> ## Documentation Index
> Fetch the complete documentation index at: https://docs.useharmonia.xyz/llms.txt
> Use this file to discover all available pages before exploring further.

# Next.js

> How Harmonia uses Next.js for its operator UI, authenticated API boundary, and deterministic workflow services.

Next.js is Harmonia's web application and server-side API runtime. React renders the dashboard, Studio, settings, calendar, monitoring, and documentation-facing product surfaces.

## How Harmonia uses it

* App Router pages under `src/app/` render the dashboard and operator workflows.
* Route handlers under `src/app/api/` expose tenant-scoped browser APIs and authenticated internal worker endpoints.
* Server libraries under `src/lib/` enforce tenancy, decisions, Firestore transitions, integration contracts, and A2UI hydration.
* `src/proxy.ts` applies the application request boundary.
* `src/instrumentation.ts` initializes server tracing when telemetry is enabled.

Next.js does not run agent reasoning. It invokes the private worker and renders validated results persisted in Firestore.

## Security and authority

Browser input is parsed with strict schemas. Tenant identity is derived server-side. Strategy and effect decisions use dedicated endpoints bound to exact persisted payloads; natural-language chat cannot bypass those handlers.

## Failure behavior

Routes return explicit non-success responses for invalid input, authorization failures, conflicts, and unavailable dependencies. UI success is derived from persisted state, not optimistic model output.

## Evidence status

The production build and Vitest suite verify the local application contract. A reachable preview proves only the deployed web revision unless correlated worker and cloud-service evidence is captured.

## Relevant files

* `src/app/`
* `src/lib/`
* `src/proxy.ts`
* `src/instrumentation.ts`
* `next.config.ts`