Custom Authentication Adapters Guide

Overview
Server-Side Adapters
Client-Side Adapters
Common Patterns
Testing Adapters
Complete Examples

Overview

For the framework-agnostic linkage model (ensureCustomer(externalRef) from your backend after your own IdP login), see Customer linkage (bring your own client and IdP). SolvaPay SDK uses authentication adapters to extract user IDs from requests. There are two types of adapters:

Server-Side Adapters (@solvapay/auth) - Extract user IDs from HTTP requests in API routes
Client-Side Adapters (@solvapay/react) - Extract user IDs and tokens from client-side auth state

Server-Side Adapters

Server-side adapters are used with @solvapay/server to extract user IDs from HTTP requests in API routes, Express endpoints, and other server-side contexts.

Interface

import type { AuthAdapter } from '@solvapay/auth'

interface AuthAdapter {
  /**
   * Extract the authenticated user ID from a request.
   * Should never throw - return null if authentication fails or is missing.
   */
  getUserIdFromRequest(req: Request | RequestLike): Promise<string | null>
}

Basic Example: JWT Token Adapter

import type { AuthAdapter } from '@solvapay/auth'
import jwt from 'jsonwebtoken'

class JWTAuthAdapter implements AuthAdapter {
  constructor(private secret: string) {}

  async getUserIdFromRequest(req: Request | RequestLike): Promise<string | null> {
    try {
      // Extract token from Authorization header
      const authHeader = req.headers.get?.('authorization') || (req.headers as any).authorization

      if (!authHeader || !authHeader.startsWith('Bearer ')) {
        return null
      }

      const token = authHeader.replace('Bearer ', '')

      // Verify and decode token
      const decoded = jwt.verify(token, this.secret) as { userId: string }

      return decoded.userId || null
    } catch (error) {
      // Never throw - return null on error
      return null
    }
  }
}

// Usage
const authAdapter = new JWTAuthAdapter(process.env.JWT_SECRET!)
const userId = await authAdapter.getUserIdFromRequest(request)

Example: Session-Based Adapter

import type { AuthAdapter } from '@solvapay/auth'
import { getSession } from 'your-session-library'

class SessionAuthAdapter implements AuthAdapter {
  async getUserIdFromRequest(req: Request | RequestLike): Promise<string | null> {
    try {
      // Extract session from request
      const session = await getSession(req)

      if (!session || !session.userId) {
        return null
      }

      return session.userId
    } catch (error) {
      return null
    }
  }
}

Example: Custom Header Adapter

import type { AuthAdapter } from '@solvapay/auth'

class HeaderAuthAdapter implements AuthAdapter {
  constructor(private headerName: string = 'x-user-id') {}

  async getUserIdFromRequest(req: Request | RequestLike): Promise<string | null> {
    const userId = req.headers.get?.(this.headerName) || (req.headers as any)[this.headerName]

    return userId || null
  }
}

// Usage
const authAdapter = new HeaderAuthAdapter('x-customer-id')

Using with SolvaPay Server SDK

import { createSolvaPay } from '@solvapay/server'
import type { AuthAdapter } from '@solvapay/auth'

const authAdapter: AuthAdapter = {
  async getUserIdFromRequest(req) {
    // Your custom logic
    return userId || null
  },
}

const solvaPay = createSolvaPay({ apiKey: process.env.SOLVAPAY_SECRET_KEY })
const payable = solvaPay.payable({ product: 'prd_myapi' })

// Use with Express
app.post(
  '/api/tasks',
  payable.http(createTask, {
    getCustomerRef: async req => {
      const userId = await authAdapter.getUserIdFromRequest(req)
      if (!userId) throw new Error('Unauthorized')
      return userId
    },
  }),
)

// Use with Next.js
export const POST = payable.next(createTask, {
  getCustomerRef: async req => {
    const userId = await authAdapter.getUserIdFromRequest(req)
    if (!userId) throw new Error('Unauthorized')
    return userId
  },
})

Client-Side Adapters

Client-side adapters are used with @solvapay/react to extract user IDs and tokens from client-side authentication state.

Interface

import type { AuthAdapter } from '@solvapay/react'

interface AuthAdapter {
  /**
   * Get the authentication token
   */
  getToken(): Promise<string | null>

  /**
   * Get the user ID (for cache key)
   */
  getUserId(): Promise<string | null>
}

Basic Example: LocalStorage Adapter

import type { AuthAdapter } from '@solvapay/react';

class LocalStorageAuthAdapter implements AuthAdapter {
  async getToken(): Promise<string | null> {
    return localStorage.getItem('auth-token');
  }

  async getUserId(): Promise<string | null> {
    const token = await this.getToken();
    if (!token) return null;

    try {
      // Decode JWT token (client-side)
      const payload = JSON.parse(atob(token.split('.')[1]));
      return payload.userId || null;
    } catch {
      return null;
    }
  }
}

// Usage with SolvaPayProvider
import { SolvaPayProvider } from '@solvapay/react';

function App() {
  const adapter = new LocalStorageAuthAdapter();

  return (
    <SolvaPayProvider config={{ auth: { adapter } }}>
      <YourApp />
    </SolvaPayProvider>
  );
}

Example: Context-Based Adapter

import type { AuthAdapter } from '@solvapay/react';
import { useContext } from 'react';
import { AuthContext } from './AuthContext';

function createContextAuthAdapter(): AuthAdapter {
  return {
    async getToken() {
      // Access auth context
      const { token } = useContext(AuthContext);
      return token || null;
    },

    async getUserId() {
      const { user } = useContext(AuthContext);
      return user?.id || null;
    },
  };
}

// Usage
function App() {
  const adapter = createContextAuthAdapter();

  return (
    <SolvaPayProvider config={{ auth: { adapter } }}>
      <YourApp />
    </SolvaPayProvider>
  );
}

Example: Async Storage Adapter (React Native)

import type { AuthAdapter } from '@solvapay/react'
import AsyncStorage from '@react-native-async-storage/async-storage'

class AsyncStorageAuthAdapter implements AuthAdapter {
  async getToken(): Promise<string | null> {
    return await AsyncStorage.getItem('auth-token')
  }

  async getUserId(): Promise<string | null> {
    const token = await this.getToken()
    if (!token) return null

    try {
      const payload = JSON.parse(atob(token.split('.')[1]))
      return payload.userId || null
    } catch {
      return null
    }
  }
}

Common Patterns

Pattern 1: JWT Token with User ID Extraction

// Server-side
class JWTAdapter implements AuthAdapter {
  constructor(private secret: string) {}

  async getUserIdFromRequest(req: Request | RequestLike): Promise<string | null> {
    const token = this.extractToken(req)
    if (!token) return null

    try {
      const decoded = jwt.verify(token, this.secret) as { userId: string }
      return decoded.userId
    } catch {
      return null
    }
  }

  private extractToken(req: Request | RequestLike): string | null {
    const authHeader = req.headers.get?.('authorization') || (req.headers as any).authorization
    return authHeader?.replace('Bearer ', '') || null
  }
}

// Server-side
class CookieAuthAdapter implements AuthAdapter {
  async getUserIdFromRequest(req: Request | RequestLike): Promise<string | null> {
    // Extract from cookies
    const cookies = this.parseCookies(req)
    const sessionId = cookies['session-id']

    if (!sessionId) return null

    // Look up session in database/cache
    const session = await this.getSession(sessionId)
    return session?.userId || null
  }

  private parseCookies(req: Request | RequestLike): Record<string, string> {
    const cookieHeader = req.headers.get?.('cookie') || (req.headers as any).cookie
    if (!cookieHeader) return {}

    return cookieHeader.split(';').reduce(
      (acc, cookie) => {
        const [key, value] = cookie.trim().split('=')
        acc[key] = value
        return acc
      },
      {} as Record<string, string>,
    )
  }

  private async getSession(sessionId: string): Promise<{ userId: string } | null> {
    // Your session lookup logic
    return null
  }
}

Pattern 3: API Key with User Mapping

// Server-side
class APIKeyAuthAdapter implements AuthAdapter {
  constructor(private keyToUserId: Map<string, string>) {}

  async getUserIdFromRequest(req: Request | RequestLike): Promise<string | null> {
    const apiKey = req.headers.get?.('x-api-key') || (req.headers as any)['x-api-key']

    if (!apiKey) return null

    return this.keyToUserId.get(apiKey) || null
  }
}

Testing Adapters

Mock Adapter for Testing

// Server-side mock
class MockAuthAdapter implements AuthAdapter {
  constructor(private mockUserId: string | null = 'test-user-123') {}

  async getUserIdFromRequest(req: Request | RequestLike): Promise<string | null> {
    // Allow override via header
    const headerUserId =
      req.headers.get?.('x-mock-user-id') || (req.headers as any)['x-mock-user-id']

    return headerUserId || this.mockUserId
  }
}

// Client-side mock
class MockClientAuthAdapter implements AuthAdapter {
  constructor(private mockUserId: string | null = 'test-user-123') {}

  async getToken(): Promise<string | null> {
    return 'mock-token'
  }

  async getUserId(): Promise<string | null> {
    return this.mockUserId
  }
}

Testing with Adapters

import { describe, it, expect } from 'vitest'
import { MockAuthAdapter } from './MockAuthAdapter'

describe('AuthAdapter', () => {
  it('should extract user ID from request', async () => {
    const adapter = new MockAuthAdapter('user-123')
    const request = new Request('http://localhost', {
      headers: { 'x-mock-user-id': 'user-123' },
    })

    const userId = await adapter.getUserIdFromRequest(request)
    expect(userId).toBe('user-123')
  })

  it('should return null for missing auth', async () => {
    const adapter = new MockAuthAdapter(null)
    const request = new Request('http://localhost')

    const userId = await adapter.getUserIdFromRequest(request)
    expect(userId).toBeNull()
  })
})

Complete Examples

Example 1: Firebase Auth Adapter (Client-Side)

import type { AuthAdapter } from '@solvapay/react';
import { getAuth } from 'firebase/auth';

class FirebaseAuthAdapter implements AuthAdapter {
  async getToken(): Promise<string | null> {
    const auth = getAuth();
    const user = auth.currentUser;

    if (!user) return null;

    return await user.getIdToken();
  }

  async getUserId(): Promise<string | null> {
    const auth = getAuth();
    return auth.currentUser?.uid || null;
  }
}

// Usage
function App() {
  const adapter = new FirebaseAuthAdapter();

  return (
    <SolvaPayProvider config={{ auth: { adapter } }}>
      <YourApp />
    </SolvaPayProvider>
  );
}

Example 2: Auth0 (Next.js App Router, v4)

Use @solvapay/auth/auth0 with @solvapay/next/middleware and @solvapay/react/auth0 on the client. The canonical runnable reference is examples/nextjs-auth0. Auth0 setup: create a Regular Web Application in the Auth0 Dashboard (httpOnly session cookie — not SPA/Native). If your IdP uses a short access-token TTL at the integrator edge (e.g. 24h), that stays on your side; SolvaPay keys customers on the stable Auth0 sub, so token expiry does not invalidate the customer mapping. Server middleware (proxy.ts at project root):

import { createAuth0AuthMiddleware } from '@solvapay/next/middleware'
import { auth0 } from './lib/auth0'

export const proxy = createAuth0AuthMiddleware({
  auth0,
  authRoutePrefix: '/auth',
})

export const config = {
  matcher: ['/((?!_next/static|_next/image|favicon.ico|sitemap.xml|robots.txt).*)'],
}

The middleware forwards two headers to downstream SolvaPay route handlers server-side only:

x-user-id — Auth0 sub, used as externalRef / customer reference
Authorization: Bearer <id_token> — optional Auth0 ID token for email/name on first customer create

Never send Auth0 access tokens from the browser to SolvaPay APIs. SolvaPay billing calls use your sk_* secret server-side. Client provider:

import { SolvaPayProvider } from '@solvapay/react'
import { createAuth0ClientAuthAdapter } from '@solvapay/react/auth0'
import { useUser } from '@auth0/nextjs-auth0'

function App({ children }: { children: React.ReactNode }) {
  const { user } = useUser()
  const adapter = createAuth0ClientAuthAdapter({ userId: user?.sub })

  return <SolvaPayProvider config={{ auth: { adapter } }}>{children}</SolvaPayProvider>
}

Auth0 keeps the session in an httpOnly cookie. The client adapter reports whether a user is signed in; proxy.ts bridges the session into x-user-id for API routes.

Example 3: Custom OAuth Adapter

import type { AuthAdapter } from '@solvapay/auth'

class OAuthAdapter implements AuthAdapter {
  constructor(
    private tokenEndpoint: string,
    private clientId: string,
    private clientSecret: string,
  ) {}

  async getUserIdFromRequest(req: Request | RequestLike): Promise<string | null> {
    const token = this.extractToken(req)
    if (!token) return null

    try {
      // Verify token with OAuth provider
      const response = await fetch(this.tokenEndpoint, {
        method: 'POST',
        headers: {
          'Content-Type': 'application/x-www-form-urlencoded',
        },
        body: new URLSearchParams({
          client_id: this.clientId,
          client_secret: this.clientSecret,
          token,
        }),
      })

      const data = await response.json()
      return data.user_id || null
    } catch {
      return null
    }
  }

  private extractToken(req: Request | RequestLike): string | null {
    const authHeader = req.headers.get?.('authorization') || (req.headers as any).authorization
    return authHeader?.replace('Bearer ', '') || null
  }
}

Best Practices

Never Throw: Adapters should never throw exceptions. Return null if authentication fails.
Handle Errors Gracefully: Catch all errors and return null instead of throwing.
Cache When Possible: Cache expensive operations (like token verification) when appropriate.
Type Safety: Use TypeScript for better type safety and developer experience.
Test Thoroughly: Write tests for your adapters, including edge cases.
Documentation: Document your adapter’s behavior and requirements.

Next Steps

Express.js Integration Guide - Use adapters with Express
Next.js Integration Guide - Use adapters with Next.js
React Integration Guide - Use adapters with React
Webhooks - Handle auth-related purchase events
API Reference - Full API documentation

TypeScript SDK

Table of Contents

Overview

Server-Side Adapters

Interface

Basic Example: JWT Token Adapter

Example: Session-Based Adapter

Example: Custom Header Adapter

Using with SolvaPay Server SDK

Client-Side Adapters

Interface

Basic Example: LocalStorage Adapter

Example: Context-Based Adapter

Example: Async Storage Adapter (React Native)

Common Patterns

Pattern 1: JWT Token with User ID Extraction

Pattern 3: API Key with User Mapping

Testing Adapters

Mock Adapter for Testing

Testing with Adapters

Complete Examples

Example 1: Firebase Auth Adapter (Client-Side)

Example 2: Auth0 (Next.js App Router, v4)

Example 3: Custom OAuth Adapter

Best Practices

Next Steps

​Table of Contents

​Overview

​Server-Side Adapters

​Interface

​Basic Example: JWT Token Adapter

​Example: Session-Based Adapter

​Example: Custom Header Adapter

​Using with SolvaPay Server SDK

​Client-Side Adapters

​Interface

​Basic Example: LocalStorage Adapter

​Example: Context-Based Adapter

​Example: Async Storage Adapter (React Native)

​Common Patterns

​Pattern 1: JWT Token with User ID Extraction

​Pattern 2: Cookie-Based Authentication

​Pattern 3: API Key with User Mapping

​Testing Adapters

​Mock Adapter for Testing

​Testing with Adapters

​Complete Examples

​Example 1: Firebase Auth Adapter (Client-Side)

​Example 2: Auth0 (Next.js App Router, v4)

​Example 3: Custom OAuth Adapter

​Best Practices

​Next Steps

Table of Contents

Overview

Server-Side Adapters

Interface

Basic Example: JWT Token Adapter

Example: Session-Based Adapter

Example: Custom Header Adapter

Using with SolvaPay Server SDK

Client-Side Adapters

Interface

Basic Example: LocalStorage Adapter

Example: Context-Based Adapter

Example: Async Storage Adapter (React Native)

Common Patterns

Pattern 1: JWT Token with User ID Extraction

Pattern 2: Cookie-Based Authentication

Pattern 3: API Key with User Mapping

Testing Adapters

Mock Adapter for Testing

Testing with Adapters

Complete Examples

Example 1: Firebase Auth Adapter (Client-Side)

Example 2: Auth0 (Next.js App Router, v4)

Example 3: Custom OAuth Adapter

Best Practices

Next Steps