diff --git a/CHANGELOG.md b/CHANGELOG.md
index ffa3b25..a8b2cae 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -219,6 +219,18 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   account visible to the login (every sub-account for a main login,
   just the login itself for a sub-account).
 
+### Changed
+
+- `internal/usage`: add a `(t Traffic) IsSummary() bool` helper so
+  callers no longer rely on the `Day == 0` magic number to distinguish
+  the monthly summary row from per-day entries; the table renderer is
+  switched over too. Document on `Space` that `UsedWebspace` is the sum
+  of the four sub-buckets so future readers do not double-count.
+
+- `kasapi-cli usage traffic`: pre-validate `--year` (must be in
+  `[2000, currentYear+1]`) and `--month` (must be `1..12`) instead of
+  forwarding obvious typos to KAS. Closes #45.
+
 ### Fixed
 
 - `internal/auth/source.go`: sharpen the `Heartbeat` doc comment.
diff --git a/internal/cli/usage.go b/internal/cli/usage.go
index d3905d9..d0ab638 100644
--- a/internal/cli/usage.go
+++ b/internal/cli/usage.go
@@ -1,11 +1,20 @@
 package cli
 
 import (
+	"fmt"
+	"time"
+
 	"github.com/spf13/cobra"
 
 	"github.com/chmmou/kasapi-cli/internal/usage"
 )
 
+// minTrafficYear is the lower bound for --year. KAS predates this date
+// but the CLI is the kasapi-cli era, so 2000 is generous enough to be
+// indistinguishable from "no validation" for any realistic input while
+// still rejecting obvious typos like 200 or 20256.
+const minTrafficYear = 2000
+
 // NewUsageCmd returns the "kasapi-cli usage" subcommand tree:
 // space (get_space), space-detail (get_space_usage),
 // traffic (get_traffic).
@@ -76,6 +85,17 @@ func newUsageTrafficCmd(opts *RootOptions) *cobra.Command {
 		Use:   "traffic",
 		Short: "Show monthly HTTP/FTP traffic (get_traffic)",
 		Args:  cobra.NoArgs,
+		PreRunE: func(_ *cobra.Command, _ []string) error {
+			maxYear := time.Now().Year() + 1
+			if year != 0 && (year < minTrafficYear || year > maxYear) {
+				return fmt.Errorf("--year must be between %d and %d, got %d",
+					minTrafficYear, maxYear, year)
+			}
+			if month != 0 && (month < 1 || month > 12) {
+				return fmt.Errorf("--month must be between 1 and 12, got %d", month)
+			}
+			return nil
+		},
 		RunE: func(cmd *cobra.Command, _ []string) error {
 			api, err := BuildAPIClient(opts)
 			if err != nil {
diff --git a/internal/cli/usage_test.go b/internal/cli/usage_test.go
new file mode 100644
index 0000000..1227d85
--- /dev/null
+++ b/internal/cli/usage_test.go
@@ -0,0 +1,43 @@
+package cli_test
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+
+	"github.com/chmmou/kasapi-cli/internal/cli"
+)
+
+func TestUsageTrafficRejectsInvalidFlags(t *testing.T) {
+	t.Parallel()
+	cases := []struct {
+		name string
+		args []string
+		want string
+	}{
+		{"month negative", []string{"usage", "traffic", "--month", "-1"}, "--month must be between 1 and 12"},
+		{"month too high", []string{"usage", "traffic", "--month", "13"}, "--month must be between 1 and 12"},
+		{"year too low", []string{"usage", "traffic", "--year", "1999"}, "--year must be between"},
+		{"year typo", []string{"usage", "traffic", "--year", "20256"}, "--year must be between"},
+	}
+	for _, tc := range cases {
+		tc := tc
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			root, opts := cli.NewRootCmd()
+			root.AddCommand(cli.NewUsageCmd(opts))
+
+			var buf bytes.Buffer
+			root.SetOut(&buf)
+			root.SetErr(&buf)
+			root.SetArgs(tc.args)
+			err := root.Execute()
+			if err == nil {
+				t.Fatalf("Execute returned nil, want error containing %q", tc.want)
+			}
+			if !strings.Contains(err.Error(), tc.want) {
+				t.Errorf("err = %v, want substring %q", err, tc.want)
+			}
+		})
+	}
+}
diff --git a/internal/usage/usage.go b/internal/usage/usage.go
index b2dd841..c44cb75 100644
--- a/internal/usage/usage.go
+++ b/internal/usage/usage.go
@@ -21,6 +21,10 @@ type Caller interface {
 // resource type. KAS reports webspace and max_webspace as xsd:int and
 // the per-resource breakdowns as xsd:string-encoded numbers; we parse
 // both into int64 so the per-account totals add up without overflow.
+//
+// UsedWebspace is the sum of UsedHTDocsSpace, UsedChrootSpace,
+// UsedDatabaseSpace, and UsedMailaccountSpace — do not add the
+// sub-buckets to UsedWebspace when computing totals.
 type Space struct {
 	AccountLogin         string `json:"account_login" yaml:"account_login"`
 	LastCalculation      int64  `json:"last_calculation" yaml:"last_calculation"`
@@ -68,6 +72,11 @@ type Traffic struct {
 	Comment      string `json:"comment,omitempty" yaml:"comment,omitempty"`
 }
 
+// IsSummary reports whether t is the monthly summary row that KAS
+// emits alongside the per-day entries. The summary is identified by a
+// zero Day; literal day-zero never occurs in get_traffic responses.
+func (t Traffic) IsSummary() bool { return t.Day == 0 }
+
 // TrafficList is the typed payload of get_traffic; satisfies
 // cli.Tabular.
 type TrafficList []Traffic
@@ -292,7 +301,7 @@ func (l TrafficList) TableRows() [][]string {
 	rows := make([][]string, 0, len(l))
 	for _, t := range l {
 		day := "*"
-		if t.Day != 0 {
+		if !t.IsSummary() {
 			day = fmt.Sprintf("%02d", t.Day)
 		}
 		rows = append(rows, []string{
diff --git a/internal/usage/usage_test.go b/internal/usage/usage_test.go
index 5530ef7..5f6778a 100644
--- a/internal/usage/usage_test.go
+++ b/internal/usage/usage_test.go
@@ -145,6 +145,12 @@ func TestDecodeTraffic(t *testing.T) {
 	if day.Day != 1 {
 		t.Errorf("day.Day = %d, want 1", day.Day)
 	}
+	if !summary.IsSummary() {
+		t.Errorf("summary.IsSummary() = false, want true")
+	}
+	if day.IsSummary() {
+		t.Errorf("day.IsSummary() = true, want false")
+	}
 }
 
 func TestClientSpace(t *testing.T) {