@@ -0,0 +1,18 @@
+---
+'@objectstack/spec': patch
+---
+
+fix(spec)!: the engine-side projection declarations (`EngineQueryOptionsSchema.fields`, the RPC request's `select`, the legacy `DataEngineQueryOptionsSchema.select` and the vector-find request's `fields`) declare the projection their transport siblings declare, instead of any list (#26954)
+
+Clause-②: no (narrowing) — the published engine-side declarations of a read's projection (`EngineQueryOptionsSchema.fields`; the deprecated `select` of the RPC query options reached through `DataEngineFindRequestSchema`, `DataEngineFindOneRequestSchema` and `DataEngineRequestSchema`; the deprecated `DataEngineQueryOptionsSchema.select`; and `DataEngineVectorFindRequestSchema.fields`, also an arm of `DataEngineRequestSchema`) refuse a list naming no field, which they admitted before. Nothing is widened and no key is added, removed or renamed.
+Metadata-protocol: none
+
+<!-- adr-0087: not-required (no-migration-prescription) No key of any schema is added, removed, renamed or re-shaped and no metadata type or stored shape moves: these are engine and RPC request declarations, never stored, and no metadata type embeds them. A caller names at least one field or leaves the projection out; there is nothing for a migration to rewrite and no tombstone. -->
+
+**BREAKING** accept-set narrowing of published declarations (`@objectstack/spec/data`). No runtime in this repository parses any of these members, so no request served here changes; what can break is code outside it that parses with these exported schemas, or a client generated from their JSON Schema, and sent an empty projection.
+
+- **Before.** `EngineQueryOptionsSchema.fields`, the RPC query's `select`, `DataEngineQueryOptionsSchema.select` and `DataEngineVectorFindRequestSchema.fields` admitted any list of strings: `[]`, and a list with a blank or whitespace-only name.
+- **Now.** A list must hold at least one name, and no name may be blank (JSON Schema `minItems: 1`, and a non-blank pattern on each name). All four read this rule from the same declaration as `QueryTransportParamsSchema.select` / `$select` and `GetDataRequestSchema.select`, so the refusal messages are the same.
+- **Unchanged.** The inferred types (`string[]`), a named projection, and leaving the projection out. The in-process ObjectQL engine does not parse its options with this schema: `engine.find(object, { fields: [] })` still reads every column, as before.
+
+**The remedy.** Name at least one field, or leave `fields` / `select` out to read every column.
@@ -25,6 +25,7 @@ import {
   DataEngineVectorFindRequestSchema,
   DataEngineRequestSchema,
   DroppedFieldsEventSchema,
+  QueryTransportParamsSchema,
 } from './data-engine.zod';
 import { QuerySchema } from './query.zod';
 
@@ -1242,3 +1243,104 @@ describe('DroppedFieldsEventSchema.reason — why a write dropped submitted fiel
     expect(issue?.path).toEqual(['reason']);
   });
 });
+
+describe('the engine-side projection declarations admit exactly what their transport siblings admit', () => {
+  /**
+   * [#26954] `EngineQueryOptionsSchema.fields` and the RPC request's
+   * deprecated `select` (`DataEngineFindRequestSchema.query`, and so
+   * `DataEngineFindOneRequestSchema` and the `DataEngineRequestSchema` union)
+   * take the projection's array arm from the one declaration their transport
+   * siblings use: at least one name, none blank. `fields` shares
+   * `FieldNodeSchema` with `$select`; `select` shares `z.string()` with the
+   * bare `select`. So each refusal reads exactly as its sibling's does.
+   */
+  const REFUSED: ReadonlyArray<readonly [string, string[], ReadonlyArray<string | number>]> = [
+    ['an empty list', [], []],
+    ['a list with a blank name', ['name', ''], [1]],
+    ['a list with a whitespace name', ['name', '  '], [1]],
+  ];
+  const KEPT: ReadonlyArray<readonly [string, string[]]> = [
+    ['one name', ['name']],
+    ['two names', ['name', 'email']],
+  ];
+
+  /** The first issue's message and path when `value` sits under `key` of a transport bag. */
+  const sibling = (key: 'select' | '$select', value: unknown) => {
+    const parsed = QueryTransportParamsSchema.safeParse({ [key]: value });
+    expect(parsed.success, `${key} sibling refuses ${JSON.stringify(value)}`).toBe(false);
+    return parsed.success ? undefined : parsed.error.issues[0];
+  };
+
+  it.each(REFUSED)('EngineQueryOptionsSchema.fields refuses %s, naming the path, as `$select` does', (_name, value, tail) => {
+    const parsed = EngineQueryOptionsSchema.safeParse({ fields: value });
+    expect(parsed.success).toBe(false);
+    const issue = parsed.success ? undefined : parsed.error.issues[0];
+    expect(issue?.path).toEqual(['fields', ...tail]);
+    const twin = sibling('$select', value);
+    expect(issue?.message).toBe(twin?.message);
+    expect(twin?.path).toEqual(['$select', ...tail]);
+  });
+
+  it.each(KEPT)('EngineQueryOptionsSchema.fields keeps %s, unchanged', (_name, value) => {
+    expect(EngineQueryOptionsSchema.parse({ fields: value }).fields).toEqual(value);
+  });
+
+  for (const [label, schema, method] of [
+    ['DataEngineFindRequestSchema', DataEngineFindRequestSchema, 'find'],
+    ['DataEngineFindOneRequestSchema', DataEngineFindOneRequestSchema, 'findOne'],
+    ['DataEngineRequestSchema', DataEngineRequestSchema, 'find'],
+  ] as const) {
+    it.each(REFUSED)(`${label}: query.select refuses %s, naming the path, as the bare \`select\` does`, (_name, value, tail) => {
+      const parsed = schema.safeParse({ method, object: 'account', query: { select: value } });
+      expect(parsed.success).toBe(false);
+      const issue = parsed.success ? undefined : parsed.error.issues[0];
+      expect(issue?.path).toEqual(['query', 'select', ...tail]);
+      expect(issue?.message).toBe(sibling('select', value)?.message);
+    });
+
+    it.each(REFUSED)(`${label}: query.fields refuses %s, naming the path`, (_name, value, tail) => {
+      const parsed = schema.safeParse({ method, object: 'account', query: { fields: value } });
+      expect(parsed.success).toBe(false);
+      const issue = parsed.success ? undefined : parsed.error.issues[0];
+      expect(issue?.path).toEqual(['query', 'fields', ...tail]);
+      expect(issue?.message).toBe(sibling('$select', value)?.message);
+    });
+
+    it.each(KEPT)(`${label}: query.select keeps %s, folded onto \`fields\``, (_name, value) => {
+      const parsed = schema.parse({ method, object: 'account', query: { select: value } }) as { query?: Record<string, unknown> };
+      expect(parsed.query).toEqual({ fields: value });
+    });
+  }
+
+  // The legacy options bag's `select` and the vector-find request's `fields`
+  // are `z.string()` lists, so their sibling is the bare `select`.
+  it.each(REFUSED)('DataEngineQueryOptionsSchema.select refuses %s, naming the path, as the bare `select` does', (_name, value, tail) => {
+    const parsed = DataEngineQueryOptionsSchema.safeParse({ select: value });
+    expect(parsed.success).toBe(false);
+    const issue = parsed.success ? undefined : parsed.error.issues[0];
+    expect(issue?.path).toEqual(['select', ...tail]);
+    expect(issue?.message).toBe(sibling('select', value)?.message);
+  });
+
+  it.each(KEPT)('DataEngineQueryOptionsSchema.select keeps %s, unchanged', (_name, value) => {
+    expect(DataEngineQueryOptionsSchema.parse({ select: value }).select).toEqual(value);
+  });
+
+  for (const [label, schema] of [
+    ['DataEngineVectorFindRequestSchema', DataEngineVectorFindRequestSchema],
+    ['DataEngineRequestSchema', DataEngineRequestSchema],
+  ] as const) {
+    it.each(REFUSED)(`${label}: vectorFind fields refuses %s, naming the path, as the bare \`select\` does`, (_name, value, tail) => {
+      const parsed = schema.safeParse({ method: 'vectorFind', object: 'doc', vector: [0.1, 0.2], fields: value });
+      expect(parsed.success).toBe(false);
+      const issue = parsed.success ? undefined : parsed.error.issues[0];
+      expect(issue?.path).toEqual(['fields', ...tail]);
+      expect(issue?.message).toBe(sibling('select', value)?.message);
+    });
+
+    it.each(KEPT)(`${label}: vectorFind fields keeps %s, unchanged`, (_name, value) => {
+      const parsed = schema.parse({ method: 'vectorFind', object: 'doc', vector: [0.1, 0.2], fields: value }) as { fields?: string[] };
+      expect(parsed.fields).toEqual(value);
+    });
+  }
+});
@@ -75,6 +75,41 @@ export const BaseEngineOptionsSchema = lazySchema(() => z.object({
   context: ExecutionContextSchema.partial().optional(),
 }));
 
+/**
+ * [#26909] The projection's accept set — the `select` / `$select` spellings
+ * of `fields` — as the doors that read it judge it (`projectionFieldsValue` in
+ * `@objectstack/metadata-protocol`, shared by the list's `findData` and the
+ * by-id `getData`). A comma string is split, trimmed and emptied of blank
+ * members there, and a value naming NO field is refused with a 400: absent
+ * means "every column", so an empty projection read as absent is a widening.
+ *
+ * The string arm is that rule as a pattern: `[^\s,]` holds iff some comma
+ * segment has a non-whitespace character, and `\s` is exactly the set
+ * `trim()` strips. A pattern, not a refinement, so the published JSON Schema
+ * states it too. The array arm is not split: the door keeps every member,
+ * refuses `[]`, and checks every member against the object's fields, where a
+ * blank name is never one.
+ *
+ * `HttpFindQueryParamsSchema.select` and `GetDataRequestSchema.select`
+ * (`../api/protocol.zod.ts`) read these arms off the `QueryTransportParamsSchema`
+ * members rather than restating them, so the three declarations of one door
+ * rule cannot drift apart.
+ *
+ * [#26954] The engine-side declarations of the same projection —
+ * `EngineQueryOptionsSchema.fields`, `RpcQueryOptionsSchema.select`, the
+ * legacy `DataEngineQueryOptionsSchema.select` and
+ * `DataEngineVectorFindRequestSchema.fields` — take the array arm from here
+ * too. It is declared ahead of both because
+ * `RpcQueryOptionsSchema` extends `EngineQueryOptionsSchema` at module load,
+ * which runs the lazy factory before any later `const` exists.
+ */
+const PROJECTION_NAMES_NO_FIELD = 'Name at least one field';
+const TransportProjectionStringSchema = z.string().regex(/[^\s,]/, PROJECTION_NAMES_NO_FIELD);
+const transportProjectionArray = <T extends z.ZodString>(member: T) =>
+  z.array(member.regex(/\S/, 'A field name cannot be blank')).min(1, PROJECTION_NAMES_NO_FIELD);
+const PROJECTION_ACCEPT_SET = 'at least one field name; a value naming no field (blank, only commas and spaces, '
+  + 'an empty list, or a list with a blank name) is refused';
+
 // ==========================================================================
 // 2. method: FIND (QueryAST-aligned)
 // ==========================================================================
@@ -101,8 +136,8 @@ export const EngineQueryOptionsSchema = lazySchema(() => BaseEngineOptionsSchema
   /** Filter conditions (WHERE) — standard QueryAST `where` */
   where: z.union([z.record(z.string(), z.unknown()), FilterConditionSchema]).optional(),
 
-  /** Fields to retrieve (SELECT) — standard QueryAST `fields` */
-  fields: z.array(FieldNodeSchema).optional(),
+  /** Fields to retrieve (SELECT) — standard QueryAST `fields`; at least one, none blank (#26954) */
+  fields: transportProjectionArray(FieldNodeSchema).optional(),
 
   /** Sorting instructions (ORDER BY) — standard QueryAST `orderBy` */
   orderBy: z.array(SortNodeSchema).optional(),
@@ -179,8 +214,8 @@ export const EngineQueryOptionsSchema = lazySchema(() => BaseEngineOptionsSchema
 export const DataEngineQueryOptionsSchema = lazySchema(() => BaseEngineOptionsSchema.extend({
   /** @deprecated Use `where` (EngineQueryOptionsSchema) */
   filter: DataEngineFilterSchema.optional(),
-  /** @deprecated Use `fields` (EngineQueryOptionsSchema) */
-  select: z.array(z.string()).optional(),
+  /** @deprecated Use `fields` (EngineQueryOptionsSchema) — the same projection rule (#26954) */
+  select: transportProjectionArray(z.string()).optional(),
   /** @deprecated Use `orderBy` (EngineQueryOptionsSchema) */
   sort: DataEngineSortSchema.optional(),
   limit: z.number().int().min(1).optional(),
@@ -774,8 +809,8 @@ function foldRpcQueryOptions(input: object, ctx: z.core.$RefinementCtx): EngineQ
  */
 const RpcQueryOptionsSchema = EngineQueryOptionsSchema.extend({
   ...RpcLegacyFilterMixin,
-  /** @deprecated Use `fields` */
-  select: z.array(z.string()).optional(),
+  /** @deprecated Use `fields` — the same projection rule (#26954) */
+  select: transportProjectionArray(z.string()).optional(),
   /** @deprecated Use `orderBy` */
   sort: DataEngineSortSchema.optional(),
   /** @deprecated Use `offset` */
@@ -907,33 +942,6 @@ const TransportFilterValueSchema = lazySchema(() => z.union([FilterArraySchema,
  */
 const TransportCountValueSchema = lazySchema(() => z.union([z.boolean(), z.enum(['true', 'false'])]));
 
-/**
- * [#26909] The projection's accept set — the `select` / `$select` spellings
- * of `fields` — as the doors that read it judge it (`projectionFieldsValue` in
- * `@objectstack/metadata-protocol`, shared by the list's `findData` and the
- * by-id `getData`). A comma string is split, trimmed and emptied of blank
- * members there, and a value naming NO field is refused with a 400: absent
- * means "every column", so an empty projection read as absent is a widening.
- *
- * The string arm is that rule as a pattern: `[^\s,]` holds iff some comma
- * segment has a non-whitespace character, and `\s` is exactly the set
- * `trim()` strips. A pattern, not a refinement, so the published JSON Schema
- * states it too. The array arm is not split: the door keeps every member,
- * refuses `[]`, and checks every member against the object's fields, where a
- * blank name is never one.
- *
- * `HttpFindQueryParamsSchema.select` and `GetDataRequestSchema.select`
- * (`../api/protocol.zod.ts`) read these arms off the members below rather
- * than restating them, so the three declarations of one door rule cannot
- * drift apart.
- */
-const PROJECTION_NAMES_NO_FIELD = 'Name at least one field';
-const TransportProjectionStringSchema = z.string().regex(/[^\s,]/, PROJECTION_NAMES_NO_FIELD);
-const transportProjectionArray = <T extends z.ZodString>(member: T) =>
-  z.array(member.regex(/\S/, 'A field name cannot be blank')).min(1, PROJECTION_NAMES_NO_FIELD);
-const PROJECTION_ACCEPT_SET = 'at least one field name; a value naming no field (blank, only commas and spaces, '
-  + 'an empty list, or a list with a blank name) is refused';
-
 /**
  * [#25269] The page window — `limit` / `offset` and every spelling that folds
  * onto them (`top`, `$top`, `skip`, `$skip`) — as the Data API list doors
@@ -1513,8 +1521,8 @@ export const DataEngineVectorFindRequestSchema = lazySchema(() => z.object({
   vector: z.array(z.number()),
   /** Optional pre-filter (Metadata filtering) — standard QueryAST `where` */
   where: z.union([z.record(z.string(), z.unknown()), FilterConditionSchema]).optional(),
-  /** Fields to retrieve — standard QueryAST `fields` */
-  fields: z.array(z.string()).optional(),
+  /** Fields to retrieve — standard QueryAST `fields`; the same projection rule (#26954) */
+  fields: transportProjectionArray(z.string()).optional(),
   /** Number of results */
   limit: z.number().int().default(5).optional(),
   /** Minimum similarity score (0-1) or distance threshold */