diff --git a/frontend/src/lib/components/copilot/chat/ToolCodeDiffView.svelte b/frontend/src/lib/components/copilot/chat/ToolCodeDiffView.svelte
new file mode 100644
index 0000000000..6f74ff7a80
--- /dev/null
+++ b/frontend/src/lib/components/copilot/chat/ToolCodeDiffView.svelte
@@ -0,0 +1,170 @@
+<script lang="ts">
+	import type { ToolCodeDiff } from './shared'
+	import {
+		toolDiffLines,
+		visibleToolDiffRows,
+		type CharacterRange,
+		type ToolDiffLine
+	} from './toolCodeDiff'
+	import { toolCodeDiffLanguage } from './toolCodeDiffLanguage'
+	import { highlightedSourceLines } from './toolCodeDiffHighlight'
+	import hljs from 'highlight.js/lib/core'
+	import HighlightTheme from '$lib/components/HighlightTheme.svelte'
+	import { Button } from '$lib/components/common'
+
+	interface Props {
+		diff: ToolCodeDiff
+		streaming?: boolean
+		diffLines?: ToolDiffLine[]
+	}
+
+	let { diff, streaming = false, diffLines }: Props = $props()
+
+	let expansions = $state<Map<string, number>>(new Map())
+	const diffRows = $derived(diffLines ?? toolDiffLines(diff, streaming))
+	const visibleRows = $derived(visibleToolDiffRows(diffRows, expansions))
+	const language = $derived(toolCodeDiffLanguage(diff.lang))
+	const lastLines = $derived.by(() => {
+		let old = 0
+		let current = 0
+		for (const row of diffRows) {
+			if (row.oldLine) old = Math.max(old, row.oldLine)
+			if (row.newLine) current = Math.max(current, row.newLine)
+		}
+		return { old, current }
+	})
+	// Both number columns fit the longest line number, so it never runs into the +/- column.
+	const lineNumberWidth = $derived(
+		`max(0.875rem, ${String(Math.max(lastLines.old, lastLines.current)).length}ch)`
+	)
+	const highlighted = $derived.by(() => {
+		if (!hljs.getLanguage(language.name)) hljs.registerLanguage(language.name, language.register)
+
+		return {
+			original: highlightedSourceLines(diff.before, language.name, lastLines.old),
+			modified: highlightedSourceLines(diff.after, language.name, lastLines.current)
+		}
+	})
+
+	function highlightedLine(row: ToolDiffLine): string {
+		const source = row.kind === 'removed' ? highlighted.original : highlighted.modified
+		const lineNumber = row.kind === 'removed' ? row.oldLine : row.newLine
+		return lineNumber === undefined ? '' : (source[lineNumber - 1] ?? '')
+	}
+
+	function expand(key: string): void {
+		expansions = new Map(expansions).set(key, (expansions.get(key) ?? 0) + 1)
+	}
+
+	// Tab stops are measured from the line start, so a range's width is its end column minus its
+	// start column, never the width of the range text on its own. Counts UTF-16 units, the unit
+	// of Monaco's ranges and of `slice`, so an emoji ahead of a change does not shift it.
+	function displayColumns(content: string): number {
+		let columns = 0
+		for (let index = 0; index < content.length; index++) {
+			columns += content[index] === '\t' ? 4 - (columns % 4) : 1
+		}
+		return columns
+	}
+
+	function rangeStyle(content: string, range: CharacterRange): string {
+		const left = displayColumns(content.slice(0, range.start))
+		const width = range.extendsToEnd
+			? 'calc(100% - ' + left + 'ch)'
+			: displayColumns(content.slice(0, range.start + range.length)) - left + 'ch'
+		return 'left: ' + left + 'ch; width: ' + width
+	}
+</script>
+
+<HighlightTheme />
+
+<div
+	class="tool-code-diff max-h-[400px] overflow-auto bg-surface-tertiary text-xs leading-[18px] text-primary"
+	style="--line-number: {lineNumberWidth}"
+>
+	{#each visibleRows as row, index ('key' in row ? `${row.kind}:${row.key}` : `line:${row.oldLine}:${row.newLine}:${index}`)}
+		{#if row.kind === 'collapsed' || row.kind === 'omitted'}
+			<Button
+				unifiedSize="2xs"
+				variant="subtle"
+				btnClasses="grid min-h-7 w-full grid-cols-[var(--line-number)_calc(2.0625rem_+_var(--line-number))_minmax(0,1fr)] items-center gap-0 rounded-none bg-surface-secondary px-0 py-1 text-left text-xs font-normal leading-[18px] text-hint hover:bg-surface-hover"
+				onclick={() => expand(row.key)}
+			>
+				<span class="text-right">...</span>
+				<span class="grid grid-cols-[1.125rem_var(--line-number)_0.9375rem]">
+					<span></span><span class="text-right">...</span><span>...</span>
+				</span>
+				<span>{row.count} {row.kind === 'collapsed' ? 'unchanged lines' : 'more lines'}</span>
+			</Button>
+		{:else}
+			<div
+				class="diff-line-{row.kind} grid grid-cols-[var(--line-number)_calc(2.0625rem_+_var(--line-number))_minmax(0,1fr)]"
+			>
+				<span class="text-right text-hint">{row.oldLine ?? ''}</span>
+				<span class="grid grid-cols-[1.125rem_var(--line-number)_0.9375rem] text-hint">
+					<span></span><span class="text-right">{row.newLine ?? ''}</span><span
+						>{row.kind === 'added' ? '+' : row.kind === 'removed' ? '-' : ''}</span
+					>
+				</span>
+				<span
+					class="relative min-w-0 whitespace-pre {row.kind === 'added'
+						? 'bg-green-500/20'
+						: row.kind === 'removed'
+							? 'bg-red-500/20'
+							: ''}"
+				>
+					{#each row.changedRanges ?? [] as range}
+						<span
+							class="pointer-events-none absolute top-0 z-0 h-[18px] {row.kind === 'added'
+								? 'bg-green-500/25'
+								: 'bg-red-500/20'}"
+							style={rangeStyle(row.content, range)}
+						></span>
+					{/each}
+					<span class="relative z-[1]">{@html highlightedLine(row) || '&nbsp;'}</span>
+				</span>
+			</div>
+		{/if}
+	{/each}
+</div>
+
+<style>
+	.tool-code-diff {
+		font-family: Menlo, Monaco, 'Courier New', monospace;
+		/* Must match the tab stops `displayColumns` uses to place intraline highlights. */
+		tab-size: 4;
+	}
+
+	.diff-line-added > span:nth-child(-n + 2),
+	.diff-line-removed > span:nth-child(-n + 2) {
+		@apply text-hint;
+	}
+
+	.tool-code-diff :global(.hljs-keyword),
+	.tool-code-diff :global(.hljs-literal),
+	.tool-code-diff :global(.hljs-built_in) {
+		@apply text-blue-700;
+	}
+
+	.tool-code-diff :global(.hljs-string) {
+		@apply text-red-700;
+	}
+
+	.tool-code-diff :global(.hljs-title.class_),
+	.tool-code-diff :global(.hljs-type) {
+		@apply text-cyan-800;
+	}
+
+	.tool-code-diff :global(.hljs-title.function_) {
+		@apply text-primary;
+	}
+
+	.tool-code-diff :global(.hljs-attr),
+	.tool-code-diff :global(.hljs-subst) {
+		@apply text-primary;
+	}
+
+	.tool-code-diff :global(.hljs-number) {
+		@apply text-green-700;
+	}
+</style>
diff --git a/frontend/src/lib/components/copilot/chat/ToolDiffCard.svelte b/frontend/src/lib/components/copilot/chat/ToolDiffCard.svelte
new file mode 100644
index 0000000000..a2ab9d341c
--- /dev/null
+++ b/frontend/src/lib/components/copilot/chat/ToolDiffCard.svelte
@@ -0,0 +1,103 @@
+<script lang="ts">
+	import type { ToolDisplayMessage } from './shared'
+	import ChatCollapsibleCard from './ChatCollapsibleCard.svelte'
+	import ToolCodeDiffView from './ToolCodeDiffView.svelte'
+	import ToolPreviewCard from './ToolPreviewCard.svelte'
+	import {
+		argumentsCarryWholeFile,
+		diffLineCounts,
+		toolCodeDiff,
+		toolDiffLineCounts,
+		toolDiffLines
+	} from './toolCodeDiff'
+
+	interface Props {
+		message: ToolDisplayMessage
+	}
+
+	let { message }: Props = $props()
+
+	const diff = $derived(toolCodeDiff(message))
+	const isStreaming = $derived(Boolean(message.isStreamingArguments))
+	const hasDiff = $derived(Boolean(diff && (diff.before !== '' || diff.after !== '')))
+	// The card body is a bordered box: opened with nothing in it, it draws an empty line.
+	const hasBody = $derived(hasDiff || Boolean(message.error))
+
+	// Keyed by call id: a bare flag would carry the expansion onto the next message that
+	// reuses this instance. An active or failed call opens by itself, and remains open
+	// when it settles so the new diff does not disappear under the user. A call whose arguments
+	// are a whole file (`write_script`, full-code `edit_code`) opens only on failure. The key
+	// also changes on failure, so a collapse made while running cannot hide the error.
+	const toggleKey = $derived(`${message.tool_call_id}:${message.error ? 'error' : ''}`)
+	let toggled = $state<{ id: string; open: boolean } | undefined>(undefined)
+	const opensByDefault = $derived(
+		Boolean(
+			message.error ||
+				(hasDiff &&
+					!argumentsCarryWholeFile(message) &&
+					(message.isLoading || message.isQueued || message.isStreamingArguments))
+		)
+	)
+	$effect(() => {
+		if (opensByDefault && toggled?.id !== toggleKey) {
+			toggled = { id: toggleKey, open: true }
+		}
+	})
+	const expanded = $derived(
+		hasBody && (toggled?.id === toggleKey ? toggled.open : opensByDefault)
+	)
+	const lines = $derived(diff && expanded ? toolDiffLines(diff, isStreaming) : undefined)
+	const counts = $derived(
+		diff && expanded
+			? isStreaming
+				? diffLineCounts(diff, true)
+				: toolDiffLineCounts(lines ?? [])
+			: undefined
+	)
+
+	const isRunning = $derived(Boolean(message.isLoading && !message.needsConfirmation))
+	const showPreviewChip = $derived(
+		Boolean(message.previewCard && !message.isLoading && !message.error)
+	)
+</script>
+
+{#snippet headerRight()}
+	<div class="flex items-center gap-2 shrink-0">
+		{#if counts && (counts.added > 0 || counts.removed > 0)}
+			<span class="font-mono text-2xs">
+				<span class="text-green-600 dark:text-green-400">+{counts.added}</span>
+				<span class="text-red-600 dark:text-red-400">−{counts.removed}</span>
+			</span>
+		{/if}
+		{#if showPreviewChip && message.previewCard}
+			<ToolPreviewCard card={message.previewCard} />
+		{/if}
+	</div>
+{/snippet}
+
+<ChatCollapsibleCard
+	label={message.content}
+	{expanded}
+	onToggle={() => (toggled = { id: toggleKey, open: !expanded })}
+	toggleable={hasBody}
+	shimmer={isRunning}
+	class={message.isQueued && !message.error
+		? 'opacity-60 hover:opacity-100 transition-opacity'
+		: ''}
+	labelClass="truncate"
+	contentClass="p-0 overflow-hidden space-y-0"
+	{headerRight}
+>
+	{#if diff && hasDiff && expanded}
+		<ToolCodeDiffView {diff} diffLines={lines} streaming={isStreaming} />
+	{/if}
+	{#if message.error}
+		<div
+			class="px-3 py-2 text-2xs text-red-600 dark:text-red-400 whitespace-pre-wrap break-words {hasDiff
+				? 'border-t border-border-light'
+				: ''}"
+		>
+			{message.error}
+		</div>
+	{/if}
+</ChatCollapsibleCard>
diff --git a/frontend/src/lib/components/copilot/chat/ToolExecutionDisplay.svelte b/frontend/src/lib/components/copilot/chat/ToolExecutionDisplay.svelte
index 8d945f94d1..d5fee5a4d9 100644
--- a/frontend/src/lib/components/copilot/chat/ToolExecutionDisplay.svelte
+++ b/frontend/src/lib/components/copilot/chat/ToolExecutionDisplay.svelte
@@ -40,6 +40,8 @@
 	import ToolPreviewCard from './ToolPreviewCard.svelte'
 	import AskUserQuestionDisplay from './AskUserQuestionDisplay.svelte'
 	import RunScriptCard from './RunScriptCard.svelte'
+	import ToolDiffCard from './ToolDiffCard.svelte'
+	import { hasToolCodeDiff } from './toolCodeDiff'
 	import WebSearchSourcesDisplay from './WebSearchSourcesDisplay.svelte'
 	import ExpandableImage from '$lib/components/common/image/ExpandableImage.svelte'
 	import McpServerIcon from '$lib/components/mcp/McpServerIcon.svelte'
@@ -132,6 +134,7 @@
 	// A call that inspected a run rather than starting one gets the same card, bound to
 	// the job it named — what happened in a run reads the same either way.
 	const isRunCard = $derived(Boolean(message.runForm || message.inspectedRun))
+	const isDiffCard = $derived(Boolean(message.codeDiff) || hasToolCodeDiff(message.toolName))
 
 	// The preview chip sits on the header row (to the right of the tool-call text);
 	// shown once the tool settled, never while loading/erroring/awaiting confirmation.
@@ -170,6 +173,8 @@
 	</div>
 {:else if isRunCard}
 	<RunScriptCard {message} />
+{:else if isDiffCard}
+	<ToolDiffCard {message} />
 {:else if planState}
 	<!-- Same lean shape as a tool call below: a header row that collapses into the
 	     transcript, with everything else in one box under it. -->
diff --git a/frontend/src/lib/components/copilot/chat/global/core.test.ts b/frontend/src/lib/components/copilot/chat/global/core.test.ts
index 7a86e0ba76..a1755bcaa9 100644
--- a/frontend/src/lib/components/copilot/chat/global/core.test.ts
+++ b/frontend/src/lib/components/copilot/chat/global/core.test.ts
@@ -2024,6 +2024,24 @@ describe('global AI tools', () => {
 		})
 	})
 
+	it('records a creation diff for a new script draft', async () => {
+		const statuses: any[] = []
+
+		await callGlobalTool(
+			'write_script',
+			{
+				path: 'f/scripts/new-script',
+				language: 'python3',
+				content: 'print("hello")'
+			},
+			{ ...toolCallbacks, setToolStatus: (_toolId, status) => statuses.push(status) }
+		)
+
+		expect(statuses).toContainEqual({
+			codeDiff: { before: '', after: 'print("hello")', lang: 'python' }
+		})
+	})
+
 	it('tells a code app from a drag-and-drop app', async () => {
 		vi.mocked(AppService.listApps).mockResolvedValueOnce([
 			{ path: 'f/apps/code', summary: 'Code app', raw_app: true },
@@ -3080,12 +3098,17 @@ describe('global AI tools', () => {
 			kind: 'script'
 		} as any)
 
-		await callGlobalTool('write_script', {
-			path: 'f/scripts/existing',
-			summary: 'new summary',
-			language: 'bun',
-			content: 'new content'
-		})
+		const statuses: any[] = []
+		await callGlobalTool(
+			'write_script',
+			{
+				path: 'f/scripts/existing',
+				summary: 'new summary',
+				language: 'bun',
+				content: 'new content'
+			},
+			{ ...toolCallbacks, setToolStatus: (_toolId, status) => statuses.push(status) }
+		)
 
 		expect(
 			getBackendDraft<any>('script', 'f/scripts/existing', { workspace: WORKSPACE })
@@ -3097,6 +3120,9 @@ describe('global AI tools', () => {
 			content: 'new content',
 			language: 'bun'
 		})
+		expect(statuses).toContainEqual({
+			codeDiff: { before: 'old deployed content', after: 'new content', lang: 'typescript' }
+		})
 	})
 
 	describe('stale-draft deploy guard and rebase', () => {
diff --git a/frontend/src/lib/components/copilot/chat/global/core.ts b/frontend/src/lib/components/copilot/chat/global/core.ts
index 5b0490c724..a4581bd173 100644
--- a/frontend/src/lib/components/copilot/chat/global/core.ts
+++ b/frontend/src/lib/components/copilot/chat/global/core.ts
@@ -137,8 +137,10 @@ import {
 	type RunFormDisplay,
 	type Tool,
 	type ToolCallbacks,
+	type ToolCodeDiff,
 	type ToolDisplayAction
 } from '../shared'
+import { scriptLangToEditorLang } from '$lib/scripts'
 import { searchDocsTool, readDocsPageTool } from '../docs/core'
 import { createDbSchemaTool } from '../script/core'
 import type { ContextElement } from '../context'
@@ -5029,10 +5031,12 @@ function finishAppDraftWrite(
 function finishDraftWrite(
 	result: DraftPersistResult,
 	existed: boolean,
-	ctx: WriteDraftCtx
+	ctx: WriteDraftCtx,
+	codeDiff?: ToolCodeDiff
 ): string {
 	const failure = draftWriteFailure(result, ctx)
 	if (failure) return failure
+	if (codeDiff) ctx.toolCallbacks.setToolStatus(ctx.toolId, { codeDiff })
 	ctx.toolCallbacks.onItemModified?.(result.itemKind, result.storagePath)
 	maybeAttachPreviewCard(ctx, result.itemKind, result.item.path)
 	const stored = result.item
@@ -5078,7 +5082,11 @@ async function writeDraft<T, A>(
 	path: string,
 	args: A,
 	ctx: WriteDraftCtx,
-	opts: { triggerKind?: TriggerKind; override?: boolean } = {}
+	opts: {
+		triggerKind?: TriggerKind
+		override?: boolean
+		codeDiff?: ToolCodeDiff | ((base: T | undefined, draft: T) => ToolCodeDiff | undefined)
+	} = {}
 ): Promise<string> {
 	const { workspace } = ctx
 	startDraftWrite(ctx, type, path)
@@ -5097,7 +5105,8 @@ async function writeDraft<T, A>(
 		triggerKind: opts.triggerKind,
 		force: opts.override
 	})
-	return finishDraftWrite(result, existed, ctx)
+	const codeDiff = typeof opts.codeDiff === 'function' ? opts.codeDiff(base, draft) : opts.codeDiff
+	return finishDraftWrite(result, existed, ctx, codeDiff)
 }
 
 type ScriptDraftArgs = {
@@ -5150,8 +5159,21 @@ const SCRIPT_SPEC: WriteSpec<NewScript, ScriptDraftArgs> = {
 	}
 }
 
-function writeScriptDraft(args: ScriptDraftArgs, ctx: WriteDraftCtx): Promise<string> {
-	return writeDraft(SCRIPT_SPEC, 'script', args.path, args, ctx, { override: args.override })
+function writeScriptDraft(
+	args: ScriptDraftArgs,
+	ctx: WriteDraftCtx,
+	codeDiff?: ToolCodeDiff
+): Promise<string> {
+	return writeDraft(SCRIPT_SPEC, 'script', args.path, args, ctx, {
+		override: args.override,
+		codeDiff:
+			codeDiff ??
+			((base, draft) => ({
+				before: base?.content ?? '',
+				after: draft.content,
+				lang: scriptLangToEditorLang(draft.language)
+			}))
+	})
 }
 
 type FlowDraftArgs = {
@@ -5368,7 +5390,8 @@ async function editScript(
 			language: base.language,
 			content: updated
 		},
-		ctx
+		ctx,
+		{ before: base.content, after: updated, lang: scriptLangToEditorLang(base.language) }
 	)
 }
 
diff --git a/frontend/src/lib/components/copilot/chat/script/core.test.ts b/frontend/src/lib/components/copilot/chat/script/core.test.ts
new file mode 100644
index 0000000000..f235ae3796
--- /dev/null
+++ b/frontend/src/lib/components/copilot/chat/script/core.test.ts
@@ -0,0 +1,50 @@
+import { describe, expect, it, vi } from 'vitest'
+
+vi.mock('monaco-editor', () => ({
+	editor: {},
+	languages: {},
+	KeyCode: {},
+	Uri: { parse: (value: string) => ({ toString: () => value }) },
+	MarkerSeverity: { Error: 8, Warning: 4, Info: 2, Hint: 1 }
+}))
+vi.mock('@codingame/monaco-vscode-standalone-typescript-language-features', () => ({
+	getTypeScriptWorker: async () => async () => ({}),
+	typescriptVersion: 'test'
+}))
+vi.mock('@codingame/monaco-vscode-languages-service-override', () => ({ default: () => ({}) }))
+vi.mock('$lib/components/vscode', () => ({}))
+
+import { editCodeToolWithDiff } from './core'
+
+describe('editCodeToolWithDiff', () => {
+	it('records the complete script diff after applying replacements', async () => {
+		const statuses: unknown[] = []
+		const applyCode = vi.fn(async () => {})
+
+		await editCodeToolWithDiff.fn({
+			args: { diffs: [{ old_string: 'return 1', new_string: 'return 2' }] },
+			helpers: {
+				getScriptOptions: () => ({
+					code: 'export function main() {\n\treturn 1\n}',
+					lang: 'bun',
+					path: 'f/example',
+					args: {}
+				}),
+				applyCode
+			},
+			toolCallbacks: {
+				setToolStatus: (_toolId, status) => statuses.push(status)
+			},
+			toolId: 'edit-1'
+		})
+
+		expect(applyCode).toHaveBeenCalledTimes(2)
+		expect(statuses.at(-1)).toMatchObject({
+			codeDiff: {
+				before: 'export function main() {\n\treturn 1\n}',
+				after: 'export function main() {\n\treturn 2\n}',
+				lang: 'typescript'
+			}
+		})
+	})
+})
diff --git a/frontend/src/lib/components/copilot/chat/script/core.ts b/frontend/src/lib/components/copilot/chat/script/core.ts
index 622f182eae..0c8b3f9a0e 100644
--- a/frontend/src/lib/components/copilot/chat/script/core.ts
+++ b/frontend/src/lib/components/copilot/chat/script/core.ts
@@ -27,6 +27,7 @@ import type { ReviewChangesOpts } from '../monaco-adapter'
 import { copilotInfo, getCurrentModel } from '$lib/aiStore'
 import { get } from 'svelte/store'
 import { getDbSchemas } from '$lib/components/apps/components/display/dbtable/metadata'
+import { scriptLangToEditorLang } from '$lib/scripts'
 import { getScriptPrompt, getWorkflowAsCodePrompt } from '$system_prompts'
 
 // Score threshold for npm packages search filtering
@@ -826,7 +827,12 @@ export const editCodeToolWithDiff: Tool<ScriptChatHelpers> = {
 
 			toolCallbacks.setToolStatus(toolId, {
 				content: `Code changes applied`,
-				result: 'Success'
+				result: 'Success',
+				codeDiff: {
+					before: oldCode,
+					after: updatedCode,
+					lang: scriptLangToEditorLang(scriptOptions.lang)
+				}
 			})
 			return `Applied changes to the script editor.`
 		} catch (error) {
@@ -878,7 +884,12 @@ export const editCodeTool: Tool<ScriptChatHelpers> = {
 
 			toolCallbacks.setToolStatus(toolId, {
 				content: 'Code changes applied',
-				result: 'Success'
+				result: 'Success',
+				codeDiff: {
+					before: oldCode,
+					after: args.code,
+					lang: scriptLangToEditorLang(scriptOptions.lang)
+				}
 			})
 			return 'Code has been applied to the script editor.'
 		} catch (error) {
diff --git a/frontend/src/lib/components/copilot/chat/shared.ts b/frontend/src/lib/components/copilot/chat/shared.ts
index 772284865d..83a51b527c 100644
--- a/frontend/src/lib/components/copilot/chat/shared.ts
+++ b/frontend/src/lib/components/copilot/chat/shared.ts
@@ -610,6 +610,13 @@ export type WebSearchSource = {
 	title?: string
 }
 
+export type ToolCodeDiff = {
+	before: string
+	after: string
+	/** Monaco language id. */
+	lang: string
+}
+
 /** The result shape any tool returns to have it rendered as a web search card. */
 export type WebSearchResult = {
 	sources: WebSearchSource[]
@@ -725,6 +732,9 @@ export type ToolDisplayMessage = {
 	 * always-visible card that opens (or focuses) the item's preview in the
 	 * session side panel. Set only for session chats — the side panel is their surface. */
 	previewCard?: { kind: PreviewCardKind; path: string }
+	/** Whole text on both sides of an edit this call saved. Kept in full rather than as a
+	 * reference: drafts are overwritten in place, so the sides cannot be fetched back later. */
+	codeDiff?: ToolCodeDiff
 	planArtifactId?: string
 	/** The version this card's proposal wrote, so a card scrolled far up still opens the plan
 	 * it proposed rather than what the document became. */
diff --git a/frontend/src/lib/components/copilot/chat/toolCodeDiff.test.ts b/frontend/src/lib/components/copilot/chat/toolCodeDiff.test.ts
new file mode 100644
index 0000000000..5077585a4f
--- /dev/null
+++ b/frontend/src/lib/components/copilot/chat/toolCodeDiff.test.ts
@@ -0,0 +1,411 @@
+import { describe, expect, it } from 'vitest'
+import hljs from 'highlight.js/lib/core'
+import {
+	argumentsCarryWholeFile,
+	diffLineCounts,
+	hasToolCodeDiff,
+	toolCodeDiff,
+	toolDiffLines,
+	visibleToolDiffRows
+} from './toolCodeDiff'
+import { highlightedSourceLines } from './toolCodeDiffHighlight'
+import { TOOL_CODE_DIFF_LANGUAGES, toolCodeDiffLanguage } from './toolCodeDiffLanguage'
+import type { ToolDisplayMessage } from './shared'
+
+function message(overrides: Partial<ToolDisplayMessage>): ToolDisplayMessage {
+	return {
+		role: 'tool',
+		tool_call_id: 'call-1',
+		content: 'Updated script "f/example" as a draft',
+		...overrides
+	}
+}
+
+describe('toolCodeDiff', () => {
+	it('uses the saved full script sides after an edit succeeds', () => {
+		const codeDiff = {
+			before: 'const before = true\n',
+			after: 'const after = true\n',
+			lang: 'typescript'
+		}
+
+		expect(
+			toolCodeDiff(
+				message({
+					toolName: 'edit_script',
+					parameters: { old_string: 'before', new_string: 'after' },
+					codeDiff
+				})
+			)
+		).toBe(codeDiff)
+	})
+
+	it('uses edit arguments until the full script sides are available', () => {
+		expect(
+			toolCodeDiff(
+				message({
+					toolName: 'edit_script',
+					parameters: { old_string: 'before', new_string: 'after' }
+				})
+			)
+		).toEqual({ before: 'before', after: 'after', lang: 'plaintext' })
+	})
+
+	it('renders streamed edit arguments before their JSON object is complete', () => {
+		expect(
+			toolCodeDiff(
+				message({
+					toolName: 'edit_script',
+					parameters: '{"old_string":"before\\nline","new_string":"after'
+				})
+			)
+		).toEqual({ before: 'before\nline', after: 'after', lang: 'plaintext' })
+	})
+
+	it('uses each replacement from an in-editor edit while its arguments stream', () => {
+		expect(
+			toolCodeDiff(
+				message({
+					toolName: 'edit_code',
+					parameters: {
+						diffs: [
+							{ old_string: 'first old', new_string: 'first new' },
+							{ old_string: 'second old', new_string: 'second new' }
+						]
+					}
+				})
+			)
+		).toEqual({
+			before: 'first old\nsecond old',
+			after: 'first new\nsecond new',
+			lang: 'plaintext'
+		})
+	})
+
+	it('uses each replacement while in-editor edit arguments are incomplete', () => {
+		expect(
+			toolCodeDiff(
+				message({
+					toolName: 'edit_code',
+					parameters:
+						'{"diffs":[{"old_string":"first old","new_string":"first new"},{"old_string":"second old","new_string":"second new'
+				})
+			)
+		).toEqual({
+			before: 'first old\nsecond old',
+			after: 'first new\nsecond new',
+			lang: 'plaintext'
+		})
+	})
+
+	it('shows a streamed full-code editor update without replacements', () => {
+		expect(
+			toolCodeDiff(message({ toolName: 'edit_code', parameters: '{"code":"console.log(1)' }))
+		).toEqual({ before: '', after: 'console.log(1)', lang: 'plaintext' })
+	})
+
+	it('shows streamed full-script updates before the saved diff is available', () => {
+		expect(
+			toolCodeDiff(
+				message({ toolName: 'write_script', parameters: '{"content":"print(\\\"hello\\\")' })
+			)
+		).toEqual({ before: '', after: 'print("hello")', lang: 'plaintext' })
+	})
+
+	it('only marks tools with a defined argument diff as diff-capable', () => {
+		expect(hasToolCodeDiff('edit_script')).toBe(true)
+		expect(hasToolCodeDiff('edit_code')).toBe(true)
+		expect(hasToolCodeDiff('write_script')).toBe(true)
+		expect(hasToolCodeDiff('__proto__')).toBe(false)
+		expect(hasToolCodeDiff('toString')).toBe(false)
+		expect(hasToolCodeDiff(undefined)).toBe(false)
+	})
+
+	it('ignores tool names inherited from Object.prototype', () => {
+		expect(toolCodeDiff(message({ toolName: 'constructor', parameters: {} }))).toBeUndefined()
+	})
+
+	it('counts added and removed lines independently', () => {
+		expect(
+			diffLineCounts({
+				before: 'one\ntwo\nthree\n',
+				after: 'one\nnew\nthree\nfour\n',
+				lang: 'plaintext'
+			})
+		).toEqual({
+			added: 2,
+			removed: 1
+		})
+	})
+
+	it('keeps a final newline as a diffable line', () => {
+		expect(toolDiffLines({ before: 'one', after: 'one\n', lang: 'plaintext' })).toEqual([
+			{ kind: 'context', content: 'one', oldLine: 1, newLine: 1 },
+			{ kind: 'added', content: '', newLine: 2 }
+		])
+	})
+
+	it('does not add a final-newline row when creating or deleting a file', () => {
+		expect(toolDiffLines({ before: '', after: 'one\n', lang: 'plaintext' })).toEqual([
+			{ kind: 'added', content: 'one', newLine: 1, changedRanges: [{ start: 0, length: 3 }] }
+		])
+		expect(toolDiffLines({ before: 'one\n', after: '', lang: 'plaintext' })).toEqual([
+			{ kind: 'removed', content: 'one', oldLine: 1, changedRanges: [{ start: 0, length: 3 }] }
+		])
+	})
+
+	it('keeps both gutters correct across removed and added lines', () => {
+		expect(
+			toolDiffLines({
+				before: 'one\ntwo\nthree\n',
+				after: 'one\nnew\nthree\nfour\n',
+				lang: 'plaintext'
+			})
+		).toEqual([
+			{ kind: 'context', content: 'one', oldLine: 1, newLine: 1 },
+			{
+				kind: 'removed',
+				content: 'two',
+				oldLine: 2,
+				newLine: undefined,
+				changedRanges: [{ start: 0, length: 3 }]
+			},
+			{
+				kind: 'added',
+				content: 'new',
+				oldLine: undefined,
+				newLine: 2,
+				changedRanges: [{ start: 0, length: 3 }]
+			},
+			{ kind: 'context', content: 'three', oldLine: 3, newLine: 3 },
+			{
+				kind: 'added',
+				content: 'four',
+				oldLine: undefined,
+				newLine: 4,
+				changedRanges: [{ start: 0, length: 4 }]
+			}
+		])
+	})
+
+	it('keeps unchanged trailing lines out of the change counts', () => {
+		const diff = { before: 'one\ntwo\nthree', after: 'one\nnew\nthree', lang: 'plaintext' }
+
+		expect(diffLineCounts(diff)).toEqual({ added: 1, removed: 1 })
+		expect(toolDiffLines(diff).filter((line) => line.content === 'three')).toEqual([
+			{ kind: 'context', content: 'three', oldLine: 3, newLine: 3 }
+		])
+	})
+
+	it('does not report changes for identical files', () => {
+		const diff = { before: 'one\ntwo\nthree', after: 'one\ntwo\nthree', lang: 'plaintext' }
+
+		expect(diffLineCounts(diff)).toEqual({ added: 0, removed: 0 })
+		expect(toolDiffLines(diff)).toEqual([
+			{ kind: 'context', content: 'one', oldLine: 1, newLine: 1 },
+			{ kind: 'context', content: 'two', oldLine: 2, newLine: 2 },
+			{ kind: 'context', content: 'three', oldLine: 3, newLine: 3 }
+		])
+	})
+
+	it('renders a newly created file', () => {
+		expect(toolDiffLines({ before: '', after: 'print("hello")', lang: 'python' })).toEqual([
+			{
+				kind: 'added',
+				content: 'print("hello")',
+				oldLine: undefined,
+				newLine: 1,
+				changedRanges: [{ start: 0, length: 14 }]
+			}
+		])
+	})
+
+	it('renders a deleted file', () => {
+		expect(toolDiffLines({ before: 'print("goodbye")', after: '', lang: 'python' })).toEqual([
+			{
+				kind: 'removed',
+				content: 'print("goodbye")',
+				oldLine: 1,
+				newLine: undefined,
+				changedRanges: [{ start: 0, length: 16 }]
+			}
+		])
+	})
+
+	it('uses Monaco line alignment for reordered lines', () => {
+		expect(
+			toolDiffLines({
+				before: 'const a = 1\nconst b = 2',
+				after: 'const b = 2\nconst a = 1',
+				lang: 'typescript'
+			})
+		).toEqual([
+			{
+				kind: 'added',
+				content: 'const b = 2',
+				newLine: 1,
+				changedRanges: [{ start: 0, length: 11, extendsToEnd: true }]
+			},
+			{ kind: 'context', content: 'const a = 1', oldLine: 1, newLine: 2 },
+			{
+				kind: 'removed',
+				content: 'const b = 2',
+				oldLine: 2,
+				changedRanges: [{ start: 0, length: 11 }]
+			}
+		])
+	})
+
+	it('avoids expensive alignment while arguments stream', () => {
+		const before = Array.from({ length: 1_000 }, (_, index) => `before ${index}`).join('\n')
+		const after = Array.from({ length: 1_000 }, (_, index) => `after ${index}`).join('\n')
+
+		expect(toolDiffLines({ before, after, lang: 'plaintext' }, true)).toHaveLength(400)
+		expect(diffLineCounts({ before, after, lang: 'plaintext' }, true)).toEqual({
+			added: 1_000,
+			removed: 1_000
+		})
+	})
+
+	it('diffs a small edit in a large file', () => {
+		const beforeLines = Array.from({ length: 600 }, (_, index) => `line ${index}`)
+		const afterLines = beforeLines.map((line, index) => (index === 400 ? 'changed' : line))
+		const diff = { before: beforeLines.join('\n'), after: afterLines.join('\n'), lang: 'plaintext' }
+		const lines = toolDiffLines(diff)
+
+		expect(lines.filter((line) => line.kind !== 'context')).toMatchObject([
+			{ kind: 'removed', content: 'line 400', oldLine: 401 },
+			{ kind: 'added', content: 'changed', newLine: 401 }
+		])
+		expect(lines[399]).toEqual({ kind: 'context', content: 'line 399', oldLine: 400, newLine: 400 })
+		expect(diffLineCounts(diff)).toEqual({ added: 1, removed: 1 })
+	})
+
+	it('reports a large rewrite as a whole-file change', () => {
+		const before = Array.from({ length: 1_000 }, (_, index) => `before ${index}`).join('\n')
+		const after = Array.from({ length: 1_000 }, (_, index) => `after ${index}`).join('\n')
+
+		expect(diffLineCounts({ before, after, lang: 'plaintext' })).toEqual({
+			added: 1_000,
+			removed: 1_000
+		})
+	})
+
+	it('keeps an empty streamed replacement paired with its own edit', () => {
+		const parameters =
+			'{"diffs":[{"old_string":"a","new_string":""},{"old_string":"b","new_string":"c"}]}'
+
+		expect(toolCodeDiff(message({ toolName: 'edit_code', parameters }))).toEqual({
+			before: 'a\nb',
+			after: '\nc',
+			lang: 'plaintext'
+		})
+	})
+
+	it('recognizes calls whose arguments are a whole file', () => {
+		const call = (
+			toolName: string,
+			parameters: unknown,
+			codeDiff?: ToolDisplayMessage['codeDiff']
+		) => argumentsCarryWholeFile(message({ toolName, parameters, codeDiff }))
+
+		expect(call('write_script', { path: 'f/a', content: '' })).toBe(true)
+		expect(call('edit_code', '{"code":"x')).toBe(true)
+		expect(
+			call('edit_code', { code: 'x' }, { before: 'old', after: 'x', lang: 'typescript' })
+		).toBe(true)
+		expect(call('edit_code', { diffs: [{ old_string: 'a', new_string: 'b' }] })).toBe(false)
+		expect(call('edit_script', { old_string: 'a', new_string: 'b' })).toBe(false)
+	})
+
+	it('shows each row of a small edit once', () => {
+		const lines = toolDiffLines({ before: 'a\nb\nc\n', after: 'a\nB\nc\n', lang: 'plaintext' })
+
+		expect(visibleToolDiffRows(lines, new Map())).toEqual(lines)
+		expect(lines.map((line) => line.kind)).toEqual(['context', 'removed', 'added', 'context'])
+	})
+
+	it('bounds the rows of a large rewrite', () => {
+		const before = Array.from({ length: 1_000 }, (_, index) => `before ${index}`).join('\n')
+		const after = Array.from({ length: 1_000 }, (_, index) => `after ${index}`).join('\n')
+		const lines = toolDiffLines({ before, after, lang: 'plaintext' })
+		const rows = visibleToolDiffRows(lines, new Map())
+
+		expect(rows).toHaveLength(802)
+		expect(rows[400]).toEqual({ kind: 'omitted', key: 'removed:0', count: 600 })
+		expect(rows[401]).toMatchObject({ kind: 'added', newLine: 1 })
+		expect(rows[801]).toEqual({ kind: 'omitted', key: 'added:1000', count: 600 })
+
+		// One click on a run reveals 400 more rows; the overall cap still holds until `end` is clicked.
+		const expanded = visibleToolDiffRows(lines, new Map([['removed:0', 1]]))
+		expect(expanded).toHaveLength(1_001)
+		expect(expanded[800]).toEqual({ kind: 'omitted', key: 'removed:0', count: 200 })
+		expect(expanded[1_000]).toEqual({ kind: 'omitted', key: 'end', count: 801 })
+		expect(
+			visibleToolDiffRows(
+				lines,
+				new Map([
+					['removed:0', 1],
+					['end', 1]
+				])
+			)
+		).toHaveLength(1_202)
+	})
+
+	it('highlights each line within the scope of the whole source', () => {
+		const typescript = toolCodeDiffLanguage('typescript')
+		hljs.registerLanguage(typescript.name, typescript.register)
+		const code = 'const s = `a\nb`\nconst n = 1\n'
+
+		const lines = highlightedSourceLines(code, typescript.name)
+		expect(lines).toHaveLength(3)
+		expect(lines[0]).toBe(
+			'<span class="hljs-keyword">const</span> s = <span class="hljs-string">`a</span>'
+		)
+		expect(lines[1]).toMatch(/^<span class="hljs-string">b`<\/span>/)
+		expect(highlightedSourceLines(code, typescript.name, 2)).toEqual(lines.slice(0, 2))
+		expect(highlightedSourceLines('a\nb', typescript.name)).toEqual(['a', 'b'])
+		expect(highlightedSourceLines('a\nb\n', typescript.name)).toEqual(['a', 'b'])
+		expect(highlightedSourceLines('', typescript.name)).toEqual([])
+
+		const large = Array.from({ length: 2_001 }, () => 'const a = 1 < 2').join('\n') + '\n'
+		const plain = highlightedSourceLines(large, typescript.name)
+		expect(plain).toHaveLength(2_001)
+		expect(plain[0]).toBe('const a = 1 &lt; 2')
+
+		const minified = highlightedSourceLines('x'.repeat(100_001), typescript.name)
+		expect(minified).toEqual(['x'.repeat(10_000) + '…'])
+	})
+
+	it('has a highlighter for every supported editor language', () => {
+		expect(TOOL_CODE_DIFF_LANGUAGES).toEqual(
+			expect.arrayContaining([
+				'typescript',
+				'javascript',
+				'python',
+				'json',
+				'yaml',
+				'sql',
+				'shell',
+				'powershell',
+				'php',
+				'rust',
+				'graphql',
+				'csharp',
+				'nu',
+				'java',
+				'r',
+				'go',
+				'ruby',
+				'plaintext'
+			])
+		)
+		for (const language of TOOL_CODE_DIFF_LANGUAGES) {
+			const highlighter = toolCodeDiffLanguage(language)
+			hljs.registerLanguage(highlighter.name, highlighter.register)
+			expect(() =>
+				hljs.highlight('const changed = true', { language: highlighter.name })
+			).not.toThrow()
+		}
+	})
+})
diff --git a/frontend/src/lib/components/copilot/chat/toolCodeDiff.ts b/frontend/src/lib/components/copilot/chat/toolCodeDiff.ts
new file mode 100644
index 0000000000..2e5d8bf13c
--- /dev/null
+++ b/frontend/src/lib/components/copilot/chat/toolCodeDiff.ts
@@ -0,0 +1,381 @@
+// This is Monaco's advanced diff engine; it supplies the same multi-line inner ranges.
+import { DefaultLinesDiffComputer } from '@codingame/monaco-vscode-api/vscode/vs/editor/common/diff/defaultLinesDiffComputer/defaultLinesDiffComputer'
+import type { ToolCodeDiff, ToolDisplayMessage } from './shared'
+
+// Builds a diff from a call's own arguments, for the moments the saved `codeDiff` does not
+// exist: while the arguments stream, when the call failed, and for transcripts saved before
+// calls recorded one. Argument diffs have no language or surrounding lines.
+const ARGS_DIFF_BY_TOOL: Record<string, (params: unknown) => ToolCodeDiff | undefined> = {
+	edit_script: (params) => argumentDiff(streamingEditArguments(params)),
+	write_script: (params) => fullContentArgumentDiff(params, 'content'),
+	edit_code: (params) => {
+		if (
+			params &&
+			typeof params === 'object' &&
+			Array.isArray((params as { diffs?: unknown }).diffs)
+		) {
+			const diffs = (params as { diffs: unknown[] }).diffs
+			return argumentDiffs(diffs.map(streamingEditArguments))
+		}
+		if (typeof params === 'string') {
+			return (
+				argumentDiffs(streamingEditArgumentsArray(params)) ??
+				fullContentArgumentDiff(params, 'code')
+			)
+		}
+		return argumentDiff(streamingEditArguments(params)) ?? fullContentArgumentDiff(params, 'code')
+	}
+}
+
+function fullContentArgumentDiff(params: unknown, key: string): ToolCodeDiff | undefined {
+	const content =
+		params && typeof params === 'object'
+			? (params as Record<string, unknown>)[key]
+			: typeof params === 'string'
+				? partialJsonString(params, key)
+				: undefined
+	return typeof content === 'string' ? { before: '', after: content, lang: 'plaintext' } : undefined
+}
+
+function argumentDiff(
+	edit: { old_string?: string; new_string?: string } | undefined
+): ToolCodeDiff | undefined {
+	return typeof edit?.old_string === 'string' || typeof edit?.new_string === 'string'
+		? {
+				before: typeof edit.old_string === 'string' ? edit.old_string : '',
+				after: typeof edit.new_string === 'string' ? edit.new_string : '',
+				lang: 'plaintext'
+			}
+		: undefined
+}
+
+function argumentDiffs(
+	edits: ({ old_string?: string; new_string?: string } | undefined)[]
+): ToolCodeDiff | undefined {
+	const diffs = edits.map(argumentDiff).filter((diff): diff is ToolCodeDiff => diff !== undefined)
+	if (diffs.length === 0) return undefined
+	return {
+		before: diffs.map((diff) => diff.before).join('\n'),
+		after: diffs.map((diff) => diff.after).join('\n'),
+		lang: 'plaintext'
+	}
+}
+
+function streamingEditArguments(
+	params: unknown
+): { old_string?: string; new_string?: string } | undefined {
+	if (params && typeof params === 'object')
+		return params as { old_string?: string; new_string?: string }
+	if (typeof params !== 'string') return undefined
+
+	return {
+		old_string: partialJsonString(params, 'old_string'),
+		new_string: partialJsonString(params, 'new_string')
+	}
+}
+
+function streamingEditArgumentsArray(
+	partialJson: string
+): { old_string?: string; new_string?: string }[] {
+	const oldStrings = partialJsonStrings(partialJson, 'old_string')
+	const newStrings = partialJsonStrings(partialJson, 'new_string')
+	return Array.from({ length: Math.max(oldStrings.length, newStrings.length) }, (_, index) => ({
+		old_string: oldStrings[index],
+		new_string: newStrings[index]
+	}))
+}
+
+function partialJsonString(partialJson: string, key: string): string | undefined {
+	return partialJsonStrings(partialJson, key)[0]
+}
+
+function partialJsonStrings(partialJson: string, key: string): string[] {
+	const matches = partialJson.matchAll(new RegExp(`"${key}"\\s*:\\s*"((?:[^"\\\\]|\\\\.)*)`, 'g'))
+	const result: string[] = []
+	// An empty string is a real value (a deletion): skipping it would pair the next edit's
+	// `new_string` with this edit's `old_string`.
+	for (const match of matches) {
+		try {
+			result.push(JSON.parse(`"${match[1]}"`))
+		} catch {
+			continue
+		}
+	}
+	return result
+}
+
+export function hasToolCodeDiff(toolName: string | undefined): boolean {
+	return toolName !== undefined && Object.hasOwn(ARGS_DIFF_BY_TOOL, toolName)
+}
+
+export function toolCodeDiff(message: ToolDisplayMessage): ToolCodeDiff | undefined {
+	if (message.codeDiff) return message.codeDiff
+	if (!message.toolName) return undefined
+	return Object.hasOwn(ARGS_DIFF_BY_TOOL, message.toolName)
+		? ARGS_DIFF_BY_TOOL[message.toolName](message.parameters)
+		: undefined
+}
+
+// Decided by tool and argument shape, never by the saved `codeDiff`: an overwrite's saved diff
+// has a before side, and clearing a script leaves both argument sides empty, yet both calls sent
+// a whole file.
+export function argumentsCarryWholeFile(message: ToolDisplayMessage): boolean {
+	if (message.toolName === 'write_script') return true
+	if (message.toolName !== 'edit_code') return false
+	const params = message.parameters
+	if (typeof params === 'string') return /"code"\s*:/.test(params)
+	return Boolean(params && typeof params === 'object' && 'code' in params)
+}
+
+export function diffLineCounts(
+	diff: ToolCodeDiff,
+	streaming = false
+): { added: number; removed: number } {
+	if (streaming) {
+		return { added: lines(diff.after).length, removed: lines(diff.before).length }
+	}
+
+	return toolDiffLineCounts(toolDiffLines(diff))
+}
+
+export function toolDiffLineCounts(lines: ToolDiffLine[]): { added: number; removed: number } {
+	let added = 0
+	let removed = 0
+	for (const line of lines) {
+		if (line.kind === 'added') added++
+		else if (line.kind === 'removed') removed++
+	}
+	return { added, removed }
+}
+
+export type ToolDiffLine = {
+	kind: 'context' | 'added' | 'removed'
+	content: string
+	oldLine?: number
+	newLine?: number
+	changedRanges?: CharacterRange[]
+}
+
+export type CharacterRange = { start: number; length: number; extendsToEnd?: boolean }
+
+const STREAMING_PREVIEW_LINES = 200
+
+function lines(value: string): string[] {
+	const result = value.split('\n')
+	if (result.at(-1) === '') result.pop()
+	return result
+}
+
+function hasFinalNewline(value: string): boolean {
+	return value.endsWith('\n')
+}
+
+export function toolDiffLines(diff: ToolCodeDiff, streaming = false): ToolDiffLine[] {
+	const before = lines(diff.before)
+	const after = lines(diff.after)
+	if (streaming) return streamingDiffLines(before, after)
+
+	const result: ToolDiffLine[] = []
+	const monacoBefore = monacoLines(diff.before)
+	const monacoAfter = monacoLines(diff.after)
+	// Runs on the UI thread. Cost follows the amount changed, not file size: only a large
+	// rewrite reaches the cap, and Monaco then reports the whole file as one change.
+	const changes = new DefaultLinesDiffComputer().computeDiff(monacoBefore, monacoAfter, {
+		ignoreTrimWhitespace: false,
+		maxComputationTimeMs: 200,
+		computeMoves: false,
+		extendToSubwords: false
+	}).changes
+	const removedRanges = new Map<number, CharacterRange[]>()
+	const addedRanges = new Map<number, CharacterRange[]>()
+	for (const change of changes) {
+		for (const innerChange of change.innerChanges ?? []) {
+			addCharacterRanges(removedRanges, monacoBefore, innerChange.originalRange)
+			addCharacterRanges(addedRanges, monacoAfter, innerChange.modifiedRange)
+		}
+	}
+
+	let oldIndex = 0
+	let newIndex = 0
+	for (const change of changes) {
+		const oldStart = change.original.startLineNumber - 1
+		const newStart = change.modified.startLineNumber - 1
+		appendContextLines(
+			result,
+			before,
+			after,
+			oldIndex,
+			newIndex,
+			Math.min(oldStart - oldIndex, newStart - newIndex)
+		)
+		appendChangedLines(
+			result,
+			'removed',
+			before,
+			oldStart,
+			change.original.endLineNumberExclusive - 1,
+			removedRanges
+		)
+		appendChangedLines(
+			result,
+			'added',
+			after,
+			newStart,
+			change.modified.endLineNumberExclusive - 1,
+			addedRanges
+		)
+		oldIndex = Math.min(change.original.endLineNumberExclusive - 1, before.length)
+		newIndex = Math.min(change.modified.endLineNumberExclusive - 1, after.length)
+	}
+	const trailingContext = Math.max(0, Math.min(before.length - oldIndex, after.length - newIndex))
+	appendContextLines(result, before, after, oldIndex, newIndex, trailingContext)
+	oldIndex += trailingContext
+	newIndex += trailingContext
+	appendChangedLines(result, 'removed', before, oldIndex, before.length, removedRanges)
+	appendChangedLines(result, 'added', after, newIndex, after.length, addedRanges)
+	if (
+		diff.before !== '' &&
+		diff.after !== '' &&
+		hasFinalNewline(diff.before) !== hasFinalNewline(diff.after)
+	) {
+		result.push(
+			hasFinalNewline(diff.before)
+				? { kind: 'removed', content: '', oldLine: before.length + 1 }
+				: { kind: 'added', content: '', newLine: after.length + 1 }
+		)
+	}
+
+	return result
+}
+
+export type VisibleToolDiffRow =
+	| ToolDiffLine
+	| { kind: 'collapsed'; key: string; count: number }
+	| { kind: 'omitted'; key: string; count: number }
+
+const CONTEXT_LINES = 3
+// Every visible row is a DOM row with no virtualization, and only context collapses: a whole-file
+// rewrite would otherwise render every line of both sides. Two capped runs fit under the overall
+// cap, so a rewrite still shows its added side. Each expansion of an omitted row reveals one more
+// cap's worth, so no single click renders an unbounded number of rows.
+const MAX_CHANGED_RUN_ROWS = 400
+const MAX_VISIBLE_ROWS = 1_000
+
+// `expansions` counts the clicks on each collapsed or omitted row, by its key.
+export function visibleToolDiffRows(
+	lines: ToolDiffLine[],
+	expansions: ReadonlyMap<string, number>
+): VisibleToolDiffRow[] {
+	const result: VisibleToolDiffRow[] = []
+	for (let index = 0; index < lines.length; ) {
+		const start = index
+		const kind = lines[start].kind
+		while (index < lines.length && lines[index].kind === kind) index++
+		const count = index - start
+
+		if (kind !== 'context') {
+			const key = `${kind}:${start}`
+			const shown = MAX_CHANGED_RUN_ROWS * (1 + (expansions.get(key) ?? 0))
+			result.push(...lines.slice(start, Math.min(index, start + shown)))
+			if (count > shown) result.push({ kind: 'omitted', key, count: count - shown })
+			continue
+		}
+
+		const key = `${lines[start].oldLine}:${lines[start].newLine}:${count}`
+		if (count <= CONTEXT_LINES * 2 + 1 || expansions.has(key)) {
+			result.push(...lines.slice(start, index))
+			continue
+		}
+		result.push(...lines.slice(start, start + CONTEXT_LINES))
+		result.push({ kind: 'collapsed', key, count: count - CONTEXT_LINES * 2 })
+		result.push(...lines.slice(index - CONTEXT_LINES, index))
+	}
+
+	const maxRows = MAX_VISIBLE_ROWS * (1 + (expansions.get('end') ?? 0))
+	if (result.length <= maxRows) return result
+	const hidden = result
+		.slice(maxRows)
+		.reduce((total, row) => total + ('count' in row ? row.count : 1), 0)
+	return [...result.slice(0, maxRows), { kind: 'omitted', key: 'end', count: hidden }]
+}
+
+function streamingDiffLines(before: string[], after: string[]): ToolDiffLine[] {
+	return [
+		...before
+			.slice(0, STREAMING_PREVIEW_LINES)
+			.map((content, index) => ({ kind: 'removed' as const, content, oldLine: index + 1 })),
+		...after
+			.slice(0, STREAMING_PREVIEW_LINES)
+			.map((content, index) => ({ kind: 'added' as const, content, newLine: index + 1 }))
+	]
+}
+
+function monacoLines(value: string): string[] {
+	const result = lines(value)
+	return result.length ? result : ['']
+}
+
+function appendContextLines(
+	result: ToolDiffLine[],
+	before: string[],
+	after: string[],
+	oldStart: number,
+	newStart: number,
+	count: number
+): void {
+	for (let index = 0; index < count; index++) {
+		result.push({
+			kind: 'context',
+			content: before[oldStart + index],
+			oldLine: oldStart + index + 1,
+			newLine: newStart + index + 1
+		})
+	}
+}
+
+function appendChangedLines(
+	result: ToolDiffLine[],
+	kind: 'added' | 'removed',
+	source: string[],
+	start: number,
+	end: number,
+	ranges: Map<number, CharacterRange[]>
+): void {
+	for (let index = start; index < Math.min(end, source.length); index++) {
+		const changedRanges = ranges.get(index + 1)
+		result.push({
+			kind,
+			content: source[index],
+			...(kind === 'added' ? { newLine: index + 1 } : { oldLine: index + 1 }),
+			...(changedRanges?.length ? { changedRanges } : {})
+		})
+	}
+}
+
+function addCharacterRanges(
+	ranges: Map<number, CharacterRange[]>,
+	lines: string[],
+	range: {
+		startLineNumber: number
+		startColumn: number
+		endLineNumber: number
+		endColumn: number
+	}
+): void {
+	for (
+		let lineNumber = range.startLineNumber;
+		lineNumber <= Math.min(range.endLineNumber, lines.length);
+		lineNumber++
+	) {
+		const content = lines[lineNumber - 1]
+		const start = lineNumber === range.startLineNumber ? range.startColumn - 1 : 0
+		const extendsToEnd = lineNumber < range.endLineNumber
+		const end = extendsToEnd ? content.length : range.endColumn - 1
+		if (end > start) {
+			ranges.set(lineNumber, [
+				...(ranges.get(lineNumber) ?? []),
+				{ start, length: end - start, ...(extendsToEnd ? { extendsToEnd } : {}) }
+			])
+		}
+	}
+}
diff --git a/frontend/src/lib/components/copilot/chat/toolCodeDiffHighlight.ts b/frontend/src/lib/components/copilot/chat/toolCodeDiffHighlight.ts
new file mode 100644
index 0000000000..b63d002e9e
--- /dev/null
+++ b/frontend/src/lib/components/copilot/chat/toolCodeDiffHighlight.ts
@@ -0,0 +1,57 @@
+import hljs from 'highlight.js/lib/core'
+import { escapeHtml } from '$lib/utils'
+
+// highlight.js runs on the UI thread at roughly 15 ms per 1,000 lines (50 KB), and must see the
+// whole source for multi-line strings and comments to scope correctly. Larger sides, including
+// minified one-line sources, render as plain text with each line cut at MAX_PLAIN_LINE_CHARS.
+const MAX_HIGHLIGHTED_LINES = 2_000
+const MAX_HIGHLIGHTED_CHARS = 100_000
+const MAX_PLAIN_LINE_CHARS = 10_000
+
+// Highlights the whole source before splitting it, so a line inside a multi-line string or
+// comment keeps that scope. Only the first `lineLimit` lines are highlighted: a streamed source
+// grows on every delta while the preview shows a bounded prefix.
+export function highlightedSourceLines(
+	code: string,
+	languageName: string,
+	lineLimit = Infinity
+): string[] {
+	if (code === '' || lineLimit <= 0) return []
+	const sourceLines = code.split('\n')
+	const truncated = sourceLines.length > lineLimit
+	const shownLines = truncated ? sourceLines.slice(0, lineLimit) : sourceLines
+	const source = truncated ? shownLines.join('\n') : code
+	if (shownLines.length > MAX_HIGHLIGHTED_LINES || source.length > MAX_HIGHLIGHTED_CHARS) {
+		const plain = shownLines.map((line) =>
+			escapeHtml(
+				line.length > MAX_PLAIN_LINE_CHARS ? line.slice(0, MAX_PLAIN_LINE_CHARS) + '…' : line
+			)
+		)
+		if (!truncated && code.endsWith('\n')) plain.pop()
+		return plain
+	}
+	const highlighted = hljs.highlight(source, { language: languageName }).value
+	return splitHighlightedLines(highlighted, !truncated && code.endsWith('\n'))
+}
+
+// Returns one entry per source line, closing the spans still open at each line end and
+// reopening them on the next line so every entry is balanced HTML.
+export function splitHighlightedLines(highlighted: string, hasFinalNewline: boolean): string[] {
+	const result: string[] = []
+	const openTags: string[] = []
+	let line = ''
+
+	for (const token of highlighted.split(/(<[^>]+>|\n)/)) {
+		if (token === '\n') {
+			result.push(line + '</span>'.repeat(openTags.length))
+			line = openTags.join('')
+		} else {
+			line += token
+			if (token.startsWith('<span')) openTags.push(token)
+			else if (token === '</span>') openTags.pop()
+		}
+	}
+
+	if (!hasFinalNewline) result.push(line)
+	return result
+}
diff --git a/frontend/src/lib/components/copilot/chat/toolCodeDiffLanguage.ts b/frontend/src/lib/components/copilot/chat/toolCodeDiffLanguage.ts
new file mode 100644
index 0000000000..ddf2464136
--- /dev/null
+++ b/frontend/src/lib/components/copilot/chat/toolCodeDiffLanguage.ts
@@ -0,0 +1,43 @@
+import { csharp, go, java, json, plaintext, type LanguageType } from 'svelte-highlight/languages'
+import graphql from 'svelte-highlight/languages/graphql'
+import javascript from 'svelte-highlight/languages/javascript'
+import php from 'svelte-highlight/languages/php'
+import python from 'svelte-highlight/languages/python'
+import powershell from 'svelte-highlight/languages/powershell'
+import r from 'svelte-highlight/languages/r'
+import ruby from 'svelte-highlight/languages/ruby'
+import rust from 'svelte-highlight/languages/rust'
+import shell from 'svelte-highlight/languages/shell'
+import sql from 'svelte-highlight/languages/sql'
+import typescript from 'svelte-highlight/languages/typescript'
+import yaml from 'svelte-highlight/languages/yaml'
+
+// Keyed by the editor language `scriptLangToEditorLang` returns, not the script language.
+// for related places search: ADD_NEW_LANG
+const LANGUAGE_BY_MONACO: Record<string, LanguageType<string>> = {
+	typescript,
+	javascript,
+	python,
+	json,
+	yaml,
+	sql,
+	shell,
+	powershell,
+	php,
+	rust,
+	graphql,
+	csharp,
+	nu: python,
+	java,
+	r,
+	go,
+	ruby,
+	text: plaintext,
+	plaintext
+}
+
+export const TOOL_CODE_DIFF_LANGUAGES = Object.keys(LANGUAGE_BY_MONACO)
+
+export function toolCodeDiffLanguage(language: string): LanguageType<string> {
+	return LANGUAGE_BY_MONACO[language] ?? plaintext
+}
diff --git a/frontend/src/routes/test_dev/diff-comparison/+page.svelte b/frontend/src/routes/test_dev/diff-comparison/+page.svelte
new file mode 100644
index 0000000000..c473dbcb64
--- /dev/null
+++ b/frontend/src/routes/test_dev/diff-comparison/+page.svelte
@@ -0,0 +1,139 @@
+<script lang="ts">
+	import { Button } from '$lib/components/common'
+	import DiffEditor from '$lib/components/DiffEditor.svelte'
+	import ToolCodeDiffView from '$lib/components/copilot/chat/ToolCodeDiffView.svelte'
+	import type { ToolCodeDiff } from '$lib/components/copilot/chat/shared'
+
+	type ComparisonCase = {
+		id: string
+		label: string
+		diff: ToolCodeDiff
+	}
+
+	const stableLines = Array.from(
+		{ length: 14 },
+		(_, index) => `	const stableValue${index + 1} = getStableValue(${index + 1})`
+	)
+
+	const cases: ComparisonCase[] = [
+		{
+			id: 'typescript-replacement',
+			label: 'TypeScript replacement',
+			diff: {
+				lang: 'typescript',
+				before: `export function greeting(name: string) {
+	return \`Hello, \${name}\`
+}`,
+				after: `export function greeting(name: string) {
+	const message = \`Hello, \${name}!\`
+	return message
+}`
+			}
+		},
+		{
+			id: 'long-unchanged-sections',
+			label: 'Long unchanged sections',
+			diff: {
+				lang: 'typescript',
+				before: [
+					'export function buildSummary() {',
+					...stableLines,
+					'\treturn formatSummary("draft")',
+					'}'
+				].join('\n'),
+				after: [
+					'export function buildSummary() {',
+					...stableLines,
+					'\treturn formatSummary("published")',
+					'}'
+				].join('\n')
+			}
+		},
+		{
+			id: 'json-structure',
+			label: 'JSON structure',
+			diff: {
+				lang: 'json',
+				before: `{
+	"name": "daily-report",
+	"schedule": "0 9 * * 1-5",
+	"enabled": false
+}`,
+				after: `{
+	"name": "daily-report",
+	"schedule": "0 9 * * 1-5",
+	"timezone": "Europe/Paris",
+	"enabled": true
+}`
+			}
+		},
+		{
+			id: 'python-add-remove',
+			label: 'Python additions and removals',
+			diff: {
+				lang: 'python',
+				before: `def notify(users):
+	for user in users:
+		send_email(user, "Weekly update")
+
+	return len(users)`,
+				after: `def notify(users):
+	active_users = [user for user in users if user.active]
+	for user in active_users:
+		send_email(user, "Your weekly update")
+
+	return len(active_users)`
+			}
+		}
+	]
+
+	let activeCaseId = $state(cases[0].id)
+	let activeCase = $derived(cases.find((item) => item.id === activeCaseId) ?? cases[0])
+</script>
+
+<svelte:head>
+	<title>Diff comparison</title>
+</svelte:head>
+
+<main class="mx-auto max-w-[1440px] p-6">
+	<div class="mb-5 flex flex-wrap gap-2">
+		{#each cases as item}
+			<Button
+				unifiedSize="xs"
+				variant="subtle"
+				selected={activeCaseId === item.id}
+				aria-pressed={activeCaseId === item.id}
+				onclick={() => (activeCaseId = item.id)}>{item.label}</Button
+			>
+		{/each}
+	</div>
+
+	<div class="grid gap-5 lg:grid-cols-2">
+		<section class="min-w-0">
+			<h1 class="mb-2 text-sm font-semibold">Monaco</h1>
+			<div class="h-[400px] overflow-hidden border border-slate-200 bg-white">
+				{#key activeCase.id}
+					<DiffEditor
+						className="h-full"
+						defaultLang={activeCase.diff.lang}
+						defaultModifiedLang={activeCase.diff.lang}
+						defaultOriginal={activeCase.diff.before}
+						defaultModified={activeCase.diff.after}
+						inlineDiff
+						open
+						readOnly
+					/>
+				{/key}
+			</div>
+		</section>
+
+		<section class="min-w-0">
+			<h2 class="mb-2 text-sm font-semibold">Native</h2>
+			<div class="h-[400px] overflow-auto border border-slate-200 bg-white">
+				{#key activeCase.id}
+					<ToolCodeDiffView diff={activeCase.diff} />
+				{/key}
+			</div>
+		</section>
+	</div>
+</main>