Exec & Eval

The first word in a fence info string is the language. The remaining words form a middleware chain read left-to-right. Standard renderers only read the first word, so the modifiers stay invisible everywhere else.

```bash silent timeout=30s exec
git diff --stat
```

Built-in modifiers

• exec — run the block as a subprocess and render stdout.
• eval — run JavaScript/TypeScript in-process as an Effection operation.
• silent — execute but suppress rendered output.
• persist — keep resources created by an eval block alive for the component invocation.
• timeout=30s — cancel a long-running block.
• daemon — start a long-running subprocess tied to the component invocation.

LLM sampling is not a fence modifier — it happens through the <Sample> component installed by provider middleware.

Eval blocks share bindings

eval blocks run in a shared binding environment for the current component. Top-level bindings export automatically to later blocks, and bare {name} interpolation inside any executable block reads from them.

```ts eval
const port = yield* findFreePort();
const baseUrl = `http://127.0.0.1:${port}`;
```

```bash daemon exec
./server --port {port}
```

Rendering from eval

• output("...") renders text into the document from an eval block.
• renderChildren() and render(markdown) render nested content intentionally.

Daemons

daemon exec starts a long-lived process, returns control immediately, and is torn down by structured concurrency when the component invocation completes — no manual cleanup. Combined with readiness polling, this is how provider components run local model servers.

Next: LLM providers →