@@ -0,0 +1,33 @@
+---
+"@objectstack/plugin-security": minor
+"@objectstack/rest": minor
+---
+
+feat: `security.getReadableFields` query surface for export column projection (#3547, #3391 follow-up)
+
+The REST export route projected its columns by inferring readability from the
+first chunk of already-masked data rows (#3498). That has two known
+compromises: a readable column whose first-chunk values are all null (and thus
+omitted by the driver) drops out of the header, and an empty result set leaves
+nothing to narrow. This adds the long-term-correct path.
+
+- **plugin-security** — the `security` service gains
+  `getReadableFields(object, context)`. It resolves the caller's permission
+  sets and builds the field-permission map with the SAME evaluator +
+  `requiredPermissions` fold the read middleware's `FieldMasker` uses (and the
+  same on-behalf-of delegator intersection, fail-closed on a dangling
+  delegator), then returns every schema field NOT masked non-readable — the
+  exact complement of what the mask deletes, so it can never drift from
+  data-plane FLS. Computed from schema + context, never from data rows: immune
+  to null values and empty result sets. A system context bypasses FLS; an
+  unresolvable schema returns `undefined` so callers fall back.
+- **rest** — the `GET /data/:object/export` route asks the environment's
+  `security` service for `getReadableFields(object, context)` and projects the
+  schema-derived header to that set BEFORE streaming. When no security service
+  is reachable (no plugin-security / single-kernel without a provider) it
+  degrades to the existing masked-row inference, so there is zero regression.
+  Explicit `?fields=` requests are still honored verbatim.
+
+Contract-neutral: export columns already equal list's readable columns
+(`export ⊆ list`, #3391); this makes the projection authoritative instead of
+inferred.
@@ -0,0 +1,116 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+/**
+ * getReadableFields query surface (#3547) — the authoritative column
+ * projection for a read-derived export (`export ⊆ list`, #3391). It must
+ * return exactly the fields the read middleware's FieldMasker would NOT delete
+ * for the caller, computed from the object SCHEMA + context (never from data
+ * rows) so it is immune to null values / empty result sets.
+ */
+
+import { describe, it, expect, vi } from 'vitest';
+import { SecurityPlugin } from './security-plugin.js';
+import type { PermissionSet } from '@objectstack/spec/security';
+
+/**
+ * Minimal plugin harness: a metadata `list()` that surfaces the permission
+ * sets, `get()`/`ql.getSchema()` that surface the object schema. Mirrors the
+ * shape used by security-plugin.test.ts so the plugin boots identically.
+ */
+function bootPlugin(permissionSets: PermissionSet[], objectFields: string[], fallback = 'restricted') {
+  const fields: Record<string, any> = {};
+  for (const f of objectFields) fields[f] = { name: f };
+  const schema: any = { name: 'deal', label: 'Deal', systemFields: false, fields };
+  const ql: any = {
+    registerMiddleware: () => {},
+    getSchema: (name: string) => (name === 'deal' ? schema : null),
+    findOne: async () => null,
+  };
+  const metadata: any = {
+    get: async (_type: string, name: string) => (name === 'deal' ? schema : null),
+    list: async () => permissionSets,
+  };
+  const services: Record<string, any> = { manifest: { register: vi.fn() }, objectql: ql, metadata };
+  const ctx: any = {
+    logger: { info: vi.fn(), warn: vi.fn(), error: vi.fn() },
+    registerService: vi.fn(),
+    getService: (name: string) => {
+      if (!(name in services)) throw new Error(`service not registered: ${name}`);
+      return services[name];
+    },
+  };
+  const plugin = new SecurityPlugin({ fallbackPermissionSet: fallback });
+  return { plugin, ctx };
+}
+
+// A permission set that reads `deal` but masks the `secret` field.
+const RESTRICTED: PermissionSet = {
+  name: 'restricted',
+  label: 'Restricted',
+  objects: { deal: { allowRead: true } },
+  fields: { 'deal.secret': { readable: false, editable: false } },
+} as any;
+
+// A permission set with no field-level rules → every field readable.
+const OPEN: PermissionSet = {
+  name: 'open',
+  label: 'Open',
+  objects: { deal: { allowRead: true } },
+} as any;
+
+describe('SecurityPlugin.getReadableFields (#3547)', () => {
+  it('returns all schema fields MINUS those masked non-readable — the FieldMasker complement', async () => {
+    const { plugin, ctx } = bootPlugin([RESTRICTED], ['id', 'name', 'secret']);
+    await plugin.init(ctx);
+    await plugin.start(ctx);
+
+    const readable = await plugin.getReadableFields('deal', { userId: 'u1', permissions: ['restricted'] });
+    expect(readable).toBeDefined();
+    expect(readable).toContain('id');
+    expect(readable).toContain('name');
+    // `secret` is masked (readable:false) → excluded, exactly as maskResults deletes it.
+    expect(readable).not.toContain('secret');
+  });
+
+  it('a field with no permission entry passes through (allow-list only enumerates named fields)', async () => {
+    // `open` names no fields → nothing is masked → all schema fields readable.
+    const { plugin, ctx } = bootPlugin([OPEN], ['id', 'name', 'secret'], 'open');
+    await plugin.init(ctx);
+    await plugin.start(ctx);
+
+    const readable = await plugin.getReadableFields('deal', { userId: 'u1', permissions: ['open'] });
+    expect(new Set(readable)).toEqual(new Set(['id', 'name', 'secret']));
+  });
+
+  it('a system context bypasses FLS → the full field set (mirrors the middleware isSystem skip)', async () => {
+    const { plugin, ctx } = bootPlugin([RESTRICTED], ['id', 'name', 'secret']);
+    await plugin.init(ctx);
+    await plugin.start(ctx);
+
+    const readable = await plugin.getReadableFields('deal', { isSystem: true });
+    expect(new Set(readable)).toEqual(new Set(['id', 'name', 'secret']));
+  });
+
+  it('returns undefined when the object schema cannot be resolved (caller falls back)', async () => {
+    const { plugin, ctx } = bootPlugin([RESTRICTED], ['id', 'name', 'secret']);
+    await plugin.init(ctx);
+    await plugin.start(ctx);
+
+    const readable = await plugin.getReadableFields('unknown_object', { userId: 'u1', permissions: ['restricted'] });
+    expect(readable).toBeUndefined();
+  });
+
+  it('is computed from the schema, not from data rows — immune to null/empty result sets', async () => {
+    // No data rows exist in this harness at all (ql.findOne → null); the
+    // readable set is still complete, proving it is derived from schema+context.
+    const { plugin, ctx } = bootPlugin([RESTRICTED], ['id', 'name', 'secret', 'amount']);
+    await plugin.init(ctx);
+    await plugin.start(ctx);
+
+    const readable = await plugin.getReadableFields('deal', { userId: 'u1', permissions: ['restricted'] });
+    // `amount` is readable and would be entirely absent from an empty result —
+    // it survives here because the projection never consults rows.
+    expect(readable).toContain('amount');
+    expect(readable).not.toContain('secret');
+  });
+});
@@ -546,6 +546,12 @@ export class SecurityPlugin implements Plugin {
       };
       ctx.registerService('security', {
         getReadFilter: (object: string, context?: any) => this.getReadFilter(object, context),
+        // [#3547] Readable-field projection for a context — the authoritative
+        // column set for a read-derived export (`export ⊆ list`, #3391).
+        // Same field mask as the read middleware (no drift). The REST export
+        // route uses it to project columns instead of inferring readability
+        // from already-masked data rows. See getReadableFields.
+        getReadableFields: (object: string, context?: any) => this.getReadableFields(object, context),
         // [ADR-0046 §6.7] Effective permission-set NAMES for a caller — the
         // primitive the REST read layer needs to evaluate a permission-set-
         // gated book/doc audience ({ permissionSet: '…' }). Same resolution
@@ -571,7 +577,7 @@ export class SecurityPlugin implements Plugin {
         dismissAudienceBindingSuggestion: (callerContext: any, id: string) =>
           dismissAudienceBindingSuggestion(suggestionDeps, callerContext, id),
       });
-      ctx.logger.info('[security] registered "security" service (getReadFilter, explain, audience-binding suggestions) — ADR-0021 D-C / ADR-0090 D5/D6/D9');
+      ctx.logger.info('[security] registered "security" service (getReadFilter, getReadableFields, explain, audience-binding suggestions) — ADR-0021 D-C / ADR-0090 D5/D6/D9 / #3547');
     } catch (e) {
       ctx.logger.warn?.('[security] failed to register "security" service', {
         error: (e as Error).message,
@@ -2044,6 +2050,71 @@ export class SecurityPlugin implements Plugin {
     }
   }
 
+  /**
+   * [#3547] Query surface: the field names the caller MAY READ on `object`
+   * under `context`. This is the authoritative column projection for a
+   * read-derived export (`export ⊆ list`, #3391) — the REST export route
+   * consumes it to project columns directly, instead of inferring readability
+   * from already-masked data rows (which loses an all-readable-but-all-null
+   * column and falls back to the full schema on an empty result set, #3498).
+   *
+   * Same-source-as-read-middleware, so it can never drift from data-plane FLS:
+   * it resolves the caller's permission sets via
+   * {@link resolvePermissionSetsForContext}, builds the field-permission map
+   * with the SAME evaluator + `requiredPermissions` fold the read mask uses,
+   * intersects the on-behalf-of delegator's mask (ADR-0090 D10, fail-closed on
+   * a dangling delegator), then returns every schema field NOT marked
+   * non-readable — the exact complement of what {@link FieldMasker.maskResults}
+   * deletes (fields without an explicit permission entry pass through).
+   *
+   * Returns `undefined` when the object schema can't be resolved, so the caller
+   * falls back to its own projection. A system context (`context.isSystem`)
+   * bypasses FLS and returns the full field set, mirroring the middleware's
+   * `isSystem` skip.
+   */
+  async getReadableFields(object: string, context?: any): Promise<string[] | undefined> {
+    const objectName = String(object ?? '');
+    if (!objectName) return undefined;
+    // The field universe — the SAME source the RLS field pass uses (ObjectQL's
+    // live SchemaRegistry first, metadata artifact fallback). `null` → schema
+    // not resolvable → let the caller fall back rather than guess.
+    const fieldNameSet = await this.getObjectFieldNames(this.metadata, objectName, this.ql);
+    if (!fieldNameSet) return undefined;
+    const allFields = [...fieldNameSet];
+    // System operations bypass FLS (mirrors the middleware's isSystem skip).
+    if (context?.isSystem) return allFields;
+
+    const permissionSets = await this.resolvePermissionSetsForContext(context);
+    // No sets resolved (e.g. unauthenticated) → no field mask applies, exactly
+    // as the middleware (getFieldPermissions([]) === {} → nothing deleted).
+    if (permissionSets.length === 0) return allFields;
+
+    const secMeta = await this.getObjectSecurityMeta(objectName);
+    let fieldPerms = this.permissionEvaluator.getFieldPermissions(objectName, permissionSets);
+    fieldPerms = this.foldFieldRequiredPermissions(fieldPerms, secMeta.fieldRequiredPermissions, permissionSets);
+
+    // [ADR-0090 D10] On an on-behalf-of read the readable set must NOT widen
+    // past what the DELEGATOR can read — intersect the delegator's field mask
+    // too. A dangling delegator fails CLOSED (expose no columns), the same
+    // fail-closed stance the CRUD middleware takes on a 'missing' delegator.
+    if (context?.onBehalfOf?.userId) {
+      const del = await resolveDelegatorContext(this.ql, context);
+      if (del.kind === 'missing') return [];
+      if (del.kind === 'resolved') {
+        const delegatorSets = await this.resolvePermissionSetsForContext(del.context);
+        let delFieldPerms = this.permissionEvaluator.getFieldPermissions(objectName, delegatorSets);
+        delFieldPerms = this.foldFieldRequiredPermissions(delFieldPerms, secMeta.fieldRequiredPermissions, delegatorSets);
+        fieldPerms = intersectFieldMasks(fieldPerms, delFieldPerms);
+      }
+    }
+
+    // Readable = every schema field NOT explicitly masked non-readable. A field
+    // with no permission entry passes through (the field allow-list only
+    // enumerates fields it names) — the exact complement of maskResults' delete
+    // set, so the export header matches list's readable columns by construction.
+    return allFields.filter((f) => fieldPerms[f]?.readable !== false);
+  }
+
   /**
    * Resolve the effective permission sets for an execution context — positions +
    * explicit permission sets, with the configured baseline applied both as an
 
@@ -383,3 +383,110 @@ describe('export route — FLS column projection (#3391 blocking)', () => {
     }
   });
 });
+
+// ===========================================================================
+// #3547: export column projection via the security service's getReadableFields
+// — the LONG-TERM correct path that replaces inferring readability from masked
+// data rows (#3498). The route asks `security.getReadableFields(object, ctx)`
+// for the readable column set BEFORE streaming, so the header is derived from
+// the schema + context, never from row content — immune to an all-null
+// readable column and to an empty result set.
+// ===========================================================================
+describe('export route — FLS column projection via getReadableFields (#3547)', () => {
+  // Boot a real engine + protocol, seed tasks, and wire a RestServer whose
+  // `security` service (host provider) resolves to the supplied
+  // getReadableFields — mirroring how plugin-security registers the service.
+  async function bootWithSecurity(opts: {
+    getReadableFields: (object: string, context?: any) => string[] | undefined;
+    tasks?: Array<Record<string, unknown>>;
+  }) {
+    const { driver } = makeMemoryDriver();
+    const engine = new ObjectQL();
+    engine.registerDriver(driver, true);
+    await engine.init();
+    engine.registry.registerObject(USER as any);
+    engine.registry.registerObject(TASK as any);
+    await engine.insert('user', { id: 'u1', name: '张三' });
+    const tasks = opts.tasks ?? [
+      { id: '1', title: '写代码', done: true, priority: 'high', due: '2026-06-30T00:00:00.000Z', owner: 'u1' },
+      { id: '2', title: '写文档', done: false, priority: 'low', due: '2026-07-01T00:00:00.000Z', owner: 'u1' },
+    ];
+    for (const t of tasks) await engine.insert('task', t);
+    const protocol = new ObjectStackProtocolImplementation(engine as any);
+    // 16th positional ctor arg is `securityServiceProvider`.
+    const securityServiceProvider = async () => ({ getReadableFields: opts.getReadableFields });
+    const rest = new RestServer(
+      createMockServer() as any,
+      protocol as any,
+      { api: { requireAuth: false } } as any,
+      undefined, // kernelManager
+      undefined, // envRegistry
+      undefined, // defaultEnvironmentIdProvider
+      undefined, // authServiceProvider
+      undefined, // objectQLProvider
+      undefined, // emailServiceProvider
+      undefined, // sharingServiceProvider
+      undefined, // reportsServiceProvider
+      undefined, // approvalsServiceProvider
+      undefined, // sharingRulesServiceProvider
+      undefined, // i18nServiceProvider
+      undefined, // analyticsServiceProvider
+      undefined, // settingsServiceProvider
+      undefined, // serviceExistsProvider
+      securityServiceProvider,
+    );
+    rest.registerRoutes();
+    const route = rest.getRoutes().find(
+      (r: any) => r.method === 'GET' && r.path === '/api/v1/data/:object/export',
+    );
+    return { engine, route };
+  }
+
+  it('projects columns from getReadableFields — drops a masked field even though rows still carry it', async () => {
+    // The service says `title` is NOT readable; every row still HAS a title
+    // value. The route must drop 标题 from the header — proving the projection
+    // comes from the service, not the row keys (the masked-row inference would
+    // have kept 标题 because it is present in the rows).
+    const { route } = await bootWithSecurity({
+      getReadableFields: () => ['id', 'done', 'priority', 'due', 'owner'],
+    });
+    const csv = makeRes();
+    await route.handler({ params: { object: 'task' }, query: { format: 'csv' } } as any, csv.res);
+    const header = parseCsv(csv.chunks.join(''))[0];
+    expect(header).not.toContain('标题');
+    expect(header).toEqual(['ID', '完成', '优先级', '截止', '负责人']);
+  });
+
+  it('keeps a readable column that is absent from every row (null-value immunity)', async () => {
+    // All tasks omit `due` → the masked-row inference would DROP 截止 from the
+    // header (no row carries the key). getReadableFields lists it, so the
+    // column survives — the header no longer depends on row content.
+    const { route } = await bootWithSecurity({
+      getReadableFields: () => ['id', 'title', 'done', 'priority', 'due', 'owner'],
+      tasks: [
+        { id: '1', title: 'A', done: true, priority: 'high', owner: 'u1' }, // no `due`
+        { id: '2', title: 'B', done: false, priority: 'low', owner: 'u1' }, // no `due`
+      ],
+    });
+    const csv = makeRes();
+    await route.handler({ params: { object: 'task' }, query: { format: 'csv' } } as any, csv.res);
+    const header = parseCsv(csv.chunks.join(''))[0];
+    expect(header).toContain('截止'); // survives despite being absent from every row
+    expect(header).toEqual(['ID', '标题', '完成', '优先级', '截止', '负责人']);
+  });
+
+  it('explicit ?fields= is still honored verbatim (service projection only narrows schema-derived headers)', async () => {
+    // getReadableFields would drop `title`, but an explicit request wins — the
+    // projection only applies to schema-derived headers (fieldsFromSchema).
+    const { route } = await bootWithSecurity({
+      getReadableFields: () => ['id', 'done'],
+    });
+    const csv = makeRes();
+    await route.handler(
+      { params: { object: 'task' }, query: { format: 'csv', fields: 'id,title' } } as any,
+      csv.res,
+    );
+    const header = parseCsv(csv.chunks.join(''))[0];
+    expect(header).toEqual(['ID', '标题']); // requested columns kept as asked
+  });
+});