diff --git a/checker/definition.go b/checker/definition.go
index ecd4e6c..7d26d4a 100644
--- a/checker/definition.go
+++ b/checker/definition.go
@@ -34,8 +34,8 @@ func Definition() *sdk.CheckerDefinition {
 					Id:      "mode",
 					Type:    "string",
 					Label:   "Mode",
-					Default: "both",
-					Choices: []string{"c2s", "s2s", "both"},
+					Default: string(ModeBoth),
+					Choices: validModes,
 				},
 				{
 					Id:      "timeout",
diff --git a/checker/interactive.go b/checker/interactive.go
index 02c44b4..561ee91 100644
--- a/checker/interactive.go
+++ b/checker/interactive.go
@@ -3,6 +3,7 @@ package checker
 import (
 	"errors"
 	"net/http"
+	"slices"
 	"strconv"
 	"strings"
 
@@ -23,8 +24,8 @@ func (p *xmppProvider) RenderForm() []sdk.CheckerOptionField {
 			Id:      "mode",
 			Type:    "string",
 			Label:   "Mode",
-			Default: "both",
-			Choices: []string{"c2s", "s2s", "both"},
+			Default: string(ModeBoth),
+			Choices: validModes,
 		},
 		{
 			Id:      "timeout",
@@ -46,12 +47,10 @@ func (p *xmppProvider) ParseForm(r *http.Request) (sdk.CheckerOptions, error) {
 	opts := sdk.CheckerOptions{"domain": domain}
 
 	if mode := strings.TrimSpace(r.FormValue("mode")); mode != "" {
-		switch mode {
-		case "c2s", "s2s", "both":
-			opts["mode"] = mode
-		default:
+		if !slices.Contains(validModes, mode) {
 			return nil, errors.New("mode must be one of: c2s, s2s, both")
 		}
+		opts["mode"] = mode
 	}
 
 	if to := strings.TrimSpace(r.FormValue("timeout")); to != "" {
diff --git a/checker/rule.go b/checker/rule.go
index 8c5853a..3b0886c 100644
--- a/checker/rule.go
+++ b/checker/rule.go
@@ -3,6 +3,7 @@ package checker
 import (
 	"context"
 	"fmt"
+	"slices"
 	"strings"
 
 	sdk "git.happydns.org/checker-sdk-go/checker"
@@ -24,7 +25,7 @@ func (r *xmppRule) Description() string {
 
 func (r *xmppRule) ValidateOptions(opts sdk.CheckerOptions) error {
 	if v, ok := opts["mode"]; ok {
-		if s, ok := v.(string); ok && s != "" && s != "c2s" && s != "s2s" && s != "both" {
+		if s, ok := v.(string); ok && s != "" && !slices.Contains(validModes, s) {
 			return fmt.Errorf(`mode must be "c2s", "s2s", or "both"`)
 		}
 	}
diff --git a/checker/tls_related_test.go b/checker/tls_related_test.go
index 13e13c0..dbd3a46 100644
--- a/checker/tls_related_test.go
+++ b/checker/tls_related_test.go
@@ -57,7 +57,8 @@ func TestRule_FoldsTLSCritIntoAggregate(t *testing.T) {
 			}),
 		},
 	}
-	state := (&xmppRule{}).Evaluate(context.Background(), obs, sdk.CheckerOptions{"domain": "example.com", "mode": "both"})
+	states := (&xmppRule{}).Evaluate(context.Background(), obs, sdk.CheckerOptions{"domain": "example.com", "mode": "both"})
+	state := states[0]
 	if state.Status != sdk.StatusCrit {
 		t.Fatalf("expected StatusCrit due to TLS chain invalid, got %s (%s)", state.Status, state.Message)
 	}
@@ -71,7 +72,8 @@ func TestRule_IgnoresUnrelatedTLSObs(t *testing.T) {
 		xmpp:    healthyXMPPData(),
 		related: nil,
 	}
-	state := (&xmppRule{}).Evaluate(context.Background(), obs, sdk.CheckerOptions{"domain": "example.com", "mode": "both"})
+	states := (&xmppRule{}).Evaluate(context.Background(), obs, sdk.CheckerOptions{"domain": "example.com", "mode": "both"})
+	state := states[0]
 	if state.Status != sdk.StatusOK {
 		t.Fatalf("expected StatusOK without related TLS issues, got %s (%s)", state.Status, state.Message)
 	}
diff --git a/checker/types.go b/checker/types.go
index 062717c..94474b0 100644
--- a/checker/types.go
+++ b/checker/types.go
@@ -19,8 +19,11 @@ type XMPPMode string
 const (
 	ModeClient XMPPMode = "c2s"
 	ModeServer XMPPMode = "s2s"
+	ModeBoth   XMPPMode = "both"
 )
 
+var validModes = []string{string(ModeClient), string(ModeServer), string(ModeBoth)}
+
 // XMPPData is the full observation stored per run.
 type XMPPData struct {
 	Domain    string           `json:"domain"`