Tool annotations
Tool annotations are optional hints on an MCP tool, such as readOnlyHint, that tell a client how it behaves; they are the server's claims, not guarantees. Most tools in our MCP dataset carry annotations on October 13, 2026: 91% set readOnlyHint and 85% set destructiveHint. The sample is the 425 open servers we track across every store, not only the 105 behind ChatGPT and Claude listings, with 7,777 tools.
- readOnlyHint set
- 91%
- openWorldHint set
- 86%
- destructiveHint set
- 85%
- idempotentHint set
- 66%
More from our data
Most tools say they only read, and few say they can destroy data:
| Annotation | Tools that set it | Set to true |
|---|---|---|
readOnlyHint | 91% | 66% |
destructiveHint | 85% | 9% |
idempotentHint | 66% | 52% |
openWorldHint | 86% | 28% |
title | 55% | n/a |
An annotation is the server's own claim. A tool marked read-only can still change data if its code does, so treat annotations as labels to check, not as permissions.