diff --git a/pkg/sdkserver/localsdk.go b/pkg/sdkserver/localsdk.go
index 0289f8de40..26bdd26bbc 100644
--- a/pkg/sdkserver/localsdk.go
+++ b/pkg/sdkserver/localsdk.go
@@ -588,7 +588,7 @@ func (l *LocalSDKServer) GetPlayerCapacity(_ context.Context, _ *alpha.Empty) (*
 	return result, nil
 }
 
-// GetCounter returns a Counter. Returns NOT_FOUND if the counter does not exist.
+// GetCounter returns a Counter. Returns not found if the counter does not exist.
 // [Stage:Alpha]
 // [FeatureFlag:CountsAndLists]
 func (l *LocalSDKServer) GetCounter(ctx context.Context, in *alpha.GetCounterRequest) (*alpha.Counter, error) {
@@ -596,6 +596,10 @@ func (l *LocalSDKServer) GetCounter(ctx context.Context, in *alpha.GetCounterReq
 		return nil, errors.Errorf("%s not enabled", runtime.FeatureCountsAndLists)
 	}
 
+	if in == nil {
+		return nil, errors.Errorf("invalid argument. GetCounterRequest cannot be nil")
+	}
+
 	l.logger.WithField("name", in.Name).Info("Getting Counter")
 	l.recordRequest("getcounter")
 	l.gsMutex.RLock()
@@ -604,18 +608,69 @@ func (l *LocalSDKServer) GetCounter(ctx context.Context, in *alpha.GetCounterReq
 	if counter, ok := l.gs.Status.Counters[in.Name]; ok {
 		return &alpha.Counter{Name: in.Name, Count: counter.Count, Capacity: counter.Capacity}, nil
 	}
-	return nil, errors.Errorf("NOT_FOUND. %s Counter not found", in.Name)
+	return nil, errors.Errorf("not found. %s Counter not found", in.Name)
 }
 
-// UpdateCounter returns the updated Counter.
+// UpdateCounter updates the given Counter. Unlike the SDKServer, this LocalSDKServer UpdateCounter
+// does not batch requests, and directly updates the localsdk gameserver.
+// Returns error if the Counter does not exist (name cannot be updated).
+// Returns error if the Count is out of range [0,Capacity].
 // [Stage:Alpha]
 // [FeatureFlag:CountsAndLists]
 func (l *LocalSDKServer) UpdateCounter(ctx context.Context, in *alpha.UpdateCounterRequest) (*alpha.Counter, error) {
-	// TODO(#2716): Implement me
-	return nil, errors.Errorf("Unimplemented -- UpdateCounter coming soon")
+	if !runtime.FeatureEnabled(runtime.FeatureCountsAndLists) {
+		return nil, errors.Errorf("%s not enabled", runtime.FeatureCountsAndLists)
+	}
+
+	if in.CounterUpdateRequest == nil {
+		return nil, errors.Errorf("invalid argument. CounterUpdateRequest cannot be nil")
+	}
+
+	name := in.CounterUpdateRequest.Name
+
+	l.logger.WithField("name", name).Info("Updating Counter")
+	l.recordRequest("updateCounter")
+	l.gsMutex.Lock()
+	defer l.gsMutex.Unlock()
+
+	counter, ok := l.gs.Status.Counters[name]
+	if !ok {
+		return nil, errors.Errorf("not found. %s Counter not found", name)
+	}
+
+	tmpCounter := alpha.Counter{Name: name, Count: counter.Count, Capacity: counter.Capacity}
+	// Set Capacity
+	if in.CounterUpdateRequest.Capacity != nil {
+		if in.CounterUpdateRequest.Capacity.GetValue() < 0 {
+			return nil, errors.Errorf("out of range. Capacity must be greater than or equal to 0. Found Capacity: %d",
+				in.CounterUpdateRequest.Capacity.GetValue())
+		}
+		tmpCounter.Capacity = in.CounterUpdateRequest.Capacity.GetValue()
+	}
+	// Set Count
+	if in.CounterUpdateRequest.Count != nil {
+		if in.CounterUpdateRequest.Count.GetValue() < 0 || in.CounterUpdateRequest.Count.GetValue() > tmpCounter.Capacity {
+			return nil, errors.Errorf("out of range. Count must be within range [0,Capacity]. Found Count: %d, Capacity: %d",
+				in.CounterUpdateRequest.Count.GetValue(), tmpCounter.Capacity)
+		}
+	}
+	// Increment or Decrement Count
+	if in.CounterUpdateRequest.CountDiff != 0 {
+		tmpCounter.Count += in.CounterUpdateRequest.CountDiff
+		if tmpCounter.Count < 0 || tmpCounter.Count > tmpCounter.Capacity {
+			return nil, errors.Errorf("out of range. Count must be within range [0,Capacity]. Found Count: %d, Capacity: %d",
+				tmpCounter.Count, tmpCounter.Capacity)
+		}
+	}
+
+	// Write newly updated List to gameserverstatus.
+	counter.Capacity = tmpCounter.Capacity
+	counter.Count = tmpCounter.Count
+	l.gs.Status.Counters[name] = counter
+	return &tmpCounter, nil
 }
 
-// GetList returns a List. Returns NOT_FOUND if the List does not exist.
+// GetList returns a List. Returns not found if the List does not exist.
 // [Stage:Alpha]
 // [FeatureFlag:CountsAndLists]
 func (l *LocalSDKServer) GetList(ctx context.Context, in *alpha.GetListRequest) (*alpha.List, error) {
@@ -631,13 +686,13 @@ func (l *LocalSDKServer) GetList(ctx context.Context, in *alpha.GetListRequest)
 	if list, ok := l.gs.Status.Lists[in.Name]; ok {
 		return &alpha.List{Name: in.Name, Capacity: list.Capacity, Values: list.Values}, nil
 	}
-	return nil, errors.Errorf("NOT_FOUND. %s List not found", in.Name)
+	return nil, errors.Errorf("not found. %s List not found", in.Name)
 }
 
-// UpdateList returns the updated List. Returns NOT_FOUND if the List does not exist (name cannot be updated).
+// UpdateList returns the updated List. Returns not found if the List does not exist (name cannot be updated).
 // **THIS WILL OVERWRITE ALL EXISTING LIST.VALUES WITH ANY REQUEST LIST.VALUES**
 // Use AddListValue() or RemoveListValue() for modifying the List.Values field.
-// Returns INVALID_ARGUMENT if the field mask path(s) are not field(s) of the List.
+// Returns invalid argument if the field mask path(s) are not field(s) of the List.
 // If a field mask path(s) is specified, but the value is not set in the request List object,
 // then the default value for the variable will be set (i.e. 0 for "capacity", empty list for "values").
 // [Stage:Alpha]
@@ -648,7 +703,7 @@ func (l *LocalSDKServer) UpdateList(ctx context.Context, in *alpha.UpdateListReq
 	}
 
 	if in.List == nil || in.UpdateMask == nil {
-		return nil, errors.Errorf("INVALID_ARGUMENT. List: %v and UpdateMask %v cannot be nil", in.List, in.UpdateMask)
+		return nil, errors.Errorf("invalid argument. List: %v and UpdateMask %v cannot be nil", in.List, in.UpdateMask)
 	}
 
 	l.logger.WithField("name", in.List.Name).Info("Updating List")
@@ -657,13 +712,13 @@ func (l *LocalSDKServer) UpdateList(ctx context.Context, in *alpha.UpdateListReq
 	defer l.gsMutex.Unlock()
 
 	// TODO: https://google.aip.dev/134, "Update masks must support a special value *, meaning full replacement."
-	// Check if the UpdateMask paths are valid, return INVALID_ARGUMENT if not.
+	// Check if the UpdateMask paths are valid, return invalid argument if not.
 	if !in.UpdateMask.IsValid(in.List.ProtoReflect().Interface()) {
-		return nil, errors.Errorf("INVALID_ARGUMENT. Field Mask Path(s): %v are invalid for List. Use valid field name(s): %v", in.UpdateMask.GetPaths(), in.List.ProtoReflect().Descriptor().Fields())
+		return nil, errors.Errorf("invalid argument. Field Mask Path(s): %v are invalid for List. Use valid field name(s): %v", in.UpdateMask.GetPaths(), in.List.ProtoReflect().Descriptor().Fields())
 	}
 
 	if in.List.Capacity < 0 || in.List.Capacity > apiserver.ListMaxCapacity {
-		return nil, errors.Errorf("OUT_OF_RANGE. Capacity must be within range [0,1000]. Found Capacity: %d", in.List.Capacity)
+		return nil, errors.Errorf("out of range. Capacity must be within range [0,1000]. Found Capacity: %d", in.List.Capacity)
 	}
 
 	name := in.List.Name
@@ -678,20 +733,20 @@ func (l *LocalSDKServer) UpdateList(ctx context.Context, in *alpha.UpdateListReq
 		proto.Merge(tmpList, in.List)
 		// Verify that Capacity >= len(tmpList.values)
 		if tmpList.Capacity < int64(len(tmpList.Values)) {
-			return nil, errors.Errorf("OUT_OF_RANGE. Capacity must be great than or equal to the size of the List of values. Found Capacity: %d, List Size: %d", tmpList.Capacity, len(tmpList.Values))
+			return nil, errors.Errorf("out of range. Capacity must be great than or equal to the size of the List of values. Found Capacity: %d, List Size: %d", tmpList.Capacity, len(tmpList.Values))
 		}
 		// Write newly updated List to gameserverstatus.
 		l.gs.Status.Lists[name].Capacity = tmpList.Capacity
 		l.gs.Status.Lists[name].Values = tmpList.Values
 		return &alpha.List{Name: name, Capacity: l.gs.Status.Lists[name].Capacity, Values: l.gs.Status.Lists[name].Values}, nil
 	}
-	return nil, errors.Errorf("NOT_FOUND. %s List not found", name)
+	return nil, errors.Errorf("not found. %s List not found", name)
 }
 
 // AddListValue appends a value to the end of a List and returns updated List.
-// Returns NOT_FOUND if the List does not exist.
-// Returns ALREADY_EXISTS if the value is already in the List.
-// Returns OUT_OF_RANGE if the List is already at Capacity.
+// Returns not found if the List does not exist.
+// Returns already exists if the value is already in the List.
+// Returns out of range if the List is already at Capacity.
 // [Stage:Alpha]
 // [FeatureFlag:CountsAndLists]
 func (l *LocalSDKServer) AddListValue(ctx context.Context, in *alpha.AddListValueRequest) (*alpha.List, error) {
@@ -707,24 +762,24 @@ func (l *LocalSDKServer) AddListValue(ctx context.Context, in *alpha.AddListValu
 	if list, ok := l.gs.Status.Lists[in.Name]; ok {
 		// Verify room to add another value
 		if list.Capacity <= int64(len(list.Values)) {
-			return nil, errors.Errorf("OUT_OF_RANGE. No available capacity. Current Capacity: %d, List Size: %d", list.Capacity, len(list.Values))
+			return nil, errors.Errorf("out of range. No available capacity. Current Capacity: %d, List Size: %d", list.Capacity, len(list.Values))
 		}
 		// Verify value does not already exist in the list
 		for _, val := range l.gs.Status.Lists[in.Name].Values {
 			if in.Value == val {
-				return nil, errors.Errorf("ALREADY_EXISTS. Value: %s already in List: %s", in.Value, in.Name)
+				return nil, errors.Errorf("already exists. Value: %s already in List: %s", in.Value, in.Name)
 			}
 		}
 		// Add new value to gameserverstatus.
 		l.gs.Status.Lists[in.Name].Values = append(l.gs.Status.Lists[in.Name].Values, in.Value)
 		return &alpha.List{Name: in.Name, Capacity: l.gs.Status.Lists[in.Name].Capacity, Values: l.gs.Status.Lists[in.Name].Values}, nil
 	}
-	return nil, errors.Errorf("NOT_FOUND. %s List not found", in.Name)
+	return nil, errors.Errorf("not found. %s List not found", in.Name)
 }
 
 // RemoveListValue removes a value from a List and returns updated List.
-// Returns NOT_FOUND if the List does not exist.
-// Returns NOT_FOUND if the value is not in the List.
+// Returns not found if the List does not exist.
+// Returns not found if the value is not in the List.
 // [Stage:Alpha]
 // [FeatureFlag:CountsAndLists]
 func (l *LocalSDKServer) RemoveListValue(ctx context.Context, in *alpha.RemoveListValueRequest) (*alpha.List, error) {
@@ -746,9 +801,9 @@ func (l *LocalSDKServer) RemoveListValue(ctx context.Context, in *alpha.RemoveLi
 				return &alpha.List{Name: in.Name, Capacity: l.gs.Status.Lists[in.Name].Capacity, Values: l.gs.Status.Lists[in.Name].Values}, nil
 			}
 		}
-		return nil, errors.Errorf("NOT_FOUND. Value: %s not found in List: %s", in.Value, in.Name)
+		return nil, errors.Errorf("not found. Value: %s not found in List: %s", in.Value, in.Name)
 	}
-	return nil, errors.Errorf("NOT_FOUND. %s List not found", in.Name)
+	return nil, errors.Errorf("not found. %s List not found", in.Name)
 }
 
 // Close tears down all the things
diff --git a/pkg/sdkserver/localsdk_test.go b/pkg/sdkserver/localsdk_test.go
index 1575c4cb99..3b1f562203 100644
--- a/pkg/sdkserver/localsdk_test.go
+++ b/pkg/sdkserver/localsdk_test.go
@@ -33,6 +33,7 @@ import (
 	"github.com/stretchr/testify/assert"
 	"google.golang.org/protobuf/testing/protocmp"
 	"google.golang.org/protobuf/types/known/fieldmaskpb"
+	"google.golang.org/protobuf/types/known/wrapperspb"
 	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
 	"k8s.io/apimachinery/pkg/util/wait"
 )
@@ -662,7 +663,7 @@ func TestLocalSDKServerGetCounter(t *testing.T) {
 		},
 		"Counter does not exist": {
 			name:    "noName",
-			wantErr: errors.Errorf("NOT_FOUND. %s Counter not found", "noName"),
+			wantErr: errors.Errorf("not found. %s Counter not found", "noName"),
 		},
 	}
 
@@ -683,6 +684,158 @@ func TestLocalSDKServerGetCounter(t *testing.T) {
 	}
 }
 
+func TestLocalSDKServerUpdateCounter(t *testing.T) {
+	t.Parallel()
+
+	runtime.FeatureTestMutex.Lock()
+	defer runtime.FeatureTestMutex.Unlock()
+	assert.NoError(t, runtime.ParseFeatures(string(runtime.FeatureCountsAndLists)+"=true"))
+
+	counters := map[string]agonesv1.CounterStatus{
+		"sessions": {Count: 1, Capacity: 100},
+		"players":  {Count: 100, Capacity: 100},
+		"lobbies":  {Count: 0, Capacity: 0},
+		"games":    {Count: 5, Capacity: 10},
+		"npcs":     {Count: 0, Capacity: 10},
+	}
+	fixture := &agonesv1.GameServer{
+		ObjectMeta: metav1.ObjectMeta{Name: "stuff"},
+		Status: agonesv1.GameServerStatus{
+			Counters: counters,
+		},
+	}
+
+	path, err := gsToTmpFile(fixture)
+	assert.NoError(t, err)
+	l, err := NewLocalSDKServer(path, "")
+	assert.Nil(t, err)
+
+	stream := newGameServerMockStream()
+	go func() {
+		err := l.WatchGameServer(&sdk.Empty{}, stream)
+		assert.Nil(t, err)
+	}()
+	assertInitialWatchUpdate(t, stream)
+
+	// wait for watching to begin
+	err = wait.PollUntilContextTimeout(context.Background(), time.Second, 10*time.Second, true, func(ctx context.Context) (bool, error) {
+		found := false
+		l.updateObservers.Range(func(_, _ interface{}) bool {
+			found = true
+			return false
+		})
+		return found, nil
+	})
+	assert.NoError(t, err)
+
+	testScenarios := map[string]struct {
+		updateRequest *alpha.UpdateCounterRequest
+		want          *alpha.Counter
+		wantErr       error
+	}{
+		"Set Counter Capacity": {
+			updateRequest: &alpha.UpdateCounterRequest{
+				CounterUpdateRequest: &alpha.CounterUpdateRequest{
+					Name:     "lobbies",
+					Capacity: wrapperspb.Int64(10),
+				}},
+			want: &alpha.Counter{
+				Name: "lobbies", Count: 0, Capacity: 10,
+			},
+		},
+		"Set Counter Count": {
+			updateRequest: &alpha.UpdateCounterRequest{
+				CounterUpdateRequest: &alpha.CounterUpdateRequest{
+					Name:      "npcs",
+					CountDiff: 10,
+				}},
+			want: &alpha.Counter{
+				Name: "npcs", Count: 10, Capacity: 10,
+			},
+		},
+		"Decrement Counter Count": {
+			updateRequest: &alpha.UpdateCounterRequest{
+				CounterUpdateRequest: &alpha.CounterUpdateRequest{
+					Name:      "games",
+					CountDiff: -5,
+				}},
+			want: &alpha.Counter{
+				Name: "games", Count: 0, Capacity: 10,
+			},
+		},
+		"Cannot Decrement Counter": {
+			updateRequest: &alpha.UpdateCounterRequest{
+				CounterUpdateRequest: &alpha.CounterUpdateRequest{
+					Name:      "sessions",
+					CountDiff: -2,
+				}},
+			wantErr: errors.Errorf("out of range. Count must be within range [0,Capacity]. Found Count: %d, Capacity: %d", -1, 100),
+		},
+		"Cannot Increment Counter": {
+			updateRequest: &alpha.UpdateCounterRequest{
+				CounterUpdateRequest: &alpha.CounterUpdateRequest{
+					Name:      "players",
+					CountDiff: 1,
+				}},
+			wantErr: errors.Errorf("out of range. Count must be within range [0,Capacity]. Found Count: %d, Capacity: %d", 101, 100),
+		},
+		"Counter does not exist": {
+			updateRequest: &alpha.UpdateCounterRequest{
+				CounterUpdateRequest: &alpha.CounterUpdateRequest{
+					Name:      "dragons",
+					CountDiff: 1,
+				}},
+			wantErr: errors.Errorf("not found. %s Counter not found", "dragons"),
+		},
+		"request Counter is nil": {
+			updateRequest: &alpha.UpdateCounterRequest{
+				CounterUpdateRequest: nil,
+			},
+			wantErr: errors.Errorf("invalid argument. CounterUpdateRequest cannot be nil"),
+		},
+		"capacity is less than zero": {
+			updateRequest: &alpha.UpdateCounterRequest{
+				CounterUpdateRequest: &alpha.CounterUpdateRequest{
+					Name:     "lobbies",
+					Capacity: wrapperspb.Int64(-1),
+				}},
+			wantErr: errors.Errorf("out of range. Capacity must be greater than or equal to 0. Found Capacity: %d", -1),
+		},
+		"count is less than zero": {
+			updateRequest: &alpha.UpdateCounterRequest{
+				CounterUpdateRequest: &alpha.CounterUpdateRequest{
+					Name:  "players",
+					Count: wrapperspb.Int64(-1),
+				}},
+			wantErr: errors.Errorf("out of range. Count must be within range [0,Capacity]. Found Count: %d, Capacity: %d", -1, 100),
+		},
+		"count is greater than capacity": {
+			updateRequest: &alpha.UpdateCounterRequest{
+				CounterUpdateRequest: &alpha.CounterUpdateRequest{
+					Name:  "players",
+					Count: wrapperspb.Int64(101),
+				}},
+			wantErr: errors.Errorf("out of range. Count must be within range [0,Capacity]. Found Count: %d, Capacity: %d", 101, 100),
+		},
+	}
+
+	for testName, testScenario := range testScenarios {
+		t.Run(testName, func(t *testing.T) {
+			got, err := l.UpdateCounter(context.Background(), testScenario.updateRequest)
+			// Check tests expecting non-errors
+			if testScenario.want != nil {
+				assert.NoError(t, err)
+				if diff := cmp.Diff(testScenario.want, got, protocmp.Transform()); diff != "" {
+					t.Errorf("Unexpected difference:\n%v", diff)
+				}
+			} else {
+				// Check tests expecting errors
+				assert.ErrorContains(t, err, testScenario.wantErr.Error())
+			}
+		})
+	}
+}
+
 func TestLocalSDKServerGetList(t *testing.T) {
 	t.Parallel()
 
@@ -734,7 +887,7 @@ func TestLocalSDKServerGetList(t *testing.T) {
 		},
 		"List does not exist": {
 			name:    "noName",
-			wantErr: errors.Errorf("NOT_FOUND. %s List not found", "noName"),
+			wantErr: errors.Errorf("not found. %s List not found", "noName"),
 		},
 	}
 
@@ -867,21 +1020,21 @@ func TestLocalSDKServerUpdateList(t *testing.T) {
 				},
 				UpdateMask: &fieldmaskpb.FieldMask{Paths: []string{"capacity"}},
 			},
-			wantErr: errors.Errorf("NOT_FOUND. %s List not found", "dragons"),
+			wantErr: errors.Errorf("not found. %s List not found", "dragons"),
 		},
 		"request List is nil": {
 			updateRequest: &alpha.UpdateListRequest{
 				List:       nil,
 				UpdateMask: &fieldmaskpb.FieldMask{},
 			},
-			wantErr: errors.Errorf("INVALID_ARGUMENT. List: %v and UpdateMask %v cannot be nil", nil, &fieldmaskpb.FieldMask{}),
+			wantErr: errors.Errorf("invalid argument. List: %v and UpdateMask %v cannot be nil", nil, &fieldmaskpb.FieldMask{}),
 		},
 		"request UpdateMask is nil": {
 			updateRequest: &alpha.UpdateListRequest{
 				List:       &alpha.List{},
 				UpdateMask: nil,
 			},
-			wantErr: errors.Errorf("INVALID_ARGUMENT. List: %v and UpdateMask %v cannot be nil", &alpha.Counter{}, nil),
+			wantErr: errors.Errorf("invalid argument. List: %v and UpdateMask %v cannot be nil", &alpha.List{}, nil),
 		},
 		"updateMask contains invalid path": {
 			updateRequest: &alpha.UpdateListRequest{
@@ -890,7 +1043,7 @@ func TestLocalSDKServerUpdateList(t *testing.T) {
 				},
 				UpdateMask: &fieldmaskpb.FieldMask{Paths: []string{"foo"}},
 			},
-			wantErr: errors.Errorf("INVALID_ARGUMENT. Field Mask Path(s): [foo] are invalid for List. Use valid field name(s): "),
+			wantErr: errors.Errorf("invalid argument. Field Mask Path(s): [foo] are invalid for List. Use valid field name(s): "),
 		},
 		"updateMask is empty": {
 			updateRequest: &alpha.UpdateListRequest{
@@ -899,7 +1052,7 @@ func TestLocalSDKServerUpdateList(t *testing.T) {
 				},
 				UpdateMask: &fieldmaskpb.FieldMask{Paths: []string{""}},
 			},
-			wantErr: errors.Errorf("INVALID_ARGUMENT. Field Mask Path(s): [] are invalid for List. Use valid field name(s): "),
+			wantErr: errors.Errorf("invalid argument. Field Mask Path(s): [] are invalid for List. Use valid field name(s): "),
 		},
 		"capacity is less than zero": {
 			updateRequest: &alpha.UpdateListRequest{
@@ -909,7 +1062,7 @@ func TestLocalSDKServerUpdateList(t *testing.T) {
 				},
 				UpdateMask: &fieldmaskpb.FieldMask{Paths: []string{"capacity"}},
 			},
-			wantErr: errors.Errorf("OUT_OF_RANGE. Capacity must be within range [0,1000]. Found Capacity: %d", int64(-1)),
+			wantErr: errors.Errorf("out of range. Capacity must be within range [0,1000]. Found Capacity: %d", -1),
 		},
 		"capacity greater than max capacity (1000)": {
 			updateRequest: &alpha.UpdateListRequest{
@@ -919,7 +1072,7 @@ func TestLocalSDKServerUpdateList(t *testing.T) {
 				},
 				UpdateMask: &fieldmaskpb.FieldMask{Paths: []string{"capacity"}},
 			},
-			wantErr: errors.Errorf("OUT_OF_RANGE. Capacity must be within range [0,1000]. Found Capacity: %d", int64(1001)),
+			wantErr: errors.Errorf("out of range. Capacity must be within range [0,1000]. Found Capacity: %d", 1001),
 		},
 		"capacity is less than List length": {
 			updateRequest: &alpha.UpdateListRequest{
@@ -929,7 +1082,7 @@ func TestLocalSDKServerUpdateList(t *testing.T) {
 				},
 				UpdateMask: &fieldmaskpb.FieldMask{Paths: []string{"capacity"}},
 			},
-			wantErr: errors.Errorf("OUT_OF_RANGE. Capacity must be great than or equal to the size of the List of values. Found Capacity: %d, List Size: %d", int64(1), int64(2)),
+			wantErr: errors.Errorf("out of range. Capacity must be great than or equal to the size of the List of values. Found Capacity: %d, List Size: %d", 1, 2),
 		},
 	}
 
@@ -1007,21 +1160,21 @@ func TestLocalSDKServerAddListValue(t *testing.T) {
 			addRequest: &alpha.AddListValueRequest{
 				Name: "dragons",
 			},
-			wantErr: errors.Errorf("NOT_FOUND. %s List not found", "dragons"),
+			wantErr: errors.Errorf("not found. %s List not found", "dragons"),
 		},
 		"add more values than capacity": {
 			addRequest: &alpha.AddListValueRequest{
 				Name:  "hacks",
 				Value: "hack3",
 			},
-			wantErr: errors.Errorf("OUT_OF_RANGE. No available capacity. Current Capacity: %d, List Size: %d", int64(2), int64(2)),
+			wantErr: errors.Errorf("out of range. No available capacity. Current Capacity: %d, List Size: %d", int64(2), int64(2)),
 		},
 		"add existing value": {
 			addRequest: &alpha.AddListValueRequest{
 				Name:  "lemmings",
 				Value: "lemming1",
 			},
-			wantErr: errors.Errorf("ALREADY_EXISTS. Value: %s already in List: %s", "lemming1", "lemmings"),
+			wantErr: errors.Errorf("already exists. Value: %s already in List: %s", "lemming1", "lemmings"),
 		},
 	}
 
@@ -1099,14 +1252,14 @@ func TestLocalSDKServerRemoveListValue(t *testing.T) {
 			removeRequest: &alpha.RemoveListValueRequest{
 				Name: "dragons",
 			},
-			wantErr: errors.Errorf("NOT_FOUND. %s List not found", "dragons"),
+			wantErr: errors.Errorf("not found. %s List not found", "dragons"),
 		},
 		"value does not exist": {
 			removeRequest: &alpha.RemoveListValueRequest{
 				Name:  "items",
 				Value: "item3",
 			},
-			wantErr: errors.Errorf("NOT_FOUND. Value: %s not found in List: %s", "item3", "items"),
+			wantErr: errors.Errorf("not found. Value: %s not found in List: %s", "item3", "items"),
 		},
 	}
 
diff --git a/pkg/sdkserver/sdkserver.go b/pkg/sdkserver/sdkserver.go
index 42b5e59e5c..74b5e3d94d 100644
--- a/pkg/sdkserver/sdkserver.go
+++ b/pkg/sdkserver/sdkserver.go
@@ -849,6 +849,7 @@ func (s *SDKServer) GetCounter(ctx context.Context, in *alpha.GetCounterRequest)
 }
 
 // UpdateCounter collapses all UpdateCounterRequests for a given Counter into a single request.
+// UpdateCounterRequest must be one and only one of Capacity, Count, or CountDiff.
 // Returns error if the Counter does not exist (name cannot be updated).
 // Returns error if the Count is out of range [0,Capacity].
 // [Stage:Alpha]