# Packages Source: https://submilli.ai/docs/packages.md Authorship: AI-assisted. A Package in Submilli is like a package in npm or pip, a library you install and import. The language is TypeScript, but npm packages can't be used. Submilli resets the ecosystem with Packages built for AI agents. Each function that reaches outside names its operation and asks the Blueprint before it acts. Why the reset is worth it comes later in this chapter. In the quickstart you wrote one with a single function. Packages are also Submilli's answer to MCP servers. Other code-execution platforms take the MCP servers you run and turn them into an interface the agent's code can call. Submilli can do that too. Declare a server in the Blueprint and it becomes a Package, with each tool a function the Blueprint can allow or deny. But MCP was designed for tool calling. Most MCP servers publish no output schema, so a program can't know the shape of what a tool returns, and a rule over an MCP tool can see only the tool's name. A Package needs no server to deploy or maintain, calls the API directly, returns typed values, and tells the runtime what each call means. A Package can be one you write for an internal system, one you write for a third-party service you consume, or one someone else published. Any Package in a Git repository [installs straight from it](https://submilli.ai/docs/blueprints/start-a-blueprint.md). Submilli publishes [curated Packages](https://submilli.ai/docs/reference/curated-packages.md) that way for common services such as GitHub, Slack, Google Drive, Linear, Notion, and others. ## Why not npm Generated code can't import npm packages or Node.js modules. An npm package is written for Node, and Node gives it the whole operating system: files, sockets, processes, anything a system call can reach. Submilli is designed for agents, and the ways a program can reach the outside world are designed for that. A program gets a small set of operations, each named and checked. An npm package also has no [semantic permissions](https://submilli.ai/docs/blueprints.md#semantic-permission-model) (no `check` calls), so a Blueprint would have nothing to govern. You pay by wrapping your systems as Packages. ## A Package, from the inside Here is one operation of the billing Package, the one the previous chapter's rules were about: ```typescript title="package/src/lib.ts (fragment)" import { check } from "submilli:security"; import { post } from "submilli:http"; import secrets from "submilli:secrets"; const BASE = "https://billing.internal.example.com/v1"; /** * Add a goodwill credit to a customer's account. * @param customerId The customer's id in the billing system, such as `cus_northwind`. * @param amount The credit, in cents; must be positive. * @returns The credit as recorded. * @capability acme.com/credits.apply { customerId: string, customerClass: string, amount: number } */ export function applyCredit(customerId: string, amount: number): Credit { // The call names a customer. Whether that customer is premium is a fact // about the account, so the package looks it up before asking. const customerClass = lookUpClass(customerId); check("acme.com/credits.apply", { customerId, customerClass, amount }); const key = secrets.get("BILLING_API_KEY"); if (key === null) { throw new Error("BILLING_API_KEY is not configured for this blueprint"); } const headers = new Map(); headers.set("Authorization", "Bearer " + key); const response = post(BASE + "/customers/" + customerId + "/credits", { amount }, headers); if (!response.ok) { throw new Error("billing API failed: HTTP " + response.status.toString()); } return JSON.parse(response.body) as Credit; } ``` Two lines make it an operation. The `@capability` tag **declares** it with a name and the fields a rule may test. The `check` call, from `submilli:security`, **enforces** it. It takes the capability's name and those fields, asks the Blueprint whether the caller may do this with these values, and throws `PermissionDeniedError` if not. :::tip[Did you know?] The compiler keeps the `@capability` tag and the `check` call in step. When you build a Package, it compares the fields the tag declares with the fields the call passes, and warns when they disagree: a field in one and not the other, a tag with no check, a check with no tag. ```text warning: payload key `customerClass` missing from `@capability` binding --> package/src/lib.ts:24:37 | 23 | const customerClass = lookUpClass(customerId); 24 | check("acme.com/credits.apply", { customerId, customerClass, amount }); | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ``` ::: This is the [semantic permission model](https://submilli.ai/docs/blueprints.md#semantic-permission-model) from the Package's side. The Package decides what the operation means and which facts describe it, and hands them to the runtime typed: the customer, the amount, and the customer's class, which the call didn't carry and the Package looked up. A Blueprint can then say "premium customers only", and nobody had to read a payload. ## What the agent's program sees To the model, a Package is an import. Before it writes a program, the agent searches for Packages and reads a Package's documentation. You can do the same from the CLI: ``` submilli search billing ``` ```text @acme/billing — Credits for one customer of Acme's billing service. ``` ``` submilli docs @acme/billing ``` ```typescript // @acme/billing — Credits for one customer of Acme's billing service. /** * Add a goodwill credit to a customer's account. * @param customerId The customer's id in the billing system, such as `cus_northwind`. * @param amount The credit, in cents; must be positive. * @capability acme.com/credits.apply { customerId: string, customerClass: string, amount: number } * @returns The credit as recorded. */ function applyCredit(customerId: string, amount: number): Credit; /** * A credit applied to a customer's account. */ interface Credit { /** * Amount in cents. */ amount: number; /** * The customer credited. */ customerId: string; } ``` The agent's documentation tool returns the same declarations, doc comments included, together with the Package's readme. Then it writes the program: ```typescript title="credit.ts" import { applyCredit } from "@acme/billing"; function main(): string { const credit = applyCredit("cus_northwind", 1500); return `credited ${credit.amount} cents`; } ``` Calls are synchronous. There is no `await`, and the program gets the value back. Run under the previous chapter's Blueprint, bound to `cus_northwind`: ```text credited 1500 cents ``` The program never sees the billing API, its URL, or the key that authenticates the request. The Package holds all three. When the Blueprint refuses the call, the program gets the error you saw in the quickstart, and the model reads it. ## The tools to build one `submilli build` is to a Package what `npm` is to a Node project. It scaffolds the project, compiles it, derives what it can be granted from the `@capability` tags, runs its tests, and installs it where programs can import it. And with the skill installed, your coding assistant does the writing. Give it a service's API documentation and what the agent may do, and it writes the Package, the readme the model reads, and the tests, then tests it under a Blueprint. The how-to pages on Packages take each step in turn, starting with [start a project](https://submilli.ai/docs/packages/start-a-project.md), and the [build a Package](https://submilli.ai/docs/tutorials/build-a-package.md) tutorial walks through doing it with your assistant. Next: [the server](https://submilli.ai/docs/server.md), the process that compiles the program and answers every check.