Check the CSV before the import job.
ReadyRows separates rules that can be checked from the file itself from rules that live only inside your Salesforce org.
Choose the operation first
Insert creates new records. Update needs a Salesforce record ID for the record being changed. Upsert needs a record ID or an External ID field. ReadyRows changes its validation based on the operation you select.
Data Import Wizard vs Data Loader
Salesforce documents a 50,000-record limit for Data Import Wizard. ReadyRows checks that limit when Wizard is selected and separately warns about large Data Loader files instead of applying one generic limit to both tools.
What ReadyRows can check locally
The fixer checks CSV structure, blank or duplicate headers, malformed rows, 15/18-character ID shapes, emails, Boolean-like values, likely dates, blank rows and delimiter cleanup. It exports import-ready rows, rejected rows and an issue report.
What still depends on your Salesforce org
Required fields, picklist options, custom validation rules, duplicate rules and External ID definitions are account-specific. ReadyRows flags this limitation instead of guessing values that may be wrong.