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 A 401 occurs when: The API key is missing from the request headers The key has been revoked or rotated in the Anthropic Console Environment variables (ANTHROPIC_API_KEY) are not set or contain whitespace/newlines You ar...

Fix Claude API Error 401 — Authentication Failed

Written by Michael Lip · Solo founder of Zovo · $400K+ on Upwork · 100% JSS Join 50+ builders · More at zovo.one

Last updated: April 14, 2026

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"
  }
}

This blocks all API requests until the authentication issue is resolved.

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The key has been revoked or rotated in the Anthropic Console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 Exists and Is Valid

# 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

If the output shows length 0 or an unexpected prefix, your key is not set correctly.

Step 2 — Set the API Key Correctly

macOS / Linux (bash/zsh):

# Add to your shell profile (~/.zshrc or ~/.bashrc)
export ANTHROPIC_API_KEY="sk-ant-api03-your-key-here"

# Reload the profile
source ~/.zshrc  # or source ~/.bashrc

Windows (PowerShell):

# Set for current session
$env:ANTHROPIC_API_KEY = "sk-ant-api03-your-key-here"

# Set permanently
[System.Environment]::SetEnvironmentVariable("ANTHROPIC_API_KEY", "sk-ant-api03-your-key-here", "User")

In code (Python SDK):

import anthropic

# Option 1: SDK reads ANTHROPIC_API_KEY automatically
client = anthropic.Anthropic()

# Option 2: Pass explicitly (not recommended for production)
client = anthropic.Anthropic(api_key="sk-ant-api03-...")

Step 3 — Verify the Header Format

If you are making raw HTTP requests, ensure you 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"}]}'

# Wrong — this will return 401
curl https://api.anthropic.com/v1/messages \
  -H "Authorization: Bearer $ANTHROPIC_API_KEY" \
  ...

Step 4 — Verify It Works

curl -s -o /dev/null -w "%{http_code}" 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":32,"messages":[{"role":"user","content":"test"}]}'

Expected output:

Common Variations

Scenario	Cause	Quick Fix
401 in CI/CD pipeline	Secret not injected	Check GitHub Actions / CI secret configuration
401 after key rotation	Old key cached	Restart application, clear env cache
401 with SDK	`ANTHROPIC_API_KEY` not exported	Add `export` before the variable in shell profile
401 with trailing newline	Copy-paste artifact	`export ANTHROPIC_API_KEY=$(echo -n "sk-ant-...")`
401 on Bedrock/Vertex	Wrong auth method	Use AWS/GCP credentials, not Anthropic API key

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.

Fix Claude API Error 500 — Internal Server Error — Server-side error troubleshooting
Fix Claude API Rate Limit Errors — HTTP 429 rate limiting solutions
Claude API Error 400 Invalid Request Fix — Browse all API error guides

Last verified: 2026-04-14. Found an issue? Open a GitHub issue.