diff --git a/.changeset/undefined-comparand-prescription-position-safe.md b/.changeset/undefined-comparand-prescription-position-safe.md
new file mode 100644
index 0000000000..25e5aaf86b
--- /dev/null
+++ b/.changeset/undefined-comparand-prescription-position-safe.md
@@ -0,0 +1,18 @@
+---
+"@objectstack/spec": patch
+---
+
+fix(spec): the `undefined` comparand refusal prescribes the null predicate by its ruled spellings (#14426)
+
+`parseFilterAST`'s comparand-type door refuses an `undefined` comparand at every
+position. Its prescription read "Write null for the null predicate, or omit the
+key" — position-agnostic advice that, followed at `{ $gt: undefined }`, produced
+`{ $gt: null }`, which the 2026-09-01 ruling refuses one door over (and, at an
+`$in` / `$nin` / `$between` member, produced the list shapes refused on
+2026-08-31). Two loud refusals to reach one right answer.
+
+The sentence now names the null predicate by its complete spellings —
+`{"$eq": null}` / `{"$ne": null}` — or omit the key, so following it never lands
+in a refusal at any position the sentence is emitted at. No accept/refuse
+behaviour changes: same envelope (`INVALID_FILTER` / 400), same path, same
+accepted-set and NOT-applied sentences.
diff --git a/packages/drivers/driver-memory/src/memory-matcher-null-value-and-comparand.test.ts b/packages/drivers/driver-memory/src/memory-matcher-null-value-and-comparand.test.ts
index c55b71bfbb..dfb86f4517 100644
--- a/packages/drivers/driver-memory/src/memory-matcher-null-value-and-comparand.test.ts
+++ b/packages/drivers/driver-memory/src/memory-matcher-null-value-and-comparand.test.ts
@@ -60,14 +60,18 @@
  * asserted here, now for the ruling's own reason — ⛔「不单独修一个到不了的
  * 路径」 — and they were measured byte-identical across this repair too.
  *
- * ⚠️ A null COMPARAND in an ORDERING position (`{$gte: null}`) is absent for
- * the ORIGINAL reason, and it is the one such position the contract still
- * ACCEPTS: the 2026-08-31 ruling refused the three siblings and #5332's
- * landing had already recorded this one in writing as a position "no ruling
- * covers". #13553's guard is scoped to leave those cells exactly where it
- * found them, so pinning them here — in either direction — would prejudge a
- * ruling nobody has made. The invariance is proven in the PR, not asserted
- * here.
+ * ⚠️ A null COMPARAND in an ORDERING position (`{$gte: null}`) is likewise
+ * ABSENT from this file. When #13553 landed it was the one such position the
+ * contract still ACCEPTED (the 2026-08-31 ruling had refused the three
+ * siblings, and #5332's landing had recorded this one in writing as "no
+ * ruling covers"), so #13553's guard was scoped to leave those cells exactly
+ * where it found them rather than prejudge a ruling. The maintainer ruled it
+ * on 2026-09-01 (option A, #14080): the shape is now REFUSED at the
+ * contract's validation entrance, the same door as the list positions, with
+ * the negative pin in `memory-null-ordering-comparand-unreachable.test.ts`.
+ * Its cells stay unasserted here, now for the ruling's own reason — ⛔「不单独修
+ * matcher(死代码)」— and no ordering-vs-null semantics is defined anywhere,
+ * so nothing here says what either face would have answered.
  */
 
 import { describe, it, expect, beforeAll, afterAll } from 'vitest';
diff --git a/packages/drivers/driver-memory/src/memory-matcher.ts b/packages/drivers/driver-memory/src/memory-matcher.ts
index b347fa8da1..eb0cabd7b1 100644
--- a/packages/drivers/driver-memory/src/memory-matcher.ts
+++ b/packages/drivers/driver-memory/src/memory-matcher.ts
@@ -337,16 +337,26 @@ function checkCondition(value: any, condition: any): boolean {
         // same reason — a rule spelled over "the value is null" alone would
         // reach arms whose no-value answer is ruled elsewhere.
         //
-        // ⛔ A no-value COMPARAND is excluded from this guard, deliberately, so
-        // those cells keep TODAY's answer rather than being decided here.
-        // `$gt: null` is the one null-comparand position the contract still
-        // ACCEPTS (measured at `parseFilterAST`): the 2026-08-31 ruling refused
-        // the three siblings — `$in` / `$nin` null members and `$between`'s
-        // null endpoints (#13357) — and #5332's landing had already recorded
-        // this position in writing as one "no ruling covers". Deciding it in an
-        // operator arm would pick a camp the platform declined to pick, and its
-        // sibling was settled by REFUSING the shape rather than by answering
-        // it.
+        // ⛔ A no-value COMPARAND is excluded from this guard, deliberately.
+        // When #13553 landed, `$gt: null` was the one null-comparand position
+        // the contract still ACCEPTED (measured at `parseFilterAST`): the
+        // 2026-08-31 ruling had refused the three siblings — `$in` / `$nin`
+        // null members and `$between`'s null endpoints (#13357) — and #5332's
+        // landing had recorded this one in writing as "no ruling covers", so
+        // deciding it here would have picked a camp the platform declined to
+        // pick. That reason is gone: ruled 2026-09-01 (option A, #14080), the
+        // shape is REFUSED at the contract's validation entrance
+        // (`assertListComparandShapes`, inside `parseFilterAST` and at the
+        // engine seam), the same door and envelope as its siblings. These
+        // cells are now constructively unreachable through the compile face,
+        // and the exclusion stays for the ruling's own reason — ⛔「不单独修
+        // matcher(死代码)」— with no ordering-vs-null semantics defined
+        // anywhere. Negative pin:
+        // `memory-null-ordering-comparand-unreachable.test.ts`. A direct
+        // caller that skips the compile face meets only this package's own
+        // `assertFilterConditionShape`, which deliberately does not carry the
+        // rule (⛔ 不做跨后端对齐工程) — the honest boundary, not a cell for an
+        // arm to decide.
         if (value === null && ORDERING_OPERATORS.has(op)
             && target !== null && target !== undefined) {
             return false;
diff --git a/packages/spec/src/data/filter-comparand-type.test.ts b/packages/spec/src/data/filter-comparand-type.test.ts
index f3227fd3af..b1ecae1a61 100644
--- a/packages/spec/src/data/filter-comparand-type.test.ts
+++ b/packages/spec/src/data/filter-comparand-type.test.ts
@@ -131,6 +131,34 @@ describe('refusals — the measured divergence rows die at the door', () => {
     expect(err?.message).toMatch(/omit/);
   });
 
+  it('the undefined prescription is position-safe: it names the null predicate by its ruled spellings, at every position it is emitted at (#14426)', () => {
+    // "Write null for the null predicate" was position-agnostic advice: followed
+    // at `$gt: undefined` it produced `$gt: null`, refused one door over since
+    // the 2026-09-01 ruling; at an `$in` member it produced `$in: [null]`,
+    // refused since 2026-08-31. The sentence names COMPLETE spellings instead —
+    // the pair the ruling names — so following it never lands in a refusal.
+    const positions: Array<[Record<string, unknown>, string]> = [
+      [{ owner: undefined }, 'where.owner'],
+      [{ owner: { $eq: undefined } }, 'where.owner.$eq'],
+      [{ owner: { $gt: undefined } }, 'where.owner.$gt'],
+      [{ owner: { $lte: undefined } }, 'where.owner.$lte'],
+      [{ owner: { $in: [undefined] } }, 'where.owner.$in[0]'],
+    ];
+    for (const [where, path] of positions) {
+      const err = refusalOf(() => normalizeFilterComparandTypes(where));
+      expect(err?.code, path).toBe('INVALID_FILTER');
+      expect(err?.status, path).toBe(400);
+      expect(err?.message, path).toContain(path);
+      expect(err?.message, path).toContain('{"$eq": null}');
+      expect(err?.message, path).toContain('{"$ne": null}');
+      expect(err?.message, path).toMatch(/omit the key/);
+      // The defect's own spelling: an instruction to write a bare null INTO the
+      // position the sentence was emitted at.
+      expect(err?.message, path).not.toMatch(/Write null\b/);
+      expect(err?.message.length, path).toBeLessThan(500); // the client bound (#5423)
+    }
+  });
+
   it('refuses a PLAIN OBJECT where a scalar operator comparand belongs — the SQL family already did', () => {
     const err = refusalOf(() => normalizeFilterComparandTypes({ qty: { $eq: { a: 1 } } }));
     expect(err?.code).toBe('INVALID_FILTER');
diff --git a/packages/spec/src/data/filter-comparand-type.ts b/packages/spec/src/data/filter-comparand-type.ts
index a1fc08d7f9..f449a25074 100644
--- a/packages/spec/src/data/filter-comparand-type.ts
+++ b/packages/spec/src/data/filter-comparand-type.ts
@@ -255,15 +255,24 @@ function invalidComparandError(context: string | undefined, message: string): Er
 const NOT_APPLIED =
   'The filter was NOT applied, and an unapplied filter would have returned the UNFILTERED result set.';
 
-/** `undefined` gets its own sentence — it is the one refused value that arrives by ACCIDENT. */
+/**
+ * `undefined` gets its own sentence — it is the one refused value that arrives
+ * by ACCIDENT. Its prescription names the null predicate by COMPLETE spellings
+ * (`{"$eq": null}` / `{"$ne": null}`, the pair the 2026-09-01 ruling names),
+ * never as "write null": this sentence is emitted at every comparand position,
+ * and at `$gt` / `$gte` / `$lt` / `$lte` — or an `$in` / `$nin` / `$between`
+ * member — "write null" produced exactly the null shapes refused one door over
+ * (2026-08-31, 2026-09-01). Position-safe means following it never lands in a
+ * refusal, whatever position it was emitted at (#14426).
+ */
 function undefinedComparandRefusal(context: string | undefined, path: string): Error {
   return invalidComparandError(
     context,
     `Filter comparand at ${path} is undefined. { key: undefined } cannot be told apart from an ` +
       `omitted key, yet the two mean OPPOSITE things (a predicate vs no constraint) — one ` +
-      `backend even encoded it as MATCH EVERYTHING. Write null for the null predicate, or omit ` +
-      `the key. A comparison value must be ${ACCEPTED_FILTER_COMPARAND_TYPES_SENTENCE}. ` +
-      NOT_APPLIED,
+      `backend even encoded it as MATCH EVERYTHING. Write the null predicate — {"$eq": null} / ` +
+      `{"$ne": null} — or omit the key. A comparison value must be ` +
+      `${ACCEPTED_FILTER_COMPARAND_TYPES_SENTENCE}. ${NOT_APPLIED}`,
   );
 }