Add tool output truncation with line/byte limits

- Add truncate.ts utility with truncateHead/truncateTail functions
- Both respect 2000 line and 30KB limits (whichever hits first)
- read: head truncation, returns truncation info in details
- bash: tail truncation, writes full output to temp file if large
- grep: head truncation + 100 match limit
- find: head truncation + 1000 result limit
- ls: head truncation + 500 entry limit
- tool-execution.ts displays truncation notices in warning color
- All tools return clean output + structured truncation in details

packages/coding-agent/src/tools/truncate.ts

@@ -0,0 +1,252 @@
+/**
+ * Shared truncation utilities for tool outputs.
+ *
+ * Truncation is based on two independent limits - whichever is hit first wins:
+ * - Line limit (default: 2000 lines)
+ * - Byte limit (default: 30KB)
+ */
+
+export const DEFAULT_MAX_LINES = 2000;
+export const DEFAULT_MAX_BYTES = 30 * 1024; // 30KB
+
+export interface TruncationResult {
+	/** The truncated content */
+	content: string;
+	/** Whether truncation occurred */
+	truncated: boolean;
+	/** Which limit was hit: "lines", "bytes", or null if not truncated */
+	truncatedBy: "lines" | "bytes" | null;
+	/** Total number of lines in the original content */
+	totalLines: number;
+	/** Total number of bytes in the original content */
+	totalBytes: number;
+	/** Number of lines in the truncated output */
+	outputLines: number;
+	/** Number of bytes in the truncated output */
+	outputBytes: number;
+	/** Human-readable truncation notice (empty if not truncated) */
+	notice: string;
+}
+
+export interface TruncationOptions {
+	/** Maximum number of lines (default: 2000) */
+	maxLines?: number;
+	/** Maximum number of bytes (default: 30KB) */
+	maxBytes?: number;
+}
+
+/**
+ * Format bytes as human-readable size.
+ */
+function formatSize(bytes: number): string {
+	if (bytes < 1024) {
+		return `${bytes}B`;
+	} else if (bytes < 1024 * 1024) {
+		return `${(bytes / 1024).toFixed(1)}KB`;
+	} else {
+		return `${(bytes / (1024 * 1024)).toFixed(1)}MB`;
+	}
+}
+
+/**
+ * Generate a truncation notice.
+ */
+function makeNotice(
+	direction: "head" | "tail",
+	truncatedBy: "lines" | "bytes",
+	totalLines: number,
+	totalBytes: number,
+	outputLines: number,
+	outputBytes: number,
+): string {
+	const totalSize = formatSize(totalBytes);
+	const outputSize = formatSize(outputBytes);
+	const directionText = direction === "head" ? "first" : "last";
+
+	if (truncatedBy === "lines") {
+		return `[Truncated: ${totalLines} lines / ${totalSize} total, showing ${directionText} ${outputLines} lines]`;
+	} else {
+		return `[Truncated: ${totalLines} lines / ${totalSize} total, showing ${directionText} ${outputSize}]`;
+	}
+}
+
+/**
+ * Truncate content from the head (keep first N lines/bytes).
+ * Suitable for file reads where you want to see the beginning.
+ */
+export function truncateHead(content: string, options: TruncationOptions = {}): TruncationResult {
+	const maxLines = options.maxLines ?? DEFAULT_MAX_LINES;
+	const maxBytes = options.maxBytes ?? DEFAULT_MAX_BYTES;
+
+	const totalBytes = Buffer.byteLength(content, "utf-8");
+	const lines = content.split("\n");
+	const totalLines = lines.length;
+
+	// Check if no truncation needed
+	if (totalLines <= maxLines && totalBytes <= maxBytes) {
+		return {
+			content,
+			truncated: false,
+			truncatedBy: null,
+			totalLines,
+			totalBytes,
+			outputLines: totalLines,
+			outputBytes: totalBytes,
+			notice: "",
+		};
+	}
+
+	// Determine which limit we'll hit first
+	const outputLinesArr: string[] = [];
+	let outputBytesCount = 0;
+	let truncatedBy: "lines" | "bytes" = "lines";
+
+	for (let i = 0; i < lines.length && i < maxLines; i++) {
+		const line = lines[i];
+		const lineBytes = Buffer.byteLength(line, "utf-8") + (i > 0 ? 1 : 0); // +1 for newline
+
+		if (outputBytesCount + lineBytes > maxBytes) {
+			truncatedBy = "bytes";
+			// If this is the first line and it alone exceeds maxBytes, include partial
+			if (i === 0) {
+				const truncatedLine = truncateStringToBytes(line, maxBytes);
+				outputLinesArr.push(truncatedLine);
+				outputBytesCount = Buffer.byteLength(truncatedLine, "utf-8");
+			}
+			break;
+		}
+
+		outputLinesArr.push(line);
+		outputBytesCount += lineBytes;
+	}
+
+	// If we exited due to line limit
+	if (outputLinesArr.length >= maxLines && outputBytesCount <= maxBytes) {
+		truncatedBy = "lines";
+	}
+
+	const outputContent = outputLinesArr.join("\n");
+	const finalOutputBytes = Buffer.byteLength(outputContent, "utf-8");
+
+	return {
+		content: outputContent,
+		truncated: true,
+		truncatedBy,
+		totalLines,
+		totalBytes,
+		outputLines: outputLinesArr.length,
+		outputBytes: finalOutputBytes,
+		notice: makeNotice("head", truncatedBy, totalLines, totalBytes, outputLinesArr.length, finalOutputBytes),
+	};
+}
+
+/**
+ * Truncate content from the tail (keep last N lines/bytes).
+ * Suitable for bash output where you want to see the end (errors, final results).
+ */
+export function truncateTail(content: string, options: TruncationOptions = {}): TruncationResult {
+	const maxLines = options.maxLines ?? DEFAULT_MAX_LINES;
+	const maxBytes = options.maxBytes ?? DEFAULT_MAX_BYTES;
+
+	const totalBytes = Buffer.byteLength(content, "utf-8");
+	const lines = content.split("\n");
+	const totalLines = lines.length;
+
+	// Check if no truncation needed
+	if (totalLines <= maxLines && totalBytes <= maxBytes) {
+		return {
+			content,
+			truncated: false,
+			truncatedBy: null,
+			totalLines,
+			totalBytes,
+			outputLines: totalLines,
+			outputBytes: totalBytes,
+			notice: "",
+		};
+	}
+
+	// Work backwards from the end
+	const outputLinesArr: string[] = [];
+	let outputBytesCount = 0;
+	let truncatedBy: "lines" | "bytes" = "lines";
+
+	for (let i = lines.length - 1; i >= 0 && outputLinesArr.length < maxLines; i--) {
+		const line = lines[i];
+		const lineBytes = Buffer.byteLength(line, "utf-8") + (outputLinesArr.length > 0 ? 1 : 0); // +1 for newline
+
+		if (outputBytesCount + lineBytes > maxBytes) {
+			truncatedBy = "bytes";
+			// If this is the first line we're adding and it alone exceeds maxBytes, include partial
+			if (outputLinesArr.length === 0) {
+				// Take the end of the line
+				const truncatedLine = truncateStringToBytesFromEnd(line, maxBytes);
+				outputLinesArr.unshift(truncatedLine);
+				outputBytesCount = Buffer.byteLength(truncatedLine, "utf-8");
+			}
+			break;
+		}
+
+		outputLinesArr.unshift(line);
+		outputBytesCount += lineBytes;
+	}
+
+	// If we exited due to line limit
+	if (outputLinesArr.length >= maxLines && outputBytesCount <= maxBytes) {
+		truncatedBy = "lines";
+	}
+
+	const outputContent = outputLinesArr.join("\n");
+	const finalOutputBytes = Buffer.byteLength(outputContent, "utf-8");
+
+	return {
+		content: outputContent,
+		truncated: true,
+		truncatedBy,
+		totalLines,
+		totalBytes,
+		outputLines: outputLinesArr.length,
+		outputBytes: finalOutputBytes,
+		notice: makeNotice("tail", truncatedBy, totalLines, totalBytes, outputLinesArr.length, finalOutputBytes),
+	};
+}
+
+/**
+ * Truncate a string to fit within a byte limit (from the start).
+ * Handles multi-byte UTF-8 characters correctly.
+ */
+function truncateStringToBytes(str: string, maxBytes: number): string {
+	const buf = Buffer.from(str, "utf-8");
+	if (buf.length <= maxBytes) {
+		return str;
+	}
+
+	// Find a valid UTF-8 boundary
+	let end = maxBytes;
+	while (end > 0 && (buf[end] & 0xc0) === 0x80) {
+		end--;
+	}
+
+	return buf.slice(0, end).toString("utf-8");
+}
+
+/**
+ * Truncate a string to fit within a byte limit (from the end).
+ * Handles multi-byte UTF-8 characters correctly.
+ */
+function truncateStringToBytesFromEnd(str: string, maxBytes: number): string {
+	const buf = Buffer.from(str, "utf-8");
+	if (buf.length <= maxBytes) {
+		return str;
+	}
+
+	// Start from the end, skip maxBytes back
+	let start = buf.length - maxBytes;
+
+	// Find a valid UTF-8 boundary (start of a character)
+	while (start < buf.length && (buf[start] & 0xc0) === 0x80) {
+		start++;
+	}
+
+	return buf.slice(start).toString("utf-8");
+}