Setup Convex with Sigma Auth

Guide for integrating Sigma Auth (Bitcoin-native authentication) into a Convex application using the @sigma-auth/better-auth-plugin package with @convex-dev/better-auth.

For general Better Auth concepts (session management, plugins, hooks), see Skill(better-auth-best-practices). This skill covers Convex + Sigma specific wiring and deployment pitfalls.

When to Use

• Building a Convex backend with Better Auth
• Adding Bitcoin-native auth to a Convex app
• Implementing OAuth flow with auth.sigmaidentity.com
• Integrating BAP (Bitcoin Attestation Protocol) identity
• If you are not using Convex, follow Mode B in setup-nextjs instead

Installation

bun add @convex-dev/better-auth@0.10.10 better-auth@1.4.9 @sigma-auth/better-auth-plugin

@convex-dev/better-auth@0.10.10 has a strict peer dependency on better-auth@1.4.9.

Quick Start

1. Convex Component Registration (convex/convex.config.ts)

Register the Better Auth component. Its tables (user, session, account, verification) are isolated from your app tables.

import betterAuth from "@convex-dev/better-auth/convex.config";
import { defineApp } from "convex/server";

const app = defineApp();
app.use(betterAuth);

export default app;

2. Convex Auth Config (convex/auth.config.ts)

import { getAuthConfigProvider } from "@convex-dev/better-auth/auth-config";
import type { AuthConfig } from "convex/server";

export default {
  providers: [getAuthConfigProvider()],
} satisfies AuthConfig;

3. Environment Variables

Next.js app (.env.local)

NEXT_PUBLIC_CONVEX_URL=https://your-deployment.convex.cloud
NEXT_PUBLIC_CONVEX_SITE_URL=https://your-deployment.convex.site
NEXT_PUBLIC_SIGMA_CLIENT_ID=your-app-name
NEXT_PUBLIC_SIGMA_AUTH_URL=https://auth.sigmaidentity.com

Convex deployment (CLI)

npx convex env set BETTER_AUTH_URL "https://your-site-url"
npx convex env set BETTER_AUTH_SECRET "your-random-secret"
npx convex env set NEXT_PUBLIC_SIGMA_CLIENT_ID "your-app-name"
npx convex env set NEXT_PUBLIC_SIGMA_AUTH_URL "https://auth.sigmaidentity.com"
npx convex env set SIGMA_MEMBER_PRIVATE_KEY "your-member-wif-key"

Use --prod for production: npx convex env set VAR "value" --prod

4. Server Configuration (convex/auth.ts)

Add the sigmaCallbackPlugin to your Better Auth server configuration. This runs inside the Convex environment.

import { createClient, type GenericCtx } from "@convex-dev/better-auth";
import { convex } from "@convex-dev/better-auth/plugins";
import { sigmaCallbackPlugin } from "@sigma-auth/better-auth-plugin/server";
import { betterAuth } from "better-auth/minimal";
import { components } from "./_generated/api";
import type { DataModel } from "./_generated/dataModel";
import { query } from "./_generated/server";
import authConfig from "./auth.config";

export const authComponent = createClient<DataModel>(components.betterAuth);

export const createAuth = (ctx: GenericCtx<DataModel>) => {
  const siteUrl = process.env.BETTER_AUTH_URL!;

  return betterAuth({
    baseURL: siteUrl,
    secret: process.env.BETTER_AUTH_SECRET,
    database: authComponent.adapter(ctx),
    trustedOrigins: [siteUrl],
    plugins: [
      convex({ authConfig }),
      sigmaCallbackPlugin(),
    ],
  });
};

export const getCurrentUser = query({
  args: {},
  handler: async (ctx) => {
    return authComponent.getAuthUser(ctx);
  },
});

Important: Use "better-auth/minimal" (NOT "better-auth") for the Convex runtime.

5. Next.js Auth Handler (lib/auth-server.ts)

Proxy /api/auth/* requests to Convex.

import { convexBetterAuthNextJs } from "@convex-dev/better-auth/nextjs";

const convexUrl = process.env.NEXT_PUBLIC_CONVEX_URL!;
const convexSiteUrl = process.env.NEXT_PUBLIC_CONVEX_SITE_URL!;

export const {
  handler,
  preloadAuthQuery,
  isAuthenticated,
  getToken,
  fetchAuthQuery,
  fetchAuthMutation,
  fetchAuthAction,
} = convexBetterAuthNextJs({ convexUrl, convexSiteUrl });

6. API Route (app/api/auth/[...all]/route.ts)

import { handler } from "@/lib/auth-server";

export const { GET, POST } = handler;

7. Client Configuration (lib/auth-client.ts)

import { convexClient } from "@convex-dev/better-auth/client/plugins";
import { sigmaClient } from "@sigma-auth/better-auth-plugin/client";
import { createAuthClient } from "better-auth/react";

export const authClient = createAuthClient({
  plugins: [convexClient(), sigmaClient()],
});

export const { signIn, signOut, useSession } = authClient;

8. Convex Provider (components/convex-provider.tsx)

Replace the standard ConvexProvider with the auth-aware provider.

"use client";

import { ConvexBetterAuthProvider } from "@convex-dev/better-auth/react";
import { ConvexReactClient } from "convex/react";
import type { ReactNode } from "react";
import { authClient } from "@/lib/auth-client";

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

export function ConvexClientProvider({
  children,
  initialToken,
}: {
  children: ReactNode;
  initialToken?: string | null;
}) {
  return (
    <ConvexBetterAuthProvider
      client={convex}
      authClient={authClient}
      initialToken={initialToken}
    >
      {children}
    </ConvexBetterAuthProvider>
  );
}

9. Sign-In Component

"use client";
import { signIn } from "@/lib/auth-client";

export function SignInButton() {
  return (
    <button onClick={() => signIn.sigma({
      clientId: process.env.NEXT_PUBLIC_SIGMA_CLIENT_ID!,
      // callbackURL defaults to /auth/sigma/callback
    })}>
      Sign in with Sigma
    </button>
  );
}

How It Works

1. Client: The sigmaClient initiates the OAuth flow, redirecting to auth.sigmaidentity.com.
2. Next.js Proxy: The convexBetterAuthNextJs handler proxies /api/auth/* requests to Convex HTTP actions.
3. Convex Plugin: The sigmaCallbackPlugin running in Convex:
   • Intercepts the callback.
   • Exchanges the authorization code for tokens using the server-side SIGMA_MEMBER_PRIVATE_KEY.
   • Creates or updates the user in the Convex database (via Better Auth's internal adapter).
   • Establishes a session.
4. Session: The ConvexBetterAuthProvider reads session state and provides it to your app via useSession().

Local Install (Admin & Organization Plugins)

The default "supported plugins" for @convex-dev/better-auth do not include admin or organization. To use these plugins, you must use the Local Install approach, which replaces the npm component with a local component definition that you control.

Full documentation: https://labs.convex.dev/better-auth/features/local-install

Why Local Install?

The standard @convex-dev/better-auth component ships a fixed schema. Plugins like admin and organization add additional tables and fields that the standard component does not know about. The local install lets you generate a schema that includes these plugin tables, then wire up the adapter yourself.

Step-by-Step

a. Create Local Component Config (convex/betterAuth/convex.config.ts)

Replace the npm component with a local component definition:

import { defineComponent } from "convex/server";
const component = defineComponent("betterAuth");
export default component;

b. Update App Config (convex/convex.config.ts)

Point the app at your local component instead of the npm package:

import { defineApp } from "convex/server";
import betterAuth from "./betterAuth/convex.config";

const app = defineApp();
app.use(betterAuth);

export default app;

c. Refactor Auth Config (convex/auth.ts)

Split into two functions: createAuthOptions (safe for module-load analysis) and createAuth (validates env vars at runtime).

Critical pattern -- env() vs must(): The createApi call in the adapter imports createAuthOptions at module load time, before Convex env vars are available. If createAuthOptions throws on a missing env var, the push will fail. Use an env() helper that returns "" instead of throwing. The createAuth function runs at request time, so it can validate with must().

import { createClient, type GenericCtx } from "@convex-dev/better-auth";
import { convex } from "@convex-dev/better-auth/plugins";
import { sigmaCallbackPlugin } from "@sigma-auth/better-auth-plugin/server";
import { betterAuth } from "better-auth/minimal";
import { admin } from "better-auth/plugins/admin";
import { organization } from "better-auth/plugins/organization";
import { components } from "./_generated/api";
import type { DataModel } from "./_generated/dataModel";
import { query } from "./_generated/server";
import authConfig from "./auth.config";
import authSchema from "./betterAuth/schema";

// Returns "" instead of throwing -- safe for module-load analysis
function env(name: string): string {
  return process.env[name] ?? "";
}

// Throws if missing -- use only at request time
function must(name: string): string {
  const v = process.env[name];
  if (!v) throw new Error(`Missing env var: ${name}`);
  return v;
}

export const authComponent = createClient<DataModel, typeof authSchema>(
  components.betterAuth,
  { local: { schema: authSchema } },
);

// Safe for module-load: never throws on missing env vars
export function createAuthOptions(ctx?: GenericCtx<DataModel>) {
  const siteUrl = env("BETTER_AUTH_URL");
  return {
    baseURL: siteUrl,
    secret: env("BETTER_AUTH_SECRET"),
    database: ctx ? authComponent.adapter(ctx) : undefined,
    trustedOrigins: [siteUrl, "http://localhost:3000"].filter(Boolean),
    plugins: [
      convex({ authConfig }),
      sigmaCallbackPlugin(),
      admin(),
      organization(),
    ],
  };
}

// Request-time only: validates env vars, then creates the auth instance
export const createAuth = (ctx: GenericCtx<DataModel>) => {
  must("BETTER_AUTH_URL");
  must("BETTER_AUTH_SECRET");
  return betterAuth({
    ...createAuthOptions(ctx),
    database: authComponent.adapter(ctx),
  });
};

export const getCurrentUser = query({
  args: {},
  handler: async (ctx) => {
    return authComponent.getAuthUser(ctx);
  },
});

d. Generate the Schema

Create a temporary file convex/betterAuth/auth.ts for schema generation:

import { createAuth } from "../auth";
export const auth = createAuth({} as any);

Then generate the schema:

cd convex/betterAuth
BETTER_AUTH_URL=http://localhost:3000 BETTER_AUTH_SECRET=dummy npx @better-auth/cli generate -y

Delete the temporary file immediately after generation:

rm convex/betterAuth/auth.ts

Convex analyzes all .ts files in the convex/ tree at push time. If auth.ts remains, the push will fail because createAuth({} as any) triggers env var access during module analysis.

e. Create the Adapter (convex/betterAuth/adapter.ts)

Wire the generated schema to the auth options:

import { createApi } from "@convex-dev/better-auth";
import { createAuthOptions } from "../auth";
import schema from "./schema";

export const {
  create,
  findOne,
  findMany,
  updateOne,
  updateMany,
  deleteOne,
  deleteMany,
} = createApi(schema, createAuthOptions);

f. Client-Side Plugins (lib/auth-client.ts)

Add the admin and organization client plugins:

import { convexClient } from "@convex-dev/better-auth/client/plugins";
import { sigmaClient } from "@sigma-auth/better-auth-plugin/client";
import { createAuthClient } from "better-auth/react";
import { adminClient } from "better-auth/client/plugins";
import { organizationClient } from "better-auth/client/plugins";

export const authClient = createAuthClient({
  plugins: [
    convexClient(),
    sigmaClient(),
    adminClient(),
    organizationClient(),
  ],
});

export const { signIn, signOut, useSession } = authClient;

Regenerating the Schema After Plugin Changes

Whenever you add, remove, or update plugins, regenerate the schema:

# Re-create the temporary auth.ts
cat > convex/betterAuth/auth.ts << 'EOF'
import { createAuth } from "../auth";
export const auth = createAuth({} as any);
EOF

# Generate
cd convex/betterAuth
BETTER_AUTH_URL=http://localhost:3000 BETTER_AUTH_SECRET=dummy npx @better-auth/cli generate -y

# Clean up
rm convex/betterAuth/auth.ts

Critical Deployment Checklist

Before going live, verify ALL of these. Missing any one causes silent auth failures:

1. Understand the Two Convex URLs (They Are Different!)

Variable	Value	Purpose
NEXT_PUBLIC_CONVEX_URL	https://<deployment>.convex.cloud	Client SDK connection (queries, mutations, subscriptions)
NEXT_PUBLIC_CONVEX_SITE_URL	https://<deployment>.convex.site	HTTP actions URL (auth proxy forwards requests here)

CRITICAL: NEXT_PUBLIC_CONVEX_SITE_URL must be the .convex.site URL. If you set it to your app domain (e.g., https://myapp.com), the auth proxy (convexBetterAuthNextJs) will loop back to itself instead of forwarding to Convex. This causes infinite redirects or timeouts during sign-in.

2. Set ALL Env Vars on Convex PRODUCTION

bunx convex env set targets the DEV deployment by default. Production requires --prod:

# PRODUCTION (the deployed app uses these)
bunx convex env set BETTER_AUTH_URL "https://your-domain.com" --prod
bunx convex env set BETTER_AUTH_SECRET "$(openssl rand -hex 32)" --prod
bunx convex env set NEXT_PUBLIC_SIGMA_CLIENT_ID "your-app-name" --prod
bunx convex env set NEXT_PUBLIC_SIGMA_AUTH_URL "https://auth.sigmaidentity.com" --prod
bunx convex env set SIGMA_MEMBER_PRIVATE_KEY "your-wif-key" --prod

# Verify they're actually set
bunx convex env list --prod

Without --prod, your production app has NO env vars and auth silently fails.

3. Deploy Convex Functions to Production

Running bunx convex dev only pushes functions to the dev deployment. Production needs:

bunx convex deploy --yes

Without this, your production Convex deployment has no functions, queries, or HTTP actions.

4. Set Vercel Env Vars Correctly

On Vercel (Settings > Environment Variables), set:

NEXT_PUBLIC_CONVEX_URL=https://<deployment>.convex.cloud
NEXT_PUBLIC_CONVEX_SITE_URL=https://<deployment>.convex.site

Common mistake: Setting NEXT_PUBLIC_CONVEX_SITE_URL to your app domain (https://myapp.com). This breaks the auth proxy.

5. Better Auth baseURL = Your App Domain

In convex/auth.ts, baseURL (from BETTER_AUTH_URL) must be your user-facing domain:

baseURL: process.env.BETTER_AUTH_URL, // e.g., "https://myapp.com"

NOT the .convex.site URL. This is used for OAuth redirect URIs.

6. Use Lazy Handler Initialization

The auth route handler must defer env var access to request time:

// src/app/api/auth/[...all]/route.ts
import { convexBetterAuthNextJs } from "@convex-dev/better-auth/nextjs";

function createHandler() {
  const convexUrl = process.env.NEXT_PUBLIC_CONVEX_URL;
  const convexSiteUrl = process.env.NEXT_PUBLIC_CONVEX_SITE_URL;
  if (!convexUrl || !convexSiteUrl) {
    throw new Error(
      "NEXT_PUBLIC_CONVEX_URL and NEXT_PUBLIC_CONVEX_SITE_URL must be set",
    );
  }
  return convexBetterAuthNextJs({ convexUrl, convexSiteUrl }).handler;
}

export const GET = async (req: Request) => {
  const { GET } = createHandler();
  return GET(req);
};

export const POST = async (req: Request) => {
  const { POST } = createHandler();
  return POST(req);
};

Without this, convexBetterAuthNextJs throws at build time when env vars aren't available yet.

7. Auth UI Must Exist

A bare sign-in button is not sufficient. Set up proper auth pages:

bunx shadcn@latest add login-05 signup-05

Then adapt for Sigma auth: remove email/password fields, keep the branded layout. Use your project logo, not the default icons.

Also add sign-in/sign-out controls to the app shell (sidebar footer, header) so users can access auth from anywhere.

8. Protect Routes from Unauthenticated Access

Don't blindly redirect unauthenticated users into the app. Use Next.js middleware to check auth and redirect to /login. See Skill(better-auth-best-practices) for the middleware pattern. Key: use betterFetch("/api/auth/get-session", { baseURL: request.nextUrl.origin }) and match all routes except api, _next, login, signup.

---

Troubleshooting

403 on Token Exchange (CSRF / trustedOrigins)

Symptom: OAuth flow succeeds but callback returns "Token Exchange Failed - Server returned 403". Better Auth logs: Invalid origin: https://your-preview-url.vercel.app

Root Cause: Better Auth's CSRF protection rejects POST requests from origins not in trustedOrigins. This is common on Vercel preview deployments with dynamic URLs.

Fix: Add Vercel's auto-set env vars to your Better Auth config's trustedOrigins in convex/auth.ts. You'll need to set VERCEL_URL and VERCEL_BRANCH_URL as Convex env vars (Convex doesn't get these automatically like Vercel does):

trustedOrigins: [
  siteUrl,
  process.env.VERCEL_URL ? `https://${process.env.VERCEL_URL}` : "",
  process.env.VERCEL_BRANCH_URL ? `https://${process.env.VERCEL_BRANCH_URL}` : "",
  "http://localhost:3000",
].filter(Boolean),

Missing Convex Env Vars

Convex env vars are separate from Vercel env vars. Set them with:

npx convex env set VAR_NAME "value"          # Dev deployment
npx convex env set VAR_NAME "value" --prod   # Production deployment
npx convex env list                          # Verify dev
npx convex env list --prod                   # Verify production

Callback URL Mismatch

Ensure your callback URL is registered in Sigma for every domain you deploy to:

• http://localhost:3000/auth/sigma/callback (local dev)
• https://your-domain.com/auth/sigma/callback (production)
• Vercel preview URLs as needed

Security

• Private Key: The SIGMA_MEMBER_PRIVATE_KEY is critical for signing token exchange requests. Ensure it is stored securely in Convex environment variables and never exposed to the client.
• PKCE: Handled automatically by the client/server plugin combination.
• Component Isolation: Better Auth tables (user, session, account, verification) are isolated from your app tables via the Convex component system.

Reference

Full documentation: https://github.com/b-open-io/better-auth-plugin