diff --git a/tools/ui/src/lib/components/app/chat/ChatAttachments/ChatAttachmentsPreview/ChatAttachmentsPreview.svelte b/tools/ui/src/lib/components/app/chat/ChatAttachments/ChatAttachmentsPreview/ChatAttachmentsPreview.svelte
index 0efe052b1f..8e89491723 100644
--- a/tools/ui/src/lib/components/app/chat/ChatAttachments/ChatAttachmentsPreview/ChatAttachmentsPreview.svelte
+++ b/tools/ui/src/lib/components/app/chat/ChatAttachments/ChatAttachmentsPreview/ChatAttachmentsPreview.svelte
@@ -5,6 +5,7 @@
 		ChatAttachmentsPreviewNavButtons,
 		ChatAttachmentsPreviewThumbnailStrip
 	} from '$lib/components/app';
+	import { UI_DATA_ATTRS } from '$lib/constants';
 	import { modelsStore } from '$lib/stores';
 	import {
 		createBase64DataUrl,
@@ -90,7 +91,7 @@
 		const index = currentIndex;
 
 		setTimeout(() => {
-			const thumbnail = document.querySelector(`[data-thumbnail-index="${index}"]`);
+			const thumbnail = document.querySelector(`[${UI_DATA_ATTRS.THUMBNAIL_INDEX}="${index}"]`);
 
 			thumbnail?.scrollIntoView({ behavior: 'smooth', block: 'nearest', inline: 'center' });
 		}, 0);
diff --git a/tools/ui/src/lib/components/app/chat/ChatAttachments/ChatAttachmentsPreview/ChatAttachmentsPreviewThumbnailStrip.svelte b/tools/ui/src/lib/components/app/chat/ChatAttachments/ChatAttachmentsPreview/ChatAttachmentsPreviewThumbnailStrip.svelte
index 60b72077d8..366c8372b9 100644
--- a/tools/ui/src/lib/components/app/chat/ChatAttachments/ChatAttachmentsPreview/ChatAttachmentsPreviewThumbnailStrip.svelte
+++ b/tools/ui/src/lib/components/app/chat/ChatAttachments/ChatAttachmentsPreview/ChatAttachmentsPreviewThumbnailStrip.svelte
@@ -1,7 +1,7 @@
 <script lang="ts">
 	import { FileText, Music, Video } from '@lucide/svelte';
 	import { HorizontalScrollCarousel } from '$lib/components/app/misc';
-	import { ICON_CLASS_DEFAULT } from '$lib/constants';
+	import { ICON_CLASS_DEFAULT, UI_DATA_ATTRS } from '$lib/constants';
 
 	interface PreviewItem {
 		id: string;
@@ -36,7 +36,7 @@
 		<HorizontalScrollCarousel class="max-w-full">
 			{#each items as item, index (item.id)}
 				<button
-					data-thumbnail-index={index}
+					{...{ [UI_DATA_ATTRS.THUMBNAIL_INDEX]: index }}
 					class={[
 						'relative flex-shrink-0 cursor-pointer overflow-hidden rounded border-2 bg-black/80 backdrop-blur-sm transition-all hover:opacity-90',
 						index === currentIndex ? 'border-white' : 'border-transparent opacity-60',
diff --git a/tools/ui/src/lib/components/app/chat/ChatForm/ChatForm.svelte b/tools/ui/src/lib/components/app/chat/ChatForm/ChatForm.svelte
index 49c3dfe205..a9f721e47f 100644
--- a/tools/ui/src/lib/components/app/chat/ChatForm/ChatForm.svelte
+++ b/tools/ui/src/lib/components/app/chat/ChatForm/ChatForm.svelte
@@ -109,7 +109,7 @@
 	}: Props = $props();
 
 	// Component References
-	// Shared handle of the two input renderers (textarea + contenteditable).
+	// Shared handle of the two input renderers (plain textarea + rich chat form input).
 	type ChatInputHandle = {
 		focus(): void;
 		resetHeight(): void;
@@ -125,11 +125,11 @@
 		$state(undefined);
 	let inputRef: ChatInputHandle | undefined = $state(undefined);
 
-	// Render-mode gate: the plain textarea by default, the contenteditable
+	// Render-mode gate: the plain textarea by default, the rich chat form input
 	// while the buffer carries a `file://` mention link or a complete code
 	// span (badges and code chips need a DOM the textarea cannot provide).
 	// Demotes back once neither remains.
-	let useContenteditable = $state(false);
+	let useRichInput = $state(false);
 
 	// Audio Recording State
 	let isRecording = $state(false);
@@ -241,16 +241,15 @@
 	}
 
 	$effect(() => {
-		const wantContenteditable =
-			containsFileMentionLink(value ?? '') || containsCodeSpan(value ?? '');
+		const wantRichInput = containsFileMentionLink(value ?? '') || containsCodeSpan(value ?? '');
 
-		if (useContenteditable === wantContenteditable) return;
+		if (useRichInput === wantRichInput) return;
 
 		if (!caretOffsetPinned) {
 			pendingCaretOffset = inputRef?.getCaretOffset() ?? (value ?? '').length;
 		}
 
-		useContenteditable = wantContenteditable;
+		useRichInput = wantRichInput;
 		queueCaretRestore();
 	});
 
@@ -314,7 +313,7 @@
 
 			// Caret inside a fenced code block (closed, or still open
 			// while being typed): Enter adds a line, never submits. The
-			// contenteditable consumes this case locally; this gate
+			// rich chat form input consumes this case locally; this gate
 			// covers the plain textarea, where skipping submit lets the
 			// native newline through.
 			if (!isModifier && isOffsetInCodeBlock(value ?? '', inputRef?.getCaretOffset() ?? 0)) {
@@ -507,9 +506,9 @@
 		value = built.newValue;
 		onValueChange?.(built.newValue);
 
-		// Already in contenteditable mode: no renderer flip, so the swap
+		// Already in rich chat form input mode: no renderer flip, so the swap
 		// effect's caret restore never runs.
-		if (useContenteditable) {
+		if (useRichInput) {
 			queueCaretRestore();
 		}
 	}
@@ -614,7 +613,7 @@
 				onPaste={handlePaste}
 				{disabled}
 				{placeholder}
-				{useContenteditable}
+				{useRichInput}
 			/>
 
 			{#if mcpResourceStore.hasAttachments}
diff --git a/tools/ui/src/lib/components/app/chat/ChatForm/ChatFormCurrentWorkingDirectory/ChatFormCurrentWorkingDirectory.svelte b/tools/ui/src/lib/components/app/chat/ChatForm/ChatFormCurrentWorkingDirectory/ChatFormCurrentWorkingDirectory.svelte
index 78e5b5cb99..856b05cb02 100644
--- a/tools/ui/src/lib/components/app/chat/ChatForm/ChatFormCurrentWorkingDirectory/ChatFormCurrentWorkingDirectory.svelte
+++ b/tools/ui/src/lib/components/app/chat/ChatForm/ChatFormCurrentWorkingDirectory/ChatFormCurrentWorkingDirectory.svelte
@@ -4,7 +4,7 @@
 	import { FolderOpen } from '@lucide/svelte';
 	import SearchInput from '$lib/components/app/forms/SearchInput.svelte';
 	import * as Popover from '$lib/components/ui/popover';
-	import { DEFAULT_MOBILE_BREAKPOINT, HOME_TILDE, SEARCH } from '$lib/constants';
+	import { DEFAULT_MOBILE_BREAKPOINT, HOME_TILDE, SEARCH, UI_DATA_ATTRS } from '$lib/constants';
 	import { BuiltInTool, GlobSearchType, KeyboardKey } from '$lib/enums';
 	import { useDebouncedSearch } from '$lib/hooks/use-debounced-search.svelte';
 	import { usePickerNavigation } from '$lib/hooks/use-picker-navigation.svelte';
@@ -120,7 +120,7 @@
 	});
 
 	useScrollActiveRow({
-		dataIndex: 'result',
+		dataAttr: UI_DATA_ATTRS.RESULT_INDEX,
 		getContainer: () => listContainer,
 		getCount: () => queryResults.length,
 		getIndex: () => nav.hoveredIndex,
diff --git a/tools/ui/src/lib/components/app/chat/ChatForm/ChatFormCurrentWorkingDirectory/ChatFormCurrentWorkingDirectoryResultsList.svelte b/tools/ui/src/lib/components/app/chat/ChatForm/ChatFormCurrentWorkingDirectory/ChatFormCurrentWorkingDirectoryResultsList.svelte
index 32de06189e..e8087d967e 100644
--- a/tools/ui/src/lib/components/app/chat/ChatForm/ChatFormCurrentWorkingDirectory/ChatFormCurrentWorkingDirectoryResultsList.svelte
+++ b/tools/ui/src/lib/components/app/chat/ChatForm/ChatFormCurrentWorkingDirectory/ChatFormCurrentWorkingDirectoryResultsList.svelte
@@ -1,6 +1,7 @@
 <script lang="ts">
 	import { Folder } from '@lucide/svelte';
 	import { cn } from '$lib/components/ui/utils';
+	import { UI_DATA_ATTRS } from '$lib/constants';
 	import { highlightMatch } from '$lib/utils';
 	import { fly } from 'svelte/transition';
 
@@ -46,7 +47,7 @@
 		{#each results as path, index (path)}
 			<button
 				type="button"
-				data-result-index={index}
+				{...{ [UI_DATA_ATTRS.RESULT_INDEX]: index }}
 				data-highlighted={index === hoveredIndex ? '' : undefined}
 				class={cn(
 					'relative flex w-full cursor-pointer items-center gap-2 rounded-sm px-2 py-1.5 text-sm outline-hidden select-none data-highlighted:bg-accent data-highlighted:text-accent-foreground'
diff --git a/tools/ui/src/lib/components/app/chat/ChatForm/ChatFormInput/ChatFormInput.svelte b/tools/ui/src/lib/components/app/chat/ChatForm/ChatFormInput/ChatFormInput.svelte
index 2b92b30fec..b708ae0475 100644
--- a/tools/ui/src/lib/components/app/chat/ChatForm/ChatFormInput/ChatFormInput.svelte
+++ b/tools/ui/src/lib/components/app/chat/ChatForm/ChatFormInput/ChatFormInput.svelte
@@ -10,7 +10,7 @@
 		onPaste?: (event: ClipboardEvent) => void;
 		placeholder?: string;
 		value?: string;
-		useContenteditable?: boolean;
+		useRichInput?: boolean;
 	}
 
 	let {
@@ -20,7 +20,7 @@
 		onKeydown,
 		onPaste,
 		placeholder = 'Ask anything...',
-		useContenteditable = false,
+		useRichInput = false,
 		value = $bindable('')
 	}: Props = $props();
 
@@ -30,32 +30,30 @@
 	// The two renderers share one imperative handle (focus/caret/height), so
 	// the parent can drive whichever variant is mounted through this one.
 	export function getElement() {
-		return useContenteditable ? richRef?.getElement() : basicRef?.getElement();
+		return useRichInput ? richRef?.getElement() : basicRef?.getElement();
 	}
 
 	export function focus() {
-		if (useContenteditable) richRef?.focus();
+		if (useRichInput) richRef?.focus();
 		else basicRef?.focus();
 	}
 
 	export function resetHeight() {
-		if (useContenteditable) richRef?.resetHeight();
+		if (useRichInput) richRef?.resetHeight();
 		else basicRef?.resetHeight();
 	}
 
 	export function getCaretOffset(): number {
-		return useContenteditable
-			? (richRef?.getCaretOffset() ?? 0)
-			: (basicRef?.getCaretOffset() ?? 0);
+		return useRichInput ? (richRef?.getCaretOffset() ?? 0) : (basicRef?.getCaretOffset() ?? 0);
 	}
 
 	export function setCaretOffset(offset: number) {
-		if (useContenteditable) richRef?.setCaretOffset(offset);
+		if (useRichInput) richRef?.setCaretOffset(offset);
 		else basicRef?.setCaretOffset(offset);
 	}
 </script>
 
-{#if useContenteditable}
+{#if useRichInput}
 	<ChatFormInputRich
 		bind:this={richRef}
 		class={className}
diff --git a/tools/ui/src/lib/components/app/chat/ChatForm/ChatFormInput/ChatFormInputBasic.svelte b/tools/ui/src/lib/components/app/chat/ChatForm/ChatFormInput/ChatFormInputBasic.svelte
index 623744d8af..e0c08c721f 100644
--- a/tools/ui/src/lib/components/app/chat/ChatForm/ChatFormInput/ChatFormInputBasic.svelte
+++ b/tools/ui/src/lib/components/app/chat/ChatForm/ChatFormInput/ChatFormInputBasic.svelte
@@ -48,7 +48,7 @@
 		}
 	}
 
-	// Plain-text caret offsets, shared with the contenteditable variant so
+	// Plain-text caret offsets, shared with the rich chat form input variant so
 	// the picker/paste flows can address either renderer through one handle.
 	export function getCaretOffset(): number {
 		if (!textareaElement) return 0;
diff --git a/tools/ui/src/lib/components/app/chat/ChatForm/ChatFormInput/ChatFormInputRich.svelte b/tools/ui/src/lib/components/app/chat/ChatForm/ChatFormInput/ChatFormInputRich.svelte
index f25c3c95cd..d87817adb7 100644
--- a/tools/ui/src/lib/components/app/chat/ChatForm/ChatFormInput/ChatFormInputRich.svelte
+++ b/tools/ui/src/lib/components/app/chat/ChatForm/ChatFormInput/ChatFormInputRich.svelte
@@ -1,6 +1,6 @@
 <script lang="ts">
-	import { CODE_BLOCK } from '$lib/constants';
-	import { ColorMode } from '$lib/enums';
+	import { CODE_BLOCK, CODE_TOKEN_ATTR, UI_DATA_ATTRS } from '$lib/constants';
+	import { BooleanString, ChatFormInputRichTokenKind, ColorMode } from '$lib/enums';
 	import { isMobile } from '$lib/stores';
 	import type { ChatFormInputRichToken } from '$lib/types';
 	import type { SourceHistoryEntry } from '$lib/utils';
@@ -53,7 +53,7 @@
 	// browser's native undo stack.
 	const history = new SourceHistory();
 
-	// Browsers disagree on what an empty contenteditable contains (`<br>`,
+	// Browsers disagree on what an empty rich chat form input contains (`<br>`,
 	// `<div><br></div>`, or nothing), so emptiness is decided by the
 	// serialized source, not the DOM shape.
 	function syncEmptyState(serialized?: string) {
@@ -61,7 +61,7 @@
 
 		const source = serialized ?? serializeContent(rootElement);
 
-		rootElement.dataset.empty = source.length === 0 ? 'true' : 'false';
+		rootElement.dataset.empty = source.length === 0 ? BooleanString.TRUE : BooleanString.FALSE;
 	}
 
 	function renderTokens(tokens: ChatFormInputRichToken[]) {
@@ -69,7 +69,7 @@
 
 		const caret = rangeToTextOffset(rootElement, safeRange());
 
-		// eslint-disable-next-line svelte/no-dom-manipulating -- the token layer is owned imperatively; Svelte renders only the contenteditable host, never its children
+		// eslint-disable-next-line svelte/no-dom-manipulating -- the token layer is owned imperatively; Svelte renders only the rich chat form input host, never its children
 		rootElement.replaceChildren(buildFragment(tokens));
 
 		syncCodeBlockHatches(rootElement);
@@ -127,7 +127,9 @@
 	}
 
 	function highlightCodeBlocks(root: HTMLElement) {
-		for (const el of root.querySelectorAll<HTMLElement>('code[data-code-token="code_block"]')) {
+		for (const el of root.querySelectorAll<HTMLElement>(
+			`code[${CODE_TOKEN_ATTR}="${ChatFormInputRichTokenKind.CODE_BLOCK}"]`
+		)) {
 			highlightCodeBlockElement(el);
 		}
 	}
@@ -151,7 +153,10 @@
 		}
 
 		while (node && node !== rootElement) {
-			if (node instanceof HTMLElement && node.dataset.codeToken === 'code_block') {
+			if (
+				node instanceof HTMLElement &&
+				node.getAttribute(CODE_TOKEN_ATTR) === ChatFormInputRichTokenKind.CODE_BLOCK
+			) {
 				const caret = rangeToTextOffset(rootElement, range);
 
 				if (highlightCodeBlockElement(node)) {
@@ -189,11 +194,13 @@
 	 * (deduped via the data attribute) swapped on mode change.
 	 */
 	function loadHighlightTheme(isDark: boolean) {
-		document.querySelectorAll('style[data-highlight-theme-preview]').forEach((s) => s.remove());
+		document
+			.querySelectorAll(`style[${UI_DATA_ATTRS.HIGHLIGHT_THEME_PREVIEW}]`)
+			.forEach((s) => s.remove());
 
 		const style = document.createElement('style');
 
-		style.setAttribute('data-highlight-theme-preview', 'true');
+		style.setAttribute(UI_DATA_ATTRS.HIGHLIGHT_THEME_PREVIEW, BooleanString.TRUE);
 		style.textContent = isDark ? githubDarkCss : githubLightCss;
 
 		document.head.appendChild(style);
@@ -311,7 +318,7 @@
 					source[source.length - 2] !== '\n' &&
 					last?.nodeType === Node.TEXT_NODE
 				) {
-					// eslint-disable-next-line svelte/no-dom-manipulating -- the token layer is owned imperatively; Svelte renders only the contenteditable host, never its children
+					// eslint-disable-next-line svelte/no-dom-manipulating -- the token layer is owned imperatively; Svelte renders only the rich chat form input host, never its children
 					rootElement.appendChild(document.createTextNode('\n'));
 					restoreCaret(source.length);
 					resizeHeight();
@@ -404,7 +411,10 @@
 			let node: Node | null = container.parentNode;
 
 			while (node && node !== rootElement) {
-				if (node instanceof HTMLElement && node.dataset.codeToken === 'code_block') {
+				if (
+					node instanceof HTMLElement &&
+					node.getAttribute(CODE_TOKEN_ATTR) === ChatFormInputRichTokenKind.CODE_BLOCK
+				) {
 					const tail = document.createRange();
 
 					tail.setStart(container, offset);
@@ -462,7 +472,11 @@
 
 		const first = rootElement.firstChild;
 
-		if (!(first instanceof HTMLElement) || first.dataset.codeToken !== 'code_block') return false;
+		if (
+			!(first instanceof HTMLElement) ||
+			first.getAttribute(CODE_TOKEN_ATTR) !== ChatFormInputRichTokenKind.CODE_BLOCK
+		)
+			return false;
 
 		const range = safeRange();
 
@@ -483,7 +497,7 @@
 			if (firstLineEnd !== -1 && caret > firstLineEnd) return false;
 		}
 
-		// eslint-disable-next-line svelte/no-dom-manipulating -- the token layer is owned imperatively; Svelte renders only the contenteditable host, never its children
+		// eslint-disable-next-line svelte/no-dom-manipulating -- the token layer is owned imperatively; Svelte renders only the rich chat form input host, never its children
 		rootElement.prepend(document.createElement('br'));
 		restoreCaret(0, extend);
 
@@ -507,7 +521,11 @@
 
 		const second = first.nextSibling;
 
-		if (!(second instanceof HTMLElement) || second.dataset.codeToken !== 'code_block') return;
+		if (
+			!(second instanceof HTMLElement) ||
+			second.getAttribute(CODE_TOKEN_ATTR) !== ChatFormInputRichTokenKind.CODE_BLOCK
+		)
+			return;
 
 		const range = safeRange();
 		const onHatch =
diff --git a/tools/ui/src/lib/components/app/chat/ChatForm/ChatFormPickers/ChatFormPicker/ChatFormPickerList.svelte b/tools/ui/src/lib/components/app/chat/ChatForm/ChatFormPickers/ChatFormPicker/ChatFormPickerList.svelte
index 3ebb29b9b3..160c14ce8e 100644
--- a/tools/ui/src/lib/components/app/chat/ChatForm/ChatFormPickers/ChatFormPicker/ChatFormPickerList.svelte
+++ b/tools/ui/src/lib/components/app/chat/ChatForm/ChatFormPickers/ChatFormPicker/ChatFormPickerList.svelte
@@ -1,7 +1,7 @@
 <script lang="ts" generics="T">
 	import { SearchInput } from '$lib/components/app';
 	import ScrollArea from '$lib/components/ui/scroll-area/scroll-area.svelte';
-	import { CHAT_FORM_POPOVER_MAX_HEIGHT } from '$lib/constants';
+	import { CHAT_FORM_POPOVER_MAX_HEIGHT, UI_DATA_ATTRS } from '$lib/constants';
 	import { useScrollActiveRow } from '$lib/hooks/use-scroll-active-row.svelte';
 	import type { Snippet } from 'svelte';
 
@@ -55,7 +55,7 @@
 	// selectedIndex/items.length are untracked so hover and result replacement
 	// never re-fire the scroll; keyboard nav is the only path that bumps the trigger.
 	useScrollActiveRow({
-		dataIndex: 'picker',
+		dataAttr: UI_DATA_ATTRS.PICKER_INDEX,
 		getContainer: () => listContainer,
 		getCount: () => items.length,
 		getIndex: () => selectedIndex,
diff --git a/tools/ui/src/lib/components/app/chat/ChatForm/ChatFormPickers/ChatFormPicker/ChatFormPickerListItem.svelte b/tools/ui/src/lib/components/app/chat/ChatForm/ChatFormPickers/ChatFormPicker/ChatFormPickerListItem.svelte
index 51a51f8080..045534f488 100644
--- a/tools/ui/src/lib/components/app/chat/ChatForm/ChatFormPickers/ChatFormPicker/ChatFormPickerListItem.svelte
+++ b/tools/ui/src/lib/components/app/chat/ChatForm/ChatFormPickers/ChatFormPicker/ChatFormPickerListItem.svelte
@@ -1,4 +1,5 @@
 <script lang="ts">
+	import { UI_DATA_ATTRS } from '$lib/constants';
 	import type { Snippet } from 'svelte';
 
 	interface Props {
@@ -24,7 +25,7 @@
 
 <button
 	type="button"
-	data-picker-index={dataIndex}
+	{...{ [UI_DATA_ATTRS.PICKER_INDEX]: dataIndex }}
 	{disabled}
 	{onclick}
 	{onmouseenter}
diff --git a/tools/ui/src/lib/components/app/chat/index.ts b/tools/ui/src/lib/components/app/chat/index.ts
index d9e408c96e..2de8a6ace1 100644
--- a/tools/ui/src/lib/components/app/chat/index.ts
+++ b/tools/ui/src/lib/components/app/chat/index.ts
@@ -120,7 +120,7 @@ export { default as ChatAttachmentsPreviewCurrentItem } from './ChatAttachments/
  * Used by ChatScreenForm and ChatMessageEditForm for both new conversations and message editing.
  *
  * **Architecture:**
- * - Composes ChatFormInput (a plain textarea, or a contenteditable for
+ * - Composes ChatFormInput (a plain textarea, or a ChatFormInputRich for
  *   messages with file mention links), ChatFormActions, and ChatFormPickerMcpPrompts
  * - Manages file upload state via `uploadedFiles` bindable prop
  * - Integrates with ModelsSelectorDropdown for model selection in router mode
@@ -268,10 +268,10 @@ export { default as ChatFormMcpResourcesList } from './ChatForm/ChatFormMcpResou
 
 /**
  * The message editor. Renders a plain auto-resizing textarea by default,
- * or a contenteditable that renders `[name](file://...)` mention links as
+ * or a ChatFormInputRich that renders `[name](file://...)` mention links as
  * inline chips (keeping the value as the markdown source string) once a
  * mention link lands in the buffer. The variant is selected via the
- * `useContenteditable` prop; both share one imperative handle.
+ * `useRichInput` prop; both share one imperative handle.
  */
 export { default as ChatFormInput } from './ChatForm/ChatFormInput/ChatFormInput.svelte';
 
@@ -384,14 +384,14 @@ export { default as ChatFormPickerListItemSkeleton } from './ChatForm/ChatFormPi
  * tool, scoped to the conversation cwd (or server home when unset).
  * Selection splices a `[name](file:///<abs path>)` link into the input.
  */
-export { default as ChatFormMentionPicker } from './ChatForm/ChatFormPickers/ChatFormPickerMention.svelte';
+export { default as ChatFormPickerMention } from './ChatForm/ChatFormPickers/ChatFormPickerMention.svelte';
 
 /**
  * `/`-triggered slash-command picker. Lists the available slash commands
  * (`/prompt`, `/cwd`, `/model`) filtered by the typed query; selection
  * hands the command to the parent for dispatch.
  */
-export { default as ChatFormCommandPicker } from './ChatForm/ChatFormPickers/ChatFormPickerCommand.svelte';
+export { default as ChatFormPickerCommand } from './ChatForm/ChatFormPickers/ChatFormPickerCommand.svelte';
 
 /**
  * Hosts the chat-form pickers (slash-command, MCP prompt, file mention)
diff --git a/tools/ui/src/lib/components/app/content/MarkdownContent/MarkdownContent.svelte b/tools/ui/src/lib/components/app/content/MarkdownContent/MarkdownContent.svelte
index cadd9f210b..2a1d617ba2 100644
--- a/tools/ui/src/lib/components/app/content/MarkdownContent/MarkdownContent.svelte
+++ b/tools/ui/src/lib/components/app/content/MarkdownContent/MarkdownContent.svelte
@@ -25,14 +25,12 @@
 		DialogMermaidPreview
 	} from '$lib/components/app';
 	import {
-		BOOL_TRUE_STRING,
 		CODE_BLOCK_CLASS,
-		DATA_ERROR_BOUND_ATTR,
-		DATA_ERROR_HANDLED_ATTR,
 		DIAGRAM_VIEW_MODE_ATTR,
 		DIAGRAM_VIEW_RENDERED,
 		DIAGRAM_VIEW_SOURCE,
 		IMAGE_NOT_ERROR_BOUND_SELECTOR,
+		MARKDOWN_DATA_ATTRS,
 		MERMAID_BLOCK_CLASS,
 		MERMAID_LANGUAGE,
 		MERMAID_RENDERED_ATTR,
@@ -42,7 +40,7 @@
 		SVG,
 		TOGGLE_SOURCE_BTN_CLASS
 	} from '$lib/constants';
-	import { ColorMode, UrlProtocol } from '$lib/enums';
+	import { BooleanString, ColorMode, UrlProtocol } from '$lib/enums';
 	import { FileTypeText } from '$lib/enums/files.enums';
 	import { createAutoScrollController } from '$lib/hooks/use-auto-scroll.svelte';
 	import { settingsStore } from '$lib/stores';
@@ -486,13 +484,19 @@
 			const copyButton = wrapper.querySelector<HTMLButtonElement>('.copy-code-btn');
 			const previewButton = wrapper.querySelector<HTMLButtonElement>('.preview-code-btn');
 
-			if (copyButton && copyButton.dataset.listenerBound !== 'true') {
-				copyButton.dataset.listenerBound = 'true';
+			if (
+				copyButton &&
+				copyButton.getAttribute(MARKDOWN_DATA_ATTRS.LISTENER_BOUND) !== BooleanString.TRUE
+			) {
+				copyButton.setAttribute(MARKDOWN_DATA_ATTRS.LISTENER_BOUND, BooleanString.TRUE);
 				copyButton.addEventListener('click', handleCopyClick);
 			}
 
-			if (previewButton && previewButton.dataset.listenerBound !== 'true') {
-				previewButton.dataset.listenerBound = 'true';
+			if (
+				previewButton &&
+				previewButton.getAttribute(MARKDOWN_DATA_ATTRS.LISTENER_BOUND) !== BooleanString.TRUE
+			) {
+				previewButton.setAttribute(MARKDOWN_DATA_ATTRS.LISTENER_BOUND, BooleanString.TRUE);
 				previewButton.addEventListener('click', handlePreviewClick);
 			}
 		}
@@ -508,7 +512,7 @@
 		const images = containerRef.querySelectorAll<HTMLImageElement>(IMAGE_NOT_ERROR_BOUND_SELECTOR);
 
 		for (const img of images) {
-			img.dataset[DATA_ERROR_BOUND_ATTR] = BOOL_TRUE_STRING;
+			img.setAttribute(MARKDOWN_DATA_ATTRS.ERROR_BOUND, BooleanString.TRUE);
 			img.addEventListener('error', handleImageError);
 		}
 	}
@@ -691,7 +695,7 @@
 
 		// Mark nodes immediately to prevent duplicate renders if called again during streaming.
 		// This avoids needing a guard that would block node discovery.
-		nodes.forEach((node) => node.setAttribute(MERMAID_RENDERED_ATTR, 'true'));
+		nodes.forEach((node) => node.setAttribute(MERMAID_RENDERED_ATTR, BooleanString.TRUE));
 
 		// Read mode before await so Svelte tracks it reactively.
 		const isDark = mode.current === ColorMode.DARK;
@@ -738,7 +742,7 @@
 		if (nodes.length === 0) return;
 
 		nodes.forEach((node) => {
-			node.setAttribute(SVG.RENDERED_ATTR, 'true');
+			node.setAttribute(SVG.RENDERED_ATTR, BooleanString.TRUE);
 
 			const source = node.getAttribute(SVG.SOURCE_ATTR) ?? node.textContent ?? '';
 			const clean = sanitizeSvg(source);
@@ -765,11 +769,11 @@
 		// Don't handle data URLs or already-handled images
 		if (
 			img.src.startsWith(UrlProtocol.DATA) ||
-			img.dataset[DATA_ERROR_HANDLED_ATTR] === BOOL_TRUE_STRING
+			img.getAttribute(MARKDOWN_DATA_ATTRS.ERROR_HANDLED) === BooleanString.TRUE
 		)
 			return;
 
-		img.dataset[DATA_ERROR_HANDLED_ATTR] = BOOL_TRUE_STRING;
+		img.setAttribute(MARKDOWN_DATA_ATTRS.ERROR_HANDLED, BooleanString.TRUE);
 
 		const src = img.src;
 		// Create fallback element
@@ -869,13 +873,16 @@
 		: ''}"
 >
 	{#each renderedBlocks as block (block.id)}
-		<div class="markdown-block" data-block-id={block.id}>
+		<div class="markdown-block" {...{ [MARKDOWN_DATA_ATTRS.BLOCK_ID]: block.id }}>
 			{@html block.html}
 		</div>
 	{/each}
 
 	{#if unstableBlockHtml}
-		<div class="markdown-block markdown-block--unstable" data-block-id="unstable">
+		<div
+			class="markdown-block markdown-block--unstable"
+			{...{ [MARKDOWN_DATA_ATTRS.BLOCK_ID]: 'unstable' }}
+		>
 			<!-- eslint-disable-next-line no-at-html-tags -->
 			{@html unstableBlockHtml}
 		</div>
diff --git a/tools/ui/src/lib/components/app/content/MarkdownContent/markdown-handlers.ts b/tools/ui/src/lib/components/app/content/MarkdownContent/markdown-handlers.ts
index bd6f28ee7f..0a1db19093 100644
--- a/tools/ui/src/lib/components/app/content/MarkdownContent/markdown-handlers.ts
+++ b/tools/ui/src/lib/components/app/content/MarkdownContent/markdown-handlers.ts
@@ -3,7 +3,14 @@
  * Uses dependency injection pattern to avoid direct component state access.
  */
 
-import { MERMAID_BLOCK_CLASS, MERMAID_SYNTAX_ATTR, MERMAID_WRAPPER_CLASS } from '$lib/constants';
+import {
+	CODE_BLOCK_CLASS,
+	MARKDOWN_DATA_ATTRS,
+	MERMAID_BLOCK_CLASS,
+	MERMAID_SYNTAX_ATTR,
+	MERMAID_WRAPPER_CLASS
+} from '$lib/constants';
+import { BooleanString } from '$lib/enums';
 import { copyCodeToClipboard, copyToClipboard } from '$lib/utils';
 
 export interface PreviewState {
@@ -40,11 +47,11 @@ export function createHandleCopyClick() {
 
 		if (!target) return;
 
-		const wrapper = target.closest('.code-block-wrapper');
+		const wrapper = target.closest(`.${CODE_BLOCK_CLASS.WRAPPER}`);
 
 		if (!wrapper) return;
 
-		const codeElement = wrapper.querySelector<HTMLElement>('code[data-code-id]');
+		const codeElement = wrapper.querySelector<HTMLElement>(`code[${MARKDOWN_DATA_ATTRS.CODE_ID}]`);
 
 		if (!codeElement) return;
 
@@ -86,16 +93,16 @@ export function createHandlePreviewClick(previewState: PreviewState) {
 
 		if (!target) return;
 
-		const wrapper = target.closest('.code-block-wrapper');
+		const wrapper = target.closest(`.${CODE_BLOCK_CLASS.WRAPPER}`);
 
 		if (!wrapper) return;
 
-		const codeElement = wrapper.querySelector<HTMLElement>('code[data-code-id]');
+		const codeElement = wrapper.querySelector<HTMLElement>(`code[${MARKDOWN_DATA_ATTRS.CODE_ID}]`);
 
 		if (!codeElement) return;
 
 		const rawCode = codeElement.textContent ?? '';
-		const languageLabel = wrapper.querySelector<HTMLElement>('.code-language');
+		const languageLabel = wrapper.querySelector<HTMLElement>(`.${CODE_BLOCK_CLASS.LANGUAGE}`);
 		const language = languageLabel?.textContent?.trim() || 'text';
 
 		previewState.setPreviewCode(rawCode);
@@ -112,8 +119,8 @@ export function createHandleMermaidClick(mermaidState: MermaidPreviewState) {
 	return async function handleMermaidClick(event: MouseEvent) {
 		const target = event.target as HTMLElement;
 		// Check if clicking on copy or preview button in mermaid block
-		const copyBtn = target.closest(`.${MERMAID_WRAPPER_CLASS} .copy-code-btn`);
-		const previewBtn = target.closest(`.${MERMAID_WRAPPER_CLASS} .preview-code-btn`);
+		const copyBtn = target.closest(`.${MERMAID_WRAPPER_CLASS} .${CODE_BLOCK_CLASS.COPY_BTN}`);
+		const previewBtn = target.closest(`.${MERMAID_WRAPPER_CLASS} .${CODE_BLOCK_CLASS.PREVIEW_BTN}`);
 
 		if (copyBtn || previewBtn) {
 			const wrapper = target.closest(`.${MERMAID_WRAPPER_CLASS}`);
@@ -189,15 +196,17 @@ export function createHandleMermaidPreviewOpenChange(mermaidState: MermaidPrevie
 export function createHandleImageError(
 	renderedBlocksState: RenderedBlocksState,
 	IMAGE_NOT_ERROR_BOUND_SELECTOR: string,
-	DATA_ERROR_BOUND_ATTR: string,
-	BOOL_TRUE_STRING: string
+	errorBoundAttr: string,
+	booleanString: BooleanString
 ) {
 	return async function handleImageError(event: Event) {
 		const img = event.target as HTMLImageElement;
 
 		if (!img) return;
 
-		const blockId = img.closest('[data-block-id]')?.getAttribute('data-block-id');
+		const blockId = img
+			.closest(`[${MARKDOWN_DATA_ATTRS.BLOCK_ID}]`)
+			?.getAttribute(MARKDOWN_DATA_ATTRS.BLOCK_ID);
 
 		if (!blockId) return;
 
@@ -206,19 +215,22 @@ export function createHandleImageError(
 		if (!block) return;
 
 		// Skip if already handled
-		if (img.dataset[DATA_ERROR_BOUND_ATTR] === BOOL_TRUE_STRING) return;
+		if (img.getAttribute(errorBoundAttr) === booleanString) return;
 
-		img.dataset[DATA_ERROR_BOUND_ATTR] = BOOL_TRUE_STRING;
+		img.setAttribute(errorBoundAttr, booleanString);
 
 		// Get the fallback HTML and replace the image
-		const fallbackHtml = `<div class="image-error-placeholder" data-original-src="${img.src}">
+		const fallbackHtml = `<div class="image-error-placeholder" ${MARKDOWN_DATA_ATTRS.ORIGINAL_SRC}="${img.src}">
 			<span class="image-error-icon">⚠️</span>
 			<span class="image-error-text">Failed to load image</span>
 		</div>`;
 		// Replace the img element with fallback in the block's HTML
 		const newHtml = block.html.replace(/img[^>]*src=["']([^"']*)[^>]*>/g, (match, src) => {
 			if (src === img.src) {
-				return fallbackHtml.replace('data-original-src=""', `data-original-src="${src}"`);
+				return fallbackHtml.replace(
+					`${MARKDOWN_DATA_ATTRS.ORIGINAL_SRC}=""`,
+					`${MARKDOWN_DATA_ATTRS.ORIGINAL_SRC}="${src}"`
+				);
 			}
 
 			return match;
@@ -243,19 +255,27 @@ export function createSetupCodeBlockActions(
 	return function setupCodeBlockActions(containerRef: HTMLElement | null) {
 		if (!containerRef) return;
 
-		const wrappers = containerRef.querySelectorAll<HTMLElement>('.code-block-wrapper');
+		const wrappers = containerRef.querySelectorAll<HTMLElement>(`.${CODE_BLOCK_CLASS.WRAPPER}`);
 
 		for (const wrapper of wrappers) {
-			const copyButton = wrapper.querySelector<HTMLButtonElement>('.copy-code-btn');
-			const previewButton = wrapper.querySelector<HTMLButtonElement>('.preview-code-btn');
+			const copyButton = wrapper.querySelector<HTMLButtonElement>(`.${CODE_BLOCK_CLASS.COPY_BTN}`);
+			const previewButton = wrapper.querySelector<HTMLButtonElement>(
+				`.${CODE_BLOCK_CLASS.PREVIEW_BTN}`
+			);
 
-			if (copyButton && copyButton.dataset.listenerBound !== 'true') {
-				copyButton.dataset.listenerBound = 'true';
+			if (
+				copyButton &&
+				copyButton.getAttribute(MARKDOWN_DATA_ATTRS.LISTENER_BOUND) !== BooleanString.TRUE
+			) {
+				copyButton.setAttribute(MARKDOWN_DATA_ATTRS.LISTENER_BOUND, BooleanString.TRUE);
 				copyButton.addEventListener('click', handleCopyClick);
 			}
 
-			if (previewButton && previewButton.dataset.listenerBound !== 'true') {
-				previewButton.dataset.listenerBound = 'true';
+			if (
+				previewButton &&
+				previewButton.getAttribute(MARKDOWN_DATA_ATTRS.LISTENER_BOUND) !== BooleanString.TRUE
+			) {
+				previewButton.setAttribute(MARKDOWN_DATA_ATTRS.LISTENER_BOUND, BooleanString.TRUE);
 				previewButton.addEventListener('click', handlePreviewClick);
 			}
 		}
@@ -269,8 +289,8 @@ export function createSetupCodeBlockActions(
 export function createSetupImageErrorHandlers(
 	handleImageError: (event: Event) => void,
 	IMAGE_NOT_ERROR_BOUND_SELECTOR: string,
-	DATA_ERROR_BOUND_ATTR: string,
-	BOOL_TRUE_STRING: string
+	errorBoundAttr: string,
+	booleanString: BooleanString
 ) {
 	return function setupImageErrorHandlers(containerRef: HTMLElement | null) {
 		if (!containerRef) return;
@@ -278,7 +298,7 @@ export function createSetupImageErrorHandlers(
 		const images = containerRef.querySelectorAll<HTMLImageElement>(IMAGE_NOT_ERROR_BOUND_SELECTOR);
 
 		for (const img of images) {
-			img.dataset[DATA_ERROR_BOUND_ATTR] = BOOL_TRUE_STRING;
+			img.setAttribute(errorBoundAttr, booleanString);
 			img.addEventListener('error', handleImageError);
 		}
 	};
diff --git a/tools/ui/src/lib/components/app/content/MarkdownContent/markdown-utils.ts b/tools/ui/src/lib/components/app/content/MarkdownContent/markdown-utils.ts
index 96fcac6cec..9e2c0f4f8c 100644
--- a/tools/ui/src/lib/components/app/content/MarkdownContent/markdown-utils.ts
+++ b/tools/ui/src/lib/components/app/content/MarkdownContent/markdown-utils.ts
@@ -2,6 +2,7 @@
  * Utility functions for markdown processing in MarkdownContent component.
  */
 
+import { MARKDOWN_DATA_ATTRS } from '$lib/constants';
 import type { RootContent as HastRootContent } from 'hast';
 
 /**
@@ -69,7 +70,7 @@ export function getCodeInfoFromTarget(target: HTMLElement): CodeInfo | null {
 		return null;
 	}
 
-	const codeElement = wrapper.querySelector<HTMLElement>('code[data-code-id]');
+	const codeElement = wrapper.querySelector<HTMLElement>(`code[${MARKDOWN_DATA_ATTRS.CODE_ID}]`);
 
 	if (!codeElement) {
 		console.error('No code element found in wrapper');
diff --git a/tools/ui/src/lib/components/app/content/MarkdownContent/plugins/rehype/enhance-code-blocks.ts b/tools/ui/src/lib/components/app/content/MarkdownContent/plugins/rehype/enhance-code-blocks.ts
index 54d401c116..f42ab1c07c 100644
--- a/tools/ui/src/lib/components/app/content/MarkdownContent/plugins/rehype/enhance-code-blocks.ts
+++ b/tools/ui/src/lib/components/app/content/MarkdownContent/plugins/rehype/enhance-code-blocks.ts
@@ -17,7 +17,7 @@ import {
 	createWrapper,
 	generateBlockId
 } from './code-block-utils';
-import { CODE_BLOCK_CLASS } from '$lib/constants';
+import { CODE_BLOCK_CLASS, MARKDOWN_DATA_ATTRS } from '$lib/constants';
 import type { Element, ElementContent, Root } from 'hast';
 import type { Plugin } from 'unified';
 import { visit } from 'unist-util-visit';
@@ -65,16 +65,18 @@ export const rehypeEnhanceCodeBlocks: Plugin<[], Root> = () => {
 
 			codeElement.properties = {
 				...codeElement.properties,
-				'data-code-id': codeId
+				[MARKDOWN_DATA_ATTRS.CODE_ID]: codeId
 			};
 
-			const actions: Element[] = [createCopyButton(codeId, 'data-code-id', 'Copy code')];
+			const actions: Element[] = [
+				createCopyButton(codeId, MARKDOWN_DATA_ATTRS.CODE_ID, 'Copy code')
+			];
 
 			if (language.toLowerCase() === 'html') {
-				actions.push(createPreviewButton(codeId, 'data-code-id', 'Preview code'));
+				actions.push(createPreviewButton(codeId, MARKDOWN_DATA_ATTRS.CODE_ID, 'Preview code'));
 			}
 
-			const header = createBlockHeader(language, codeId, 'data-code-id', actions);
+			const header = createBlockHeader(language, codeId, MARKDOWN_DATA_ATTRS.CODE_ID, actions);
 			const wrapper = createWrapper(
 				header,
 				node,
diff --git a/tools/ui/src/lib/components/app/content/MarkdownContent/plugins/rehype/file-badge.ts b/tools/ui/src/lib/components/app/content/MarkdownContent/plugins/rehype/file-badge.ts
index 7bc5ca3b8e..ab50d437ca 100644
--- a/tools/ui/src/lib/components/app/content/MarkdownContent/plugins/rehype/file-badge.ts
+++ b/tools/ui/src/lib/components/app/content/MarkdownContent/plugins/rehype/file-badge.ts
@@ -1,6 +1,6 @@
 /**
  * Rehype plugin that rewrites `file://` markdown anchors into the inline
- * mention chip, sharing the class string with the contenteditable
+ * mention chip, sharing the class string with the ChatFormInputRich
  * tokenizer via `$lib/constants`.
  *
  * The chip is presentational: `file://` navigation is blocked from
diff --git a/tools/ui/src/lib/components/app/content/SyntaxHighlightedCode.svelte b/tools/ui/src/lib/components/app/content/SyntaxHighlightedCode.svelte
index 829c846034..a879197a28 100644
--- a/tools/ui/src/lib/components/app/content/SyntaxHighlightedCode.svelte
+++ b/tools/ui/src/lib/components/app/content/SyntaxHighlightedCode.svelte
@@ -1,7 +1,7 @@
 <script lang="ts">
 	import { browser } from '$app/environment';
-	import { SYNTAX_CODE_SCROLL_AT_BOTTOM_THRESHOLD_PX } from '$lib/constants';
-	import { ColorMode } from '$lib/enums';
+	import { SYNTAX_CODE_SCROLL_AT_BOTTOM_THRESHOLD_PX, UI_DATA_ATTRS } from '$lib/constants';
+	import { BooleanString, ColorMode } from '$lib/enums';
 	import { highlightCode } from '$lib/utils';
 	import githubLightCss from 'highlight.js/styles/github.css?inline';
 	import githubDarkCss from 'highlight.js/styles/github-dark.css?inline';
@@ -38,13 +38,15 @@
 	function loadHighlightTheme(isDark: boolean) {
 		if (!browser) return;
 
-		const existingThemes = document.querySelectorAll('style[data-highlight-theme-preview]');
+		const existingThemes = document.querySelectorAll(
+			`style[${UI_DATA_ATTRS.HIGHLIGHT_THEME_PREVIEW}]`
+		);
 
 		existingThemes.forEach((style) => style.remove());
 
 		const style = document.createElement('style');
 
-		style.setAttribute('data-highlight-theme-preview', 'true');
+		style.setAttribute(UI_DATA_ATTRS.HIGHLIGHT_THEME_PREVIEW, BooleanString.TRUE);
 		style.textContent = isDark ? githubDarkCss : githubLightCss;
 
 		document.head.appendChild(style);
diff --git a/tools/ui/src/lib/components/app/dialogs/DialogMcpServerAddNew.svelte b/tools/ui/src/lib/components/app/dialogs/DialogMcpServerAddNew.svelte
index d8d6704253..9123dcef99 100644
--- a/tools/ui/src/lib/components/app/dialogs/DialogMcpServerAddNew.svelte
+++ b/tools/ui/src/lib/components/app/dialogs/DialogMcpServerAddNew.svelte
@@ -4,14 +4,12 @@
 	import { Button } from '$lib/components/ui/button';
 	import * as Dialog from '$lib/components/ui/dialog';
 	import {
-		BOOL_FALSE_STRING,
-		BOOL_TRUE_STRING,
 		DISMISSED_RECOMMENDED_MCP_SERVERS_LOCALSTORAGE_KEY,
 		HEADERS,
 		MCP_SERVER_ID_PREFIX,
 		RECOMMENDED_MCP_SERVERS
 	} from '$lib/constants';
-	import { HealthCheckStatus } from '$lib/enums';
+	import { BooleanString, HealthCheckStatus } from '$lib/enums';
 	import { conversationsStore, mcpStore } from '$lib/stores';
 	import { canonicalizeServerUrl, parseHeadersToArray, uuid } from '$lib/utils';
 
@@ -97,9 +95,9 @@
 
 		if (!raw) return false;
 
-		if (raw === BOOL_TRUE_STRING) return true;
+		if (raw === BooleanString.TRUE) return true;
 
-		if (raw === BOOL_FALSE_STRING) return false;
+		if (raw === BooleanString.FALSE) return false;
 
 		try {
 			const parsed = JSON.parse(raw);
@@ -116,7 +114,7 @@
 		if (browser) {
 			localStorage.setItem(
 				DISMISSED_RECOMMENDED_MCP_SERVERS_LOCALSTORAGE_KEY,
-				dismissed ? BOOL_TRUE_STRING : BOOL_FALSE_STRING
+				dismissed ? BooleanString.TRUE : BooleanString.FALSE
 			);
 		}
 	}
diff --git a/tools/ui/src/lib/components/app/misc/ConversationSelection.svelte b/tools/ui/src/lib/components/app/misc/ConversationSelection.svelte
index a3096ffb33..66f6b2cb89 100644
--- a/tools/ui/src/lib/components/app/misc/ConversationSelection.svelte
+++ b/tools/ui/src/lib/components/app/misc/ConversationSelection.svelte
@@ -3,6 +3,7 @@
 	import { Button } from '$lib/components/ui/button';
 	import { Checkbox } from '$lib/components/ui/checkbox';
 	import { ScrollArea } from '$lib/components/ui/scroll-area';
+	import { UI_DATA_ATTRS } from '$lib/constants';
 	import { useMarqueeSelection } from '$lib/hooks/use-marquee-selection.svelte';
 	import { SvelteSet } from 'svelte/reactivity';
 
@@ -138,7 +139,7 @@
 								class="cursor-pointer border-b transition-colors hover:bg-muted/50 {checked
 									? 'bg-muted/75'
 									: ''}"
-								data-conversation-row={conv.id}
+								{...{ [UI_DATA_ATTRS.CONVERSATION_ROW]: conv.id }}
 								onmousedown={(event) => marquee.rowMouseDown(conv.id, event)}
 								onclick={(event) => marquee.rowClick(conv.id, event.shiftKey)}
 							>
diff --git a/tools/ui/src/lib/components/app/navigation/SidebarNavigation/SidebarNavigationConversationItem.svelte b/tools/ui/src/lib/components/app/navigation/SidebarNavigation/SidebarNavigationConversationItem.svelte
index 5190badd67..a847e0aa92 100644
--- a/tools/ui/src/lib/components/app/navigation/SidebarNavigation/SidebarNavigationConversationItem.svelte
+++ b/tools/ui/src/lib/components/app/navigation/SidebarNavigation/SidebarNavigationConversationItem.svelte
@@ -15,7 +15,7 @@
 	import { TruncatedText } from '$lib/components/app';
 	import { Checkbox } from '$lib/components/ui/checkbox';
 	import * as Tooltip from '$lib/components/ui/tooltip';
-	import { FORK_TREE_DEPTH_PADDING, ICON_CLASS_DEFAULT } from '$lib/constants';
+	import { FORK_TREE_DEPTH_PADDING, ICON_CLASS_DEFAULT, UI_DATA_ATTRS } from '$lib/constants';
 	import { RouterService } from '$lib/services/router.service';
 	import { chatStore, conversationsStore } from '$lib/stores';
 	import { onMount } from 'svelte';
@@ -154,14 +154,13 @@
 	});
 </script>
 
-<!-- svelte-ignore a11y_mouse_events_have_key_events -->
 <button
 	class="group flex min-h-9 w-full cursor-pointer items-center justify-between space-x-3 rounded-lg py-1.5 text-left transition-colors hover:bg-foreground/10 {isActive
 		? 'bg-foreground/5 text-accent-foreground'
 		: ''} {isSelected ? 'bg-primary/10 hover:bg-primary/15' : ''} {isSelectionMode
 		? 'is-selection-mode'
 		: ''} px-2"
-	data-conversation-row={conversation.id}
+	{...{ [UI_DATA_ATTRS.CONVERSATION_ROW]: conversation.id }}
 	onclick={(e) => handleSelect(e)}
 	onmouseover={handleMouseOver}
 	onmouseleave={handleMouseLeave}
diff --git a/tools/ui/src/lib/components/app/settings/SettingsChatMobileHeader.svelte b/tools/ui/src/lib/components/app/settings/SettingsChatMobileHeader.svelte
index 7392b04460..58617956a0 100644
--- a/tools/ui/src/lib/components/app/settings/SettingsChatMobileHeader.svelte
+++ b/tools/ui/src/lib/components/app/settings/SettingsChatMobileHeader.svelte
@@ -1,6 +1,7 @@
 <script lang="ts">
 	import { ChevronLeft, ChevronRight, Settings } from '@lucide/svelte';
-	import { ICON_CLASS_DEFAULT } from '$lib/constants';
+	import { ICON_CLASS_DEFAULT, UI_DATA_ATTRS } from '$lib/constants';
+	import { BooleanString } from '$lib/enums';
 	import { useScrollCarousel } from '$lib/hooks/use-scroll-carousel.svelte';
 	import type { SettingsSection, SettingsSectionTitle } from '$lib/types';
 	import { onMount, tick } from 'svelte';
@@ -20,7 +21,9 @@
 		await tick();
 
 		if (carousel.scrollContainer) {
-			const activeTab = carousel.scrollContainer.querySelector('[data-active="true"]');
+			const activeTab = carousel.scrollContainer.querySelector(
+				`[${UI_DATA_ATTRS.ACTIVE}="${BooleanString.TRUE}"]`
+			);
 
 			if (activeTab instanceof HTMLElement) {
 				carousel.scrollToCenter(activeTab);
@@ -66,7 +69,7 @@
 								)
 									? 'bg-accent text-accent-foreground'
 									: 'text-muted-foreground'}"
-								data-active={isActive(section)}
+								{...{ [UI_DATA_ATTRS.ACTIVE]: isActive(section) }}
 								href={getHref(section)}
 								onclick={(e: MouseEvent) => {
 									carousel.scrollToCenter(e.currentTarget as HTMLElement);
@@ -82,7 +85,7 @@
 								)
 									? 'bg-accent text-accent-foreground'
 									: 'text-muted-foreground'}"
-								data-active={isActive(section)}
+								{...{ [UI_DATA_ATTRS.ACTIVE]: isActive(section) }}
 								onclick={(e: MouseEvent) => {
 									onSectionChange?.(section.title);
 									carousel.scrollToCenter(e.currentTarget as HTMLElement);
diff --git a/tools/ui/src/lib/constants/chat-form.constants.ts b/tools/ui/src/lib/constants/chat-form.constants.ts
index 6c413ed70a..9fb786f927 100644
--- a/tools/ui/src/lib/constants/chat-form.constants.ts
+++ b/tools/ui/src/lib/constants/chat-form.constants.ts
@@ -1,3 +1,6 @@
+/** Data attribute that tags ChatFormInputRich code spans and blocks. */
+export const CODE_TOKEN_ATTR = 'data-code-token';
+
 export const INITIAL_FILE_SIZE = 0;
 export const PROMPT_CONTENT_SEPARATOR = '\n\n';
 export const CLIPBOARD_CONTENT_QUOTE_PREFIX = '"';
diff --git a/tools/ui/src/lib/constants/headers.constants.ts b/tools/ui/src/lib/constants/headers.constants.ts
index f40df05710..d477fc8783 100644
--- a/tools/ui/src/lib/constants/headers.constants.ts
+++ b/tools/ui/src/lib/constants/headers.constants.ts
@@ -1,3 +1,6 @@
+/** Number of trailing characters to keep visible when partially redacting mcp-session-id */
+const MCP_SESSION_ID_VISIBLE_CHARS = 5;
+
 /** HTTP header handling for API and MCP requests. */
 export const HEADERS = {
 	/** Canonical casing for the Authorization header (RFC 7235) */
@@ -7,7 +10,7 @@ export const HEADERS = {
 	/** Content-Type HTTP header name */
 	CONTENT_TYPE: 'Content-Type',
 	/** Partial-redaction rules for MCP headers: header name -> visible trailing chars */
-	PARTIAL_REDACT: new Map<string, number>([['mcp-session-id', 5]]),
+	PARTIAL_REDACT: new Map<string, number>([['mcp-session-id', MCP_SESSION_ID_VISIBLE_CHARS]]),
 
 	/** Header names whose values should be redacted in diagnostic logs */
 	REDACTED: new Set([
diff --git a/tools/ui/src/lib/constants/markdown.constants.ts b/tools/ui/src/lib/constants/markdown.constants.ts
index c8289c39a1..298a2f1108 100644
--- a/tools/ui/src/lib/constants/markdown.constants.ts
+++ b/tools/ui/src/lib/constants/markdown.constants.ts
@@ -1,8 +1,14 @@
 export const IMAGE_NOT_ERROR_BOUND_SELECTOR = 'img:not([data-error-bound])';
-export const DATA_ERROR_BOUND_ATTR = 'errorBound';
-export const DATA_ERROR_HANDLED_ATTR = 'errorHandled';
-export const BOOL_TRUE_STRING = 'true';
-export const BOOL_FALSE_STRING = 'false';
+
+/** Data attributes for the markdown renderer DOM contract. */
+export const MARKDOWN_DATA_ATTRS = {
+	BLOCK_ID: 'data-block-id',
+	CODE_ID: 'data-code-id',
+	ERROR_BOUND: 'data-error-bound',
+	ERROR_HANDLED: 'data-error-handled',
+	LISTENER_BOUND: 'data-listener-bound',
+	ORIGINAL_SRC: 'data-original-src'
+} as const;
 
 /** Markdown structural markers used by `looksLikeMarkdown`. Inline / line-level. */
 export const MARKDOWN = {
diff --git a/tools/ui/src/lib/constants/mention-badge.constants.ts b/tools/ui/src/lib/constants/mention-badge.constants.ts
index a9ef963c39..e69211ec85 100644
--- a/tools/ui/src/lib/constants/mention-badge.constants.ts
+++ b/tools/ui/src/lib/constants/mention-badge.constants.ts
@@ -1,6 +1,6 @@
 /**
  * Shared visual contract between the two DOM-only badge paths (the
- * contenteditable tokenizer + the rehype plugin). Svelte cannot be
+ * ChatFormInputRich tokenizer + the rehype plugin). Svelte cannot be
  * mounted at the per-keystroke tokenizer hot path nor from a hast tree,
  * so both emit the badge with the same class string literal; Tailwind's
  * scanner picks it up in both sources.
@@ -10,6 +10,13 @@ export const MENTION_BADGE_CLASSNAME =
 
 export const MENTION_BADGE_ICON_CLASSNAME = 'h-3 w-3 shrink-0';
 
+/** Full `data-*` attribute names that tag ChatFormInputRich mention badges. */
+export const MENTION_BADGE_DATA_ATTRS = {
+	BADGE: 'data-mention-badge',
+	NAME: 'data-mention-name',
+	PATH: 'data-mention-path'
+} as const;
+
 /** Regex flag that makes the mention scanner walk every link in a message instead of the first. */
 export const MENTION_LINK_SCAN_FLAGS = 'g';
 
diff --git a/tools/ui/src/lib/constants/ui.constants.ts b/tools/ui/src/lib/constants/ui.constants.ts
index 3374b6e008..e4889649f4 100644
--- a/tools/ui/src/lib/constants/ui.constants.ts
+++ b/tools/ui/src/lib/constants/ui.constants.ts
@@ -7,6 +7,16 @@ import type { DesktopIconStripItem } from '$lib/types';
 export const FORK_TREE_DEPTH_PADDING = 8;
 export const SYSTEM_MESSAGE_PLACEHOLDER = 'System message';
 
+/** Data attributes for app-level DOM contracts. */
+export const UI_DATA_ATTRS = {
+	ACTIVE: 'data-active',
+	CONVERSATION_ROW: 'data-conversation-row',
+	HIGHLIGHT_THEME_PREVIEW: 'data-highlight-theme-preview',
+	PICKER_INDEX: 'data-picker-index',
+	RESULT_INDEX: 'data-result-index',
+	THUMBNAIL_INDEX: 'data-thumbnail-index'
+} as const;
+
 export const TOOL_GROUP_LABELS = {
 	[ToolSource.BUILTIN]: 'Built-in',
 	[ToolSource.CUSTOM]: 'JSON Schema',
diff --git a/tools/ui/src/lib/enums/boolean-string.enums.ts b/tools/ui/src/lib/enums/boolean-string.enums.ts
new file mode 100644
index 0000000000..d6b7ac8892
--- /dev/null
+++ b/tools/ui/src/lib/enums/boolean-string.enums.ts
@@ -0,0 +1,5 @@
+/** String representation of a boolean used in data attributes and persisted values. */
+export enum BooleanString {
+	TRUE = 'true',
+	FALSE = 'false'
+}
diff --git a/tools/ui/src/lib/enums/index.ts b/tools/ui/src/lib/enums/index.ts
index 5d0884b986..7ad7df7b8d 100644
--- a/tools/ui/src/lib/enums/index.ts
+++ b/tools/ui/src/lib/enums/index.ts
@@ -33,6 +33,8 @@ export {
 
 export { SessionRecordType } from './conversation-import.enums';
 
+export { BooleanString } from './boolean-string.enums';
+
 export { ReasoningEffort } from './reasoning-effort.enums';
 
 export {
diff --git a/tools/ui/src/lib/hooks/use-marquee-selection.svelte.ts b/tools/ui/src/lib/hooks/use-marquee-selection.svelte.ts
index 05436ecc8d..800327c43c 100644
--- a/tools/ui/src/lib/hooks/use-marquee-selection.svelte.ts
+++ b/tools/ui/src/lib/hooks/use-marquee-selection.svelte.ts
@@ -9,6 +9,7 @@
  * matches what the user sees on screen.
  */
 
+import { UI_DATA_ATTRS } from '$lib/constants';
 import { SvelteSet } from 'svelte/reactivity';
 
 interface UseMarqueeSelectionOptions {
@@ -18,8 +19,8 @@ interface UseMarqueeSelectionOptions {
 	orderedIds: () => string[];
 	/** Document listeners attach only while the getter returns true. */
 	enabled: () => boolean;
-	/** DOM attribute key (after the `data-` prefix) that marks selectable rows. */
-	attributeName?: () => string;
+	/** Full `data-*` attribute that marks selectable rows. */
+	dataAttr?: () => string;
 	/** Minimum pixel distance before a press becomes a marquee drag. */
 	dragThresholdPx?: number;
 }
@@ -36,16 +37,8 @@ export function useMarqueeSelection(options: UseMarqueeSelectionOptions) {
 	let dragMode: 'add' | 'remove' | null = null;
 	let suppressNextClick = false;
 
-	function resolveAttributeName(): string {
-		return options.attributeName?.() ?? 'conversation-row';
-	}
-
-	/**
-	 * `dataset` keys are camelCased. `data-conversation-row` -> `conversationRow`.
-	 * We resolve the attribute name once per call and read via the camelCase key.
-	 */
-	function datasetKey(key: string = resolveAttributeName()): string {
-		return key.replace(/-([a-z])/g, (_, c) => c.toUpperCase());
+	function resolveDataAttr(): string {
+		return options.dataAttr?.() ?? UI_DATA_ATTRS.CONVERSATION_ROW;
 	}
 
 	function decideDragMode(startingRowId: string | null, currentlySelected: ReadonlySet<string>) {
@@ -78,9 +71,8 @@ export function useMarqueeSelection(options: UseMarqueeSelectionOptions) {
 	}
 
 	function findRowAtPoint(x: number, y: number): string | null {
-		const attr = resolveAttributeName();
-		const selector = `[data-${attr}]`;
-		const key = datasetKey(attr);
+		const attr = resolveDataAttr();
+		const selector = `[${attr}]`;
 
 		let bestMatch: HTMLElement | null = null;
 		let bestCenterDistance = Infinity;
@@ -89,7 +81,7 @@ export function useMarqueeSelection(options: UseMarqueeSelectionOptions) {
 			const rect = row.getBoundingClientRect();
 
 			if (y >= rect.top && y <= rect.bottom && x >= rect.left && x <= rect.right) {
-				return row.dataset[key] ?? null;
+				return row.getAttribute(attr);
 			}
 
 			if (x >= rect.left && x <= rect.right) {
@@ -102,13 +94,12 @@ export function useMarqueeSelection(options: UseMarqueeSelectionOptions) {
 			}
 		}
 
-		return bestMatch ? (bestMatch.dataset[key] ?? null) : null;
+		return bestMatch ? bestMatch.getAttribute(attr) : null;
 	}
 
 	function updateMarqueeRect(currentX: number, currentY: number) {
-		const attr = resolveAttributeName();
-		const selector = `[data-${attr}]`;
-		const key = datasetKey(attr);
+		const attr = resolveDataAttr();
+		const selector = `[${attr}]`;
 		const selected = options.selectedIds();
 		const left = Math.min(dragStartX, currentX);
 		const top = Math.min(dragStartY, currentY);
@@ -117,7 +108,7 @@ export function useMarqueeSelection(options: UseMarqueeSelectionOptions) {
 		const visibleIds = new SvelteSet(options.orderedIds());
 
 		for (const row of document.querySelectorAll<HTMLElement>(selector)) {
-			const id = row.dataset[key];
+			const id = row.getAttribute(attr);
 
 			if (!id || !visibleIds.has(id)) continue;
 
diff --git a/tools/ui/src/lib/hooks/use-scroll-active-row.svelte.ts b/tools/ui/src/lib/hooks/use-scroll-active-row.svelte.ts
index 3e7cf05440..d353c93743 100644
--- a/tools/ui/src/lib/hooks/use-scroll-active-row.svelte.ts
+++ b/tools/ui/src/lib/hooks/use-scroll-active-row.svelte.ts
@@ -11,8 +11,8 @@ export interface UseScrollActiveRowOptions {
 	getContainer: () => HTMLDivElement | null;
 	getIndex: () => number;
 	getCount: () => number;
-	/** Attribute prefix, e.g. 'picker' for `[data-picker-index="0"]`. */
-	dataIndex: string;
+	/** Full data attribute marking the row, e.g. `data-picker-index`. */
+	dataAttr: string;
 }
 
 export function useScrollActiveRow(opts: UseScrollActiveRowOptions) {
@@ -41,9 +41,7 @@ export function useScrollActiveRow(opts: UseScrollActiveRowOptions) {
 
 			if (!container || index < 0 || index >= opts.getCount()) return;
 
-			const row = container.querySelector(
-				`[data-${opts.dataIndex}-index="${index}"]`
-			) as HTMLElement | null;
+			const row = container.querySelector(`[${opts.dataAttr}="${index}"]`) as HTMLElement | null;
 
 			row?.scrollIntoView({ block: 'nearest', inline: 'nearest' });
 		});
diff --git a/tools/ui/src/lib/services/migration.service.ts b/tools/ui/src/lib/services/migration.service.ts
index 3dcbbed827..2626a42b3c 100644
--- a/tools/ui/src/lib/services/migration.service.ts
+++ b/tools/ui/src/lib/services/migration.service.ts
@@ -29,7 +29,7 @@ import {
 	STORAGE_APP_NAME,
 	STORAGE_APP_NAME_DEPRECATED
 } from '$lib/constants';
-import { MessageRole } from '$lib/enums';
+import { BooleanString, MessageRole } from '$lib/enums';
 import Dexie from 'dexie';
 
 // Types
@@ -613,10 +613,10 @@ const configTypesMigration: Migration = {
 		// schema rejects them. No config string field holds exactly "true"/"false", so the
 		// match is unambiguous.
 		for (const key of Object.keys(config)) {
-			if (config[key] === 'true') {
+			if (config[key] === BooleanString.TRUE) {
 				config[key] = true;
 				changed = true;
-			} else if (config[key] === 'false') {
+			} else if (config[key] === BooleanString.FALSE) {
 				config[key] = false;
 				changed = true;
 			}
diff --git a/tools/ui/src/lib/types/index.ts b/tools/ui/src/lib/types/index.ts
index 497bfd415e..562b63e980 100644
--- a/tools/ui/src/lib/types/index.ts
+++ b/tools/ui/src/lib/types/index.ts
@@ -182,7 +182,7 @@ export type {
 	GlobSearchChildResult
 } from './glob';
 
-// Contenteditable token types (chat form)
+// ChatFormInputRich token types (chat form)
 export type { ChatFormInputRichToken } from './chat-form-input-rich';
 
 // Agentic types
diff --git a/tools/ui/src/lib/utils/chat-form-input-rich-tokenizer.ts b/tools/ui/src/lib/utils/chat-form-input-rich-tokenizer.ts
index 7af350b24d..c09afd018b 100644
--- a/tools/ui/src/lib/utils/chat-form-input-rich-tokenizer.ts
+++ b/tools/ui/src/lib/utils/chat-form-input-rich-tokenizer.ts
@@ -30,12 +30,14 @@ import {
 	getMentionBadgeLabel
 } from './mention-badge';
 import {
+	CODE_TOKEN_ATTR,
 	MENTION_BADGE_CLASSNAME,
+	MENTION_BADGE_DATA_ATTRS,
 	MENTION_BADGE_ICON_CLASSNAME,
 	MENTION_BADGE_SVG_ATTRIBUTES,
 	SETTINGS_KEYS
 } from '$lib/constants';
-import { ChatFormInputRichTokenKind } from '$lib/enums';
+import { BooleanString, ChatFormInputRichTokenKind } from '$lib/enums';
 import { settingsStore } from '$lib/stores/settings.svelte';
 import { toolsStore } from '$lib/stores/tools.svelte';
 import type { ChatFormInputRichToken } from '$lib/types/chat-form-input-rich';
@@ -168,7 +170,8 @@ function pushTextAndBadgeTokens(input: string, tokens: ChatFormInputRichToken[])
 
 function isCodeBlockElement(node: Node | null): node is HTMLElement {
 	return (
-		node instanceof HTMLElement && node.dataset.codeToken === ChatFormInputRichTokenKind.CODE_BLOCK
+		node instanceof HTMLElement &&
+		node.getAttribute(CODE_TOKEN_ATTR) === ChatFormInputRichTokenKind.CODE_BLOCK
 	);
 }
 
@@ -210,9 +213,9 @@ export function serializeContent(root: HTMLElement): string {
 
 			const el = child as HTMLElement;
 
-			if (el.dataset.mentionBadge === 'true') {
-				const name = el.dataset.mentionName ?? '';
-				const path = el.dataset.mentionPath ?? '';
+			if (el.getAttribute(MENTION_BADGE_DATA_ATTRS.BADGE) === BooleanString.TRUE) {
+				const name = el.getAttribute(MENTION_BADGE_DATA_ATTRS.NAME) ?? '';
+				const path = el.getAttribute(MENTION_BADGE_DATA_ATTRS.PATH) ?? '';
 
 				if (name && path) {
 					if (pendingBlockBoundary) {
@@ -227,8 +230,10 @@ export function serializeContent(root: HTMLElement): string {
 				continue;
 			}
 
-			if (el.dataset.codeToken !== undefined) {
-				const isBlock = el.dataset.codeToken === ChatFormInputRichTokenKind.CODE_BLOCK;
+			const codeToken = el.getAttribute(CODE_TOKEN_ATTR);
+
+			if (codeToken !== null) {
+				const isBlock = codeToken === ChatFormInputRichTokenKind.CODE_BLOCK;
 
 				if (isBlock && (pendingBlockBoundary || !first)) out += '\n';
 
@@ -297,8 +302,8 @@ export function domMatchesTokens(root: HTMLElement, tokens: ChatFormInputRichTok
 			if (child.nodeType !== Node.ELEMENT_NODE) continue;
 
 			const el = child as HTMLElement;
-			const isBadge = el.dataset.mentionBadge === 'true';
-			const isCode = el.dataset.codeToken !== undefined;
+			const isBadge = el.getAttribute(MENTION_BADGE_DATA_ATTRS.BADGE) === BooleanString.TRUE;
+			const isCode = el.getAttribute(CODE_TOKEN_ATTR) !== null;
 
 			if (!isBadge && !isCode) {
 				if (!walk(el)) return false;
@@ -313,15 +318,15 @@ export function domMatchesTokens(root: HTMLElement, tokens: ChatFormInputRichTok
 			if (isBadge) {
 				if (token.kind !== ChatFormInputRichTokenKind.BADGE) return false;
 
-				if (token.name !== (el.dataset.mentionName ?? '')) return false;
+				if (token.name !== (el.getAttribute(MENTION_BADGE_DATA_ATTRS.NAME) ?? '')) return false;
 
-				if (token.path !== (el.dataset.mentionPath ?? '')) return false;
+				if (token.path !== (el.getAttribute(MENTION_BADGE_DATA_ATTRS.PATH) ?? '')) return false;
 
 				continue;
 			}
 
 			const codeKind: ChatFormInputRichTokenKind =
-				el.dataset.codeToken === ChatFormInputRichTokenKind.CODE_BLOCK
+				el.getAttribute(CODE_TOKEN_ATTR) === ChatFormInputRichTokenKind.CODE_BLOCK
 					? ChatFormInputRichTokenKind.CODE_BLOCK
 					: ChatFormInputRichTokenKind.CODE_INLINE;
 
@@ -430,8 +435,11 @@ export function rangeToTextOffset(root: HTMLElement, range: Range | null): numbe
 				total += 1;
 			}
 
-			if (el.dataset.mentionBadge === 'true') {
-				const len = badgeSourceLength(el.dataset.mentionName ?? '', el.dataset.mentionPath ?? '');
+			if (el.getAttribute(MENTION_BADGE_DATA_ATTRS.BADGE) === BooleanString.TRUE) {
+				const len = badgeSourceLength(
+					el.getAttribute(MENTION_BADGE_DATA_ATTRS.NAME) ?? '',
+					el.getAttribute(MENTION_BADGE_DATA_ATTRS.PATH) ?? ''
+				);
 
 				if (len === 0) continue;
 
@@ -447,8 +455,10 @@ export function rangeToTextOffset(root: HTMLElement, range: Range | null): numbe
 				continue;
 			}
 
-			if (el.dataset.codeToken !== undefined) {
-				const isBlock = el.dataset.codeToken === ChatFormInputRichTokenKind.CODE_BLOCK;
+			const codeToken = el.getAttribute(CODE_TOKEN_ATTR);
+
+			if (codeToken !== null) {
+				const isBlock = codeToken === ChatFormInputRichTokenKind.CODE_BLOCK;
 
 				if (isBlock && !first) {
 					if (!atOrBeforeCaret(el, 0)) {
@@ -562,7 +572,7 @@ export function buildFragment(tokens: ChatFormInputRichToken[]): DocumentFragmen
 		) {
 			const code = document.createElement('code');
 
-			code.dataset.codeToken = token.kind;
+			code.setAttribute(CODE_TOKEN_ATTR, token.kind);
 			code.textContent = token.text;
 			fragment.appendChild(code);
 
@@ -578,9 +588,9 @@ export function buildFragment(tokens: ChatFormInputRichToken[]): DocumentFragmen
 
 		const badge = document.createElement('span');
 
-		badge.dataset.mentionBadge = 'true';
-		badge.dataset.mentionName = token.name;
-		badge.dataset.mentionPath = token.path;
+		badge.setAttribute(MENTION_BADGE_DATA_ATTRS.BADGE, BooleanString.TRUE);
+		badge.setAttribute(MENTION_BADGE_DATA_ATTRS.NAME, token.name);
+		badge.setAttribute(MENTION_BADGE_DATA_ATTRS.PATH, token.path);
 		badge.title = decodeFileLinkPath(token.path);
 		badge.className = MENTION_BADGE_CLASSNAME;
 		badge.contentEditable = 'false';
@@ -876,8 +886,11 @@ export function textOffsetToRange(root: HTMLElement, offset: number): Range {
 
 			const el = child as HTMLElement;
 
-			if (el.dataset.mentionBadge === 'true') {
-				const len = badgeSourceLength(el.dataset.mentionName ?? '', el.dataset.mentionPath ?? '');
+			if (el.getAttribute(MENTION_BADGE_DATA_ATTRS.BADGE) === BooleanString.TRUE) {
+				const len = badgeSourceLength(
+					el.getAttribute(MENTION_BADGE_DATA_ATTRS.NAME) ?? '',
+					el.getAttribute(MENTION_BADGE_DATA_ATTRS.PATH) ?? ''
+				);
 
 				if (len === 0) continue;
 
@@ -915,8 +928,10 @@ export function textOffsetToRange(root: HTMLElement, offset: number): Range {
 				continue;
 			}
 
-			if (el.dataset.codeToken !== undefined) {
-				const isBlock = el.dataset.codeToken === ChatFormInputRichTokenKind.CODE_BLOCK;
+			const codeToken = el.getAttribute(CODE_TOKEN_ATTR);
+
+			if (codeToken !== null) {
+				const isBlock = codeToken === ChatFormInputRichTokenKind.CODE_BLOCK;
 
 				if (isBlock && (pendingBlockBoundary || !first)) {
 					pendingBlockBoundary = false;
diff --git a/tools/ui/src/lib/utils/index.ts b/tools/ui/src/lib/utils/index.ts
index abe2142724..3fb701b323 100644
--- a/tools/ui/src/lib/utils/index.ts
+++ b/tools/ui/src/lib/utils/index.ts
@@ -207,7 +207,7 @@ export {
 	type CommandDismissSnapshot
 } from './command-token';
 
-// Tokenization for the chat-form contenteditable (mention links + code spans <-> chip DOM)
+// Tokenization for the ChatFormInputRich (mention links + code spans <-> chip DOM)
 export {
 	tokenizeContent,
 	containsCodeSpan,
@@ -223,10 +223,10 @@ export {
 	leadingBadgeEdgeOffset
 } from './chat-form-input-rich-tokenizer';
 
-// Source-space undo/redo history for the chat-form contenteditable
+// Source-space undo/redo history for the ChatFormInputRich
 export { SourceHistory, type SourceHistoryEntry } from './source-history';
 
-// Mention-badge visual contract (used by the contenteditable / rehype
+// Mention-badge visual contract (used by the ChatFormInputRich / rehype
 // DOM paths that build the same chip without a Svelte mount)
 export {
 	containsFileMentionLink,
diff --git a/tools/ui/src/lib/utils/source-history.ts b/tools/ui/src/lib/utils/source-history.ts
index 12b273b243..32995ae034 100644
--- a/tools/ui/src/lib/utils/source-history.ts
+++ b/tools/ui/src/lib/utils/source-history.ts
@@ -1,5 +1,5 @@
 /**
- * Source-space undo/redo history for the chat-form contenteditable, whose
+ * Source-space undo/redo history for the ChatFormInputRich, whose
  * imperative DOM rebuilds destroy the browser's native undo stack.
  * Entries record the state BEFORE an edit; edits within `groupWindowMs`
  * extend the open group so a typing burst undoes as a unit, while
diff --git a/tools/ui/tests/client/chat-form-enter-code-block.svelte.test.ts b/tools/ui/tests/client/chat-form-enter-code-block.svelte.test.ts
index e7c1779276..485dc39655 100644
--- a/tools/ui/tests/client/chat-form-enter-code-block.svelte.test.ts
+++ b/tools/ui/tests/client/chat-form-enter-code-block.svelte.test.ts
@@ -2,7 +2,7 @@
 // fenced-code-block flow: while the caret sits inside a fenced
 // block region - closed, or still OPEN while the user is typing
 // one - plain Enter adds a line instead of submitting the message.
-// The textarea path is covered here end-to-end (the contenteditable
+// The textarea path is covered here end-to-end (the ChatFormInputRich
 // consumes the same case locally; see chat-form-input-rich).
 
 import ChatFormTestWrapper from './components/ChatFormTestWrapper.svelte';
diff --git a/tools/ui/tests/client/chat-form-input-rich-blocks.svelte.test.ts b/tools/ui/tests/client/chat-form-input-rich-blocks.svelte.test.ts
index ab7fa01334..7c59ff951c 100644
--- a/tools/ui/tests/client/chat-form-input-rich-blocks.svelte.test.ts
+++ b/tools/ui/tests/client/chat-form-input-rich-blocks.svelte.test.ts
@@ -1,4 +1,4 @@
-// Guards the newline contract of the chat-form contenteditable: browsers
+// Guards the newline contract of the ChatFormInputRich: browsers
 // restructure the flat DOM on Enter (`<div>` wrappers, `<br>` shapes) and
 // serialization must fold those back into `\n` so the emitted value never
 // diverges from what is on screen.
@@ -13,7 +13,7 @@ const SOURCE = 'see [docs](file:///a/b) here';
 function editableIn(container: HTMLElement): HTMLElement {
 	const el = container.querySelector('[role="textbox"]');
 
-	if (!(el instanceof HTMLElement)) throw new Error('contenteditable not rendered');
+	if (!(el instanceof HTMLElement)) throw new Error('ChatFormInputRich not rendered');
 
 	return el;
 }
diff --git a/tools/ui/tests/client/chat-form-input-rich-undo.svelte.test.ts b/tools/ui/tests/client/chat-form-input-rich-undo.svelte.test.ts
index a89a888e02..00f07f4820 100644
--- a/tools/ui/tests/client/chat-form-input-rich-undo.svelte.test.ts
+++ b/tools/ui/tests/client/chat-form-input-rich-undo.svelte.test.ts
@@ -1,4 +1,4 @@
-// Guards the editing-key contract of the chat-form contenteditable:
+// Guards the editing-key contract of the ChatFormInputRich:
 // undo/redo is replayed from source snapshots (the token rebuilds destroy
 // the native undo stack), and Tab is NOT intercepted (WCAG 2.1.2 no
 // keyboard trap), matching the plain textarea.
@@ -13,7 +13,7 @@ const SOURCE = 'see [docs](file:///a/b)';
 function editableIn(container: HTMLElement): HTMLElement {
 	const el = container.querySelector('[role="textbox"]');
 
-	if (!(el instanceof HTMLElement)) throw new Error('contenteditable not rendered');
+	if (!(el instanceof HTMLElement)) throw new Error('ChatFormInputRich not rendered');
 
 	return el;
 }
diff --git a/tools/ui/tests/client/chat-form-input-rich.svelte.test.ts b/tools/ui/tests/client/chat-form-input-rich.svelte.test.ts
index c5120fb198..d3403030d5 100644
--- a/tools/ui/tests/client/chat-form-input-rich.svelte.test.ts
+++ b/tools/ui/tests/client/chat-form-input-rich.svelte.test.ts
@@ -1,4 +1,4 @@
-// Guards the clipboard contract of the chat-form contenteditable:
+// Guards the clipboard contract of the ChatFormInputRich:
 // copy/cut expose the markdown SOURCE of the selection (each badge
 // contributes its full `[name](file://...)` link) and pasting such
 // markdown re-renders the badges.
@@ -16,7 +16,7 @@ const BADGE_SELECTOR = '[data-mention-badge="true"]';
 function editableIn(container: HTMLElement): HTMLElement {
 	const el = container.querySelector('[role="textbox"]');
 
-	if (!(el instanceof HTMLElement)) throw new Error('contenteditable not rendered');
+	if (!(el instanceof HTMLElement)) throw new Error('ChatFormInputRich not rendered');
 
 	return el;
 }
diff --git a/tools/ui/tests/client/chat-form-mention-picker-gate.svelte.test.ts b/tools/ui/tests/client/chat-form-mention-picker-gate.svelte.test.ts
index a6ff29597a..fc06e5d2c1 100644
--- a/tools/ui/tests/client/chat-form-mention-picker-gate.svelte.test.ts
+++ b/tools/ui/tests/client/chat-form-mention-picker-gate.svelte.test.ts
@@ -3,7 +3,7 @@
 // it, the picker still opens but explains why instead of firing searches
 // that would only fail with "Search failed".
 
-import ChatFormMentionPicker from '$lib/components/app/chat/ChatForm/ChatFormPickers/ChatFormPickerMention.svelte';
+import ChatFormPickerMention from '$lib/components/app/chat/ChatForm/ChatFormPickers/ChatFormPickerMention.svelte';
 import { DISABLED_TOOL_KEYS_LOCALSTORAGE_KEY } from '$lib/constants';
 import { BuiltInTool } from '$lib/enums';
 import { toolsStore } from '$lib/stores/tools.svelte';
@@ -25,7 +25,7 @@ function setBuiltinTools(defs: OpenAIToolDefinition[]) {
 }
 
 function renderPicker() {
-	return render(ChatFormMentionPicker, {
+	return render(ChatFormPickerMention, {
 		isOpen: true,
 		onClose: () => {},
 		onSelect: () => {},
@@ -39,7 +39,7 @@ afterEach(() => {
 	localStorage.removeItem(DISABLED_TOOL_KEYS_LOCALSTORAGE_KEY);
 });
 
-describe('ChatFormMentionPicker file_glob_search gate', () => {
+describe('ChatFormPickerMention file_glob_search gate', () => {
 	it('explains that file search is unavailable when the server has no tools', async () => {
 		setBuiltinTools([]);
 		renderPicker();