diff --git a/staging/src/k8s.io/apimachinery/pkg/runtime/serializer/cbor/internal/modes/appendixa_test.go b/staging/src/k8s.io/apimachinery/pkg/runtime/serializer/cbor/internal/modes/appendixa_test.go
index dec784b2526..7442e584079 100644
--- a/staging/src/k8s.io/apimachinery/pkg/runtime/serializer/cbor/internal/modes/appendixa_test.go
+++ b/staging/src/k8s.io/apimachinery/pkg/runtime/serializer/cbor/internal/modes/appendixa_test.go
@@ -121,7 +121,6 @@ func TestAppendixA(t *testing.T) {
 		{
 			example: hex("c249010000000000000000"),
 			reject:  "decoding tagged positive bigint value to interface{} can't reproduce this value without losing distinction between float and integer",
-			fixme:   "decoding bigint to interface{} must not produce math/big.Int",
 		},
 		{
 			example: hex("3bffffffffffffffff"),
@@ -130,7 +129,6 @@ func TestAppendixA(t *testing.T) {
 		{
 			example: hex("c349010000000000000000"),
 			reject:  "-18446744073709551617 overflows int64 and falling back to float64 (as with JSON) loses distinction between float and integer",
-			fixme:   "decoding negative bigint to interface{} must not produce math/big.Int",
 		},
 		{
 			example: hex("20"),
diff --git a/staging/src/k8s.io/apimachinery/pkg/runtime/serializer/cbor/internal/modes/decode.go b/staging/src/k8s.io/apimachinery/pkg/runtime/serializer/cbor/internal/modes/decode.go
index df8a33a0745..14c56970569 100644
--- a/staging/src/k8s.io/apimachinery/pkg/runtime/serializer/cbor/internal/modes/decode.go
+++ b/staging/src/k8s.io/apimachinery/pkg/runtime/serializer/cbor/internal/modes/decode.go
@@ -96,6 +96,10 @@ var Decode cbor.DecMode = func() cbor.DecMode {
 		// representation (RFC 8259 Section 6).
 		NaN: cbor.NaNDecodeForbidden,
 		Inf: cbor.InfDecodeForbidden,
+
+		// Reject the arbitrary-precision integer tags because they can't be faithfully
+		// roundtripped through the allowable Unstructured types.
+		BignumTag: cbor.BignumTagForbidden,
 	}.DecMode()
 	if err != nil {
 		panic(err)
diff --git a/staging/src/k8s.io/apimachinery/pkg/runtime/serializer/cbor/internal/modes/decode_test.go b/staging/src/k8s.io/apimachinery/pkg/runtime/serializer/cbor/internal/modes/decode_test.go
index 3472ec436b6..e10468a79d7 100644
--- a/staging/src/k8s.io/apimachinery/pkg/runtime/serializer/cbor/internal/modes/decode_test.go
+++ b/staging/src/k8s.io/apimachinery/pkg/runtime/serializer/cbor/internal/modes/decode_test.go
@@ -739,29 +739,25 @@ func TestDecode(t *testing.T) {
 
 		group(t, "unsigned bignum", []test{
 			{
-				name:  "rejected",
-				in:    hex("c249010000000000000000"), // 2(18446744073709551616)
-				fixme: "decoding cbor data tagged with 2 produces big.Int instead of rejecting",
-				assertOnError: func(t *testing.T, e error) {
-					// TODO: Once this can pass, make the assertion stronger.
-					if e == nil {
-						t.Error("expected non-nil error")
+				name: "rejected",
+				in:   hex("c249010000000000000000"), // 2(18446744073709551616)
+				assertOnError: assertOnConcreteError(func(t *testing.T, e *cbor.UnacceptableDataItemError) {
+					if diff := cmp.Diff(&cbor.UnacceptableDataItemError{CBORType: "tag", Message: "bignum"}, e); diff != "" {
+						t.Errorf("unexpected error diff:\n%s", diff)
 					}
-				},
+				}),
 			},
 		})
 
 		group(t, "negative bignum", []test{
 			{
-				name:  "rejected",
-				in:    hex("c349010000000000000000"), // 3(-18446744073709551617)
-				fixme: "decoding cbor data tagged with 3 produces big.Int instead of rejecting",
-				assertOnError: func(t *testing.T, e error) {
-					// TODO: Once this can pass, make the assertion stronger.
-					if e == nil {
-						t.Error("expected non-nil error")
+				name: "rejected",
+				in:   hex("c349010000000000000000"), // 3(-18446744073709551617)
+				assertOnError: assertOnConcreteError(func(t *testing.T, e *cbor.UnacceptableDataItemError) {
+					if diff := cmp.Diff(&cbor.UnacceptableDataItemError{CBORType: "tag", Message: "bignum"}, e); diff != "" {
+						t.Errorf("unexpected error diff:\n%s", diff)
 					}
-				},
+				}),
 			},
 		})
 
diff --git a/staging/src/k8s.io/apimachinery/pkg/runtime/serializer/cbor/internal/modes/encode.go b/staging/src/k8s.io/apimachinery/pkg/runtime/serializer/cbor/internal/modes/encode.go
index 16a05c5e57d..ac839a53521 100644
--- a/staging/src/k8s.io/apimachinery/pkg/runtime/serializer/cbor/internal/modes/encode.go
+++ b/staging/src/k8s.io/apimachinery/pkg/runtime/serializer/cbor/internal/modes/encode.go
@@ -37,11 +37,10 @@ var Encode cbor.EncMode = func() cbor.EncMode {
 		NaNConvert: cbor.NaNConvertReject,
 		InfConvert: cbor.InfConvertReject,
 
-		// Prefer encoding math/big.Int to one of the 64-bit integer types if it fits. When
-		// later decoded into Unstructured, the set of allowable concrete numeric types is
-		// limited to int64 and float64, so the distinction between big integer and integer
-		// can't be preserved.
-		BigIntConvert: cbor.BigIntConvertShortest,
+		// Error on attempt to encode math/big.Int values, which can't be faithfully
+		// roundtripped through Unstructured in general (the dynamic numeric types allowed
+		// in Unstructured are limited to float64 and int64).
+		BigIntConvert: cbor.BigIntConvertReject,
 
 		// MarshalJSON for time.Time writes RFC3339 with nanos.
 		Time: cbor.TimeRFC3339Nano,
diff --git a/staging/src/k8s.io/apimachinery/pkg/runtime/serializer/cbor/internal/modes/encode_test.go b/staging/src/k8s.io/apimachinery/pkg/runtime/serializer/cbor/internal/modes/encode_test.go
index 1d9b8e47880..e9d8906e502 100644
--- a/staging/src/k8s.io/apimachinery/pkg/runtime/serializer/cbor/internal/modes/encode_test.go
+++ b/staging/src/k8s.io/apimachinery/pkg/runtime/serializer/cbor/internal/modes/encode_test.go
@@ -18,6 +18,8 @@ package modes_test
 
 import (
 	"fmt"
+	"math/big"
+	"reflect"
 	"testing"
 
 	"github.com/fxamacker/cbor/v2"
@@ -53,6 +55,15 @@ func TestEncode(t *testing.T) {
 			want:          []byte{0xa1, 0x41, 0x41, 0x02}, // {"A": 2}
 			assertOnError: assertNilError,
 		},
+		{
+			name: "math/big.Int values are rejected",
+			in:   big.NewInt(1),
+			assertOnError: assertOnConcreteError(func(t *testing.T, got *cbor.UnsupportedTypeError) {
+				if want := (&cbor.UnsupportedTypeError{Type: reflect.TypeFor[big.Int]()}); *want != *got {
+					t.Errorf("unexpected error, got %#v (%q), want %#v (%q)", got, got.Error(), want, want.Error())
+				}
+			}),
+		},
 	} {
 		encModes := tc.modes
 		if len(encModes) == 0 {
@@ -68,7 +79,7 @@ func TestEncode(t *testing.T) {
 			t.Run(fmt.Sprintf("mode=%s/%s", modeName, tc.name), func(t *testing.T) {
 				out, err := encMode.Marshal(tc.in)
 				tc.assertOnError(t, err)
-				if diff := cmp.Diff(tc.want, out); diff != "" {
+				if diff := cmp.Diff(tc.want, out, cmp.Comparer(func(a, b reflect.Type) bool { return a == b })); diff != "" {
 					t.Errorf("unexpected output:\n%s", diff)
 				}
 			})