diff --git a/snapshot/src/core/cache.ts b/snapshot/src/core/cache.ts
index c3c315d..f5870fb 100644
--- a/snapshot/src/core/cache.ts
+++ b/snapshot/src/core/cache.ts
@@ -4,13 +4,23 @@ export interface CacheState {
 }
 
 export async function readCache(path: string): Promise<CacheState | undefined> {
+  let text: string
   try {
-    const text = await Deno.readTextFile(path)
-    return JSON.parse(text) as CacheState
+    text = await Deno.readTextFile(path)
   } catch (err) {
     if (err instanceof Deno.errors.NotFound) return undefined
     throw err
   }
+  const parsed = JSON.parse(text) as unknown
+  if (
+    !parsed ||
+    typeof parsed !== 'object' ||
+    typeof (parsed as { lastKnownGoodCount?: unknown }).lastKnownGoodCount !== 'number' ||
+    !Array.isArray((parsed as { deletedCoords?: unknown }).deletedCoords)
+  ) {
+    throw new Error('Cache-File hat unbekanntes Format — bitte loeschen und neu starten')
+  }
+  return parsed as CacheState
 }
 
 export async function writeCache(path: string, state: CacheState): Promise<void> {
diff --git a/snapshot/tests/cache.test.ts b/snapshot/tests/cache.test.ts
index 4b66e17..2eb630c 100644
--- a/snapshot/tests/cache.test.ts
+++ b/snapshot/tests/cache.test.ts
@@ -17,3 +17,18 @@ Deno.test('writeCache + readCache: round-trip', async () => {
   const out = await readCache(path)
   assertEquals(out, state)
 })
+
+Deno.test('readCache wirft bei korruptem cache-file', async () => {
+  const dir = await Deno.makeTempDir()
+  const path = join(dir, 'cache.json')
+  await Deno.writeTextFile(path, '{"unsinn": 42}')
+  let threw = false
+  try {
+    await readCache(path)
+  } catch (err) {
+    threw = true
+    if (!(err instanceof Error)) throw err
+    if (!err.message.includes('Cache-File')) throw err
+  }
+  if (!threw) throw new Error('readCache haette werfen sollen')
+})