Developer Writes a Svelte Compiler in Rust With an AI Agent in 106 Evenings and $700

A developer on Habr has finished building a Svelte compiler in Rust with the help of an AI agent, a year after abandoning the same task while working alone. The second attempt took 106 evenings and $700 in API costs, while the first took 5.5 months and produced no result.

Why rewrite the Svelte compiler in Rust

The official Svelte compiler is written in JavaScript and TypeScript, which becomes a bottleneck as a project scales. According to the author of the Habr post, everything slowed down on his team: the build, the dev server, type checking via svelte-check and the LSP language server.

Rust is a systems language with performance comparable to C++, and it could speed up the entire development loop several times over. Svelte is popular precisely for its speed on the client: it compiles templates into optimized JavaScript at build time, bypassing the virtual DOM. Rewriting the compiler in Rust means getting the same speed on the development side too.

Key project parameters:

• Problem: slow build, dev-server, svelte-check and LSP
• Solution: write an alternative compiler in Rust
• First attempt: 5.5 months of solo work — abandoned
• Second attempt: 106 evenings with an AI agent, $700 in API costs

Why a compiler is hard to write alone

Writing a compiler is one of the most labor-intensive tasks in programming. It requires lexical analysis, building an abstract syntax tree (AST), semantic analysis, code generation and compatibility with the plugin ecosystem. Even for a relatively small framework, that means hundreds of hours of work.

For 5.5 months the developer made progress on his own, until the work stalled. Projects like this demand holding a great deal of context in your head at once: parser state, grammar edge cases, dozens of interdependent components. After any long pause, rebuilding that working context becomes a weeks-long task in itself.

The project was abandoned. A year went by.

How the AI agent changed the outcome

When he returned to the project, the developer brought in an AI agent. The compiler was finished in 106 evenings at a total cost of $700 in API requests.

"I poked at it by hand for five and a half months and gave up.

A year later I came back with an AI agent and finished it in 106 evenings and $700," the author writes in his Habr post.

That averages about $6.6 per evening, comparable to the price of a coffee. The AI agent took over exactly the kind of work that wears people out most in manual development: boilerplate code, debugging cycles and refactoring of repetitive patterns. The developer remained the project's architect and made the key decisions.

In the end, the $700 investment turned into a real piece of infrastructure for the whole team: a compiler that speeds up the very tools that had been getting in the way from the start.

What this means

The story shows a new practical role for AI agents in development: not a replacement for the programmer, but a tool for getting through the "valley of death" of complex projects — that stage of voluminous routine work where most personal projects stall for good. A Svelte compiler in Rust that proved too much to tackle alone became a working tool for an investment of $700.

Frequently asked questions

How much did it cost to write a

Svelte compiler with an AI agent?

According to the author, total API spending came to $700 over 106 evenings of work, about $6.6 per work session.

Why use Rust for a Svelte compiler?

Rust is a systems language with C++-level performance. Rewriting the compiler from JavaScript/TypeScript to Rust means substantially faster builds, LSP and svelte-check — precisely the tools that slowed down the team's workflow.