<!-- source-hash: 89ad4456d6f452acc2ba185db7b6f3b5 -->
Constructs branded fallback cover-image URLs for entity cards (onboarding guides, blog posts, releases, etc.) that have no image, centralizing all URL-building logic so consumers never concatenate og-placeholder URLs themselves.

## Key Components

### Interfaces

- **`OgPlaceholderEndpoints`** — Slice of `ChatRuntime.endpoints` needed for base URL resolution; accepts `ogPlaceholderUrl` (explicit) or `imageProxyUrlPrefix` (derived fallback).
- **`BuildOgPlaceholderOptions`** — Options for `site` label, `aspect` ratio (`'wide'` | `'square'`), and explicit `width`/`height` pixel overrides.

### Functions

- **`resolveOgPlaceholderBase(endpoints?)`** *(internal)* — Resolves the route base via three-tier priority: explicit `ogPlaceholderUrl` → derived from `imageProxyUrlPrefix` (swaps `/image-proxy` for `/og-placeholder`) → same-origin relative `/api/og-placeholder`.
- **`buildOgPlaceholderUrl(endpoints, title, options?)`** — Primary entry point. Resolves the base URL, preserves any pre-existing query params, and appends `title`, `site`, and dimension params. SSR- and browser-safe; always returns a usable URL.

## Usage Example

```typescript
import { buildOgPlaceholderUrl } from './og-placeholder'

const endpoints = {
  imageProxyUrlPrefix: 'https://api.example.com/image-proxy',
}

// Wide fallback (1200×630, route default)
const wideUrl = buildOgPlaceholderUrl(endpoints, 'Getting Started Guide', {
  site: 'Docs',
  aspect: 'wide',
})
// → https://api.example.com/og-placeholder?title=Getting+Started+Guide&site=Docs

// Square fallback for compact chat-inline slots (1024×1024)
const squareUrl = buildOgPlaceholderUrl(endpoints, 'Release v2.0', {
  aspect: 'square',
})
// → https://api.example.com/og-placeholder?title=Release+v2.0&w=1024&h=1024

// Explicit dimensions win over aspect
const customUrl = buildOgPlaceholderUrl(endpoints, 'Case Study', {
  width: 800,
  height: 400,
})
```

**Base resolution priority:** `ogPlaceholderUrl` → derived from `imageProxyUrlPrefix` → `/api/og-placeholder` (same-origin fallback). Per-platform brand colors are resolved server-side — never baked into the URL.