The Definitive Guide to User Authentication in Convex with Next.js (featuring Clerk)

Building with Convex usually feels like a cheat code. You have a real-time database, automatic caching, and type-safe serverless functions all wrapped in a developer experience that just works.

But for many developers, that momentum hits a wall the moment they need to answer one question: "Who is this user?"

Authentication is often cited as the hardest part of modern app development. Unlike traditional monolithic backends where you own the session cookie, Convex operates on a "Bring Your Own Auth" (BYOA) model. This flexibility is powerful, but it puts the burden of configuration on you.

If you've been thinking, "Convex has been great… but auth is holding me back," this guide is for you. We are going to demystify the process, integrate Clerk—the standard for modern Next.js auth—and secure your Convex backend step-by-step.

Understanding the "Bring Your Own Auth" Model

Before we write code, you need to understand how Convex knows who your user is. Convex does not store passwords or manage sessions. Instead, it relies on the OpenID Connect standard.

Here is the flow:

1. The Identity Provider (Clerk): Your frontend logs the user in via Clerk. Clerk issues a JSON Web Token (JWT).
2. The Handshake: Your Convex client (in Next.js) passes this token to the Convex backend with every request.
3. The Validation: Convex checks the token's signature against Clerk's public keys (defined in your configuration).
4. The Identity: If valid, Convex populates the ctx.auth object in your functions.

This architecture delegates security to experts (Clerk) while keeping your backend logic close to your data (Convex).

Step 1: Setting up Clerk in Next.js

We assume you have a Next.js App Router project with Convex initialized. If not, check our initialization guide first.

First, install the Clerk SDK:

npm install @clerk/nextjs

Environment Configuration

In your .env.local file, add your Clerk keys (found in the Clerk Dashboard) and your Convex URL:

# .env.local
NEXT_PUBLIC_CONVEX_URL="https://your-convex-url.convex.cloud"
NEXT_PUBLIC_CLERK_PUBLISHABLE_KEY="pk_test_..."
CLERK_SECRET_KEY="sk_test_..."

Middleware Protection

In the Next.js App Router, middleware.ts is the gatekeeper. Create this file in your root directory to secure your routes:

// middleware.ts
import { clerkMiddleware } from "@clerk/nextjs/server";

export default clerkMiddleware();

export const config = {
  matcher: [
    // Skip Next.js internals and all static files, unless found in search params
    '/((?!_next|[^?]*\\.(?:html?|css|js(?!on)|jpe?g|webp|png|gif|svg|ttf|woff2?|ico|csv|docx?|xlsx?|zip|webmanifest)).*)',
    // Always run for API routes
    '/(api|trpc)(.*)',
  ],
};

Step 2: The Convex-Clerk Glue

This is where most developers get stuck. You need to tell the Convex React client to use Clerk's authentication state. We do this by replacing the standard ConvexProvider with ConvexProviderWithClerk.

Update your client provider file (usually app/ConvexClientProvider.tsx):

// app/ConvexClientProvider.tsx
"use client";

import { ReactNode } from "react";
import { ConvexReactClient } from "convex/react";
import { ConvexProviderWithClerk } from "convex/react-clerk";
import { ClerkProvider, useAuth } from "@clerk/nextjs";

const convex = new ConvexReactClient(process.env.NEXT_PUBLIC_CONVEX_URL!);

export default function ConvexClientProvider({
  children,
}: {
  children: ReactNode;
}) {
  return (
    <ClerkProvider publishableKey={process.env.NEXT_PUBLIC_CLERK_PUBLISHABLE_KEY}>
      <ConvexProviderWithClerk client={convex} useAuth={useAuth}>
        {children}
      </ConvexProviderWithClerk>
    </ClerkProvider>
  );
}

This wrapper automatically handles fetching the JWT from Clerk and ensuring the Convex WebSocket connection is authenticated.

Step 3: Server-Side Configuration

Now, we need to teach your Convex backend to trust tokens signed by Clerk.

Create a file named auth.config.ts inside your convex/ folder.

// convex/auth.config.ts
export default {
  providers: [
    {
      domain: "https://your-clerk-issuer-url.clerk.accounts.dev",
      applicationID: "convex",
    },
  ],
};

Crucial: The domain must match the "Issuer" URL found in your Clerk Dashboard (under API Keys > Advanced > JWT Templates usually, or the main API Keys page). It usually looks like https://clerk.your-project.com or https://pleasing-poodle-12.clerk.accounts.dev.

Once saved, run npx convex dev. Convex will detect the config and sync with Clerk's public keys.

Step 4: Securing Your Functions

With the plumbing in place, you can now secure your data. The ctx.auth object is your gateway to user identity.

1. The Guard Pattern (Queries & Mutations)

The most common pattern is throwing an error if a user isn't logged in.

// convex/todos.ts
import { query, mutation } from "./_generated/server";
import { v } from "convex/values";

// A secure query to get user's todos
export const getMyTodos = query({
  args: {},
  handler: async (ctx) => {
    const identity = await ctx.auth.getUserIdentity();

    if (!identity) {
      throw new Error("Unauthorized");
    }

    // `identity.subject` is the unique User ID from Clerk
    return await ctx.db
      .query("todos")
      .withIndex("by_user", (q) => q.eq("userId", identity.subject))
      .collect();
  },
});

// A secure mutation to add a todo
export const addTodo = mutation({
  args: { text: v.string() },
  handler: async (ctx, args) => {
    const identity = await ctx.auth.getUserIdentity();

    if (!identity) {
      throw new Error("Unauthorized");
    }

    await ctx.db.insert("todos", {
      text: args.text,
      userId: identity.subject, // Link data to the user
      completed: false,
    });
  },
});

2. Handling Optional Auth

Sometimes you want to show public data but enhance it if the user is logged in.

export const getPublicPost = query({
  args: { postId: v.id("posts") },
  handler: async (ctx, args) => {
    const post = await ctx.db.get(args.postId);
    const identity = await ctx.auth.getUserIdentity();

    // If logged in and owner, return extra draft content
    if (identity && post?.authorId === identity.subject) {
       return { ...post, ...post.draftContent };
    }

    return post;
  },
});

Step 5: Managing User Data in Convex

The identity object gives you the Clerk User ID (subject), email, and name. However, for a real application, you often need to store user profiles in your own database (Convex) to attach application-specific data (like role, credits, or preferences).

The "Sync" Pattern

A robust way to handle this is ensuring a User document exists in Convex whenever a mutation is called.

// convex/users.ts
import { mutation } from "./_generated/server";

export const storeUser = mutation({
  args: {},
  handler: async (ctx) => {
    const identity = await ctx.auth.getUserIdentity();
    if (!identity) {
      throw new Error("Called storeUser without authentication present");
    }

    // Check if we've already stored this user
    const user = await ctx.db
      .query("users")
      .withIndex("by_token", (q) => q.eq("tokenIdentifier", identity.tokenIdentifier))
      .unique();

    if (user !== null) {
      // If we differ from Clerk data, update. Otherwise, do nothing.
      if (user.name !== identity.name) {
        await ctx.db.patch(user._id, { name: identity.name });
      }
      return user._id;
    }

    // If it's a new user, create them
    return await ctx.db.insert("users", {
      name: identity.name!,
      tokenIdentifier: identity.tokenIdentifier,
      role: "user", // Default role
    });
  },
});

You can trigger this mutation inside a useEffect on your frontend immediately after login, or utilize Clerk Webhooks calling a Convex HTTP Action for a purely backend synchronization (ideal for production).

Best Practices & Pitfalls

1. Always use indexes: When querying data by userId, ensure you have defined an index in your schema.ts.

   // convex/schema.ts
   defineTable("todos", {
     text: v.string(),
     userId: v.string(),
   }).index("by_user", ["userId"])
   

2. Trust ctx.auth, not client arguments: Never pass a userId as an argument from the client to a mutation. A malicious user can easily change that argument. Always derive the ID from ctx.auth.getUserIdentity().
3. Token Refresh: The ConvexProviderWithClerk handles token refreshing automatically. You don't need to manually manage JWT expiration.

Conclusion

Authentication doesn't have to be the bottleneck of your project. By leveraging Convex's specific authentication hooks and Clerk's robust management, you can implement a secure, scalable auth system in under an hour.

You now have a backend that knows who your users are, secures their data, and scales effortlessly. The "auth wall" is gone. Go build something amazing.