Your file never leaves this computer. Checking, fixing and splitting run in this browser tab. Nothing is uploaded, so SSNs and EINs stay with you. You can disconnect from the internet after the page loads and it still works for CSV and FIRE files.
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Combine two reports (QuickBooks contact list + 1099 totals)

QuickBooks Online has no single report with names, tax IDs, addresses and 1099 totals. Export the Vendor Contact List (with Tax ID and address columns) and the 1099 Contractor Summary, drop one above and the other here. Rows are joined by vendor name.

Match the exact header of your IRS template (recommended)

IRIS checks the header row word for word, and the IRS can change it each tax year. In the portal, open Upload CSV with Form Data, download the blank template for your form and tax year, and drop it here. The tool then writes your files under that exact header. It remembers the header row, never your data.

Free tool by IRIS CSV Tool. Files are checked in your browser and never uploaded, not even to the site showing this checker.