diff --git a/packages/cultcache-ts/CHANGELOG.md b/packages/cultcache-ts/CHANGELOG.md
index 53eb5867..8328cb86 100644
--- a/packages/cultcache-ts/CHANGELOG.md
+++ b/packages/cultcache-ts/CHANGELOG.md
@@ -20,7 +20,7 @@ All notable changes to `@gamecult/cultcache-ts` are recorded here, following
 - A store whose canonical-schema catalog slot is not a string is refused as `StoreUnreadableError`.
 - The cache holds one record per key (variants:ruling:cache-refuses-retype). A `put` or `putEnvelope` that lands a second type under a key the cache holds, or under a key a store carries foreign, is refused with `SchemaConflictError` naming the key, even with no store (a memory-only cache used to hold both types at the key). A type that owns the held record's schema id (as for the store) replaces it, unless the held record is in a store other than the written type's home: that rename is refused with `SchemaConflictError` (remove the record first). Two stores holding one key, registered or foreign, refuse the load and admit nothing. A store's `delete` removes the record whose stored schema id the removal's type owns, whatever the arrived record's `type` string.
 - A store holding a record no registered type claims now loads (it used to throw "No schema is registered..."): the cache does not serve the record, lists it in `foreignRecords()`, keeps its bytes through every write, and refuses a `put` or a `delete` at its key with `SchemaConflictError` (variants:ruling:unheld-removal-refused-at-commit). Registering a type that claims its schema id serves it at the next `pullAllBackingStores`.
-- A record whose payload carries a slot past its reader's declared slots, at any depth (the record, a nested record, a list element, a union arm, a unit arm), is now held read-only: it loads and reads, and every `put`, `putEnvelope`, `update` or `delete` of its key is refused with `SchemaConflictError` before a store is touched (cultlib-gaps:ruling:extra-trailing-slots, as the C# reference). A load used to skip the extra slots silently and a later write shed them. The refusal names the schema, the key, the type that was read with extra slots and both slot counts, never a slot value, and carries them as `SchemaConflictError.unreadSlots`. An untouched record keeps its bytes through writes of other keys. Missing trailing slots keep their declared defaults and mark nothing. A record put through the cache never carries a mark. A mark at depth 0 is consented to only when the record arrived under a catalog entry that lists those slots and the reader lists that id compatible (`unreadSlots.explained`); every other mark refuses whatever is declared. New: `CultCache.isWritable(definition, key)` (live, advisory), `CultCache.unreadSlots(definition, key)` and the exported `CultCacheUnreadSlots`. Documents with no slot codec (a plain map, a user formatter) never mark. Implied semver: a minor bump (0.16.0) while the package is 0.x; a major one at 1.0 and after.
+- A record whose payload carries a slot past its reader's declared slots, at any depth (the record, a nested record, a list element, a union arm, a unit arm), is now held read-only: it loads and reads, and every `put`, `putEnvelope`, `update` or `delete` of its key is refused with `SchemaConflictError` before a store is touched (cultlib-gaps:ruling:extra-trailing-slots, as the C# reference). A load used to skip the extra slots silently and a later write shed them. The refusal names the schema, the key, the type that was read with extra slots and both slot counts, never a slot value, and carries them as `SchemaConflictError.unreadSlots`. An untouched record keeps its bytes through writes of other keys. Missing trailing slots keep their declared defaults and mark nothing. A `put` never marks (the value is encoded by this runtime); a `putEnvelope` of a payload carrying extra slots lands its bytes and holds the record read-only. A mark at depth 0 is consented to only when the record arrived under a schema id other than the reader's own that the reader lists compatible, and that id's catalog entry lists those slots (a writer that adds a slot and keeps the reader's id is never consented to) (`unreadSlots.explained`); every other mark refuses whatever is declared. New: `CultCache.isWritable(definition, key)` (live, advisory), `CultCache.unreadSlots(definition, key)` and the exported `CultCacheUnreadSlots`. Documents with no slot codec (a plain map, a user formatter) never mark. Implied semver: a minor bump (0.16.0) while the package is 0.x; a major one at 1.0 and after.
 
 ### Added
 
diff --git a/packages/cultcache-ts/src/cult-cache.ts b/packages/cultcache-ts/src/cult-cache.ts
index e370e2e8..3f15d378 100644
--- a/packages/cultcache-ts/src/cult-cache.ts
+++ b/packages/cultcache-ts/src/cult-cache.ts
@@ -54,6 +54,8 @@ type HydratedEntry = CultCacheEnvelope & {
   lookups: EntryLookups;
   // What the payload decode read past: set when the record was admitted, and it makes the record read-only unless explained.
   unreadSlots?: CultCacheUnreadSlots;
+  // The schema id the record arrived under, before it was admitted under its reader's: what an unread-slots refusal names.
+  storedSchemaId?: string;
 };
 
 // Everything a pull replaces. Built whole, then swapped in with one assignment.
@@ -88,11 +90,15 @@ type DerivedTypeLookups = {
   readonly indexMaps: Map<string, Map<string, string>>;
 };
 
-// The consent a compatible-id declaration gives (cultlib-gaps:ruling:slot-mark-consent): a mark is explained only when it is at
+// The consent a compatible-id declaration gives (cultlib-gaps:ruling:slot-mark-consent): a mark is explained only when the record
+// arrived under an id other than the reader's own that the reader lists compatible, it is at
 // depth 0 and the schema comparison of the catalog entry the record arrived with against the reader's own reported the slots it
 // carries as ignored, so its slot count is at most the highest ignored slot plus one. A record written or put through the cache
 // arrives under the reader's own entry, which ignores nothing, so no mark of it is explained.
-function explainedBy(noted: NotedSlots, reader: CultCacheSchemaCatalogEntry, arrived: CultCacheSchemaCatalogEntry | undefined): boolean {
+function explainedBy(noted: NotedSlots, reader: CultCacheSchemaCatalogEntry, arrivedId: string, arrived: CultCacheSchemaCatalogEntry | undefined): boolean {
+  if (arrivedId === reader.schemaId || !(reader.compatibleSchemaIds ?? []).includes(arrivedId)) {
+    return false;
+  }
   const declared = new Set((reader.members ?? []).map((member) => member.slot));
   const ignored = (arrived?.members ?? []).map((member) => member.slot).filter((slot) => !declared.has(slot));
   return noted.depth0Only && noted.slotCount <= Math.max(-1, ...ignored) + 1;
@@ -599,7 +605,6 @@ export class CultCache {
     });
   }
 
-  // What the last pull found that no registered type claims, by key ordinal. A later registration claims a record at the next pull.
   /** What the record's payload carried past this build's declared slots, or undefined: the held record is read-only unless `explained`. */
   unreadSlots(definition: AnyCultCacheDocumentDefinition, key: string): CultCacheUnreadSlots | undefined {
     return this.#hydrated.entries.get(this.#entryId(this.#requireDefinition(definition).definition.type, key))?.unreadSlots;
@@ -612,10 +617,21 @@ export class CultCache {
    */
   isWritable(definition: AnyCultCacheDocumentDefinition, key: string): boolean {
     const registered = this.#requireDefinition(definition);
-    return !this.#hydrated.foreign.some((record) => record.key === key)
-      && this.#unreadRefusal(this.#hydrated.entries.get(this.#entryId(registered.definition.type, key))) === undefined;
+    const type = registered.definition.type;
+    if (this.#hydrated.foreign.some((record) => record.key === key)
+      || this.#unreadRefusal(this.#hydrated.entries.get(this.#entryId(type, key))) !== undefined) {
+      return false;
+    }
+    // The record of another type a write would restamp (or be refused by): the write's own questions, writeNow's #refuseRetype.
+    const other = this.#heldUnderOtherType(this.#hydrated, type, key);
+    const written: CultCacheEnvelope = {
+      key, type, payload: documentPayloadFromBytes(new Uint8Array(0)), storedAt: "",
+      schemaId: registered.catalogEntry.schemaId, catalogEntry: registered.catalogEntry,
+    };
+    return other === undefined || (writtenOwns(written, schemaIdForEnvelope(other)) && this.#unreadRefusal(other) === undefined);
   }
 
+  // What the last pull found that no registered type claims, by key ordinal. A later registration claims a record at the next pull.
   foreignRecords(): readonly CultCacheForeignRecord[] {
     return [...this.#hydrated.foreign].sort((left, right) => compareOrdinal(left.key, right.key));
   }
@@ -765,7 +781,7 @@ export class CultCache {
       return undefined;
     }
     return new SchemaConflictError(
-      held.schemaId ?? held.type,
+      held.storedSchemaId ?? held.schemaId ?? held.type,
       [held.catalogEntry.schemaName],
       held.key,
       `${held.catalogEntry.schemaName} record ${held.key} is read-only: ${unread.typeName} was read with ${unread.slotCount} slots where this build declares ${unread.declaredCount} ` +
@@ -1078,7 +1094,8 @@ export class CultCache {
       catalogEntry: registered.catalogEntry,
       value,
       lookups,
-      ...(noted ? { unreadSlots: { ...noted, explained: explainedBy(noted, registered.catalogEntry, entry.catalogEntry) } } : {}),
+      storedSchemaId: entry.schemaId ?? entry.type,
+      ...(noted ? { unreadSlots: { ...noted, explained: explainedBy(noted, registered.catalogEntry, entry.schemaId ?? entry.type, entry.catalogEntry) } } : {}),
     };
   }
 
diff --git a/packages/cultcache-ts/test/slot-readonly.test.ts b/packages/cultcache-ts/test/slot-readonly.test.ts
index 262362b0..364adc67 100644
--- a/packages/cultcache-ts/test/slot-readonly.test.ts
+++ b/packages/cultcache-ts/test/slot-readonly.test.ts
@@ -5,7 +5,9 @@ import { join } from "node:path";
 import { test } from "node:test";
 
 import { decode, encode } from "@msgpack/msgpack";
+import { z } from "zod";
 
+import { codecs } from "../src";
 import { CultCache } from "../src/cult-cache";
 import { defineDocumentType, documentPayloadBytes } from "../src/document";
 import { SingleFileMessagePackBackingStore, readSingleFileStore } from "../src/single-file-messagepack-backing-store";
@@ -19,6 +21,7 @@ import {
 // read-only, and every write or removal of its key is refused before a store is touched. The vectors are written by the C#
 // reference: tests/vectors/union-parity/README.md, "Extra trailing slot" and "Slot stores".
 
+const { slots, string } = codecs;
 const SLOT_VALUE = "extra-slot";
 
 function mark(typeName: string, slotCount: number, declaredCount: number, depth0Only: boolean, explained = false, occurrences = 1): CultCacheUnreadSlots {
@@ -129,6 +132,7 @@ test("a write by a type that owns the held record's id restamps it, and is refus
     const held = { ...envelopeOf(await read(file)), type: older.type, schemaId: "tests.surface_older.v1" };
     const cache = CultCache.builder().withDocumentType(older).withDocumentType(newer).withGenericStore(memoryStore([held])).build();
     await cache.pullAllBackingStores();
+    assert.equal(cache.isWritable(newer, "surface"), lands, `${file}: isWritable agrees with the write by the type that owns the id`);
     const write = cache.put(newer, "surface", golden);
     if (lands) {
       await write;
@@ -225,7 +229,11 @@ test("consent: a write of the explained record lands; the over-consent write is
 
   for (const key of ["over-consent", "nested"]) {
     const refused = await openStore("extra-slot-consent.cc", true);
-    await assert.rejects(() => refused.cache.put(refused.surface, key, golden), SchemaConflictError, key);
+    await assert.rejects(
+      () => refused.cache.put(refused.surface, key, golden),
+      (error: unknown) => error instanceof SchemaConflictError && error.schemaId === "tests.union_surface.wide",
+      `${key}: the refusal names the id the record was stored under`,
+    );
   }
 });
 
@@ -243,3 +251,44 @@ test("carried: after writing another record, the marked record's bytes, schema i
     assert.deepEqual(now.catalogEntry, was.catalogEntry, key);
   }
 });
+
+// A writer that adds a slot and keeps its schema id (TS ids are hand-written; C# ids hash the members): the reader of that
+// same id has declared nothing, so it consents to nothing.
+test("a wider writer under the reader's own schema id is read-only, and a real round trip does not shed the slot", async () => {
+  const doc = (columns: 1 | 2, compatibleSchemaIds: string[] = []) => defineDocumentType({
+    type: "tests.own_id",
+    schemaId: "tests.own_id.v1",
+    schemaName: "tests.own_id",
+    compatibleSchemaIds,
+    schema: z.object({ n: z.string(), extra: z.string().optional() }),
+    codec: columns === 2
+      ? slots<{ n: string; extra: string }>({ 0: ["n", string], 1: ["extra", string] }, "Own")
+      : slots<{ n: string }>({ 0: ["n", string] }, "Own"),
+    name: "n",
+  });
+  const path = join(await mkdtemp(join(tmpdir(), "slot-readonly-")), "own.cc");
+  const wide = doc(2);
+  const writer = CultCache.builder().withDocumentType(wide).withGenericStore(new SingleFileMessagePackBackingStore(path)).build();
+  await writer.put(wide, "k", { n: "n1", extra: "kept" });
+  const bytes = await readFile(path);
+
+  for (const compatible of [[], ["tests.own_id.v1"]]) {
+    const reader = doc(1, compatible);
+    const cache = CultCache.builder().withDocumentType(reader).withGenericStore(new SingleFileMessagePackBackingStore(path)).build();
+    await cache.pullAllBackingStores();
+    assert.deepEqual(cache.unreadSlots(reader, "k"), mark("Own", 2, 1, true), "unexplained: the arrived id is the reader's own");
+    assert.equal(cache.isWritable(reader, "k"), false);
+    await assert.rejects(() => cache.put(reader, "k", { n: "n2" }), SchemaConflictError);
+    await assert.rejects(() => cache.delete(reader, "k"), SchemaConflictError);
+    assert.equal(Buffer.compare(bytes, await readFile(path)), 0, "the file still holds the slot");
+  }
+});
+
+test("a record that arrived under an id the reader does not list compatible is never consented to", async () => {
+  const path = await copyOf("extra-slot-consent.cc");
+  const reader = surfaceDocument("tests.union_surface.v2"); // resolves by type name; lists nothing
+  const cache = CultCache.builder().withDocumentType(reader).withGenericStore(new SingleFileMessagePackBackingStore(path)).build();
+  await cache.pullAllBackingStores();
+  assert.equal(cache.unreadSlots(reader, "consented")?.explained, false);
+  assert.equal(cache.isWritable(reader, "consented"), false);
+});