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@amadeni/better-auth-kit

Shared Better Auth base for Amadeni apps: Convex + Next.js, passwordless magic link sign-in via Resend, closed registration with initial-admin bootstrap.

The kit extracts the hardened reference implementation into reusable building blocks:

  • a hardened Better Auth options factory (storeToken: 'hashed', database-backed rate limiting — not overridable)
  • the branded German magic link email with the interstitial /login?token=… flow (mail scanners cannot consume tokens)
  • eligibility hooks for /sign-in/magic-link and /magic-link/verify
  • the lazy-linking user.onCreate trigger (app users table stays the source of truth)
  • Next.js middleware, catch-all route handlers, and server helpers
  • the fleet-wide email canonicalization and error codes with German client-side messages

Install

pnpm add @amadeni/better-auth-kit better-auth@1.6.23 @convex-dev/better-auth@0.12.5 convex

react and next are optional peers — only needed for the ./client and ./next entry points.

Entry points

Import Runs in Contents
@amadeni/better-auth-kit Convex functions options factory, Resend sender, eligibility, lazy-link trigger, hashing, URLs
@amadeni/better-auth-kit/client Browser/React auth client factory, sign-in error parsing + German messages
@amadeni/better-auth-kit/next Next.js server middleware, route decision, route handlers, auth server wrapper

The root entry never imports from react or next, so it is safe inside the Convex component directory.

Recommended Setup

The Better Auth component is installed locally in the app repo — this scaffold stays in the app (generated code and app-specific wiring cannot live in a package):

convex/
  auth.config.ts                 # getAuthConfigProvider()
  auth.ts                        # runtime instance (see below)
  betterAuth/
    convex.config.ts             # defineComponent('betterAuth')
    schema.ts                    # generated: npx auth generate --output ./schema.ts
    adapter.ts                   # createApi(schema, () => options)
    auth.ts                      # CLI-only betterAuth(options) for schema generation
    _generated/                  # convex codegen
src/
  middleware.ts
  app/api/auth/[...all]/route.ts

convex/betterAuth/authOptions.ts

import { createAmadeniAuthOptions } from '@amadeni/better-auth-kit';
import type { BetterAuthOptions } from 'better-auth/minimal';

export const createAuthOptions = createAmadeniAuthOptions;

// Static options for schema generation and the component adapter only.
export const options = createAmadeniAuthOptions() as BetterAuthOptions;

convex/auth.ts

import {
  AUTH_ERROR_CODES,
  canonicalizeAuthEmail,
  createAmadeniAuthOptions,
  createEligibilityHook,
  createLazyLinkTrigger,
  createResendMagicLinkSender,
  evaluateLoginEligibility,
} from '@amadeni/better-auth-kit';
import { createClient, type GenericCtx } from '@convex-dev/better-auth';
import { getAuthConfigProvider } from '@convex-dev/better-auth/auth-config';
import { betterAuth } from 'better-auth/minimal';
import { components, internal } from './_generated/api';
import authSchema from './betterAuth/schema';

const linkUser = createLazyLinkTrigger({
  findAppUserByEmail: async (ctx, email) =>
    ctx.db
      .query('users')
      .withIndex('by_email', q => q.eq('email', email))
      .unique(),
  hasAnyAppUser: async ctx => (await ctx.db.query('users').first()) !== null,
  linkComponentUser: async (ctx, { authUserId, appUserId }) => {
    await ctx.runMutation(components.betterAuth.adapter.updateOne, {
      input: {
        model: 'user',
        where: [{ field: '_id', value: authUserId }],
        update: { userId: appUserId },
      },
    });
  },
  bootstrapInitialAdmin: async (ctx, { email }) =>
    ctx.db.insert('users', { email, role: 'admin' }),
});

export const authComponent = createClient(components.betterAuth, {
  local: { schema: authSchema },
  triggers: {
    user: {
      onCreate: async (ctx, authUser) => {
        await linkUser(ctx, authUser);
      },
    },
  },
  authFunctions: internal.auth,
});

export const createAuth = (ctx: GenericCtx<DataModel>) =>
  betterAuth({
    ...createAmadeniAuthOptions({
      baseURL: process.env.SITE_URL,
      authConfig: { providers: [getAuthConfigProvider()] },
      sendMagicLink: createResendMagicLinkSender({
        brand: {
          productName: 'Example App',
          from: 'Example App <noreply@mail.example.com>',
        },
      }),
    }),
    database: authComponent.adapter(ctx),
    hooks: {
      before: createEligibilityHook({
        isEligible: async email =>
          'runQuery' in ctx
            ? ctx.runQuery(internal.auth.checkLoginEligibility, { email })
            : // Fail closed: a context that cannot run the check must not
              // let the request through.
              { ok: false, error: AUTH_ERROR_CODES.SIGN_IN_LINK_UNAVAILABLE },
      }),
    },
  });

Implement checkLoginEligibility as an internalQuery with evaluateLoginEligibility — same closed-registration rules as the trigger, checked before the email is sent.

convex/auth.config.ts

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

const authConfig = {
  providers: [getAuthConfigProvider()],
} satisfies AuthConfig;

export default authConfig;

src/middleware.ts

import { createAuthNextMiddleware } from '@amadeni/better-auth-kit/next';

export default createAuthNextMiddleware();

// Next.js statically parses `config` at build time — the matcher MUST be an
// inline literal here; importing AUTH_MIDDLEWARE_MATCHER breaks `next build`.
export const config = {
  matcher: ['/((?!.*\\..*|_next).*)', '/', '/(api|trpc)(.*)'],
};

Keep the literal in sync with the kit via a drift-guard test:

// src/middleware.test.ts
import { AUTH_MIDDLEWARE_MATCHER } from '@amadeni/better-auth-kit/next';
import { config } from './middleware';

test('middleware matcher matches the kit constant', () => {
  expect(config.matcher).toEqual(AUTH_MIDDLEWARE_MATCHER);
});

src/app/api/auth/[...all]/route.ts

import { createAuthRouteHandlers } from '@amadeni/better-auth-kit/next';
import { authServer } from '@/lib/auth-server';

export const { GET, POST } = createAuthRouteHandlers(authServer.handler);

src/lib/auth-server.ts and src/lib/auth-client.ts

// auth-server.ts
import { createAuthServer } from '@amadeni/better-auth-kit/next';

export const authServer = createAuthServer({
  convexUrl: process.env.NEXT_PUBLIC_CONVEX_URL!,
  convexSiteUrl: process.env.NEXT_PUBLIC_CONVEX_SITE_URL!,
});

// auth-client.ts
import { createAmadeniAuthClient } from '@amadeni/better-auth-kit/client';

export const authClient = createAmadeniAuthClient();

Environment variables

Variable Where Purpose
SITE_URL Convex deployment Public app URL; base for magic link URLs
BETTER_AUTH_SECRET Convex deployment Better Auth signing secret
AUTH_RESEND_KEY Convex deployment Resend API key (fallbacks: AUTH_RESEND_API_KEY, RESEND_API_KEY)
INITIAL_ADMIN_EMAIL Convex deployment Bootstrap admin while the users table is empty
NEXT_PUBLIC_CONVEX_URL Next.js Convex deployment URL (*.convex.cloud)
NEXT_PUBLIC_CONVEX_SITE_URL Next.js Convex HTTP actions URL (*.convex.site)
AMADENI_DEV_AUTH_ENABLED Convex deployment Dev deployments only — enables deterministic dev logins

Deterministic dev logins (createDevAuth)

WARNING: AMADENI_DEV_AUTH_ENABLED=true turns deployment env access into login ability. It must NEVER be set on a production deployment. assertDevAuthEnabled additionally refuses production-shaped CONVEX_DEPLOYMENT values (prod, prod:*, production:*) even when the flag leaks — but the flag simply has no business existing on prod.

Automated pipelines (visual review, e2e, @amadeni/dev-contract) need a login without an email round-trip. createDevAuth mints a single-use magic link token and writes its hashed verification row directly into the Better Auth component — the regular /api/auth/magic-link/verify endpoint then completes the sign-in with real sessions and real cookies. No auth-config deviation, nothing to clean up.

// convex/dev/auth.ts — app wiring (generic factory, injected persistence)
import { v } from 'convex/values';
import {
  createDevAuth,
  requireDevAuthCliIdentity,
} from '@amadeni/better-auth-kit';
import { action, internalMutation } from '../_generated/server';
import { components, internal } from '../_generated/api';

const devAuth = createDevAuth({
  createVerification: (ctx, input) =>
    ctx.runMutation(components.betterAuth.adapter.create, { input }),
  // Ensure the app user exists and is eligible BEFORE the token is minted,
  // otherwise the verify-time eligibility hook denies the login.
  ensureUser: (ctx, { email, name }) =>
    ctx.runMutation(internal.dev.auth.ensureDevUserInternal, { email, name }),
  defaultEmail: 'dev@amadeni.local',
});

export const createDevToken = action({
  args: { email: v.optional(v.string()) },
  handler: async (ctx, args) => {
    // CLI-only surface: `convex run --identity '{"issuer": ..., "subject": ...}'`
    await requireDevAuthCliIdentity(ctx, {
      issuer: 'my-app-dev-auth',
      subject: 'dev-auth-cli',
    });
    return await devAuth.issueToken(ctx, args); // gate + user + token
  },
});

Consume the token via devAuth.buildVerifyUrl({ token, origin }) — a fetch(url, { redirect: 'manual' }) returns the session cookies (better-auth.session_token, better-auth.convex_jwt) in Set-Cookie. @amadeni/dev-contract packages exactly this flow as a CLI with a verified-login readiness gate.

Resilient Convex token refresh (createConvexTokenManager)

After laptop standby or a background tab the Convex JWT expires; on wake the Convex reconnect races the token refresh, getSession-style requests can hang on a half-dead network, and the Convex client never retries once a token fetch returned null — users see error pages or a stuck "renewing sign-in" screen until they reload. The /client token manager fixes this fleet-wide:

  • Proactive refresh on visibilitychange → visible and online events plus a background interval — the token is fresh again before Convex reconnects.
  • Stale-while-revalidate + backoff: a not-yet-expired token keeps being served while refreshes retry (1s/2s/5s/15s, each attempt capped by a timeout). Unauthenticated is reported only once the token expired AND retries are exhausted — or the backend answers 401/403.
  • Automatic recovery: after Convex gave up (saw null), the next successful refresh bumps an auth epoch, which rotates the fetchAccessToken identity and makes ConvexProviderWithAuth call client.setAuth again — the automated version of "reload fixes it".
// src/lib/convex-token.ts
import {
  createBetterAuthTokenFetcher,
  createConvexAuthHooks,
  createConvexTokenManager,
} from '@amadeni/better-auth-kit/client';
import { authClient } from './auth-client';

export const tokenManager = createConvexTokenManager({
  fetchToken: createBetterAuthTokenFetcher(authClient),
});
export const { useAuth, useAuthToken } = createConvexAuthHooks(tokenManager);

// src/app/ConvexClientProvider.tsx
<ConvexProviderWithAuth client={convex} useAuth={useAuth}>

// Imperative consumers (HTTP uploads etc.):
const token = await tokenManager.getToken({ forceRefresh: gotA401 });

// Intermediate states become visible only after a few seconds:
const showPending = useDelayedAuthPending(authLost, 3_000);

Seed the SSR token with tokenManager.prime(initialToken); call tokenManager.clear() on explicit sign-out.

Security

This package is the auth base of the whole fleet, which makes it a supply-chain target. Deliberate posture:

  • Zero runtime dependencies. Everything is a peer dependency the app already ships. No postinstall scripts, no telemetry, and no network calls except the single explicit fetch to https://api.resend.com/emails in createResendMagicLinkSender.
  • Exact peer pins for the auth stack. better-auth@1.6.23 and @convex-dev/better-auth@0.12.5 are pinned exactly: the kit mirrors internals of these versions (token hashing, verification storage, internal adapter lookups), and silent minor upgrades of an auth library are a risk, not a feature. Upgrades happen as deliberate kit releases.
  • Consumers cannot weaken the core. createAmadeniAuthOptions always sets storeToken: 'hashed' (a database leak must not leak live sign-in tokens) and rateLimit: { storage: 'database' } (in-memory rate limiting never fires on the stateless Convex runtime). The magic link and convex plugins are always wired and cannot be replaced through extraPlugins: Better Auth merges plugin endpoints last-wins (and only logs endpoint conflicts), so the kit orders its plugins last and additionally throws if an extra plugin carries the reserved id magic-link or convex.
  • No open redirect through the envelope unwrap. unwrapRedirectEnvelope only issues redirects to the request's own origin (or origins explicitly allow-listed via allowedRedirectOrigins); cross-origin envelopes pass through unchanged.
  • Scanner-proof links. Emails link to the /login?token=… interstitial; only an explicit user click consumes the token at the verify endpoint.
  • Fail-closed verify re-check. createEligibilityHook re-validates eligibility on /magic-link/verify via the hashed-token lookup, so a user deleted after the email was sent cannot complete the sign-in.

Development

pnpm install
pnpm run ci    # prettier + eslint + tsc + cspell + vitest

Releases: pnpm run release (patch), release:minor, release:major.

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Shared better-auth foundation for Amadeni Convex + Next.js projects — hardened magic-link auth kit

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