diff --git a/src/background/index.ts b/src/background/index.ts
index dfac677..c069243 100644
--- a/src/background/index.ts
+++ b/src/background/index.ts
@@ -98,6 +98,17 @@ const NOTIFICATION_STARTUP_PREFIX = 'mokosh-startup-';
 const NOTIFICATION_RECOVERY_PREFIX = 'mokosh-recovery-';
 const POPUP_HTML_PATH = 'src/popup/index.html';
 
+// ─── Plan 01-10 onboarding constants (D-17-onboarding) ───────────────
+// Project naming standard: SCREAMING_SNAKE for true constants. These
+// drive the first-install welcome-tab flow (chrome.runtime.onInstalled
+// → openWelcomeIfFirstInstall; flag persisted via chrome.storage.local).
+// The 'onboarding-completed' key is the storage-schema identifier
+// preserved across SW respawns (and surveyed by the unit test in
+// tests/background/onboarding.test.ts Test A).
+const ONBOARDING_FLAG = 'onboarding-completed';
+const ONBOARDING_INSTALLED_AT = 'installed-at';
+const WELCOME_PATH = 'src/welcome/welcome.html';
+
 // Plan 01-12 Wave 4: operator-facing copy fallbacks for the notification
 // title (extName) + the two notification messages. Same `|| fallback`
 // pattern as the popup — unit-test contexts without chrome.i18n stub
@@ -217,6 +228,58 @@ async function ensureOffscreen() {
   }
 }
 
+/**
+ * Open the welcome page on first install (Plan 01-10 D-17-onboarding).
+ *
+ * The 'D-17-onboarding' suffix disambiguates from D-17-port-lifecycle
+ * per CONTEXT.md lines 540-545. Trigger conditions (all must hold):
+ *   - details.reason === 'install' (NOT 'update' / 'chrome_update' /
+ *     'shared_module_update');
+ *   - chrome.storage.local key 'onboarding-completed' NOT === true.
+ *
+ * Side effects on first install:
+ *   - chrome.tabs.create({url: chrome.runtime.getURL('src/welcome/welcome.html')})
+ *   - chrome.storage.local.set({'onboarding-completed': true,
+ *     'installed-at': Date.now()})
+ *
+ * Failure mode: any thrown chrome.* call is caught + logged via
+ * logger.warn. The welcome tab failing is NOT fatal — the toolbar
+ * onClicked path (D-16-toolbar) remains the operator's start path and
+ * is independent of the onboarding flow.
+ *
+ * Architectural note: the fetch of 'onboarding-completed' uses the
+ * EXACT string key (no array form) so the unit-test contract in
+ * tests/background/onboarding.test.ts Test A's assertion
+ * "saw chrome.storage.local.get('onboarding-completed')" holds. The
+ * storage-schema cross-version-compat pin (I-02 lesson preserved from
+ * prior plan draft) lives in that test.
+ */
+async function openWelcomeIfFirstInstall(
+  details: chrome.runtime.InstalledDetails,
+): Promise<void> {
+  if (details.reason !== 'install') {
+    return;
+  }
+  try {
+    const stored = await chrome.storage.local.get(ONBOARDING_FLAG);
+    if (stored[ONBOARDING_FLAG] === true) {
+      logger.log('Onboarding already completed; skipping welcome tab.');
+      return;
+    }
+    const url = chrome.runtime.getURL(WELCOME_PATH);
+    await chrome.tabs.create({ url });
+    await chrome.storage.local.set({
+      [ONBOARDING_FLAG]: true,
+      [ONBOARDING_INSTALLED_AT]: Date.now(),
+    });
+    logger.log(
+      'Welcome tab opened (D-17-onboarding); onboarding flag set.',
+    );
+  } catch (err) {
+    logger.warn('openWelcomeIfFirstInstall failed:', err);
+  }
+}
+
 // Outer-bound buffer fetch budget. Larger than the legacy
 // BUFFER_FETCH_TIMEOUT_MS (was 2 s; per-port-attempt) because the new
 // architecture covers MULTIPLE port-replacement retries inside one outer
@@ -1002,6 +1065,14 @@ chrome.runtime.onInstalled.addListener((details) => {
     logger.warn('IDB cleanup failed:', e);
   }
   initialize();
+  // Plan 01-10 D-17-onboarding: open welcome tab on first install.
+  // Fire-and-forget — the helper logs its own errors and rejected
+  // promises are caught at the .catch boundary so they cannot escape
+  // the synchronous listener. The toolbar onClicked start path
+  // (D-16-toolbar) is independent of this flow.
+  openWelcomeIfFirstInstall(details).catch((err) => {
+    logger.warn('openWelcomeIfFirstInstall threw:', err);
+  });
 });
 
 // Запуск при старте Service Worker