diff --git a/src/host/fiber.ts b/src/host/fiber.ts
new file mode 100644
index 0000000..87bacf2
--- /dev/null
+++ b/src/host/fiber.ts
@@ -0,0 +1,17 @@
+export interface Fiber<I> {
+  instance: I;
+  tag: string;
+  props: Record<string, unknown>;
+  key: string | undefined;
+  children: Fiber<I>[];
+  parent: Fiber<I> | null;
+  effect: Effect<I> | null;
+  signalDisposers: (() => void)[];
+  prevProps: Record<string, unknown> | null;
+}
+
+export type Effect<I> =
+  | { type: "update"; payload: unknown }
+  | { type: "insert"; before: Fiber<I> | null }
+  | { type: "move"; before: Fiber<I> | null }
+  | { type: "remove" };
\ No newline at end of file
diff --git a/src/mod.ts b/src/mod.ts
index 2044767..2af5285 100644
--- a/src/mod.ts
+++ b/src/mod.ts
@@ -19,5 +19,7 @@ export { ValuePointer, selectNode, setNode } from "./core/pointer.js";
 export { createRoot as createHostRoot } from "./host/config.js";
 export type { HostConfig, Root } from "./host/config.js";
 
+export type { Fiber, Effect } from "./host/fiber.js";
+
 export { TransformRegistry, childCtx, matchesSchema, ctx as transformCtx } from "./transform/registry.js";
 export type { TransformContext, TransformFn, TransformRule } from "./transform/registry.js";
\ No newline at end of file
diff --git a/test/fiber.test.ts b/test/fiber.test.ts
new file mode 100644
index 0000000..a2ca9e6
--- /dev/null
+++ b/test/fiber.test.ts
@@ -0,0 +1,192 @@
+import { describe, it, expect, expectTypeOf } from "vitest";
+import type { Fiber, Effect } from "../src/host/fiber.js";
+
+describe("Fiber<I> interface", () => {
+  it("has all required fields", () => {
+    const fiber: Fiber<string> = {
+      instance: "inst-1",
+      tag: "div",
+      props: { class: "test" },
+      key: "a",
+      children: [],
+      parent: null,
+      effect: null,
+      signalDisposers: [],
+      prevProps: null,
+    };
+    expect(fiber.instance).toBe("inst-1");
+    expect(fiber.tag).toBe("div");
+    expect(fiber.props.class).toBe("test");
+    expect(fiber.key).toBe("a");
+    expect(fiber.children).toEqual([]);
+    expect(fiber.parent).toBeNull();
+    expect(fiber.effect).toBeNull();
+    expect(fiber.signalDisposers).toEqual([]);
+    expect(fiber.prevProps).toBeNull();
+  });
+
+  it("supports key as undefined", () => {
+    const fiber: Fiber<string> = {
+      instance: "inst-2",
+      tag: "span",
+      props: {},
+      key: undefined,
+      children: [],
+      parent: null,
+      effect: null,
+      signalDisposers: [],
+      prevProps: null,
+    };
+    expect(fiber.key).toBeUndefined();
+  });
+
+  it("supports parent reference", () => {
+    const parent: Fiber<string> = {
+      instance: "parent-inst",
+      tag: "div",
+      props: {},
+      key: undefined,
+      children: [],
+      parent: null,
+      effect: null,
+      signalDisposers: [],
+      prevProps: null,
+    };
+    const child: Fiber<string> = {
+      instance: "child-inst",
+      tag: "span",
+      props: {},
+      key: "child-1",
+      children: [],
+      parent,
+      effect: null,
+      signalDisposers: [],
+      prevProps: null,
+    };
+    parent.children.push(child);
+    expect(child.parent).toBe(parent);
+    expect(parent.children[0]).toBe(child);
+  });
+
+  it("supports signalDisposers array", () => {
+    const disposed: string[] = [];
+    const fiber: Fiber<string> = {
+      instance: "inst",
+      tag: "div",
+      props: {},
+      key: undefined,
+      children: [],
+      parent: null,
+      effect: null,
+      signalDisposers: [() => disposed.push("a"), () => disposed.push("b")],
+      prevProps: null,
+    };
+    fiber.signalDisposers.forEach((d) => d());
+    expect(disposed).toEqual(["a", "b"]);
+  });
+});
+
+describe("Effect<I> union type", () => {
+  it("update effect", () => {
+    const effect: Effect<string> = { type: "update", payload: { class: "new" } };
+    expect(effect.type).toBe("update");
+    expect((effect as { type: "update"; payload: unknown }).payload).toEqual({ class: "new" });
+  });
+
+  it("insert effect with before=null (append)", () => {
+    const effect: Effect<string> = { type: "insert", before: null };
+    expect(effect.type).toBe("insert");
+  });
+
+  it("insert effect with before=fiber", () => {
+    const before: Fiber<string> = {
+      instance: "existing",
+      tag: "span",
+      props: {},
+      key: undefined,
+      children: [],
+      parent: null,
+      effect: null,
+      signalDisposers: [],
+      prevProps: null,
+    };
+    const effect: Effect<string> = { type: "insert", before };
+    expect(effect.type).toBe("insert");
+    expect(effect.before).toBe(before);
+  });
+
+  it("move effect with before=fiber", () => {
+    const before: Fiber<string> = {
+      instance: "target",
+      tag: "span",
+      props: {},
+      key: undefined,
+      children: [],
+      parent: null,
+      effect: null,
+      signalDisposers: [],
+      prevProps: null,
+    };
+    const effect: Effect<string> = { type: "move", before };
+    expect(effect.type).toBe("move");
+    expect(effect.before).toBe(before);
+  });
+
+  it("move effect with before=null (append at end)", () => {
+    const effect: Effect<string> = { type: "move", before: null };
+    expect(effect.type).toBe("move");
+  });
+
+  it("remove effect", () => {
+    const effect: Effect<string> = { type: "remove" };
+    expect(effect.type).toBe("remove");
+  });
+
+  it("effect field on Fiber accepts all variants", () => {
+    const fiber: Fiber<number> = {
+      instance: 1,
+      tag: "div",
+      props: {},
+      key: undefined,
+      children: [],
+      parent: null,
+      effect: { type: "update", payload: "x" },
+      signalDisposers: [],
+      prevProps: {},
+    };
+    expect(fiber.effect!.type).toBe("update");
+
+    fiber.effect = { type: "remove" };
+    expect(fiber.effect.type).toBe("remove");
+
+    fiber.effect = { type: "insert", before: null };
+    expect(fiber.effect.type).toBe("insert");
+
+    fiber.effect = { type: "move", before: null };
+    expect(fiber.effect.type).toBe("move");
+
+    fiber.effect = null;
+    expect(fiber.effect).toBeNull();
+  });
+});
+
+describe("Fiber re-export from barrel", () => {
+  it("Fiber and Effect types are importable from src/mod.ts", async () => {
+    type _FiberCheck = import("../src/mod.js").Fiber<string>;
+    type _EffectCheck = import("../src/mod.js").Effect<string>;
+    const _fiber: _FiberCheck = {
+      instance: "inst",
+      tag: "div",
+      props: {},
+      key: undefined,
+      children: [],
+      parent: null,
+      effect: null,
+      signalDisposers: [],
+      prevProps: null,
+    };
+    const _effect: _EffectCheck = { type: "remove" };
+    expect(_fiber.instance).toBe("inst");
+    expect(_effect.type).toBe("remove");
+  });
+});
\ No newline at end of file