From 1d60aa005fd1640362975abe21e37d39d0da3ea9 Mon Sep 17 00:00:00 2001
From: andig <andi@evcc.io>
Date: Sun, 18 Jan 2026 10:48:34 +0100
Subject: [PATCH] Modbus: ensure service result is rounded (#26756)

---
 util/service/helper.go      | 19 +++++++++----------
 util/service/modbus.go      | 12 +++++-------
 util/service/modbus_test.go |  5 +++--
 3 files changed, 17 insertions(+), 19 deletions(-)

diff --git a/util/service/helper.go b/util/service/helper.go
index d5f82e590..e3cc9f635 100644
--- a/util/service/helper.go
+++ b/util/service/helper.go
@@ -3,26 +3,25 @@ package service
 import (
 	"encoding/json"
 	"net/http"
+	"strconv"
 	"strings"
 
 	"github.com/evcc-io/evcc/util"
 	"github.com/spf13/cast"
 )
 
-// applyCast applies optional type casting
-func applyCast(value any, castType string) any {
+// toString converts to canonical string representation
+func toString(value any, castType string) string {
+	res := value
 	switch strings.ToLower(castType) {
 	case "int":
-		return cast.ToInt64(value)
-	case "float":
-		return cast.ToFloat64(value)
+		res = cast.ToInt64(value)
 	case "bool":
-		return cast.ToBool(value)
-	case "string":
-		return cast.ToString(value)
-	default:
-		return value
+		res = cast.ToBool(value)
+	case "float":
+		return strconv.FormatFloat(cast.ToFloat64(value), 'g', 3, 64)
 	}
+	return cast.ToString(res)
 }
 
 // jsonWrite writes a JSON response
diff --git a/util/service/modbus.go b/util/service/modbus.go
index 826e50de1..213ce7f17 100644
--- a/util/service/modbus.go
+++ b/util/service/modbus.go
@@ -18,7 +18,7 @@ import (
 
 // Simple cache for service responses
 type cacheEntry struct {
-	value     any
+	value     string
 	timestamp time.Time
 }
 
@@ -88,20 +88,18 @@ func modbusRead(w http.ResponseWriter, req *http.Request) {
 		return
 	}
 
-	// Apply optional cast
-	if query.ResultType != "" {
-		value = applyCast(value, query.ResultType)
-	}
+	// Convert to string
+	res := toString(value, query.ResultType)
 
 	// Store in cache
 	mu.Lock()
 	cache[cacheKey] = cacheEntry{
-		value:     value,
+		value:     res,
 		timestamp: time.Now(),
 	}
 	mu.Unlock()
 
-	jsonWrite(w, []string{cast.ToString(value)})
+	jsonWrite(w, []string{res})
 }
 
 // readRegisterValue reads a modbus register value by reusing the modbus plugin
diff --git a/util/service/modbus_test.go b/util/service/modbus_test.go
index 53cab99af..b9939e3a2 100644
--- a/util/service/modbus_test.go
+++ b/util/service/modbus_test.go
@@ -5,6 +5,7 @@ import (
 	"net/http/httptest"
 	"testing"
 
+	"github.com/spf13/cast"
 	"github.com/stretchr/testify/assert"
 )
 
@@ -139,8 +140,8 @@ func TestApplyCast(t *testing.T) {
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			result := applyCast(tt.value, tt.castType)
-			assert.Equal(t, tt.expected, result)
+			result := toString(tt.value, tt.castType)
+			assert.Equal(t, cast.ToString(tt.expected), result)
 		})
 	}
 }