diff --git a/client/src/components/LiveLogView.tsx b/client/src/components/LiveLogView.tsx
index 873196d..07d00be 100644
--- a/client/src/components/LiveLogView.tsx
+++ b/client/src/components/LiveLogView.tsx
@@ -152,6 +152,7 @@ export default function LiveLogView({
   const [modal, setModal] = useState<LiveModal>('none')
   const [weatherExpanded, setWeatherExpanded] = useState(false)
   const [weatherOwmLoading, setWeatherOwmLoading] = useState(false)
+  const [isOnline, setIsOnline] = useState(navigator.onLine)
   const [commentText, setCommentText] = useState('')
   const [valueInput, setValueInput] = useState('')
   const [valueInputSecondary, setValueInputSecondary] = useState('')
@@ -269,6 +270,17 @@ export default function LiveLogView({
     }
   }, [logbookId, applyLoadedEntry, t])
 
+  useEffect(() => {
+    const handleOnline = () => setIsOnline(true)
+    const handleOffline = () => setIsOnline(false)
+    window.addEventListener('online', handleOnline)
+    window.addEventListener('offline', handleOffline)
+    return () => {
+      window.removeEventListener('online', handleOnline)
+      window.removeEventListener('offline', handleOffline)
+    }
+  }, [])
+
   useEffect(() => {
     void runInit()
     return () => {
@@ -503,6 +515,10 @@ export default function LiveLogView({
 
   const handleFetchOwmWeather = () => {
     if (!entryId || busy || weatherOwmLoading) return
+    if (!isOnline) {
+      void showAlert(t('logs.weather_offline'), t('logs.live_weather_owm_btn'))
+      return
+    }
 
     const position = getLastPositionFixWithin(
       events,
@@ -533,6 +549,10 @@ export default function LiveLogView({
             { analyticsSource: 'live_log' }
           )
         } catch (err) {
+          if (err instanceof WeatherApiError && err.code === 'OFFLINE') {
+            void showAlert(t('logs.weather_offline'), t('logs.live_weather_owm_btn'))
+            return
+          }
           if (err instanceof WeatherApiError && err.code === 'NO_KEY') {
             void showAlert(t('settings.no_key'), t('logs.live_weather_owm_btn'))
             return
diff --git a/client/src/components/LogEntryEditor.tsx b/client/src/components/LogEntryEditor.tsx
index d3600f0..3ff8ab3 100644
--- a/client/src/components/LogEntryEditor.tsx
+++ b/client/src/components/LogEntryEditor.tsx
@@ -538,7 +538,7 @@ export default function LogEntryEditor({
   }, [logbookId])
 
   useEffect(() => {
-    if (!canSignSkipper || readOnly) {
+    if (!canSignSkipper || readOnly || !isOnline) {
       setAiSummaryRemaining(null)
       return
     }
@@ -553,7 +553,7 @@ export default function LogEntryEditor({
         console.warn('Failed to load AI summary usage:', err)
       })
     return () => { cancelled = true }
-  }, [canSignSkipper, readOnly, logbookId, entryId])
+  }, [canSignSkipper, readOnly, isOnline, logbookId, entryId])
 
   useEffect(() => {
     const seq = ++entryHashSeqRef.current
@@ -986,6 +986,10 @@ export default function LogEntryEditor({
         showAlert('GPS capturing failed, and no location name is entered in "Ort / Hafen" or "Start-Hafen" to look up coordinates.')
         return
       }
+      if (!isOnline) {
+        showAlert(t('logs.weather_offline'))
+        return
+      }
 
       try {
         const data = await fetchOpenWeatherCurrent(
@@ -999,6 +1003,10 @@ export default function LogEntryEditor({
           showAlert(`Coordinates loaded for "${locationQuery}" via OpenWeatherMap.`)
         }
       } catch (e) {
+        if (e instanceof WeatherApiError && e.code === 'OFFLINE') {
+          showAlert(t('logs.weather_offline'))
+          return
+        }
         if (e instanceof WeatherApiError && e.code === 'NO_KEY') {
           showAlert(t('settings.no_key'))
           return
@@ -1025,6 +1033,11 @@ export default function LogEntryEditor({
   }
 
   const handleFetchWeather = async () => {
+    if (!isOnline) {
+      showAlert(t('logs.weather_offline'))
+      return
+    }
+
     const localToday = new Date()
     const todayStr = `${localToday.getFullYear()}-${String(localToday.getMonth() + 1).padStart(2, '0')}-${String(localToday.getDate()).padStart(2, '0')}`
 
@@ -1066,6 +1079,10 @@ export default function LogEntryEditor({
 
       showAlert(t('settings.weather_success'))
     } catch (err) {
+      if (err instanceof WeatherApiError && err.code === 'OFFLINE') {
+        showAlert(t('logs.weather_offline'))
+        return
+      }
       if (err instanceof WeatherApiError && err.code === 'NO_KEY') {
         showAlert(t('settings.no_key'))
         return
@@ -1079,6 +1096,10 @@ export default function LogEntryEditor({
 
   const handleGenerateAiSummary = async () => {
     if (!canSignSkipper || readOnly || aiSummaryLoading) return
+    if (!isOnline) {
+      setAiSummaryError(t('logs.ai_summary_offline'))
+      return
+    }
     if (aiSummaryRemaining === 0) {
       setAiSummaryError(t('logs.ai_summary_error_rate_limited'))
       return
@@ -1135,7 +1156,9 @@ export default function LogEntryEditor({
       })
     } catch (err) {
       if (err instanceof TravelDaySummaryApiError) {
-        if (err.code === 'NO_KEY') {
+        if (err.code === 'OFFLINE') {
+          setAiSummaryError(t('logs.ai_summary_offline'))
+        } else if (err.code === 'NO_KEY') {
           setAiSummaryError(t('logs.ai_summary_error_no_key'))
         } else if (err.code === 'RATE_LIMITED') {
           setAiSummaryError(t('logs.ai_summary_error_rate_limited'))
diff --git a/client/src/i18n/locales/da.json b/client/src/i18n/locales/da.json
index 283a2bc..c81e906 100644
--- a/client/src/i18n/locales/da.json
+++ b/client/src/i18n/locales/da.json
@@ -358,6 +358,7 @@
       "event_remarks": "Bemærkninger / hændelser",
       "gps_btn": "Hent GPS-koordinater",
       "weather_btn": "OpenWeatherMap Kald vejret op",
+      "weather_offline": "OpenWeatherMap kræver internetforbindelse. Du er offline lige nu.",
       "event_wind_pressure": "Lufttryk (hPa)",
       "event_heel": "Krængning (°)",
       "event_sails": "Sejlhåndtering/motor",
@@ -381,6 +382,7 @@
       "ai_summary_error_no_key": "Ingen OpenRouter API-nøgle konfigureret på serveren.",
       "ai_summary_error_rate_limited": "Maksimalt antal genereringer nået for denne rejsedag.",
       "ai_summary_error_forbidden": "Kun skipperen må generere AI-resuméer.",
+      "ai_summary_offline": "AI-resumé kræver internetforbindelse. Du er offline lige nu.",
       "photos_title": "Vedhæftede billeder (E2E-krypteret)",
       "photo_caption_label": "Fotobeskrivelse/etiket (valgfrit)",
       "photo_caption_placeholder": "f.eks. at sætte sejl nær indsejlingen til havnen",
diff --git a/client/src/i18n/locales/de.json b/client/src/i18n/locales/de.json
index ecb2346..b316719 100644
--- a/client/src/i18n/locales/de.json
+++ b/client/src/i18n/locales/de.json
@@ -358,6 +358,7 @@
       "event_remarks": "Bemerkungen / Vorkommnisse",
       "gps_btn": "GPS-Koordinaten abrufen",
       "weather_btn": "OpenWeatherMap Wetter abrufen",
+      "weather_offline": "OpenWeatherMap erfordert eine Internetverbindung. Du bist derzeit offline.",
       "event_wind_pressure": "Luftdruck (hPa)",
       "event_heel": "Krängung (°)",
       "event_sails": "Segelführung / Motor",
@@ -381,6 +382,7 @@
       "ai_summary_error_no_key": "Kein OpenRouter API-Schlüssel auf dem Server konfiguriert.",
       "ai_summary_error_rate_limited": "Maximale Anzahl an Generierungen für diesen Reisetag erreicht.",
       "ai_summary_error_forbidden": "Nur der Skipper darf KI-Zusammenfassungen generieren.",
+      "ai_summary_offline": "Die KI-Zusammenfassung erfordert eine Internetverbindung. Du bist derzeit offline.",
       "photos_title": "Foto-Anhänge (E2E-verschlüsselt)",
       "photo_caption_label": "Foto-Beschreibung / Label (Optional)",
       "photo_caption_placeholder": "z.B. Segel setzen nahe Hafeneinfahrt",
diff --git a/client/src/i18n/locales/en.json b/client/src/i18n/locales/en.json
index cdf8659..020d5d2 100644
--- a/client/src/i18n/locales/en.json
+++ b/client/src/i18n/locales/en.json
@@ -358,6 +358,7 @@
       "event_remarks": "Remarks / Events",
       "gps_btn": "Get GPS Location",
       "weather_btn": "Fetch OpenWeatherMap Weather",
+      "weather_offline": "OpenWeatherMap requires an internet connection. You are currently offline.",
       "event_wind_pressure": "Barometer (hPa)",
       "event_heel": "Heel Angle (°)",
       "event_sails": "Sails / Motor Status",
@@ -381,6 +382,7 @@
       "ai_summary_error_no_key": "No OpenRouter API key configured on the server.",
       "ai_summary_error_rate_limited": "Maximum number of generations reached for this travel day.",
       "ai_summary_error_forbidden": "Only the skipper may generate AI summaries.",
+      "ai_summary_offline": "AI summary generation requires an internet connection. You are currently offline.",
       "photos_title": "Photo Attachments (E2E Encrypted)",
       "photo_caption_label": "Photo Caption / Label (Optional)",
       "photo_caption_placeholder": "e.g. Setting sails near harbor entrance",
diff --git a/client/src/i18n/locales/nb.json b/client/src/i18n/locales/nb.json
index e02244c..f543dac 100644
--- a/client/src/i18n/locales/nb.json
+++ b/client/src/i18n/locales/nb.json
@@ -358,6 +358,7 @@
       "event_remarks": "Merknader / hendelser",
       "gps_btn": "Hent GPS-koordinater",
       "weather_btn": "OpenWeatherMap Ring opp været",
+      "weather_offline": "OpenWeatherMap krever internettforbindelse. Du er frakoblet.",
       "event_wind_pressure": "Lufttrykk (hPa)",
       "event_heel": "Helning (°)",
       "event_sails": "Seilhåndtering / motor",
@@ -381,6 +382,7 @@
       "ai_summary_error_no_key": "Ingen OpenRouter API-nøkkel konfigurert på serveren.",
       "ai_summary_error_rate_limited": "Maksimalt antall genereringer nådd for denne reisedagen.",
       "ai_summary_error_forbidden": "Kun skipperen kan generere AI-sammendrag.",
+      "ai_summary_offline": "AI-sammendrag krever internettforbindelse. Du er frakoblet.",
       "photos_title": "Bildevedlegg (E2E-kryptert)",
       "photo_caption_label": "Fotobeskrivelse/etikett (valgfritt)",
       "photo_caption_placeholder": "f.eks. å sette seil nær innseilingen til havnen",
diff --git a/client/src/i18n/locales/sv.json b/client/src/i18n/locales/sv.json
index 03004bf..b4cbe56 100644
--- a/client/src/i18n/locales/sv.json
+++ b/client/src/i18n/locales/sv.json
@@ -358,6 +358,7 @@
       "event_remarks": "Anmärkningar / incidenter",
       "gps_btn": "Hämta GPS-koordinater",
       "weather_btn": "OpenWeatherMap Ring upp väder",
+      "weather_offline": "OpenWeatherMap kräver internetanslutning. Du är offline.",
       "event_wind_pressure": "Lufttryck (hPa)",
       "event_heel": "Krängning (°)",
       "event_sails": "Segelhantering / motor",
@@ -381,6 +382,7 @@
       "ai_summary_error_no_key": "Ingen OpenRouter API-nyckel konfigurerad på servern.",
       "ai_summary_error_rate_limited": "Maximalt antal genereringar nått för denna resedag.",
       "ai_summary_error_forbidden": "Endast skepparen får generera AI-sammanfattningar.",
+      "ai_summary_offline": "AI-sammanfattning kräver internetanslutning. Du är offline.",
       "photos_title": "Fotobilagor (E2E-krypterade)",
       "photo_caption_label": "Fotobeskrivning/etikett (valfritt)",
       "photo_caption_placeholder": "t.ex. sätta segel nära hamninloppet",
diff --git a/client/src/services/aiSummary.ts b/client/src/services/aiSummary.ts
index 7651344..511c910 100644
--- a/client/src/services/aiSummary.ts
+++ b/client/src/services/aiSummary.ts
@@ -5,11 +5,11 @@ import { sortLogEventsByTime, type LogEventPayload } from '../utils/logEntryPayl
 import { PlausibleEvents, trackPlausibleEvent } from './analytics.js'
 
 export class TravelDaySummaryApiError extends Error {
-  code: 'NO_KEY' | 'FORBIDDEN' | 'RATE_LIMITED' | 'REQUEST_FAILED'
+  code: 'NO_KEY' | 'FORBIDDEN' | 'RATE_LIMITED' | 'OFFLINE' | 'REQUEST_FAILED'
 
   constructor(
     message: string,
-    code: 'NO_KEY' | 'FORBIDDEN' | 'RATE_LIMITED' | 'REQUEST_FAILED' = 'REQUEST_FAILED'
+    code: 'NO_KEY' | 'FORBIDDEN' | 'RATE_LIMITED' | 'OFFLINE' | 'REQUEST_FAILED' = 'REQUEST_FAILED'
   ) {
     super(message)
     this.name = 'TravelDaySummaryApiError'
@@ -146,6 +146,10 @@ export async function generateTravelDaySummary(params: {
   language: string
   context: TravelDaySummaryContext
 }): Promise<{ summary: string; remainingAttempts: number; maxAttempts: number }> {
+  if (!navigator.onLine) {
+    throw new TravelDaySummaryApiError('Offline', 'OFFLINE')
+  }
+
   const controller = new AbortController()
   const timeoutId = window.setTimeout(() => controller.abort(), SUMMARY_FETCH_TIMEOUT_MS)
 
diff --git a/client/src/services/weather.test.ts b/client/src/services/weather.test.ts
index b765291..a7b6685 100644
--- a/client/src/services/weather.test.ts
+++ b/client/src/services/weather.test.ts
@@ -44,6 +44,17 @@ describe('fetchOpenWeatherCurrent', () => {
     })
   })
 
+  it('throws OFFLINE when navigator.onLine is false', async () => {
+    vi.stubGlobal('navigator', { ...navigator, onLine: false })
+
+    const { fetchOpenWeatherCurrent, WeatherApiError } = await import('./weather.js')
+    const err = await fetchOpenWeatherCurrent({ lat: '54', lon: '10' }).catch((e) => e)
+    expect(err).toBeInstanceOf(WeatherApiError)
+    expect((err as InstanceType<typeof WeatherApiError>).code).toBe('OFFLINE')
+
+    expect(apiFetch).not.toHaveBeenCalled()
+  })
+
   it('does not track when the API request fails', async () => {
     apiFetch.mockResolvedValue({
       ok: false,
diff --git a/client/src/services/weather.ts b/client/src/services/weather.ts
index 5b0f5ad..9b3e55c 100644
--- a/client/src/services/weather.ts
+++ b/client/src/services/weather.ts
@@ -7,9 +7,9 @@ import {
 } from './analytics.js'
 
 export class WeatherApiError extends Error {
-  code: 'NO_KEY' | 'REQUEST_FAILED'
+  code: 'NO_KEY' | 'OFFLINE' | 'REQUEST_FAILED'
 
-  constructor(message: string, code: 'NO_KEY' | 'REQUEST_FAILED' = 'REQUEST_FAILED') {
+  constructor(message: string, code: 'NO_KEY' | 'OFFLINE' | 'REQUEST_FAILED' = 'REQUEST_FAILED') {
     super(message)
     this.name = 'WeatherApiError'
     this.code = code
@@ -26,6 +26,10 @@ export async function fetchOpenWeatherCurrent(
   },
   options?: { analyticsSource: OwmAnalyticsSource }
 ): Promise<Record<string, unknown>> {
+  if (!navigator.onLine) {
+    throw new WeatherApiError('Offline', 'OFFLINE')
+  }
+
   const searchParams = new URLSearchParams()
 
   if (params.lat && params.lon) {