diff --git a/snapshot/src/core/cover-probe.ts b/snapshot/src/core/cover-probe.ts
new file mode 100644
index 0000000..10a47d3
--- /dev/null
+++ b/snapshot/src/core/cover-probe.ts
@@ -0,0 +1,23 @@
+export interface ProbeResult {
+  reachable: boolean
+  status: number
+}
+
+export type HeadFetcher = (url: string) => Promise<{ ok: boolean; status: number }>
+
+export const defaultHeadFetcher: HeadFetcher = async (url) => {
+  const resp = await fetch(url, { method: 'HEAD' })
+  return { ok: resp.ok, status: resp.status }
+}
+
+export async function probeCover(
+  url: string,
+  fetcher: HeadFetcher = defaultHeadFetcher,
+): Promise<ProbeResult> {
+  try {
+    const r = await fetcher(url)
+    return { reachable: r.ok, status: r.status }
+  } catch {
+    return { reachable: false, status: 0 }
+  }
+}
diff --git a/snapshot/tests/cover-probe.test.ts b/snapshot/tests/cover-probe.test.ts
new file mode 100644
index 0000000..34499a1
--- /dev/null
+++ b/snapshot/tests/cover-probe.test.ts
@@ -0,0 +1,22 @@
+import { assertEquals } from '@std/assert'
+import { probeCover, type HeadFetcher } from '../src/core/cover-probe.ts'
+
+Deno.test('probeCover: 200 -> reachable=true', async () => {
+  const fetcher: HeadFetcher = async () => ({ ok: true, status: 200 })
+  const r = await probeCover('https://blossom.example/abc.jpg', fetcher)
+  assertEquals(r, { reachable: true, status: 200 })
+})
+
+Deno.test('probeCover: 404 -> reachable=false', async () => {
+  const fetcher: HeadFetcher = async () => ({ ok: false, status: 404 })
+  const r = await probeCover('https://blossom.example/abc.jpg', fetcher)
+  assertEquals(r, { reachable: false, status: 404 })
+})
+
+Deno.test('probeCover: network error -> reachable=false', async () => {
+  const fetcher: HeadFetcher = async () => {
+    throw new Error('ECONNREFUSED')
+  }
+  const r = await probeCover('https://blossom.example/abc.jpg', fetcher)
+  assertEquals(r, { reachable: false, status: 0 })
+})