The spec is inconsistent about whether outputSchema must have type: "object" at the root level.

Prose docs (does not require object)

docs/specification/draft/server/tools.mdx describes outputSchema without any type restriction:

• outputSchema: Optional JSON Schema defining expected output structure
  • Follows the JSON Schema usage guidelines
  • Defaults to 2020-12 if no $schema field is present

This contrasts with inputSchema, which explicitly requires type: "object".

JSON schema (requires object)

schema/draft/schema.json requires type: "object":

"outputSchema": {
    "properties": {
        "type": {
            "const": "object",
            "type": "string"
        }
    },
    "required": ["type"],
    ...
}

Implementations

The Python SDK implemented the prose interpretation, allowing arrays and primitives. The TypeScript SDK implemented the schema interpretation, rejecting non-object schemas. This breaks interoperability for tools that want to return arrays (e.g. list operations) or primitives.

See: modelcontextprotocol/typescript-sdk#1149

Proposal

We should make things consistent one way or the other. I don't mind too much either way but we should choose one!

For making it permissive: It's an awkward restriction that makes defining JSON schemas more fiddly. Additionally, this would be less likely to break existing implementations.

For making it object: I'm not sure how many people are using the Python SDK in the permissive way which would be a breaking change, but changing it to require type: "object" would bring it into harmony with inputSchema.