ui: added single line reasoning preview (#23601)

* webui: added single line reasoning preview.

* patch: reduce width slightly for the previewing section

* refactor: move formatter constants to the right file

* feat: reimplement reasoning preview with throttled dynamic per-line rendering

* chore: fix spacing

Co-authored-by: Aleksander Grygier <aleksander.grygier@gmail.com>

* chore: refactor to requested changes

* refactor: grouped by capture pattern instead of block-level + inline

* ui: fax interrupt state only trigger for 1st reasoning message

* chore: make reasoning preview respects showThoughtInProgress setting

* chore; newline at EOF

Co-authored-by: Aleksander Grygier <aleksander.grygier@gmail.com>

* fix: thread rawContent so collapsible content can handle compute preview

* patch: showThoughtInProgress accidentally blocks rawContent being passed

* chore: fix lint

* chore: change smoke test

---------

Co-authored-by: Aleksander Grygier <aleksander.grygier@gmail.com>

tools/ui/src/lib/components/app/chat/ChatMessages/ChatMessageAgenticContent.svelte

@@ -31,7 +31,8 @@
 		agenticPendingPermissionRequest,
 		agenticResolvePermission,
 		agenticPendingContinueRequest,
-		agenticResolveContinue
+		agenticResolveContinue,
+		agenticLastError
 	} from '$lib/stores/agentic.svelte';
 	import { config } from '$lib/stores/settings.svelte';
 
@@ -56,6 +57,10 @@
 	const showToolCallInProgress = $derived(config().showToolCallInProgress as boolean);
 	const showThoughtInProgress = $derived(config().showThoughtInProgress as boolean);
 
+	const hasReasoningError = $derived(
+		isLastAssistantMessage ? !!agenticLastError(message.convId) : false
+	);
+
 	let permissionDismissed = $state(false);
 
 	const pendingPermission = $derived(
@@ -293,11 +298,21 @@
 			</div>
 		</CollapsibleContentBlock>
 	{:else if section.type === AgenticSectionType.REASONING}
+		{@const reasoningSubtitle = section.wasInterrupted
+			? hasReasoningError
+				? 'Error'
+				: 'Cancelled'
+			: isStreaming
+				? ''
+				: undefined}
+
 		<CollapsibleContentBlock
 			open={isExpanded(index, section)}
 			class="my-2"
 			icon={Brain}
 			title="Reasoning"
+			subtitle={reasoningSubtitle}
+			rawContent={section.content}
 			onToggle={() => toggleExpanded(index, section)}
 		>
 			<div class="pt-3">
@@ -308,7 +323,7 @@
 		</CollapsibleContentBlock>
 	{:else if section.type === AgenticSectionType.REASONING_PENDING}
 		{@const reasoningTitle = isStreaming ? 'Reasoning...' : 'Reasoning'}
-		{@const reasoningSubtitle = isStreaming ? '' : 'incomplete'}
+		{@const reasoningSubtitle = isStreaming ? '' : hasReasoningError ? 'Error' : 'Cancelled'}
 
 		<CollapsibleContentBlock
 			open={isExpanded(index, section)}
@@ -316,6 +331,7 @@
 			icon={Brain}
 			title={reasoningTitle}
 			subtitle={reasoningSubtitle}
+			rawContent={section.content}
 			{isStreaming}
 			onToggle={() => toggleExpanded(index, section)}
 		>

tools/ui/src/lib/components/app/content/CollapsibleContentBlock.svelte

@@ -4,6 +4,9 @@
 	import { buttonVariants } from '$lib/components/ui/button/index.js';
 	import { Card } from '$lib/components/ui/card';
 	import { createAutoScrollController } from '$lib/hooks/use-auto-scroll.svelte';
+	import { useThrottle } from '$lib/hooks/use-throttle.svelte';
+	import { formatReasoningPreview } from '$lib/utils';
+	import { config } from '$lib/stores/settings.svelte';
 	import type { Snippet } from 'svelte';
 	import type { Component } from 'svelte';
 
@@ -14,6 +17,8 @@
 		iconClass?: string;
 		title: string;
 		subtitle?: string;
+		preview?: string;
+		rawContent?: string;
 		isStreaming?: boolean;
 		onToggle?: () => void;
 		children: Snippet;
@@ -26,6 +31,8 @@
 		iconClass = 'h-4 w-4',
 		title,
 		subtitle,
+		preview,
+		rawContent,
 		isStreaming = false,
 		onToggle,
 		children
@@ -33,6 +40,20 @@
 
 	let contentContainer: HTMLDivElement | undefined = $state();
 
+	const showThoughtInProgress = $derived(config().showThoughtInProgress as boolean);
+
+	let previewKey = useThrottle(() => rawContent ?? preview ?? '', 500);
+	let displayedPreview = $state('');
+	let displayedOverflow = $state(0);
+
+	$effect(() => {
+		void previewKey.key;
+		const content = rawContent ?? preview ?? '';
+		const result = formatReasoningPreview(content);
+		displayedPreview = result.preview;
+		displayedOverflow = result.overflow;
+	});
+
 	const autoScroll = createAutoScrollController();
 
 	$effect(() => {
@@ -58,16 +79,31 @@
 	class={className}
 >
 	<Card class="gap-0 border-muted bg-muted/30 py-0">
-		<Collapsible.Trigger class="flex w-full cursor-pointer items-center justify-between p-3">
-			<div class="flex items-center gap-2 text-muted-foreground">
-				{#if IconComponent}
-					<IconComponent class={iconClass} />
-				{/if}
+		<Collapsible.Trigger class="flex w-full cursor-pointer items-start justify-between gap-2 p-3">
+			<div class="flex min-w-0 items-center gap-2">
+				<div class="flex items-center gap-2 text-muted-foreground">
+					{#if IconComponent}
+						<IconComponent class={iconClass} />
+					{/if}
 
-				<span class="font-mono text-sm font-medium">{title}</span>
+					<span class="font-mono text-sm font-medium">{title}</span>
 
-				{#if subtitle}
-					<span class="text-xs italic">{subtitle}</span>
+					{#if subtitle}
+						<span class="text-xs italic">{subtitle}</span>
+					{/if}
+				</div>
+
+				{#if displayedPreview && !showThoughtInProgress}
+					<div class="flex min-w-0 items-baseline justify-between gap-2">
+						<div class="w-3/4 truncate text-xs text-muted-foreground/80">
+							{displayedPreview}
+						</div>
+						{#if displayedOverflow > 0}
+							<span class="shrink-0 text-xs text-muted-foreground/60"
+								>{displayedOverflow}+ chars</span
+							>
+						{/if}
+					</div>
 				{/if}
 			</div>
 

tools/ui/src/lib/constants/formatters.ts

@@ -6,3 +6,30 @@ export const MEDIUM_DURATION_THRESHOLD = 10;
 
 /** Default display value when no performance time is available */
 export const DEFAULT_PERFORMANCE_TIME = '0s';
+
+/** Max length before reasoning preview is truncated */
+export const MAX_PREVIEW_LENGTH = 120;
+
+export const STRIP_MARKDOWN_CAPTURE_PATTERNS: [RegExp, string][] = [
+	[/^```(.*)/gm, '$1'],
+	[/(.*)```$/gm, '$1'],
+	[/`([^`]*)`/g, '$1'],
+	[/\*\*(.*?)\*\*/g, '$1'],
+	[/__(.*?)__/g, '$1'],
+	[/\*(.*?)\*/g, '$1'],
+	[/_(.*?)_/g, '$1']
+];
+
+/* eslint-disable no-misleading-character-class */
+export const STRIP_MARKDOWN_INLINE_REGEX = new RegExp(
+	[
+		'<[^>]*>',
+		'^>\\s*',
+		'^#{1,6}\\s+',
+		'^[\\s]*[-*+]\\s+',
+		'^[\\s]*\\d+[.)]\\s+',
+		'[\\u{1F600}-\\u{1F64F}\\u{1F300}-\\u{1F5FF}\\u{1F680}-\\u{1F6FF}\\u{1F1E0}-\\u{1F1FF}\\u{2600}-\\u{26FF}\\u{2700}-\\u{27BF}\\u{FE00}-\\u{FE0F}\\u{1F900}-\\u{1F9FF}\\u{1FA00}-\\u{1FA6F}\\u{1FA70}-\\u{1FAFF}\\u{200D}\\u{20E3}\\u{231A}-\\u{231B}\\u{23E9}-\\u{23F3}\\u{23F8}-\\u{23FA}\\u{25AA}-\\u{25AB}\\u{25B6}\\u{25C0}\\u{25FB}-\\u{25FE}\\u{2934}-\\u{2935}\\u{2B05}-\\u{2B07}\\u{2B1B}-\\u{2B1C}\\u{2B50}\\u{2B55}\\u{3030}\\u{303D}\\u{3297}\\u{3299}]'
+	].join('|'),
+	'gmu'
+);
+/* eslint-enable no-misleading-character-class */

tools/ui/src/lib/hooks/use-throttle.svelte.ts

@@ -0,0 +1,32 @@
+/**
+ * Creates a reactive throttle key that increments when `getValue()` changes
+ * and the throttle window has elapsed since the last increment.
+ *
+ * Useful for throttling animations that should not fire on every rapid update.
+ *
+ * @param getValue - A reactive getter for the value to watch
+ * @param ms - Throttle window in milliseconds
+ * @returns A reactive number that increments when the throttled value changes
+ */
+export function useThrottle(getValue: () => string | undefined, ms: number) {
+	let key = $state(0);
+	let throttleEnd = $state(0);
+	let lastValue: string | undefined = getValue();
+
+	$effect(() => {
+		const value = getValue();
+		if (value === lastValue) return;
+		const now = Date.now();
+		if (now >= throttleEnd) {
+			lastValue = value;
+			key++;
+			throttleEnd = now + ms;
+		}
+	});
+
+	return {
+		get key() {
+			return key;
+		}
+	};
+}

tools/ui/src/lib/utils/agentic.ts

@@ -18,6 +18,7 @@ export interface AgenticSection {
 	toolArgs?: string;
 	toolResult?: string;
 	toolResultExtras?: DatabaseMessageExtra[];
+	wasInterrupted?: boolean;
 }
 
 /**
@@ -51,7 +52,8 @@ function deriveSingleTurnSections(
 		const isPending = isStreaming && !hasContentAfterReasoning;
 		sections.push({
 			type: isPending ? AgenticSectionType.REASONING_PENDING : AgenticSectionType.REASONING,
-			content: message.reasoningContent
+			content: message.reasoningContent,
+			wasInterrupted: !isStreaming && !hasContentAfterReasoning
 		});
 	}
 

tools/ui/src/lib/utils/formatters.ts

@@ -3,7 +3,11 @@ import {
 	SECONDS_PER_MINUTE,
 	SECONDS_PER_HOUR,
 	SHORT_DURATION_THRESHOLD,
-	MEDIUM_DURATION_THRESHOLD
+	MEDIUM_DURATION_THRESHOLD,
+	MAX_PREVIEW_LENGTH,
+	STRIP_MARKDOWN_INLINE_REGEX,
+	STRIP_MARKDOWN_CAPTURE_PATTERNS,
+	NEWLINE_SEPARATOR
 } from '$lib/constants';
 
 /**
@@ -151,3 +155,33 @@ export function formatAttachmentText(
 	const header = extra ? `${name} (${extra})` : name;
 	return `\n\n--- ${label}: ${header} ---\n${content}`;
 }
+
+export function formatReasoningPreview(content: string): { preview: string; overflow: number } {
+	if (!content) return { preview: '', overflow: 0 };
+
+	const lines = content.split(NEWLINE_SEPARATOR);
+	let lastLine = '';
+
+	for (let i = lines.length - 1; i >= 0; i--) {
+		let cleaned = lines[i].trim();
+		if (!cleaned) continue;
+
+		cleaned = cleaned.replace(STRIP_MARKDOWN_INLINE_REGEX, '');
+		for (const [pattern, replacement] of STRIP_MARKDOWN_CAPTURE_PATTERNS) {
+			cleaned = cleaned.replace(pattern, replacement);
+		}
+
+		if (cleaned.length > 0) {
+			lastLine = cleaned;
+			break;
+		}
+	}
+
+	const fullLength = lastLine.length;
+	const overflow = Math.max(0, fullLength - MAX_PREVIEW_LENGTH);
+	if (fullLength > MAX_PREVIEW_LENGTH) {
+		lastLine = lastLine.slice(0, MAX_PREVIEW_LENGTH) + '...';
+	}
+
+	return { preview: lastLine, overflow };
+}

tools/ui/src/lib/utils/index.ts

@@ -76,7 +76,8 @@ export {
 	formatJsonPretty,
 	formatTime,
 	formatPerformanceTime,
-	formatAttachmentText
+	formatAttachmentText,
+	formatReasoningPreview
 } from './formatters';
 
 // IME utilities