assistant, Shopify Sidekick, is changing how merchants build and edit store designs, and that means the underlying theme architecture has to work better for AI agents too. Shopify says 20% of merchants are using Sidekick to edit themes, generating 25 million edits this year alone. When 25 million edits are happening, the platform cannot afford templates that are basically “auto-saved output” instead of a developer surface.

For context, Horizon’s core issue was complexity created by serialization. Shopify product director for storefronts Ben Sehl wrote on X that “Every leading model already understands HTML. It is expressive, local, and token-efficient,” and that Shopify’s previous JSON-centric approach created a developer-experience cost: “you could no longer understand a page by reading one file.” His framing is important for executives because it turns a tooling problem into a design-system problem. JSON is not inherently bad, Shopify argues, but configuration that relies on a constrained vocabulary becomes harder for both humans and agents to interpret without context.

So Shopify is trying to satisfy the same requirements for agents that humans have demanded for years: readable code, explicit contracts, and helpful feedback. In its new architecture, Shopify reorganized its theme structure around those principles, while keeping existing Liquid themes under a “forever API.” That matters because theme ecosystems are sticky. If you break compatibility, you don’t just upset developers, you disrupt your merchant base. Shopify’s approach is essentially: modernize the base theme experience for the next interaction layer (agents) without detonating the existing marketplace.

The technical moves are where the strategy turns into engineering reality. In the new base theme, Liquid gets “a new syntax and parser,” built with added discipline to execute more complex instructions on the back end. What merchants see is not a wall of opaque JSON, but composable, typed blocks that “fit easily alongside ordinary HTML.” Shopify says when merchants peek into the templates directory, they will see plain-text files of code that are easier for both people and machines to parse, not long JSON strings.

Shopify also introduces a generation model for each custom theme. Each user-designed theme gets a special directory plus files with instructions and pointers for how additional artifacts should be generated. An expanded doc tag includes examples and instructions focused on contracts and snippets. Central to the system is a new composable block tag containing nested parameters for page components. Shopify notes React developers will recognize those parameters as similar to React props, but without the need for a virtual DOM. There’s also a “partial primitive” that allows updating a specific region of a page without rerendering the entire page.

To reduce UI wiring complexity, Shopify adds standard actions, described as event triggers for a site like updating the shopping cart. Sehl wrote that “This one primitive will let us remove thousands of lines of reactivity code from Horizon.” Shopify’s team also created 20 new rules to govern generated themes, covering contracts, structure, validation, complexity, nesting, and file-size limits. This is a subtle but major shift. By treating contracts and validation as first-class, Shopify is essentially building guardrails that agents can follow, not just code that agents can scrape.

Finally, Shopify is leaning into more expressive Liquid capabilities. Liquid now allows logical capabilities such as Boolean expressions, infix operators with precedence, and literal arrays and objects. Shopify even says that “at some point in the future, the Tailwind CSS framework will even be supported.” For executives, the “second-order” signal here is bigger than Tailwind support. Shopify is aligning theme authoring with what AI agents handle well: semantically-rich, human-readable structure, plus explicit constraints that reduce ambiguity.

There is an industry conversation baked into this too. Shopify’s post includes a note that AI experts are gradually coming around to the idea that dense, symbolic-heavy code may not be the best diet for AIs for non-coding tasks, and that LLMs often do better with more easily-understandable prose. Shopify cites an example where SQL was easier to process than the DSLs of 17 tools, and it contrasts the industry’s shift from XML to leaner JSON a decade ago. Whether you care about the debate or not, the business takeaway is that agent-first product features are forcing platforms to revisit foundational developer experiences.

So the question for boards and leaders is not “will AI change frontend code.” It already has. The question is how quickly you can turn agent-generated changes into reliable artifacts with contracts, validation, and readable surfaces. Shopify is betting that cleaner code is not a throwback, it is an infrastructure upgrade for the era where “edits” are made by both people and agents.