Optimize message chunking performance and sync platform limits with 2026 standards

pkg/channels/discord.go

@@ -123,7 +123,7 @@ func (c *DiscordChannel) Send(ctx context.Context, msg bus.OutboundMessage) erro
 
 	limit := c.config.MaxMessageLength
 	if limit <= 0 {
-		limit = 1900
+		limit = 1900 // Discord limit is 2000 chars
 	}
 
 	chunks := utils.SplitMessage(msg.Content, limit)

pkg/channels/line.go

@@ -512,7 +512,7 @@ func (c *LINEChannel) Send(ctx context.Context, msg bus.OutboundMessage) error {
 
 	limit := c.config.MaxMessageLength
 	if limit <= 0 {
-		limit = 4000
+		limit = 4500 // LINE individual message bubbles are capped at 5,000 chars
 	}
 
 	chunks := utils.SplitMessage(msg.Content, limit)
@@ -525,7 +525,7 @@ func (c *LINEChannel) Send(ctx context.Context, msg bus.OutboundMessage) error {
 
 		if i == 0 && hasReplyToken {
 			if err := c.sendReply(ctx, tokenEntry.token, chunk, currentQuoteToken); err == nil {
-				logger.DebugCF("line", "Message chunk sent via Reply API", map[string]interface{}{
+				logger.DebugCF("line", "Message chunk sent via Reply API", map[string]any{
 					"chat_id": msg.ChatID,
 					"quoted":  currentQuoteToken != "",
 					"chunk":   i + 1,

pkg/channels/slack.go

@@ -121,7 +121,7 @@ func (c *SlackChannel) Send(ctx context.Context, msg bus.OutboundMessage) error
 
 	limit := c.config.MaxMessageLength
 	if limit <= 0 {
-		limit = 3000
+		limit = 3500 // Slack has a 4,000 char limit (individual blocks may be 3k)
 	}
 
 	chunks := utils.SplitMessage(msg.Content, limit)

pkg/channels/telegram.go

@@ -170,10 +170,11 @@ func (c *TelegramChannel) Send(ctx context.Context, msg bus.OutboundMessage) err
 	}
 
 	// Telegram HTML tags (like <pre><code>) can expand content significantly.
-	// We use a safe headroom for the markdown-based split.
+	// We use a safe headroom for the markdown-based split, but never exceed
+	// the configured limit.
 	effectiveMarkdownLimit := limit - 500
-	if effectiveMarkdownLimit < 500 {
+	if effectiveMarkdownLimit < 1 {
-		effectiveMarkdownLimit = 500
+		effectiveMarkdownLimit = 1
 	}
 
 	chunks := utils.SplitMessage(msg.Content, effectiveMarkdownLimit)
@@ -199,8 +200,12 @@ func (c *TelegramChannel) Send(ctx context.Context, msg bus.OutboundMessage) err
 		tgMsg.ParseMode = telego.ModeHTML
 
 		if _, err = c.bot.SendMessage(ctx, tgMsg); err != nil {
-			logger.ErrorCF("telegram", "HTML parse failed, falling back to plain text", map[string]any{
+			logger.ErrorCF("telegram", "failed to send message in HTML mode, falling back to plain text", map[string]any{
 				"error":        err.Error(),
+				"chat_id":      msg.ChatID,
+				"chunk_index":  i,
+				"chunk_total":  len(chunks),
+				"chunk_length": len(chunk),
 			})
 			tgMsg.ParseMode = ""
 			tgMsg.Text = chunk // Use raw chunk if HTML fails

pkg/utils/message.go

@@ -28,10 +28,12 @@ func SplitMessage(content string, maxLen int) []string {
 	}
 
 	runes := []rune(content)
+	startIndex := 0
 
-	for len(runes) > 0 {
+	for startIndex < len(runes) {
-		if len(runes) <= maxLen {
+		remainingRunes := runes[startIndex:]
-			messages = append(messages, string(runes))
+		if len(remainingRunes) <= maxLen {
+			messages = append(messages, string(remainingRunes))
 			break
 		}
 
@@ -41,113 +43,124 @@ func SplitMessage(content string, maxLen int) []string {
 			effectiveLimit = maxLen / 2
 		}
 
-		// Find natural split point within the effective limit
+		// Find natural split point within the effective limit from the current startIndex
-		// We pass the full slice and a limit so findLastSentenceBoundaryRunes can look ahead.
+		// We pass the full slice so findLastSentenceBoundaryRunes can look ahead past the window
-		msgEnd := findLastSentenceBoundaryRunes(runes, effectiveLimit, 300)
+		msgEndOffset := findLastSentenceBoundaryRunes(runes, startIndex+effectiveLimit, 300)
-		if msgEnd <= 0 {
+		if msgEndOffset <= startIndex {
-			msgEnd = findLastNewlineRunes(runes[:effectiveLimit], 200)
+			msgEndOffset = findLastNewlineRunes(remainingRunes[:effectiveLimit], 200)
+			if msgEndOffset >= 0 {
+				msgEndOffset += startIndex
 			}
-		if msgEnd <= 0 {
-			msgEnd = findLastSpaceRunes(runes[:effectiveLimit], 100)
 		}
-		if msgEnd <= 0 {
+		if msgEndOffset <= startIndex {
-			msgEnd = effectiveLimit
+			msgEndOffset = findLastSpaceRunes(remainingRunes[:effectiveLimit], 100)
+			if msgEndOffset >= 0 {
+				msgEndOffset += startIndex
+			}
+		}
+		if msgEndOffset <= startIndex {
+			msgEndOffset = startIndex + effectiveLimit
 		}
 
 		// Check if this would end with an incomplete code block
-		candidate := runes[:msgEnd]
+		candidateRunes := runes[startIndex:msgEndOffset]
-		unclosedIdx := findLastUnclosedCodeBlockRunes(candidate)
+		unclosedIdx := findLastUnclosedCodeBlockRunes(candidateRunes)
 
 		if unclosedIdx >= 0 {
-			// Message would end with incomplete code block
+			// Absolute index of the unclosed fence
-			// Try to extend up to maxLen to include the closing ```
+			absUnclosedIdx := startIndex + unclosedIdx
-			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.
 
-					// Find the header end (first newline after the opening ```)
+			// Try to extend up to maxLen to include the closing ```
+			closingIdx := findNextClosingCodeBlockRunes(runes, msgEndOffset)
+			if closingIdx > 0 && closingIdx <= startIndex+maxLen {
+				msgEndOffset = closingIdx
+			} else {
+				// Find first newline after opening fence to extract header
 				headerEnd := -1
-					for i := unclosedIdx; i < len(runes); i++ {
+				for i := absUnclosedIdx; i < len(runes); i++ {
 					if runes[i] == '\n' {
 						headerEnd = i
 						break
 					}
 				}
-					
 				if headerEnd == -1 {
-						headerEnd = unclosedIdx + 3
+					headerEnd = absUnclosedIdx + 3
 				} else {
-						// include newline
+					headerEnd++ // include newline
-						headerEnd++
 				}
+				header := strings.TrimSpace(string(runes[absUnclosedIdx:headerEnd]))
 
-					header := strings.TrimSpace(string(runes[unclosedIdx:headerEnd]))
+				if msgEndOffset > headerEnd+20 {
-
+					innerLimit := maxLen - 5
-					// If we have a reasonable amount of content after the header, split inside
+					betterEnd := findLastSentenceBoundaryRunes(runes, startIndex+innerLimit, 300)
-					if msgEnd > headerEnd+20 {
-						// Find a better split point closer to maxLen
-						innerLimit := maxLen - 5 // Leave room for "\n```"
-						betterEnd := findLastSentenceBoundaryRunes(runes, innerLimit, 300)
 					if betterEnd <= headerEnd {
-							betterEnd = findLastNewlineRunes(runes[:innerLimit], 200)
+						betterEnd = findLastNewlineRunes(runes[startIndex:startIndex+innerLimit], 200)
+						if betterEnd >= 0 {
+							betterEnd += startIndex
+						}
 					}
 
 					if betterEnd > headerEnd {
-							msgEnd = betterEnd
+						msgEndOffset = betterEnd
 					} else {
-							msgEnd = innerLimit
+						msgEndOffset = startIndex + innerLimit
 					}
 
-						chunkStr := string(runes[:msgEnd])
+					chunkStr := string(runes[startIndex:msgEndOffset])
 					messages = append(messages, strings.TrimRight(chunkStr, " \t\n\r")+"\n```")
 
-						nextChunkStart := string(runes[msgEnd:])
+					// Move startIndex to msgEndOffset but "inject" the header for the next iteration.
-						content = strings.TrimSpace(header + "\n" + nextChunkStart)
+					// We prepend the header and a newline.
-						runes = []rune(content)
+					injectedHeader := header + "\n"
+					nextRunes := append([]rune(injectedHeader), runes[msgEndOffset:]...)
+					runes = append(runes[:0], nextRunes...) // Reuse capacity
+					startIndex = 0
 					continue
 				}
 
-					// Otherwise, try to split before the code block starts
+				// Try to split before the code block
-					newEnd := findLastSentenceBoundaryRunes(runes, unclosedIdx, 300)
+				newEnd := findLastSentenceBoundaryRunes(runes, absUnclosedIdx, 300)
-					if newEnd <= 0 {
+				if newEnd <= startIndex {
-						newEnd = findLastNewlineRunes(runes[:unclosedIdx], 200)
+					newEnd = findLastNewlineRunes(runes[startIndex:absUnclosedIdx], 200)
+					if newEnd >= 0 {
+						newEnd += startIndex
 					}
-					if newEnd <= 0 {
-						newEnd = findLastSpaceRunes(runes[:unclosedIdx], 100)
 				}
-					if newEnd > 0 {
+				if newEnd <= startIndex {
-						msgEnd = newEnd
+					newEnd = findLastSpaceRunes(runes[startIndex:absUnclosedIdx], 100)
+					if newEnd >= 0 {
+						newEnd += startIndex
+					}
+				}
+
+				if newEnd > startIndex {
+					msgEndOffset = newEnd
 				} else {
-						// If we can't split before, we MUST split inside (last resort)
+					// Hard split inside (last resort)
-						if unclosedIdx > 20 {
+					msgEndOffset = startIndex + maxLen - 5
-							msgEnd = unclosedIdx
+					chunkStr := string(runes[startIndex:msgEndOffset])
-						} else {
-							msgEnd = maxLen - 5
-							chunkStr := string(runes[:msgEnd])
 					messages = append(messages, strings.TrimRight(chunkStr, " \t\n\r")+"\n```")
 
-							nextChunkStart := string(runes[msgEnd:])
+					injectedHeader := header + "\n"
-							content = strings.TrimSpace(header + "\n" + nextChunkStart)
+					nextRunes := append([]rune(injectedHeader), runes[msgEndOffset:]...)
-							runes = []rune(content)
+					runes = append(runes[:0], nextRunes...)
+					startIndex = 0
 					continue
 				}
 			}
 		}
-			}
+
+		if msgEndOffset <= startIndex {
+			msgEndOffset = startIndex + effectiveLimit
 		}
 
-		if msgEnd <= 0 {
+		messages = append(messages, string(runes[startIndex:msgEndOffset]))
-			msgEnd = effectiveLimit
-		}
 
-		messages = append(messages, string(runes[:msgEnd]))
+		// Advance startIndex and skip leading whitespace for next chunk
-		nextContent := strings.TrimSpace(string(runes[msgEnd:]))
+		startIndex = msgEndOffset
-		runes = []rune(nextContent)
+		for startIndex < len(runes) && (runes[startIndex] == ' ' || runes[startIndex] == '\n' || runes[startIndex] == '\t' || runes[startIndex] == '\r') {
+			startIndex++
+		}
 	}
 
 	return messages

pkg/utils/message_test.go

@@ -99,6 +99,22 @@ func TestSplitMessage(t *testing.T) {
 				}
 			},
 		},
+		{
+			name: "Prefer sentence boundary",
+			// Content is: 1700 'a's, then ". ", then 500 'b's.
+			// Effective limit with maxLen=2000 is 1800. 1700 is well within it.
+			content:      strings.Repeat("a", 1700) + ". " + strings.Repeat("b", 500),
+			maxLen:       2000,
+			expectChunks: 2,
+			checkContent: func(t *testing.T, chunks []string) {
+				if len([]rune(chunks[0])) != 1701 { // 1700 'a's + '.'
+					t.Errorf("Expected chunk 0 to be 1701 runes (split at period), got %d %q", len([]rune(chunks[0])), chunks[0])
+				}
+				if !strings.HasSuffix(chunks[0], ".") {
+					t.Errorf("Chunk 0 should end with a period, got suffix: %q", chunks[0][len(chunks[0])-5:])
+				}
+			},
+		},
 	}
 
 	for _, tc := range tests {