diff --git a/snapshot/src/core/checks.ts b/snapshot/src/core/checks.ts
new file mode 100644
index 0000000..d2fc97b
--- /dev/null
+++ b/snapshot/src/core/checks.ts
@@ -0,0 +1,36 @@
+export interface CheckInput {
+  relaysQueried: number
+  relaysResponded: number
+  eventCount: number
+  minEvents: number
+  lastKnownGoodCount: number | undefined
+  newDeletionsCount: number
+  allowShrink: boolean
+}
+
+export function runChecks(input: CheckInput): void {
+  const quorum = Math.ceil(input.relaysQueried * 0.6)
+  if (input.relaysResponded < quorum) {
+    throw new Error(
+      `Relay-Quorum nicht erreicht: ${input.relaysResponded}/${input.relaysQueried} ` +
+        `(brauche mindestens ${quorum})`,
+    )
+  }
+  if (input.eventCount < input.minEvents) {
+    throw new Error(
+      `Event-Count ${input.eventCount} unter min-events ${input.minEvents}`,
+    )
+  }
+  if (input.lastKnownGoodCount !== undefined && !input.allowShrink) {
+    const drop = input.lastKnownGoodCount - input.eventCount
+    const dropPct = drop / input.lastKnownGoodCount
+    if (dropPct > 0.2 && drop > input.newDeletionsCount) {
+      throw new Error(
+        `Event-Count-Drop ${drop} (${(dropPct * 100).toFixed(0)}%) gegenueber ` +
+          `last-known-good ${input.lastKnownGoodCount}, ` +
+          `nur ${input.newDeletionsCount} korrespondierende kind:5. ` +
+          `Override mit --allow-shrink falls bewusst.`,
+      )
+    }
+  }
+}
diff --git a/snapshot/tests/checks.test.ts b/snapshot/tests/checks.test.ts
new file mode 100644
index 0000000..65547c0
--- /dev/null
+++ b/snapshot/tests/checks.test.ts
@@ -0,0 +1,59 @@
+import { assertEquals, assertThrows } from '@std/assert'
+import { runChecks } from '../src/core/checks.ts'
+
+Deno.test('runChecks: weniger als 60% relays geantwortet -> hard-fail', () => {
+  assertThrows(
+    () => runChecks({
+      relaysQueried: 5, relaysResponded: 2,
+      eventCount: 27, minEvents: 1, lastKnownGoodCount: undefined,
+      newDeletionsCount: 0, allowShrink: false,
+    }),
+    Error, 'Relay-Quorum',
+  )
+})
+
+Deno.test('runChecks: event-count unter min-events -> hard-fail', () => {
+  assertThrows(
+    () => runChecks({
+      relaysQueried: 5, relaysResponded: 5,
+      eventCount: 0, minEvents: 1, lastKnownGoodCount: undefined,
+      newDeletionsCount: 0, allowShrink: false,
+    }),
+    Error, 'min-events',
+  )
+})
+
+Deno.test('runChecks: drop > 20% ohne kind:5 -> hard-fail', () => {
+  assertThrows(
+    () => runChecks({
+      relaysQueried: 5, relaysResponded: 5,
+      eventCount: 20, minEvents: 1, lastKnownGoodCount: 27,
+      newDeletionsCount: 0, allowShrink: false,
+    }),
+    Error, 'Event-Count-Drop',
+  )
+})
+
+Deno.test('runChecks: drop > 20% mit korrespondierenden kind:5 -> ok', () => {
+  runChecks({
+    relaysQueried: 5, relaysResponded: 5,
+    eventCount: 20, minEvents: 1, lastKnownGoodCount: 27,
+    newDeletionsCount: 7, allowShrink: false,
+  })
+})
+
+Deno.test('runChecks: --allow-shrink umgeht drop-check', () => {
+  runChecks({
+    relaysQueried: 5, relaysResponded: 5,
+    eventCount: 1, minEvents: 1, lastKnownGoodCount: 27,
+    newDeletionsCount: 0, allowShrink: true,
+  })
+})
+
+Deno.test('runChecks: erstlauf ohne cache + min-events=1 -> ok', () => {
+  runChecks({
+    relaysQueried: 5, relaysResponded: 5,
+    eventCount: 1, minEvents: 1, lastKnownGoodCount: undefined,
+    newDeletionsCount: 0, allowShrink: false,
+  })
+})