diff --git a/‎.changeset/21333-objectql-boolean-comparand-door.md‎
Lines changed: 31 additions & 0 deletions b/‎.changeset/21333-objectql-boolean-comparand-door.md‎
Lines changed: 31 additions & 0 deletions
diff --git a/‎.changeset/21333-spec-boolean-comparand-contract.md‎
Lines changed: 19 additions & 0 deletions b/‎.changeset/21333-spec-boolean-comparand-contract.md‎
Lines changed: 19 additions & 0 deletions
diff --git a/‎packages/objectql/src/boolean-comparand-declared-type-door.ts‎
Lines changed: 111 additions & 0 deletions b/‎packages/objectql/src/boolean-comparand-declared-type-door.ts‎
Lines changed: 111 additions & 0 deletions
@@ -0,0 +1,31 @@
+---
+"@objectstack/objectql": minor
+---
+
+fix(objectql)!: a comparand against a declared boolean field is narrowed to its boolean at the engine's filter door, and any string other than "true" / "false" / "1" / "0" is refused with `INVALID_FILTER` / 400
+
+Clause-②: yes (narrowing)
+
+<!-- adr-0087: not-required (no-migration-prescription) a refusal and a narrowing of filter COMPARANDS at the engine's query door, against a declared boolean or toggle field. No authorable key, spelling, export or stored shape moves: every query shape, every FilterCondition and every object definition parse as before, @objectstack/objectql exports nothing new and nothing less, and no stored row is read or rewritten. What is refused is a comparand that matched no row (and every row under $ne) on InMemoryDriver and on SqlDriver over SQLite (PostgreSQL and MySQL not measured), and which boolean a caller meant by "yes" is not something a ledger entry can decide. The other categories are closed on facts: the package publishes (not unpublished); no ADR-0087 id covers a filter comparand, and this diff adds none (not registered / already-registered); and the change is runtime behaviour, not a declaration (not runtime-interface-only / type-surface-only). -->
+
+**BREAKING**: this narrows what a filter may compare a declared `boolean` or `toggle` field with, at every filter position and through every door that reaches the engine's filter walk (`engine.find` / `findOne` / `count` / `aggregate` / `update` / `delete`, and every spelling the data API hands it). It ships as `minor` under the launch-window convention for accept-set narrowings. No export or published type changes.
+
+**What was accepted before.** A string compared with a boolean field was neither refused nor read as a boolean: the engine handed it to the driver as written, and every answer was a 200. Measured on two rows (one `true`, one `false`) on InMemoryDriver and on SqlDriver over SQLite, through `engine.find`, `engine.aggregate` and the protocol's `findData` with each spelling the `POST /api/v1/data/:object/query` and `GET /api/v1/data/:object` routes hand it:
+
+- `"true"` (implicit, `$eq`, `$in`) and `"false"` (implicit), and both through `?filter=`, `?$filter=`, the filter AST and the bare query parameter (`?flag=true`), matched no row on either driver;
+- `$ne "true"` and `$nin ["true"]` returned both rows, the true row included;
+- `"yes"` matched no row, and `$ne "yes"` both rows;
+- `1`, `"1"`, `0` and `"0"` at `where` (and `"1"` / `"0"` through every spelling above) matched the right row on SQLite and no row on InMemoryDriver (`$ne 1` returned both rows there);
+- the per-aggregation `filter` and `having` (the engine's own evaluator) answered `"true"` with no row and no group, and `$ne "true"` with every one.
+
+**What is answered now.** At `where` (both spellings), the per-aggregation `filter` and `having`, on every verb that collects a filter, before any driver is asked for a row:
+
+- `true` / `false` are handed to the driver as written;
+- `1` / `0`, `"1"` / `"0"` and `"true"` / `"false"` are narrowed to `true` / `false`, so every driver receives the one boolean each names. Measured on InMemoryDriver and on SqlDriver over SQLite, `?flag=true` and `?flag=1` now return the true row; any other driver receives the same narrowed boolean by mechanism (PostgreSQL and MySQL not measured);
+- any other string, a different letter case (`"TRUE"`), surrounding whitespace, a blank and a `{placeholder}` included, is refused `INVALID_FILTER` / 400. The message names the field, its declared type, the comparand and its position, and says what is wrong with it.
+
+The accepted set is the one the record validator already admits when a boolean field is WRITTEN. The rule lives in `@objectstack/spec/data`'s `filter-boolean-comparand-declared-type.ts`, and the engine applies it in the same walk that judges number comparands.
+
+**The remedy.** Write `true` or `false`. In a querystring, where every value is a string, write `true` / `false` or `1` / `0`.
+
+**Unchanged.** A boolean comparand, `null` (the null test) and the flag operators (`$null`, `$exists`, `$empty`) answer as before, and so does every comparand against a field that is not boolean. A number other than `1` / `0` against a boolean field is still handed to the driver as written. A filter on a `formula` field is still refused one step earlier, as before.
@@ -0,0 +1,19 @@
+---
+"@objectstack/spec": minor
+---
+
+feat(spec): the boolean-comparand declared-type contract in `@objectstack/spec/data` — the comparands a declared boolean field accepts in a filter, the boolean each narrows to, and the refusal words
+
+Clause-②: yes
+
+**What it declares.** `filter-boolean-comparand-declared-type.ts`, the boolean twin of `filter-number-comparand-declared-type.ts`:
+
+- `BOOLEAN_COMPARAND_SPELLINGS`: the accepted non-boolean spellings, `1` / `0`, `"1"` / `"0"` and `"true"` / `"false"`, each with the boolean it narrows to. This is the set the record validator admits when a boolean field is written. `readBooleanComparand` reads a comparand by it, and names why a string is not one (`NON_BOOLEAN_STRING_FORMS`: `empty`, `padded`, `letter-case`, `placeholder`, `not-a-boolean`).
+- `BOOLEAN_COMPARAND_DOOR_JUDGED_TYPES` (`BOOLEAN_VALUE_TYPES` itself), and the judged positions, which are the number door's lists by identity.
+- `booleanComparandFieldVerdict` and `booleanComparandDoorVerdict`, the pure verdict: `narrows`, `door-refusal` (`INVALID_FILTER` / 400), `passes` or `deferred`.
+- `booleanComparandRefusalMessage`: the refusal words, inside the 500-character client bound.
+- `BOOLEAN_COMPARAND_READING_CASES`, `BOOLEAN_COMPARAND_DOOR_FIXTURE` and the derived `BOOLEAN_COMPARAND_DOOR_CASES`, for a door's suite to drive.
+
+**What the verdict answers `door-refusal` for.** A string other than the four accepted ones, compared with a declared boolean field, at the value positions of a filter (the implicit comparand, `$eq` / `$ne` / `$gt` / `$gte` / `$lt` / `$lte`, and each member of `$in` / `$nin` / `$between`).
+
+**What moves for consumers.** Nothing in this package refuses or narrows a filter, and every existing export is unchanged. The door that applies the verdict ships in the same release in `@objectstack/objectql`, whose changeset states what changes for a caller.
@@ -0,0 +1,111 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+/**
+ * [#21333] The BOOLEAN-comparand arm of the engine's one field-aware filter
+ * walk — the boolean twin of the number-comparand door (#20351), at the same
+ * seam, the same positions and the same three filter positions (`where` on
+ * both spellings, the per-aggregation `filter`, `having`).
+ *
+ * ## What it answers
+ *
+ * A comparand against a declared boolean field (`boolean`, `toggle`, a
+ * `formula` returning `boolean`):
+ *
+ * - `true` / `false` pass, as written;
+ * - `1` / `0`, `"1"` / `"0"` and `"true"` / `"false"` NARROW to `true` /
+ *   `false`, copy-on-write, so every backend receives the one boolean each
+ *   spelling names — a bare query parameter (`?active=true`) is always a
+ *   string, and before this arm `"true"` matched no row on any driver while
+ *   `1` matched no row on InMemoryDriver;
+ * - any other string (`"yes"`, `"TRUE"`, `""`, a `{placeholder}`) is refused
+ *   `INVALID_FILTER` / 400, naming the field and its declared type, before
+ *   any driver is resolved.
+ *
+ * The contract — the accepted spellings, the pure verdict, the refusal words,
+ * the case table — is lane (1), `@objectstack/spec/data`'s
+ * `filter-boolean-comparand-declared-type.ts`. ⛔ Nothing here reads a
+ * spelling: the verdict does.
+ *
+ * ## Why an arm and not a door of its own
+ *
+ * `number-comparand-declared-type-door.ts`'s `walkCondition` is the one filter
+ * walk the engine runs at all three positions with each column's declaration
+ * in hand, and every position-specific fact (the site kind, the relation arm,
+ * the copy-on-write discipline, the depth bound and the combinators descended)
+ * lives in it. A second walk would redraw every one of those boundaries. So
+ * the walk asks this arm at each field key the number arm does not judge (the
+ * two classes are disjoint), exactly as it asks the no-operator-object arm
+ * (`no-operator-object-door.ts`) — and, like that module, ⛔ nothing here
+ * walks a filter.
+ *
+ * ## One door for every surface
+ *
+ * Every REST spelling reaches the walk unchanged: the `POST …/query` body's
+ * `where`, the `filter` / `$filter` JSON and the `FilterArray` sugar
+ * (`parseFilterAST` lowers it first), and the bare query parameters, which
+ * `metadata-protocol`'s `findData` folds into an implicit `where` of strings.
+ * ⛔ No per-door coercion: the REST layer and the protocol hand the strings
+ * through, and this arm is the only place one becomes a boolean.
+ *
+ * @see booleanComparandDoorVerdict — the pure verdict (lane 1, `@objectstack/spec`).
+ * @see https://github.com/objectstack-ai/objectstack/issues/21333
+ */
+
+import {
+  booleanComparandDoorVerdict,
+  booleanComparandFieldVerdict,
+  booleanComparandRefusalMessage,
+  type BooleanComparandDoorFieldMeta,
+  type BooleanComparandRefusalSite,
+} from '@objectstack/spec/data';
+
+/** A comparand the arm refuses: the site the contract's words are written from. */
+export type NonBooleanComparand = BooleanComparandRefusalSite;
+
+/** The arm's answer for one comparand: kept (possibly narrowed), or refused at a site. */
+export type BooleanArmAnswer =
+  | { readonly refused: false; readonly value: unknown }
+  | { readonly refused: true; readonly site: NonBooleanComparand };
+
+/**
+ * The field meta the arm judges, or `null` when it does not judge this
+ * declaration — the spec's field verdict decides (`deferred` and
+ * `not-judged` are both "nothing to judge here"), never a list here.
+ */
+export function booleanArmFieldMeta(meta: BooleanComparandDoorFieldMeta | null): BooleanComparandDoorFieldMeta | null {
+  return meta !== null && booleanComparandFieldVerdict(meta) === 'judged' ? meta : null;
+}
+
+/**
+ * One comparand at a judged position: the spec's verdict, routed. `aggregated`
+ * is the walk's site fact — `having`'s columns are the aggregated row's, not a
+ * declared field — and only ever written when true.
+ */
+export function judgeBooleanComparand(
+  meta: BooleanComparandDoorFieldMeta,
+  field: string,
+  comparand: unknown,
+  path: string,
+  aggregated: boolean,
+): BooleanArmAnswer {
+  const verdict = booleanComparandDoorVerdict(meta, comparand);
+  if (verdict.verdict === 'narrows') return { refused: false, value: verdict.value };
+  if (verdict.verdict !== 'door-refusal') return { refused: false, value: comparand };
+  return {
+    refused: true,
+    site: {
+      field,
+      declaredType: meta.type,
+      ...(meta.returnType === undefined ? {} : { returnType: meta.returnType }),
+      path,
+      value: comparand,
+      form: verdict.form,
+      ...(aggregated ? { aggregated: true as const } : {}),
+    },
+  };
+}
+
+/** The arm's refusal words — the contract's, behind the engine's caller prefix. */
+export function nonBooleanComparandRefusalMessage(site: NonBooleanComparand, context: string): string {
+  return booleanComparandRefusalMessage(site, context);
+}