diff --git a/packages/framework/src/define-handler.test.ts b/packages/framework/src/define-handler.test.ts
new file mode 100644
index 0000000..b065548
--- /dev/null
+++ b/packages/framework/src/define-handler.test.ts
@@ -0,0 +1,109 @@
+import { expect, expectTypeOf, test } from "vitest";
+
+import { defineHandler } from "./define-handler";
+import type { HandlerModule } from "./index";
+
+declare module "./index" {
+	// eslint-disable-next-line @typescript-eslint/consistent-type-definitions
+	interface RouteMap {
+		// eslint-disable-next-line @typescript-eslint/naming-convention
+		"/api/health": Record<string, never>;
+		// eslint-disable-next-line @typescript-eslint/naming-convention
+		"/api/users/[id]": { id: string };
+	}
+}
+
+// --- TDD Slice 1: identity at runtime ---
+
+test("defineHandler returns config unchanged (identity)", () => {
+	const config = {
+		// eslint-disable-next-line @typescript-eslint/naming-convention
+		GET: () => Response.json({ ok: true }),
+	};
+
+	const result = defineHandler("/api/health")(config);
+
+	expect(result).toBe(config);
+});
+
+// --- TDD Slice 2: supports all HTTP methods ---
+
+test("defineHandler accepts all HTTP method handlers", () => {
+	const config = {
+		// eslint-disable-next-line @typescript-eslint/naming-convention
+		GET: () => new Response("get"),
+		// eslint-disable-next-line @typescript-eslint/naming-convention
+		POST: () => new Response("post"),
+		// eslint-disable-next-line @typescript-eslint/naming-convention
+		PUT: () => new Response("put"),
+		// eslint-disable-next-line @typescript-eslint/naming-convention
+		PATCH: () => new Response("patch"),
+		// eslint-disable-next-line @typescript-eslint/naming-convention
+		DELETE: () => new Response("delete"),
+	};
+
+	const result = defineHandler("/api/health")(config);
+
+	expect(result).toBe(config);
+});
+
+// --- TDD Slice 3: all methods are optional ---
+
+test("defineHandler allows omitting all methods", () => {
+	const result = defineHandler("/api/health")({});
+
+	expect(result).toEqual({});
+});
+
+// --- TDD Slice 4: type — params are typed from RouteMap ---
+
+test("handler receives typed params from RouteMap", () => {
+	defineHandler("/api/users/[id]")({
+		// eslint-disable-next-line @typescript-eslint/naming-convention
+		GET: (ctx) => {
+			expectTypeOf(ctx.params).toEqualTypeOf<{ id: string }>();
+			expectTypeOf(ctx.request).toEqualTypeOf<Request>();
+			return Response.json({ id: ctx.params.id });
+		},
+	});
+});
+
+// --- TDD Slice 5: type — return type is HandlerModule ---
+
+test("defineHandler returns a HandlerModule", () => {
+	const result = defineHandler("/api/users/[id]")({
+		// eslint-disable-next-line @typescript-eslint/naming-convention
+		GET: () => Response.json({ ok: true }),
+	});
+
+	expectTypeOf(result).toExtend<HandlerModule<{ id: string }>>();
+});
+
+// --- TDD Slice 6: type — handlers accept async functions ---
+
+test("handlers can return Promise<Response>", () => {
+	const result = defineHandler("/api/health")({
+		// eslint-disable-next-line @typescript-eslint/naming-convention
+		POST: async (ctx) => {
+			const body = (await ctx.request.json()) as Record<string, unknown>;
+			return Response.json({ received: body });
+		},
+	});
+
+	expectTypeOf(result.POST).toExtend<
+		| ((ctx: { request: Request; params: Record<string, never> }) => Response | Promise<Response>)
+		| undefined
+	>();
+});
+
+// --- TDD Slice 7: type — paramless route has empty params ---
+
+test("paramless route has Record<string, never> params", () => {
+	defineHandler("/api/health")({
+		// eslint-disable-next-line @typescript-eslint/naming-convention
+		GET: (ctx) => {
+			expectTypeOf(ctx.params).toEqualTypeOf<Record<string, never>>();
+			return Response.json({ ok: true });
+		},
+	});
+});
diff --git a/packages/framework/src/define-handler.ts b/packages/framework/src/define-handler.ts
new file mode 100644
index 0000000..e91f740
--- /dev/null
+++ b/packages/framework/src/define-handler.ts
@@ -0,0 +1,5 @@
+import type { HandlerModule, RouteMap } from "./core/index";
+
+export function defineHandler<TPath extends keyof RouteMap>(_path: TPath) {
+	return (config: HandlerModule<RouteMap[TPath]>) => config;
+}
diff --git a/packages/framework/src/index.ts b/packages/framework/src/index.ts
index 61ece85..008d033 100644
--- a/packages/framework/src/index.ts
+++ b/packages/framework/src/index.ts
@@ -2,6 +2,7 @@ export const VERSION = "0.0.0";
 
 export { createApp } from "./create-app";
 export type { AppConfig } from "./create-app";
+export { defineHandler } from "./define-handler";
 export { definePage } from "./define-page";
 
 export type {