diff --git a/scripts/check-skills-token-ratchet.mjs b/scripts/check-skills-token-ratchet.mjs
index 2811c044162..d8148a2e965 100644
--- a/scripts/check-skills-token-ratchet.mjs
+++ b/scripts/check-skills-token-ratchet.mjs
@@ -552,7 +552,16 @@ export const CEILINGS = new Map([
   // 2026-10-24 (project decision list item 9), verbatim and untranslated, the
   // line for item 8 of the same message elided:
   // 「9. 技能文档 security.md 的 token 上限 2543 → 2563 … 同意」.
-  ['skills/objectstack-data/rules/security.md', 2563],
+  // 2563 -> 2956 (#26179): the field permission row condition `when` — the
+  // choice rule (a lock for everyone is a validation; a lock that depends on
+  // who edits is `when` on that set's field entries), one example and the
+  // four boundaries — which the file did not teach, so an author locked the
+  // fields for every user. +395 tokens on a landed 2561, 2 absorbed by
+  // headroom, ceiling +393. The file carries no duplicated passage to cut
+  // first (measured: no sentence repeats in the file). Maintainer ruling
+  // 2026-10-27 on the decision card 「skills 修复卡的教学和 eval 都撞 token 天
+  // 花板,怎么付?」, tap A, verbatim and untranslated: 「A:先削重复,余额按实测提高」.
+  ['skills/objectstack-data/rules/security.md', 2956],
   // 3024 -> 3109 (2026-08-31 app-repo-principles raise, see the block above).
   // Severity Levels listed the three values and left the CHOICE unstated: a
   // block rests on a judgement a person made, so a machine-inferred signal — a
@@ -651,7 +660,15 @@ export const CEILINGS = new Map([
   ['skills/objectstack-ai/evals/skills-tools-knowledge.json', 1074],
   ['skills/objectstack-api/evals/endpoints-auth-routes.json', 1042],
   ['skills/objectstack-automation/evals/flows-triggers-approvals.json', 1255],
-  ['skills/objectstack-data/evals/hooks-security-seeds-datasources.json', 894],
+  // 894 -> 1095 (#26179): one regression case for the field permission `when`
+  // (the contract prompt: lock the legal terms for finance from approval on,
+  // legal keeps the edit) — expected: `when` on the finance set's entries with
+  // `readable: true`, and NO validation rule or `readonlyWhen` locking the
+  // fields for every user. +201 tokens, zero headroom to absorb any of it,
+  // ceiling +201; nothing duplicated in the file to cut first. Same ruling as
+  // the `rules/security.md` row above (2026-10-27, tap A:
+  // 「A:先削重复,余额按实测提高」).
+  ['skills/objectstack-data/evals/hooks-security-seeds-datasources.json', 1095],
   ['skills/objectstack-data/evals/objects-fields-relationships.json', 946],
   ['skills/objectstack-formula/evals/cel-predicates-formulas.json', 1046],
   ['skills/objectstack-i18n/evals/bundles-locales-coverage.json', 1004],
diff --git a/skills/objectstack-data/SKILL.md b/skills/objectstack-data/SKILL.md
index 468a5b3fd22..917b77131f0 100644
--- a/skills/objectstack-data/SKILL.md
+++ b/skills/objectstack-data/SKILL.md
@@ -218,7 +218,8 @@ export const Invoice = ObjectSchema.create({
 
 - Use `visibleWhen` to hide irrelevant fields in ObjectUI forms.
 - Use `readonlyWhen` for state-locked fields; the ObjectQL write path ignores
-  incoming changes when the predicate is `TRUE`.
+  incoming changes when the predicate is `TRUE` (for everyone; per permission
+  set → field permission `when`, rules/security.md).
 - **`readonly: true` governs the end-user surface, not trusted system writers.**
   A non-system write (REST/UI, and any `runAs:'user'` flow — the default) has
   the field **stripped** from any non-system write; the write reports
diff --git a/skills/objectstack-data/evals/hooks-security-seeds-datasources.json b/skills/objectstack-data/evals/hooks-security-seeds-datasources.json
index 6f4648942de..42a526f0eae 100644
--- a/skills/objectstack-data/evals/hooks-security-seeds-datasources.json
+++ b/skills/objectstack-data/evals/hooks-security-seeds-datasources.json
@@ -40,6 +40,16 @@
         "must_contain": ["defineSeed", "externalId", "upsert", "env:", "daysFromNow"],
         "must_not_contain": ["new Date(", "defineDataset", "externalId: 'id'"]
       }
+    },
+    {
+      "id": 5,
+      "prompt": "Finance may edit a contract's legal terms (`governing_law`, `liability_cap`) while it is a draft, but not once it is approved, signing or active. Legal still edits them.",
+      "expected_output": "Field permission entries with a row condition on the finance set: `'clm_contract.governing_law': { readable: true, editable: false, when: \"status in ['approved', 'signing', 'active']\" }` and the same for `liability_cap`; the legal set keeps its edit grant. No validation rule and no `readonlyWhen`, which would lock the fields for every user.",
+      "files": [],
+      "assertions": {
+        "must_contain": ["when:", "editable: false", "readable: true", "clm_contract.governing_law"],
+        "must_not_contain": ["validations:", "readonlyWhen"]
+      }
     }
   ]
 }
diff --git a/skills/objectstack-data/rules/security.md b/skills/objectstack-data/rules/security.md
index a6fed439cf4..1e4ca33b44c 100644
--- a/skills/objectstack-data/rules/security.md
+++ b/skills/objectstack-data/rules/security.md
@@ -116,6 +116,35 @@ bridge (warns). The compiler resolves these
   `owner_only_deletes`) and the object's `sharingModel` (ADR-0056 D1) over
   hand-written policies for the common cases.
 
+## Field permissions — a row-conditioned edit lock (`when`)
+
+A set's `fields` entry (`'OBJECT.FIELD': { readable, editable }`) may carry
+`when`, a row condition in the same canonical CEL as `rowLevelSecurity[].using`.
+**Choose by who loses the edit — one mechanism per need, never both:**
+
+- Locked for **everyone** at that stage → a [validation rule](./validation.md)
+  on the object (`readonlyWhen` likewise strips the write for everyone).
+- Locked for **some permission sets** only → `when` on the field entries of
+  each set that loses the edit; the other sets keep their grant.
+
+```typescript
+// finance may not change the contract's legal terms after approval
+fields: {
+  'clm_contract.governing_law': { readable: true, editable: false, when: "status in ['approved', 'signing', 'active']" },
+}
+```
+
+- **Edit vote only.** Reads are row-blind; `readable: false` beside `when` is
+  refused at parse time.
+- **Same object.** Fields of the entry's object vs a literal or `current_user.*`;
+  no relation reach (refused at `os validate` / publish).
+- **Pre-image.** An update is judged on the stored row, so a write cannot unlock
+  itself by moving `status` back; an insert on the row as sent.
+- **Locks, never grants.** On unmatched rows the entry is absent; another held
+  set's `editable: true` still wins (most-permissive merge).
+- Docs: https://objectstack.ai/docs/permissions/field-level-security#row-conditioned-edit-locks-when
+  and the recipe https://objectstack.ai/docs/permissions/access-recipes#data-lock-fields-for-a-position-once-a-record-reaches-a-stage
+
 ## Sensitive fields — `secret` type + `requiredPermissions`
 
 `maskingRule` is **live** (FieldMasker). Two more channels: