From 93c182e486f32180d9befe06841f3c5c4a7d46c4 Mon Sep 17 00:00:00 2001
From: andig <andi@evcc.io>
Date: Mon, 10 Aug 2026 20:42:46 +0200
Subject: [PATCH] MQTT: publish the forecast as a single JSON message (#32710)

---
 core/site_tariffs.go | 28 ++++++++++++++++++++--------
 server/mqtt.go       | 20 ++++++++++----------
 server/mqtt_test.go  | 18 ++++++++++++++++++
 3 files changed, 48 insertions(+), 18 deletions(-)

diff --git a/core/site_tariffs.go b/core/site_tariffs.go
index e9790a020..ebfa767c0 100644
--- a/core/site_tariffs.go
+++ b/core/site_tariffs.go
@@ -1,6 +1,7 @@
 package core
 
 import (
+	"encoding/json"
 	"math"
 	"time"
 
@@ -13,6 +14,24 @@ import (
 	"github.com/samber/lo"
 )
 
+// forecastDetails is the published forecast payload. It implements BytesMarshaler
+// so MQTT sends a single JSON message instead of decomposing every slot into an
+// individual topic (several thousand messages per update).
+type forecastDetails struct {
+	Co2         [][]float64   `json:"co2,omitempty"`
+	FeedIn      [][]float64   `json:"feedin,omitempty"`
+	Grid        [][]float64   `json:"grid,omitempty"`
+	Planner     [][]float64   `json:"planner,omitempty"`
+	Solar       *solarDetails `json:"solar,omitempty"`
+	Temperature [][]float64   `json:"temperature,omitempty"`
+}
+
+var _ api.BytesMarshaler = (*forecastDetails)(nil)
+
+func (fc forecastDetails) MarshalBytes() ([]byte, error) {
+	return json.Marshal(fc)
+}
+
 type solarDetails struct {
 	Scale            float64      `json:"scale"`                      // scale factor yield/forecasted today, 1 if unscaled
 	Today            dailyDetails `json:"today,omitempty"`            // tomorrow
@@ -115,14 +134,7 @@ func (site *Site) publishTariffs(greenShareHome float64, greenShareLoadpoints fl
 		site.publish(keys.TariffCo2Loadpoints, v)
 	}
 
-	fc := struct {
-		Co2         [][]float64   `json:"co2,omitempty"`
-		FeedIn      [][]float64   `json:"feedin,omitempty"`
-		Grid        [][]float64   `json:"grid,omitempty"`
-		Planner     [][]float64   `json:"planner,omitempty"`
-		Solar       *solarDetails `json:"solar,omitempty"`
-		Temperature [][]float64   `json:"temperature,omitempty"`
-	}{
+	fc := forecastDetails{
 		Co2:         forecastRates(tariff.Rates(site.GetTariff(api.TariffUsageCo2))),
 		FeedIn:      forecastRates(tariff.Rates(site.GetTariff(api.TariffUsageFeedIn))),
 		Planner:     forecastRates(tariff.Rates(site.GetTariff(api.TariffUsagePlanner))),
diff --git a/server/mqtt.go b/server/mqtt.go
index cfc8400f6..a4412d495 100644
--- a/server/mqtt.go
+++ b/server/mqtt.go
@@ -84,6 +84,16 @@ func mqttTagAttribute(attr string, f reflect.StructField) bool {
 }
 
 func (m *MQTT) publishComplex(topic string, retained bool, payload any) {
+	// unwrap first so the wrapped value is still checked for the marshalers below
+	if mm, ok := payload.(api.StructMarshaler); ok {
+		d, err := mm.MarshalStruct()
+		if err != nil {
+			m.log.ERROR.Printf("marshal struct: %v", err)
+			return
+		}
+		payload = d
+	}
+
 	if _, ok := payload.(fmt.Stringer); ok || payload == nil {
 		m.publishSingleValue(topic, retained, payload)
 		return
@@ -98,16 +108,6 @@ func (m *MQTT) publishComplex(topic string, retained bool, payload any) {
 		return
 	}
 
-	if mm, ok := payload.(api.StructMarshaler); ok {
-		if d, err := mm.MarshalStruct(); err != nil {
-			m.log.ERROR.Printf("marshal struct: %v", err)
-			return
-		} else {
-			payload = d
-			// fallthrough
-		}
-	}
-
 	switch typ := reflect.TypeOf(payload); typ.Kind() {
 	case reflect.Slice:
 		// publish count
diff --git a/server/mqtt_test.go b/server/mqtt_test.go
index 87b05d1e0..b2a2655cf 100644
--- a/server/mqtt_test.go
+++ b/server/mqtt_test.go
@@ -1,6 +1,7 @@
 package server
 
 import (
+	"encoding/json"
 	"math"
 	"slices"
 	"strconv"
@@ -8,6 +9,7 @@ import (
 	"time"
 
 	"github.com/evcc-io/evcc/core/types"
+	"github.com/evcc-io/evcc/util"
 	"github.com/samber/lo"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/suite"
@@ -97,6 +99,22 @@ func (suite *mqttSuite) TestSlice() {
 	suite.Equal([]string{"2", "10", "20"}, suite.payloads, "payloads")
 }
 
+type jsonPayload struct {
+	Foo [][]float64 `json:"foo,omitempty"`
+}
+
+func (p jsonPayload) MarshalBytes() ([]byte, error) {
+	return json.Marshal(p)
+}
+
+// a BytesMarshaler wrapped in a Sharder must still publish as a single message
+func (suite *mqttSuite) TestShardedBytesMarshaler() {
+	p := jsonPayload{Foo: [][]float64{{1, 2, 3}, {4, 5, 6}}}
+	suite.publish("test", false, util.NewSharder("test", p))
+	suite.Equal([]string{"test"}, suite.topics, "topics")
+	suite.Equal([]string{`{"foo":[[1,2,3],[4,5,6]]}`}, suite.payloads, "payloads")
+}
+
 func (suite *mqttSuite) TestNilInterface() {
 	var ptr *time.Time
 	suite.publish("test", false, ptr)