objectstack-data skill teaches deleteBehavior:'set_null' on master_detail — wrong before #9689, a parse-time rejection after it

[os-steve]

Filing unassigned — recording, not claiming. Found while implementing #9689 (parse-time rejection of deleteBehavior: 'set_null' authored on a master_detail; PR #11406).

The defect

The published objectstack-data skill teaches AI authors to write exactly the combination #9689 makes a named parse-time rejection:

• skills/objectstack-data/rules/relationships.md:157-159 — the master-detail deleteBehavior table offers set_null ("Set child reference to null | Manager → Employees") as a legal choice. This was already wrong before FieldSchema accepts deleteBehavior: 'set_null' on a master_detail, and the engine silently resolves it to cascade #9689: the engine has always resolved every non-restrict value on a master_detail to cascade (measured and pinned in packages/objectql/src/engine-cascade-delete.test.ts), so the row promised keep-the-children and delivered delete-the-children. The "Manager → Employees" example is doubly wrong — that relationship (children must survive the parent) wants a lookup, never a master_detail.
• skills/objectstack-data/rules/relationships.md:165 — deleteBehavior: 'cascade', // or 'restrict' or 'set_null' in the master-detail code sample.
• skills/objectstack-data/rules/relationships.md:405 — best practice Add Changesets and GitHub Actions automation #4: "deleteBehavior on master_detail — Always specify cascade/restrict/set_null".
• skills/objectstack-data/rules/field-types.md:72 — master_detail row: "deleteBehavior (cascade/restrict/set_null)".

After #9689 lands, an AI following this skill authors a field that fails publish with a 422 — the skill manufactures exactly the AI-authored metadata error the parse-time rejection exists to catch.

Fix shape (small, subtractive)

Remove set_null from the master-detail option set in all four spots; the table row's example belongs under lookup (where set_null is real and stays legal, #9625 escalation caveats included). Net negative or zero line delta — no expansion. The rejection message in FieldSchema is the source of truth for the wording ("a detail row cannot outlive its master; use a lookup if children must survive the parent").

Note: skills/** is a governed surface (Prime Directive #14): human-merge only, must be its own PR — which is why this is filed rather than folded into #9689's PR (a mixed diff would fork the whole PR into governance).

Refs: #9689 (the ruling and the rejection), #9625 (the engine pin), #9907 (precedent: a skill teaching pre-tightening behavior).

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Generated by Claude Code

[claude]

Triage (daily round, session session_01Kktexqp6uVuFMztvvTMf3V, 2026-08-24): lands in skills/objectstack-data/rules/relationships.md → domain:skills + pm:queue, Bug (the published skill teaches a combination that was wrong before #9689 and is a parse-time rejection after it). Governed face: draft PR + human merge. Whole-package value-density review applies; this is a correction, not an expansion — expect net-zero or net-negative lines.

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Generated by Claude Code