--- name: add-a-capability description: Learn how one contract and its handler become a command, HTTP route, MCP tool, browser RPC, and sandbox call. plain: "Define one contract, bind one handler, register it once, and every surface projects the same action." diagram: |- 1 defineContract input, output, failure 2 implement one handler 3 register capabilities tuple 4 project CLI HTTP MCP RPC code 5 test the handler --- # Add a capability > For agents: start with the [agent guide](https://ratstack.sh/llms.txt). Every page is Markdown by default; add `Accept: text/html` for HTML. New on /news: [alchemy-turbo-cache](/news). A capability is one named action with a contract and a server-side handler. The contract defines its name, input, output, failure, and metadata. The handler implements the action. Define the shared contract once. Implement its handler where the data and infrastructure live. Do not write separate business logic for each interface. Effect describes work with typed failures and required dependencies. Schema defines accepted values and their runtime checks. A service is a named interface for one job. A Layer builds services and supplies their construction dependencies. ## 1. Define the contract Put cross-process contracts in `packages/core/src/contracts.ts` and import `defineContract` from `@rat-stack/capability/contract`. 1. Define the input, output, and [expected failure](/lore/error-model) schemas. 2. Give the contract a stable name and short description. 3. Use `Schema.Struct` for the input. 4. Set honest annotations such as `readOnly`, `idempotent`, `destructive`, and `openWorld`. 5. Set `needsApproval: true` when the action needs approval. `implement` adds the `Approval` requirement and `ApprovalDenied` failure; the handler itself declares only the contract's failures. ```ts import { defineContract } from "@rat-stack/capability/contract"; import { Schema } from "effect"; class ThingError extends Schema.TaggedError< ThingError, { readonly brand: unique symbol } >()("ThingError", { id: Schema.String }) {} const ThingResult = Schema.Struct({ id: Schema.String, done: Schema.Boolean }); export const doThingContract = defineContract("doThing", { annotations: { idempotent: true, readOnly: true }, description: "Do one concrete thing", failure: ThingError, input: Schema.Struct({ id: Schema.String }), output: ThingResult, }); ``` Schemas must encode and decode without services. Keep server dependencies out of the contract module. ## 2. Implement it Import `implement` from `@rat-stack/capability/implement`. Put the handler next to its service or server-side data. Keep it small; put real work in a service or lifecycle machine. packages/core/src/inspect-file.ts at 81a12c4b86cd65bf86e4bdba98eda122d0133844; lines 14-16; highlighted none. ```typescript export const inspectFile = implement(inspectFileContract, ({ path }) => runInspectMachine(path) ); ``` The handler input comes from the contract. Its Effect requirements and expected failures stay typed. `implement` supplies the approval gate when the contract requires it. If the action needs a service, copy `packages/core/src/file-inspector.ts`. Use a `Context.Service` class. Capture dependencies in `make` and keep `static layer` beside it. ## 3. Register it Add the implementation to the `capabilities` tuple consumed by its composition root. For the CLI example, that tuple lives in `packages/core/src/inspect-file.ts`: ```ts nocheck="Imports the doThing module this skill has the reader write." import { doThing } from "./do-thing.js"; export const capabilities = [inspectFile, doThing] as const; ``` The order is public. The tuple feeds the projections and code-mode declarations. A projection builds a callable interface from capabilities. ## 4. Project the implementation The CLI, HTTP, MCP, RPC, and code-mode projections take implemented capabilities. They read names, schemas, annotations, and approval settings from `capability.contract`. - HTTP adds `POST /doThing` and updates OpenAPI. - MCP (Model Context Protocol) adds a `doThing` tool with the same schemas and flags. - The sandbox catalogue adds `tools.doThing(input)`. - Sandbox calls decode input, run the same handler, then encode the result. `toCommand` builds one CLI command from the registered tuple. Open `apps/cli/src/command.ts` only when the command needs a positional argument, custom renderer, or alias. Use `name`, `positional`, and `render` for those cases. `toCommand` adds `--json`; do not parse fields again or call the service directly. RPC means remote procedure call: a client calls a named server operation. Browser clients import contracts from `@rat-stack/core/contracts` and `toRpcGroup` from `@rat-stack/capability/rpc-group`; they do not import a handler or the server-side `toRpc` projection. ## 5. Test it Use `@effect/vitest` and run Effects with `it.effect` or `it.layer`. Do not call `Effect.run*` or `ManagedRuntime.make` in tests. 1. Test the handler's output, expected failures, annotations, and approval behavior. 2. Add projection tests when the projection changes. Check that client RPC groups can be built from contracts alone. 3. Add a command-line end-to-end test when the new capability changes the command tree or a public interface. Use `Schema.encodeEffect` to check encoded results and `Effect.flip` to inspect expected errors. ## 6. Finish ```sh pnpm turbo run check test build ``` Fix failures. Do not loosen the checks, hooks, or pinned versions. These checks and hooks form the fence that rejects prohibited code and shortcuts. ## Lore on this page - [Handle expected failures without hiding defects](/lore/error-model) - [Lifecycles are machines](/lore/lifecycles-are-machines) - [Structure an Effect codebase by domain](/lore/structure-effect-by-domain) - [OpenAPI describes the refusal too](/lore/openapi-describes-the-refusal-too) - [MCP is another surface](/lore/mcp-is-another-surface) - [The fence](/lore/the-fence) ## Linked from - learn-rat-stack → Trace a capability through Effect, XState, five projections, the CLI, and Alchemy. Try hosted search and read. → [Read page](https://ratstack.sh/skills/learn-rat-stack)