outputSchema
outputSchema is an optional JSON Schema on an MCP tool that describes the structured result the tool returns, so a client can check and parse it. In our MCP dataset (the 425 open servers we track across every store, not only the 105 behind ChatGPT and Claude listings, with 7,777 tools), 39% of tools declare an output schema on October 13, 2026, against 91% that set readOnlyHint.
- Tools that declare it
- 39%
- Servers with one or more
- 46%
- MCP dataset
- 7,777 toolsevery store we track
More from our data
Few tools say what they return: 46% of the 425 open servers declare an output schema on at least one tool.