diff --git a/.changeset/curly-fishes-tease.md b/.changeset/curly-fishes-tease.md
new file mode 100644
index 00000000000..492060ae48d
--- /dev/null
+++ b/.changeset/curly-fishes-tease.md
@@ -0,0 +1,5 @@
+---
+"@smithy/core": patch
+---
+
+fix NumericValue typecheck
diff --git a/packages/core/src/submodules/cbor/byte-printer.ts b/packages/core/src/submodules/cbor/byte-printer.ts
index e1c73c24c6c..6c4f7be2dcb 100644
--- a/packages/core/src/submodules/cbor/byte-printer.ts
+++ b/packages/core/src/submodules/cbor/byte-printer.ts
@@ -4,5 +4,42 @@
  * @deprecated for testing only, do not use in runtime.
  */
 export function printBytes(bytes: Uint8Array) {
-  return [...bytes].map((n) => ("0".repeat(8) + n.toString(2)).slice(-8) + ` (${n})`);
+  return [...bytes].map((n) => {
+    const pad = (num: number) => ("0".repeat(8) + num.toString(2)).slice(-8);
+    const b = pad(n);
+    const [maj, min] = [b.slice(0, 3), b.slice(3)];
+
+    let dmaj: string = "";
+
+    switch (maj) {
+      case "000":
+        dmaj = "0 - Uint64";
+        break;
+      case "001":
+        dmaj = "1 - Neg Uint64";
+        break;
+      case "010":
+        dmaj = "2 - unstructured bytestring";
+        break;
+      case "011":
+        dmaj = "3 - utf8 string";
+        break;
+      case "100":
+        dmaj = "4 - list";
+        break;
+      case "101":
+        dmaj = "5 - map";
+        break;
+      case "110":
+        dmaj = "6 - tag";
+        break;
+      case "111":
+        dmaj = "7 - special";
+        break;
+      default:
+        dmaj = String(parseInt(maj, 2));
+    }
+
+    return `${maj}_${min} (${dmaj}, ${parseInt(min, 2)})`;
+  });
 }
diff --git a/packages/core/src/submodules/cbor/cbor.spec.ts b/packages/core/src/submodules/cbor/cbor.spec.ts
index 89c74a2162c..8bf59268cb8 100644
--- a/packages/core/src/submodules/cbor/cbor.spec.ts
+++ b/packages/core/src/submodules/cbor/cbor.spec.ts
@@ -5,6 +5,7 @@ import JSONbig from "json-bigint";
 import * as path from "path";
 import { describe, expect, test as it } from "vitest";
 
+import { printBytes } from "./byte-printer";
 import { cbor } from "./cbor";
 import { bytesToFloat16 } from "./cbor-decode";
 import { tagSymbol } from "./cbor-types";
@@ -286,7 +287,7 @@ describe("cbor", () => {
       expect(deserialized).toEqual(bigInt);
     });
 
-    it.skip("should round-trip NumericValue to major 6 with tag 4", () => {
+    it("should round-trip NumericValue to major 6 with tag 4", () => {
       for (const bigDecimal of [
         "10000000000000000000000054.321",
         "1000000000000000000000000000000000054.134134321",
@@ -310,6 +311,15 @@ describe("cbor", () => {
         expect(deserialized).toEqual(nv);
         expect(deserialized.string).toEqual(nv.string);
       }
+
+      const bigDecimal = nv("0");
+      expect(bigDecimal).toBeInstanceOf(NumericValue);
+      expect(printBytes(cbor.serialize(bigDecimal))).toEqual([
+        "110_00100 (6 - tag, 4)",
+        "100_00010 (4 - list, 2)",
+        "000_00000 (0 - Uint64, 0)",
+        "000_00000 (0 - Uint64, 0)",
+      ]);
     });
 
     it("should round-trip sequences of big numbers", () => {
diff --git a/packages/core/src/submodules/serde/value/NumericValue.spec.ts b/packages/core/src/submodules/serde/value/NumericValue.spec.ts
index 26cbacc4dbd..f790632ba7f 100644
--- a/packages/core/src/submodules/serde/value/NumericValue.spec.ts
+++ b/packages/core/src/submodules/serde/value/NumericValue.spec.ts
@@ -16,4 +16,59 @@ describe(NumericValue.name, () => {
     expect(() => nv("-10.1")).not.toThrow();
     expect(() => nv("-.101")).not.toThrow();
   });
+
+  it("has a custom instanceof check", () => {
+    const isInstance = [
+      nv("0"),
+      nv("-0.00"),
+      new NumericValue("0", "bigDecimal"),
+      new NumericValue("-0.00", "bigDecimal"),
+      {
+        string: "abcd",
+        type: "bigDecimal",
+        constructor: {
+          name: "_NumericValue",
+        },
+      },
+      (() => {
+        const x = {};
+        Object.setPrototypeOf(x, NumericValue.prototype);
+        return x;
+      })(),
+      (() => {
+        function F() {}
+        F.prototype = Object.create(NumericValue.prototype);
+        // @ts-ignore
+        return new F();
+      })(),
+      (() => {
+        return new (class extends NumericValue {})("0", "bigDecimal");
+      })(),
+    ] as unknown[];
+
+    const isNotInstance = [
+      BigInt(0),
+      "-0.00",
+      {
+        string: "abcd",
+        type: "bigDecimal",
+        constructor: {
+          name: "_NumericValue_",
+        },
+      },
+      (() => {
+        const x = {};
+        Object.setPrototypeOf(x, NumericValue);
+        return x;
+      })(),
+    ] as unknown[];
+
+    for (const instance of isInstance) {
+      expect(instance).toBeInstanceOf(NumericValue);
+    }
+
+    for (const instance of isNotInstance) {
+      expect(instance).not.toBeInstanceOf(NumericValue);
+    }
+  });
 });
diff --git a/packages/core/src/submodules/serde/value/NumericValue.ts b/packages/core/src/submodules/serde/value/NumericValue.ts
index ad3af6e2de5..d3b081ca42a 100644
--- a/packages/core/src/submodules/serde/value/NumericValue.ts
+++ b/packages/core/src/submodules/serde/value/NumericValue.ts
@@ -58,11 +58,15 @@ export class NumericValue {
       return false;
     }
     const _nv = object as NumericValue;
-    const prototypeMatch = NumericValue.prototype.isPrototypeOf(object.constructor?.prototype);
+    const prototypeMatch = NumericValue.prototype.isPrototypeOf(object);
     if (prototypeMatch) {
       return prototypeMatch;
     }
-    if (typeof _nv.string === "string" && typeof _nv.type === "string" && _nv.constructor?.name === "NumericValue") {
+    if (
+      typeof _nv.string === "string" &&
+      typeof _nv.type === "string" &&
+      _nv.constructor?.name?.endsWith("NumericValue")
+    ) {
       return true;
     }
     return prototypeMatch;