diff --git a/src/modules/forms/validateSubmission.ts b/src/modules/forms/validateSubmission.ts
index d35feba..a19a228 100644
--- a/src/modules/forms/validateSubmission.ts
+++ b/src/modules/forms/validateSubmission.ts
@@ -34,6 +34,15 @@ export type FormValidationResult =
 /** Field block types that don't carry a submittable value. */
 const NON_DATA_BLOCKS = new Set(['message'])
 
+/**
+ * Keys injected by the captcha widget itself, not by the form definition.
+ * Cloudflare Turnstile adds a hidden <input name="cf-turnstile-response"> after
+ * a successful challenge; reCAPTCHA adds 'g-recaptcha-response'. Since the
+ * plugin drives Turnstile end-to-end, these are legitimate artifacts — they
+ * must not count as "unknown fields" and trip the anti-tampering check.
+ */
+const CAPTCHA_KEYS = new Set(['cf-turnstile-response', 'g-recaptcha-response'])
+
 /** Hard ceiling on a single field's length, independent of the form config. */
 const MAX_FIELD_LENGTH = 5000
 
@@ -95,8 +104,10 @@ export async function validateSubmission(
   }
 
   // Reject outright if the payload carried keys the form doesn't define — a
-  // sign the request wasn't produced by the rendered form.
-  const unknownKeys = Object.keys(data).filter((k) => !known.has(k))
+  // sign the request wasn't produced by the rendered form. Captcha keys are
+  // exempt: the widget injects them into the rendered form, so they're expected,
+  // not tampering.
+  const unknownKeys = Object.keys(data).filter((k) => !known.has(k) && !CAPTCHA_KEYS.has(k))
   if (unknownKeys.length > 0) {
     return { kind: 'unknown_fields', ok: false, reason: 'invalid' }
   }