diff --git a/gerbil-manager-web/README.md b/gerbil-manager-web/README.md
index 4e8df10..4359625 100644
--- a/gerbil-manager-web/README.md
+++ b/gerbil-manager-web/README.md
@@ -24,6 +24,14 @@ Erwartet die laufende GerbilManagerWebAPI unter `http://localhost:5179`
   `src/strings/de.ts` — keine Texte direkt in Komponenten hartkodieren.
 - API-Zugriffe laufen über `src/api/client.ts`.
 - Routen werden in `src/App.tsx` registriert, Seiten liegen in `src/pages/`.
+- **Mutationen** (`useMutation` aus `src/hooks/useApi.ts`): `run()` wirft NIE,
+  sondern liefert ein `MutationOutcome<T>` — `{ ok: true, value }` oder
+  `{ ok: false, error, cause }`. Aufrufer MÜSSEN verzweigen:
+  `const r = await m.run(x); if (r.ok) navigate(r.value.id)`. `m.error` treibt
+  die Inline-Anzeige; für Spezialfälle (z. B. HTTP 409) `r.cause` prüfen
+  (`r.cause instanceof ApiError && r.cause.status === 409`). So sind unbehandelte
+  Promise-Rejections an Aufrufstellen ausgeschlossen (siehe auch
+  `src/dev/unhandledRejectionGuard.ts`).
 
 ## Build
 
diff --git a/gerbil-manager-web/src/hooks/useApi.ts b/gerbil-manager-web/src/hooks/useApi.ts
index 01c635b..b0dc883 100644
--- a/gerbil-manager-web/src/hooks/useApi.ts
+++ b/gerbil-manager-web/src/hooks/useApi.ts
@@ -67,11 +67,25 @@ export function useApi<T>(loader: () => Promise<T>, deps: unknown[]): AsyncState
   return { data: resolved.data, loading, error: loading ? null : resolved.error, reload }
 }
 
-/** Wraps a mutation (create/update/delete) with pending + error state. */
+/**
+ * Outcome of a mutation. Discriminated on `ok` so callers MUST branch before
+ * touching `value` — this makes error handling compile-time-visible and means
+ * `run()` NEVER throws (no more unhandled rejections from async event handlers).
+ * Works for void mutations too (delete): success is `{ ok: true, value: undefined }`.
+ */
+export type MutationOutcome<T> =
+  | { ok: true; value: T }
+  | { ok: false; error: string; cause: unknown }
+
+/**
+ * Wraps a mutation (create/update/delete) with pending + error state.
+ * `run()` resolves to a MutationOutcome and never rejects; `error` also drives
+ * inline UI as before. Call sites: `const r = await m.run(x); if (r.ok) …`.
+ */
 export function useMutation<TArgs extends unknown[], TResult>(
   mutator: (...args: TArgs) => Promise<TResult>,
 ): {
-  run: (...args: TArgs) => Promise<TResult>
+  run: (...args: TArgs) => Promise<MutationOutcome<TResult>>
   pending: boolean
   error: string | null
 } {
@@ -79,14 +93,16 @@ export function useMutation<TArgs extends unknown[], TResult>(
   const [error, setError] = useState<string | null>(null)
 
   const run = useCallback(
-    async (...args: TArgs) => {
+    async (...args: TArgs): Promise<MutationOutcome<TResult>> => {
       setPending(true)
       setError(null)
       try {
-        return await mutator(...args)
+        const value = await mutator(...args)
+        return { ok: true, value }
       } catch (err) {
-        setError(messageFor(err))
-        throw err
+        const message = messageFor(err)
+        setError(message)
+        return { ok: false, error: message, cause: err }
       } finally {
         setPending(false)
       }
diff --git a/gerbil-manager-web/src/pages/BeckenDetailPage.tsx b/gerbil-manager-web/src/pages/BeckenDetailPage.tsx
index 452c37d..2ba666c 100644
--- a/gerbil-manager-web/src/pages/BeckenDetailPage.tsx
+++ b/gerbil-manager-web/src/pages/BeckenDetailPage.tsx
@@ -42,16 +42,14 @@ export default function BeckenDetailPage() {
   async function onDelete() {
     if (!window.confirm(t.delete.confirmMessage)) return
     setDeleteError(null)
-    try {
-      await removal.run()
+    const result = await removal.run()
+    if (result.ok) {
       navigate('/becken')
-    } catch (err) {
+    } else if (result.cause instanceof ApiError && result.cause.status === 409) {
       // Backend meldet Konflikt, wenn noch Tiere im Becken wohnen.
-      if (err instanceof ApiError && err.status === 409) {
-        setDeleteError(t.delete.conflict)
-      } else {
-        setDeleteError(removal.error ?? de.api.errors.unknown)
-      }
+      setDeleteError(t.delete.conflict)
+    } else {
+      setDeleteError(result.error)
     }
   }
 
diff --git a/gerbil-manager-web/src/pages/BeckenFormPage.tsx b/gerbil-manager-web/src/pages/BeckenFormPage.tsx
index 46702f7..f7df10c 100644
--- a/gerbil-manager-web/src/pages/BeckenFormPage.tsx
+++ b/gerbil-manager-web/src/pages/BeckenFormPage.tsx
@@ -44,8 +44,9 @@ export default function BeckenFormPage() {
       return
     }
     setErrors({})
-    const saved = await mutation.run({ name: form.name.trim(), notes: nn(form.notes) })
-    navigate(`/becken/${saved.id}`)
+    const result = await mutation.run({ name: form.name.trim(), notes: nn(form.notes) })
+    if (result.ok) navigate(`/becken/${result.value.id}`)
+    // On failure mutation.error drives the inline alert; run() never throws.
   }
 
   if (isEdit && existing.loading) return <p className="muted">{de.common.loading}</p>
diff --git a/gerbil-manager-web/src/pages/GerbilFormPage.tsx b/gerbil-manager-web/src/pages/GerbilFormPage.tsx
index ed659a9..5600f67 100644
--- a/gerbil-manager-web/src/pages/GerbilFormPage.tsx
+++ b/gerbil-manager-web/src/pages/GerbilFormPage.tsx
@@ -166,13 +166,9 @@ export default function GerbilFormPage() {
       genotype: nn(form.genotype),
       notes: nn(form.notes),
     }
-    try {
-      const saved = await mutation.run(body)
-      navigate(`/rennmaeuse/${saved.id}`)
-    } catch {
-      // Error is surfaced via mutation.error; swallow so the rejected promise
-      // from this async submit handler doesn't become an unhandled rejection.
-    }
+    const result = await mutation.run(body)
+    if (result.ok) navigate(`/rennmaeuse/${result.value.id}`)
+    // On failure mutation.error drives the inline alert; run() never throws.
   }
 
   if (isEdit && existing.loading) return <p className="muted">{de.common.loading}</p>
diff --git a/gerbil-manager-web/src/pages/KontaktDetailPage.tsx b/gerbil-manager-web/src/pages/KontaktDetailPage.tsx
index 92490d9..d0b3103 100644
--- a/gerbil-manager-web/src/pages/KontaktDetailPage.tsx
+++ b/gerbil-manager-web/src/pages/KontaktDetailPage.tsx
@@ -45,16 +45,14 @@ export default function KontaktDetailPage() {
   async function onDelete() {
     if (!window.confirm(t.delete.confirmMessage)) return
     setDeleteError(null)
-    try {
-      await removal.run()
+    const result = await removal.run()
+    if (result.ok) {
       navigate('/kontakte')
-    } catch (err) {
+    } else if (result.cause instanceof ApiError && result.cause.status === 409) {
       // Backend meldet Konflikt, wenn der Kontakt noch referenziert wird.
-      if (err instanceof ApiError && err.status === 409) {
-        setDeleteError(t.delete.conflict)
-      } else {
-        setDeleteError(removal.error ?? de.api.errors.unknown)
-      }
+      setDeleteError(t.delete.conflict)
+    } else {
+      setDeleteError(result.error)
     }
   }
 
diff --git a/gerbil-manager-web/src/pages/KontaktFormPage.tsx b/gerbil-manager-web/src/pages/KontaktFormPage.tsx
index bfe4a94..3aebe83 100644
--- a/gerbil-manager-web/src/pages/KontaktFormPage.tsx
+++ b/gerbil-manager-web/src/pages/KontaktFormPage.tsx
@@ -52,12 +52,13 @@ export default function KontaktFormPage() {
       return
     }
     setErrors({})
-    const saved = await mutation.run({
+    const result = await mutation.run({
       name: form.name.trim(),
       contactInfo: nn(form.contactInfo),
       notes: nn(form.notes),
     })
-    navigate(`/kontakte/${saved.id}`)
+    if (result.ok) navigate(`/kontakte/${result.value.id}`)
+    // On failure mutation.error drives the inline alert; run() never throws.
   }
 
   if (isEdit && existing.loading) return <p className="muted">{de.common.loading}</p>
diff --git a/gerbil-manager-web/src/pages/WurfFormPage.tsx b/gerbil-manager-web/src/pages/WurfFormPage.tsx
index 423fe11..6ba21ba 100644
--- a/gerbil-manager-web/src/pages/WurfFormPage.tsx
+++ b/gerbil-manager-web/src/pages/WurfFormPage.tsx
@@ -116,13 +116,9 @@ export default function WurfFormPage() {
       expectedGoHomeDate: nn(form.expectedGoHomeDate),
       notes: nn(form.notes),
     }
-    try {
-      const saved = await mutation.run(body)
-      navigate(`/wuerfe/${saved.id}`)
-    } catch {
-      // Error surfaced via mutation.error; swallow to avoid an unhandled
-      // rejection escaping this async submit handler.
-    }
+    const result = await mutation.run(body)
+    if (result.ok) navigate(`/wuerfe/${result.value.id}`)
+    // On failure mutation.error drives the inline alert; run() never throws.
   }
 
   if (isEdit && existing.loading) return <p className="muted">{de.common.loading}</p>