feat(tools): focus diff preview for write-to-file changes

- Add DiffUtils.createFocusedDiff to generate context-aware diffs (default 5 lines)
- Use focused diff in WriteToFileToolHandler partial and approval messages
  using diffViewProvider.originalContent + newContent
- Fall back to existing diff/content when focused diff is unavailable

This improves readability of change previews, reduces noise on large files,
and provides clearer context around edits without overwhelming the UI.

src/core/task/tools/handlers/WriteToFileToolHandler.ts

@@ -16,6 +16,7 @@ import type { IFullyManagedTool } from "../ToolExecutorCoordinator"
 import type { ToolValidator } from "../ToolValidator"
 import type { TaskConfig } from "../types/TaskConfig"
 import type { StronglyTypedUIHelpers } from "../types/UIHelpers"
+import { DiffUtils } from "../utils/DiffUtils"
 import { applyModelContentFixes } from "../utils/ModelContentProcessor"
 import { ToolDisplayUtils } from "../utils/ToolDisplayUtils"
 import { ToolResultUtils } from "../utils/ToolResultUtils"
@@ -52,13 +53,20 @@ export class WriteToFileToolHandler implements IFullyManagedTool {
 			const { relPath, absolutePath, fileExists, diff, content, newContent } = result
 
 			// Create and show partial UI message
+			// Create focused diff for the preview
+			const focusedDiff = DiffUtils.createFocusedDiff(
+				relPath,
+				config.services.diffViewProvider.originalContent || "",
+				newContent,
+			)
+
 			const sharedMessageProps: ClineSayTool = {
 				tool: fileExists ? "editedExistingFile" : "newFileCreated",
 				path: getReadablePath(
 					config.cwd,
 					uiHelpers.removeClosingTag(block, block.params.path ? "path" : "absolutePath", relPath),
 				),
-				content: diff || content,
+				content: focusedDiff || diff || content,
 				operationIsLocatedInWorkspace: await isLocatedInWorkspace(relPath),
 			}
 			const partialMessage = JSON.stringify(sharedMessageProps)
@@ -133,11 +141,18 @@ export class WriteToFileToolHandler implements IFullyManagedTool {
 
 			const { relPath, absolutePath, fileExists, diff, content, newContent, workspaceContext } = result
 
+			// Create focused diff for the preview
+			const focusedDiff = DiffUtils.createFocusedDiff(
+				relPath,
+				config.services.diffViewProvider.originalContent || "",
+				newContent,
+			)
+
 			// Handle approval flow
 			const sharedMessageProps: ClineSayTool = {
 				tool: fileExists ? "editedExistingFile" : "newFileCreated",
 				path: getReadablePath(config.cwd, relPath),
-				content: diff || content,
+				content: focusedDiff || diff || content,
 				operationIsLocatedInWorkspace: await isLocatedInWorkspace(relPath),
 			}
 			// if isEditingFile false, that means we have the full contents of the file already.

src/core/task/tools/utils/DiffUtils.ts

@@ -0,0 +1,100 @@
+import * as diff from "diff"
+
+export class DiffUtils {
+	/**
+	 * Creates a focused diff showing the changes with context
+	 * @param filename Name of the file being modified
+	 * @param oldContent Original content of the file
+	 * @param newContent New content after changes
+	 * @param contextLines Number of lines of context to show before and after changes (default: 5)
+	 * @returns Formatted diff string with context
+	 */
+	static createFocusedDiff(
+		filename: string,
+		oldContent: string = "",
+		newContent: string = "",
+		contextLines: number = 5,
+	): string {
+		const changes = diff.diffLines(oldContent, newContent, { newlineIsToken: true })
+
+		let result = `--- ${filename}\n+++ ${filename}\n`
+		let currentLine = 1
+		let inChangeBlock = false
+		let contextBuffer: string[] = []
+		let changeLines: string[] = []
+
+		const flushContextBuffer = () => {
+			if (contextBuffer.length > 0) {
+				result += contextBuffer.join("\n") + "\n"
+				contextBuffer = []
+			}
+		}
+
+		const addContextSeparator = () => {
+			if (result && !result.endsWith("...\n")) {
+				result += "...\n"
+			}
+		}
+
+		for (const part of changes) {
+			const lines = part.value.split("\n")
+			// Remove empty string that split adds when there's a trailing newline
+			if (lines[lines.length - 1] === "") {
+				lines.pop()
+			}
+
+			if (part.added || part.removed) {
+				if (!inChangeBlock) {
+					inChangeBlock = true
+					// Add context before the change
+					flushContextBuffer()
+					addContextSeparator()
+				}
+
+				const prefix = part.added ? "+" : "-"
+				changeLines.push(...lines.map((line) => (line ? `${prefix} ${line}` : prefix)))
+			} else {
+				if (inChangeBlock) {
+					// We're transitioning from changed lines to unchanged lines
+					inChangeBlock = false
+
+					// Add the changed lines
+					result += changeLines.join("\n") + "\n"
+					changeLines = []
+
+					// Add context after the change
+					const contextAfter = lines.slice(0, contextLines)
+					if (contextAfter.length > 0) {
+						result += contextAfter.map((line) => `  ${line}`).join("\n") + "\n"
+					}
+					if (lines.length > contextLines) {
+						result += "...\n"
+					}
+				} else {
+					// We're in an unchanged section, buffer the context
+					if (contextBuffer.length < contextLines) {
+						// Add to buffer if we're still collecting context
+						contextBuffer.push(...lines.map((line) => `  ${line}`))
+						// Keep only the last contextLines
+						if (contextBuffer.length > contextLines) {
+							contextBuffer = contextBuffer.slice(-contextLines)
+							if (contextBuffer[0] !== "...\n") {
+								contextBuffer.unshift("...\n")
+							}
+						}
+					}
+				}
+				currentLine += lines.length
+			}
+		}
+
+		// Handle any remaining changed lines at the end
+		if (changeLines.length > 0) {
+			flushContextBuffer()
+			addContextSeparator()
+			result += changeLines.join("\n") + "\n"
+		}
+
+		return result
+	}
+}