Table Validation

Verify the column contract of a table that was just created or written: columns present, declared types correct, nullability correct. This skill is deliberately narrow.

Target shape (important)

ValidationHook.on_end invokes this skill with the whole session, not a single table. The target you receive is a SessionTarget whose .targets is a list of table records matching this skill's targets: [{type: table}] filter. When a node writes multiple tables (CTAS scaffolding, layered ETL), loop over session.targets and run the checks below independently for each TableTarget. Transfer targets are covered by transfer-reconciliation. Emit one CheckResult per (target, check) pair so the retry prompt can tell the agent which specific table failed.

Explicitly out of scope:

• Object exists and row count > 0 — already checked by the builtin validation layer before this skill runs. The hook supplies you with those results in the precheck context; do not re-run describe_table just to confirm the table exists.
• Data-content assertions (null ratios, value ranges, accepted values, regex format, duplicates, uniqueness). CTAS from an empty source, idempotent upserts, schema-only bootstrapping, and partition scaffolding are legitimate patterns that produce zero-row tables; blocking on those would cause false positives. If you need data-content rules for a specific table, author a project-level validator skill under ./.datus/skills/ or ~/.datus/skills/ with a targets: filter.

Checks in scope

1. Column set — every expected column name appears in the actual table, and (when strict match is requested) no unexpected columns appear.
2. Types — each expected column's declared type matches.
3. Nullability — each expected column's nullability matches.

Execution checklist

Run the column-contract checks in this order. Stop on the first blocking failure for a given table, then continue with the next target table if the session contains multiple table targets.

1. Expected columns present — when the caller supplied an expected column set, every expected column must appear in describe_table output.
2. No unexpected columns — when the caller requires exact matching, flag any actual column that is not in the contract.
3. Types match — compare each expected column's declared type with the contract. Treat widening as acceptable only when the contract explicitly allows it.
4. Nullability matches — compare each expected column's nullable / NOT NULL setting with the contract.

For every executed check, report the check name, observed value, expected value or threshold, pass/fail decision, and a short failure reason.

When there is no explicit column contract

If the caller did not supply an expected column set / type map, there is nothing for this skill to check — emit the JSON block with "checks": [] and return without calling tools. The builtin layer has already confirmed existence and row count; duplicating that check here only produces false negatives when catalog/database/schema identifiers are ambiguous.

Tools

Use describe_table to introspect the target. Do not run execute_sql for counting rows or sampling data — out of scope.

Project-level validation examples

The following checks are intentionally not bundled here. Add them in a project-level validator skill under ./.datus/skills/<name>/ or ~/.datus/skills/<name>/ with kind: validator and a targets: filter when the table actually needs them:

• null ratios per column
• numeric ranges / min-max checks
• accepted value sets / enum membership
• regex / format validation
• uniqueness / duplicate-key detection
• cross-column assertions

Output

Emit the standard validator JSON block (see the output contract appended by the hook). Use severity: "blocking" only for column contract violations that would break downstream consumers. Mismatches that are cosmetic or widening-safe should be severity: "advisory".