CSI file is Challan status inquiry file which is required to validate data while preparing TDS return.

A CSI file can be download from - https://eportal.incometax.gov.in/iec/foservices/#/login or https://tin.tin.nsdl.com/oltas/servlet/TanSearch

To download CSI File Go to Quarterly Return > Select return which you are filing for like 24Q, 26Q etc.. > E-filing

      

            

Once click on Particular return, Click on Download CSI (Oltas) or Download CSI (IT)

      

To download the CSI file, click on the respective buttons: "Download CSI (Oltas)" or "Download CSI (IT)".

Hope this helps.