diff --git a/.changeset/brave-pandas-report.md b/.changeset/brave-pandas-report.md
new file mode 100644
index 0000000..af56263
--- /dev/null
+++ b/.changeset/brave-pandas-report.md
@@ -0,0 +1,21 @@
+---
+'@seamless-auth/types': minor
+---
+
+Publish a union for the WebAuthn error codes.
+
+`WEBAUTHN_ERROR_CODES`, `WebAuthnErrorCodeSchema` and `WebAuthnErrorCode` cover
+the machine-readable codes the auth API returns as the whole of an error body's
+`error` field: `attachment_not_allowed`, `synced_passkey_not_allowed`,
+`authenticator_not_allowed`, `prf_required` and `prf_output_not_allowed`.
+
+This mirrors `OAUTH_ERROR_CODES` and exists for the same reason. A consumer that
+declares its own copy of the list has no way to find out when the API adds a
+code: it degrades to generic messaging and nothing fails anywhere. Checking a
+local map against this union with `Record<WebAuthnErrorCode, true>` turns that
+silent drift into a compile error.
+
+The remaining WebAuthn failures answer with prose rather than a code, so they are
+deliberately absent.
+
+Additive. Nothing changes for a consumer that does not import it.
diff --git a/.changeset/major-chairs-knock.md b/.changeset/major-chairs-knock.md
new file mode 100644
index 0000000..a845151
--- /dev/null
+++ b/.changeset/major-chairs-knock.md
@@ -0,0 +1,2 @@
+---
+---
diff --git a/src/schemas/webauthn/schema.test.ts b/src/schemas/webauthn/schema.test.ts
index da51a33..1a35810 100644
--- a/src/schemas/webauthn/schema.test.ts
+++ b/src/schemas/webauthn/schema.test.ts
@@ -1,9 +1,12 @@
 import { describe, it, expect } from 'vitest';
 import {
+  WEBAUTHN_ERROR_CODES,
   WebAuthnAssertionStartSchema,
+  WebAuthnErrorCodeSchema,
   WebAuthnRegisterFinishSchema,
   WebAuthnRegisterStartQuerySchema,
 } from './schema.js';
+import type { WebAuthnErrorCode } from './schema.js';
 
 describe('WebAuthnRegisterStartQuerySchema', () => {
   it('coerces the string form query values', () => {
@@ -54,3 +57,30 @@ describe('WebAuthnRegisterFinishSchema', () => {
     expect(() => WebAuthnRegisterFinishSchema.parse({})).toThrow();
   });
 });
+
+describe('WebAuthnErrorCodeSchema', () => {
+  it('accepts every published code', () => {
+    for (const code of WEBAUTHN_ERROR_CODES) {
+      expect(WebAuthnErrorCodeSchema.parse(code)).toBe(code);
+    }
+  });
+
+  it('rejects a code it does not publish', () => {
+    expect(() => WebAuthnErrorCodeSchema.parse('not_a_real_code')).toThrow();
+  });
+
+  // The point of publishing the union: a consumer's own map is checked against it
+  // and stops compiling when the API adds a code. This asserts the union and the
+  // list stay in step, which is what makes that check meaningful.
+  it('keeps the union and the list in step', () => {
+    const everyCode: Record<WebAuthnErrorCode, true> = {
+      attachment_not_allowed: true,
+      synced_passkey_not_allowed: true,
+      authenticator_not_allowed: true,
+      prf_required: true,
+      prf_output_not_allowed: true,
+    };
+
+    expect(Object.keys(everyCode).sort()).toEqual([...WEBAUTHN_ERROR_CODES].sort());
+  });
+});
diff --git a/src/schemas/webauthn/schema.ts b/src/schemas/webauthn/schema.ts
index 6d84468..1c9bf6a 100644
--- a/src/schemas/webauthn/schema.ts
+++ b/src/schemas/webauthn/schema.ts
@@ -80,3 +80,39 @@ export const WebAuthnTokenSuccessSchema = z.object({
 });
 
 export type WebAuthnTokenSuccessResponse = z.infer<typeof WebAuthnTokenSuccessSchema>;
+
+/**
+ * Machine-readable codes the auth API returns as the whole of an error body's
+ * `error` field, for WebAuthn failures a client can act on.
+ *
+ * Published here for the same reason as `OAUTH_ERROR_CODES`: a consumer that
+ * declares its own copy has no way to find out when the API adds a code, and
+ * degrades to generic messaging with nothing failing anywhere. Checking a local
+ * map against this union with `Record<WebAuthnErrorCode, true>` turns that into
+ * a compile error.
+ *
+ * The remaining WebAuthn failures answer with prose rather than a code, so they
+ * are deliberately absent rather than forgotten.
+ */
+export const WEBAUTHN_ERROR_CODES = [
+  /** Registration asked for an attachment the deployment's policy does not allow. */
+  'attachment_not_allowed',
+  /** The credential is backup eligible and the deployment blocks synced passkeys. */
+  'synced_passkey_not_allowed',
+  /** The authenticator model is denied, or absent from a non-empty allow list. */
+  'authenticator_not_allowed',
+  /** Registration required a PRF-capable credential and the one offered was not. */
+  'prf_required',
+  /** An assertion carried PRF output, which must never leave the client. */
+  'prf_output_not_allowed',
+] as const;
+
+export const WebAuthnErrorCodeSchema = z.enum(WEBAUTHN_ERROR_CODES);
+
+export type WebAuthnErrorCode = z.infer<typeof WebAuthnErrorCodeSchema>;
+
+export const WebAuthnErrorResponseSchema = z.object({
+  error: z.string(),
+});
+
+export type WebAuthnErrorResponse = z.infer<typeof WebAuthnErrorResponseSchema>;