diff --git a/ui/src/ui/app-chat.ts b/ui/src/ui/app-chat.ts
index 9ace7542120..a2d5736c33e 100644
--- a/ui/src/ui/app-chat.ts
+++ b/ui/src/ui/app-chat.ts
@@ -10,6 +10,7 @@ import { loadSessions } from "./controllers/sessions.ts";
 import type { GatewayBrowserClient, GatewayHelloOk } from "./gateway.ts";
 import { normalizeBasePath } from "./navigation.ts";
 import { parseAgentSessionKey } from "./session-key.ts";
+import { normalizeLowercaseStringOrEmpty } from "./string-coerce.ts";
 import type { ChatModelOverride, ModelCatalogEntry } from "./types.ts";
 import type { SessionsListResult } from "./types.ts";
 import type { ChatAttachment, ChatQueueItem } from "./ui-types.ts";
@@ -51,7 +52,7 @@ export function isChatStopCommand(text: string) {
   if (!trimmed) {
     return false;
   }
-  const normalized = trimmed.toLowerCase();
+  const normalized = normalizeLowercaseStringOrEmpty(trimmed);
   if (normalized === "/stop") {
     return true;
   }
@@ -69,7 +70,7 @@ function isChatResetCommand(text: string) {
   if (!trimmed) {
     return false;
   }
-  const normalized = trimmed.toLowerCase();
+  const normalized = normalizeLowercaseStringOrEmpty(trimmed);
   if (normalized === "/new" || normalized === "/reset") {
     return true;
   }
diff --git a/ui/src/ui/app-render.helpers.ts b/ui/src/ui/app-render.helpers.ts
index 7205db64fa1..3e9bd70f6a4 100644
--- a/ui/src/ui/app-render.helpers.ts
+++ b/ui/src/ui/app-render.helpers.ts
@@ -16,6 +16,7 @@ import { loadSessions } from "./controllers/sessions.ts";
 import { icons } from "./icons.ts";
 import { iconForTab, pathForTab, titleForTab, type Tab } from "./navigation.ts";
 import { parseAgentSessionKey } from "./session-key.ts";
+import { normalizeLowercaseStringOrEmpty } from "./string-coerce.ts";
 import type { ThemeMode } from "./theme.ts";
 import {
   listThinkingLevelLabels,
@@ -607,7 +608,7 @@ function buildThinkingOptions(
     if (!trimmed) {
       return;
     }
-    const key = trimmed.toLowerCase();
+    const key = normalizeLowercaseStringOrEmpty(trimmed);
     if (seen.has(key)) {
       return;
     }
@@ -624,7 +625,7 @@ function buildThinkingOptions(
   };
 
   for (const label of listThinkingLevelLabels(provider)) {
-    const normalized = normalizeThinkLevel(label) ?? label.trim().toLowerCase();
+    const normalized = normalizeThinkLevel(label) ?? normalizeLowercaseStringOrEmpty(label);
     addOption(normalized);
   }
   if (currentOverride) {
@@ -807,7 +808,7 @@ function capitalize(s: string): string {
  * fallback display name.  Exported for testing.
  */
 export function parseSessionKey(key: string): SessionKeyInfo {
-  const normalized = key.toLowerCase();
+  const normalized = normalizeLowercaseStringOrEmpty(key);
 
   // ── Main session ─────────────────────────────────
   if (key === "main" || key === "agent:main:main") {
@@ -878,7 +879,7 @@ export function resolveSessionDisplayName(
 }
 
 export function isCronSessionKey(key: string): boolean {
-  const normalized = key.trim().toLowerCase();
+  const normalized = normalizeLowercaseStringOrEmpty(key);
   if (!normalized) {
     return false;
   }
@@ -946,7 +947,7 @@ export function resolveSessionOptionGroups(
     const parsed = parseAgentSessionKey(key);
     const group = parsed
       ? ensureGroup(
-          `agent:${parsed.agentId.toLowerCase()}`,
+          `agent:${normalizeLowercaseStringOrEmpty(parsed.agentId)}`,
           resolveAgentGroupLabel(state, parsed.agentId),
         )
       : ensureGroup("other", "Other Sessions");
@@ -1060,9 +1061,9 @@ function countHiddenCronSessions(sessionKey: string, sessions: SessionsListResul
 }
 
 function resolveAgentGroupLabel(state: AppViewState, agentIdRaw: string): string {
-  const normalized = agentIdRaw.trim().toLowerCase();
+  const normalized = normalizeLowercaseStringOrEmpty(agentIdRaw);
   const agent = (state.agentsList?.agents ?? []).find(
-    (entry) => entry.id.trim().toLowerCase() === normalized,
+    (entry) => normalizeLowercaseStringOrEmpty(entry.id) === normalized,
   );
   const name = agent?.identity?.name?.trim() || agent?.name?.trim() || "";
   return name && name !== agentIdRaw ? `${name} (${agentIdRaw})` : agentIdRaw;
diff --git a/ui/src/ui/app-render.ts b/ui/src/ui/app-render.ts
index 8bca6abc167..4151f47a64f 100644
--- a/ui/src/ui/app-render.ts
+++ b/ui/src/ui/app-render.ts
@@ -101,14 +101,15 @@ import {
   updateSkillEnabled,
 } from "./controllers/skills.ts";
 import { buildExternalLinkRel, EXTERNAL_LINK_TARGET } from "./external-link.ts";
-import "./components/dashboard-header.ts";
 import { icons } from "./icons.ts";
+import "./components/dashboard-header.ts";
 import { normalizeBasePath, TAB_GROUPS, subtitleForTab, titleForTab } from "./navigation.ts";
 import {
   buildAgentMainSessionKey,
   parseAgentSessionKey,
   resolveAgentIdFromSessionKey,
 } from "./session-key.ts";
+import { normalizeLowercaseStringOrEmpty } from "./string-coerce.ts";
 import { agentLogoUrl } from "./views/agents-utils.ts";
 import {
   resolveAgentConfig,
@@ -218,7 +219,7 @@ function uniquePreserveOrder(values: string[]): string[] {
     if (!normalized) {
       continue;
     }
-    const key = normalized.toLowerCase();
+    const key = normalizeLowercaseStringOrEmpty(normalized);
     if (seen.has(key)) {
       continue;
     }
diff --git a/ui/src/ui/app-tool-stream.ts b/ui/src/ui/app-tool-stream.ts
index 007b4774e98..6727b00d824 100644
--- a/ui/src/ui/app-tool-stream.ts
+++ b/ui/src/ui/app-tool-stream.ts
@@ -1,4 +1,5 @@
 import { formatUnknownText, truncateText } from "./format.ts";
+import { normalizeLowercaseStringOrEmpty } from "./string-coerce.ts";
 
 const TOOL_STREAM_LIMIT = 50;
 const TOOL_STREAM_THROTTLE_MS = 80;
@@ -53,7 +54,11 @@ function resolveModelLabel(provider: unknown, model: unknown): string | null {
   const providerValue = toTrimmedString(provider);
   if (providerValue) {
     const prefix = `${providerValue}/`;
-    if (modelValue.toLowerCase().startsWith(prefix.toLowerCase())) {
+    if (
+      normalizeLowercaseStringOrEmpty(modelValue).startsWith(
+        normalizeLowercaseStringOrEmpty(prefix),
+      )
+    ) {
       const trimmedModel = modelValue.slice(prefix.length).trim();
       if (trimmedModel) {
         return `${providerValue}/${trimmedModel}`;
diff --git a/ui/src/ui/app.ts b/ui/src/ui/app.ts
index a0959d27584..b52aadc953d 100644
--- a/ui/src/ui/app.ts
+++ b/ui/src/ui/app.ts
@@ -72,6 +72,7 @@ import type { GatewayBrowserClient, GatewayHelloOk } from "./gateway.ts";
 import type { Tab } from "./navigation.ts";
 import { resolveAgentIdFromSessionKey } from "./session-key.ts";
 import { loadSettings, type UiSettings } from "./storage.ts";
+import { normalizeLowercaseStringOrEmpty } from "./string-coerce.ts";
 import { VALID_THEME_NAMES, type ResolvedTheme, type ThemeMode, type ThemeName } from "./theme.ts";
 import type {
   AgentsListResult,
@@ -118,7 +119,7 @@ function resolveOnboardingMode(): boolean {
   if (!raw) {
     return false;
   }
-  const normalized = raw.trim().toLowerCase();
+  const normalized = normalizeLowercaseStringOrEmpty(raw);
   return normalized === "1" || normalized === "true" || normalized === "yes" || normalized === "on";
 }
 
diff --git a/ui/src/ui/chat-event-reload.ts b/ui/src/ui/chat-event-reload.ts
index 2eb211d01aa..91f5a141137 100644
--- a/ui/src/ui/chat-event-reload.ts
+++ b/ui/src/ui/chat-event-reload.ts
@@ -1,4 +1,5 @@
 import type { ChatEventPayload } from "./controllers/chat.ts";
+import { normalizeLowercaseStringOrEmpty } from "./string-coerce.ts";
 
 export function shouldReloadHistoryForFinalEvent(payload?: ChatEventPayload): boolean {
   if (!payload || payload.state !== "final") {
@@ -8,7 +9,7 @@ export function shouldReloadHistoryForFinalEvent(payload?: ChatEventPayload): bo
     return true;
   }
   const message = payload.message as Record<string, unknown>;
-  const role = typeof message.role === "string" ? message.role.toLowerCase() : "";
+  const role = normalizeLowercaseStringOrEmpty(message.role);
   if (role && role !== "assistant") {
     return true;
   }
diff --git a/ui/src/ui/chat-model-ref.ts b/ui/src/ui/chat-model-ref.ts
index deb819fa8e3..849bb679704 100644
--- a/ui/src/ui/chat-model-ref.ts
+++ b/ui/src/ui/chat-model-ref.ts
@@ -1,3 +1,4 @@
+import { normalizeLowercaseStringOrEmpty } from "./string-coerce.ts";
 import type { ModelCatalogEntry } from "./types.ts";
 
 export type ChatModelOverride =
@@ -68,8 +69,9 @@ export function resolveChatModelOverride(
   }
 
   let matchedValue = "";
+  const normalizedTrimmed = normalizeLowercaseStringOrEmpty(trimmed);
   for (const entry of catalog) {
-    if (entry.id.trim().toLowerCase() !== trimmed.toLowerCase()) {
+    if (normalizeLowercaseStringOrEmpty(entry.id) !== normalizedTrimmed) {
       continue;
     }
     const candidate = buildQualifiedChatModelValue(entry.id, entry.provider);
@@ -77,7 +79,9 @@ export function resolveChatModelOverride(
       matchedValue = candidate;
       continue;
     }
-    if (matchedValue.toLowerCase() !== candidate.toLowerCase()) {
+    if (
+      normalizeLowercaseStringOrEmpty(matchedValue) !== normalizeLowercaseStringOrEmpty(candidate)
+    ) {
       return { value: trimmed, source: "raw", reason: "ambiguous" };
     }
   }
diff --git a/ui/src/ui/chat-model-select-state.ts b/ui/src/ui/chat-model-select-state.ts
index eede05c54c8..ab1deb3fd29 100644
--- a/ui/src/ui/chat-model-select-state.ts
+++ b/ui/src/ui/chat-model-select-state.ts
@@ -5,6 +5,7 @@ import {
   normalizeChatModelOverrideValue,
   resolvePreferredServerChatModelValue,
 } from "./chat-model-ref.ts";
+import { normalizeLowercaseStringOrEmpty } from "./string-coerce.ts";
 import type { ModelCatalogEntry } from "./types.ts";
 
 type ChatModelSelectStateInput = Pick<
@@ -66,7 +67,7 @@ function buildChatModelOptions(
     if (!trimmed) {
       return;
     }
-    const key = trimmed.toLowerCase();
+    const key = normalizeLowercaseStringOrEmpty(trimmed);
     if (seen.has(key)) {
       return;
     }
diff --git a/ui/src/ui/chat/grouped-render.ts b/ui/src/ui/chat/grouped-render.ts
index f11fba066bb..4c4329c9c67 100644
--- a/ui/src/ui/chat/grouped-render.ts
+++ b/ui/src/ui/chat/grouped-render.ts
@@ -5,6 +5,7 @@ import type { AssistantIdentity } from "../assistant-identity.ts";
 import { icons } from "../icons.ts";
 import { toSanitizedMarkdownHtml } from "../markdown.ts";
 import { openExternalUrlSafe } from "../open-external-url.ts";
+import { normalizeLowercaseStringOrEmpty } from "../string-coerce.ts";
 import { detectTextDirection } from "../text-direction.ts";
 import type { MessageGroup, ToolCard } from "../types/chat-types.ts";
 import { agentLogoUrl } from "../views/agents-utils.ts";
@@ -726,10 +727,11 @@ function renderGroupedMessage(
   const m = message as Record<string, unknown>;
   const role = typeof m.role === "string" ? m.role : "unknown";
   const normalizedRole = normalizeRoleForGrouping(role);
+  const normalizedRawRole = normalizeLowercaseStringOrEmpty(role);
   const isToolResult =
     isToolResultMessage(message) ||
-    role.toLowerCase() === "toolresult" ||
-    role.toLowerCase() === "tool_result" ||
+    normalizedRawRole === "toolresult" ||
+    normalizedRawRole === "tool_result" ||
     typeof m.toolCallId === "string" ||
     typeof m.tool_call_id === "string";
 
@@ -752,7 +754,12 @@ function renderGroupedMessage(
   // Detect pure-JSON messages and render as collapsible block
   const jsonResult = markdown && !opts.isStreaming ? detectJson(markdown) : null;
 
-  const bubbleClasses = ["chat-bubble", opts.isStreaming ? "streaming" : "", "fade-in", canCopyMarkdown ? "has-copy" : ""]
+  const bubbleClasses = [
+    "chat-bubble",
+    opts.isStreaming ? "streaming" : "",
+    "fade-in",
+    canCopyMarkdown ? "has-copy" : "",
+  ]
     .filter(Boolean)
     .join(" ");
 
diff --git a/ui/src/ui/chat/message-extract.ts b/ui/src/ui/chat/message-extract.ts
index 457918a3a45..e743d8c305a 100644
--- a/ui/src/ui/chat/message-extract.ts
+++ b/ui/src/ui/chat/message-extract.ts
@@ -2,12 +2,13 @@ import { stripInboundMetadata } from "../../../../src/auto-reply/reply/strip-inb
 import { stripEnvelope } from "../../../../src/shared/chat-envelope.js";
 import { extractAssistantVisibleText as extractSharedAssistantVisibleText } from "../../../../src/shared/chat-message-content.js";
 import { stripThinkingTags } from "../format.ts";
+import { normalizeLowercaseStringOrEmpty } from "../string-coerce.ts";
 
 const textCache = new WeakMap<object, string | null>();
 const thinkingCache = new WeakMap<object, string | null>();
 
 function processMessageText(text: string, role: string): string {
-  const shouldStripInboundMetadata = role.toLowerCase() === "user";
+  const shouldStripInboundMetadata = normalizeLowercaseStringOrEmpty(role) === "user";
   if (role === "assistant") {
     return stripThinkingTags(text);
   }
diff --git a/ui/src/ui/chat/message-normalizer.ts b/ui/src/ui/chat/message-normalizer.ts
index 308f0278700..94d53f30905 100644
--- a/ui/src/ui/chat/message-normalizer.ts
+++ b/ui/src/ui/chat/message-normalizer.ts
@@ -8,6 +8,7 @@ import {
   isToolResultContentType,
   resolveToolBlockArgs,
 } from "../../../../src/chat/tool-content.js";
+import { normalizeLowercaseStringOrEmpty } from "../string-coerce.ts";
 import type { NormalizedMessage, MessageContentItem } from "../types/chat-types.ts";
 
 /**
@@ -74,7 +75,7 @@ export function normalizeMessage(message: unknown): NormalizedMessage {
  * Normalize role for grouping purposes.
  */
 export function normalizeRoleForGrouping(role: string): string {
-  const lower = role.toLowerCase();
+  const lower = normalizeLowercaseStringOrEmpty(role);
   // Preserve original casing when it's already a core role.
   if (role === "user" || role === "User") {
     return role;
@@ -102,6 +103,6 @@ export function normalizeRoleForGrouping(role: string): string {
  */
 export function isToolResultMessage(message: unknown): boolean {
   const m = message as Record<string, unknown>;
-  const role = typeof m.role === "string" ? m.role.toLowerCase() : "";
+  const role = normalizeLowercaseStringOrEmpty(m.role);
   return role === "toolresult" || role === "tool_result";
 }
diff --git a/ui/src/ui/chat/search-match.ts b/ui/src/ui/chat/search-match.ts
index 501a4ce4785..3d25bf79cf0 100644
--- a/ui/src/ui/chat/search-match.ts
+++ b/ui/src/ui/chat/search-match.ts
@@ -1,10 +1,11 @@
+import { normalizeLowercaseStringOrEmpty } from "../string-coerce.ts";
 import { extractTextCached } from "./message-extract.ts";
 
 export function messageMatchesSearchQuery(message: unknown, query: string): boolean {
-  const normalizedQuery = query.trim().toLowerCase();
+  const normalizedQuery = normalizeLowercaseStringOrEmpty(query);
   if (!normalizedQuery) {
     return true;
   }
-  const text = (extractTextCached(message) ?? "").toLowerCase();
+  const text = normalizeLowercaseStringOrEmpty(extractTextCached(message));
   return text.includes(normalizedQuery);
 }
diff --git a/ui/src/ui/chat/slash-command-executor.ts b/ui/src/ui/chat/slash-command-executor.ts
index 7534ca43186..a3289340ff0 100644
--- a/ui/src/ui/chat/slash-command-executor.ts
+++ b/ui/src/ui/chat/slash-command-executor.ts
@@ -11,6 +11,10 @@ import {
   isSubagentSessionKey,
   parseAgentSessionKey,
 } from "../session-key.ts";
+import {
+  normalizeLowercaseStringOrEmpty,
+  normalizeOptionalLowercaseString,
+} from "../string-coerce.ts";
 import {
   formatThinkingLevels,
   normalizeThinkLevel,
@@ -60,7 +64,7 @@ function normalizeVerboseLevel(raw?: string | null): "off" | "on" | "full" | und
   if (!raw) {
     return undefined;
   }
-  const key = raw.toLowerCase();
+  const key = normalizeLowercaseStringOrEmpty(raw);
   if (["off", "false", "no", "0"].includes(key)) {
     return "off";
   }
@@ -313,7 +317,7 @@ async function executeFast(
   sessionKey: string,
   args: string,
 ): Promise<SlashCommandResult> {
-  const rawMode = args.trim().toLowerCase();
+  const rawMode = normalizeLowercaseStringOrEmpty(args);
 
   if (!rawMode || rawMode === "status") {
     try {
@@ -406,6 +410,7 @@ async function executeKill(
   args: string,
 ): Promise<SlashCommandResult> {
   const target = args.trim();
+  const normalizedTarget = normalizeLowercaseStringOrEmpty(target);
   if (!target) {
     return { content: "Usage: `/kill <id|all>`" };
   }
@@ -415,7 +420,7 @@ async function executeKill(
     if (matched.length === 0) {
       return {
         content:
-          target.toLowerCase() === "all"
+          normalizedTarget === "all"
             ? "No active sub-agent sessions found."
             : `No matching sub-agent sessions found for \`${target}\`.`,
       };
@@ -435,7 +440,7 @@ async function executeKill(
       if (rejected.length === 0) {
         return {
           content:
-            target.toLowerCase() === "all"
+            normalizedTarget === "all"
               ? "No active sub-agent runs to abort."
               : `No active runs matched \`${target}\`.`,
         };
@@ -443,7 +448,7 @@ async function executeKill(
       throw rejected[0]?.reason ?? new Error("abort failed");
     }
 
-    if (target.toLowerCase() === "all") {
+    if (normalizedTarget === "all") {
       return {
         content:
           successCount === matched.length
@@ -468,13 +473,13 @@ function resolveKillTargets(
   currentSessionKey: string,
   target: string,
 ): string[] {
-  const normalizedTarget = target.trim().toLowerCase();
+  const normalizedTarget = normalizeLowercaseStringOrEmpty(target);
   if (!normalizedTarget) {
     return [];
   }
 
   const keys = new Set<string>();
-  const normalizedCurrentSessionKey = currentSessionKey.trim().toLowerCase();
+  const normalizedCurrentSessionKey = normalizeLowercaseStringOrEmpty(currentSessionKey);
   const currentParsed = parseAgentSessionKey(normalizedCurrentSessionKey);
   const currentAgentId =
     currentParsed?.agentId ??
@@ -485,7 +490,7 @@ function resolveKillTargets(
     if (!key || !isSubagentSessionKey(key)) {
       continue;
     }
-    const normalizedKey = key.toLowerCase();
+    const normalizedKey = normalizeLowercaseStringOrEmpty(key);
     const parsed = parseAgentSessionKey(normalizedKey);
     const belongsToCurrentSession = isWithinCurrentSessionSubtree(
       normalizedKey,
@@ -550,8 +555,7 @@ function buildSessionIndex(sessions: GatewaySessionRow[]): Map<string, GatewaySe
 }
 
 function normalizeSessionKey(key?: string | null): string | undefined {
-  const normalized = key?.trim().toLowerCase();
-  return normalized || undefined;
+  return normalizeOptionalLowercaseString(key);
 }
 
 function resolveEquivalentSessionKeys(
@@ -658,11 +662,11 @@ function resolveSteerSubagent(
   currentSessionKey: string,
   target: string,
 ): string[] {
-  const normalizedTarget = target.trim().toLowerCase();
+  const normalizedTarget = normalizeLowercaseStringOrEmpty(target);
   if (!normalizedTarget) {
     return [];
   }
-  const normalizedCurrentSessionKey = currentSessionKey.trim().toLowerCase();
+  const normalizedCurrentSessionKey = normalizeLowercaseStringOrEmpty(currentSessionKey);
   const currentParsed = parseAgentSessionKey(normalizedCurrentSessionKey);
   const currentAgentId =
     currentParsed?.agentId ??
@@ -675,7 +679,7 @@ function resolveSteerSubagent(
     if (!key || !isSubagentSessionKey(key)) {
       continue;
     }
-    const normalizedKey = key.toLowerCase();
+    const normalizedKey = normalizeLowercaseStringOrEmpty(key);
     const parsed = parseAgentSessionKey(normalizedKey);
     const belongsToCurrentSession = isWithinCurrentSessionSubtree(
       normalizedKey,
@@ -692,7 +696,7 @@ function resolveSteerSubagent(
       normalizedKey === normalizedTarget ||
       normalizedKey.endsWith(`:subagent:${normalizedTarget}`) ||
       normalizedKey === `subagent:${normalizedTarget}` ||
-      (session.label ?? "").toLowerCase() === normalizedTarget;
+      normalizeLowercaseStringOrEmpty(session.label) === normalizedTarget;
     if (isMatch) {
       keys.add(key);
     }
@@ -728,7 +732,7 @@ async function resolveSteerTarget(
     const rest = trimmed.slice(spaceIdx + 1).trim();
     // Skip "all" — resolveKillTargets treats it as a wildcard, but steer/redirect
     // target a single session, so "all good now" should not match subagents.
-    if (rest && maybeTarget.toLowerCase() !== "all") {
+    if (rest && normalizeLowercaseStringOrEmpty(maybeTarget) !== "all") {
       const sessions =
         context.sessionsResult ?? (await client.request<SessionsListResult>("sessions.list", {}));
       const matched = resolveSteerSubagent(sessions?.sessions ?? [], sessionKey, maybeTarget);
diff --git a/ui/src/ui/chat/slash-commands.ts b/ui/src/ui/chat/slash-commands.ts
index 3a48cf9a8b1..2ca61b759c7 100644
--- a/ui/src/ui/chat/slash-commands.ts
+++ b/ui/src/ui/chat/slash-commands.ts
@@ -4,6 +4,7 @@ import type {
   CommandArgChoice,
 } from "../../../../src/auto-reply/commands-registry.types.js";
 import type { IconName } from "../icons.ts";
+import { normalizeLowercaseStringOrEmpty } from "../string-coerce.ts";
 
 export type SlashCommandCategory = "session" | "model" | "agents" | "tools";
 
@@ -216,13 +217,13 @@ export const CATEGORY_LABELS: Record<SlashCommandCategory, string> = {
 };
 
 export function getSlashCommandCompletions(filter: string): SlashCommandDef[] {
-  const lower = filter.toLowerCase();
+  const lower = normalizeLowercaseStringOrEmpty(filter);
   const commands = lower
     ? SLASH_COMMANDS.filter(
         (cmd) =>
           cmd.name.startsWith(lower) ||
-          cmd.aliases?.some((alias) => alias.toLowerCase().startsWith(lower)) ||
-          cmd.description.toLowerCase().includes(lower),
+          cmd.aliases?.some((alias) => normalizeLowercaseStringOrEmpty(alias).startsWith(lower)) ||
+          normalizeLowercaseStringOrEmpty(cmd.description).includes(lower),
       )
     : SLASH_COMMANDS;
   return commands.toSorted((a, b) => {
@@ -266,11 +267,11 @@ export function parseSlashCommand(text: string): ParsedSlashCommand | null {
     return null;
   }
 
-  const normalizedName = name.toLowerCase();
+  const normalizedName = normalizeLowercaseStringOrEmpty(name);
   const command = SLASH_COMMANDS.find(
     (cmd) =>
       cmd.name === normalizedName ||
-      cmd.aliases?.some((alias) => alias.toLowerCase() === normalizedName),
+      cmd.aliases?.some((alias) => normalizeLowercaseStringOrEmpty(alias) === normalizedName),
   );
   if (!command) {
     return null;
diff --git a/ui/src/ui/connect-error.ts b/ui/src/ui/connect-error.ts
index 0dffd77cf91..761e7e172cd 100644
--- a/ui/src/ui/connect-error.ts
+++ b/ui/src/ui/connect-error.ts
@@ -1,5 +1,6 @@
 import { ConnectErrorDetailCodes } from "../../../src/gateway/protocol/connect-error-details.js";
 import { resolveGatewayErrorDetailCode } from "./gateway.ts";
+import { normalizeLowercaseStringOrEmpty } from "./string-coerce.ts";
 
 type ErrorWithMessageAndDetails = {
   message?: unknown;
@@ -39,7 +40,7 @@ function formatErrorFromMessageAndDetails(error: ErrorWithMessageAndDetails): st
       break;
   }
 
-  const normalized = message.trim().toLowerCase();
+  const normalized = normalizeLowercaseStringOrEmpty(message);
   if (
     normalized === "fetch failed" ||
     normalized === "failed to fetch" ||
diff --git a/ui/src/ui/controllers/chat.ts b/ui/src/ui/controllers/chat.ts
index 3fadb09a97e..4214bac4d49 100644
--- a/ui/src/ui/controllers/chat.ts
+++ b/ui/src/ui/controllers/chat.ts
@@ -2,6 +2,7 @@ import { resetToolStream } from "../app-tool-stream.ts";
 import { extractText } from "../chat/message-extract.ts";
 import { formatConnectError } from "../connect-error.ts";
 import type { GatewayBrowserClient } from "../gateway.ts";
+import { normalizeLowercaseStringOrEmpty } from "../string-coerce.ts";
 import type { ChatAttachment } from "../ui-types.ts";
 import { generateUUID } from "../uuid.ts";
 import {
@@ -20,7 +21,7 @@ function isAssistantSilentReply(message: unknown): boolean {
     return false;
   }
   const entry = message as Record<string, unknown>;
-  const role = typeof entry.role === "string" ? entry.role.toLowerCase() : "";
+  const role = normalizeLowercaseStringOrEmpty(entry.role);
   if (role !== "assistant") {
     return false;
   }
@@ -128,7 +129,7 @@ function normalizeAssistantMessage(
   const candidate = message as Record<string, unknown>;
   const roleValue = candidate.role;
   if (typeof roleValue === "string") {
-    const role = options.roleCaseSensitive ? roleValue : roleValue.toLowerCase();
+    const role = options.roleCaseSensitive ? roleValue : normalizeLowercaseStringOrEmpty(roleValue);
     if (role !== "assistant") {
       return null;
     }
diff --git a/ui/src/ui/controllers/cron.ts b/ui/src/ui/controllers/cron.ts
index f38f4c5b06d..6add0841d15 100644
--- a/ui/src/ui/controllers/cron.ts
+++ b/ui/src/ui/controllers/cron.ts
@@ -2,6 +2,7 @@ import { t } from "../../i18n/index.ts";
 import { DEFAULT_CRON_FORM } from "../app-defaults.ts";
 import { toNumber } from "../format.ts";
 import type { GatewayBrowserClient } from "../gateway.ts";
+import { normalizeLowercaseStringOrEmpty } from "../string-coerce.ts";
 import type {
   CronJob,
   CronDeliveryStatus,
@@ -900,13 +901,13 @@ export function startCronEdit(state: CronState, job: CronJob) {
 function buildCloneName(name: string, existingNames: Set<string>) {
   const base = name.trim() || "Job";
   const first = `${base} copy`;
-  if (!existingNames.has(first.toLowerCase())) {
+  if (!existingNames.has(normalizeLowercaseStringOrEmpty(first))) {
     return first;
   }
   let index = 2;
   while (index < 1000) {
     const next = `${base} copy ${index}`;
-    if (!existingNames.has(next.toLowerCase())) {
+    if (!existingNames.has(normalizeLowercaseStringOrEmpty(next))) {
       return next;
     }
     index += 1;
@@ -917,7 +918,9 @@ function buildCloneName(name: string, existingNames: Set<string>) {
 export function startCronClone(state: CronState, job: CronJob) {
   clearCronEditState(state);
   state.cronRunsJobId = job.id;
-  const existingNames = new Set(state.cronJobs.map((entry) => entry.name.trim().toLowerCase()));
+  const existingNames = new Set(
+    state.cronJobs.map((entry) => normalizeLowercaseStringOrEmpty(entry.name)),
+  );
   const cloned = jobToForm(job, state.cronForm);
   cloned.name = buildCloneName(job.name, existingNames);
   state.cronForm = cloned;
diff --git a/ui/src/ui/controllers/dreaming.ts b/ui/src/ui/controllers/dreaming.ts
index 5f05cc4cb7d..44337d2464a 100644
--- a/ui/src/ui/controllers/dreaming.ts
+++ b/ui/src/ui/controllers/dreaming.ts
@@ -1,4 +1,5 @@
 import type { GatewayBrowserClient } from "../gateway.ts";
+import { normalizeOptionalLowercaseString } from "../string-coerce.ts";
 import type { ConfigSnapshot } from "../types.ts";
 
 export type DreamingPhaseId = "light" | "deep" | "rem";
@@ -118,7 +119,7 @@ function normalizeFiniteScore(value: unknown, fallback = 0): number {
 }
 
 function normalizeStorageMode(value: unknown): DreamingStatus["storageMode"] {
-  const normalized = normalizeTrimmedString(value)?.toLowerCase();
+  const normalized = normalizeOptionalLowercaseString(normalizeTrimmedString(value));
   if (normalized === "inline" || normalized === "separate" || normalized === "both") {
     return normalized;
   }
@@ -144,7 +145,7 @@ function resolveDreamingPluginId(configValue: Record<string, unknown> | null): s
   const plugins = asRecord(configValue?.plugins);
   const slots = asRecord(plugins?.slots);
   const configuredSlot = normalizeTrimmedString(slots?.memory);
-  if (configuredSlot && configuredSlot.toLowerCase() !== "none") {
+  if (configuredSlot && normalizeOptionalLowercaseString(configuredSlot) !== "none") {
     return configuredSlot;
   }
   return DEFAULT_DREAMING_PLUGIN_ID;
diff --git a/ui/src/ui/controllers/logs.ts b/ui/src/ui/controllers/logs.ts
index 434ccbf7698..99d225e718c 100644
--- a/ui/src/ui/controllers/logs.ts
+++ b/ui/src/ui/controllers/logs.ts
@@ -1,4 +1,5 @@
 import type { GatewayBrowserClient } from "../gateway.ts";
+import { normalizeLowercaseStringOrEmpty } from "../string-coerce.ts";
 import type { LogEntry, LogLevel } from "../types.ts";
 import {
   formatMissingOperatorReadScopeMessage,
@@ -45,7 +46,7 @@ function normalizeLevel(value: unknown): LogLevel | null {
   if (typeof value !== "string") {
     return null;
   }
-  const lowered = value.toLowerCase() as LogLevel;
+  const lowered = normalizeLowercaseStringOrEmpty(value) as LogLevel;
   return LEVELS.has(lowered) ? lowered : null;
 }
 
diff --git a/ui/src/ui/controllers/usage.ts b/ui/src/ui/controllers/usage.ts
index 43bf8621066..11e1f78737a 100644
--- a/ui/src/ui/controllers/usage.ts
+++ b/ui/src/ui/controllers/usage.ts
@@ -1,5 +1,6 @@
 import { getSafeLocalStorage } from "../../local-storage.ts";
 import type { GatewayBrowserClient } from "../gateway.ts";
+import { normalizeLowercaseStringOrEmpty } from "../string-coerce.ts";
 import type { SessionsUsageResult, CostUsageSummary, SessionUsageTimeSeries } from "../types.ts";
 import type { SessionLogEntry } from "../views/usage.ts";
 import {
@@ -102,9 +103,9 @@ function normalizeGatewayCompatibilityKey(gatewayUrl?: string): string {
   try {
     const parsed = new URL(trimmed);
     const pathname = parsed.pathname === "/" ? "" : parsed.pathname;
-    return `${parsed.protocol}//${parsed.host}${pathname}`.toLowerCase();
+    return normalizeLowercaseStringOrEmpty(`${parsed.protocol}//${parsed.host}${pathname}`);
   } catch {
-    return trimmed.toLowerCase();
+    return normalizeLowercaseStringOrEmpty(trimmed);
   }
 }
 
diff --git a/ui/src/ui/external-link.ts b/ui/src/ui/external-link.ts
index 0922da638d0..e765d4404d3 100644
--- a/ui/src/ui/external-link.ts
+++ b/ui/src/ui/external-link.ts
@@ -1,3 +1,5 @@
+import { normalizeOptionalLowercaseString } from "./string-coerce.ts";
+
 const REQUIRED_EXTERNAL_REL_TOKENS = ["noopener", "noreferrer"] as const;
 
 export const EXTERNAL_LINK_TARGET = "_blank";
@@ -7,7 +9,7 @@ export function buildExternalLinkRel(currentRel?: string): string {
   const seen = new Set<string>(REQUIRED_EXTERNAL_REL_TOKENS);
 
   for (const rawToken of (currentRel ?? "").split(/\s+/)) {
-    const token = rawToken.trim().toLowerCase();
+    const token = normalizeOptionalLowercaseString(rawToken);
     if (!token || seen.has(token)) {
       continue;
     }
diff --git a/ui/src/ui/gateway.ts b/ui/src/ui/gateway.ts
index cb6bfce66ce..c3ac57b4a04 100644
--- a/ui/src/ui/gateway.ts
+++ b/ui/src/ui/gateway.ts
@@ -12,6 +12,7 @@ import {
 } from "../../../src/gateway/protocol/connect-error-details.js";
 import { clearDeviceAuthToken, loadDeviceAuthToken, storeDeviceAuthToken } from "./device-auth.ts";
 import { loadOrCreateDeviceIdentity, signDevicePayload } from "./device-identity.ts";
+import { normalizeLowercaseStringOrEmpty } from "./string-coerce.ts";
 import { generateUUID } from "./uuid.ts";
 
 export type GatewayEventFrame = {
@@ -82,7 +83,7 @@ export function isNonRecoverableAuthError(error: GatewayErrorInfo | undefined):
 function isTrustedRetryEndpoint(url: string): boolean {
   try {
     const gatewayUrl = new URL(url, window.location.href);
-    const host = gatewayUrl.hostname.trim().toLowerCase();
+    const host = normalizeLowercaseStringOrEmpty(gatewayUrl.hostname);
     const isLoopbackHost =
       host === "localhost" || host === "::1" || host === "[::1]" || host === "127.0.0.1";
     const isLoopbackIPv4 = host.startsWith("127.");
diff --git a/ui/src/ui/markdown.ts b/ui/src/ui/markdown.ts
index 6f5c69b2b30..660e51454c7 100644
--- a/ui/src/ui/markdown.ts
+++ b/ui/src/ui/markdown.ts
@@ -1,6 +1,7 @@
 import DOMPurify from "dompurify";
 import { marked } from "marked";
 import { truncateText } from "./format.ts";
+import { normalizeLowercaseStringOrEmpty } from "./string-coerce.ts";
 
 const allowedTags = [
   "a",
@@ -116,7 +117,7 @@ function installHooks() {
 
     node.setAttribute("rel", "noreferrer noopener");
     node.setAttribute("target", "_blank");
-    if (href.toLowerCase().includes("tail")) {
+    if (normalizeLowercaseStringOrEmpty(href).includes("tail")) {
       node.classList.add(TAIL_LINK_BLUR_CLASS);
     }
   });
diff --git a/ui/src/ui/navigation.ts b/ui/src/ui/navigation.ts
index c97b0259a7b..9f7ba9574ee 100644
--- a/ui/src/ui/navigation.ts
+++ b/ui/src/ui/navigation.ts
@@ -1,5 +1,6 @@
 import { t } from "../i18n/index.ts";
 import type { IconName } from "./icons.js";
+import { normalizeLowercaseStringOrEmpty } from "./string-coerce.ts";
 
 export const TAB_GROUPS = [
   { label: "chat", tabs: ["chat"] },
@@ -122,7 +123,7 @@ export function tabFromPath(pathname: string, basePath = ""): Tab | null {
       path = path.slice(base.length);
     }
   }
-  let normalized = normalizePath(path).toLowerCase();
+  let normalized = normalizeLowercaseStringOrEmpty(normalizePath(path));
   if (normalized.endsWith("/index.html")) {
     normalized = "/";
   }
@@ -145,7 +146,7 @@ export function inferBasePathFromPathname(pathname: string): string {
     return "";
   }
   for (let i = 0; i < segments.length; i++) {
-    const candidate = `/${segments.slice(i).join("/")}`.toLowerCase();
+    const candidate = normalizeLowercaseStringOrEmpty(`/${segments.slice(i).join("/")}`);
     if (PATH_TO_TAB.has(candidate)) {
       const prefix = segments.slice(0, i);
       return prefix.length ? `/${prefix.join("/")}` : "";
diff --git a/ui/src/ui/open-external-url.ts b/ui/src/ui/open-external-url.ts
index ed5a99c8678..c2bfa95978d 100644
--- a/ui/src/ui/open-external-url.ts
+++ b/ui/src/ui/open-external-url.ts
@@ -1,9 +1,11 @@
+import { normalizeLowercaseStringOrEmpty } from "./string-coerce.ts";
+
 const DATA_URL_PREFIX = "data:";
 const ALLOWED_EXTERNAL_PROTOCOLS = new Set(["http:", "https:", "blob:"]);
 const BLOCKED_DATA_IMAGE_MIME_TYPES = new Set(["image/svg+xml"]);
 
 function isAllowedDataImageUrl(url: string): boolean {
-  if (!url.toLowerCase().startsWith(DATA_URL_PREFIX)) {
+  if (!normalizeLowercaseStringOrEmpty(url).startsWith(DATA_URL_PREFIX)) {
     return false;
   }
 
@@ -13,7 +15,7 @@ function isAllowedDataImageUrl(url: string): boolean {
   }
 
   const metadata = url.slice(DATA_URL_PREFIX.length, commaIndex);
-  const mimeType = metadata.split(";")[0]?.trim().toLowerCase() ?? "";
+  const mimeType = normalizeLowercaseStringOrEmpty(metadata.split(";")[0]);
   if (!mimeType.startsWith("image/")) {
     return false;
   }
@@ -39,13 +41,15 @@ export function resolveSafeExternalUrl(
     return candidate;
   }
 
-  if (candidate.toLowerCase().startsWith(DATA_URL_PREFIX)) {
+  if (normalizeLowercaseStringOrEmpty(candidate).startsWith(DATA_URL_PREFIX)) {
     return null;
   }
 
   try {
     const parsed = new URL(candidate, baseHref);
-    return ALLOWED_EXTERNAL_PROTOCOLS.has(parsed.protocol.toLowerCase()) ? parsed.toString() : null;
+    return ALLOWED_EXTERNAL_PROTOCOLS.has(normalizeLowercaseStringOrEmpty(parsed.protocol))
+      ? parsed.toString()
+      : null;
   } catch {
     return null;
   }
diff --git a/ui/src/ui/session-key.ts b/ui/src/ui/session-key.ts
index 44bdb79343b..d78d297dc38 100644
--- a/ui/src/ui/session-key.ts
+++ b/ui/src/ui/session-key.ts
@@ -1,3 +1,8 @@
+import {
+  normalizeLowercaseStringOrEmpty,
+  normalizeOptionalLowercaseString,
+} from "./string-coerce.ts";
+
 export type ParsedAgentSessionKey = {
   agentId: string;
   rest: string;
@@ -14,7 +19,7 @@ const TRAILING_DASH_RE = /-+$/;
 export function parseAgentSessionKey(
   sessionKey: string | undefined | null,
 ): ParsedAgentSessionKey | null {
-  const raw = (sessionKey ?? "").trim().toLowerCase();
+  const raw = normalizeLowercaseStringOrEmpty(sessionKey);
   if (!raw) {
     return null;
   }
@@ -31,8 +36,7 @@ export function parseAgentSessionKey(
 }
 
 export function normalizeMainKey(value: string | undefined | null): string {
-  const trimmed = (value ?? "").trim();
-  return trimmed ? trimmed.toLowerCase() : DEFAULT_MAIN_KEY;
+  return normalizeOptionalLowercaseString(value) ?? DEFAULT_MAIN_KEY;
 }
 
 export function normalizeAgentId(value: string | undefined | null): string {
@@ -41,11 +45,10 @@ export function normalizeAgentId(value: string | undefined | null): string {
     return DEFAULT_AGENT_ID;
   }
   if (VALID_ID_RE.test(trimmed)) {
-    return trimmed.toLowerCase();
+    return normalizeLowercaseStringOrEmpty(trimmed);
   }
   return (
-    trimmed
-      .toLowerCase()
+    normalizeLowercaseStringOrEmpty(trimmed)
       .replace(INVALID_CHARS_RE, "-")
       .replace(LEADING_DASH_RE, "")
       .replace(TRAILING_DASH_RE, "")
@@ -72,9 +75,9 @@ export function isSubagentSessionKey(sessionKey: string | undefined | null): boo
   if (!raw) {
     return false;
   }
-  if (raw.toLowerCase().startsWith("subagent:")) {
+  if (normalizeLowercaseStringOrEmpty(raw).startsWith("subagent:")) {
     return true;
   }
   const parsed = parseAgentSessionKey(raw);
-  return Boolean((parsed?.rest ?? "").toLowerCase().startsWith("subagent:"));
+  return normalizeLowercaseStringOrEmpty(parsed?.rest).startsWith("subagent:");
 }
diff --git a/ui/src/ui/string-coerce.ts b/ui/src/ui/string-coerce.ts
new file mode 100644
index 00000000000..a43297b14ab
--- /dev/null
+++ b/ui/src/ui/string-coerce.ts
@@ -0,0 +1,15 @@
+export function normalizeOptionalString(value: unknown): string | undefined {
+  if (typeof value !== "string") {
+    return undefined;
+  }
+  const trimmed = value.trim();
+  return trimmed ? trimmed : undefined;
+}
+
+export function normalizeOptionalLowercaseString(value: unknown): string | undefined {
+  return normalizeOptionalString(value)?.toLowerCase();
+}
+
+export function normalizeLowercaseStringOrEmpty(value: unknown): string {
+  return normalizeOptionalLowercaseString(value) ?? "";
+}
diff --git a/ui/src/ui/thinking.ts b/ui/src/ui/thinking.ts
index 3aa12a5a55b..1e16c2bc1d0 100644
--- a/ui/src/ui/thinking.ts
+++ b/ui/src/ui/thinking.ts
@@ -1,3 +1,5 @@
+import { normalizeLowercaseStringOrEmpty } from "./string-coerce.ts";
+
 export type ThinkingCatalogEntry = {
   provider: string;
   id: string;
@@ -13,7 +15,7 @@ export function normalizeThinkingProviderId(provider?: string | null): string {
   if (!provider) {
     return "";
   }
-  const normalized = provider.trim().toLowerCase();
+  const normalized = normalizeLowercaseStringOrEmpty(provider);
   if (normalized === "z.ai" || normalized === "z-ai") {
     return "zai";
   }
@@ -31,7 +33,7 @@ export function normalizeThinkLevel(raw?: string | null): string | undefined {
   if (!raw) {
     return undefined;
   }
-  const key = raw.trim().toLowerCase();
+  const key = normalizeLowercaseStringOrEmpty(raw);
   const collapsed = key.replace(/[\s_-]+/g, "");
   if (collapsed === "adaptive" || collapsed === "auto") {
     return "adaptive";
diff --git a/ui/src/ui/tool-display.ts b/ui/src/ui/tool-display.ts
index 2b54d7d7c78..5773b94aa9f 100644
--- a/ui/src/ui/tool-display.ts
+++ b/ui/src/ui/tool-display.ts
@@ -7,6 +7,7 @@ import {
   type ToolDisplaySpec as ToolDisplaySpecBase,
 } from "../../../src/agents/tool-display-common.js";
 import type { IconName } from "./icons.ts";
+import { normalizeLowercaseStringOrEmpty } from "./string-coerce.ts";
 
 type ToolDisplaySpec = ToolDisplaySpecBase & {
   icon?: string;
@@ -120,7 +121,7 @@ export function resolveToolDisplay(params: {
   meta?: string;
 }): ToolDisplay {
   const name = normalizeToolName(params.name);
-  const key = name.toLowerCase();
+  const key = normalizeLowercaseStringOrEmpty(name);
   const spec = TOOL_MAP[key];
   const icon = (spec?.icon ?? FALLBACK.icon ?? "puzzle") as IconName;
   const title = spec?.title ?? defaultTitle(name);
diff --git a/ui/src/ui/usage-helpers.ts b/ui/src/ui/usage-helpers.ts
index 64d73d7f157..42c0686612e 100644
--- a/ui/src/ui/usage-helpers.ts
+++ b/ui/src/ui/usage-helpers.ts
@@ -51,7 +51,7 @@ const QUERY_KEYS = new Set([
   "maxmessages",
 ]);
 
-const normalizeQueryText = (value: string): string => value.trim().toLowerCase();
+const normalizeQueryText = (value: string): string => normalizeLowercaseStringOrEmpty(value);
 
 const globToRegex = (pattern: string): RegExp => {
   const escaped = pattern
@@ -62,7 +62,7 @@ const globToRegex = (pattern: string): RegExp => {
 };
 
 const parseQueryNumber = (value: string): number | null => {
-  let raw = value.trim().toLowerCase();
+  let raw = normalizeLowercaseStringOrEmpty(value);
   if (!raw) {
     return null;
   }
@@ -101,23 +101,25 @@ export const extractQueryTerms = (query: string): UsageQueryTerm[] => {
 
 const getSessionText = (session: UsageSessionQueryTarget): string[] => {
   const items: Array<string | undefined> = [session.label, session.key, session.sessionId];
-  return items.filter((item): item is string => Boolean(item)).map((item) => item.toLowerCase());
+  return items
+    .filter((item): item is string => Boolean(item))
+    .map((item) => normalizeLowercaseStringOrEmpty(item));
 };
 
 const getSessionProviders = (session: UsageSessionQueryTarget): string[] => {
   const providers = new Set<string>();
   if (session.modelProvider) {
-    providers.add(session.modelProvider.toLowerCase());
+    providers.add(normalizeLowercaseStringOrEmpty(session.modelProvider));
   }
   if (session.providerOverride) {
-    providers.add(session.providerOverride.toLowerCase());
+    providers.add(normalizeLowercaseStringOrEmpty(session.providerOverride));
   }
   if (session.origin?.provider) {
-    providers.add(session.origin.provider.toLowerCase());
+    providers.add(normalizeLowercaseStringOrEmpty(session.origin.provider));
   }
   for (const entry of session.usage?.modelUsage ?? []) {
     if (entry.provider) {
-      providers.add(entry.provider.toLowerCase());
+      providers.add(normalizeLowercaseStringOrEmpty(entry.provider));
     }
   }
   return Array.from(providers);
@@ -126,18 +128,18 @@ const getSessionProviders = (session: UsageSessionQueryTarget): string[] => {
 const getSessionModels = (session: UsageSessionQueryTarget): string[] => {
   const models = new Set<string>();
   if (session.model) {
-    models.add(session.model.toLowerCase());
+    models.add(normalizeLowercaseStringOrEmpty(session.model));
   }
   for (const entry of session.usage?.modelUsage ?? []) {
     if (entry.model) {
-      models.add(entry.model.toLowerCase());
+      models.add(normalizeLowercaseStringOrEmpty(entry.model));
     }
   }
   return Array.from(models);
 };
 
 const getSessionTools = (session: UsageSessionQueryTarget): string[] =>
-  (session.usage?.toolUsage?.tools ?? []).map((tool) => tool.name.toLowerCase());
+  (session.usage?.toolUsage?.tools ?? []).map((tool) => normalizeLowercaseStringOrEmpty(tool.name));
 
 const matchesUsageQuery = (session: UsageSessionQueryTarget, term: UsageQueryTerm): boolean => {
   const value = normalizeQueryText(term.value ?? "");
@@ -151,11 +153,11 @@ const matchesUsageQuery = (session: UsageSessionQueryTarget, term: UsageQueryTer
   const key = normalizeQueryText(term.key);
   switch (key) {
     case "agent":
-      return session.agentId?.toLowerCase().includes(value) ?? false;
+      return normalizeLowercaseStringOrEmpty(session.agentId).includes(value);
     case "channel":
-      return session.channel?.toLowerCase().includes(value) ?? false;
+      return normalizeLowercaseStringOrEmpty(session.channel).includes(value);
     case "chat":
-      return session.chatType?.toLowerCase().includes(value) ?? false;
+      return normalizeLowercaseStringOrEmpty(session.chatType).includes(value);
     case "provider":
       return getSessionProviders(session).some((provider) => provider.includes(value));
     case "model":
@@ -163,7 +165,7 @@ const matchesUsageQuery = (session: UsageSessionQueryTarget, term: UsageQueryTer
     case "tool":
       return getSessionTools(session).some((tool) => tool.includes(value));
     case "label":
-      return session.label?.toLowerCase().includes(value) ?? false;
+      return normalizeLowercaseStringOrEmpty(session.label).includes(value);
     case "key":
     case "session":
     case "id":
@@ -174,8 +176,8 @@ const matchesUsageQuery = (session: UsageSessionQueryTarget, term: UsageQueryTer
         );
       }
       return (
-        session.key.toLowerCase().includes(value) ||
-        (session.sessionId?.toLowerCase().includes(value) ?? false)
+        normalizeLowercaseStringOrEmpty(session.key).includes(value) ||
+        normalizeLowercaseStringOrEmpty(session.sessionId).includes(value)
       );
     case "has":
       switch (value) {
@@ -316,3 +318,4 @@ export function parseToolSummary(content: string) {
     cleanContent: nonToolLines.join("\n").trim(),
   };
 }
+import { normalizeLowercaseStringOrEmpty } from "./string-coerce.ts";
diff --git a/ui/src/ui/views/agents-panels-tools-skills.ts b/ui/src/ui/views/agents-panels-tools-skills.ts
index 2d7b8aa9d89..566139953bb 100644
--- a/ui/src/ui/views/agents-panels-tools-skills.ts
+++ b/ui/src/ui/views/agents-panels-tools-skills.ts
@@ -1,6 +1,7 @@
 import { html, nothing } from "lit";
 import { normalizeToolName } from "../../../../src/agents/tool-policy-shared.js";
 import { t } from "../../i18n/index.ts";
+import { normalizeLowercaseStringOrEmpty } from "../string-coerce.ts";
 import type {
   SkillStatusEntry,
   SkillStatusReport,
@@ -416,10 +417,12 @@ export function renderAgentSkills(params: {
   const usingAllowlist = allowlist !== undefined;
   const reportReady = Boolean(params.report && params.activeAgentId === params.agentId);
   const rawSkills = reportReady ? (params.report?.skills ?? []) : [];
-  const filter = params.filter.trim().toLowerCase();
+  const filter = normalizeLowercaseStringOrEmpty(params.filter);
   const filtered = filter
     ? rawSkills.filter((skill) =>
-        [skill.name, skill.description, skill.source].join(" ").toLowerCase().includes(filter),
+        normalizeLowercaseStringOrEmpty(
+          [skill.name, skill.description, skill.source].join(" "),
+        ).includes(filter),
       )
     : rawSkills;
   const groups = groupSkills(filtered);
diff --git a/ui/src/ui/views/agents-utils.ts b/ui/src/ui/views/agents-utils.ts
index e408e9af013..48c364efa08 100644
--- a/ui/src/ui/views/agents-utils.ts
+++ b/ui/src/ui/views/agents-utils.ts
@@ -4,6 +4,7 @@ import {
   normalizeToolName,
   resolveToolProfilePolicy,
 } from "../../../../src/agents/tool-policy-shared.js";
+import { normalizeLowercaseStringOrEmpty } from "../string-coerce.ts";
 import type {
   AgentIdentityResult,
   AgentsFilesListResult,
@@ -586,7 +587,7 @@ export function buildModelOptions(
   const seen = new Set<string>();
   const options: ConfiguredModelOption[] = [];
   const addOption = (value: string, label: string) => {
-    const key = value.toLowerCase();
+    const key = normalizeLowercaseStringOrEmpty(value);
     if (seen.has(key)) {
       return;
     }
@@ -607,7 +608,7 @@ export function buildModelOptions(
     }
   }
 
-  if (current && !seen.has(current.toLowerCase())) {
+  if (current && !seen.has(normalizeLowercaseStringOrEmpty(current))) {
     options.unshift({ value: current, label: `Current (${current})` });
   }
 
diff --git a/ui/src/ui/views/chat.ts b/ui/src/ui/views/chat.ts
index 2b832ad4b42..ec8154c7e96 100644
--- a/ui/src/ui/views/chat.ts
+++ b/ui/src/ui/views/chat.ts
@@ -31,6 +31,7 @@ import {
 } from "../chat/slash-commands.ts";
 import { isSttSupported, startStt, stopStt } from "../chat/speech.ts";
 import { icons } from "../icons.ts";
+import { normalizeLowercaseStringOrEmpty } from "../string-coerce.ts";
 import { detectTextDirection } from "../text-direction.ts";
 import type { GatewaySessionRow, SessionsListResult } from "../types.ts";
 import type { ChatItem, MessageGroup } from "../types/chat-types.ts";
@@ -495,12 +496,12 @@ function updateSlashMenu(value: string, requestUpdate: () => void): void {
   // Arg mode: /command <partial-arg>
   const argMatch = value.match(/^\/(\S+)\s(.*)$/);
   if (argMatch) {
-    const cmdName = argMatch[1].toLowerCase();
-    const argFilter = argMatch[2].toLowerCase();
+    const cmdName = normalizeLowercaseStringOrEmpty(argMatch[1]);
+    const argFilter = normalizeLowercaseStringOrEmpty(argMatch[2]);
     const cmd = SLASH_COMMANDS.find((c) => c.name === cmdName);
     if (cmd?.argOptions?.length) {
       const filtered = argFilter
-        ? cmd.argOptions.filter((opt) => opt.toLowerCase().startsWith(argFilter))
+        ? cmd.argOptions.filter((opt) => normalizeLowercaseStringOrEmpty(opt).startsWith(argFilter))
         : cmd.argOptions;
       if (filtered.length > 0) {
         vs.slashMenuMode = "args";
@@ -1421,13 +1422,14 @@ function groupMessages(items: ChatItem[]): Array<ChatItem | MessageGroup> {
 
     const normalized = normalizeMessage(item.message);
     const role = normalizeRoleForGrouping(normalized.role);
-    const senderLabel = role.toLowerCase() === "user" ? (normalized.senderLabel ?? null) : null;
+    const senderLabel =
+      normalizeLowercaseStringOrEmpty(role) === "user" ? (normalized.senderLabel ?? null) : null;
     const timestamp = normalized.timestamp || Date.now();
 
     if (
       !currentGroup ||
       currentGroup.role !== role ||
-      (role.toLowerCase() === "user" && currentGroup.senderLabel !== senderLabel)
+      (normalizeLowercaseStringOrEmpty(role) === "user" && currentGroup.senderLabel !== senderLabel)
     ) {
       if (currentGroup) {
         result.push(currentGroup);
@@ -1486,7 +1488,7 @@ function buildChatItems(props: ChatProps): Array<ChatItem | MessageGroup> {
       continue;
     }
 
-    if (!props.showToolCalls && normalized.role.toLowerCase() === "toolresult") {
+    if (!props.showToolCalls && normalizeLowercaseStringOrEmpty(normalized.role) === "toolresult") {
       continue;
     }
 
diff --git a/ui/src/ui/views/command-palette.ts b/ui/src/ui/views/command-palette.ts
index 7da44710aea..d514b2a20de 100644
--- a/ui/src/ui/views/command-palette.ts
+++ b/ui/src/ui/views/command-palette.ts
@@ -3,6 +3,7 @@ import { ref } from "lit/directives/ref.js";
 import { t } from "../../i18n/index.ts";
 import { SLASH_COMMANDS } from "../chat/slash-commands.ts";
 import { icons, type IconName } from "../icons.ts";
+import { normalizeLowercaseStringOrEmpty } from "../string-coerce.ts";
 
 type PaletteItem = {
   id: string;
@@ -97,11 +98,11 @@ function filteredItems(query: string): PaletteItem[] {
   if (!query) {
     return PALETTE_ITEMS;
   }
-  const q = query.toLowerCase();
+  const q = normalizeLowercaseStringOrEmpty(query);
   return PALETTE_ITEMS.filter(
     (item) =>
-      item.label.toLowerCase().includes(q) ||
-      (item.description?.toLowerCase().includes(q) ?? false),
+      normalizeLowercaseStringOrEmpty(item.label).includes(q) ||
+      normalizeLowercaseStringOrEmpty(item.description).includes(q),
   );
 }
 
diff --git a/ui/src/ui/views/config-form.node.ts b/ui/src/ui/views/config-form.node.ts
index b92543eaa98..9b0cf4f5021 100644
--- a/ui/src/ui/views/config-form.node.ts
+++ b/ui/src/ui/views/config-form.node.ts
@@ -1,6 +1,10 @@
 import { html, nothing, type TemplateResult } from "lit";
 import { formatUnknownText } from "../format.ts";
 import { icons as sharedIcons } from "../icons.ts";
+import {
+  normalizeLowercaseStringOrEmpty,
+  normalizeOptionalLowercaseString,
+} from "../string-coerce.ts";
 import type { ConfigUiHints } from "../types.ts";
 import {
   defaultValue,
@@ -218,7 +222,7 @@ export function parseConfigSearchQuery(query: string): ConfigSearchCriteria {
   const seen = new Set<string>();
   const raw = query.trim();
   const stripped = raw.replace(/(^|\s)tag:([^\s]+)/gi, (_, leading: string, token: string) => {
-    const normalized = token.trim().toLowerCase();
+    const normalized = normalizeLowercaseStringOrEmpty(token);
     if (normalized && !seen.has(normalized)) {
       seen.add(normalized);
       tags.push(normalized);
@@ -226,7 +230,7 @@ export function parseConfigSearchQuery(query: string): ConfigSearchCriteria {
     return leading;
   });
   return {
-    text: stripped.trim().toLowerCase(),
+    text: normalizeLowercaseStringOrEmpty(stripped),
     tags,
   };
 }
@@ -245,7 +249,7 @@ function normalizeTags(raw: unknown): string[] {
     if (!tag) {
       continue;
     }
-    const key = tag.toLowerCase();
+    const key = normalizeLowercaseStringOrEmpty(tag);
     if (seen.has(key)) {
       continue;
     }
@@ -277,7 +281,7 @@ function matchesText(text: string, candidates: Array<string | undefined>): boole
     return true;
   }
   for (const candidate of candidates) {
-    if (candidate && candidate.toLowerCase().includes(text)) {
+    if (normalizeOptionalLowercaseString(candidate)?.includes(text)) {
       return true;
     }
   }
@@ -288,7 +292,7 @@ function matchesTags(filterTags: string[], fieldTags: string[]): boolean {
   if (filterTags.length === 0) {
     return true;
   }
-  const normalized = new Set(fieldTags.map((tag) => tag.toLowerCase()));
+  const normalized = new Set(fieldTags.map((tag) => normalizeLowercaseStringOrEmpty(tag)));
   return filterTags.every((tag) => normalized.has(tag));
 }
 
diff --git a/ui/src/ui/views/config-form.render.ts b/ui/src/ui/views/config-form.render.ts
index 75e07a14ec1..52e60e3918d 100644
--- a/ui/src/ui/views/config-form.render.ts
+++ b/ui/src/ui/views/config-form.render.ts
@@ -1,5 +1,6 @@
 import { html, nothing } from "lit";
 import { icons } from "../icons.ts";
+import { normalizeLowercaseStringOrEmpty } from "../string-coerce.ts";
 import type { ConfigUiHints } from "../types.ts";
 import { matchesNodeSearch, parseConfigSearchQuery, renderNode } from "./config-form.node.ts";
 import { hintForPath, humanize, schemaType, type JsonSchema } from "./config-form.shared.ts";
@@ -342,9 +343,9 @@ function matchesSearch(params: {
   const meta = SECTION_META[params.key];
   const sectionMetaMatches =
     q &&
-    (params.key.toLowerCase().includes(q) ||
-      (meta?.label ? meta.label.toLowerCase().includes(q) : false) ||
-      (meta?.description ? meta.description.toLowerCase().includes(q) : false));
+    (normalizeLowercaseStringOrEmpty(params.key).includes(q) ||
+      (meta?.label ? normalizeLowercaseStringOrEmpty(meta.label).includes(q) : false) ||
+      (meta?.description ? normalizeLowercaseStringOrEmpty(meta.description).includes(q) : false));
 
   if (sectionMetaMatches && criteria.tags.length === 0) {
     return true;
diff --git a/ui/src/ui/views/config-form.shared.ts b/ui/src/ui/views/config-form.shared.ts
index b535c49e25f..e3e65477913 100644
--- a/ui/src/ui/views/config-form.shared.ts
+++ b/ui/src/ui/views/config-form.shared.ts
@@ -1,3 +1,4 @@
+import { normalizeLowercaseStringOrEmpty } from "../string-coerce.ts";
 import type { ConfigUiHint, ConfigUiHints } from "../types.ts";
 
 export type JsonSchema = {
@@ -125,7 +126,7 @@ function isEnvVarPlaceholder(value: string): boolean {
 }
 
 export function isSensitiveConfigPath(path: string): boolean {
-  const lowerPath = path.toLowerCase();
+  const lowerPath = normalizeLowercaseStringOrEmpty(path);
   const whitelisted = SENSITIVE_KEY_WHITELIST_SUFFIXES.some((suffix) => lowerPath.endsWith(suffix));
   return !whitelisted && SENSITIVE_PATTERNS.some((pattern) => pattern.test(path));
 }
diff --git a/ui/src/ui/views/logs.ts b/ui/src/ui/views/logs.ts
index 5c80be479ed..93aaf3b5ff1 100644
--- a/ui/src/ui/views/logs.ts
+++ b/ui/src/ui/views/logs.ts
@@ -1,5 +1,6 @@
 import { html, nothing } from "lit";
 import { t } from "../../i18n/index.ts";
+import { normalizeLowercaseStringOrEmpty } from "../string-coerce.ts";
 import type { LogEntry, LogLevel } from "../types.ts";
 
 const LEVELS: LogLevel[] = ["trace", "debug", "info", "warn", "error", "fatal"];
@@ -36,15 +37,14 @@ function matchesFilter(entry: LogEntry, needle: string) {
   if (!needle) {
     return true;
   }
-  const haystack = [entry.message, entry.subsystem, entry.raw]
-    .filter(Boolean)
-    .join(" ")
-    .toLowerCase();
+  const haystack = normalizeLowercaseStringOrEmpty(
+    [entry.message, entry.subsystem, entry.raw].filter(Boolean).join(" "),
+  );
   return haystack.includes(needle);
 }
 
 export function renderLogs(props: LogsProps) {
-  const needle = props.filterText.trim().toLowerCase();
+  const needle = normalizeLowercaseStringOrEmpty(props.filterText);
   const levelFiltered = LEVELS.some((level) => !props.levelFilters[level]);
   const filtered = props.entries.filter((entry) => {
     if (entry.level && !props.levelFilters[entry.level]) {
diff --git a/ui/src/ui/views/overview-hints.ts b/ui/src/ui/views/overview-hints.ts
index d4599818c48..d0a2e54f5aa 100644
--- a/ui/src/ui/views/overview-hints.ts
+++ b/ui/src/ui/views/overview-hints.ts
@@ -1,4 +1,5 @@
 import { ConnectErrorDetailCodes } from "../../../../src/gateway/protocol/connect-error-details.js";
+import { normalizeLowercaseStringOrEmpty } from "../string-coerce.ts";
 
 const AUTH_REQUIRED_CODES = new Set<string>([
   ConnectErrorDetailCodes.AUTH_REQUIRED,
@@ -40,7 +41,7 @@ export function shouldShowPairingHint(
   if (lastErrorCode === ConnectErrorDetailCodes.PAIRING_REQUIRED) {
     return true;
   }
-  return lastError.toLowerCase().includes("pairing required");
+  return normalizeLowercaseStringOrEmpty(lastError).includes("pairing required");
 }
 
 /**
@@ -66,7 +67,7 @@ export function resolveAuthHintKind(params: {
     return AUTH_REQUIRED_CODES.has(params.lastErrorCode) ? "required" : "failed";
   }
 
-  const lower = params.lastError.toLowerCase();
+  const lower = normalizeLowercaseStringOrEmpty(params.lastError);
   if (!lower.includes("unauthorized")) {
     return null;
   }
@@ -84,6 +85,6 @@ export function shouldShowInsecureContextHint(
   if (lastErrorCode) {
     return INSECURE_CONTEXT_CODES.has(lastErrorCode);
   }
-  const lower = lastError.toLowerCase();
+  const lower = normalizeLowercaseStringOrEmpty(lastError);
   return lower.includes("secure context") || lower.includes("device identity required");
 }
diff --git a/ui/src/ui/views/overview.ts b/ui/src/ui/views/overview.ts
index 58a20e475b9..d40735e5662 100644
--- a/ui/src/ui/views/overview.ts
+++ b/ui/src/ui/views/overview.ts
@@ -6,6 +6,7 @@ import { formatRelativeTimestamp, formatDurationHuman } from "../format.ts";
 import type { GatewayHelloOk } from "../gateway.ts";
 import { icons } from "../icons.ts";
 import type { UiSettings } from "../storage.ts";
+import { normalizeLowercaseStringOrEmpty } from "../string-coerce.ts";
 import type {
   AttentionItem,
   CronJob,
@@ -196,7 +197,7 @@ export function renderOverview(props: OverviewProps) {
     if (props.connected || !props.lastError || !props.warnQueryToken) {
       return null;
     }
-    const lower = props.lastError.toLowerCase();
+    const lower = normalizeLowercaseStringOrEmpty(props.lastError);
     const authFailed = lower.includes("unauthorized") || lower.includes("device identity required");
     if (!authFailed) {
       return null;
diff --git a/ui/src/ui/views/sessions.ts b/ui/src/ui/views/sessions.ts
index 7b5f73368ba..28f7e4126ec 100644
--- a/ui/src/ui/views/sessions.ts
+++ b/ui/src/ui/views/sessions.ts
@@ -4,6 +4,7 @@ import { formatRelativeTimestamp } from "../format.ts";
 import { icons } from "../icons.ts";
 import { pathForTab } from "../navigation.ts";
 import { formatSessionTokens } from "../presenter.ts";
+import { normalizeLowercaseStringOrEmpty } from "../string-coerce.ts";
 import type {
   GatewaySessionRow,
   SessionCompactionCheckpoint,
@@ -82,7 +83,7 @@ function normalizeProviderId(provider?: string | null): string {
   if (!provider) {
     return "";
   }
-  const normalized = provider.trim().toLowerCase();
+  const normalized = normalizeLowercaseStringOrEmpty(provider);
   if (normalized === "z.ai" || normalized === "z-ai") {
     return "zai";
   }
@@ -144,15 +145,15 @@ function resolveThinkLevelPatchValue(value: string, isBinary: boolean): string |
 }
 
 function filterRows(rows: GatewaySessionRow[], query: string): GatewaySessionRow[] {
-  const q = query.trim().toLowerCase();
+  const q = normalizeLowercaseStringOrEmpty(query);
   if (!q) {
     return rows;
   }
   return rows.filter((row) => {
-    const key = (row.key ?? "").toLowerCase();
-    const label = (row.label ?? "").toLowerCase();
-    const kind = (row.kind ?? "").toLowerCase();
-    const displayName = (row.displayName ?? "").toLowerCase();
+    const key = normalizeLowercaseStringOrEmpty(row.key);
+    const label = normalizeLowercaseStringOrEmpty(row.label);
+    const kind = normalizeLowercaseStringOrEmpty(row.kind);
+    const displayName = normalizeLowercaseStringOrEmpty(row.displayName);
     return key.includes(q) || label.includes(q) || kind.includes(q) || displayName.includes(q);
   });
 }
diff --git a/ui/src/ui/views/skills.ts b/ui/src/ui/views/skills.ts
index 2724918934f..5e2eb10e560 100644
--- a/ui/src/ui/views/skills.ts
+++ b/ui/src/ui/views/skills.ts
@@ -8,6 +8,7 @@ import type {
 } from "../controllers/skills.ts";
 import { clampText } from "../format.ts";
 import { resolveSafeExternalUrl } from "../open-external-url.ts";
+import { normalizeLowercaseStringOrEmpty } from "../string-coerce.ts";
 import type { SkillStatusEntry, SkillStatusReport } from "../types.ts";
 import { groupSkills } from "./skills-grouping.ts";
 import {
@@ -114,10 +115,12 @@ export function renderSkills(props: SkillsProps) {
       ? skills
       : skills.filter((s) => skillMatchesStatus(s, props.statusFilter));
 
-  const filter = props.filter.trim().toLowerCase();
+  const filter = normalizeLowercaseStringOrEmpty(props.filter);
   const filtered = filter
     ? afterStatus.filter((skill) =>
-        [skill.name, skill.description, skill.source].join(" ").toLowerCase().includes(filter),
+        normalizeLowercaseStringOrEmpty(
+          [skill.name, skill.description, skill.source].join(" "),
+        ).includes(filter),
       )
     : afterStatus;
   const groups = groupSkills(filtered);
diff --git a/ui/src/ui/views/usage-query.ts b/ui/src/ui/views/usage-query.ts
index 94dc927a564..ab8766256c4 100644
--- a/ui/src/ui/views/usage-query.ts
+++ b/ui/src/ui/views/usage-query.ts
@@ -1,3 +1,4 @@
+import { normalizeLowercaseStringOrEmpty } from "../string-coerce.ts";
 import { extractQueryTerms } from "../usage-helpers.ts";
 import { CostDailyEntry, UsageAggregates, UsageSessionEntry } from "./usageTypes.ts";
 
@@ -138,8 +139,8 @@ const buildQuerySuggestions = (
     ? [lastToken.slice(0, lastToken.indexOf(":")), lastToken.slice(lastToken.indexOf(":") + 1)]
     : ["", ""];
 
-  const key = rawKey.toLowerCase();
-  const value = rawValue.toLowerCase();
+  const key = normalizeLowercaseStringOrEmpty(rawKey);
+  const value = normalizeLowercaseStringOrEmpty(rawValue);
 
   const unique = (items: Array<string | undefined>): string[] => {
     const set = new Set<string>();
@@ -181,7 +182,7 @@ const buildQuerySuggestions = (
   const suggestions: QuerySuggestion[] = [];
   const addValues = (prefix: string, values: string[]) => {
     for (const val of values) {
-      if (!value || val.toLowerCase().includes(value)) {
+      if (!value || normalizeLowercaseStringOrEmpty(val).includes(value)) {
         suggestions.push({ label: `${prefix}:${val}`, value: `${prefix}:${val}` });
       }
     }
@@ -227,7 +228,7 @@ const applySuggestionToQuery = (query: string, suggestion: string): string => {
   return `${tokens.join(" ")} `;
 };
 
-const normalizeQueryText = (value: string): string => value.trim().toLowerCase();
+const normalizeQueryText = (value: string): string => normalizeLowercaseStringOrEmpty(value);
 
 const addQueryToken = (query: string, token: string): string => {
   const trimmed = query.trim();