diff --git a/adapters/common/attachment/__tests__/attachment-store.test.ts b/adapters/common/attachment/__tests__/attachment-store.test.ts
new file mode 100644
index 00000000..6f59c578
--- /dev/null
+++ b/adapters/common/attachment/__tests__/attachment-store.test.ts
@@ -0,0 +1,76 @@
+import { describe, it, expect, beforeEach, afterEach } from 'bun:test'
+import * as fs from 'node:fs/promises'
+import * as path from 'node:path'
+import * as os from 'node:os'
+import { AttachmentStore } from '../attachment-store.js'
+
+let tmpRoot: string
+
+beforeEach(async () => {
+  tmpRoot = await fs.mkdtemp(path.join(os.tmpdir(), 'att-store-test-'))
+})
+
+afterEach(async () => {
+  await fs.rm(tmpRoot, { recursive: true, force: true })
+})
+
+describe('AttachmentStore', () => {
+  it('writes a buffer and returns the absolute path', async () => {
+    const store = new AttachmentStore({ root: tmpRoot, retentionMs: 60_000 })
+    const target = store.resolvePath('feishu', 'sess-1', 'hello.png')
+    const written = await store.write(target, Buffer.from('PNGDATA'))
+    expect(path.isAbsolute(written)).toBe(true)
+    const content = await fs.readFile(written)
+    expect(content.toString()).toBe('PNGDATA')
+  })
+
+  it('writes under {root}/{platform}/{sessionId}/', async () => {
+    const store = new AttachmentStore({ root: tmpRoot, retentionMs: 60_000 })
+    const target = store.resolvePath('telegram', 'sess-42', 'foo.pdf')
+    expect(target).toContain(path.join('telegram', 'sess-42'))
+    expect(target.endsWith('foo.pdf')).toBe(true)
+  })
+
+  it('sanitizes unsafe filenames (strips path separators and ..)', async () => {
+    const store = new AttachmentStore({ root: tmpRoot, retentionMs: 60_000 })
+    const target = store.resolvePath('feishu', 'sess-1', '../../etc/passwd')
+    // The resulting target must still live inside the store root.
+    const root = path.resolve(tmpRoot)
+    expect(path.resolve(target).startsWith(root)).toBe(true)
+    expect(path.basename(target)).not.toContain('..')
+    expect(path.basename(target)).not.toContain('/')
+  })
+
+  it('collapses name collisions by prefixing timestamps', async () => {
+    const store = new AttachmentStore({ root: tmpRoot, retentionMs: 60_000 })
+    const a = store.resolvePath('feishu', 'sess-1', 'image.png')
+    await store.write(a, Buffer.from('first'))
+    const b = store.resolvePath('feishu', 'sess-1', 'image.png')
+    expect(b).not.toBe(a)
+    await store.write(b, Buffer.from('second'))
+    const contentB = await fs.readFile(b)
+    expect(contentB.toString()).toBe('second')
+  })
+
+  it('gc() removes files older than retentionMs and reports counts', async () => {
+    const store = new AttachmentStore({ root: tmpRoot, retentionMs: 50 })
+    const target = store.resolvePath('feishu', 'sess-1', 'stale.png')
+    await store.write(target, Buffer.from('STALE'))
+    // Age the file manually
+    const past = new Date(Date.now() - 10_000)
+    await fs.utimes(target, past, past)
+    const result = await store.gc()
+    expect(result.removed).toBe(1)
+    expect(result.bytes).toBe(5)
+    await expect(fs.access(target)).rejects.toThrow()
+  })
+
+  it('gc() keeps fresh files', async () => {
+    const store = new AttachmentStore({ root: tmpRoot, retentionMs: 60_000 })
+    const target = store.resolvePath('feishu', 'sess-1', 'fresh.png')
+    await store.write(target, Buffer.from('FRESH'))
+    const result = await store.gc()
+    expect(result.removed).toBe(0)
+    await fs.access(target)
+  })
+})
diff --git a/adapters/common/attachment/attachment-store.ts b/adapters/common/attachment/attachment-store.ts
new file mode 100644
index 00000000..12937b08
--- /dev/null
+++ b/adapters/common/attachment/attachment-store.ts
@@ -0,0 +1,106 @@
+/**
+ * Local staging directory for IM-downloaded resources.
+ *
+ * Layout: {root}/{platform}/{sessionId}/{safeName}
+ * Default root: ~/.claude/im-downloads
+ *
+ * Responsibilities:
+ *  - Generate unique, safe paths from (platform, sessionId, originalName)
+ *  - Atomic write (tmp → rename) so concurrent downloads never corrupt each other
+ *  - GC files that haven't been touched for `retentionMs` (default 24h)
+ */
+
+import * as fs from 'node:fs/promises'
+import * as fsSync from 'node:fs'
+import * as path from 'node:path'
+import * as os from 'node:os'
+import type { ImPlatform } from './attachment-types.js'
+
+export interface AttachmentStoreConfig {
+  root: string
+  retentionMs: number
+}
+
+const DEFAULT_RETENTION_MS = 24 * 60 * 60 * 1000
+
+function defaultRoot(): string {
+  return path.join(os.homedir(), '.claude', 'im-downloads')
+}
+
+/** Strip path separators / .. / control chars from a filename. */
+function sanitizeFilename(name: string): string {
+  // eslint-disable-next-line no-control-regex
+  const base = path.basename(name || '').replace(/[\x00-\x1f]/g, '')
+  const cleaned = base.replace(/[\/\\]/g, '_').replace(/\.\.+/g, '_')
+  return cleaned.trim() || 'unnamed'
+}
+
+export class AttachmentStore {
+  private readonly root: string
+  private readonly retentionMs: number
+
+  constructor(config?: Partial<AttachmentStoreConfig>) {
+    this.root = config?.root ?? defaultRoot()
+    this.retentionMs = config?.retentionMs ?? DEFAULT_RETENTION_MS
+  }
+
+  /** Compute the target path. Creates parent dirs on demand.
+   *  If a file with the same name already exists, prefix with a timestamp
+   *  to avoid clobbering. */
+  resolvePath(platform: ImPlatform, sessionId: string, name: string): string {
+    const safeSession = sanitizeFilename(sessionId)
+    const dir = path.join(this.root, platform, safeSession)
+    fsSync.mkdirSync(dir, { recursive: true })
+    const safeName = sanitizeFilename(name)
+    const candidate = path.join(dir, safeName)
+    if (!fsSync.existsSync(candidate)) return candidate
+    const { name: base, ext } = path.parse(safeName)
+    return path.join(dir, `${base}-${Date.now()}${ext}`)
+  }
+
+  /** Write atomically: stream to {target}.part, then rename. */
+  async write(target: string, data: Buffer): Promise<string> {
+    await fs.mkdir(path.dirname(target), { recursive: true })
+    const tmp = `${target}.${process.pid}.${Date.now()}.part`
+    await fs.writeFile(tmp, data)
+    await fs.rename(tmp, target)
+    return target
+  }
+
+  /** Remove files older than retentionMs. Returns summary. */
+  async gc(): Promise<{ removed: number; bytes: number }> {
+    let removed = 0
+    let bytes = 0
+    const now = Date.now()
+
+    const walk = async (dir: string): Promise<void> => {
+      let entries: Awaited<ReturnType<typeof fs.readdir>>
+      try {
+        entries = await fs.readdir(dir, { withFileTypes: true })
+      } catch {
+        return
+      }
+      for (const entry of entries) {
+        const full = path.join(dir, entry.name)
+        if (entry.isDirectory()) {
+          await walk(full)
+        } else if (entry.isFile()) {
+          try {
+            const stat = await fs.stat(full)
+            const age = now - stat.mtimeMs
+            if (age > this.retentionMs) {
+              bytes += stat.size
+              await fs.unlink(full)
+              removed++
+            }
+          } catch {
+            // ignore races
+          }
+        }
+      }
+    }
+
+    await walk(this.root).catch(() => {})
+    return { removed, bytes }
+  }
+}