From 14b422da856c9b41489b5d9db35911a66ad9ef1e Mon Sep 17 00:00:00 2001
From: premultiply <4681172+premultiply@users.noreply.github.com>
Date: Fri, 22 May 2026 21:43:03 +0200
Subject: [PATCH] Templates: render template-specific modbus defaults in
 instance mode (#29852)

---
 util/templates/template_modbus.go      | 10 ++--
 util/templates/template_modbus_test.go | 73 ++++++++++++++++++++++++++
 2 files changed, 78 insertions(+), 5 deletions(-)
 create mode 100644 util/templates/template_modbus_test.go

diff --git a/util/templates/template_modbus.go b/util/templates/template_modbus.go
index 5d4c81aa8..72b523486 100644
--- a/util/templates/template_modbus.go
+++ b/util/templates/template_modbus.go
@@ -93,13 +93,13 @@ func (t *Template) ModbusValues(renderMode int, values map[string]any) {
 			}
 
 			if defaultValue != "" {
-				// for modbus params the default value is carried
-				// using the parameter default, not the value
-				// TODO figure out why that's necessary
+				// apply the template-specific default to both the render values
+				// (so RenderModeInstance YAML reflects it) and the param definition
+				// (so the Config UI surfaces it as default). The earlier guard above
+				// ensures user-supplied values are not overwritten.
+				values[p.Name] = defaultValue
 				if renderMode == RenderModeInstance {
 					t.SetParamDefault(p.Name, defaultValue)
-				} else {
-					values[p.Name] = defaultValue
 				}
 			}
 		}
diff --git a/util/templates/template_modbus_test.go b/util/templates/template_modbus_test.go
new file mode 100644
index 000000000..3c446f4a2
--- /dev/null
+++ b/util/templates/template_modbus_test.go
@@ -0,0 +1,73 @@
+package templates
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+var renderModeNames = map[int]string{
+	RenderModeInstance: "instance",
+	RenderModeDocs:     "docs",
+	RenderModeUnitTest: "unittest",
+}
+
+// TestModbusTemplateDefaultID verifies that a template-specific modbus id
+// (e.g. Wallbe/Phoenix controllers using id 255) is rendered into the resulting
+// instance config when the user did not supply an explicit id. See #29804.
+func TestModbusTemplateDefaultID(t *testing.T) {
+	for _, mode := range []int{RenderModeInstance, RenderModeDocs, RenderModeUnitTest} {
+		t.Run(renderModeNames[mode], func(t *testing.T) {
+			tmpl, err := ByName(Charger, "phoenix-ev-eth")
+			require.NoError(t, err)
+
+			_, values, err := tmpl.RenderResult(mode, map[string]any{
+				"host": "192.168.0.8",
+				"port": 502,
+			})
+			require.NoError(t, err)
+			assert.Equal(t, "255", values["id"], "template-specific modbus id must be applied")
+		})
+	}
+}
+
+// TestModbusTemplateUserIDOverridesTemplate ensures a user-supplied id wins
+// over the template default in all render modes.
+func TestModbusTemplateUserIDOverridesTemplate(t *testing.T) {
+	for _, mode := range []int{RenderModeInstance, RenderModeDocs, RenderModeUnitTest} {
+		t.Run(renderModeNames[mode], func(t *testing.T) {
+			tmpl, err := ByName(Charger, "phoenix-ev-eth")
+			require.NoError(t, err)
+
+			_, values, err := tmpl.RenderResult(mode, map[string]any{
+				"host": "192.168.0.8",
+				"port": 502,
+				"id":   42,
+			})
+			require.NoError(t, err)
+			assert.Equal(t, "42", values["id"], "user-supplied modbus id must not be overwritten")
+		})
+	}
+}
+
+// TestWallbeTemplateCoveredByPhoenix verifies the BC migration: a config that
+// still references the removed `wallbe` templates is transparently routed to
+// the phoenix-ev-eth template via the `covers:` directive, while still
+// producing the Wallbe controller's modbus slave id 255.
+func TestWallbeTemplateCoveredByPhoenix(t *testing.T) {
+	for _, name := range []string{"wallbe", "wallbe-meter", "wallbe-pre2019", "wallbe-pre2019-meter"} {
+		t.Run(name, func(t *testing.T) {
+			tmpl, err := ByName(Charger, name)
+			require.NoError(t, err)
+			assert.Equal(t, "phoenix-ev-eth", tmpl.Template)
+
+			_, values, err := tmpl.RenderResult(RenderModeInstance, map[string]any{
+				"host": "192.168.0.8",
+				"port": 502,
+			})
+			require.NoError(t, err)
+			assert.Equal(t, "255", values["id"])
+		})
+	}
+}