diff --git a/docs/product/error-contract.md b/docs/product/error-contract.md
new file mode 100644
index 0000000..d1def36
--- /dev/null
+++ b/docs/product/error-contract.md
@@ -0,0 +1,22 @@
+# Harbor error contract v1
+
+Public API failures use one envelope:
+
+```json
+{
+  "success": false,
+  "error": {
+    "code": "PROVIDER_UNAVAILABLE",
+    "message": "Провайдер подписки временно недоступен.",
+    "retryable": true,
+    "correlationId": "6f1a63de-30f9-4dc5-b8ce-38d38c164fe3",
+    "details": "HTTP 503"
+  }
+}
+```
+
+`code`, Russian user copy, HTTP status and retry policy come from `src/shared/errors.js`. The browser maps copy and retry behavior by `code`; it does not display server-provided `details`. Unknown failures use `UNKNOWN`, never expose the raw exception, and always receive a correlation reference. Server logs use the same reference and redact complete HTTP(S) URLs.
+
+Errors are local operation results, not canonical state replacements. A failed apply keeps the previous snapshot; in particular, server existence is validated before `desiredServerId` is persisted. Frontend errors are shown beside subscription or connection controls. Only retryable codes expose `Повторить`.
+
+This is a coordinated API change: old frontends do not understand the object-valued `error` field, so frontend and control plane must be deployed together. Persisted files and volumes are unchanged. Rollback is code-only and requires no data migration.
diff --git a/src/server/dataplaneClient.js b/src/server/dataplaneClient.js
index 7499fba..fddce30 100644
--- a/src/server/dataplaneClient.js
+++ b/src/server/dataplaneClient.js
@@ -1,4 +1,5 @@
 import http from 'node:http';
+import { HarborError } from '../shared/errors.js';
 
 function request(socketPath, pathname, method = 'GET') {
   return new Promise((resolve, reject) => {
@@ -27,8 +28,15 @@ function request(socketPath, pathname, method = 'GET') {
 export function createDataplaneClient(socketPath, send = request) {
   let current = { running: false, startedAt: null };
   const update = async (pathname, method) => {
-    current = await send(socketPath, pathname, method);
-    return current;
+    try {
+      current = await send(socketPath, pathname, method);
+      return current;
+    } catch (cause) {
+      if (pathname === '/apply' || pathname === '/restart') {
+        throw new HarborError('PROCESS_START_FAILED', { cause });
+      }
+      throw cause;
+    }
   };
 
   return {
diff --git a/src/server/gatewayPresence.js b/src/server/gatewayPresence.js
index f43c1bd..7c6fbb5 100644
--- a/src/server/gatewayPresence.js
+++ b/src/server/gatewayPresence.js
@@ -1,5 +1,6 @@
 import crypto from 'node:crypto';
 import fs from 'node:fs';
+import { HarborError } from '../shared/errors.js';
 
 const NONCE_RE = /^[a-f0-9]{32}$/;
 const PROOF_RE = /^[a-f0-9]{64}$/;
@@ -51,9 +52,7 @@ function presenceProof(subscriptionUrl, nonce, gatewayId) {
 
 export function buildGatewayPresence({ appMode, subscriptionUrl, gatewayId, nonce }) {
   if (!NONCE_RE.test(String(nonce || ''))) {
-    const error = new Error('Некорректный nonce');
-    error.statusCode = 400;
-    throw error;
+    throw new HarborError('REQUEST_INVALID');
   }
 
   const subscription = String(subscriptionUrl || '').trim();
diff --git a/src/server/index.js b/src/server/index.js
index 7d45261..8e50703 100644
--- a/src/server/index.js
+++ b/src/server/index.js
@@ -1,3 +1,4 @@
+import crypto from 'node:crypto';
 import fs from 'node:fs';
 import http from 'node:http';
 import path from 'node:path';
@@ -27,6 +28,7 @@ import {
   normalizeStoredState,
   withStateV0Compatibility,
 } from '../shared/contracts/state.js';
+import { HarborError, normalizeHarborError } from '../shared/errors.js';
 
 const MAX_BODY_BYTES = 1_000_000;
 const SUBSCRIPTION_REFRESH_INTERVAL_MS = 15 * 60 * 1000;
@@ -91,10 +93,11 @@ async function withOperation(kind, operation) {
     updateStoredState((state) => state);
     return result;
   } catch (error) {
+    const harborError = normalizeHarborError(error);
     operationState = {
       ...operationState,
       status: 'failed',
-      error: error?.message || String(error),
+      error: harborError.message,
     };
     updateStoredState((state) => state);
     throw error;
@@ -103,7 +106,8 @@ async function withOperation(kind, operation) {
 
 function serializeControl(operation) {
   const result = controlOperation.then(operation, operation);
-  controlOperation = result.catch(() => {});
+  // The caller observes result; this settled tail only keeps the next operation runnable.
+  controlOperation = result.then(() => undefined, () => undefined);
   return result;
 }
 
@@ -112,6 +116,29 @@ function sendJson(res, statusCode, payload) {
   res.end(JSON.stringify(payload));
 }
 
+function redactLogDetails(value) {
+  return String(value || '').replace(/https?:\/\/\S+/gi, '[redacted-url]');
+}
+
+function sendError(res, error) {
+  const harborError = normalizeHarborError(error);
+  const correlationId = crypto.randomUUID();
+  const technical = harborError.cause?.message || harborError.details || error;
+  console.error(
+    `[control] request failed [${correlationId}] ${harborError.code}: ${redactLogDetails(technical?.message || technical)}`,
+  );
+  return sendJson(res, harborError.status, {
+    success: false,
+    error: {
+      code: harborError.code,
+      message: harborError.message,
+      retryable: harborError.retryable,
+      correlationId,
+      ...(harborError.details ? { details: harborError.details } : {}),
+    },
+  });
+}
+
 function readBody(req) {
   return new Promise((resolve, reject) => {
     const chunks = [];
@@ -122,9 +149,7 @@ function readBody(req) {
       size += chunk.length;
       if (size > MAX_BODY_BYTES) {
         tooLarge = true;
-        const error = new Error('Тело запроса слишком большое');
-        error.statusCode = 413;
-        reject(error);
+        reject(new HarborError('REQUEST_INVALID'));
         return;
       }
       chunks.push(chunk);
@@ -134,10 +159,8 @@ function readBody(req) {
       if (!chunks.length) return resolve({});
       try {
         resolve(JSON.parse(Buffer.concat(chunks).toString('utf8')));
-      } catch {
-        const error = new Error('Невалидный JSON в теле запроса');
-        error.statusCode = 400;
-        reject(error);
+      } catch (cause) {
+        reject(new HarborError('REQUEST_INVALID', { cause }));
       }
     });
     req.on('error', reject);
@@ -312,7 +335,8 @@ function refreshGatewayAutoMode({ reconfigure = true } = {}) {
 
 async function applySelectedServer(selectedTag, { persist = true } = {}) {
   const cached = readJson(settings.subscriptionCachePath, null);
-  if (!cached?.config) throw new Error('Сначала загрузите подписку');
+  if (!cached?.config) throw new HarborError('CONFIG_INVALID');
+  const nextConfig = buildActiveConfig(cached.config, selectedTag);
 
   if (persist) {
     updateStoredState((state) => ({
@@ -325,7 +349,7 @@ async function applySelectedServer(selectedTag, { persist = true } = {}) {
   const previousConfig = fs.existsSync(settings.configPath)
     ? fs.readFileSync(settings.configPath, 'utf8')
     : null;
-  writeSingboxConfig(buildActiveConfig(cached.config, selectedTag));
+  writeSingboxConfig(nextConfig);
   try {
     await startSingbox();
   } catch (error) {
@@ -348,9 +372,7 @@ function refreshSavedSubscription() {
   subscriptionRefreshPromise = (async () => {
     const initialState = readJson(settings.statePath, {});
     if (!initialState.subscriptionUrl) {
-      const error = new Error('Подписка не настроена');
-      error.statusCode = 400;
-      throw error;
+      throw new HarborError('SUBSCRIPTION_INVALID');
     }
 
     const subscriptionUrl = initialState.subscriptionUrl;
@@ -358,7 +380,7 @@ function refreshSavedSubscription() {
     return serializeControl(async () => {
       const currentState = readJson(settings.statePath, {});
       if (currentState.subscriptionUrl !== subscriptionUrl) {
-        throw new Error('Подписка была изменена во время обновления');
+        throw new HarborError('STATE_CONFLICT');
       }
 
       const selectedTag = selectRefreshedServer(currentState.selectedTag, parsed.servers);
@@ -498,11 +520,11 @@ async function handleApi(req, res) {
 
   if (req.method === 'POST' && req.url === '/api/gateway-auto') {
     if (settings.appMode !== 'client') {
-      return sendJson(res, 400, { success: false, error: 'Режим доступен только в Harbor Connect' });
+      throw new HarborError('REQUEST_INVALID');
     }
     const { enabled } = await readBody(req);
     if (typeof enabled !== 'boolean') {
-      return sendJson(res, 400, { success: false, error: 'Укажите enabled: true или false' });
+      throw new HarborError('REQUEST_INVALID');
     }
     await withOperation('gateway-auto', () => serializeControl(async () => {
       await applyGatewayAutoState(applyGatewayPreference(gatewayAutoState, enabled));
@@ -529,7 +551,7 @@ async function handleApi(req, res) {
   if (req.method === 'POST' && req.url === '/api/apply') {
     const { selectedTag = '' } = await readBody(req);
     const tag = String(selectedTag).trim();
-    if (!tag) return sendJson(res, 400, { success: false, error: 'Выберите сервер' });
+    if (!tag) throw new HarborError('REQUEST_INVALID');
     await withOperation('apply-server', () => serializeControl(() => applySelectedServer(tag)));
     return sendState(res, { selectedTag: tag });
   }
@@ -545,9 +567,7 @@ async function handleApi(req, res) {
   if (req.method === 'POST' && req.url === '/api/singbox/restart') {
     await withOperation('start', () => serializeControl(async () => {
       if (!fs.existsSync(settings.configPath)) {
-        const error = new Error('Сначала выберите сервер');
-        error.statusCode = 400;
-        throw error;
+        throw new HarborError('CONFIG_INVALID');
       }
       await singboxRuntime.restart();
       updateStoredState((state) => ({
@@ -559,7 +579,7 @@ async function handleApi(req, res) {
     return sendState(res, { singboxRunning: true });
   }
 
-  return sendJson(res, 404, { success: false, error: 'Не найдено' });
+  return sendError(res, new HarborError('ENDPOINT_NOT_FOUND'));
 }
 
 const mime = {
@@ -592,11 +612,7 @@ const server = http.createServer(async (req, res) => {
       ? await handleApi(req, res)
       : serveStatic(req, res);
   } catch (error) {
-    console.error('[control] request failed', error);
-    return sendJson(res, error.statusCode || 500, {
-      success: false,
-      error: error.message || String(error),
-    });
+    return sendError(res, error);
   }
 });
 
diff --git a/src/server/singbox.js b/src/server/singbox.js
index 516f4ca..2cbe2be 100644
--- a/src/server/singbox.js
+++ b/src/server/singbox.js
@@ -1,6 +1,7 @@
 import fs from 'node:fs';
 import path from 'node:path';
 import { settings } from './config.js';
+import { HarborError } from '../shared/errors.js';
 
 const PROXY_TYPES = new Set(['vless', 'vmess', 'trojan', 'shadowsocks', 'hysteria2']);
 const MIXED_INBOUND = 'mixed-in';
@@ -22,7 +23,7 @@ export function buildGatewayConfig(subscriptionConfig, selectedTag, { clientDire
   const vpnOutbound = directClient
     ? null
     : structuredClone(findOutbound(subscriptionConfig, selectedTag));
-  if (!directClient && !vpnOutbound) throw new Error(`Outbound не найден: ${selectedTag}`);
+  if (!directClient && !vpnOutbound) throw new HarborError('SERVER_NOT_FOUND');
   if (vpnOutbound && !vpnOutbound.tag) vpnOutbound.tag = 'vpn-out';
   if (vpnOutbound?.type === 'vless' && !vpnOutbound.packet_encoding) {
     vpnOutbound.packet_encoding = 'xudp';
diff --git a/src/server/singboxRuntime.js b/src/server/singboxRuntime.js
index 27b2c9a..be8205a 100644
--- a/src/server/singboxRuntime.js
+++ b/src/server/singboxRuntime.js
@@ -2,6 +2,7 @@ import crypto from 'node:crypto';
 import fs from 'node:fs';
 import { spawn, spawnSync } from 'node:child_process';
 import { setGatewayInterception } from './gatewayRouting.js';
+import { HarborError } from '../shared/errors.js';
 
 export function createSingboxRuntime({ configPath, gateway = false, tproxyChain = '' }) {
   let child = null;
@@ -44,16 +45,27 @@ export function createSingboxRuntime({ configPath, gateway = false, tproxyChain
 
     const check = spawnSync('sing-box', ['check', '-c', configPath], { encoding: 'utf8' });
     if (check.status !== 0) {
-      throw new Error((check.stderr || check.stdout || 'sing-box check failed').trim());
+      throw new HarborError('CONFIG_INVALID', {
+        cause: new Error((check.stderr || check.stdout || check.error?.message || 'sing-box check failed').trim()),
+      });
     }
 
     const nextHash = crypto.createHash('sha256').update(fs.readFileSync(configPath)).digest('hex');
     if (!force && child && nextHash === configHash) return state();
 
     await stop();
-    const current = spawn('sing-box', ['run', '-c', configPath], {
-      stdio: ['ignore', 'inherit', 'inherit'],
-    });
+    let current;
+    try {
+      current = spawn('sing-box', ['run', '-c', configPath], {
+        stdio: ['ignore', 'inherit', 'inherit'],
+      });
+      await new Promise((resolve, reject) => {
+        current.once('spawn', resolve);
+        current.once('error', reject);
+      });
+    } catch (cause) {
+      throw new HarborError('PROCESS_START_FAILED', { cause });
+    }
     child = current;
     configHash = nextHash;
     startedAt = new Date().toISOString();
@@ -64,7 +76,7 @@ export function createSingboxRuntime({ configPath, gateway = false, tproxyChain
       child = null;
       configHash = '';
       startedAt = null;
-      throw error;
+      throw new HarborError('PROCESS_START_FAILED', { cause: error });
     }
     current.once('exit', () => {
       if (child !== current) return;
diff --git a/src/server/subscription.js b/src/server/subscription.js
index 33be146..8453f66 100644
--- a/src/server/subscription.js
+++ b/src/server/subscription.js
@@ -1,6 +1,7 @@
 import crypto from 'node:crypto';
 import fs from 'node:fs';
 import { settings } from './config.js';
+import { HarborError } from '../shared/errors.js';
 
 const PROXY_TYPES = new Set(['vless', 'vmess', 'trojan', 'shadowsocks', 'hysteria2']);
 
@@ -40,10 +41,15 @@ export function parseUserInfo(headerValue) {
 
 export function parseVlessUrl(rawUrl) {
   if (!rawUrl.startsWith('vless://')) {
-    throw new Error('VLESS URL must start with vless://');
+    throw new HarborError('SUBSCRIPTION_INVALID');
   }
 
-  const parsed = new URL(rawUrl);
+  let parsed;
+  try {
+    parsed = new URL(rawUrl);
+  } catch (cause) {
+    throw new HarborError('SUBSCRIPTION_INVALID', { cause });
+  }
   const tag = decodeURIComponent(parsed.hash ? parsed.hash.slice(1) : 'vless-out');
   const uuid = decodeURIComponent(parsed.username || '');
   const server = parsed.hostname;
@@ -55,11 +61,11 @@ export function parseVlessUrl(rawUrl) {
   const flow = parsed.searchParams.get('flow') || '';
 
   if (!uuid || !server || !serverPort) {
-    throw new Error('VLESS URL misses uuid, host or port');
+    throw new HarborError('SUBSCRIPTION_INVALID');
   }
 
   if (!publicKey || !shortId) {
-    throw new Error('VLESS REALITY parameters pbk and sid are required');
+    throw new HarborError('SUBSCRIPTION_INVALID');
   }
 
   return {
@@ -111,7 +117,7 @@ export function parseSubscriptionBody(body) {
       .filter((line) => line.startsWith('vless://'));
 
     if (!links.length) {
-      throw new Error('Subscription does not contain JSON config or VLESS links');
+      throw new HarborError('SUBSCRIPTION_INVALID');
     }
 
     parsedConfig = {
@@ -130,7 +136,7 @@ export function parseSubscriptionBody(body) {
     }));
 
   if (!servers.length) {
-    throw new Error('No supported proxy outbounds found in subscription');
+    throw new HarborError('SUBSCRIPTION_INVALID');
   }
 
   return { config: parsedConfig, servers };
@@ -140,21 +146,26 @@ async function requestSubscription(url) {
   let parsedUrl;
   try {
     parsedUrl = new URL(url);
-  } catch {
-    throw new Error('Invalid subscription URL');
+  } catch (cause) {
+    throw new HarborError('SUBSCRIPTION_INVALID', { cause });
   }
 
   if (!['http:', 'https:'].includes(parsedUrl.protocol)) {
-    throw new Error('Subscription URL must use http or https');
+    throw new HarborError('SUBSCRIPTION_INVALID');
   }
 
-  const response = await fetch(parsedUrl, {
-    headers: subscriptionHeaders(),
-    redirect: 'follow',
-  });
+  let response;
+  try {
+    response = await fetch(parsedUrl, {
+      headers: subscriptionHeaders(),
+      redirect: 'follow',
+    });
+  } catch (cause) {
+    throw new HarborError('PROVIDER_UNAVAILABLE', { cause });
+  }
 
   if (!response.ok) {
-    throw new Error(`Subscription request failed: HTTP ${response.status}`);
+    throw new HarborError('PROVIDER_UNAVAILABLE', { details: `HTTP ${response.status}` });
   }
 
   return response;
diff --git a/src/shared/errors.js b/src/shared/errors.js
new file mode 100644
index 0000000..8f0aa81
--- /dev/null
+++ b/src/shared/errors.js
@@ -0,0 +1,33 @@
+export const ERROR_DEFINITIONS = Object.freeze({
+  CONTROL_UNREACHABLE: { status: 503, message: 'Harbor сейчас недоступен.', retryable: true },
+  REQUEST_INVALID: { status: 400, message: 'Запрос содержит некорректные данные.', retryable: false },
+  ENDPOINT_NOT_FOUND: { status: 404, message: 'Запрошенный API-метод не найден.', retryable: false },
+  SUBSCRIPTION_INVALID: { status: 400, message: 'Ссылка подписки недействительна.', retryable: false },
+  PROVIDER_UNAVAILABLE: { status: 502, message: 'Провайдер подписки временно недоступен.', retryable: true },
+  STATE_CONFLICT: { status: 409, message: 'Данные изменились во время операции.', retryable: true },
+  SERVER_NOT_FOUND: { status: 404, message: 'Выбранный сервер больше недоступен.', retryable: false },
+  CONFIG_INVALID: { status: 422, message: 'Конфигурация VPN недействительна.', retryable: false },
+  PROCESS_START_FAILED: { status: 503, message: 'Не удалось запустить VPN-процесс.', retryable: true },
+  OPERATION_IN_PROGRESS: { status: 409, message: 'Другая операция ещё выполняется.', retryable: true },
+  UNKNOWN: { status: 500, message: 'Не удалось выполнить действие.', retryable: false },
+});
+
+export function errorDefinition(code) {
+  return ERROR_DEFINITIONS[code] || ERROR_DEFINITIONS.UNKNOWN;
+}
+
+export class HarborError extends Error {
+  constructor(code, { cause, details } = {}) {
+    const definition = errorDefinition(code);
+    super(definition.message, { cause });
+    this.name = 'HarborError';
+    this.code = ERROR_DEFINITIONS[code] ? code : 'UNKNOWN';
+    this.status = definition.status;
+    this.retryable = definition.retryable;
+    this.details = details;
+  }
+}
+
+export function normalizeHarborError(error) {
+  return error instanceof HarborError ? error : new HarborError('UNKNOWN', { cause: error });
+}
diff --git a/src/web/App.jsx b/src/web/App.jsx
index b5bbe2a..dbf61af 100644
--- a/src/web/App.jsx
+++ b/src/web/App.jsx
@@ -1,7 +1,7 @@
 import React, { useEffect, useReducer, useRef, useState } from 'react';
 import { createRoot } from 'react-dom/client';
 import './styles.css';
-import { api } from './api.js';
+import { api, HarborApiError } from './api.js';
 import { ClientOverviewPage } from './components/ClientOverviewPage.jsx';
 import { BootStatePage, StaleBanner } from './components/SyncStatus.jsx';
 import {
@@ -18,7 +18,7 @@ function App() {
   );
   const [subscriptionUrl, setSubscriptionUrl] = useState('');
   const [busy, setBusy] = useState(false);
-  const [error, setError] = useState('');
+  const [error, setError] = useState(null);
   const pollGeneration = useRef(0);
 
   function setPendingTag(serverId) {
@@ -60,14 +60,21 @@ function App() {
       : '/harbor-connect.svg?v=2';
   }, [state?.mode]);
 
-  async function run(action) {
+  async function run(action, context) {
     setBusy(true);
-    setError('');
+    setError(null);
     try {
       return await applyMutation(action);
     } catch (err) {
-      setError(err.message);
-      throw err;
+      const safeError = err instanceof HarborApiError ? err : new HarborApiError();
+      setError({
+        context,
+        message: safeError.message,
+        code: safeError.code,
+        correlationId: safeError.correlationId,
+        retry: safeError.retryable ? () => run(action, context) : null,
+      });
+      return false;
     } finally {
       setBusy(false);
     }
@@ -91,11 +98,11 @@ function App() {
       const data = await api.subscription.fetch(subscriptionUrl);
       dispatch({ type: 'clear-pending-server' });
       return data;
-    });
+    }, 'subscription');
   }
 
   async function refreshSubscription() {
-    return applyMutation(api.subscription.refresh);
+    return run(api.subscription.refresh, 'subscription');
   }
 
   async function forgetSubscription() {
@@ -104,7 +111,7 @@ function App() {
       setSubscriptionUrl('');
       dispatch({ type: 'clear-pending-server' });
       return data;
-    });
+    }, 'subscription');
   }
 
   if (!state) return <BootStatePage transport={transport} onRetry={() => loadState({ retry: true })} />;
@@ -126,10 +133,10 @@ function App() {
             onFetchSubscription={fetchSubscription}
             onRefreshSubscription={refreshSubscription}
             onForgetSubscription={forgetSubscription}
-            onApply={(tag) => run(() => api.apply(tag))}
-            onRestart={() => run(api.singbox.restart)}
-            onStop={() => run(api.singbox.stop)}
-            onSetGatewayAuto={(enabled) => run(() => api.gatewayAuto.setEnabled(enabled))}
+            onApply={(tag) => run(() => api.apply(tag), 'connection')}
+            onRestart={() => run(api.singbox.restart, 'connection')}
+            onStop={() => run(api.singbox.stop, 'connection')}
+            onSetGatewayAuto={(enabled) => run(() => api.gatewayAuto.setEnabled(enabled), 'connection')}
           />
         </main>
       </div>
diff --git a/src/web/api.js b/src/web/api.js
index 8954c94..70aa7e7 100644
--- a/src/web/api.js
+++ b/src/web/api.js
@@ -1,16 +1,51 @@
-async function request(url, options = {}) {
-  const response = await fetch(url, {
-    ...options,
-    headers: {
-      'content-type': 'application/json',
-      ...(options.headers || {}),
-    },
-  });
-  const data = await response.json().catch(() => ({}));
+import { ERROR_DEFINITIONS, errorDefinition } from '../shared/errors.js';
+
+export class HarborApiError extends Error {
+  constructor(payload = {}, status = 0) {
+    const code = ERROR_DEFINITIONS[payload.code] ? payload.code : 'UNKNOWN';
+    const definition = errorDefinition(code);
+    super(definition.message);
+    this.name = 'HarborApiError';
+    this.code = code;
+    this.status = status >= 400 ? status : definition.status;
+    this.retryable = definition.retryable;
+    this.details = payload.details;
+    this.correlationId = payload.correlationId
+      || globalThis.crypto?.randomUUID?.()
+      || new Date().toISOString();
+  }
+}
+
+export function validationStatusForError(error) {
+  return error?.code === 'SUBSCRIPTION_INVALID' ? 'invalid' : 'unavailable';
+}
+
+export async function request(url, options = {}, fetchImpl = fetch) {
+  let response;
+  try {
+    response = await fetchImpl(url, {
+      ...options,
+      headers: {
+        'content-type': 'application/json',
+        ...(options.headers || {}),
+      },
+    });
+  } catch (error) {
+    if (error?.name === 'AbortError') throw error;
+    throw new HarborApiError({ code: 'CONTROL_UNREACHABLE' });
+  }
+
+  let data = {};
+  try {
+    data = await response.json();
+  } catch {
+    if (response.ok) throw new HarborApiError({ code: 'UNKNOWN' }, response.status);
+  }
   if (!response.ok || data?.success === false) {
-    const error = new Error(data?.error || `Запрос ${url} завершился ошибкой ${response.status}`);
-    error.status = response.status;
-    throw error;
+    const payload = data?.error && typeof data.error === 'object'
+      ? data.error
+      : { code: response.status >= 500 ? 'CONTROL_UNREACHABLE' : 'UNKNOWN' };
+    throw new HarborApiError(payload, response.status);
   }
   return data;
 }
diff --git a/src/web/components/ClientOverviewPage.jsx b/src/web/components/ClientOverviewPage.jsx
index 198f65d..928253b 100644
--- a/src/web/components/ClientOverviewPage.jsx
+++ b/src/web/components/ClientOverviewPage.jsx
@@ -1,6 +1,6 @@
 import React, { useEffect, useRef, useState } from 'react';
 import { flushSync } from 'react-dom';
-import { api } from '../api.js';
+import { api, validationStatusForError } from '../api.js';
 import {
   connectionAction,
   connectionDurationParts,
@@ -20,6 +20,19 @@ function CloudTooltip({ children }) {
   return <span className="client-tooltip" role="tooltip">{children}</span>;
 }
 
+function InlineError({ error, context }) {
+  if (!error || error.context !== context) return null;
+  return (
+    <div className={`client-inline-error is-${context}`} role="alert">
+      <span>{error.message}</span>
+      {error.correlationId && (
+        <small title={error.correlationId}>Код: {error.correlationId.slice(0, 8)}</small>
+      )}
+      {error.retry && <button type="button" onClick={error.retry}>Повторить</button>}
+    </div>
+  );
+}
+
 function InstructionStep({ step }) {
   if (typeof step === 'string') return step;
   return (
@@ -196,6 +209,7 @@ export function ClientOverviewPage({
   });
   const [editingSubscription, setEditingSubscription] = useState(!state?.hasSubscription);
   const [subscriptionValidation, setSubscriptionValidation] = useState({ url: '', status: 'idle' });
+  const [validationAttempt, setValidationAttempt] = useState(0);
   const [showIntro, setShowIntro] = useState(!hasSubscription);
   const [subscriptionContentReady, setSubscriptionContentReady] = useState(hasSubscription);
   const [pings, setPings] = useState({});
@@ -240,6 +254,9 @@ export function ClientOverviewPage({
   const subscriptionValidationStatus = subscriptionValidation.url === normalizedSubscriptionUrl
     ? subscriptionValidation.status
     : normalizedSubscriptionUrl ? 'checking' : 'idle';
+  const subscriptionError = subscriptionValidation.url === normalizedSubscriptionUrl
+    ? subscriptionValidation.error
+    : null;
   const subscriptionWaiting = hasSubscription && !subscriptionContentReady;
 
   useEffect(() => {
@@ -316,10 +333,32 @@ export function ClientOverviewPage({
     setSubscriptionValidation({ url: normalizedSubscriptionUrl, status: 'checking' });
     const timer = setTimeout(() => {
       api.subscription.validate(normalizedSubscriptionUrl, controller.signal)
-        .then(() => setSubscriptionValidation({ url: normalizedSubscriptionUrl, status: 'valid' }))
-        .catch(() => {
+        .then(() => setSubscriptionValidation({
+          url: normalizedSubscriptionUrl,
+          status: 'valid',
+          error: null,
+        }))
+        .catch((validationError) => {
           if (!controller.signal.aborted) {
-            setSubscriptionValidation({ url: normalizedSubscriptionUrl, status: 'invalid' });
+            setSubscriptionValidation({
+              url: normalizedSubscriptionUrl,
+              status: validationStatusForError(validationError),
+              error: {
+                context: 'subscription',
+                message: validationError.message,
+                correlationId: validationError.correlationId,
+                retry: validationError.retryable
+                  ? () => {
+                    setSubscriptionValidation({
+                      url: normalizedSubscriptionUrl,
+                      status: 'checking',
+                      error: null,
+                    });
+                    setValidationAttempt((attempt) => attempt + 1);
+                  }
+                  : null,
+              },
+            });
           }
         });
     }, 350);
@@ -328,7 +367,7 @@ export function ClientOverviewPage({
       clearTimeout(timer);
       controller.abort();
     };
-  }, [normalizedSubscriptionUrl]);
+  }, [normalizedSubscriptionUrl, validationAttempt]);
 
   useEffect(() => {
     if (!hasSubscription) {
@@ -356,7 +395,7 @@ export function ClientOverviewPage({
 
   useEffect(() => {
     if (!state?.hasSubscription) return undefined;
-    onRefreshSubscription().catch(() => {});
+    onRefreshSubscription();
     return undefined;
   }, [state?.hasSubscription]);
 
@@ -413,7 +452,7 @@ export function ClientOverviewPage({
   async function submitSubscription(event) {
     event.preventDefault();
     if (subscriptionValidationStatus !== 'valid') return;
-    await onFetchSubscription();
+    if (!await onFetchSubscription()) return;
     setSubscriptionUrl('');
     setEditingSubscription(false);
   }
@@ -434,7 +473,7 @@ export function ClientOverviewPage({
     const startedAt = performance.now();
     setRefreshingInfo(true);
     try {
-      await onRefreshSubscription();
+      if (!await onRefreshSubscription()) return;
       setUsageUpdated(false);
       requestAnimationFrame(() => setUsageUpdated(true));
       setTimeout(() => setUsageUpdated(false), 900);
@@ -451,7 +490,7 @@ export function ClientOverviewPage({
   }
 
   async function forgetSubscription() {
-    await onForgetSubscription();
+    if (!await onForgetSubscription()) return;
     setConfirmingDelete(false);
   }
 
@@ -641,11 +680,10 @@ export function ClientOverviewPage({
             </div>
             )}
           </section>
+          <InlineError error={error} context="connection" />
         </section>
         )}
 
-        {error && <p className="client-error" role="alert">{error}</p>}
-
         <div
           className={`client-form${subscriptionWaiting ? ' is-waiting' : ''}${confirmingDelete ? ' is-confirming-delete' : ''}`}
           aria-hidden={subscriptionWaiting}
@@ -759,15 +797,20 @@ export function ClientOverviewPage({
                     ? 'Сохранить подписку'
                     : subscriptionValidationStatus === 'invalid'
                       ? 'Ссылка подписки не распознана'
-                      : 'Проверяем ссылку подписки'}
+                      : subscriptionValidationStatus === 'unavailable'
+                        ? 'Проверка подписки временно недоступна'
+                        : 'Проверяем ссылку подписки'}
                   disabled={busy || subscriptionValidationStatus !== 'valid'}
                 >
                   {subscriptionValidationStatus === 'valid'
                     ? '✓'
-                    : subscriptionValidationStatus === 'invalid' ? '×' : '…'}
+                    : subscriptionValidationStatus === 'invalid'
+                      ? '×'
+                      : subscriptionValidationStatus === 'unavailable' ? '!' : '…'}
                 </button>
               )}
             </form>
+            <InlineError error={subscriptionError || error} context="subscription" />
           </div>
 
           {hasSubscription && subscriptionContentReady && hasUsage && (
diff --git a/src/web/state/harborReducer.js b/src/web/state/harborReducer.js
index b8d2bc3..62988b6 100644
--- a/src/web/state/harborReducer.js
+++ b/src/web/state/harborReducer.js
@@ -22,7 +22,7 @@ export function compatibleSnapshot(snapshot) {
 export function classifySyncError(error) {
   const status = Number(error?.status) || 0;
   if (error?.code === 'INCOMPATIBLE_API' || status === 404) return 'incompatible-api';
-  if (error?.name === 'TypeError' || status >= 500) return 'control-unreachable';
+  if (error?.code === 'CONTROL_UNREACHABLE' || error?.name === 'TypeError' || status >= 500) return 'control-unreachable';
   return 'fatal';
 }
 
diff --git a/src/web/styles.css b/src/web/styles.css
index 13ac727..a63c072 100644
--- a/src/web/styles.css
+++ b/src/web/styles.css
@@ -876,6 +876,7 @@ p {
 }
 
 .client-power-section {
+  position: relative;
   display: grid;
   justify-items: center;
   gap: 18px;
@@ -1896,17 +1897,53 @@ p {
   margin-top: 7px;
 }
 
-.client-error {
+.client-inline-error {
   position: absolute;
-  bottom: 0;
   left: 50%;
-  width: min(420px, 90vw);
+  z-index: 3;
+  width: min(360px, 90vw);
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  gap: 8px;
+  flex-wrap: wrap;
   color: oklch(0.62 0.2 28);
   font: 600 11px/1.5 'JetBrains Mono', 'SF Mono', ui-monospace, Menlo, monospace;
   text-align: center;
   transform: translateX(-50%);
 }
 
+.client-inline-error.is-connection {
+  top: calc(100% + 12px);
+}
+
+.client-inline-error.is-subscription {
+  top: calc(100% + 2px);
+}
+
+.client-inline-error small {
+  color: var(--client-muted);
+  font-size: 9px;
+  white-space: nowrap;
+}
+
+.client-inline-error button {
+  padding: 4px 7px;
+  border: 0;
+  border-radius: 6px;
+  background: color-mix(in oklch, var(--client-control) 72%, transparent);
+  color: var(--client-text);
+  font-size: 9px;
+  font-weight: 700;
+  cursor: pointer;
+  white-space: nowrap;
+}
+
+.client-inline-error button:focus-visible {
+  outline: 2px solid oklch(0.68 0.15 28);
+  outline-offset: 2px;
+}
+
 .client-copy-button {
   position: relative;
   width: 86px;
@@ -2009,6 +2046,19 @@ p {
     padding: 20px 6px;
   }
 
+  .client-power-section {
+    padding-bottom: 40px;
+  }
+
+  .client-inline-error.is-connection {
+    top: auto;
+    bottom: 0;
+  }
+
+  .client-form-content {
+    gap: 48px;
+  }
+
   .harbor-brand {
     transform: translate(-50%, calc(-50% - 28vh)) scale(2.15);
   }
diff --git a/test/server/errors.test.js b/test/server/errors.test.js
new file mode 100644
index 0000000..a7c91fc
--- /dev/null
+++ b/test/server/errors.test.js
@@ -0,0 +1,28 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+
+import {
+  ERROR_DEFINITIONS,
+  HarborError,
+  normalizeHarborError,
+} from '../../src/shared/errors.js';
+
+test('every Harbor error code has stable Russian copy and retry policy', () => {
+  for (const [code, definition] of Object.entries(ERROR_DEFINITIONS)) {
+    const error = new HarborError(code);
+    assert.equal(error.code, code);
+    assert.equal(error.message, definition.message);
+    assert.equal(error.retryable, definition.retryable);
+    assert.match(error.message, /[А-Яа-яЁё]/);
+    assert.equal(typeof error.status, 'number');
+  }
+});
+
+test('unknown failures use the safe non-retryable fallback', () => {
+  const error = normalizeHarborError(new Error('secret internal failure'));
+
+  assert.equal(error.code, 'UNKNOWN');
+  assert.equal(error.message, ERROR_DEFINITIONS.UNKNOWN.message);
+  assert.equal(error.retryable, false);
+  assert.equal(error.message.includes('secret'), false);
+});
diff --git a/test/server/state-contract.test.js b/test/server/state-contract.test.js
index c26e4bf..0233379 100644
--- a/test/server/state-contract.test.js
+++ b/test/server/state-contract.test.js
@@ -29,13 +29,18 @@ async function freePort() {
   return port;
 }
 
-async function request(port, pathname, method = 'GET', body) {
+async function rawRequest(port, pathname, method = 'GET', body) {
   const response = await fetch(`http://127.0.0.1:${port}${pathname}`, {
     method,
     headers: { 'content-type': 'application/json' },
     body: body === undefined ? undefined : JSON.stringify(body),
   });
   const payload = await response.json();
+  return { response, payload };
+}
+
+async function request(port, pathname, method = 'GET', body) {
+  const { response, payload } = await rawRequest(port, pathname, method, body);
   assert.equal(response.ok, true, JSON.stringify(payload));
   return payload;
 }
@@ -54,7 +59,7 @@ async function waitForState(port, child, stderr) {
 
 test('state v1 normalizes legacy storage and validates the canonical snapshot', () => {
   const stored = normalizeStoredState({
-    subscriptionUrl: 'https://provider.example/subscription/full-secret-token',
+    subscriptionUrl: 'https://provider.example/subscription/test',
     selectedTag: ' legacy ',
     servers: [{ tag: ' legacy ', type: 'vless', server: 'vpn.example', server_port: 443 }],
   });
@@ -95,14 +100,20 @@ test('GET and domain mutations return one state shape with monotonic revisions',
     }],
   };
   fs.mkdirSync(binDir);
-  fs.writeFileSync(path.join(binDir, 'sing-box'), `#!/usr/bin/env node
+  const singboxPath = path.join(binDir, 'sing-box');
+  const workingSingbox = `#!/usr/bin/env node
 if (process.argv[2] === 'check') process.exit(0);
 process.on('SIGTERM', () => process.exit(0));
 setInterval(() => {}, 60_000);
-`);
-  fs.chmodSync(path.join(binDir, 'sing-box'), 0o755);
+`;
+  fs.writeFileSync(singboxPath, workingSingbox);
+  fs.chmodSync(singboxPath, 0o755);
 
   const subscriptionServer = http.createServer((req, res) => {
+    if (req.url === '/unavailable') {
+      res.writeHead(503);
+      return res.end('unavailable');
+    }
     res.writeHead(200, {
       'content-type': 'application/json',
       'subscription-userinfo': 'upload=10; download=20; total=100',
@@ -110,7 +121,7 @@ setInterval(() => {}, 60_000);
     res.end(JSON.stringify(config));
   });
   const subscriptionPort = await listen(subscriptionServer);
-  const subscriptionUrl = `http://127.0.0.1:${subscriptionPort}/subscription/full-secret-token`;
+  const subscriptionUrl = `http://127.0.0.1:${subscriptionPort}/subscription/test`;
   fs.writeFileSync(path.join(dir, 'state.json'), JSON.stringify({
     subscriptionUrl,
     selectedTag: 'test-vpn',
@@ -150,6 +161,42 @@ setInterval(() => {}, 60_000);
   const stateKeys = Object.keys(initial).sort();
   let revision = initial.revision;
 
+  const invalidSubscription = await rawRequest(
+    port,
+    '/api/subscription/validate',
+    'POST',
+    { url: 'not-a-url' },
+  );
+  assert.equal(invalidSubscription.response.status, 400);
+  assert.deepEqual(
+    {
+      code: invalidSubscription.payload.error.code,
+      retryable: invalidSubscription.payload.error.retryable,
+    },
+    { code: 'SUBSCRIPTION_INVALID', retryable: false },
+  );
+  assert.equal(typeof invalidSubscription.payload.error.correlationId, 'string');
+
+  const providerUnavailable = await rawRequest(
+    port,
+    '/api/subscription/validate',
+    'POST',
+    { url: `http://127.0.0.1:${subscriptionPort}/unavailable` },
+  );
+  assert.equal(providerUnavailable.response.status, 502);
+  assert.equal(providerUnavailable.payload.error.code, 'PROVIDER_UNAVAILABLE');
+  assert.equal(providerUnavailable.payload.error.retryable, true);
+
+  const missingServer = await rawRequest(
+    port,
+    '/api/apply',
+    'POST',
+    { selectedTag: 'missing-server' },
+  );
+  assert.equal(missingServer.response.status, 404);
+  assert.equal(missingServer.payload.error.code, 'SERVER_NOT_FOUND');
+  assert.equal((await request(port, '/api/state')).selection.desiredServerId, 'test-vpn');
+
   async function stateResponse(pathname, method = 'POST', body) {
     const result = await request(port, pathname, method, body);
     assert.deepEqual(Object.keys(result.state).sort(), stateKeys);
@@ -174,10 +221,30 @@ setInterval(() => {}, 60_000);
   assert.equal(applied.state.connection.process, 'running');
   assert.equal((await mutation('/api/singbox/stop')).state.connection.desired, 'stopped');
   assert.equal((await mutation('/api/singbox/restart')).state.connection.desired, 'running');
+
+  fs.writeFileSync(singboxPath, `#!/usr/bin/env node
+if (process.argv[2] === 'check') {
+  require('node:fs').unlinkSync(process.argv[1]);
+  process.exit(0);
+}
+`);
+  fs.chmodSync(singboxPath, 0o755);
+  const processFailure = await rawRequest(port, '/api/singbox/restart', 'POST');
+  assert.equal(processFailure.response.status, 503);
+  assert.equal(processFailure.payload.error.code, 'PROCESS_START_FAILED');
+  assert.equal(processFailure.payload.error.retryable, true);
+  fs.writeFileSync(singboxPath, workingSingbox);
+  fs.chmodSync(singboxPath, 0o755);
+
   await mutation('/api/subscription/refresh');
   assert.equal((await mutation('/api/gateway-auto', 'POST', { enabled: false })).state.gatewayAuto.enabled, false);
   const forgotten = await mutation('/api/subscription', 'DELETE');
   assert.equal(forgotten.state.subscription.status, 'missing');
   assert.equal(forgotten.state.servers.length, 0);
   assert.deepEqual((await stateResponse('/api/servers/ping-all')).results, []);
+
+  const missingConfig = await rawRequest(port, '/api/singbox/restart', 'POST');
+  assert.equal(missingConfig.response.status, 422);
+  assert.equal(missingConfig.payload.error.code, 'CONFIG_INVALID');
+  assert.equal(missingConfig.payload.error.retryable, false);
 });
diff --git a/test/web/api-errors.test.js b/test/web/api-errors.test.js
new file mode 100644
index 0000000..88cf134
--- /dev/null
+++ b/test/web/api-errors.test.js
@@ -0,0 +1,61 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+
+import {
+  HarborApiError,
+  request,
+  validationStatusForError,
+} from '../../src/web/api.js';
+
+const response = (status, error) => ({
+  ok: status >= 200 && status < 300,
+  status,
+  json: async () => ({ success: false, error }),
+});
+
+test('frontend exposes retry only for retryable structured errors', async () => {
+  await assert.rejects(
+    request('/api/test', {}, async () => response(502, {
+      code: 'PROVIDER_UNAVAILABLE',
+      message: 'untrusted server copy',
+      retryable: false,
+      correlationId: 'provider-reference',
+    })),
+    (error) => {
+      assert.ok(error instanceof HarborApiError);
+      assert.equal(error.message, 'Провайдер подписки временно недоступен.');
+      assert.equal(error.retryable, true);
+      assert.equal(error.correlationId, 'provider-reference');
+      return true;
+    },
+  );
+
+  await assert.rejects(
+    request('/api/test', {}, async () => response(400, {
+      code: 'SUBSCRIPTION_INVALID',
+      retryable: true,
+    })),
+    (error) => error.retryable === false,
+  );
+});
+
+test('network failures become retryable control errors', async () => {
+  await assert.rejects(
+    request('/api/state', {}, async () => { throw new TypeError('fetch failed'); }),
+    (error) => error.code === 'CONTROL_UNREACHABLE' && error.retryable === true,
+  );
+});
+
+test('local unknown errors get a safe message and diagnostic reference', () => {
+  const error = new HarborApiError({ code: 'NOT_A_REAL_CODE' });
+
+  assert.equal(error.code, 'UNKNOWN');
+  assert.equal(error.message, 'Не удалось выполнить действие.');
+  assert.equal(typeof error.correlationId, 'string');
+  assert.ok(error.correlationId.length >= 8);
+});
+
+test('subscription validation distinguishes bad input from provider outage', () => {
+  assert.equal(validationStatusForError({ code: 'SUBSCRIPTION_INVALID' }), 'invalid');
+  assert.equal(validationStatusForError({ code: 'PROVIDER_UNAVAILABLE' }), 'unavailable');
+});