refactor(channels): unify message splitting and add per-channel worker queues

Move message splitting from individual channels (Discord) to the Manager
layer via per-channel worker goroutines. Each channel now declares its
max message length through BaseChannelOption/MessageLengthProvider, and
the Manager automatically splits oversized outbound messages before
dispatch. This prevents one slow channel from blocking all others.

- Add WithMaxMessageLength option and MessageLengthProvider interface
- Set platform-specific limits (Discord 2000, Telegram 4096, Slack 40000, etc.)
- Convert SplitMessage to rune-aware counting for correct Unicode handling
- Replace single dispatcher goroutine with per-channel buffered worker queues
- Remove Discord's internal SplitMessage call (now handled centrally)

pkg/channels/base.go

@@ -17,21 +17,55 @@ type Channel interface {
 	IsAllowed(senderID string) bool
 }
 
-type BaseChannel struct {
-	config    any
-	bus       *bus.MessageBus
-	running   atomic.Bool
-	name      string
-	allowList []string
+// BaseChannelOption is a functional option for configuring a BaseChannel.
+type BaseChannelOption func(*BaseChannel)
+
+// WithMaxMessageLength sets the maximum message length (in runes) for a channel.
+// Messages exceeding this limit will be automatically split by the Manager.
+// A value of 0 means no limit.
+func WithMaxMessageLength(n int) BaseChannelOption {
+	return func(c *BaseChannel) { c.maxMessageLength = n }
 }
 
-func NewBaseChannel(name string, config any, bus *bus.MessageBus, allowList []string) *BaseChannel {
-	return &BaseChannel{
+// MessageLengthProvider is an opt-in interface that channels implement
+// to advertise their maximum message length. The Manager uses this via
+// type assertion to decide whether to split outbound messages.
+type MessageLengthProvider interface {
+	MaxMessageLength() int
+}
+
+type BaseChannel struct {
+	config           any
+	bus              *bus.MessageBus
+	running          atomic.Bool
+	name             string
+	allowList        []string
+	maxMessageLength int
+}
+
+func NewBaseChannel(
+	name string,
+	config any,
+	bus *bus.MessageBus,
+	allowList []string,
+	opts ...BaseChannelOption,
+) *BaseChannel {
+	bc := &BaseChannel{
 		config:    config,
 		bus:       bus,
 		name:      name,
 		allowList: allowList,
 	}
+	for _, opt := range opts {
+		opt(bc)
+	}
+	return bc
+}
+
+// MaxMessageLength returns the maximum message length (in runes) for this channel.
+// A value of 0 means no limit.
+func (c *BaseChannel) MaxMessageLength() int {
+	return c.maxMessageLength
 }
 
 func (c *BaseChannel) Name() string {

pkg/channels/dingtalk/dingtalk.go

@@ -38,7 +38,7 @@ func NewDingTalkChannel(cfg config.DingTalkConfig, messageBus *bus.MessageBus) (
 		return nil, fmt.Errorf("dingtalk client_id and client_secret are required")
 	}
 
-	base := channels.NewBaseChannel("dingtalk", cfg, messageBus, cfg.AllowFrom)
+	base := channels.NewBaseChannel("dingtalk", cfg, messageBus, cfg.AllowFrom, channels.WithMaxMessageLength(20000))
 
 	return &DingTalkChannel{
 		BaseChannel:  base,

pkg/channels/discord/discord.go

@@ -41,7 +41,7 @@ func NewDiscordChannel(cfg config.DiscordConfig, bus *bus.MessageBus) (*DiscordC
 		return nil, fmt.Errorf("failed to create discord session: %w", err)
 	}
 
-	base := channels.NewBaseChannel("discord", cfg, bus, cfg.AllowFrom)
+	base := channels.NewBaseChannel("discord", cfg, bus, cfg.AllowFrom, channels.WithMaxMessageLength(2000))
 
 	return &DiscordChannel{
 		BaseChannel: base,
@@ -121,20 +121,11 @@ func (c *DiscordChannel) Send(ctx context.Context, msg bus.OutboundMessage) erro
 		return fmt.Errorf("channel ID is empty")
 	}
 
-	runes := []rune(msg.Content)
-	if len(runes) == 0 {
+	if len([]rune(msg.Content)) == 0 {
 		return nil
 	}
 
-	chunks := utils.SplitMessage(msg.Content, 2000) // Split messages into chunks, Discord length limit: 2000 chars
-
-	for _, chunk := range chunks {
-		if err := c.sendChunk(ctx, channelID, chunk); err != nil {
-			return err
-		}
-	}
-
-	return nil
+	return c.sendChunk(ctx, channelID, msg.Content)
 }
 
 func (c *DiscordChannel) sendChunk(ctx context.Context, channelID, content string) error {

pkg/channels/line/line.go

@@ -60,7 +60,7 @@ func NewLINEChannel(cfg config.LINEConfig, messageBus *bus.MessageBus) (*LINECha
 		return nil, fmt.Errorf("line channel_secret and channel_access_token are required")
 	}
 
-	base := channels.NewBaseChannel("line", cfg, messageBus, cfg.AllowFrom)
+	base := channels.NewBaseChannel("line", cfg, messageBus, cfg.AllowFrom, channels.WithMaxMessageLength(5000))
 
 	return &LINEChannel{
 		BaseChannel: base,

pkg/channels/manager.go

@@ -15,10 +15,20 @@ import (
 	"github.com/sipeed/picoclaw/pkg/config"
 	"github.com/sipeed/picoclaw/pkg/constants"
 	"github.com/sipeed/picoclaw/pkg/logger"
+	"github.com/sipeed/picoclaw/pkg/utils"
 )
 
+const defaultChannelQueueSize = 100
+
+type channelWorker struct {
+	ch    Channel
+	queue chan bus.OutboundMessage
+	done  chan struct{}
+}
+
 type Manager struct {
 	channels     map[string]Channel
+	workers      map[string]*channelWorker
 	bus          *bus.MessageBus
 	config       *config.Config
 	dispatchTask *asyncTask
@@ -32,6 +42,7 @@ type asyncTask struct {
 func NewManager(cfg *config.Config, messageBus *bus.MessageBus) (*Manager, error) {
 	m := &Manager{
 		channels: make(map[string]Channel),
+		workers:  make(map[string]*channelWorker),
 		bus:      messageBus,
 		config:   cfg,
 	}
@@ -63,6 +74,11 @@ func (m *Manager) initChannel(name, displayName string) {
 		})
 	} else {
 		m.channels[name] = ch
+		m.workers[name] = &channelWorker{
+			ch:    ch,
+			queue: make(chan bus.OutboundMessage, defaultChannelQueueSize),
+			done:  make(chan struct{}),
+		}
 		logger.InfoCF("channels", "Channel enabled successfully", map[string]any{
 			"channel": displayName,
 		})
@@ -141,8 +157,6 @@ func (m *Manager) StartAll(ctx context.Context) error {
 	dispatchCtx, cancel := context.WithCancel(ctx)
 	m.dispatchTask = &asyncTask{cancel: cancel}
 
-	go m.dispatchOutbound(dispatchCtx)
-
 	for name, channel := range m.channels {
 		logger.InfoCF("channels", "Starting channel", map[string]any{
 			"channel": name,
@@ -155,6 +169,14 @@ func (m *Manager) StartAll(ctx context.Context) error {
 		}
 	}
 
+	// Start per-channel workers
+	for name, w := range m.workers {
+		go m.runWorker(dispatchCtx, name, w)
+	}
+
+	// Start the dispatcher that reads from the bus and routes to workers
+	go m.dispatchOutbound(dispatchCtx)
+
 	logger.InfoC("channels", "All channels started")
 	return nil
 }
@@ -165,11 +187,21 @@ func (m *Manager) StopAll(ctx context.Context) error {
 
 	logger.InfoC("channels", "Stopping all channels")
 
+	// Cancel dispatcher first
 	if m.dispatchTask != nil {
 		m.dispatchTask.cancel()
 		m.dispatchTask = nil
 	}
 
+	// Close all worker queues and wait for them to drain
+	for _, w := range m.workers {
+		close(w.queue)
+	}
+	for _, w := range m.workers {
+		<-w.done
+	}
+
+	// Stop all channels
 	for name, channel := range m.channels {
 		logger.InfoCF("channels", "Stopping channel", map[string]any{
 			"channel": name,
@@ -186,6 +218,44 @@ func (m *Manager) StopAll(ctx context.Context) error {
 	return nil
 }
 
+// runWorker processes outbound messages for a single channel, splitting
+// messages that exceed the channel's maximum message length.
+func (m *Manager) runWorker(ctx context.Context, name string, w *channelWorker) {
+	defer close(w.done)
+	for {
+		select {
+		case msg, ok := <-w.queue:
+			if !ok {
+				return
+			}
+			maxLen := 0
+			if mlp, ok := w.ch.(MessageLengthProvider); ok {
+				maxLen = mlp.MaxMessageLength()
+			}
+			if maxLen > 0 && len([]rune(msg.Content)) > maxLen {
+				chunks := utils.SplitMessage(msg.Content, maxLen)
+				for _, chunk := range chunks {
+					chunkMsg := msg
+					chunkMsg.Content = chunk
+					if err := w.ch.Send(ctx, chunkMsg); err != nil {
+						logger.ErrorCF("channels", "Error sending chunk", map[string]any{
+							"channel": name, "error": err.Error(),
+						})
+					}
+				}
+			} else {
+				if err := w.ch.Send(ctx, msg); err != nil {
+					logger.ErrorCF("channels", "Error sending message", map[string]any{
+						"channel": name, "error": err.Error(),
+					})
+				}
+			}
+		case <-ctx.Done():
+			return
+		}
+	}
+}
+
 func (m *Manager) dispatchOutbound(ctx context.Context) {
 	logger.InfoC("channels", "Outbound dispatcher started")
 
@@ -206,7 +276,8 @@ func (m *Manager) dispatchOutbound(ctx context.Context) {
 			}
 
 			m.mu.RLock()
-			channel, exists := m.channels[msg.Channel]
+			_, exists := m.channels[msg.Channel]
+			w, wExists := m.workers[msg.Channel]
 			m.mu.RUnlock()
 
 			if !exists {
@@ -216,11 +287,12 @@ func (m *Manager) dispatchOutbound(ctx context.Context) {
 				continue
 			}
 
-			if err := channel.Send(ctx, msg); err != nil {
-				logger.ErrorCF("channels", "Error sending message to channel", map[string]any{
-					"channel": msg.Channel,
-					"error":   err.Error(),
-				})
+			if wExists {
+				select {
+				case w.queue <- msg:
+				case <-ctx.Done():
+					return
+				}
 			}
 		}
 	}
@@ -262,17 +334,28 @@ func (m *Manager) RegisterChannel(name string, channel Channel) {
 	m.mu.Lock()
 	defer m.mu.Unlock()
 	m.channels[name] = channel
+	m.workers[name] = &channelWorker{
+		ch:    channel,
+		queue: make(chan bus.OutboundMessage, defaultChannelQueueSize),
+		done:  make(chan struct{}),
+	}
 }
 
 func (m *Manager) UnregisterChannel(name string) {
 	m.mu.Lock()
 	defer m.mu.Unlock()
+	if w, ok := m.workers[name]; ok {
+		close(w.queue)
+		<-w.done
+	}
+	delete(m.workers, name)
 	delete(m.channels, name)
 }
 
 func (m *Manager) SendToChannel(ctx context.Context, channelName, chatID, content string) error {
 	m.mu.RLock()
-	channel, exists := m.channels[channelName]
+	_, exists := m.channels[channelName]
+	w, wExists := m.workers[channelName]
 	m.mu.RUnlock()
 
 	if !exists {
@@ -285,5 +368,16 @@ func (m *Manager) SendToChannel(ctx context.Context, channelName, chatID, conten
 		Content: content,
 	}
 
+	if wExists {
+		select {
+		case w.queue <- msg:
+			return nil
+		case <-ctx.Done():
+			return ctx.Err()
+		}
+	}
+
+	// Fallback: direct send (should not happen)
+	channel, _ := m.channels[channelName]
 	return channel.Send(ctx, msg)
 }

pkg/channels/slack/slack.go

@@ -50,7 +50,7 @@ func NewSlackChannel(cfg config.SlackConfig, messageBus *bus.MessageBus) (*Slack
 
 	socketClient := socketmode.New(api)
 
-	base := channels.NewBaseChannel("slack", cfg, messageBus, cfg.AllowFrom)
+	base := channels.NewBaseChannel("slack", cfg, messageBus, cfg.AllowFrom, channels.WithMaxMessageLength(40000))
 
 	return &SlackChannel{
 		BaseChannel:  base,

pkg/channels/telegram/telegram.go

@@ -76,7 +76,13 @@ func NewTelegramChannel(cfg *config.Config, bus *bus.MessageBus) (*TelegramChann
 		return nil, fmt.Errorf("failed to create telegram bot: %w", err)
 	}
 
-	base := channels.NewBaseChannel("telegram", telegramCfg, bus, telegramCfg.AllowFrom)
+	base := channels.NewBaseChannel(
+		"telegram",
+		telegramCfg,
+		bus,
+		telegramCfg.AllowFrom,
+		channels.WithMaxMessageLength(4096),
+	)
 
 	return &TelegramChannel{
 		BaseChannel:  base,

pkg/channels/wecom/app.go

@@ -120,7 +120,7 @@ func NewWeComAppChannel(cfg config.WeComAppConfig, messageBus *bus.MessageBus) (
 		return nil, fmt.Errorf("wecom_app corp_id, corp_secret and agent_id are required")
 	}
 
-	base := channels.NewBaseChannel("wecom_app", cfg, messageBus, cfg.AllowFrom)
+	base := channels.NewBaseChannel("wecom_app", cfg, messageBus, cfg.AllowFrom, channels.WithMaxMessageLength(2048))
 
 	return &WeComAppChannel{
 		BaseChannel:   base,

pkg/channels/wecom/bot.go

@@ -87,7 +87,7 @@ func NewWeComBotChannel(cfg config.WeComConfig, messageBus *bus.MessageBus) (*We
 		return nil, fmt.Errorf("wecom token and webhook_url are required")
 	}
 
-	base := channels.NewBaseChannel("wecom", cfg, messageBus, cfg.AllowFrom)
+	base := channels.NewBaseChannel("wecom", cfg, messageBus, cfg.AllowFrom, channels.WithMaxMessageLength(2048))
 
 	return &WeComBotChannel{
 		BaseChannel:   base,

pkg/channels/whatsapp/whatsapp.go

@@ -28,7 +28,7 @@ type WhatsAppChannel struct {
 }
 
 func NewWhatsAppChannel(cfg config.WhatsAppConfig, bus *bus.MessageBus) (*WhatsAppChannel, error) {
-	base := channels.NewBaseChannel("whatsapp", cfg, bus, cfg.AllowFrom)
+	base := channels.NewBaseChannel("whatsapp", cfg, bus, cfg.AllowFrom, channels.WithMaxMessageLength(65536))
 
 	return &WhatsAppChannel{
 		BaseChannel: base,

pkg/utils/message.go

@@ -5,11 +5,20 @@ import (
 )
 
 // SplitMessage splits long messages into chunks, preserving code block integrity.
+// The maxLen parameter is measured in runes (Unicode characters), not bytes.
 // The function reserves a buffer (10% of maxLen, min 50) to leave room for closing code blocks,
 // but may extend to maxLen when needed.
 // Call SplitMessage with the full text content and the maximum allowed length of a single message;
 // it returns a slice of message chunks that each respect maxLen and avoid splitting fenced code blocks.
 func SplitMessage(content string, maxLen int) []string {
+	if maxLen <= 0 {
+		if content == "" {
+			return nil
+		}
+		return []string{content}
+	}
+
+	runes := []rune(content)
 	var messages []string
 
 	// Dynamic buffer: 10% of maxLen, but at least 50 chars if possible
@@ -21,9 +30,9 @@ func SplitMessage(content string, maxLen int) []string {
 		codeBlockBuffer = maxLen / 2
 	}
 
-	for len(content) > 0 {
-		if len(content) <= maxLen {
-			messages = append(messages, content)
+	for len(runes) > 0 {
+		if len(runes) <= maxLen {
+			messages = append(messages, string(runes))
 			break
 		}
 
@@ -34,56 +43,66 @@ func SplitMessage(content string, maxLen int) []string {
 		}
 
 		// Find natural split point within the effective limit
-		msgEnd := findLastNewline(content[:effectiveLimit], 200)
+		msgEnd := findLastNewlineRunes(runes[:effectiveLimit], 200)
 		if msgEnd <= 0 {
-			msgEnd = findLastSpace(content[:effectiveLimit], 100)
+			msgEnd = findLastSpaceRunes(runes[:effectiveLimit], 100)
 		}
 		if msgEnd <= 0 {
 			msgEnd = effectiveLimit
 		}
 
 		// Check if this would end with an incomplete code block
-		candidate := content[:msgEnd]
-		unclosedIdx := findLastUnclosedCodeBlock(candidate)
+		candidate := runes[:msgEnd]
+		unclosedIdx := findLastUnclosedCodeBlockRunes(candidate)
 
 		if unclosedIdx >= 0 {
 			// Message would end with incomplete code block
 			// Try to extend up to maxLen to include the closing ```
-			if len(content) > msgEnd {
-				closingIdx := findNextClosingCodeBlock(content, msgEnd)
+			if len(runes) > msgEnd {
+				closingIdx := findNextClosingCodeBlockRunes(runes, msgEnd)
 				if closingIdx > 0 && closingIdx <= maxLen {
 					// Extend to include the closing ```
 					msgEnd = closingIdx
 				} else {
 					// Code block is too long to fit in one chunk or missing closing fence.
 					// Try to split inside by injecting closing and reopening fences.
-					headerEnd := strings.Index(content[unclosedIdx:], "\n")
+					candidateStr := string(candidate)
+					unclosedStr := string(runes[unclosedIdx:])
+					headerEnd := strings.Index(unclosedStr, "\n")
+					var header string
 					if headerEnd == -1 {
-						headerEnd = unclosedIdx + 3
+						header = strings.TrimSpace(string(runes[unclosedIdx : unclosedIdx+3]))
 					} else {
-						headerEnd += unclosedIdx
+						header = strings.TrimSpace(string(runes[unclosedIdx : unclosedIdx+headerEnd]))
 					}
-					header := strings.TrimSpace(content[unclosedIdx:headerEnd])
+					headerEndIdx := unclosedIdx + len([]rune(header))
+					if headerEnd != -1 {
+						headerEndIdx = unclosedIdx + headerEnd
+					}
+
+					_ = candidateStr // used above for context
 
 					// If we have a reasonable amount of content after the header, split inside
-					if msgEnd > headerEnd+20 {
+					if msgEnd > headerEndIdx+20 {
 						// Find a better split point closer to maxLen
 						innerLimit := maxLen - 5 // Leave room for "\n```"
-						betterEnd := findLastNewline(content[:innerLimit], 200)
-						if betterEnd > headerEnd {
+						betterEnd := findLastNewlineRunes(runes[:innerLimit], 200)
+						if betterEnd > headerEndIdx {
 							msgEnd = betterEnd
 						} else {
 							msgEnd = innerLimit
 						}
-						messages = append(messages, strings.TrimRight(content[:msgEnd], " \t\n\r")+"\n```")
-						content = strings.TrimSpace(header + "\n" + content[msgEnd:])
+						chunk := strings.TrimRight(string(runes[:msgEnd]), " \t\n\r") + "\n```"
+						messages = append(messages, chunk)
+						remaining := strings.TrimSpace(header + "\n" + string(runes[msgEnd:]))
+						runes = []rune(remaining)
 						continue
 					}
 
 					// Otherwise, try to split before the code block starts
-					newEnd := findLastNewline(content[:unclosedIdx], 200)
+					newEnd := findLastNewlineRunes(runes[:unclosedIdx], 200)
 					if newEnd <= 0 {
-						newEnd = findLastSpace(content[:unclosedIdx], 100)
+						newEnd = findLastSpaceRunes(runes[:unclosedIdx], 100)
 					}
 					if newEnd > 0 {
 						msgEnd = newEnd
@@ -93,8 +112,10 @@ func SplitMessage(content string, maxLen int) []string {
 							msgEnd = unclosedIdx
 						} else {
 							msgEnd = maxLen - 5
-							messages = append(messages, strings.TrimRight(content[:msgEnd], " \t\n\r")+"\n```")
-							content = strings.TrimSpace(header + "\n" + content[msgEnd:])
+							chunk := strings.TrimRight(string(runes[:msgEnd]), " \t\n\r") + "\n```"
+							messages = append(messages, chunk)
+							remaining := strings.TrimSpace(header + "\n" + string(runes[msgEnd:]))
+							runes = []rune(remaining)
 							continue
 						}
 					}
@@ -106,21 +127,22 @@ func SplitMessage(content string, maxLen int) []string {
 			msgEnd = effectiveLimit
 		}
 
-		messages = append(messages, content[:msgEnd])
-		content = strings.TrimSpace(content[msgEnd:])
+		messages = append(messages, string(runes[:msgEnd]))
+		remaining := strings.TrimSpace(string(runes[msgEnd:]))
+		runes = []rune(remaining)
 	}
 
 	return messages
 }
 
-// findLastUnclosedCodeBlock finds the last opening ``` that doesn't have a closing ```
-// Returns the position of the opening ``` or -1 if all code blocks are complete
-func findLastUnclosedCodeBlock(text string) int {
+// findLastUnclosedCodeBlockRunes finds the last opening ``` that doesn't have a closing ```
+// Returns the rune position of the opening ``` or -1 if all code blocks are complete
+func findLastUnclosedCodeBlockRunes(runes []rune) int {
 	inCodeBlock := false
 	lastOpenIdx := -1
 
-	for i := 0; i < len(text); i++ {
-		if i+2 < len(text) && text[i] == '`' && text[i+1] == '`' && text[i+2] == '`' {
+	for i := 0; i < len(runes); i++ {
+		if i+2 < len(runes) && runes[i] == '`' && runes[i+1] == '`' && runes[i+2] == '`' {
 			// Toggle code block state on each fence
 			if !inCodeBlock {
 				// Entering a code block: record this opening fence
@@ -137,41 +159,41 @@ func findLastUnclosedCodeBlock(text string) int {
 	return -1
 }
 
-// findNextClosingCodeBlock finds the next closing ``` starting from a position
-// Returns the position after the closing ``` or -1 if not found
-func findNextClosingCodeBlock(text string, startIdx int) int {
-	for i := startIdx; i < len(text); i++ {
-		if i+2 < len(text) && text[i] == '`' && text[i+1] == '`' && text[i+2] == '`' {
+// findNextClosingCodeBlockRunes finds the next closing ``` starting from a rune position
+// Returns the rune position after the closing ``` or -1 if not found
+func findNextClosingCodeBlockRunes(runes []rune, startIdx int) int {
+	for i := startIdx; i < len(runes); i++ {
+		if i+2 < len(runes) && runes[i] == '`' && runes[i+1] == '`' && runes[i+2] == '`' {
 			return i + 3
 		}
 	}
 	return -1
 }
 
-// findLastNewline finds the last newline character within the last N characters
-// Returns the position of the newline or -1 if not found
-func findLastNewline(s string, searchWindow int) int {
-	searchStart := len(s) - searchWindow
+// findLastNewlineRunes finds the last newline character within the last N runes
+// Returns the rune position of the newline or -1 if not found
+func findLastNewlineRunes(runes []rune, searchWindow int) int {
+	searchStart := len(runes) - searchWindow
 	if searchStart < 0 {
 		searchStart = 0
 	}
-	for i := len(s) - 1; i >= searchStart; i-- {
-		if s[i] == '\n' {
+	for i := len(runes) - 1; i >= searchStart; i-- {
+		if runes[i] == '\n' {
 			return i
 		}
 	}
 	return -1
 }
 
-// findLastSpace finds the last space character within the last N characters
-// Returns the position of the space or -1 if not found
-func findLastSpace(s string, searchWindow int) int {
-	searchStart := len(s) - searchWindow
+// findLastSpaceRunes finds the last space character within the last N runes
+// Returns the rune position of the space or -1 if not found
+func findLastSpaceRunes(runes []rune, searchWindow int) int {
+	searchStart := len(runes) - searchWindow
 	if searchStart < 0 {
 		searchStart = 0
 	}
-	for i := len(s) - 1; i >= searchStart; i-- {
-		if s[i] == ' ' || s[i] == '\t' {
+	for i := len(runes) - 1; i >= searchStart; i-- {
+		if runes[i] == ' ' || runes[i] == '\t' {
 			return i
 		}
 	}

pkg/utils/message_test.go

@@ -34,11 +34,15 @@ func TestSplitMessage(t *testing.T) {
 			maxLen:       2000,
 			expectChunks: 2,
 			checkContent: func(t *testing.T, chunks []string) {
-				if len(chunks[0]) > 2000 {
-					t.Errorf("Chunk 0 too large: %d", len(chunks[0]))
+				if len([]rune(chunks[0])) > 2000 {
+					t.Errorf("Chunk 0 too large: %d runes", len([]rune(chunks[0])))
 				}
-				if len(chunks[0])+len(chunks[1]) != len(longText) {
-					t.Errorf("Total length mismatch. Got %d, want %d", len(chunks[0])+len(chunks[1]), len(longText))
+				if len([]rune(chunks[0]))+len([]rune(chunks[1])) != len([]rune(longText)) {
+					t.Errorf(
+						"Total rune length mismatch. Got %d, want %d",
+						len([]rune(chunks[0]))+len([]rune(chunks[1])),
+						len([]rune(longText)),
+					)
 				}
 			},
 		},
@@ -53,11 +57,11 @@ func TestSplitMessage(t *testing.T) {
 			maxLen:       2000,
 			expectChunks: 2,
 			checkContent: func(t *testing.T, chunks []string) {
-				if len(chunks[0]) != 1750 {
-					t.Errorf("Expected chunk 0 to be 1750 length (split at newline), got %d", len(chunks[0]))
+				if len([]rune(chunks[0])) != 1750 {
+					t.Errorf("Expected chunk 0 to be 1750 runes (split at newline), got %d", len([]rune(chunks[0])))
 				}
 				if chunks[1] != strings.Repeat("b", 300) {
-					t.Errorf("Chunk 1 content mismatch. Len: %d", len(chunks[1]))
+					t.Errorf("Chunk 1 content mismatch. Len: %d", len([]rune(chunks[1])))
 				}
 			},
 		},
@@ -78,17 +82,39 @@ func TestSplitMessage(t *testing.T) {
 			},
 		},
 		{
-			name:         "Preserve Unicode characters",
-			content:      strings.Repeat("\u4e16", 1000), // 3000 bytes
+			name:         "Preserve Unicode characters (rune-aware)",
+			content:      strings.Repeat("\u4e16", 2500), // 2500 runes, 7500 bytes
 			maxLen:       2000,
 			expectChunks: 2,
 			checkContent: func(t *testing.T, chunks []string) {
-				// Just verify we didn't panic and got valid strings.
-				// Go strings are UTF-8, if we split mid-rune it would be bad,
-				// but standard slicing might do that.
-				// Let's assume standard behavior is acceptable or check if it produces invalid rune?
-				if !strings.Contains(chunks[0], "\u4e16") {
-					t.Error("Chunk should contain unicode characters")
+				// Verify chunks contain valid unicode and don't split mid-rune
+				for i, chunk := range chunks {
+					runeCount := len([]rune(chunk))
+					if runeCount > 2000 {
+						t.Errorf("Chunk %d has %d runes, exceeds maxLen 2000", i, runeCount)
+					}
+					if !strings.Contains(chunk, "\u4e16") {
+						t.Errorf("Chunk %d should contain unicode characters", i)
+					}
+				}
+				// Verify total rune count is preserved
+				totalRunes := 0
+				for _, chunk := range chunks {
+					totalRunes += len([]rune(chunk))
+				}
+				if totalRunes != 2500 {
+					t.Errorf("Total rune count mismatch. Got %d, want 2500", totalRunes)
+				}
+			},
+		},
+		{
+			name:         "Zero maxLen returns single chunk",
+			content:      "Hello world",
+			maxLen:       0,
+			expectChunks: 1,
+			checkContent: func(t *testing.T, chunks []string) {
+				if chunks[0] != "Hello world" {
+					t.Errorf("Expected original content, got %q", chunks[0])
 				}
 			},
 		},
@@ -145,7 +171,7 @@ func TestSplitMessage_CodeBlockIntegrity(t *testing.T) {
 	}
 
 	// First chunk should contain meaningful content
-	if len(chunks[0]) > 40 {
-		t.Errorf("First chunk exceeded maxLen: length %d", len(chunks[0]))
+	if len([]rune(chunks[0])) > 40 {
+		t.Errorf("First chunk exceeded maxLen: length %d runes", len([]rune(chunks[0])))
 	}
 }