diff --git a/docs/canary/concepts/builder.md b/docs/canary/concepts/builder.md
new file mode 100644
index 00000000000..e9c3dd1cf16
--- /dev/null
+++ b/docs/canary/concepts/builder.md
@@ -0,0 +1,84 @@
+---
+description: |
+  The Builder class is used to generate optimized assets for production.
+---
+
+The `Builder` class in Fresh is where you'll do everything related to builds.
+You'll typically find it being created inside your project's `dev.ts` file.
+
+```ts dev.ts
+import { Builder } from "fresh/dev";
+
+const builder = new Builder({ target: "safari12" });
+
+if (Deno.args.includes("build")) {
+  // This creates a production build
+  await builder.build();
+} else {
+  // This starts a development server with live reload
+  await builder.listen(() => import("./main.ts"));
+}
+```
+
+## Options
+
+You can customize the builder by passing options.
+
+```ts
+const builder = new Builder({
+  // Browser target for generated code. Maps to https://esbuild.github.io/api/#target
+  target?: string | string[];
+  // The root directory of the project. All other paths will be resolved
+  // against this if they're relative. (Default: `Deno.cwd()`)
+  root?: string;
+  // Where to write generated files when doing a production build.
+  // (default: `<root>/_fresh/`)
+  outDir?: string;
+  // Path to static file directory. (Default: `<root>/static/`)
+  staticDir?: string;
+  // Path to island directory. (Default: `<root>/islands`)
+  islandDir?: string;
+  // Path to routes directory. (Default: `<root>/routes`)
+  routeDir?: string;
+  // File paths which should be ignored 
+  ignore?: RegExp[];
+})
+```
+
+## Registering islands
+
+The builder is where you'll register files that contain islands. This is the
+same API that Fresh uses internally.
+
+```ts
+const builder = new Builder();
+
+// Path to local island
+builder.registerIsland("path/to/my/Island.tsx");
+// File urls work too
+builder.registerIsland("file:///path/to/my/Island.tsx");
+// Also islands from jsr
+builder.registerIsland("jsr:@marvinh-test/fresh-island");
+```
+
+## Adding build plugins
+
+The `Builder` has a very simple processing mechanism for static files.
+
+```ts
+builder.onTransformStaticFile({
+  pluginName: "My cool plugin",
+  filter: /\.css$/,
+}, (args) => {
+  // Prepend `body { background: red }` to every `.css` file
+  const code = `body { background: red } ${args.text}`;
+
+  return {
+    content: code,
+    map: undefined, // Optional: source maps
+  };
+});
+```
+
+> [info]: Only static files in `static/` or the value you set `staticDir` to
+> will be processed. The builder won't process anything else.
diff --git a/docs/canary/testing/index.md b/docs/canary/testing/index.md
index f3049b6e83e..9f3ec28831f 100644
--- a/docs/canary/testing/index.md
+++ b/docs/canary/testing/index.md
@@ -103,3 +103,37 @@ Deno.test("MyLayout - renders heading and content", async () => {
   expect(text).toContain("Hello");
 });
 ```
+
+## Testing with file routes and islands
+
+[File routes](/docs/canary/concepts//file-routing) are collected by the
+[`Builder`](/docs/canary/concepts/builder) class and not just by
+[`App`](/docs/concepts/app) alone. We can generate a snapshot and re-use it for
+many app instances in our test suite.
+
+```ts my-app.test.ts
+// Best to do this once instead of for every test case for
+// performance reasons.
+const builder = new Builder();
+const applySnapshot = await builder.build({ snapshot: "memory" });
+
+function testApp() {
+  const app = new App()
+    .get("/", () => new Response("hello"))
+    .fsRoutes();
+
+  // Applies build snapshot to this app instance.
+  applySnapshot(app);
+}
+
+Deno.test("My Test", () => {
+  const handler = testApp().handler();
+
+  const response = await handler(new Request("http://localhost"));
+  const text = await response.text();
+
+  if (text !== "hello") {
+    throw new Error("fail");
+  }
+});
+```
diff --git a/docs/toc.ts b/docs/toc.ts
index 5aa1a7d2a90..4105f1613d1 100644
--- a/docs/toc.ts
+++ b/docs/toc.ts
@@ -39,6 +39,7 @@ const toc: RawTableOfContents = {
           ["islands", "Islands", "link:canary"],
           ["static-files", "Static files", "link:canary"],
 
+          ["builder", "Builder", "link:canary"],
           ["file-routing", "File routing", "link:canary"],
         ],
       },