ResourceSchema missing size field from MCP spec

[olaservo]

Description

The MCP specification defines an optional size?: number field on the Resource interface (added in spec revision 2025-06-18, also present in 2025-11-25 and draft):

size: The size of the raw resource content, in bytes (i.e., before base64 encoding or any tokenization), if known. This can be used by Hosts to display file sizes and estimate context window usage.

The spec's JSON schema includes this field:

• https://modelcontextprotocol.io/specification/2025-06-18/schema — Resource interface has size?: number
• https://modelcontextprotocol.io/specification/2025-11-25/schema — same
• https://modelcontextprotocol.io/specification/draft/schema — same

However, the TypeScript SDK's ResourceSchema Zod definition in v1.27.0 does not include size. The schema has uri, description, mimeType, annotations, _meta, icons, name, and title — but no size.

Expected behavior

ResourceSchema should include an optional size number field to match the MCP spec, allowing servers to declare resource sizes and enabling hosts to display file sizes / estimate context window usage.

Steps to reproduce

import { ResourceSchema } from "@modelcontextprotocol/sdk/types.js";

// The Resource type does not include a `size` field,
// even though the MCP spec defines one.

Environment

• SDK version: 1.27.0 (also checked 1.26.0)
• MCP spec revisions checked: 2025-06-18, 2025-11-25, draft

---

Note

This issue was written with the help of Claude Code (claude-opus-4-6).

[haroldfabla2-hue]

Proposed Fix

I've prepared a fix for this issue! The solution adds the missing size field to ResourceSchema in packages/core/src/types/types.ts:

```typescript
/**

• The size of the raw resource content, in bytes (i.e., before base64 encoding or any tokenization), if known.
• This can be used by Hosts to display file sizes and estimate context window usage.
  */
  size: z.optional(z.number().int().positive()),
  ```

The fix is available in my fork:

• Repository: https://github.com/haroldfabla2-hue/typescript-sdk
• Branch: pr-size-field
• Commit: 40bf88a9950164427bb7b296b939bf2c1c0f679c

Would you like me to open a PR with this fix?