The output schema a client is told a tool answers with: MCP's outputSchema and the 200 response of the OpenAPI document.

A read-only tool answers with its own output_schema. Any other tool (a write tool, PhoenixKit.API.ControlFields.write_tool?/1) may answer with a preview instead: a dry run, a confirmation request or a pending approval. Its schema is therefore {"type": "object", "anyOf": [success, preview]}. It is anyOf, not oneOf, because a preview also satisfies a success schema that does not forbid extra properties, which would make oneOf reject it. The root keeps "type": "object" because MCP client libraries require an object schema there.

A tool with no output_schema has no output shape. Keys are turned into strings at every depth and the root is forced to "type": "object" (a tool answers with a map); the rest of the schema is passed through as declared, PhoenixKit.API.ToolLint is what checks that it is sound.