diff --git a/packages/react/src/index.ts b/packages/react/src/index.ts
index 4d7602e5..868ee7b1 100644
--- a/packages/react/src/index.ts
+++ b/packages/react/src/index.ts
@@ -2,3 +2,7 @@ export { createSelectorCache, createSnapshotCache } from "./snapshot-cache.ts";
 export type { SnapshotCache, SnapshotSource } from "./snapshot-cache.ts";
 export { setStoreResolver, useWrnActions, useWrnStore } from "./store-bridge.ts";
 export type { BoundStore, IslandStore, StoreResolver } from "./store-bridge.ts";
+export { IslandErrorBoundary } from "./error-boundary.tsx";
+export type { IslandErrorBoundaryProps } from "./error-boundary.tsx";
+export { islandRootCount, mountIslands, unmountIslands } from "./island-runtime.ts";
+export type { MountOptions } from "./island-runtime.ts";
diff --git a/packages/react/src/island-runtime.ts b/packages/react/src/island-runtime.ts
new file mode 100644
index 00000000..f9bb96c0
--- /dev/null
+++ b/packages/react/src/island-runtime.ts
@@ -0,0 +1,102 @@
+import { createElement, type ComponentType } from "react";
+import { createRoot, type Root } from "react-dom/client";
+import { IslandErrorBoundary } from "./error-boundary.tsx";
+
+export interface MountOptions {
+  loader: (name: string) => Promise<{ default: ComponentType<any> }>;
+  development?: boolean;
+}
+
+const roots = new Map<Element, Root>();
+
+export function islandRootCount(): number {
+  return roots.size;
+}
+
+function readProps(element: Element): Record<string, unknown> {
+  const raw = element.getAttribute("data-wrn-island-props");
+  if (!raw) return {};
+  try {
+    return JSON.parse(raw) as Record<string, unknown>;
+  } catch (error) {
+    console.error("[wrnexus] island props were not valid JSON", error);
+    return {};
+  }
+}
+
+function whenReady(element: Element, strategy: string): Promise<void> {
+  if (strategy === "visible" && typeof IntersectionObserver !== "undefined") {
+    return new Promise((resolve) => {
+      const observer = new IntersectionObserver((entries) => {
+        if (entries.some((entry) => entry.isIntersecting)) {
+          observer.disconnect();
+          resolve();
+        }
+      });
+      observer.observe(element);
+    });
+  }
+  if (strategy === "idle" && typeof requestIdleCallback !== "undefined") {
+    return new Promise((resolve) => requestIdleCallback(() => resolve()));
+  }
+  return Promise.resolve();
+}
+
+async function mountOne(element: Element, options: MountOptions): Promise<void> {
+  if (roots.has(element)) return;
+
+  const name = element.getAttribute("data-wrn-island");
+  if (!name) return;
+
+  const strategy = element.getAttribute("data-wrn-island-strategy") ?? "only";
+  await whenReady(element, strategy);
+
+  // Re-check: an await point means a concurrent mount may have claimed this
+  // element while the strategy was resolving.
+  if (roots.has(element)) return;
+
+  let Component: ComponentType<any>;
+  try {
+    Component = (await options.loader(name)).default;
+  } catch (error) {
+    console.error(`[wrnexus] failed to load island bundle for '${name}'`, error);
+    return;
+  }
+
+  if (roots.has(element)) return;
+
+  const root = createRoot(element);
+  roots.set(element, root);
+  root.render(
+    createElement(
+      IslandErrorBoundary,
+      { name, development: options.development ?? false },
+      createElement(Component, readProps(element)),
+    ),
+  );
+}
+
+/** Mounts every island marker under `root`. No-op when the page has none. */
+export async function mountIslands(root: ParentNode, options: MountOptions): Promise<void> {
+  const markers = Array.from(root.querySelectorAll("[data-wrn-island]"));
+  if (markers.length === 0) return;
+  await Promise.all(markers.map((element) => mountOne(element, options)));
+}
+
+/**
+ * Disposes island roots under `root`. Must run on client-side navigation or
+ * React roots, detached DOM, and store subscriptions leak on every route change.
+ */
+export function unmountIslands(root: ParentNode): void {
+  for (const [element, reactRoot] of [...roots]) {
+    if (element !== (root as unknown as Element) && !(root as unknown as Node).contains(element)) {
+      continue;
+    }
+    try {
+      reactRoot.unmount();
+    } catch (error) {
+      console.error("[wrnexus] island failed to unmount cleanly", error);
+    }
+    roots.delete(element);
+  }
+}
diff --git a/packages/react/test/island-runtime.test.ts b/packages/react/test/island-runtime.test.ts
new file mode 100644
index 00000000..800c83a1
--- /dev/null
+++ b/packages/react/test/island-runtime.test.ts
@@ -0,0 +1,124 @@
+import { afterEach, expect, test } from "bun:test";
+import { Window } from "happy-dom";
+import { act, createElement } from "react";
+import { islandRootCount, mountIslands, unmountIslands } from "../src/island-runtime.ts";
+
+(globalThis as any).IS_REACT_ACT_ENVIRONMENT = true;
+
+function domWith(html: string) {
+  const window = new Window();
+  window.document.body.innerHTML = html;
+  (globalThis as any).window = window;
+  (globalThis as any).document = window.document;
+  return window;
+}
+
+const loader = async () => ({
+  default: (props: { title?: string }) => createElement("span", null, props.title ?? "none"),
+});
+
+afterEach(() => {
+  const doc = (globalThis as any).document;
+  if (!doc) return;
+  act(() => {
+    unmountIslands(doc);
+  });
+});
+
+function marker(props = "{}", strategy = "only") {
+  return (
+    `<div data-wrn-island="Chart" data-wrn-island-strategy="${strategy}"` +
+    ` data-wrn-island-props='${props}'></div>`
+  );
+}
+
+test("mounts an island and passes deserialized props", async () => {
+  const window = domWith(marker('{"title":"Revenue"}'));
+
+  await act(async () => {
+    await mountIslands(window.document.body, { loader });
+  });
+
+  expect(window.document.body.textContent).toContain("Revenue");
+  expect(islandRootCount()).toBe(1);
+});
+
+test("unmounts roots and leaves no leaked roots behind", async () => {
+  const window = domWith(marker('{"title":"A"}'));
+
+  await act(async () => {
+    await mountIslands(window.document.body, { loader });
+  });
+  expect(islandRootCount()).toBe(1);
+
+  act(() => {
+    unmountIslands(window.document.body);
+  });
+  expect(islandRootCount()).toBe(0);
+});
+
+test("repeated mount/unmount cycles do not accumulate roots", async () => {
+  const window = domWith(marker());
+
+  for (let i = 0; i < 5; i += 1) {
+    await act(async () => {
+      await mountIslands(window.document.body, { loader });
+    });
+    act(() => {
+      unmountIslands(window.document.body);
+    });
+  }
+
+  expect(islandRootCount()).toBe(0);
+});
+
+test("does nothing when no island markers are present", async () => {
+  const window = domWith(`<p>plain server html</p>`);
+  await act(async () => {
+    await mountIslands(window.document.body, { loader });
+  });
+  expect(islandRootCount()).toBe(0);
+});
+
+test("mounting twice does not create a second root for the same element", async () => {
+  const window = domWith(marker());
+
+  await act(async () => {
+    await mountIslands(window.document.body, { loader });
+    await mountIslands(window.document.body, { loader });
+  });
+
+  expect(islandRootCount()).toBe(1);
+});
+
+test("a failing bundle load leaves the placeholder and mounts no root", async () => {
+  const window = domWith(marker());
+  const original = console.error;
+  console.error = () => {};
+
+  await act(async () => {
+    await mountIslands(window.document.body, {
+      loader: async () => {
+        throw new Error("network down");
+      },
+    });
+  });
+  console.error = original;
+
+  expect(islandRootCount()).toBe(0);
+  expect(window.document.querySelector("[data-wrn-island]")).not.toBeNull();
+});
+
+test("malformed props JSON falls back to empty props instead of throwing", async () => {
+  const window = domWith(marker("not-json"));
+  const original = console.error;
+  console.error = () => {};
+
+  await act(async () => {
+    await mountIslands(window.document.body, { loader });
+  });
+  console.error = original;
+
+  expect(window.document.body.textContent).toContain("none");
+  expect(islandRootCount()).toBe(1);
+});