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packages/api-stack/api/api-auth-better-auth/src/oauth/default-factory.md

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+# `default-factory`
+
+## Purpose
+
+Default `betterAuthFactory` used when an operator does not supply one via
+`BetterAuthOptions.betterAuthFactory`. Dynamically imports `better-auth`
+AND (when OAuth providers are configured) the `genericOAuth` plugin, then
+calls `betterAuth({...})` with the assembled args. The dynamic imports
+mean module-load of `@suaveplan/api-auth-better-auth` never blocks on
+`better-auth` — operators that ship a stubbed factory (unit tests, custom
+adapter shells) avoid the upstream package altogether.
+
+## Features
+
+- Dynamic `import("better-auth")` keeps cold-start fast for stub-only
+  consumers.
+- Dynamic `import("better-auth/plugins/generic-oauth")` only fires when the
+  options actually configure OAuth providers.
+- The Suaveplan-private `__suaveplan_generic_oauth` sentinel key from
+  `buildBetterAuthFactoryArgs` is stripped before forwarding so the upstream
+  package never sees our internal fields.
+- Pre-existing `plugins` arrays the operator may pass through their own
+  factory are preserved (the genericOAuth plugin is appended, not assigned).
+
+## Usage: Basic
+
+```ts
+// Production wiring — pass options without a betterAuthFactory and the
+// default fires implicitly:
+const provider = createBetterAuthProvider({
+    database: pool,
+    baseUrl: "https://api.example.com",
+    secret: process.env.BETTER_AUTH_SECRET!,
+});
+```
+
+## Usage: Advanced
+
+```ts
+import { defaultBetterAuthFactory } from "@suaveplan/api-auth-better-auth";
+
+// Operator-supplied factory that wraps the default to inject the
+// `organization` Better-Auth plugin on top of whatever the wrapper itself
+// already configures:
+const customFactory = async (args: Record<string, unknown>) => {
+    const auth = await defaultBetterAuthFactory(args);
+    return auth;
+};
+
+const provider = createBetterAuthProvider({
+    database: pool,
+    baseUrl,
+    secret,
+    betterAuthFactory: customFactory,
+});
+```
+
+## API Reference
+
+| Export | Shape | Purpose |
+| ------ | ----- | ------- |
+| `defaultBetterAuthFactory(args)` | `(Record<string, unknown>) => Promise<BetterAuthLike>` | Dynamic loader + plugin assembler |
+
+## Implementation Notes
+
+The sentinel-key pattern (`__suaveplan_generic_oauth`) is the trade-off
+between (a) keeping `buildBetterAuthFactoryArgs` synchronous and pure, and
+(b) actually loading the upstream plugin module. The args bag is built
+synchronously so callers and tests can inspect it; the plugin module is
+loaded lazily inside this factory. Stripping the sentinel keeps the
+boundary clean — Better-Auth's option validator complains about unknown
+top-level keys.
+
+The `plugins` array preservation matters because operators MAY supply a
+custom factory that adds Better-Auth's own plugins (organisation, passkeys,
+2FA, magic-link) on top of the OAuth config the wrapper itself injects. By
+appending we ensure a stub operator factory + the wrapper's genericOAuth
+config coexist correctly.