Claude API Error 401 authentication_error Fix

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The 401 authentication_error means your API key is invalid, missing, or expired. This is one of the most common errors when first setting up the Claude API.

The Error

{
  "type": "error",
  "error": {
    "type": "authentication_error",
    "message": "There's an issue with your API key."
  },
  "request_id": "req_011CSHoEeqs5C35K2UUqR7Fy"
}

Quick Fix

  1. Verify your ANTHROPIC_API_KEY environment variable is set and not empty.
  2. Confirm the key starts with sk-ant- and has not been revoked.
  3. Restart your terminal or IDE after setting the environment variable.

What Causes This

The Claude API requires a valid API key in the x-api-key header (or via the ANTHROPIC_API_KEY environment variable when using SDKs). Common causes:

Full Solution

Set the Environment Variable

# Linux / macOS -- add to ~/.bashrc or ~/.zshrc
export ANTHROPIC_API_KEY="sk-ant-your-key-here"

# Windows PowerShell
$env:ANTHROPIC_API_KEY = "sk-ant-your-key-here"

# Windows Command Prompt
set ANTHROPIC_API_KEY=sk-ant-your-key-here

After setting, restart your terminal session or run source ~/.bashrc.

Python SDK Authentication

The Python SDK reads ANTHROPIC_API_KEY automatically:

import anthropic

# Option 1: Auto-reads ANTHROPIC_API_KEY from environment
client = anthropic.Anthropic()

# Option 2: Pass key explicitly (not recommended for production)
client = anthropic.Anthropic(api_key="sk-ant-your-key-here")

TypeScript SDK Authentication

import Anthropic from "@anthropic-ai/sdk";

// Option 1: Auto-reads ANTHROPIC_API_KEY from environment
const client = new Anthropic();

// Option 2: Pass key explicitly
const client2 = new Anthropic({ apiKey: "sk-ant-your-key-here" });

Catch Authentication Errors

import anthropic

client = anthropic.Anthropic()

try:
    message = client.messages.create(
        model="claude-sonnet-4-6",
        max_tokens=1024,
        messages=[{"role": "user", "content": "Hello"}]
    )
except anthropic.AuthenticationError as e:
    print(f"401 Auth Error: {e.message}")
    print("Check your ANTHROPIC_API_KEY environment variable")

import Anthropic from "@anthropic-ai/sdk";

const client = new Anthropic();

try {
  await client.messages.create({
    model: "claude-sonnet-4-6",
    max_tokens: 1024,
    messages: [{ role: "user", content: "Hello" }]
  });
} catch (err) {
  if (err instanceof Anthropic.AuthenticationError) {
    console.error("401: Check your ANTHROPIC_API_KEY");
  }
}

Debug with Logging

Enable SDK debug logging to see exactly what headers are being sent:

# Python SDK debug mode
ANTHROPIC_LOG=debug python your_script.py

# TypeScript SDK debug mode
ANTHROPIC_LOG=debug node your_script.js

Platform-Specific Authentication

If you are using Bedrock or Vertex AI, you need the platform-specific client, not the default Anthropic client:

import anthropic

# Amazon Bedrock (uses AWS credentials, not ANTHROPIC_API_KEY)
client = anthropic.AnthropicBedrock()

# Google Vertex AI (uses Google Cloud credentials)
client = anthropic.AnthropicVertex(project_id="your-project", region="us-east5")

# Microsoft Foundry
client = anthropic.AnthropicFoundry()

Prevention

  1. Use environment variables: Never hardcode API keys. The SDK reads ANTHROPIC_API_KEY automatically.
  2. Add to .env files: Use python-dotenv or similar to load keys from .env files that are in .gitignore.
  3. Rotate keys safely: When generating a new key in the Console, update all deployments before revoking the old key.
  4. Test with a simple request: After setup, run a minimal “Hello” request to verify authentication before building complex logic.