---
title: "Preserve unknown reserved metadata with a warning"
description: "An unrecognized key in an MCP-reserved namespace may belong to a newer protocol definition. It is retained with a warning rather than assigned invented semantics. Verify the key an"
source: "https://mcpdesc.org/docs/rules/0.9.0-rc.3/mcp/meta-reserved-prefix/"
---

An unrecognized key in an MCP-reserved namespace may belong to a newer protocol definition. It is retained with a warning rather than assigned invented semantics. Verify the key and protocol version before treating it as a recognized MCP feature.

## Requirement

A \_meta key under an MCP-reserved prefix that the MCP protocol version does not define is preserved with a warning.

## Details

**Rule ID:** `mcp.meta.reserved-prefix`  
**Owner:** mcp  
**Surface:** \_meta  
**Kind:** semantic  
**Requirement level:** must  
**Status:** internal · **Enforcement:** internal

## Protocol version applicability

| 2024-11-05     | 2025-03-26     | 2025-06-18 | 2025-11-25 | 2026-07-28 |
| -------------- | -------------- | ---------- | ---------- | ---------- |
| not-applicable | not-applicable | applies    | applies    | applies    |

## Required by

* **MCP Description 0.9.0, Section 3.5**  
  * 3\. Document Structure: [req-meta-reserved-prefix](https://mcpdesc.org/docs/specification/0.9.0-rc.3/document-structure#req-meta-reserved-prefix)

## Based on

* **Tier 1 — MCP schema.ts and normative specification text:** MCP 2025-06-18 to 2026-07-28 specification, Basic, General fields, \_meta (a prefix whose second label is modelcontextprotocol or mcp is reserved for MCP use)  
  * <https://github.com/modelcontextprotocol/modelcontextprotocol/blob/dae694f61a31f7a3ec2af3aff709e11f70f61e48/docs/specification/2025-06-18/basic/index.mdx#L140-L142>
  * <https://github.com/modelcontextprotocol/modelcontextprotocol/blob/dae694f61a31f7a3ec2af3aff709e11f70f61e48/docs/specification/2025-11-25/basic/index.mdx#L208-L210>
  * <https://github.com/modelcontextprotocol/modelcontextprotocol/blob/dae694f61a31f7a3ec2af3aff709e11f70f61e48/docs/specification/2026-07-28/basic/index.mdx#L337-L339>

## Notes

A later MCP protocol version may define the key, so it is preserved rather than rejected.

## Examples

### Accepted with a warning

Source: [expected-warning/unknown-reserved-meta-key.json](https://github.com/mcpdesc/mcpdesc-specification/blob/v0.9.0-rc.3/spec/draft/fixtures/expected-warning/unknown-reserved-meta-key.json) · Selected: `/prompts/0/_meta`

```json
{
  "io.modelcontextprotocol/future-key": {
    "preserved": true
  }
}
```

## Rule Source Code

[meta.yaml](https://github.com/mcpdesc/mcpdesc-specification/blob/v0.9.0-rc.3/spec/draft/validation/rules/meta.yaml) at immutable tag [v0.9.0-rc.3](https://github.com/mcpdesc/mcpdesc-specification/blob/v0.9.0-rc.3/spec/draft).