Have an LLM build a new language toolchain

A developer got Claude to produce the compiler, runtime, and tests for 5emel while barely inspecting the code themselves.

Done with Claude· Advanced· Nov 2026

What they did
They spent a few weeks prompting Claude Opus 5.5 to assemble the tooling for 5emel, an experimental language of their own. The project grew into roughly 15,000 lines of TypeScript/JavaScript plus 3,000 lines of C. They said they scarcely opened the code, including the compiler, and instead used long experience with programming books to keep the project on track.
How it went
The end result was a functioning toolchain. When another Claude instance was shown one function and a description of most of the language, it could describe a lot of it, but not every detail.
Tips
This kind of project still leaves you with plenty to untangle yourself. Build in short passes, and pause before any step that would permanently alter files, outputs, or generated code.

Try it yourself with Claude

I want to create a small experimental language called [language name]. Its main rules are [feature 1], [feature 2], and [feature 3]. Please help me design the language, then generate a compiler or interpreter and a runtime in [TypeScript/Python/C]. Break the work into small stages, explain what each stage changes, and check the examples after every stage. If a step would delete files, overwrite major modules, or otherwise make an irreversible change, stop first and ask for my approval. Finish by summarizing the system design and listing any parts that need extra review.

Fill in the [brackets]. Agents can make mistakes, so review anything before it is sent, paid or deleted.

Read the original on news.ycombinator.com ↗

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