diff --git a/adapters/common/attachment/__tests__/attachment-limits.test.ts b/adapters/common/attachment/__tests__/attachment-limits.test.ts
new file mode 100644
index 00000000..aa497d0e
--- /dev/null
+++ b/adapters/common/attachment/__tests__/attachment-limits.test.ts
@@ -0,0 +1,46 @@
+import { describe, it, expect } from 'bun:test'
+import {
+  checkAttachmentLimit,
+  IMAGE_MAX_BYTES,
+  FILE_MAX_BYTES,
+  IMAGE_MIME_WHITELIST,
+} from '../attachment-limits.js'
+
+describe('checkAttachmentLimit', () => {
+  it('accepts a 1 MB PNG image', () => {
+    const result = checkAttachmentLimit('image', 1024 * 1024, 'image/png')
+    expect(result.ok).toBe(true)
+  })
+
+  it('rejects an 11 MB image as too_large', () => {
+    const result = checkAttachmentLimit('image', 11 * 1024 * 1024, 'image/png')
+    expect(result.ok).toBe(false)
+    if (!result.ok) {
+      expect(result.reason).toBe('too_large')
+      expect(result.hint).toContain('10')
+    }
+  })
+
+  it('rejects an unsupported image mime', () => {
+    const result = checkAttachmentLimit('image', 500_000, 'image/svg+xml')
+    expect(result.ok).toBe(false)
+    if (!result.ok) expect(result.reason).toBe('unsupported_mime')
+  })
+
+  it('accepts a 10 MB PDF file', () => {
+    const result = checkAttachmentLimit('file', 10 * 1024 * 1024, 'application/pdf')
+    expect(result.ok).toBe(true)
+  })
+
+  it('rejects a 31 MB file as too_large', () => {
+    const result = checkAttachmentLimit('file', 31 * 1024 * 1024, 'application/pdf')
+    expect(result.ok).toBe(false)
+    if (!result.ok) expect(result.reason).toBe('too_large')
+  })
+
+  it('exposes the limits as exports', () => {
+    expect(IMAGE_MAX_BYTES).toBe(10 * 1024 * 1024)
+    expect(FILE_MAX_BYTES).toBe(30 * 1024 * 1024)
+    expect(IMAGE_MIME_WHITELIST).toContain('image/png')
+  })
+})
diff --git a/adapters/common/attachment/attachment-limits.ts b/adapters/common/attachment/attachment-limits.ts
new file mode 100644
index 00000000..170ae231
--- /dev/null
+++ b/adapters/common/attachment/attachment-limits.ts
@@ -0,0 +1,59 @@
+/**
+ * Size and MIME restrictions for IM attachments.
+ *
+ * Limits chosen to sit safely under both Feishu (10 MB image / 30 MB file)
+ * and Telegram Bot API (10 MB image / 50 MB file), and under Claude API's
+ * own image size bounds.
+ */
+
+export const IMAGE_MAX_BYTES = 10 * 1024 * 1024 // 10 MB
+export const FILE_MAX_BYTES = 30 * 1024 * 1024  // 30 MB
+
+export const IMAGE_MIME_WHITELIST = [
+  'image/jpeg',
+  'image/png',
+  'image/gif',
+  'image/webp',
+  'image/heic',
+] as const
+
+export type LimitCheckResult =
+  | { ok: true }
+  | { ok: false; reason: 'too_large' | 'unsupported_mime'; hint: string }
+
+function formatMb(bytes: number): string {
+  return (bytes / (1024 * 1024)).toFixed(1)
+}
+
+export function checkAttachmentLimit(
+  kind: 'image' | 'file',
+  size: number,
+  mime?: string,
+): LimitCheckResult {
+  if (kind === 'image') {
+    if (size > IMAGE_MAX_BYTES) {
+      return {
+        ok: false,
+        reason: 'too_large',
+        hint: `📎 图片过大(${formatMb(size)} MB),请控制在 10 MB 以内`,
+      }
+    }
+    if (mime && !IMAGE_MIME_WHITELIST.includes(mime as (typeof IMAGE_MIME_WHITELIST)[number])) {
+      return {
+        ok: false,
+        reason: 'unsupported_mime',
+        hint: `📎 暂不支持此图片格式(${mime})`,
+      }
+    }
+    return { ok: true }
+  }
+  // kind === 'file'
+  if (size > FILE_MAX_BYTES) {
+    return {
+      ok: false,
+      reason: 'too_large',
+      hint: `📎 文件过大(${formatMb(size)} MB),请控制在 30 MB 以内`,
+    }
+  }
+  return { ok: true }
+}
diff --git a/adapters/common/attachment/attachment-types.ts b/adapters/common/attachment/attachment-types.ts
new file mode 100644
index 00000000..d5d152e5
--- /dev/null
+++ b/adapters/common/attachment/attachment-types.ts
@@ -0,0 +1,29 @@
+/**
+ * Shared attachment types for IM adapters.
+ */
+
+import type { AttachmentRef } from '../ws-bridge.js'
+export type { AttachmentRef }
+
+/** Platform tag — used for local staging subdir and telemetry. */
+export type ImPlatform = 'feishu' | 'telegram'
+
+/** Result of downloading an IM resource into the local stage dir. */
+export interface LocalAttachment {
+  kind: 'image' | 'file'
+  name: string        // original filename, or synthesized if none
+  path: string        // absolute path on disk (under ~/.claude/im-downloads)
+  size: number        // bytes
+  mimeType: string    // detected or provided
+  buffer: Buffer      // raw bytes (kept so caller can choose base64 vs path)
+}
+
+/** Pending outbound media found in Agent stream output. */
+export interface PendingUpload {
+  id: string          // fingerprint, used for dedup
+  source:
+    | { kind: 'base64'; data: string; mime: string }
+    | { kind: 'path'; path: string; mime?: string }
+    | { kind: 'url'; url: string; mime?: string }
+  alt?: string
+}