diff --git a/snapshot/src/core/cache.ts b/snapshot/src/core/cache.ts
new file mode 100644
index 0000000..c3c315d
--- /dev/null
+++ b/snapshot/src/core/cache.ts
@@ -0,0 +1,18 @@
+export interface CacheState {
+  lastKnownGoodCount: number
+  deletedCoords: string[]
+}
+
+export async function readCache(path: string): Promise<CacheState | undefined> {
+  try {
+    const text = await Deno.readTextFile(path)
+    return JSON.parse(text) as CacheState
+  } catch (err) {
+    if (err instanceof Deno.errors.NotFound) return undefined
+    throw err
+  }
+}
+
+export async function writeCache(path: string, state: CacheState): Promise<void> {
+  await Deno.writeTextFile(path, JSON.stringify(state, null, 2) + '\n')
+}
diff --git a/snapshot/tests/cache.test.ts b/snapshot/tests/cache.test.ts
new file mode 100644
index 0000000..4b66e17
--- /dev/null
+++ b/snapshot/tests/cache.test.ts
@@ -0,0 +1,19 @@
+import { assertEquals } from '@std/assert'
+import { join } from '@std/path'
+import { readCache, writeCache, type CacheState } from '../src/core/cache.ts'
+
+Deno.test('readCache: file fehlt -> undefined', async () => {
+  const dir = await Deno.makeTempDir()
+  const path = join(dir, 'cache.json')
+  const cache = await readCache(path)
+  assertEquals(cache, undefined)
+})
+
+Deno.test('writeCache + readCache: round-trip', async () => {
+  const dir = await Deno.makeTempDir()
+  const path = join(dir, 'cache.json')
+  const state: CacheState = { lastKnownGoodCount: 27, deletedCoords: ['30023:P:dead'] }
+  await writeCache(path, state)
+  const out = await readCache(path)
+  assertEquals(out, state)
+})