Fix Claude API Error 401 -- Authentication Failed

TL;DR: A 401 error means your API key is missing, invalid, or expired. Verify your key, check environment variables, and ensure the correct header format.

The Problem

When calling the Claude API, you receive an HTTP 401 response:

{
  "type": "error",
  "error": {
    "type": "authentication_error",
    "message": "Invalid API key"
  }
}

Why This Happens

The Anthropic API requires a valid API key in the x-api-key header. A 401 occurs when:

• The API key is missing from the request headers
• The key has been revoked or rotated in the Anthropic Console
• Environment variables (ANTHROPIC_API_KEY) are not set or contain whitespace/newlines
• You are using the wrong header name (e.g., Authorization: Bearer instead of x-api-key)

The Fix

Step 1: Verify Your API Key

# Check if the environment variable is set
echo "Key length: ${#ANTHROPIC_API_KEY}"
echo "Key prefix: ${ANTHROPIC_API_KEY:0:10}..."
# Key should start with "sk-ant-" and be ~100+ characters

Step 2: Set the API Key Correctly

macOS / Linux (bash/zsh):

export ANTHROPIC_API_KEY="sk-ant-api03-your-key-here"
source ~/.zshrc

In code (Python SDK):

import anthropic
# SDK reads ANTHROPIC_API_KEY automatically
client = anthropic.Anthropic()

Step 3: Verify the Header Format

If making raw HTTP requests, use x-api-key (not Authorization):

# Correct
curl https://api.anthropic.com/v1/messages \
  -H "x-api-key: $ANTHROPIC_API_KEY" \
  -H "anthropic-version: 2023-06-01" \
  -H "content-type: application/json" \
  -d '{"model":"claude-sonnet-4-5-20250514","max_tokens":128,"messages":[{"role":"user","content":"Hi"}]}'

Common Variations

Prevention

• Never hardcode API keys: Use environment variables or a secrets manager.
• Rotate keys periodically: Generate new keys in the Anthropic Console and update your environment.
• Test auth separately: Before debugging complex requests, verify auth with a minimal test call.