How do you split a CSV and keep the header in every file?

Question

Sheet Reserve · Accepted Answer

Parse the input into CSV records, separate the header, and divide the data records into chunks. Write the header before each chunk. Avoid splitting on physical lines because a quoted cell can contain a line break. Sheet Reserve’s CSV splitter performs this in the browser and downloads a ZIP containing the smaller files.

Five data records split into groups of two produce three files containing 2, 2, and 1 data records. A repeated header is additional to each part’s data-row count.
What should you check after splitting CSV files?
Add the data-row counts across all parts and compare the sum with the original. Confirm that each header is identical and that a record with an embedded newline remains in one file.