diff --git a/packages/cultnet-ts/CHANGELOG.md b/packages/cultnet-ts/CHANGELOG.md
index 3e8ad1fa..64ee5a3b 100644
--- a/packages/cultnet-ts/CHANGELOG.md
+++ b/packages/cultnet-ts/CHANGELOG.md
@@ -7,6 +7,7 @@ All notable changes to `@gamecult/cultnet-ts` are recorded here, following
 
 ### Added
 
+- `cultNetWriteNotAuthorizedError(recordKey, reason?)`: the typed `write_not_authorized` refusal (code and `details.recordKey`), byte-identical to the C# `ForRemoteWriteRefused` vector in `error-vectors.json`. `CultNetDocumentRegistry.canonicalSchemaId(schemaId)` resolves the id a put or delete writes under, for a server's write authority.
 - `cultNetTextError(error)`: the text-only `cultnet.error.v0` refusal, with `routingHint`, `code` and `details` null. The legacy-union error decode uses it, so there is one place that spells the null members.
 - `CultNetRudpSocketTransportServer`: one UDP socket serving many peers, each remote address and port with its own reliable session (`peerConnected`, `frame`, `peerDisconnected` events; `send(peer, ...)` answers on the peer a frame came from). A second client no longer replaces the first; Connects beyond `maxPeers` (default 256) are dropped, and `close()` resolves once every peer's goodbye has left the socket. The shape follows the Rust `CultNetRudpServerHub` and C# `CultNetRudpSocketTransportServer`, and the wire format is unchanged. The optional `peerTimeoutMs` (idle peers ended and told so; default off) exists only in TypeScript. `CultNetRudpSocketTransportConnection` in `server` mode still serves exactly one peer.
 
diff --git a/packages/cultnet-ts/src/contracts.ts b/packages/cultnet-ts/src/contracts.ts
index 9ec805ff..bc6a7b72 100644
--- a/packages/cultnet-ts/src/contracts.ts
+++ b/packages/cultnet-ts/src/contracts.ts
@@ -1,3 +1,4 @@
+import { isNullOrWhiteSpaceCSharp } from "./cultmesh-authority";
 import type { DocumentPayload } from "@gamecult/cultcache-ts/document";
 import Ajv2020, { type ValidateFunction } from "ajv/dist/2020.js";
 
@@ -204,6 +205,20 @@ export function cultNetTextError(error: string): CultNetErrorMessage {
   return { schemaVersion: "cultnet.error.v0", error, routingHint: null, code: null, details: null };
 }
 
+/**
+ * The refusal for a peer write its server's authority did not grant (C# `CultNetErrorMessage.ForRemoteWriteRefused`):
+ * code `write_not_authorized`, `details` naming the record in `recordKey`. A blank `reason` leaves the text bare.
+ */
+export function cultNetWriteNotAuthorizedError(recordKey: string, reason?: string | null): CultNetErrorMessage {
+  return {
+    schemaVersion: "cultnet.error.v0",
+    error: typeof reason === "string" && !isNullOrWhiteSpaceCSharp(reason) ? `write_not_authorized: ${reason}` : "write_not_authorized",
+    routingHint: null,
+    code: "write_not_authorized",
+    details: { field: "recordKey", value: recordKey, asOf: null, current: null },
+  };
+}
+
 export interface CultNetErrorRoutingHint {
   shard: Record<string, unknown> | null;
   reason: string;
diff --git a/packages/cultnet-ts/src/index.ts b/packages/cultnet-ts/src/index.ts
index 7857e72d..94199d02 100644
--- a/packages/cultnet-ts/src/index.ts
+++ b/packages/cultnet-ts/src/index.ts
@@ -18,6 +18,7 @@ export {
   type CultNetRawPayloadEncoding,
   CULTNET_ERROR_CODES,
   cultNetTextError,
+  cultNetWriteNotAuthorizedError,
   type CultNetErrorCode,
   type CultNetErrorDetails,
   type CultNetErrorMessage,
diff --git a/packages/cultnet-ts/src/replication.ts b/packages/cultnet-ts/src/replication.ts
index 9676d0f0..32ee4163 100644
--- a/packages/cultnet-ts/src/replication.ts
+++ b/packages/cultnet-ts/src/replication.ts
@@ -270,6 +270,20 @@ export class CultNetDocumentRegistry {
     );
   }
 
+  /**
+   * The canonical schema id a put or delete naming `schemaId` writes under, from the id alone, or `undefined` when no
+   * binding takes it (or several claim it). A server asks its write authority about this id, not the alias a peer sent.
+   */
+  canonicalSchemaId(schemaId: string): string | undefined {
+    try {
+      const binding = this.getBySchemaId(schemaId);
+      return binding ? schemaIdForBinding(binding) : undefined;
+    } catch (error) {
+      if (error instanceof SchemaConflictError) return undefined;
+      throw error;
+    }
+  }
+
   async applyDocumentDeleteMessage(
     cache: CultCache,
     message: CultNetDocumentDeleteMessage,
diff --git a/packages/cultnet-ts/test/cultnet-error.test.ts b/packages/cultnet-ts/test/cultnet-error.test.ts
index 42ef0753..e234f105 100644
--- a/packages/cultnet-ts/test/cultnet-error.test.ts
+++ b/packages/cultnet-ts/test/cultnet-error.test.ts
@@ -8,6 +8,7 @@ import { decode, encode } from "@msgpack/msgpack";
 import {
   CULTNET_ERROR_CODES,
   cultNetSchemas,
+  cultNetWriteNotAuthorizedError,
   encodeCultNetMessageForWire,
   parseCultNetMessage,
   type CultNetErrorMessage,
@@ -81,3 +82,18 @@ test("a code outside the closed enum is refused, as is a malformed details objec
     /cultnet\.error\.v0/,
   );
 });
+
+test("the write_not_authorized factory writes the C# vector's bytes and names the key", () => {
+  const vector = vectors.find((v) => v.name === "write_not_authorized")!;
+  assert.equal(hexOf(wireOf(cultNetWriteNotAuthorizedError("some:key", "not granted"))), vector.messagePackHex);
+  assert.deepEqual(cultNetWriteNotAuthorizedError("some:key", "not granted"), vector.message);
+});
+
+test("a write_not_authorized refusal with no reason, or a blank one, says only the code", () => {
+  assert.equal(cultNetWriteNotAuthorizedError("k").error, "write_not_authorized");
+  assert.equal(cultNetWriteNotAuthorizedError("k", "  ").error, "write_not_authorized");
+  assert.equal(cultNetWriteNotAuthorizedError("k", null).error, "write_not_authorized");
+  // C# string.IsNullOrWhiteSpace counts U+0085 as white space and not U+FEFF; JS trim() disagrees on both.
+  assert.equal(cultNetWriteNotAuthorizedError("k", "\u0085").error, "write_not_authorized");
+  assert.equal(cultNetWriteNotAuthorizedError("k", "\ufeff").error, "write_not_authorized: \ufeff");
+});
diff --git a/packages/cultnet-ts/test/cultnet.test.ts b/packages/cultnet-ts/test/cultnet.test.ts
index 1bcfca0d..37d963a8 100644
--- a/packages/cultnet-ts/test/cultnet.test.ts
+++ b/packages/cultnet-ts/test/cultnet.test.ts
@@ -3078,3 +3078,15 @@ test("document put message omits unset optional keys so the encoded wire has no
   assert.equal(stamped.document.sourceRuntimeId, "r");
   assert.deepEqual(stamped.document.tags, ["t"]);
 });
+
+test("CultNet registry names the canonical schema id of a compatible alias, and none for an unknown or conflicted id", () => {
+  const shape = z.object({ body: z.string() });
+  const owner = defineDocumentType({ type: "p3.owner", schemaId: "p3.owner.v2", compatibleSchemaIds: ["p3.owner.v1"], schema: shape });
+  const first = defineDocumentType({ type: "p3.first", schemaId: "p3.first", compatibleSchemaIds: ["p3.shared"], schema: shape });
+  const second = defineDocumentType({ type: "p3.second", schemaId: "p3.second", compatibleSchemaIds: ["p3.shared"], schema: shape });
+  const registry = new CultNetDocumentRegistry([owner, first, second].map(definition => defineCultNetDocumentBinding({ definition })));
+  assert.equal(registry.canonicalSchemaId("p3.owner.v1"), "p3.owner.v2");
+  assert.equal(registry.canonicalSchemaId("p3.owner.v2"), "p3.owner.v2");
+  assert.equal(registry.canonicalSchemaId("p3.nobody"), undefined);
+  assert.equal(registry.canonicalSchemaId("p3.shared"), undefined, "an id several bindings claim resolves to none");
+});