diff --git a/internal/agent/tools/param_validate.go b/internal/agent/tools/param_validate.go
index 0c97f4864b7..4c1a96ba979 100644
--- a/internal/agent/tools/param_validate.go
+++ b/internal/agent/tools/param_validate.go
@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"strings"
+	"unicode/utf8"
 )
 
 // ValidationError describes a specific parameter validation failure.
@@ -132,14 +133,16 @@ func validateProperty(name string, val any, prop map[string]any) []ValidationErr
 	// String length bounds
 	if targetType == "string" {
 		if s, ok := val.(string); ok {
-			if minLen, ok := getFloat(prop, "minLength"); ok && float64(len(s)) < minLen {
+			// JSON Schema measures string length in Unicode code points, not bytes.
+			length := float64(utf8.RuneCountInString(s))
+			if minLen, ok := getFloat(prop, "minLength"); ok && length < minLen {
 				errs = append(errs, ValidationError{
 					Param: name,
 					Message: fmt.Sprintf("parameter '%s' must have at least %d characters",
 						name, int(minLen)),
 				})
 			}
-			if maxLen, ok := getFloat(prop, "maxLength"); ok && float64(len(s)) > maxLen {
+			if maxLen, ok := getFloat(prop, "maxLength"); ok && length > maxLen {
 				errs = append(errs, ValidationError{
 					Param: name,
 					Message: fmt.Sprintf("parameter '%s' must have at most %d characters",
diff --git a/internal/agent/tools/param_validate_unicode_test.go b/internal/agent/tools/param_validate_unicode_test.go
new file mode 100644
index 00000000000..cd10164c3c3
--- /dev/null
+++ b/internal/agent/tools/param_validate_unicode_test.go
@@ -0,0 +1,41 @@
+package tools
+
+import (
+	"encoding/json"
+	"strings"
+	"testing"
+)
+
+func TestValidateParamsUnicodeLength(t *testing.T) {
+	cases := []struct {
+		name, value, constraint, wantError string
+	}{
+		{"Chinese maximum", "你好", `"maxLength":2`, ""},
+		{"emoji maximum", "😀", `"maxLength":1`, ""},
+		{"accent maximum", "é", `"maxLength":1`, ""},
+		{"Chinese minimum", "你", `"minLength":2`, "at least 2 characters"},
+		{"emoji minimum", "😀", `"minLength":2`, "at least 2 characters"},
+		{"mixed exact length", "A你😀", `"minLength":3,"maxLength":3`, ""},
+		{"combining code points", "e\u0301", `"minLength":2,"maxLength":2`, ""},
+		{"too many code points", "你好啊", `"maxLength":2`, "at most 2 characters"},
+		{"empty minimum", "", `"minLength":1`, "at least 1 characters"},
+		{"ASCII exact length", "abc", `"minLength":3,"maxLength":3`, ""},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			args, err := json.Marshal(map[string]string{"text": tc.value})
+			if err != nil {
+				t.Fatal(err)
+			}
+			schema := json.RawMessage(`{"type":"object","properties":{"text":{"type":"string",` + tc.constraint + `}}}`)
+			errs := ValidateParams(args, schema)
+			if tc.wantError == "" {
+				if len(errs) != 0 {
+					t.Fatalf("valid code-point length rejected: %v", errs)
+				}
+			} else if len(errs) != 1 || errs[0].Param != "text" || !strings.Contains(errs[0].Message, tc.wantError) {
+				t.Fatalf("expected %q for text, got %v", tc.wantError, errs)
+			}
+		})
+	}
+}