Cometchat Core

Ground truth: the installed @cometchat/chat-uikit-react@^6 + @cometchat/chat-sdk-javascript@^4 package types (node_modules/@cometchat/chat-uikit-react) + docs/ui-kit/react. Official docs: https://www.cometchat.com/docs/ui-kit/react/overview · Docs MCP: claude mcp add --transport http cometchat-docs https://www.cometchat.com/docs/mcp (or fetch the URL directly on agents without MCP). Verify any non-obvious symbol against the installed package types before relying on it.

Purpose

This is the foundational skill for every CometChat React UI Kit v6 integration. It teaches Claude HOW CometChat works -- initialization, login, CSS, environment variables, SSR safety, and the provider pattern -- so Claude can write project-appropriate code instead of relying on templates.

Read this skill first, before any framework or placement skill.

When to use

• Any React-family integration: Vite + React, Next.js (App or Pages Router), React Router v6/v7, Astro with React islands.
• BEFORE loading any framework-specific patterns skill (cometchat-react-patterns, cometchat-nextjs-patterns, etc.) — those layer on top of this.
• When the user is asking about CometChat init, login, env vars, CSS, provider pattern, SSR safety, or production auth.

When NOT to use

• React Native (Expo or bare) — load cometchat-native-core instead. The wrappers, env-var prefixes, and lifecycle are different.
• Angular — load cometchat-angular-core. Uses APP_INITIALIZER + CUSTOM_ELEMENTS_SCHEMA + environment.ts, not React's provider chain.
• Native Android (V5 or V6) — load cometchat-android-v5-core or cometchat-android-v6-core. Kotlin init, local.properties credentials.
• Native iOS (V5) — load cometchat-ios-core. Swift init, CocoaPods/SPM.
• Flutter (V5 or V6) — load cometchat-flutter-v5-core or cometchat-flutter-v6-core. Dart init via pubspec.yaml.
• Backend-only token-mint server work — load cometchat-production for the REST-API token recipes; this skill is client-side.

Common Rationalizations — and why they're wrong

A two-column anti-skip defense. Every excuse below has been used by agents (or could plausibly be) to skip rules in this skill. The rebuttal cites the validated incident.

Red flags — signs you're misapplying this skill

• The agent emitted setError(String(e)) anywhere in code — should be setError(formatCometChatError(e)) per §6 (ENG-35719).
• The agent's plan (Step 3f) doesn't include cometchat/errors.ts as a created file.
• The agent emitted bare CometChatUIKit.login(uid) without the in-flight-promise guard (ensureLoggedIn) — guaranteed StrictMode race in dev.
• A <style> or CSS rule targets internal class names like .cometchat-conversation-list-item (anti-pattern §8.3) instead of the --cometchat-* CSS variables.
• The agent picked cometchat-uid-1 without asking the user (ENG-35717 — the bot-flag failure mode).
• The agent's env file uses the wrong framework prefix (e.g., VITE_* in a Next.js project, or EXPO_PUBLIC_* in a Vite project).
• The provider doesn't gate {children} on isReady — chat components mount before init completes and throw CometChat is not initialized errors.

Verification — before declaring this skill applied

Run through this checklist before saying "done" for any task that touched cometchat-core territory:

• grep -nE "setError\(String\(" src/ returns ZERO matches (ENG-35719 — must use formatCometChatError).
• src/cometchat/errors.ts exists and exports formatCometChatError + logCometChatError.
• The provider has an ensureLoggedIn-style in-flight guard (no bare CometChatUIKit.login calls).
• tsc --noEmit passes against the project's existing tsconfig.
• Env vars use the detected framework's prefix (VITE_* / NEXT_PUBLIC_* / PUBLIC_* / EXPO_PUBLIC_*), and .env (or .env.local for Next.js) is in .gitignore.
• CSS imports happen once at the entry / root (per framework section in §3).
• If the user is on Next.js: the page that owns CometChat components has "use client" (App Router) or is dynamic-imported with ssr: false (App Router only inside another Client Component, per the Next.js 15+ rule verified 2026-06-02).
• The user confirmed the dev-mode UID via the dispatcher's Step 3d.1 prompt — NOT silently picked.

1. Initialization

CometChat must be initialized exactly once before any UI component renders. Initialization is asynchronous and must complete fully before mounting any CometChat* component.

File-based init with cometchat-settings.json (recommended)

Version requirement (ENG-35866 — Skills Telemetry). CometChatUIKit.initFromSettings(settings) reads a cometchat-settings.json object and lets the SDK self-report integrationSource = "ai-agent" to /user_sessions. It ships GA in @cometchat/chat-uikit-react >= 6.5.2 + @cometchat/chat-sdk-javascript >= 4.1.11 (npm latest). On an older UI Kit the method does not exist — use the UIKitSettingsBuilder fallback below.

Step 1 — create cometchat-settings.json at the project root. Fill appId / region / credentials.authKey from the CLI provision setup output; leave everything else at the defaults below. This is the single source of credentials — no second copy to keep in sync.

{
  "appId": "APP_ID_HERE",
  "region": "us",
  "credentials": {
    "authKey": "AUTH_KEY_HERE"
  },
  "chatSDK": {
    "presenceSubscription": {
      "type": "ALL_USERS",
      "roles": []
    },
    "autoEstablishSocketConnection": true,
    "adminHost": null,
    "clientHost": null
  },
  "callsSDK": {
    "host": null,
    "adminHost": null,
    "clientHost": null,
    "callsHost": null
  },
  "uiKit": {
    "subscribePresenceForAllUsers": true
  }
}

Step 2 — init by importing the JSON as a build-time module. Vite, CRA, Next.js, and Astro all have resolveJsonModule on by default, so the import is type-safe with no extra config:

// initFromSettings ships GA in @cometchat/chat-uikit-react >= 6.5.2 (ENG-35866)
import { CometChatUIKit } from "@cometchat/chat-uikit-react";
import cometchatSettings from "./cometchat-settings.json"; // adjust path to the file's location

await CometChatUIKit.initFromSettings(cometchatSettings);

• Commit cometchat-settings.json — do not gitignore it (the file is part of the integration). Its authKey is an optional demo/POC credential: a quick-start affordance so a PM or developer can see working chat before the backend auth-token flow is wired (that flow often waits on internal approvals). Because the file is committed to source control, treat the key as public — use a dedicated demo CometChat app (a committed key trips secret scanners and stays in git history; never reuse a production app's key). Switch to a server-minted authToken via loginWithAuthToken before production, where authKey must not ship.
• The same module-flag / useEffect / entry-point placement rules in the rest of this section apply unchanged — just swap CometChatUIKit.init(settings) for CometChatUIKit.initFromSettings(cometchatSettings).

The UIKitSettingsBuilder (fallback — UI Kit before file-based init)

import { CometChatUIKit, UIKitSettingsBuilder } from "@cometchat/chat-uikit-react";

const settings = new UIKitSettingsBuilder()
  .setAppId(APP_ID)       // Required. String from the CometChat dashboard.
  .setRegion(REGION)       // Required. "us", "eu", "in", etc.
  .setAuthKey(AUTH_KEY)    // Required for dev mode. Omit in production (use auth tokens).
  .subscribePresenceForAllUsers() // Optional but recommended -- enables online/offline indicators.
  .build();

Init must happen once

Use a module-level flag to prevent double-init. This is critical because React StrictMode in development calls effects twice:

let initialized = false;

async function initCometChat(): Promise<void> {
  if (initialized) return;
  initialized = true;

  // Fail loud if env vars didn't load. Empty credentials otherwise surface
  // much later as a cryptic `ERROR_API_KEY_NOT_FOUND` / failed init that's hard
  // to trace back to a missing/mis-prefixed .env. (audit P0-5)
  if (!APP_ID || !REGION || !AUTH_KEY) {
    throw new Error(
      "CometChat credentials are empty — check .env and the framework's env prefix " +
        "(VITE_ / NEXT_PUBLIC_ / PUBLIC_), and restart the dev server after editing .env.",
    );
  }

  const settings = new UIKitSettingsBuilder()
    .setAppId(APP_ID)
    .setRegion(REGION)
    .setAuthKey(AUTH_KEY)
    .subscribePresenceForAllUsers()
    .build();

  await CometChatUIKit.init(settings);
}

Init must be in useEffect (React components) or before mount (entry files)

In a useEffect (Next.js, Astro, React Router SSR):

useEffect(() => {
  initCometChat()
    .then(() => loginUser())
    .then(() => setReady(true))
    .catch((e) => setError(String(e)));
}, []);

At the entry point (Vite/CRA -- no SSR):

// main.tsx -- runs once, before React mounts
CometChatUIKit.init(settings)
  ?.then(() => CometChatUIKit.login("cometchat-uid-1"))
  .then(() => mount())
  .catch((e) => mountError(String(e)));

The init-at-entry pattern works for Vite/CRA because main.tsx only runs in the browser. For frameworks with SSR (Next.js, Astro, React Router v7 SSR), you MUST use the useEffect pattern because the module runs on the server first.

2. Login

Development mode

Use CometChatUIKit.login(uid) with a test UID. Every new CometChat app comes with five pre-created test users: cometchat-uid-1 through cometchat-uid-5.

const user = await CometChatUIKit.getLoggedinUser();
if (!user) {
  await CometChatUIKit.login("cometchat-uid-1");
}

⚠️ login() is safe to call sequentially, NOT concurrently

A subtle but important distinction:

• Sequential (first login() completes, then second is called): the SDK's second call returns immediately with the already-logged-in user. Safe.
• Concurrent (a second login() fires while the first is still in-flight): the SDK throws "Please wait until the previous login request ends." The user sees a red error on the page, has to refresh, and only then does it work (because the first session is now cached).

This is exactly the case that React 18 StrictMode triggers in development: effects run mount → unmount → mount, so a useEffect that calls login() fires twice with no time for the first call to finish. Production builds don't double-mount, but any code path that can call login() from two places simultaneously hits the same error.

Guard concurrent login with a module-level in-flight promise:

let loginInFlight: Promise<unknown> | null = null;

async function ensureLoggedIn(
  uid: string,
  authToken?: string,
): Promise<void> {
  const existing = await CometChatUIKit.getLoggedinUser();
  // Same user already logged in → nothing to do (sequential case).
  if (existing && existing.getUid?.() === uid) return;
  // A DIFFERENT user is logged in (account switch, or logout → login-as-other).
  // Log out first: otherwise the SDK keeps the old session and login() silently
  // no-ops, so the app shows the previous account. (Switching accounts requires
  // an explicit logout — login() is a no-op against an existing session.)
  if (existing) await CometChatUIKit.logout();
  if (loginInFlight) {                   // concurrent case — reuse pending promise
    await loginInFlight;
    return;
  }
  loginInFlight = authToken
    ? CometChatUIKit.loginWithAuthToken(authToken)
    : CometChatUIKit.login(uid);
  try {
    await loginInFlight;
  } finally {
    loginInFlight = null;
  }
}

Call ensureLoggedIn() from the provider / effect instead of CometChatUIKit.login() directly. Both StrictMode mounts resolve against the same promise, so only one login request actually hits the server.

Why not just a boolean flag? A boolean would require extra wait-loop code to handle "login started but not finished yet." A cached promise handles that automatically — await on the same promise is free for all callers.

Getting the current logged-in UID in app code

When your integration code needs the current user's UID (for example, to decide which conversation to target, or to filter by sender), always fetch it from the SDK — never hardcode a UID like "cometchat-uid-1".

Two getters, for different contexts. Default to the sync version — it matches the v6 sample app and works for almost all app code, because by the time UI components render, the kit's init + login flow is already complete:

// ✓ Preferred — sync, returns User | null directly. Use this in app code.
import { CometChatUIKitLoginListener } from "@cometchat/chat-uikit-react";
const me = CometChatUIKitLoginListener.getLoggedInUser();  // note capital `I` in `InUser`
const myUid = me?.getUid();

// Fallback — async, for the bootstrap path where init may not be complete
// (e.g., inside the provider's init effect, or before the first login resolves).
const me = await CometChatUIKit.getLoggedinUser();
const myUid = me?.getUid();

The sync CometChatUIKitLoginListener.getLoggedInUser() is the right call from any component that mounts AFTER login completes — which is virtually all of them, since the dispatcher's recipes put login on a dedicated route or in the provider's init effect that gates rendering. Reach for the async CometChatUIKit.getLoggedinUser() only when you're inside that init effect itself.

Casing matters. Note getLogged**In**User (capital I) on the LoginListener vs getLogged**in**User (lowercase i) on CometChatUIKit — both casings exist in the kit, they're different methods.

Hardcoding "cometchat-uid-1" only works in the dev mode login call (CometChatUIKit.login("cometchat-uid-1")) because you're choosing who to log in as. Once logged in, the getters are the source of truth — useful when the logged-in user comes from production auth (a real user ID, not a test UID), or when the user logs out and logs in as someone else.

Production mode

Use CometChatUIKit.loginWithAuthToken(token) with a token obtained from your backend. The backend generates the token using the CometChat REST API with your AUTH_TOKEN (not the client-side AUTH_KEY).

// Fetch token from YOUR backend, which calls CometChat's REST API
const response = await fetch("/api/cometchat-token", {
  method: "POST",
  headers: { "Content-Type": "application/json" },
  body: JSON.stringify({ uid: currentUser.id }),
});
const { token } = await response.json();

await CometChatUIKit.loginWithAuthToken(token);

For the full production auth setup, use npx @cometchat/skills-cli production-auth. Never hardcode auth keys in source code that ships to production.

Logout

await CometChatUIKit.logout();

Call this when the user signs out of your application. This clears CometChat's local session.

3. CSS

Import once at the app root

import "@cometchat/chat-uikit-react/css-variables.css";

This import MUST appear exactly once, at the highest level of your application:

Theming with CSS variables

All CometChat components respect --cometchat-* CSS variables. Override them on a parent element or :root:

:root {
  --cometchat-primary-color: #6851d6;
  --cometchat-background-color-01: #ffffff;
  --cometchat-text-color-primary: #141414;
  --cometchat-font-family: "Inter", sans-serif;
  --cometchat-border-radius-lg: 12px;
}

Never target internal class names

CometChat's internal class names (like .cometchat-message-bubble__wrapper) are not part of the public API and may change between versions. Always use CSS variables for customization. The only exception is when explicitly copying patterns from the v6 sample app that use documented BEM class names.

4. Environment variables

Each framework has its own convention for exposing env vars to client-side code. CometChat needs three variables: APP_ID, REGION, and AUTH_KEY.

Per-framework naming

The three variables

.env file placement

5. SSR safety

All CometChat UI Kit components are browser-only. They access window, document, and browser APIs during import. Rendering them on the server will crash.

Framework-specific SSR prevention

Next.js (App Router):

Mark the file containing CometChat components with "use client" at the top. Use next/dynamic with ssr: false if the component is imported from a server component:

"use client";
// This entire file only runs in the browser

import { CometChatConversations } from "@cometchat/chat-uikit-react";

Or from a server component:

import dynamic from "next/dynamic";

const ChatView = dynamic(() => import("./ChatView"), { ssr: false });

Next.js (Pages Router):

Use next/dynamic with ssr: false:

import dynamic from "next/dynamic";

const CometChatNoSSR = dynamic(() => import("../components/CometChatNoSSR"), {
  ssr: false,
});

Astro:

Use the client:only="react" directive. This prevents the component from rendering during Astro's static build:

---
import ChatPanel from "../components/ChatPanel";
---
<ChatPanel client:only="react" />

React Router v7 (SSR mode):

Use React.lazy() with Suspense in a clientLoader or useEffect guard:

import { lazy, Suspense } from "react";

const ChatView = lazy(() => import("./ChatView"));

export default function ChatRoute() {
  const [mounted, setMounted] = useState(false);
  useEffect(() => setMounted(true), []);

  if (!mounted) return null;
  return (
    <Suspense fallback={<div>Loading chat...</div>}>
      <ChatView />
    </Suspense>
  );
}

React (Vite / CRA):

No SSR concerns. These are client-only by nature. Import and use directly.

6. Provider pattern

Instead of inlining init/login logic in every component, create a reusable CometChatProvider that handles initialization, login, and ready-state gating. Wrap your chat UI with it.

// CometChatProvider.tsx
"use client"; // Required for Next.js App Router; harmless in other frameworks

import React, { useEffect, useState, createContext, useContext } from "react";
import { CometChatUIKit, UIKitSettingsBuilder } from "@cometchat/chat-uikit-react";

interface CometChatContextValue {
  isReady: boolean;
  error: string | null;
}

const CometChatContext = createContext<CometChatContextValue>({
  isReady: false,
  error: null,
});

export const useCometChat = () => useContext(CometChatContext);

// Module-level state: shared across all mounts so React 18 StrictMode's
// double-invocation of effects doesn't fire init or login twice.
let initialized = false;
let loginInFlight: Promise<unknown> | null = null;

async function ensureLoggedIn(
  uid: string,
  authToken?: string,
): Promise<void> {
  const existing = await CometChatUIKit.getLoggedinUser();
  if (existing) return;
  if (loginInFlight) {
    // A prior StrictMode mount (or another effect) already started login —
    // reuse its promise instead of calling login() a second time, which
    // throws "Please wait until the previous login request ends."
    await loginInFlight;
    return;
  }
  loginInFlight = authToken
    ? CometChatUIKit.loginWithAuthToken(authToken)
    : CometChatUIKit.login(uid);
  try {
    await loginInFlight;
  } finally {
    loginInFlight = null;
  }
}

// ENG-35715 telemetry hook — opt-in callback for init/login lifecycle.
// Fires at six phases. If `telemetry` prop is not passed, all calls are no-ops.
export type CometChatTelemetryEvent =
  | { phase: "init_started";    appId: string; region: string }
  | { phase: "init_finished";   durationMs: number }
  | { phase: "init_failed";     error: unknown }
  | { phase: "login_started";   uid: string; mode: "authKey" | "authToken" }
  | { phase: "login_finished";  durationMs: number }
  | { phase: "login_failed";    error: unknown };

interface CometChatProviderProps {
  appId: string;
  region: string;
  authKey?: string;
  authToken?: string;
  uid?: string;
  telemetry?: (event: CometChatTelemetryEvent) => void;   // ← opt-in lifecycle hook
  children: React.ReactNode;
}

export function CometChatProvider({
  appId,
  region,
  authKey,
  authToken,
  uid = "cometchat-uid-1",
  telemetry,
  children,
}: CometChatProviderProps) {
  const [isReady, setIsReady] = useState(false);
  const [error, setError] = useState<string | null>(null);

  useEffect(() => {
    async function setup() {
      const initStart = performance.now();
      try {
        if (!initialized) {
          initialized = true;
          telemetry?.({ phase: "init_started", appId, region });
          const builder = new UIKitSettingsBuilder()
            .setAppId(appId)
            .setRegion(region)
            .subscribePresenceForAllUsers();

          if (authKey) {
            builder.setAuthKey(authKey);
          }

          const settings = builder.build();
          await CometChatUIKit.init(settings);
          telemetry?.({ phase: "init_finished", durationMs: performance.now() - initStart });
        }

        const loginStart = performance.now();
        telemetry?.({ phase: "login_started", uid, mode: authToken ? "authToken" : "authKey" });
        await ensureLoggedIn(uid, authToken);
        telemetry?.({ phase: "login_finished", durationMs: performance.now() - loginStart });

        setIsReady(true);
      } catch (e) {
        // If init never completed, this is an init_failed; otherwise login_failed.
        const phase = isReady ? "login_failed" : "init_failed";
        telemetry?.({ phase, error: e } as CometChatTelemetryEvent);
        setError(formatCometChatError(e));
      }
    }

    setup();
  }, [appId, region, authKey, authToken, uid, telemetry]);

  if (error) {
    return (
      <div style={{ color: "red", padding: 16, fontFamily: "monospace" }}>
        CometChat Error: {error}
      </div>
    );
  }

  if (!isReady) {
    return null; // Or a loading spinner
  }

  return (
    <CometChatContext.Provider value={{ isReady, error }}>
      {children}
    </CometChatContext.Provider>
  );
}

Usage

// In your app layout or route wrapper:
<CometChatProvider
  appId={import.meta.env.VITE_COMETCHAT_APP_ID}
  region={import.meta.env.VITE_COMETCHAT_REGION}
  authKey={import.meta.env.VITE_COMETCHAT_AUTH_KEY}
  // Optional — wire your analytics on each lifecycle event (ENG-35715):
  telemetry={(event) => {
    if (event.phase === "init_finished" || event.phase === "login_finished") {
      analytics.track(`cometchat.${event.phase}`, { ms: event.durationMs });
    } else if (event.phase === "init_failed" || event.phase === "login_failed") {
      analytics.trackError(`cometchat.${event.phase}`, { error: String(event.error) });
    } else {
      analytics.track(`cometchat.${event.phase}`, event);
    }
  }}
>
  <ChatPage />
</CometChatProvider>

The provider pattern keeps init/login logic in one place. Chat components inside <CometChatProvider> are guaranteed to render only after init and login succeed.

Pretty-print errors — DO NOT String(error) (ENG-35719)

CometChat.CometChatException objects look like { code: "ERROR_API_KEY_NOT_FOUND", message: "Auth Key cannot be empty", details: ..., source: ... } (in some kit versions the fields are errorCode / errorDescription). Calling String(e) on them yields "[object Object]" — the integrator then has to open devtools, copy the error to the console, and JSON.stringify it by hand just to read the message. Testers consistently flag this as the most frustrating moment of the first-run integration.

Emit this helper in cometchat/errors.ts and reuse it from the provider, login screen, and any feature module that catches a kit error:

// cometchat/errors.ts
export function formatCometChatError(e: unknown): string {
  if (e == null) return "Unknown CometChat error.";
  const err = e as Record<string, unknown>;
  const code =
    (err.code as string | undefined) ??
    (err.errorCode as string | undefined);
  const message =
    (err.message as string | undefined) ??
    (err.errorDescription as string | undefined);
  if (code && message) return `[CometChat ${code}] ${message}`;
  if (message) return `[CometChat] ${message}`;
  try {
    return `[CometChat] ${JSON.stringify(e)}`;
  } catch {
    return `[CometChat] ${String(e)}`;
  }
}

const KNOWN_DOC_HINTS: Record<string, string> = {
  ERROR_API_KEY_NOT_FOUND:
    "Auth Key is missing or invalid. Check your env vars (VITE_COMETCHAT_AUTH_KEY / EXPO_PUBLIC_COMETCHAT_AUTH_KEY) and confirm the key in Dashboard → App → Credentials.",
  ERR_UID_NOT_FOUND:
    "The UID you're logging in with doesn't exist in this CometChat app. Create the user in Dashboard → Users, or pick one of the pre-seeded test UIDs (cometchat-uid-1 .. cometchat-uid-5).",
  ERR_AUTH_TOKEN_NOT_FOUND:
    "Auth token is empty or expired. Re-mint it from your backend via the CometChat REST API.",
  AUTH_ERR_BOT:
    "This UID is flagged as a Bot in the dashboard — auth-key login is refused for bot users by design. Toggle Bot OFF in Dashboard → Users, or pick a non-bot UID.",
};

export function logCometChatError(e: unknown): void {
  const formatted = formatCometChatError(e);
  console.error(formatted, e);
  const code = (e as { code?: string; errorCode?: string })?.code ??
               (e as { code?: string; errorCode?: string })?.errorCode;
  if (code && KNOWN_DOC_HINTS[code]) {
    console.warn(`[CometChat hint] ${KNOWN_DOC_HINTS[code]}`);
  }
}

Then in the provider:

} catch (e) {
  logCometChatError(e);
  setError(formatCometChatError(e));
}

And in the error renderer:

if (error) {
  return (
    <div role="alert" style={{ color: "#b91c1c", padding: 16, fontFamily: "ui-monospace, monospace", whiteSpace: "pre-wrap" }}>
      <strong>CometChat failed to initialize.</strong>
      <div style={{ marginTop: 8 }}>{error}</div>
      <div style={{ marginTop: 8, fontSize: 12, opacity: 0.7 }}>
        See the browser console for the full error object and any [CometChat hint] line above it.
      </div>
    </div>
  );
}

This three-part recipe — formatCometChatError for the visible message, logCometChatError for the console (with a docs hint for known codes), and a multi-line render that points at the console — turns "ERR_API_KEY_NOT_FOUND" from a 10-minute debugging session into a 10-second fix.

⚠️ Replace setError(String(e)) everywhere. The two earlier examples in this skill (§1 init-in-useEffect, §6 provider's setup() catch) use setError(String(e)) for compactness — when you scaffold the real integration, emit formatCometChatError(e) instead and import it from cometchat/errors.ts. The same applies to RN (cometchat-native-core), Angular (cometchat-angular-core), and the native cores — the helper is platform-agnostic, just adapt the file extension.

7. RTL, i18n, and accessibility

These three concerns share one property: the UI Kit handles them out of the box, but a careless customization can break them. Read this before writing custom views, composer actions, or header replacements.

RTL (right-to-left)

The UI Kit reads dir="rtl" from the document root. If the project already sets <html dir="rtl"> (or toggles it dynamically for Arabic/Hebrew locales), CometChat components flip automatically — message bubbles mirror, avatars swap sides, icons rotate where appropriate. No CometChat-specific config needed.

To test: add <html dir="rtl"> to index.html (or set it via JS in Next.js App Router: <html dir="rtl"> in app/layout.tsx). Reload — the conversation list avatar + text should render on the right, message bubbles mirror, the composer input aligns right.

When customizing: if you replace a default view (e.g. a custom message bubble), test it in both LTR and RTL. The UI Kit's components use logical properties (margin-inline-start, padding-inline-end) — your custom components should too, or they'll break RTL.

i18n (translations)

The UI Kit has a built-in CometChatLocalize utility that covers ~40 languages out of the box. Initialize it once, alongside CometChatUIKit.init():

import { CometChatLocalize } from "@cometchat/chat-uikit-react";

CometChatLocalize.init({
  language: "es",  // or "fr", "de", "ar", "hi", etc.
});

For a dynamic language switcher, call CometChatLocalize.setCurrentLanguage(newLang) when the user picks a language (NOT setLocale — that method doesn't exist in v6). The UI Kit re-renders with the new strings.

To override a string: there's no nested resources: { en: {...} } option. Use either translationsForLanguage at init (a FLAT key→value map for the chosen language), or the standalone CometChatLocalize.addTranslation({...}):

// (a) at init — flat map for the active language
CometChatLocalize.init({
  language: "en",
  translationsForLanguage: {
    "type a message": "Write your message…",
    "start a conversation": "Say hi 👋",
  },
});

// (b) anytime after init
CometChatLocalize.addTranslation({
  "type a message": "Write your message…",
});

Full translation key list lives under node_modules/@cometchat/chat-uikit-react/dist/types/resources/ (types) or the docs MCP. Don't invent keys — unknown keys fall through to the default.

Accessibility

Default components ship with:

• aria-label on icon-only buttons (send, attach, call, etc.)
• role="listbox" + role="option" on conversation / user / group lists
• Keyboard navigation: Tab to focus, Enter to activate, Esc to close modals
• Focus management: opening a thread view moves focus to the thread header; closing returns focus to the trigger

Rules when customizing:

1. Replacing an icon-only button? Add aria-label="<verb>" (e.g. aria-label="Send message").
2. Replacing a list item? Keep role="option" + aria-selected on the wrapping element.
3. Replacing the composer? Preserve the <textarea> with an accessible <label> (visible or aria-label), and keep Enter/Shift+Enter behavior.
4. Replacing a modal? Trap focus inside the modal while open, restore focus to the trigger on close, and add role="dialog" + aria-modal="true" + a labelled heading.
5. Color contrast: when theming with custom colors, verify text contrast ≥ 4.5:1 against background. A low-saturation primary color on a white background breaks AA contrast.

For deep customization (e.g. a fully custom message bubble), the a11y responsibility shifts to the custom component — the UI Kit only guarantees it for its own defaults. Test with a screen reader (VoiceOver on macOS, NVDA on Windows) and keyboard-only navigation before shipping.

8. Anti-patterns

These are specific things NOT to do. Each one causes real bugs that are hard to debug.

1. Do NOT call CometChatUIKit.init() during render. Init is async and has side effects. Calling it during render causes infinite re-render loops. Always call in useEffect or before createRoot.

2. Do NOT import css-variables.css in multiple files. Duplicate imports cause CSS specificity conflicts and doubled variable declarations. Import it exactly once at the app root.

3. Do NOT render CometChat components before init completes. Components assume the SDK is initialized. Rendering before init finishes causes "CometChat is not initialized" runtime errors. Use the provider pattern or a ready-state gate.

4. Do NOT hardcode AUTH_KEY in source files. The auth key is a secret. Use environment variables during development. Use auth tokens in production.

5. Guard concurrent login() calls with a module-level in-flight promise. login() is only safe to call sequentially. Two login() calls overlapping (e.g. React 18 StrictMode's double effect) throw "Please wait until the previous login request ends." Cache the first login's promise at module scope and await that from subsequent callers. See the ensureLoggedIn helper in section 2 and section 6's provider pattern.

6. Do NOT render CometChat components in a server-side context. All components require browser APIs. In Next.js, always use "use client". In Astro, always use client:only="react".

7. Do NOT target CometChat's internal CSS class names for styling. These are not part of the public API. Use --cometchat-* CSS variables instead. Internal classes change between minor versions.

8. Do NOT create CometChat components without a container that has explicit dimensions. CometChat components fill 100% of their container. If the container has no height, the components collapse to zero height. Always set height, min-height, or use flexbox/grid to give the container dimensions.

9. Do NOT re-initialize CometChat when navigating between routes. Init should happen once at the app level (in the provider or entry file), not per-route. Re-initializing causes flickering and dropped WebSocket connections.

10. Do NOT invent component names. CometChat exports specific components with specific names. Check the cometchat-components skill before writing any <CometChat*> JSX. Using a wrong name (e.g., <CometChatChat>, <CometChatMessenger>) causes a build error.

11. Do NOT wrap CometChat components in a transformed container. Per the CSS spec, any non-none transform on an element creates a new containing block for position: fixed descendants. CometChat UI Kit renders several overlays as position: fixed (message options menu, emoji picker, file preview, reactions popover, thread panel) and expects them to anchor to the viewport. Wrapping the chat in a container that uses transform: translateX(...) — a common pattern for slide-in drawers / sidebars — reparents those overlays to the drawer, causing them to appear clipped, offset, or drift mid-animation.

    This includes Tailwind's translate-x-* utilities — translate-x-full, -translate-x-full, translate-x-0, translate-x-[420px], etc. all compile to transform: translateX(...) and trigger the same bug. Same for -translate-y-*, translate-*, scale-*, rotate-*, skew-*, transform-*, and any transition-transform utility applied to a container wrapping CometChat components. If you see yourself reaching for any Tailwind class in the transform: family on a drawer/sidebar/modal that contains chat UI, stop.

    Animate the right / left offset instead, or use margin-right: isOpen ? 0 : -<width>. In Tailwind: toggle between right-0 and a negative right-[-420px] with transition-[right] instead of transition-transform.

    Same rule applies to filter, perspective, backdrop-filter, and will-change: transform — any of those also trigger the containing-block takeover. See cometchat-placement's drawer and widget patterns for the correct right-offset animation.

9. Docs MCP (recommended, not required)

The CometChat docs MCP provides runtime access to the latest documentation, including prop types, callback signatures, request builder methods, SDK events, CSS variable names, and error decoders.

Installation

claude mcp add --transport http cometchat-docs https://www.cometchat.com/docs/mcp

For other clients, see: https://www.cometchat.com/docs/mcp-server

When to use

• Looking up a prop's exact type or default value
• Finding callback signatures (e.g., what onItemClick passes)
• Checking request builder methods (e.g., ConversationsRequestBuilder.setLimit)
• Understanding SDK events (e.g., CometChatMessageEvents.ccMessageSent)
• Verifying CSS variable names before writing overrides
• Decoding error messages (e.g., "INVALID_AUTH_KEY")

When NOT to use

• For component names and basic props -- use the cometchat-components skill instead (it works offline)
• For init/login/CSS patterns -- they are in this skill
• For placement patterns -- they are in the cometchat-placement skill
• For anything the CLI handles -- the CLI templates are the source of truth for those paths

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