feat(tools): extract shared truncateOutput utility and apply to diagnostic output

Moved truncateOutput and formatBytes from shell.ts to new shared truncate.ts module. Applied truncation to type error diagnostics in editFile, multiEdit, and writeFile tools (4KB limit) to prevent excessively large tool responses.

Author: wgraham17

src/backend/tools/base/truncate.ts

@@ -0,0 +1,37 @@
+/**
+ * Shared output truncation utilities
+ */
+
+/** Size of head portion to preserve when truncating */
+const HEAD_SIZE = 1024; // 1KB
+
+/** Size of tail portion to preserve when truncating */
+const TAIL_SIZE = 3072; // 3KB
+
+/**
+ * Format byte count for display (KB with one decimal for >= 1024, bytes otherwise)
+ */
+export function formatBytes(bytes: number): string {
+	if (bytes >= 1024) {
+		return `${(bytes / 1024).toFixed(1)}KB`;
+	}
+	return `${bytes} bytes`;
+}
+
+/**
+ * Truncate output using head+tail strategy if it exceeds max size.
+ * Uses HEAD_SIZE:TAIL_SIZE ratio (1:3) scaled to maxSize for truncation bounds.
+ */
+export function truncateOutput(output: string, maxSize: number): string {
+	if (output.length <= maxSize) {
+		return output;
+	}
+	// Scale head/tail sizes proportionally to maxSize, maintaining 1:3 ratio
+	const totalParts = HEAD_SIZE + TAIL_SIZE; // 4KB
+	const headSize = Math.floor((HEAD_SIZE / totalParts) * maxSize);
+	const tailSize = maxSize - headSize;
+
+	const omittedBytes = output.length - headSize - tailSize;
+	const omissionMsg = `\n... [${formatBytes(omittedBytes)} omitted] ...\n`;
+	return output.slice(0, headSize) + omissionMsg + output.slice(-tailSize);
+}

src/backend/tools/pulsing/editFile.ts

@@ -6,6 +6,7 @@ import { existsSync, readFileSync, writeFileSync } from "node:fs";
 import { isAbsolute, join, normalize } from "node:path";
 import { z } from "zod";
 import { log } from "@/backend/logger";
+import { truncateOutput } from "../base/truncate";
 import { getEffectiveRoot } from "../base/utils";
 import {
 	REASON_DESCRIPTION,
@@ -202,7 +203,7 @@ Note: You are working in an isolated git worktree. Changes are isolated until pu
 			let diagnosticOutput = "";
 			const diagnostics = await getDiagnostics(context, fullPath);
 			if (diagnostics) {
-				diagnosticOutput = `\n\n⚠️ Type errors:\n${diagnostics}`;
+				diagnosticOutput = `\n\n⚠️ Type errors:\n${truncateOutput(diagnostics, 4 * 1024)}`;
 			}
 
 			// Build output with hook output appended if non-empty

src/backend/tools/pulsing/multiEdit.ts

@@ -6,6 +6,7 @@ import { existsSync, readFileSync, writeFileSync } from "node:fs";
 import { isAbsolute, join, normalize } from "node:path";
 import { z } from "zod";
 import { log } from "@/backend/logger";
+import { truncateOutput } from "../base/truncate";
 import { getEffectiveRoot } from "../base/utils";
 import {
 	REASON_DESCRIPTION,
@@ -250,7 +251,7 @@ Note: You are working in an isolated git worktree. Changes are isolated until pu
 			let diagnosticOutput = "";
 			const diagnostics = await getDiagnostics(context, fullPath);
 			if (diagnostics) {
-				diagnosticOutput = `\n\n⚠️ Type errors:\n${diagnostics}`;
+				diagnosticOutput = `\n\n⚠️ Type errors:\n${truncateOutput(diagnostics, 4 * 1024)}`;
 			}
 
 			// Extract context using shared helper with range merging

src/backend/tools/pulsing/shell.ts

@@ -6,6 +6,7 @@ import { z } from "zod";
 import { log } from "@/backend/logger";
 import { shellApprovalService } from "@/backend/services/shell-approval";
 import { collectPartialOutput, dumpTimeoutLog } from "../base/timeoutLog";
+import { truncateOutput } from "../base/truncate";
 import { getEffectiveRoot } from "../base/utils";
 import {
 	REASON_DESCRIPTION,
@@ -23,12 +24,6 @@ const FULL_OUTPUT_SIZE = 64 * 1024; // 64KB
 /** Default output size for truncated output */
 const DEFAULT_OUTPUT_SIZE = 4 * 1024; // 4KB
 
-/** Size of head portion to preserve when truncating */
-const HEAD_SIZE = 1024; // 1KB
-
-/** Size of tail portion to preserve when truncating */
-const TAIL_SIZE = 3072; // 3KB
-
 // =============================================================================
 // Schema
 // =============================================================================
@@ -59,34 +54,6 @@ export type ShellInput = z.infer<typeof shellInputSchema>;
 // Helper Functions
 // =============================================================================
 
-/**
- * Format byte count for display (KB with one decimal for >= 1024, bytes otherwise)
- */
-function formatBytes(bytes: number): string {
-	if (bytes >= 1024) {
-		return `${(bytes / 1024).toFixed(1)}KB`;
-	}
-	return `${bytes} bytes`;
-}
-
-/**
- * Truncate output using head+tail strategy if it exceeds max size.
- * Uses HEAD_SIZE:TAIL_SIZE ratio (1:3) scaled to maxSize for truncation bounds.
- */
-function truncateOutput(output: string, maxSize: number): string {
-	if (output.length <= maxSize) {
-		return output;
-	}
-	// Scale head/tail sizes proportionally to maxSize, maintaining 1:3 ratio
-	const totalParts = HEAD_SIZE + TAIL_SIZE; // 4KB
-	const headSize = Math.floor((HEAD_SIZE / totalParts) * maxSize);
-	const tailSize = maxSize - headSize;
-
-	const omittedBytes = output.length - headSize - tailSize;
-	const omissionMsg = `\n... [${formatBytes(omittedBytes)} omitted] ...\n`;
-	return output.slice(0, headSize) + omissionMsg + output.slice(-tailSize);
-}
-
 /**
  * Get the shell command args for the current platform
  */

src/backend/tools/pulsing/writeFile.ts

@@ -12,6 +12,7 @@ import {
 import { dirname, isAbsolute, join, normalize } from "node:path";
 import { z } from "zod";
 import { log } from "@/backend/logger";
+import { truncateOutput } from "../base/truncate";
 import { getEffectiveRoot } from "../base/utils";
 import {
 	REASON_DESCRIPTION,
@@ -110,7 +111,7 @@ Note: You are working in an isolated git worktree. Changes are isolated until pu
 			let diagnosticOutput = "";
 			const diagnostics = await getDiagnostics(context, fullPath);
 			if (diagnostics) {
-				diagnosticOutput = `\n\n⚠️ Type errors:\n${diagnostics}`;
+				diagnosticOutput = `\n\n⚠️ Type errors:\n${truncateOutput(diagnostics, 4 * 1024)}`;
 			}
 
 			// Build output with hook output appended if non-empty