diff --git a/extension/src/service-worker/vault.ts b/extension/src/service-worker/vault.ts
index 044ede8..2c10a74 100644
--- a/extension/src/service-worker/vault.ts
+++ b/extension/src/service-worker/vault.ts
@@ -3,7 +3,7 @@
 
 import type { SessionHandle } from '../../wasm/relicario_wasm';
 import type { GitHost } from './git-host';
-import type { Item, ItemId, Manifest, ManifestEntry, VaultSettings } from '../shared/types';
+import type { AttachmentRef, Item, ItemId, Manifest, ManifestEntry, VaultSettings } from '../shared/types';
 
 // eslint-disable-next-line @typescript-eslint/no-explicit-any
 let wasm: any = null;
@@ -140,3 +140,83 @@ export function findByHostname(
     .filter(([, e]) => e.trashed_at === undefined)
     .filter(([, e]) => (e.icon_hint ?? '').toLowerCase() === h);
 }
+
+// --- Attachment helpers ---
+
+/// Sync the manifest entry's attachment_summaries for a single item.
+/// Reads the current manifest + item; if either is missing, does nothing.
+async function syncManifestAttachments(
+  git: GitHost,
+  handle: SessionHandle,
+  itemId: ItemId,
+  item: Item,
+  message: string,
+): Promise<void> {
+  const manifest = await fetchAndDecryptManifest(git, handle);
+  const entry = manifest.items[itemId];
+  if (!entry) return;
+  entry.attachment_summaries = item.attachments.map((a) => ({
+    id: a.id,
+    filename: a.filename,
+    mime_type: a.mime_type,
+    size: a.size,
+  }));
+  await encryptAndWriteManifest(git, handle, manifest, message);
+}
+
+/// Add an AttachmentRef to an existing item.
+/// Reads + decrypts the item, appends to attachments[], re-encrypts and
+/// writes both items/<id>.enc and the manifest entry's attachment_summaries.
+/// Throws if the item is not found.
+export async function addAttachmentToItem(
+  git: GitHost,
+  handle: SessionHandle,
+  itemId: ItemId,
+  ref: AttachmentRef,
+): Promise<void> {
+  let item: Item;
+  try {
+    item = await fetchAndDecryptItem(git, handle, itemId);
+  } catch {
+    throw new Error(`addAttachmentToItem: item ${itemId} not found`);
+  }
+  item.attachments = [...item.attachments, ref];
+  item.modified = Math.floor(Date.now() / 1000);
+  const commitMsg = `attach ${ref.filename} to item ${itemId}`;
+  await encryptAndWriteItem(git, handle, itemId, item, commitMsg);
+  await syncManifestAttachments(git, handle, itemId, item, `manifest: sync attachments for ${itemId}`);
+}
+
+/// Remove attachments matching the given IDs from the item.
+/// Returns the removed AttachmentRefs (so the caller can deleteBlob the
+/// underlying bytes). IDs not present in the item's attachments are
+/// silently ignored.
+export async function removeAttachmentsFromItem(
+  git: GitHost,
+  handle: SessionHandle,
+  itemId: ItemId,
+  idsToRemove: string[],
+): Promise<AttachmentRef[]> {
+  let item: Item;
+  try {
+    item = await fetchAndDecryptItem(git, handle, itemId);
+  } catch {
+    throw new Error(`removeAttachmentsFromItem: item ${itemId} not found`);
+  }
+  const removeSet = new Set(idsToRemove);
+  const removed: AttachmentRef[] = [];
+  const kept: AttachmentRef[] = [];
+  for (const att of item.attachments) {
+    if (removeSet.has(att.id)) {
+      removed.push(att);
+    } else {
+      kept.push(att);
+    }
+  }
+  item.attachments = kept;
+  item.modified = Math.floor(Date.now() / 1000);
+  const commitMsg = `remove ${removed.length} attachment(s) from item ${itemId}`;
+  await encryptAndWriteItem(git, handle, itemId, item, commitMsg);
+  await syncManifestAttachments(git, handle, itemId, item, `manifest: sync attachments for ${itemId}`);
+  return removed;
+}