@@ -0,0 +1,12 @@
+---
+"@objectstack/spec": minor
+---
+
+`ObjectSchema.attachedOnRead`: an object declares the blocks a service attaches to each row it serves, computed per caller on read and never stored
+
+Clause-②: yes (widening: a new optional `ObjectSchema` key)
+
+- **The key.** `attachedOnRead` is an optional map from block name to that block's leaves, each leaf naming its value type: `attachedOnRead: { viewer: { can_act: 'boolean', can_override: 'boolean', is_submitter: 'boolean' } }`. Block names and leaf keys take the field-name grammar (lowercase snake_case). A leaf's type is one of `number`, `text`, `boolean` or `date`, the four value types `Field.returnType` declares.
+- **It is not a field.** It provisions no column, and no driver, form, list view, export, write path or translation bundle reads it. Its reader is the shared build validator, `@objectstack/lint`'s expression rule over `@objectstack/formula` (what `os build` and `os validate` run): `record.<block>` resolves, and `record.<block>.<leaf>` resolves only to a leaf the block declares (see the `@objectstack/formula` and `@objectstack/lint` entries). No other field-existence check reads it.
+- **Refused at parse:** a leaf type outside the four, a leaf that is a nested block or a field definition, a block or leaf name outside the grammar, a block that names no leaf, and a block that repeats a field name the object declares. Each refusal is located at the offending key.
+- **Nothing to migrate.** The key is optional and nothing writes it by default; an object without it parses and validates exactly as before.
@@ -0,0 +1,12 @@
+---
+"@objectstack/formula": minor
+"@objectstack/lint": patch
+"@objectstack/metadata-core": patch
+---
+
+The shared build validator (`@objectstack/lint`'s expression rule over `@objectstack/formula`) judges `record.<block>.<leaf>` against an object's declared read attachments (`ObjectSchema.attachedOnRead`)
+
+- **`@objectstack/lint`.** The field index the expression rule builds now adds each block an object declares under `attachedOnRead` to the names `record.<x>` resolves to, and hands the shared validator the block's declared leaves. A predicate such as `record.viewer.can_act` on an object that declares the `viewer` block is accepted where it was refused as an unknown field.
+- **`@objectstack/formula`.** `ExprSchemaHint` gains an optional `attachedOnRead` (block name → declared leaf keys). When a `record.` or `previous.` reference names a declared block, its next segment must be a leaf the block declares. Anything else is refused with the existing `unknown-field` code: `field` is the dotted path as written (`viewer.can_actt`), `suggestion` the nearest declared leaf (`viewer.can_act`), and the new optional `block` and `leaves` params carry the leaves the block declares, which the message names. No new refusal code.
+- **`@objectstack/metadata-core`.** The ADR-0106 field-level-security masker classifies the new top-level key as passed through unchanged (`OBJECT_REFERENCE_POSITIONS.attachedOnRead`): a block name and its leaf keys never name a field, so no caller's served object definition loses or gains anything.
+- **Unchanged.** An object that declares no block keeps every verdict it had, and a second segment off any other field is not judged. Index access, a method call on a block and a third segment stay unjudged.
@@ -948,6 +948,7 @@ Metadata query with filtering, sorting, and pagination
 | **datasource** | `string` | optional (default: `"default"`) | Target Datasource ID. "default" is the primary DB. |
 | **external** | `{ remoteName?: string; remoteSchema?: string; writable?: boolean; columnMap?: Record<string, string>; … }` | optional | Remote table binding for federated (external) objects. |
 | **fields** | `Record<string, { name?: string; label?: string; type: Enum<'text' \| 'textarea' \| 'email' \| 'url' \| 'phone' \| 'password' \| 'secret' \| …>; description?: string; … }>` | ✅ | Field definitions map. Keys must be snake_case identifiers; "__proto__", "constructor" and "prototype" are refused. |
+| **attachedOnRead** | `Record<string, Record<string, Enum<'number' \| 'text' \| 'boolean' \| 'date'>>>` | optional | Blocks a service attaches to each row it serves, computed per caller on read and never stored: block name → `{ leaf key → value type (number \| text \| boolean \| date) }`. NOT a field — no column, form, list view, export, write path or translation bundle reads it, and a block name may not repeat a declared field name. Its reader is the shared build validator (`@objectstack/lint` over `@objectstack/formula`, as `os build` / `os validate` run it): `record.<block>` resolves, and `record.<block>.<leaf>` resolves only to a leaf the block declares. |
 | **indexes** | `{ name?: string; fields: string[]; unique?: false \| 'global' \| 'organization' }[]` | optional | Database performance indexes |
 | **fieldGroups** | `{ key: string; label: string; icon?: string; description?: string; … }[]` | optional | Ordered list of field groups (array order = display order). See ObjectFieldGroupSchema. |
 | **tenancy** | `{ enabled: boolean; tenantField?: string }` | optional | Multi-tenancy configuration for SaaS applications |
 
@@ -152,6 +152,7 @@ const result = ApiMethod.parse(data);
 | **datasource** | `string` | optional (default: `"default"`) | Target Datasource ID. "default" is the primary DB. |
 | **external** | `{ remoteName?: string; remoteSchema?: string; writable?: boolean; columnMap?: Record<string, string>; … }` | optional | Remote table binding for federated (external) objects. |
 | **fields** | `Record<string, { name?: string; label?: string; type: Enum<'text' \| 'textarea' \| 'email' \| 'url' \| 'phone' \| 'password' \| 'secret' \| …>; description?: string; … }>` | ✅ | Field definitions map. Keys must be snake_case identifiers; "__proto__", "constructor" and "prototype" are refused. |
+| **attachedOnRead** | `Record<string, Record<string, Enum<'number' \| 'text' \| 'boolean' \| 'date'>>>` | optional | Blocks a service attaches to each row it serves, computed per caller on read and never stored: block name → `{ leaf key → value type (number \| text \| boolean \| date) }`. NOT a field — no column, form, list view, export, write path or translation bundle reads it, and a block name may not repeat a declared field name. Its reader is the shared build validator (`@objectstack/lint` over `@objectstack/formula`, as `os build` / `os validate` run it): `record.<block>` resolves, and `record.<block>.<leaf>` resolves only to a leaf the block declares. |
 | **indexes** | `{ name?: string; fields: string[]; unique?: false \| 'global' \| 'organization' }[]` | optional | Database performance indexes |
 | **fieldGroups** | `{ key: string; label: string; icon?: string; description?: string; … }[]` | optional | Ordered list of field groups (array order = display order). See ObjectFieldGroupSchema. |
 | **tenancy** | `{ enabled: boolean; tenantField?: string }` | optional | Multi-tenancy configuration for SaaS applications |
 
@@ -329,6 +329,7 @@ Create a new object
 | **datasource** | `string` | optional (default: `"default"`) | Target Datasource ID. "default" is the primary DB. |
 | **external** | `{ remoteName?: string; remoteSchema?: string; writable?: boolean; columnMap?: Record<string, string>; … }` | optional | Remote table binding for federated (external) objects. |
 | **fields** | `Record<string, { name?: string; label?: string; type: Enum<'text' \| 'textarea' \| 'email' \| 'url' \| 'phone' \| 'password' \| 'secret' \| …>; description?: string; … }>` | ✅ | Field definitions map. Keys must be snake_case identifiers; "__proto__", "constructor" and "prototype" are refused. |
+| **attachedOnRead** | `Record<string, Record<string, Enum<'number' \| 'text' \| 'boolean' \| 'date'>>>` | optional | Blocks a service attaches to each row it serves, computed per caller on read and never stored: block name → `{ leaf key → value type (number \| text \| boolean \| date) }`. NOT a field — no column, form, list view, export, write path or translation bundle reads it, and a block name may not repeat a declared field name. Its reader is the shared build validator (`@objectstack/lint` over `@objectstack/formula`, as `os build` / `os validate` run it): `record.<block>` resolves, and `record.<block>.<leaf>` resolves only to a leaf the block declares. |
 | **indexes** | `{ name?: string; fields: string[]; unique?: false \| 'global' \| 'organization' }[]` | optional | Database performance indexes |
 | **fieldGroups** | `{ key: string; label: string; icon?: string; description?: string; … }[]` | optional | Ordered list of field groups (array order = display order). See ObjectFieldGroupSchema. |
 | **tenancy** | `{ enabled: boolean; tenantField?: string }` | optional | Multi-tenancy configuration for SaaS applications |
@@ -616,6 +617,7 @@ Create a new object
 | **datasource** | `string` | optional (default: `"default"`) | Target Datasource ID. "default" is the primary DB. |
 | **external** | `{ remoteName?: string; remoteSchema?: string; writable?: boolean; columnMap?: Record<string, string>; … }` | optional | Remote table binding for federated (external) objects. |
 | **fields** | `Record<string, { name?: string; label?: string; type: Enum<'text' \| 'textarea' \| 'email' \| 'url' \| 'phone' \| 'password' \| 'secret' \| …>; description?: string; … }>` | ✅ | Field definitions map. Keys must be snake_case identifiers; "__proto__", "constructor" and "prototype" are refused. |
+| **attachedOnRead** | `Record<string, Record<string, Enum<'number' \| 'text' \| 'boolean' \| 'date'>>>` | optional | Blocks a service attaches to each row it serves, computed per caller on read and never stored: block name → `{ leaf key → value type (number \| text \| boolean \| date) }`. NOT a field — no column, form, list view, export, write path or translation bundle reads it, and a block name may not repeat a declared field name. Its reader is the shared build validator (`@objectstack/lint` over `@objectstack/formula`, as `os build` / `os validate` run it): `record.<block>` resolves, and `record.<block>.<leaf>` resolves only to a leaf the block declares. |
 | **indexes** | `{ name?: string; fields: string[]; unique?: false \| 'global' \| 'organization' }[]` | optional | Database performance indexes |
 | **fieldGroups** | `{ key: string; label: string; icon?: string; description?: string; … }[]` | optional | Ordered list of field groups (array order = display order). See ObjectFieldGroupSchema. |
 | **tenancy** | `{ enabled: boolean; tenantField?: string }` | optional | Multi-tenancy configuration for SaaS applications |
 
@@ -235,6 +235,26 @@ const PINS: { readonly [C in ExpressionRefusalCode]: readonly [Pin<C>, ...Pin<C>
       params: { field: 'zzz' },
       message: 'unknown field `zzz`',
     },
+    {
+      // A declared read attachment's second segment (#22386): the same code,
+      // the dotted path, and the optional clause naming the declared leaves.
+      produce: () =>
+        onlyError('predicate', 'record.viewer.can_actt', {
+          fields: ['status', 'viewer'],
+          objectName: 'approval',
+          attachedOnRead: { viewer: ['can_act', 'is_submitter'] },
+        }),
+      params: {
+        field: 'viewer.can_actt',
+        objectName: 'approval',
+        suggestion: 'viewer.can_act',
+        block: 'viewer',
+        leaves: ['can_act', 'is_submitter'],
+      },
+      message:
+        'unknown field `viewer.can_actt` on `approval` (the read attachment `viewer` declares `can_act`, `is_submitter`) ' +
+        '— did you mean `viewer.can_act`?',
+    },
   ],
   'unknown-role': [
     {
 
@@ -96,8 +96,21 @@ export interface ExpressionRefusalParams {
   };
   /** The CEL engine refused a source that holds a template brace `{ref}`. */
   'cel-template-brace': { readonly role: CelFieldRole; readonly detail: string; readonly ref: string };
-  /** `record.<field>` naming no field of the object. */
-  'unknown-field': { readonly field: string; readonly objectName?: string; readonly suggestion?: string };
+  /**
+   * `record.<field>` naming no field of the object — or, for a declared read
+   * attachment (`ExprSchemaHint.attachedOnRead`), `record.<block>.<leaf>`
+   * naming no leaf of the block: then `field` is the dotted path as written
+   * and `suggestion` the nearest declared leaf, also dotted. `block` and
+   * `leaves` are present together, exactly when the message carries the clause
+   * naming the leaves the block declares.
+   */
+  'unknown-field': {
+    readonly field: string;
+    readonly objectName?: string;
+    readonly suggestion?: string;
+    readonly block?: string;
+    readonly leaves?: readonly string[];
+  };
   /** A role-membership test naming a role outside the catalog; `catalog` is every valid role. */
   'unknown-role': { readonly name: string; readonly suggestion?: string; readonly catalog: readonly string[] };
   /** `name.…` one typo away from a root the surface binds; `roots` are the surface's declared roots. */
 
@@ -0,0 +1,132 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+/**
+ * `ExprSchemaHint.attachedOnRead` — the second segment of
+ * `record.<block>.<leaf>` judged against a declared read attachment (#22211
+ * ruling A, #22386).
+ *
+ * A read attachment is a block a service sets on each row it serves, computed
+ * per caller and never stored — plugin-approvals' `viewer: { can_act,
+ * can_override, is_submitter }` on `sys_approval_request` is the worked case.
+ * The block name resolves like a field (the caller lists it in `fields`); the
+ * hint only adds the judgement of the segment after it, under the EXISTING
+ * `unknown-field` code — no new code, and nothing keyed on the name `viewer`.
+ *
+ * Each verdict is asserted on its `code` and `params` (the named subject and
+ * the remedy), the machine-readable half of the refusal.
+ */
+
+import { describe, it, expect } from 'vitest';
+
+import { validateExpression, type ExprSchemaHint } from './validate';
+
+const LEAVES = ['can_act', 'can_override', 'is_submitter'] as const;
+
+/** An object that declares the `viewer` read attachment, as the lint field index hands it over. */
+const DECLARED: ExprSchemaHint = {
+  objectName: 'sys_approval_request',
+  fields: ['status', 'submitter_id', 'viewer'],
+  attachedOnRead: { viewer: LEAVES },
+  scope: 'record',
+};
+
+/** The same object WITHOUT the key — today's hint, byte for byte. */
+const UNDECLARED: ExprSchemaHint = {
+  objectName: 'sys_approval_request',
+  fields: ['status', 'submitter_id'],
+  scope: 'record',
+};
+
+function refusalsOf(source: string, schema: ExprSchemaHint) {
+  return validateExpression('predicate', source, schema).errors.map((e) => ({ code: e.code, params: e.params }));
+}
+
+describe('ExprSchemaHint.attachedOnRead — record.<block>.<leaf>', () => {
+  it('accepts a leaf the block declares', () => {
+    for (const leaf of LEAVES) {
+      const r = validateExpression('predicate', `record.viewer.${leaf} == true`, DECLARED);
+      expect(r.errors, leaf).toEqual([]);
+      expect(r.ok).toBe(true);
+    }
+    // The shipped shape: an OR of two leaves and a status read, one predicate.
+    expect(
+      validateExpression('predicate', "record.status == 'pending' && (record.viewer.can_act || record.viewer.can_override)", DECLARED).errors,
+    ).toEqual([]);
+  });
+
+  it('refuses a misspelt leaf under the existing `unknown-field` code, naming the declared leaves', () => {
+    expect(refusalsOf('record.viewer.can_actt == true', DECLARED)).toEqual([
+      {
+        code: 'unknown-field',
+        params: {
+          field: 'viewer.can_actt',
+          objectName: 'sys_approval_request',
+          suggestion: 'viewer.can_act',
+          block: 'viewer',
+          leaves: ['can_act', 'can_override', 'is_submitter'],
+        },
+      },
+    ]);
+  });
+
+  it('refuses a leaf with no near declared spelling, still naming every declared leaf', () => {
+    expect(refusalsOf('record.viewer.approved', DECLARED)).toEqual([
+      {
+        code: 'unknown-field',
+        params: {
+          field: 'viewer.approved',
+          objectName: 'sys_approval_request',
+          block: 'viewer',
+          leaves: ['can_act', 'can_override', 'is_submitter'],
+        },
+      },
+    ]);
+  });
+
+  it('judges the `previous` root the same way, and reports one leaf once', () => {
+    expect(refusalsOf('previous.viewer.can_actt || record.viewer.can_actt', DECLARED)).toEqual([
+      {
+        code: 'unknown-field',
+        params: {
+          field: 'viewer.can_actt',
+          objectName: 'sys_approval_request',
+          suggestion: 'viewer.can_act',
+          block: 'viewer',
+          leaves: ['can_act', 'can_override', 'is_submitter'],
+        },
+      },
+    ]);
+  });
+
+  it('refuses an undeclared block exactly as today — the FIRST segment, not the second', () => {
+    expect(refusalsOf('record.viewr.can_act == true', DECLARED)).toEqual([
+      { code: 'unknown-field', params: { field: 'viewr', objectName: 'sys_approval_request', suggestion: 'viewer' } },
+    ]);
+  });
+
+  it('an object without the key keeps today’s verdicts (control)', () => {
+    // The block is unknown as a FIELD, exactly as before the key existed …
+    expect(refusalsOf('record.viewer.can_act == true', UNDECLARED)).toEqual([
+      { code: 'unknown-field', params: { field: 'viewer', objectName: 'sys_approval_request' } },
+    ]);
+    // … and a second segment off a known field is not judged at all.
+    expect(refusalsOf('record.submitter_id.name == "x"', UNDECLARED)).toEqual([]);
+    // Same expression, same verdict, with or without an EMPTY declaration.
+    expect(refusalsOf('record.submitter_id.name == "x"', { ...UNDECLARED, attachedOnRead: {} })).toEqual([]);
+  });
+
+  it('judges only a declared block: a second segment off any other field stays unjudged', () => {
+    expect(refusalsOf('record.submitter_id.nmae == "x"', DECLARED)).toEqual([]);
+  });
+
+  it('leaves a method call, an index read and a third segment unjudged — missed catches, never false refusals', () => {
+    expect(refusalsOf("record.viewer['can_actt'] == true", DECLARED)).toEqual([]);
+    expect(refusalsOf("record.viewer.split(',') == []", DECLARED)).toEqual([]);
+    expect(refusalsOf('record.viewer.can_act.foo == true', DECLARED)).toEqual([]);
+  });
+
+  it('reads the declaration by OWN key only — an inherited name is never a block', () => {
+    const hint: ExprSchemaHint = { ...DECLARED, fields: [...(DECLARED.fields ?? []), 'constructor'] };
+    expect(refusalsOf('record.constructor.name == "x"', hint)).toEqual([]);
+  });
+});