Fix Anthropic Claude API CORS Errors in Frontend Apps

Why Are You Seeing a CORS Error with the Anthropic API?

If you try to call the Anthropic Claude API directly from a frontend application (such as a React, Vue, Angular, or vanilla JavaScript application running in the browser), your browser will block the request. You will see a console error resembling:

Access to fetch at 'https://api.anthropic.com/v1/messages' from origin 'http://localhost:3000' has been blocked by CORS policy.

This behavior is intentional. Anthropic does not support Cross-Origin Resource Sharing (CORS) from web browsers to protect your API credentials. If Anthropic allowed client-side browser requests, developers would have to embed their secret API keys directly into their frontend code. Anyone visiting your website could easily extract your API key from the network tab, inspect your source code, and use your credits to run up massive bills.

To resolve this issue, you must route your API requests through a secure server-side environment.

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Step-by-Step Fixes for Anthropic CORS Errors

1. Build a Simple Backend Proxy Server The most secure and standard way to solve a CORS error is to build a lightweight backend proxy server. Your frontend code communicates with your backend server, and your backend server communicates with the Anthropic API.

Here is how to set up a quick Node.js and Express proxy server:

1. Initialize a new Node.js project: `bash npm init -y npm install express cors dotenv @anthropic-ai/sdk ` 2. Create a .env file in your root folder to securely store your API key: `env ANTHROPIC_API_KEY=your_actual_api_key_here PORT=5000 ` 3. Create a server.js file and add the following code: `javascript const express = require('express'); const cors = require('cors'); const Anthropic = require('@anthropic-ai/sdk'); require('dotenv').config();

const app = express(); app.use(express.json());

// Configure CORS to only allow requests from your frontend domain const allowedOrigins = ['http://localhost:3000', 'https://yourwebsite.com']; app.use(cors({ origin: function (origin, callback) { if (!origin || allowedOrigins.indexOf(origin) !== -1) { callback(null, true); } else { callback(new Error('Blocked by CORS policy')); } } }));

const anthropic = new Anthropic({ apiKey: process.env.ANTHROPIC_API_KEY, });

app.post('/api/claude', async (req, res) => { try { const { messages, model } = req.body; const response = await anthropic.messages.create({ model: model || 'claude-3-5-sonnet-latest', max_tokens: 1024, messages: messages, }); res.json(response); } catch (error) { console.error('API Error:', error); res.status(500).json({ error: 'Failed to communicate with Claude' }); } });

const PORT = process.env.PORT || 5000; app.listen(PORT, () => console.log(Proxy running on port ${PORT})); `

2. Point Your Frontend Fetch Requests to Your Proxy Once your proxy server is running, change your frontend API call. Instead of hitting the Anthropic endpoint directly, hit your local server's endpoint.

Incorrect (Causes CORS Error): `javascript // Do not do this on the frontend fetch('https://api.anthropic.com/v1/messages', { headers: { 'x-api-key': 'sk-ant-...', // EXPOSES KEY 'content-type': 'application/json' } }); `

Correct (Secure & Resolves CORS): `javascript // Fetching from your backend proxy instead async function askClaude(prompt) { try { const response = await fetch('http://localhost:5000/api/claude', { method: 'POST', headers: { 'Content-Type': 'application/json', }, body: JSON.stringify({ messages: [{ role: 'user', content: prompt }], model: 'claude-3-5-sonnet-latest' }), }); const data = await response.json(); console.log(data); } catch (error) { console.error('Error contacting backend proxy:', error); } } `

3. Use Serverless Functions (Next.js, Vercel, or AWS Lambda) If you do not want to manage a dedicated Express server, you can use serverless functions or API routes. If you are using Next.js, create an API route to handle the server-side integration seamlessly.

Create a file at app/api/chat/route.js (Next.js App Router):

`javascript import { NextResponse } from 'next/server'; import Anthropic from '@anthropic-ai/sdk';

const anthropic = new Anthropic({ apiKey: process.env.ANTHROPIC_API_KEY, });

export async function POST(request) { try { const { messages } = await request.json(); const response = await anthropic.messages.create({ model: 'claude-3-5-sonnet-latest', max_tokens: 1024, messages: messages, }); return NextResponse.json(response); } catch (error) { return NextResponse.json({ error: error.message }, { status: 500 }); } } ` Your frontend can then fetch directly from /api/chat without running into any CORS blocks.

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When to Escalate If you have routed your calls through a secure backend server but still get network connectivity issues, check the Anthropic Status page. If the service is healthy, but you receive error codes, confirm your backend environment variables are loading correctly. If you accidentally pushed your API keys to a public GitHub repository prior to fixing the CORS error, go immediately to your Anthropic console, revoke the exposed key, and generate a new one to prevent unauthorized use.