Empty inputSchema requires the type key

[pusewicz]

When defining a tool without any arguments taken, it should be allowed to not have to define the input_schema at all, and the default value for such a scenario should be { type: "object" } as per the schema.

https://github.com/modelcontextprotocol/modelcontextprotocol/blob/16d91abb68bbc824944c5bda4ac2e2ccaaa74b0f/schema/2025-06-18/schema.ts#L896-L900

Motivation and Context

When testing a tool without the input_schema defined, the mcp-inspector returns an error when trying to list the tool:

Error
[ { "code": "invalid_type", "expected": "object", "received": "undefined", "path": [ "tools", 1, "inputSchema" ], "message": "Required" } ]

How Has This Been Tested?

Added new tests and locally using the mcp-inspector.

Breaking Changes

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[koic]

The linter appears to be failing in CI. It can be checked locally by running bundle exec rake.

[koic]

CI passed. Can you squash your commits into one?

[pusewicz]

@koic Done!

[koic]

Thanks!