Encountering a connection error, especially when working with an API like api.anthropic.com, can be disruptive and frustrating, especially if your project depends heavily on real-time access and communication. One of the commonly reported issues is the “Failed to Connect to api.anthropic.com: ERR_BAD_REQUEST” error. This specific error indicates a problem with how the request is being sent or how it is being handled by the server, typically pointing toward misconfigurations, incorrect headers, or request formatting issues.

TL;DR

The “ERR_BAD_REQUEST” error usually stems from malformed API requests, incorrect headers, or authentication problems. To fix it, check the endpoint URL, ensure required request headers (like the API key) are correctly configured, and validate the body formatting—especially in JSON. Also, verify your network settings, any active firewall/proxy configurations, and that your API version is up-to-date. Following a step-by-step diagnostic can pinpoint and resolve the issue efficiently.

Understanding the “ERR_BAD_REQUEST” Error

This error is part of a broader family of HTTP 400-level errors, which generally signify that the request being sent to a server is invalid or “bad” in its structure, syntax, or encoding. Specifically, in this context, it appears when attempting to interact with Anthropic’s API—commonly used for AI language models like Claude—via HTTP or HTTPS.

“ERR_BAD_REQUEST” is not always self-explanatory and can come from a variety of issues. These stretch from client-side errors, such as malformed JSON or missing headers, to network misconfigurations on the user’s device or server issues from Anthropic’s side that you might not control.

Step-by-Step Guide to Fix the Error

1. Check the Endpoint URL

Make sure you are hitting the correct and fully-qualified domain name. The official endpoint for Anthropic API is typically:

https://api.anthropic.com/v1/complete

Check your base URL in client configuration or code snippets. Make sure there’s no trailing slash unless specified, and that the version number (e.g., /v1/) is correct for your intended use.

2. Ensure API Key Is Included and Correct

A missing or incorrect API key is one of the most common causes of this error. Anthropic’s API requires an API key included in the Authorization header of your HTTP request.

Example:

Authorization: Bearer YOUR_API_KEY_HERE

If your API key is expired, not well-formed, or the header is misspelled, the request might fail with a “bad request” response.

3. Validate Your Request Headers

Besides the Authorization header, you need to correctly set the Content-Type header:

Content-Type: application/json

Missing this header or using the wrong MIME type (like text/plain or application/x-www-form-urlencoded) might cause your request to break.

4. Inspect the Payload Format

Most often, users forget to serialize the payload properly as JSON, or they include incorrect values or missing keys. Here’s a correct example of sending a POST payload to the /complete endpoint:

{
  "model": "claude-v1",
  "prompt": "Explain quantum physics in simple terms.",
  "max_tokens_to_sample": 200
}

Be sure the attribute names and data types match what the API expects. Any deviation, such as using single quotes instead of double quotes, can result in a malformed request.

5. Check for Firewall or Proxy Issues

If you’re operating behind a proxy or firewall, such configurations may intercept, alter, or block HTTP traffic. In some enterprise environments, outbound HTTPS connections need whitelisting or additional authentication.

  • Try connecting without the proxy (if applicable).
  • Use tools like curl or Postman to test a direct connection.
  • Ensure port 443 is open for outbound HTTPS requests.

6. Test Using Curl or Postman

Using an API testing tool like Postman or the curl command-line utility can help eliminate issues in your app code or development environment.

curl https://api.anthropic.com/v1/complete \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"model":"claude-v1","prompt":"Hi there","max_tokens_to_sample":100}'

If these tools work but your code does not, the issue is likely within your application.

7. Check API Version and Deprecation

Anthropic may change or version its API over time. If you’re using an outdated endpoint or deprecated structure, you might hit issues. Visit the official Anthropic API documentation and confirm you’re using the latest endpoint version with supported schema.

8. Review Server Response Logs

Whenever possible, inspect the HTTP response body returned alongside the error. Often, even with a 400 error, the body includes a JSON-formatted response indicating what field or part of the request is causing issues.

Example:

{
  "error": {
    "message": "prompt is required",
    "type": "invalid_request_error"
  }
}

9. Update (or Downgrade) Your HTTP Client

Some issues arise due to bugs in the HTTP libraries being used—especially in less maintained environments or SDKs. Try updating your HTTP client library (e.g., Axios, Python requests, fetch, etc.) or testing with another one entirely.

10. Contact Anthropic Support If All Else Fails

If the problem persists despite correct configuration, authenticated access, and validated requests, the issue could be on Anthropic’s side. Reach out to Anthropic’s support team, ideally with:

  • The exact request payload and endpoint
  • The full error message and stack traces
  • Timestamps and request IDs (if available)

Frequently Asked Questions (FAQ)

Q1: What does “ERR_BAD_REQUEST” mean exactly?

It means that the request sent to the server is invalid. This could be due to wrong headers, malformed JSON, incorrect endpoints, or missing parameters.

Q2: Can the error be due to a blocked connection?

Yes. Firewalls, proxies, or security software could block the connection or modify the request structure. Always try to test with a direct Internet connection to confirm.

Q3: What status code accompanies this error?

It typically corresponds with an HTTP 400 status—the formal indicator of a bad request.

Q4: Is this an issue with the Anthropic server?

It’s possible but not common. This error usually happens client-side unless the server is experiencing issues, in which case Anthropic might publish a notice or status alert.

Q5: Can trying the API in Postman help?

Absolutely. Testing through Postman or curl helps isolate the cause outside your codebase. It’s an excellent way to verify that the API is reachable and your request structure works.

Q6: Where can I find the latest Anthropic API docs?

You can find them at Anthropic’s official documentation.

Conclusion

Diagnosing and fixing the “Failed to Connect to api.anthropic.com: ERR_BAD_REQUEST” error requires a methodical approach. It usually stems from client-side configurations, malformed requests, or authentication errors—but by carefully checking each component of your setup, you’ll likely resolve it quickly. Always consult the API documentation for backward compatibility and examples, and use testing tools to validate your inputs. If stuck, detailed communication with support can pinpoint the issue.