diff --git a/keyserver/src/utils/validation-utils.js b/keyserver/src/utils/validation-utils.js
--- a/keyserver/src/utils/validation-utils.js
+++ b/keyserver/src/utils/validation-utils.js
@@ -1,8 +1,6 @@
 // @flow
 
-import _mapKeys from 'lodash/fp/mapKeys.js';
-import _mapValues from 'lodash/fp/mapValues.js';
-import type { TType, TInterface } from 'tcomb';
+import type { TType } from 'tcomb';
 
 import type { PolicyType } from 'lib/facts/policies.js';
 import {
@@ -17,16 +15,17 @@
   tPlatform,
   tPlatformDetails,
   assertWithValidator,
-  tID,
+  convertToNewIDSchema,
+  keyserverPrefixID,
+  convertClientIDsToServerIDs,
+  convertObject,
+  convertServerIDsToClientIDs,
 } from 'lib/utils/validation-utils.js';
 
 import { fetchNotAcknowledgedPolicies } from '../fetchers/policy-acknowledgment-fetchers.js';
 import { verifyClientSupported } from '../session/version.js';
 import type { Viewer } from '../session/viewer.js';
 
-const convertToNewIDSchema = false;
-const keyserverPrefixID = '256';
-
 async function validateInput<T>(
   viewer: Viewer,
   inputValidator: TType<T>,
@@ -42,11 +41,15 @@
     !isWebPlatform(viewer.platformDetails?.platform) &&
     convertToNewIDSchema
   ) {
-    return convertClientIDsToServerIDs(
-      keyserverPrefixID,
-      inputValidator,
-      convertedInput,
-    );
+    try {
+      return convertClientIDsToServerIDs(
+        keyserverPrefixID,
+        inputValidator,
+        convertedInput,
+      );
+    } catch (err) {
+      throw new ServerError(err.message);
+    }
   }
 
   return convertedInput;
@@ -80,39 +83,6 @@
   return data;
 }
 
-function convertServerIDsToClientIDs<T>(
-  serverPrefixID: string,
-  outputValidator: TType<T>,
-  data: T,
-): T {
-  const conversionFunction = id => {
-    if (id.indexOf('|') !== -1) {
-      console.warn(`Server id '${id}' already has a prefix`);
-      return id;
-    }
-    return `${serverPrefixID}|${id}`;
-  };
-
-  return convertObject(outputValidator, data, [tID], conversionFunction);
-}
-
-function convertClientIDsToServerIDs<T>(
-  serverPrefixID: string,
-  outputValidator: TType<T>,
-  data: T,
-): T {
-  const prefix = serverPrefixID + '|';
-  const conversionFunction = id => {
-    if (id.startsWith(prefix)) {
-      return id.substr(prefix.length);
-    }
-
-    throw new ServerError('invalid_client_id_prefix');
-  };
-
-  return convertObject(outputValidator, data, [tID], conversionFunction);
-}
-
 function checkInputValidator<T>(inputValidator: TType<T>, input: mixed): T {
   if (inputValidator.is(input)) {
     return assertWithValidator(input, inputValidator);
@@ -227,87 +197,6 @@
   return null;
 }
 
-function convertObject<T, I>(
-  validator: TType<I>,
-  input: I,
-  typesToConvert: $ReadOnlyArray<TType<T>>,
-  conversionFunction: T => T,
-): I {
-  if (input === null || input === undefined) {
-    return input;
-  }
-
-  // While they should be the same runtime object,
-  // `TValidator` is `TType<T>` and `validator` is `TType<I>`.
-  // Having them have different types allows us to use `assertWithValidator`
-  // to change `input` flow type
-  const TValidator = typesToConvert[typesToConvert.indexOf(validator)];
-  if (TValidator && TValidator.is(input)) {
-    const TInput = assertWithValidator(input, TValidator);
-    const converted = conversionFunction(TInput);
-    return assertWithValidator(converted, validator);
-  }
-
-  if (validator.meta.kind === 'maybe' || validator.meta.kind === 'subtype') {
-    return convertObject(
-      validator.meta.type,
-      input,
-      typesToConvert,
-      conversionFunction,
-    );
-  }
-  if (validator.meta.kind === 'interface' && typeof input === 'object') {
-    const recastValidator: TInterface<typeof input> = (validator: any);
-    const result = {};
-    for (const key in input) {
-      const innerValidator = recastValidator.meta.props[key];
-      result[key] = convertObject(
-        innerValidator,
-        input[key],
-        typesToConvert,
-        conversionFunction,
-      );
-    }
-    return assertWithValidator(result, recastValidator);
-  }
-  if (validator.meta.kind === 'union') {
-    for (const innerValidator of validator.meta.types) {
-      if (innerValidator.is(input)) {
-        return convertObject(
-          innerValidator,
-          input,
-          typesToConvert,
-          conversionFunction,
-        );
-      }
-    }
-    return input;
-  }
-  if (validator.meta.kind === 'list' && Array.isArray(input)) {
-    const innerValidator = validator.meta.type;
-    return (input.map(value =>
-      convertObject(innerValidator, value, typesToConvert, conversionFunction),
-    ): any);
-  }
-  if (validator.meta.kind === 'dict' && typeof input === 'object') {
-    const domainValidator = validator.meta.domain;
-    const codomainValidator = validator.meta.codomain;
-    if (typesToConvert.includes(domainValidator)) {
-      input = _mapKeys(key => conversionFunction(key))(input);
-    }
-    return _mapValues(value =>
-      convertObject(
-        codomainValidator,
-        value,
-        typesToConvert,
-        conversionFunction,
-      ),
-    )(input);
-  }
-
-  return input;
-}
-
 async function policiesValidator(
   viewer: Viewer,
   policies: $ReadOnlyArray<PolicyType>,
@@ -339,8 +228,5 @@
   sanitizeInput,
   findFirstInputMatchingValidator,
   checkClientSupported,
-  convertServerIDsToClientIDs,
-  convertClientIDsToServerIDs,
-  convertObject,
   policiesValidator,
 };
diff --git a/keyserver/src/utils/validation-utils.test.js b/keyserver/src/utils/validation-utils.test.js
--- a/keyserver/src/utils/validation-utils.test.js
+++ b/keyserver/src/utils/validation-utils.test.js
@@ -2,14 +2,9 @@
 
 import t from 'tcomb';
 
-import { tPassword, tShape, tID } from 'lib/utils/validation-utils.js';
+import { tPassword, tShape } from 'lib/utils/validation-utils.js';
 
-import {
-  convertServerIDsToClientIDs,
-  sanitizeInput,
-  redactedString,
-  convertClientIDsToServerIDs,
-} from './validation-utils.js';
+import { sanitizeInput, redactedString } from './validation-utils.js';
 
 describe('sanitization', () => {
   it('should redact a string', () => {
@@ -77,46 +72,3 @@
     expect(sanitizeInput(validator, object)).toStrictEqual(redacted);
   });
 });
-
-describe('id conversion', () => {
-  it('should convert string id', () => {
-    const validator = tShape({ id: tID });
-    const serverData = { id: '1' };
-    const clientData = { id: '0|1' };
-
-    expect(
-      convertServerIDsToClientIDs('0', validator, serverData),
-    ).toStrictEqual(clientData);
-    expect(
-      convertClientIDsToServerIDs('0', validator, clientData),
-    ).toStrictEqual(serverData);
-  });
-
-  it('should convert a complex type', () => {
-    const validator = tShape({ ids: t.dict(tID, t.list(tID)) });
-    const serverData = { ids: { '1': ['11', '12'], '2': [], '3': ['13'] } };
-    const clientData = {
-      ids: { '0|1': ['0|11', '0|12'], '0|2': [], '0|3': ['0|13'] },
-    };
-
-    expect(
-      convertServerIDsToClientIDs('0', validator, serverData),
-    ).toStrictEqual(clientData);
-    expect(
-      convertClientIDsToServerIDs('0', validator, clientData),
-    ).toStrictEqual(serverData);
-  });
-
-  it('should convert a refinement', () => {
-    const validator = t.refinement(tID, () => true);
-    const serverData = '1';
-    const clientData = '0|1';
-
-    expect(
-      convertServerIDsToClientIDs('0', validator, serverData),
-    ).toStrictEqual(clientData);
-    expect(
-      convertClientIDsToServerIDs('0', validator, clientData),
-    ).toStrictEqual(serverData);
-  });
-});
diff --git a/lib/utils/validation-utils.js b/lib/utils/validation-utils.js
--- a/lib/utils/validation-utils.js
+++ b/lib/utils/validation-utils.js
@@ -1,6 +1,8 @@
 // @flow
 
 import invariant from 'invariant';
+import _mapKeys from 'lodash/fp/mapKeys.js';
+import _mapValues from 'lodash/fp/mapValues.js';
 import t from 'tcomb';
 import type {
   TStructProps,
@@ -101,6 +103,123 @@
   return (data: any);
 }
 
+const convertToNewIDSchema = false;
+const keyserverPrefixID = '256';
+
+function convertServerIDsToClientIDs<T>(
+  serverPrefixID: string,
+  outputValidator: TType<T>,
+  data: T,
+): T {
+  const conversionFunction = id => {
+    if (id.indexOf('|') !== -1) {
+      console.warn(`Server id '${id}' already has a prefix`);
+      return id;
+    }
+    return `${serverPrefixID}|${id}`;
+  };
+
+  return convertObject(outputValidator, data, [tID], conversionFunction);
+}
+
+function convertClientIDsToServerIDs<T>(
+  serverPrefixID: string,
+  outputValidator: TType<T>,
+  data: T,
+): T {
+  const prefix = serverPrefixID + '|';
+  const conversionFunction = id => {
+    if (id.startsWith(prefix)) {
+      return id.substr(prefix.length);
+    }
+
+    throw new Error('invalid_client_id_prefix');
+  };
+
+  return convertObject(outputValidator, data, [tID], conversionFunction);
+}
+
+function convertObject<T, I>(
+  validator: TType<I>,
+  input: I,
+  typesToConvert: $ReadOnlyArray<TType<T>>,
+  conversionFunction: T => T,
+): I {
+  if (input === null || input === undefined) {
+    return input;
+  }
+
+  // While they should be the same runtime object,
+  // `TValidator` is `TType<T>` and `validator` is `TType<I>`.
+  // Having them have different types allows us to use `assertWithValidator`
+  // to change `input` flow type
+  const TValidator = typesToConvert[typesToConvert.indexOf(validator)];
+  if (TValidator && TValidator.is(input)) {
+    const TInput = assertWithValidator(input, TValidator);
+    const converted = conversionFunction(TInput);
+    return assertWithValidator(converted, validator);
+  }
+
+  if (validator.meta.kind === 'maybe' || validator.meta.kind === 'subtype') {
+    return convertObject(
+      validator.meta.type,
+      input,
+      typesToConvert,
+      conversionFunction,
+    );
+  }
+  if (validator.meta.kind === 'interface' && typeof input === 'object') {
+    const recastValidator: TInterface<typeof input> = (validator: any);
+    const result = {};
+    for (const key in input) {
+      const innerValidator = recastValidator.meta.props[key];
+      result[key] = convertObject(
+        innerValidator,
+        input[key],
+        typesToConvert,
+        conversionFunction,
+      );
+    }
+    return assertWithValidator(result, recastValidator);
+  }
+  if (validator.meta.kind === 'union') {
+    for (const innerValidator of validator.meta.types) {
+      if (innerValidator.is(input)) {
+        return convertObject(
+          innerValidator,
+          input,
+          typesToConvert,
+          conversionFunction,
+        );
+      }
+    }
+    return input;
+  }
+  if (validator.meta.kind === 'list' && Array.isArray(input)) {
+    const innerValidator = validator.meta.type;
+    return (input.map(value =>
+      convertObject(innerValidator, value, typesToConvert, conversionFunction),
+    ): any);
+  }
+  if (validator.meta.kind === 'dict' && typeof input === 'object') {
+    const domainValidator = validator.meta.domain;
+    const codomainValidator = validator.meta.codomain;
+    if (typesToConvert.includes(domainValidator)) {
+      input = _mapKeys(key => conversionFunction(key))(input);
+    }
+    return _mapValues(value =>
+      convertObject(
+        codomainValidator,
+        value,
+        typesToConvert,
+        conversionFunction,
+      ),
+    )(input);
+  }
+
+  return input;
+}
+
 export {
   tBool,
   tString,
@@ -123,4 +242,9 @@
   tMediaMessageVideo,
   tMediaMessageMedia,
   assertWithValidator,
+  convertToNewIDSchema,
+  keyserverPrefixID,
+  convertClientIDsToServerIDs,
+  convertServerIDsToClientIDs,
+  convertObject,
 };
diff --git a/lib/utils/validation-utils.test.js b/lib/utils/validation-utils.test.js
--- a/lib/utils/validation-utils.test.js
+++ b/lib/utils/validation-utils.test.js
@@ -1,9 +1,15 @@
 // @flow
 
+import t from 'tcomb';
+
 import {
   tMediaMessagePhoto,
   tMediaMessageVideo,
   tNumEnum,
+  tShape,
+  tID,
+  convertServerIDsToClientIDs,
+  convertClientIDsToServerIDs,
 } from './validation-utils.js';
 import { threadTypes } from '../types/thread-types-enum.js';
 import { values } from '../utils/objects.js';
@@ -140,3 +146,46 @@
     });
   });
 });
+
+describe('id conversion', () => {
+  it('should convert string id', () => {
+    const validator = tShape({ id: tID });
+    const serverData = { id: '1' };
+    const clientData = { id: '0|1' };
+
+    expect(
+      convertServerIDsToClientIDs('0', validator, serverData),
+    ).toStrictEqual(clientData);
+    expect(
+      convertClientIDsToServerIDs('0', validator, clientData),
+    ).toStrictEqual(serverData);
+  });
+
+  it('should convert a complex type', () => {
+    const validator = tShape({ ids: t.dict(tID, t.list(tID)) });
+    const serverData = { ids: { '1': ['11', '12'], '2': [], '3': ['13'] } };
+    const clientData = {
+      ids: { '0|1': ['0|11', '0|12'], '0|2': [], '0|3': ['0|13'] },
+    };
+
+    expect(
+      convertServerIDsToClientIDs('0', validator, serverData),
+    ).toStrictEqual(clientData);
+    expect(
+      convertClientIDsToServerIDs('0', validator, clientData),
+    ).toStrictEqual(serverData);
+  });
+
+  it('should convert a refinement', () => {
+    const validator = t.refinement(tID, () => true);
+    const serverData = '1';
+    const clientData = '0|1';
+
+    expect(
+      convertServerIDsToClientIDs('0', validator, serverData),
+    ).toStrictEqual(clientData);
+    expect(
+      convertClientIDsToServerIDs('0', validator, clientData),
+    ).toStrictEqual(serverData);
+  });
+});