> For AI agents: the complete documentation index is available at /llms.txt, the full documentation bundle is available at /llms-full.txt.

# bff.runtimeFramework

- **Type:** `'hono' | 'effect'`
- **Default:** `'effect'`

:::tip
Please refer to the [Enable BFF](/guides/advanced-features/bff/function.md#enable-bff) section in Basic Usage to enable BFF functionality first.
:::

Controls which runtime framework Modern.js uses for BFF routing.

```ts title="modern.config.ts"
export default defineConfig({
  bff: {
    runtimeFramework: 'effect',
  },
});
```

- Set to `'effect'` to run BFF only from `api/index` (Effect HttpApi runtime).
- Set to `'hono'` to run BFF only from file-convention handlers under `api/lambda/**`.

There is no implicit fallback between runtimes. Choose one runtime mode per app.
Generated UltraModern apps use strict Effect APIs by default and set
`bff.effect.strictEffectApproach: true`; `hono` and `api/lambda/**` are not
valid generated UltraModern HTTP API paths.