Dev Server

sema web serves a Sema web app in the browser with one command — no bundler, no npm install, no hand-rolled backend for llm/*. It bundles the browser runtime (the WASM VM + JS), serves your app, reloads the page when you edit a file, and proxies llm/* calls to real providers using server-side keys.

sema web app.sema

  Sema Web dev server
  → http://127.0.0.1:3000
  serving app.sema from /path/to/your/app

That's it — the browser opens to your running app.

What it does

• Serves the runtime. The WASM interpreter and the @sema-lang/* JS modules are embedded in the sema binary and served under /__sema/, wired up with a generated <script type="importmap">. No node_modules at runtime.
• Serves your app. Your entry .sema (and anything next to it) is served from /app/. A single-file app is fetched as source and evaluated by the WASM VM in the browser. An app that uses import is compiled to a .vfs archive on the server (the browser has no filesystem to resolve imports against) and served from /__build/app.vfs; an import that does not resolve is a build error shown in the page.
• Hot reloads. The server watches your app directory. The page polls /__dev/poll; when a file changes, the page reloads and fetches the new source (or a rebuilt archive).
• Proxies llm/*. Browser llm/* calls are answered by the dev server using the API keys in your environment (see LLM proxy).

Options

sema web app.sema [OPTIONS]

Option	Default	Description
-p, --port <n>	3000	Port to serve on. Advances to the next free port if taken.
--host <h>	127.0.0.1	Address to bind. A non-loopback host exposes the unauthenticated LLM proxy to the network; the server prints a warning.
--no-open	—	Don't open a browser automatically. The browser is also not opened when stdout is not a terminal.
--no-llm	—	Disable the built-in LLM proxy.

Hot reload

Edit your .sema file and the browser repaints. Reload is a full page reload — the app re-runs from a clean slate, so there's no stale state. Values persisted with store/* (localStorage) survive; in-memory reactive state resets.

The page short-polls the server for changes, so reload works without a persistent connection. There is no separate watch process to run.

LLM proxy

Because API keys can't live in the browser, the dev server answers llm/* for you. It speaks the same protocol as @sema-lang/llm-proxy, so your app code is identical in development and production — only the proxy URL differs. In dev, it's wired to the dev server automatically.

;; In the browser — no keys, no config. The dev server calls the real provider.
(llm/complete "Summarize this in one line: ..." {:model "claude-haiku-4-5-20251001"})

Keys come from your environment, exactly as with the sema CLI (ANTHROPIC_API_KEY, OPENAI_API_KEY, …). Disable the proxy with --no-llm.

The dev proxy has no authentication, rate limiting, or CORS configuration; it is meant for a loopback address on your own machine. Use @sema-lang/llm-proxy for anything reachable by others. Two small differences from that package: a malformed body is answered with 400 {"error": ...} without a code field, and GET /models returns the configured provider names rather than model names.

Streaming (llm/chat-stream) delivers tokens progressively — they render as they arrive from the provider.

One stream at a time

The built-in HTTP server is single-threaded, so a live stream holds it for its duration: other requests (including hot-reload polls and a second stream) queue until it finishes. Fine for a single-user dev loop; true concurrency is a planned improvement.

Error overlay

Compile and runtime errors in your app appear as an overlay across the top of the page (and in the console), so you don't have to hunt for them. Click the overlay to dismiss it; the next successful reload clears it.

Multi-file apps

Multi-file apps work with no extra steps. A single-file app runs from raw source (the browser compiles it directly); an app that (import ...)s other .sema modules is compiled to a .vfs archive on the fly — the same artifact sema build --target web produces — and rebuilt on each reload. The dev server picks the right mode automatically; imports just resolve.

How it compares

sema web is the zero-config path for developing an app. When you're ready to ship, sema build --target web produces a compiled .vfs you serve as static files, and @sema-lang/llm-proxy provides the production proxy for Vercel/Netlify/Cloudflare/Node.