Studio Integration Gets HTTP 400 With an HTML Error Page While the Same Call Works in Postman
What does "Studio Integration Gets HTTP 400 With an HTML Error Page While the Same Call Works in Postman" mean in Workday Studio?
The remote service answered with a generic HTML error page instead of the JSON or XML the integration expected. The request was rejected by something sitting in front of the API, so it never reached the application that would have returned a real error message.
Quick fix
Set an explicit User-Agent and Accept header on the HTTP Out step so they match what the working client sends, then retry. If it still fails, ask the endpoint owner to confirm your tenant's outbound addresses are allowlisted.
Permanent fix
Agree a stable contract with the endpoint owner covering which source addresses are permitted, which headers are required, and how they will notify you before tightening the rules. Then add a response content-type check in the integration so an HTML body is caught and reported as a rejected request, rather than being handed to a parser that expected JSON or XML. That check is what turns this from a confusing failure into a clear one.
Troubleshooting
- Read the body, not just the status code. An HTML document carrying a generic 'An error occurred' message is a web server or reverse proxy default page, not an API response, which tells you the request was stopped before the API ever saw it.
- Note whether the same call succeeds from a desktop client such as Postman. If it does, the request itself is valid and the difference lies in where it originates from or how it is framed.
- Compare the two requests header by header. Pay attention to Host, User-Agent, Accept, Content-Type and any custom headers, since a proxy will often reject a request that carries no User-Agent or an unexpected one.
- Check whether the failure started suddenly with no change on your side. That points at the remote end tightening its rules rather than at your configuration.
- Confirm the outbound addresses your tenant calls from are still permitted by the remote firewall or web application firewall. A desktop client tests from your own address, which may be allowed while the cloud runtime's addresses are not.
- Look at whether protocol expectations changed at the remote end, for example a newly enforced minimum TLS version or a required SNI value.