---
title: "Use valid metadata key names"
description: "MCP metadata keys follow a grammar for an optional dotted prefix and a local name. Check separators and permitted characters when a key is rejected. Reserved-name context and value"
source: "https://mcpdesc.org/docs/rules/0.9.0-rc.3/mcp/meta-key-format/"
---

MCP metadata keys follow a grammar for an optional dotted prefix and a local name. Check separators and permitted characters when a key is rejected. Reserved-name context and value checks are separate from this key-format check.

## Requirement

A \_meta key has an optional prefix of dot-separated labels followed by a slash, and a name that begins and ends with an alphanumeric character.

## Details

**Rule ID:** `mcp.meta.key-format`  
**Owner:** mcp  
**Surface:** \_meta  
**Kind:** structural  
**Requirement level:** must  
**Protocol versions:** independent of the MCP protocol version  
**Document schema locations:** `#/$defs/metaObject`  
**Status:** internal · **Enforcement:** internal

## Required by

* **MCP Description 0.9.0, Section 3.5 (the same grammar applies in every protocol version)**  
  * 3\. Document Structure: [req-meta-keys](https://mcpdesc.org/docs/specification/0.9.0-rc.3/document-structure#req-meta-keys)

## Based on

* **Tier 1 — MCP schema.ts and normative specification text:** MCP 2025-06-18 to 2026-07-28 specification, Basic, General fields, \_meta key name format  
  * <https://github.com/modelcontextprotocol/modelcontextprotocol/blob/dae694f61a31f7a3ec2af3aff709e11f70f61e48/docs/specification/2025-06-18/basic/index.mdx#L134-L147>
  * <https://github.com/modelcontextprotocol/modelcontextprotocol/blob/dae694f61a31f7a3ec2af3aff709e11f70f61e48/docs/specification/2025-11-25/basic/index.mdx#L201-L215>
  * <https://github.com/modelcontextprotocol/modelcontextprotocol/blob/dae694f61a31f7a3ec2af3aff709e11f70f61e48/docs/specification/2026-07-28/basic/index.mdx#L330-L446>

## Notes

The document schema rejects malformed keys first; the semantic key check applies only to direct callers of the semantic validator.

## Examples

### Invalid example

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

```json
{
  "name": "inspect",
  "inputSchema": {
    "type": "object"
  },
  "_meta": {
    "com.example/bad key": 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).