diff --git a/packages/core-shared/src/instrumentation/with-span.test.ts b/packages/core-shared/src/instrumentation/with-span.test.ts
index bb87a03..91ccac4 100644
--- a/packages/core-shared/src/instrumentation/with-span.test.ts
+++ b/packages/core-shared/src/instrumentation/with-span.test.ts
@@ -1,6 +1,7 @@
-import { describe, it, expect, vi } from "vitest";
+import { describe, it, expect, expectTypeOf, vi } from "vitest";
 import { withSpan } from "@/instrumentation/with-span";
 import type { ITracer, ISpan, SpanOpts } from "@/instrumentation/tracer.interface";
+import type { Instrumented } from "@/conformance/brands";
 
 function makeRecordingTracer() {
   const calls: SpanOpts[] = [];
@@ -60,3 +61,12 @@ describe("withSpan", () => {
     expect(calls.every((c) => c.name === "test.same")).toBe(true);
   });
 });
+
+describe("withSpan — brand", () => {
+  it("returns an Instrumented<F>", () => {
+    const { tracer } = makeRecordingTracer();
+    const fn = async (a: number) => a + 1;
+    const wrapped = withSpan(tracer, { name: "test.brand", op: "use-case" }, fn);
+    expectTypeOf(wrapped).toMatchTypeOf<Instrumented<typeof fn>>();
+  });
+});
diff --git a/packages/core-shared/src/instrumentation/with-span.ts b/packages/core-shared/src/instrumentation/with-span.ts
index 5867660..eeb6b09 100644
--- a/packages/core-shared/src/instrumentation/with-span.ts
+++ b/packages/core-shared/src/instrumentation/with-span.ts
@@ -1,12 +1,16 @@
 import type { ITracer, SpanOpts } from "./tracer.interface";
+import type { Instrumented } from "../conformance/brands";
 
 export function withSpan<Args extends unknown[], R>(
   tracer: ITracer,
   opts: SpanOpts | ((args: Args) => SpanOpts),
   fn: (...args: Args) => Promise<R>,
-): (...args: Args) => Promise<R> {
-  return (...args) => {
+): Instrumented<(...args: Args) => Promise<R>> {
+  const wrapped: (...args: Args) => Promise<R> = (...args) => {
     const resolved = typeof opts === "function" ? opts(args) : opts;
     return tracer.startSpan(resolved, () => fn(...args));
   };
+  // Cast is the only runtime concession — the brand is a phantom type;
+  // there is no real `__instrumented` property at runtime.
+  return wrapped as Instrumented<(...args: Args) => Promise<R>>;
 }