# Build Agents with the SDK

> Navigate the SDK contracts used to author Agent behavior, durable execution, Tools, Resources, and analytics.

Use `@constal/sdk` inside the immutable Agent package. The smallest useful Agent reads accepted history, makes one model turn, and returns a result that the runtime records durably:

## Minimal Agent {#minimal-agent}

```ts
import { agent, type HistoryView } from "@constal/sdk";

export default agent({
  id: "support", version: "1.0.0", model: "model",
  async onMessage(message, ctx) {
    const history = await ctx.ledger.view<HistoryView>("history");
    const turn = await ctx.turn({
      system: "Resolve the request accurately.",
      objective: message,
      context: { priorFacts: history.facts },
    });
    return { answer: turn.message.content };
  },
});
```

`model` is a logical Resource binding, not caller-selected provider input. `ctx.resources` contains only accepted CRNs, and Credential bytes never enter Agent code.

## Add a durable boundary {#durable-boundary}

```ts
async onMessage(message, ctx) {
  if (message.kind === "checkpoint") {
    return ctx.commit({ kind: "checkpoint", payload: message.payload });
  }
  return { accepted: true };
}
```

Use [Durable Agent patterns](/docs/agents/durable.md) for waits and subtasks, [Resources SDK guide](/docs/resources/sdk.md) for effects, and [Analytics SDK guide](/docs/analytics/sdk.md) for typed events. Deployment remains a Console, CLI, or Platform API operation; it is intentionally absent from `Ctx`.