diff --git a/lib/utils/user-info-extraction-utils.js b/lib/utils/user-info-extraction-utils.js
--- a/lib/utils/user-info-extraction-utils.js
+++ b/lib/utils/user-info-extraction-utils.js
@@ -6,16 +6,32 @@
 import type { CallSingleKeyserverEndpointResponse } from '../keyserver-conn/call-single-keyserver-endpoint.js';
 import { mixedRawThreadInfoValidator } from '../permissions/minimally-encoded-raw-thread-info-validators.js';
 import type { Endpoint } from '../types/endpoints.js';
+import type { MixedRawThreadInfos } from '../types/thread-types.js';
 import { userInfoValidator } from '../types/user-types.js';
+import type { UserInfo } from '../types/user-types.js';
 import { endpointValidators } from '../types/validators/endpoint-validators.js';
 import { extractUserIDsFromPayload } from '../utils/conversion-utils.js';
 import { tID, tShape } from '../utils/validation-utils.js';
 
+type AdditionalCookieChange = {
+  +threadInfos: MixedRawThreadInfos,
+  +userInfos: $ReadOnlyArray<UserInfo>,
+  +cookieInvalidated?: boolean,
+  sessionID?: string,
+  cookie?: string,
+};
+
+type AdditionalResponseFields = {
+  +cookieChange: AdditionalCookieChange,
+  +error?: string,
+  +payload?: Object,
+  +success: boolean,
+};
+
 function extendResponderValidatorBase(inputValidator: TType<*>): TType<*> {
-  return tShape({
-    ...inputValidator.meta.props,
+  const additionalResponseFieldsValidator = tShape<AdditionalResponseFields>({
     cookieChange: t.maybe(
-      tShape({
+      tShape<AdditionalCookieChange>({
         threadInfos: t.dict(tID, mixedRawThreadInfoValidator),
         userInfos: t.list(userInfoValidator),
         cookieInvalidated: t.maybe(t.Boolean),
@@ -27,6 +43,23 @@
     payload: t.maybe(t.Object),
     success: t.maybe(t.Boolean),
   });
+
+  if (inputValidator.meta.kind === 'union') {
+    const newTypes = [];
+    for (const innerValidator of inputValidator.meta.types) {
+      const newInnerValidator = extendResponderValidatorBase(innerValidator);
+      newTypes.push(newInnerValidator);
+    }
+    // $FlowIgnore
+    inputValidator.meta.types = newTypes;
+
+    return inputValidator;
+  }
+
+  return tShape({
+    ...inputValidator.meta.props,
+    ...additionalResponseFieldsValidator.meta.props,
+  });
 }
 
 const extendResponderValidator = _memoize(extendResponderValidatorBase);