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packages/api-stack/api/api-graphql/src/directives/directives.md

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+# `directives`
+
+<!-- begin:auto-docs -->
+_(no submodules)_
+<!-- end:auto-docs -->
+
+## Purpose
+
+Host-supplied GraphQL directive descriptors that engine adapters install at
+schema-build time. The contract layer ships two: `@auth(scope: String!)` for
+per-field authorisation and `@rateLimit(rpm: Int!)` for per-field rate
+limiting. Plugins reference them in SDL; engines wire the descriptor's
+`handler` into the resolver pipeline.
+
+Keeping the descriptors here (rather than in each engine adapter) means a
+plugin author writes one piece of SDL and gets the same enforcement
+semantics from every concrete engine. Engine adapters install the SDL via
+the directive `sdl` string and dispatch to the descriptor's `handler` from
+their resolver wrapper.
+
+## Features
+
+- `authDirective` — checks `ctx.principal.scopes` against the declared scope
+  and throws when missing. Locations: `FIELD_DEFINITION`.
+- `rateLimitDirective` — validates the `rpm` integer bound; the actual
+  rate-limit accounting is the engine adapter's responsibility (the
+  contract body asserts the argument shape).
+- `HOST_DIRECTIVES` — frozen map keyed by directive name for engine
+  adapter consumption.
+
+## Usage: Basic
+
+```ts
+import { authDirective } from "@suaveplan/api-graphql/directives";
+
+// SDL declaration:
+//   directive @auth(scope: String!) on FIELD_DEFINITION
+//
+// Used in a plugin's typeDefs:
+//   type Query { invoices: [Invoice!]! @auth(scope: "billing:read") }
+console.log(authDirective.sdl);
+```
+
+## Usage: Advanced
+
+Engine adapters install every host directive in one pass:
+
+```ts
+import { HOST_DIRECTIVES } from "@suaveplan/api-graphql/directives";
+
+for (const directive of Object.values(HOST_DIRECTIVES)) {
+    schemaBuilder.addDirective({
+        name: directive.name,
+        sdl: directive.sdl,
+        run: directive.handler,
+    });
+}
+```
+
+The `handler` is the contract-layer enforcement body. The `@auth` handler
+reads `ctx.principal.scopes` (the typed `readonly Scope[]`) and raises a
+plain `Error` when the required scope is absent — engine adapters wrap that
+in the engine's GraphQL error type before returning it on the wire.
+
+## API Reference
+
+- `authDirective: DirectiveDescriptor<{ scope: string }>` — descriptor for
+  `@auth`.
+- `rateLimitDirective: DirectiveDescriptor<{ rpm: number }>` — descriptor
+  for `@rateLimit`.
+- `HOST_DIRECTIVES` — frozen `{ auth, rateLimit }` lookup.
+
+## Implementation Notes
+
+The descriptors carry **contract bodies**, not full implementations. A
+production engine adapter wraps each resolver in a check that runs the
+handler before invoking the resolver. For `@rateLimit`, the engine adapter
+also wires the per-tenant counter; the contract-layer handler only enforces
+the `rpm > 0` invariant so a misconfigured directive surfaces early.
+
+Adding a new host directive: add a `DirectiveDescriptor`, append it to
+`HOST_DIRECTIVES`, and document the SDL in the package README. The
+`api-graphql-yoga` and `api-graphql-apollo` packages pick up new
+descriptors automatically through the frozen map.