Port truncation logic from coding-agent to mom

- Add truncate.ts with 2000 lines / 50KB limits
- Update bash tool with tail truncation and temp file output
- Update read tool with head truncation and offset hints
- Remove redundant context history truncation (tools already provide actionable hints)

fixes #155

packages/mom/src/tools/bash.ts

@@ -1,6 +1,19 @@
+import { randomBytes } from "node:crypto";
+import { createWriteStream } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
 import type { AgentTool } from "@mariozechner/pi-ai";
 import { Type } from "@sinclair/typebox";
 import type { Executor } from "../sandbox.js";
+import { DEFAULT_MAX_BYTES, DEFAULT_MAX_LINES, formatSize, type TruncationResult, truncateTail } from "./truncate.js";
+
+/**
+ * Generate a unique temp file path for bash output
+ */
+function getTempFilePath(): string {
+	const id = randomBytes(8).toString("hex");
+	return join(tmpdir(), `mom-bash-${id}.log`);
+}
 
 const bashSchema = Type.Object({
 	label: Type.String({ description: "Brief description of what this command does (shown to user)" }),
@@ -8,18 +21,26 @@ const bashSchema = Type.Object({
 	timeout: Type.Optional(Type.Number({ description: "Timeout in seconds (optional, no default timeout)" })),
 });
 
+interface BashToolDetails {
+	truncation?: TruncationResult;
+	fullOutputPath?: string;
+}
+
 export function createBashTool(executor: Executor): AgentTool<typeof bashSchema> {
 	return {
 		name: "bash",
 		label: "bash",
-		description:
-			"Execute a bash command in the current working directory. Returns stdout and stderr. Optionally provide a timeout in seconds.",
+		description: `Execute a bash command in the current working directory. Returns stdout and stderr. Output is truncated to last ${DEFAULT_MAX_LINES} lines or ${DEFAULT_MAX_BYTES / 1024}KB (whichever is hit first). If truncated, full output is saved to a temp file. Optionally provide a timeout in seconds.`,
 		parameters: bashSchema,
 		execute: async (
 			_toolCallId: string,
 			{ command, timeout }: { label: string; command: string; timeout?: number },
 			signal?: AbortSignal,
 		) => {
+			// Track output for potential temp file writing
+			let tempFilePath: string | undefined;
+			let tempFileStream: ReturnType<typeof createWriteStream> | undefined;
+
 			const result = await executor.exec(command, { timeout, signal });
 			let output = "";
 			if (result.stdout) output += result.stdout;
@@ -28,11 +49,49 @@ export function createBashTool(executor: Executor): AgentTool<typeof bashSchema>
 				output += result.stderr;
 			}
 
-			if (result.code !== 0) {
-				throw new Error(`${output}\n\nCommand exited with code ${result.code}`.trim());
+			const totalBytes = Buffer.byteLength(output, "utf-8");
+
+			// Write to temp file if output exceeds limit
+			if (totalBytes > DEFAULT_MAX_BYTES) {
+				tempFilePath = getTempFilePath();
+				tempFileStream = createWriteStream(tempFilePath);
+				tempFileStream.write(output);
+				tempFileStream.end();
 			}
 
-			return { content: [{ type: "text", text: output || "(no output)" }], details: undefined };
+			// Apply tail truncation
+			const truncation = truncateTail(output);
+			let outputText = truncation.content || "(no output)";
+
+			// Build details with truncation info
+			let details: BashToolDetails | undefined;
+
+			if (truncation.truncated) {
+				details = {
+					truncation,
+					fullOutputPath: tempFilePath,
+				};
+
+				// Build actionable notice
+				const startLine = truncation.totalLines - truncation.outputLines + 1;
+				const endLine = truncation.totalLines;
+
+				if (truncation.lastLinePartial) {
+					// Edge case: last line alone > 50KB
+					const lastLineSize = formatSize(Buffer.byteLength(output.split("\n").pop() || "", "utf-8"));
+					outputText += `\n\n[Showing last ${formatSize(truncation.outputBytes)} of line ${endLine} (line is ${lastLineSize}). Full output: ${tempFilePath}]`;
+				} else if (truncation.truncatedBy === "lines") {
+					outputText += `\n\n[Showing lines ${startLine}-${endLine} of ${truncation.totalLines}. Full output: ${tempFilePath}]`;
+				} else {
+					outputText += `\n\n[Showing lines ${startLine}-${endLine} of ${truncation.totalLines} (${formatSize(DEFAULT_MAX_BYTES)} limit). Full output: ${tempFilePath}]`;
+				}
+			}
+
+			if (result.code !== 0) {
+				throw new Error(`${outputText}\n\nCommand exited with code ${result.code}`.trim());
+			}
+
+			return { content: [{ type: "text", text: outputText }], details };
 		},
 	};
 }