diff --git a/application/api/.eslintrc.json b/application/api/.eslintrc.json
index 4f0fbb07..bb85d170 100644
--- a/application/api/.eslintrc.json
+++ b/application/api/.eslintrc.json
@@ -10,6 +10,7 @@
     "db": "readonly",
     "bus": "readonly",
     "domain": "readonly",
-    "metarhia": "readonly"
+    "metarhia": "readonly",
+    "DomainError": "readonly"
   }
 }
diff --git a/application/api/auth.2/error.js b/application/api/auth.2/error.js
new file mode 100644
index 00000000..d658cb99
--- /dev/null
+++ b/application/api/auth.2/error.js
@@ -0,0 +1,16 @@
+({
+  AuthError: class AuthError extends DomainError {
+    static ALREADY_EXISTS() {
+      return new AuthError(409, { message: 'User already exists' });
+    }
+    static INVALID_CREDENTIALS() {
+      return new AuthError(400, { message: 'Incorrect logic or password' });
+    }
+  },
+  onError(error) {
+    const { AuthError } = api.auth.error;
+    if (error instanceof AuthError) return error;
+    // handle unexpected, non domain error and return based on error handling logic
+    return error;
+  },
+});
diff --git a/application/api/auth.2/register.js b/application/api/auth.2/register.js
index 1547ff22..6ac6ef9e 100644
--- a/application/api/auth.2/register.js
+++ b/application/api/auth.2/register.js
@@ -1,8 +1,10 @@
 ({
   access: 'public',
   method: async ({ login, password, fullName }) => {
+    const { AuthError } = api.auth.error;
     const hash = await metarhia.metautil.hashPassword(password);
-    await api.auth.provider.registerUser(login, hash, fullName);
+    const created = await api.auth.provider.registerUser(login, hash, fullName);
+    if (!created) return AuthError.ALREADY_EXISTS();
     const token = await context.client.startSession();
     return { status: 'success', token };
   },
diff --git a/application/api/auth.2/signin.js b/application/api/auth.2/signin.js
index 742ce384..d3c61632 100644
--- a/application/api/auth.2/signin.js
+++ b/application/api/auth.2/signin.js
@@ -1,11 +1,12 @@
 ({
   access: 'public',
   method: async ({ login, password }) => {
+    const { AuthError } = api.auth.error;
     const user = await api.auth.provider.getUser(login);
-    if (!user) throw new Error('Incorrect login or password');
+    if (!user) return AuthError.INVALID_CREDENTIALS();
     const { accountId, password: hash } = user;
     const valid = await metarhia.metautil.validatePassword(password, hash);
-    if (!valid) throw new Error('Incorrect login or password');
+    if (!valid) return AuthError.INVALID_CREDENTIALS();
     console.log(`Logged user: ${login}`);
     const token = api.auth.provider.generateToken();
     const data = { accountId: user.accountId };
diff --git a/application/api/example.1/add.js b/application/api/example.1/add.js
index 080f3d2e..ec791eed 100644
--- a/application/api/example.1/add.js
+++ b/application/api/example.1/add.js
@@ -5,9 +5,20 @@
   },
 
   method: async ({ a, b }) => {
+    const { ExampleError } = api.example.error;
+    if (typeof a !== 'number') return ExampleError.INVALID_ARG_A();
+    if (typeof b !== 'number') return ExampleError.INVALID_ARG_B();
     const result = a + b;
     return result;
   },
 
   returns: 'number',
+
+  onError(error) {
+    const { ExampleError } = api.example.error;
+    // in theory there's no need to trigger onError handler
+    if (error instanceof ExampleError) return error;
+    // handle unexpected, non domain error and return based on error handling logic
+    return error;
+  },
 });
diff --git a/application/api/example.1/citiesByCountry.js b/application/api/example.1/citiesByCountry.js
index 95c89f8f..b9e8fb9e 100644
--- a/application/api/example.1/citiesByCountry.js
+++ b/application/api/example.1/citiesByCountry.js
@@ -1,4 +1,6 @@
 async ({ countryId }) => {
+  const { ExampleError } = api.example.error;
+  if (!countryId) return ExampleError.MISSING_COUNTRY_ID();
   const fields = ['cityId', 'name'];
   const where = { countryId };
   const data = await db.pg.select('City', fields, where);
diff --git a/application/api/example.1/error.js b/application/api/example.1/error.js
index e89dd604..a8ae9007 100644
--- a/application/api/example.1/error.js
+++ b/application/api/example.1/error.js
@@ -1 +1,13 @@
-async () => new Error('Return error');
+({
+  ExampleError: class ExampleError extends DomainError {
+    static INVALID_ARG_A() {
+      return new ExampleError(400, { message: 'Invalid argument: a' });
+    }
+    static INVALID_ARG_B() {
+      return new ExampleError(400, { message: 'Invalid argument: b' });
+    }
+    static MISSING_COUNTRY_ID() {
+      return new ExampleError(400, { message: 'Missing country id' });
+    }
+  },
+});
diff --git a/types/global.d.ts b/types/global.d.ts
index c30a8220..c9569bd3 100644
--- a/types/global.d.ts
+++ b/types/global.d.ts
@@ -23,3 +23,27 @@ declare global {
     const pg: Database;
   }
 }
+
+export interface ErrorOptions {
+  code?: number | string;
+  cause?: Error;
+}
+
+export class Error extends global.Error {
+  constructor(message: string, options?: number | string | ErrorOptions);
+  message: string;
+  stack: string;
+  code?: number | string;
+  cause?: Error;
+}
+
+type Errors = Record<string, string>;
+
+export class DomainError extends Error {
+  constructor(code?: string, options?: number | string | ErrorOptions);
+  message: string;
+  stack: string;
+  code?: number | string;
+  cause?: Error;
+  toError(errors: Errors): Error;
+}