Prompts

Everything your app tells the model in its own words lives in src/harness/prompts/, one file per text. Nothing is a string in code. The folder is read again on every render, so an edit reaches the next question, with no restart and no rebuild.

The files

A prompt is a pair: <name>.system.eta is the system prompt, <name>.user.eta the user turn, rendered with the same input. In the basic template:

Code renders them with prompt(name, input) — a { systemPrompt, content } pair that spreads straight into an agent — or render(file, input) for one file:

const settled = yield* useAgent({
  ...prompt(trunk ? "synthesize-extend" : "synthesize", { query, notes }),
  parent: spine,
  // …
});

The frame

Every system file opens by handing itself to one frame:

<% layout("./framed", { writesTheAnswer: true }) %>
You are an article synthesist. …

framed.eta puts what the app is for first and — for a stage that writes the words a reader gets — what its answers must do last:

<%~ [it.purpose, it.body, it.writesTheAnswer ? it.answers : ""].map((part) => (part ?? "").trim()).filter(Boolean).join("\n\n") %>

The two sentences come from src/harness/instructions.ts, empty as shipped:

export const INSTRUCTIONS = {
  purpose: "You help maintenance engineers investigate equipment failures.",
  answers: "Lead with the likely cause. Always name the part number.",
};

purpose reaches every agent that thinks about the question; answers only the one writing the reader's words. Unlike the prompt files, instructions.ts is read when a run starts.

What you can write

Templates are Eta, which means JavaScript: <%= it.x %> writes a value, <% … %> runs a statement, <%~ … %> inserts text as it is. Escaping is off — this is prose for a model, not HTML.

<% /* a numbered list from an array */ %>
<% it.topics.forEach((t, i) => { %><%= i + 1 %>. <%= t %>
<% }) %>

<% /* computed text: counts, plurals */ %>
<%= it.articles.length %> article<%= it.articles.length === 1 ? "" : "s" %>

<% /* a conditional part */ %>
<% if (it.notes.trim()) { %>Notes so far:
<%= it.notes %><% } %>

A missing input

A top-level key a template reads that its input does not give renders as empty, and one line in the engine's log says so. It never puts the word undefined in front of the model, and it never costs a reader the run.

npm test renders every file against its input contract, so a key a file reads that its stage does not pass fails the tests before any model loads.

Two rules

• Say nothing about the format. No role markers, no <think>, no tool JSON. The framework adds the model's own, so a file that spells them is wrong for the next model.
• Say nothing the harness does not know. A worked example is fine; a fact the run did not surface is not.

Related

• Build your first harness — edit a prompt and ask again.
• Structured output — when the answer should be a value, not prose.