Validate CSV before import using a fixed JSON column specification. Checks missing/extra columns, required cells, string/integer/decimal/boolean/date types, uniqueness, allowed values and numeric/row-count bounds. Returns pass/fail with row/column error codes. POST JSON with csv and spec.columns; schemas/examples attached. Full contract: /openapi.json. Max 2,000 rows, 50 columns, 256 KiB; first 50 failures. No formulas, cross-column rules, fact checking or data repair.