diff --git a/packages/core-shared/src/instrumentation/with-span.test.ts b/packages/core-shared/src/instrumentation/with-span.test.ts
index 91ccac4..1667cbd 100644
--- a/packages/core-shared/src/instrumentation/with-span.test.ts
+++ b/packages/core-shared/src/instrumentation/with-span.test.ts
@@ -2,6 +2,7 @@ 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";
+import { isInstrumented } from "@/conformance/brand-runtime";
 
 function makeRecordingTracer() {
   const calls: SpanOpts[] = [];
@@ -70,3 +71,13 @@ describe("withSpan — brand", () => {
     expectTypeOf(wrapped).toMatchTypeOf<Instrumented<typeof fn>>();
   });
 });
+
+describe("withSpan — runtime brand", () => {
+  it("attaches __instrumented as a non-enumerable property on the wrapped function", async () => {
+    const { tracer } = makeRecordingTracer();
+    const fn = async (a: number) => a + 1;
+    const wrapped = withSpan(tracer, { name: "test.brand", op: "use-case" }, fn);
+    expect(isInstrumented(wrapped)).toBe(true);
+    expect(Object.keys(wrapped)).not.toContain("__instrumented");
+  });
+});
diff --git a/packages/core-shared/src/instrumentation/with-span.ts b/packages/core-shared/src/instrumentation/with-span.ts
index 0685b0c..839a4ea 100644
--- a/packages/core-shared/src/instrumentation/with-span.ts
+++ b/packages/core-shared/src/instrumentation/with-span.ts
@@ -1,5 +1,6 @@
 import type { ITracer, SpanOpts } from "./tracer.interface";
 import type { Instrumented } from "../conformance/brands";
+import { attachBrand } from "../conformance/brand-runtime";
 
 export function withSpan<Args extends unknown[], R, Extra extends object>(
   tracer: ITracer,
@@ -20,7 +21,8 @@ export function withSpan<Args extends unknown[], R>(
     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.
+  attachBrand(wrapped, "__instrumented");
+  // Cast is the type-level concession — the brand is now also a non-enumerable
+  // runtime property attached above by `attachBrand`.
   return wrapped as Instrumented<(...args: Args) => Promise<R>>;
 }