Fix Claude API Error 401 (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  • The key has been revoked or rotated in the Anthropic Console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:

200

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.
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Try it: Paste your error into our Error Diagnostic for an instant fix.

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

Common Questions

What causes fix claude api error 401 issues?

Common causes include misconfigured settings, outdated dependencies, and environment conflicts. Check your project configuration and ensure all dependencies are up to date.

How do I prevent this error from recurring?

Set up automated checks in your development workflow. Use Claude Code’s built-in validation tools to catch configuration issues before they reach production.

Does this fix work on all operating systems?

The core fix applies to macOS, Linux, and Windows. Some path-related adjustments may be needed depending on your OS. Check the platform-specific notes in the guide above.