diff --git a/src/app/(campaign)/campanha/webauthn/login/route.ts b/src/app/(campaign)/campanha/webauthn/login/route.ts
index 53fd2ccb..ac9124df 100644
--- a/src/app/(campaign)/campanha/webauthn/login/route.ts
+++ b/src/app/(campaign)/campanha/webauthn/login/route.ts
@@ -2,6 +2,7 @@ import config from '@payload-config'
 import { NextResponse } from 'next/server'
 import { getPayload } from 'payload'
 
+import { CAMPAIGN_BIOMETRIC_LOGIN_ERROR_MESSAGE } from '@/lib/campaignAuthCopy'
 import type { CampaignWebAuthnLoginResponse } from '@/lib/campaignWebAuthn'
 import { campaignWebAuthnLoginSchema } from '@/lib/schemas/campaignWebAuthn'
 import { setCampaignAuthCookie } from '@/utilities/campaignAuth'
@@ -32,7 +33,7 @@ export const POST = campaignJsonMutationRoute(
       CAMPAIGN_WEBAUTHN_ACCOUNT_UNAVAILABLE_MESSAGE,
       CAMPAIGN_WEBAUTHN_ACCOUNT_LOCKED_MESSAGE,
     ],
-    genericMessage: 'Não foi possível entrar com a biometria. Use sua senha.',
+    genericMessage: CAMPAIGN_BIOMETRIC_LOGIN_ERROR_MESSAGE,
   },
   async ({ credential }): Promise<NextResponse<CampaignWebAuthnLoginResponse>> => {
     const payload = await getPayload({ config })
diff --git a/src/components/campaign/auth/CampaignBiometricLoginButton.tsx b/src/components/campaign/auth/CampaignBiometricLoginButton.tsx
index d4a64aeb..d3963759 100644
--- a/src/components/campaign/auth/CampaignBiometricLoginButton.tsx
+++ b/src/components/campaign/auth/CampaignBiometricLoginButton.tsx
@@ -8,11 +8,12 @@ import { useCampaignBiometricsAvailable } from '@/components/campaign/auth/useCa
 import { Button } from '@/components/ui/button'
 import { FieldError } from '@/components/ui/field'
 import { Spinner } from '@/components/ui/Spinner'
-import { CAMPAIGN_BIOMETRIC_LOGIN_LABEL } from '@/lib/campaignAuthCopy'
+import {
+  CAMPAIGN_BIOMETRIC_LOGIN_ERROR_MESSAGE,
+  CAMPAIGN_BIOMETRIC_LOGIN_LABEL,
+} from '@/lib/campaignAuthCopy'
 import { CampaignWebAuthnError } from '@/lib/campaignWebAuthnSupport'
 
-const FALLBACK_ERROR_MESSAGE = 'Não foi possível entrar com a biometria. Use sua senha.'
-
 /**
  * Password-less sign-in on the login screen. It is mounted only when the server
  * says the relying party is configured for this origin (Vercel previews are
@@ -47,7 +48,9 @@ export const CampaignBiometricLoginButton = () => {
       // A cancelled prompt is a decision, not a failure: say it plainly and
       // leave the password form untouched.
       setErrorMessage(
-        error instanceof CampaignWebAuthnError ? error.message : FALLBACK_ERROR_MESSAGE,
+        error instanceof CampaignWebAuthnError
+          ? error.message
+          : CAMPAIGN_BIOMETRIC_LOGIN_ERROR_MESSAGE,
       )
     }
     setPending(false)
diff --git a/src/lib/campaignAuthCopy.ts b/src/lib/campaignAuthCopy.ts
index 9b8f821d..25079e2f 100644
--- a/src/lib/campaignAuthCopy.ts
+++ b/src/lib/campaignAuthCopy.ts
@@ -70,6 +70,14 @@ export const CAMPAIGN_BIOMETRIC_ENROLLED_MESSAGE =
 export const CAMPAIGN_BIOMETRIC_ENROLL_ERROR_MESSAGE =
   'Não foi possível cadastrar este aparelho. Tente novamente.'
 
+/**
+ * Twin of the one above for the sign-in half: the route's `genericMessage` and
+ * the login button's fallback are the same sentence in the same slot, reached
+ * depending on whether the server answered an envelope or the network broke.
+ */
+export const CAMPAIGN_BIOMETRIC_LOGIN_ERROR_MESSAGE =
+  'Não foi possível entrar com a biometria. Use sua senha.'
+
 export const CAMPAIGN_BIOMETRIC_REMOVED_MESSAGE =
   'Aparelho removido. Ele não entra mais com biometria.'
 
diff --git a/src/lib/campaignJsonRequest.ts b/src/lib/campaignJsonRequest.ts
index c1eb094b..78df5714 100644
--- a/src/lib/campaignJsonRequest.ts
+++ b/src/lib/campaignJsonRequest.ts
@@ -1,8 +1,9 @@
 /**
- * Transport half of the list cells' quick edits: the preamble every control
- * repeated around `fetch` (same-origin credentials, JSON headers, the cast of
- * the discriminated response). Deliberately knows nothing about revert, retry
- * or debounce — that is `useCampaignCellAutosave`'s, or the caller's.
+ * Transport half of every `/campanha` JSON POST — the list cells' quick edits
+ * and the WebAuthn ceremonies: the preamble each one repeated around `fetch`
+ * (same-origin credentials, JSON headers, the cast of the discriminated
+ * response). Deliberately knows nothing about revert, retry or debounce — that
+ * is `useCampaignCellAutosave`'s, or the caller's.
  *
  * `ok` travels next to the payload because a route can answer a real body with
  * a non-200 status (`engagement-level` returns its `blocked` state as 409), so
diff --git a/src/lib/campaignWebAuthnClient.ts b/src/lib/campaignWebAuthnClient.ts
index 17ca191b..1cf6f7d1 100644
--- a/src/lib/campaignWebAuthnClient.ts
+++ b/src/lib/campaignWebAuthnClient.ts
@@ -32,22 +32,20 @@ import {
 import { CampaignWebAuthnError } from '@/lib/campaignWebAuthnSupport'
 
 /**
- * Transport is `postCampaignJson`; this adds the two things the ceremonies need
- * on top of it. The status is dropped on purpose — a 403/409/500 from these
- * routes still carries the typed envelope, so the discriminant decides — and a
- * response that is not JSON at all (proxy error page, offline) surfaces as one
- * sentence instead of a `SyntaxError` about position 0. A rejection that is not
- * a parse failure is a network failure and is rethrown untouched.
+ * Transport is `postCampaignJson`; the only thing added here is dropping `ok`,
+ * because a 403/409/500 from these routes still carries the typed envelope and
+ * the discriminant is what decides. The `{}` default is load-bearing: the route
+ * shell always parses a body, so omitting it would answer 400.
+ *
+ * Nothing is caught. A transport failure — offline, or a proxy answering HTML
+ * that `response.json()` rejects — bubbles as a plain `Error`, which is the
+ * channel `CampaignWebAuthnError` reserves for "the island shows its own copy".
+ * An earlier version rewrote that case as "Resposta inválida do servidor." and
+ * the sentence never reached a screen, because all three islands read `message`
+ * only off a `CampaignWebAuthnError`.
  */
-const postJson = async <Response>(path: string, body?: unknown): Promise<Response> => {
-  try {
-    const { payload } = await postCampaignJson<Response>(path, body ?? {})
-    return payload
-  } catch (error) {
-    if (error instanceof SyntaxError) throw new Error('Resposta inválida do servidor.')
-    throw error
-  }
-}
+const postJson = async <Response>(path: string, body?: unknown): Promise<Response> =>
+  (await postCampaignJson<Response>(path, body ?? {})).payload
 
 /**
  * `navigator.credentials` rejects with DOM exceptions whose names are the only
diff --git a/src/utilities/campaignJsonMutationRoute.ts b/src/utilities/campaignJsonMutationRoute.ts
index ea4028db..e36af26c 100644
--- a/src/utilities/campaignJsonMutationRoute.ts
+++ b/src/utilities/campaignJsonMutationRoute.ts
@@ -73,8 +73,9 @@ const campaignJsonMutationErrorResponse = (
 
 /**
  * Builds the `POST` of a `/campanha/**` JSON mutation route (the list cells'
- * quick edits): same-origin check, body parse, schema parse and error mapping,
- * leaving each route with its schema, its action call and its success payload.
+ * quick edits, and the WebAuthn ceremonies): same-origin check, body parse,
+ * schema parse and error mapping, leaving each route with its schema, its
+ * action call and its success payload.
  *
  * It is a wrapper and not a `guard(request)` helper because the origin check is
  * the kind of line a sixth route forgets, and forgetting it fails OPEN — a