---
title: Custom sources
description: Plug any backend into Blume with custom() — pass an object that implements the ContentSource interface, or a built-in engine with custom serializers.
---

Any object implementing the `ContentSource` interface can be passed to `custom()`, which is how an adapter with custom serializers — or any backend not built in — plugs in without its SDK touching the core install:

```ts blume.config.ts
import { defineConfig } from "blume";
import { custom, filesystem } from "blume/sources";
import { sanitySource } from "blume/sources/sanity.ts";

export default defineConfig({
  content: {
    sources: [
      filesystem({ root: "docs" }),
      custom(
        sanitySource({
          name: "guides",
          prefix: "guides",
          projectId: "abc123",
          dataset: "production",
          query: `*[_type == "guide"]`,
          // Map custom Portable Text blocks to Blume components
          serializers: {
            callout: (block) => `<Callout>${block.text}</Callout>`,
          },
        })
      ),
    ],
  },
});
```

A source built with one of Blume's engine factories, like `sanitySource` above, is rebuilt on the running command's context, so it reads drafts under `--preview` and keeps its snapshot in `.blume/cache` the way the built-in adapter does. A source of your own can do the same by implementing `withContext(ctx)` and returning itself rebuilt on that context.

`custom()` takes a `ContentSource` instance rather than an options object, so there is nothing to pass the [shared options](/docs/content/sources#adapters) `prefix` or `pollInterval` to. The source sets its own `prefix` property, and re-fetching is whatever its `watch` method implements. It carries a live instance rather than plain data, so it declares no runtime dependency or secret of its own — the instance manages those itself.

A custom source that reads local files should set `sourcePath` on each entry and `contentRoot` on the source itself. `sourcePath` names the file in diagnostics and resolves relative images beside it; `contentRoot` bounds the git `log` that dates pages, so without it the source's pages get no git-derived ["Last updated" date](/docs/configuration#last-modified).