Motivation and Context

See #588. This PR adds a backwards-compatible change to registerTool on McpServer to allow any Zod schema extending ZodType<object> instead of requiring inputSchema and outputSchema to conform to ZodRawShape (:= Record<string, ZodType<any>>).

The underlying schema validation and zod-to-json-schema dependency already support this, so adding this plumbing expands SDK expressiveness and flexibility. Specifically, this unblocks MCP servers' ability to define tool schemas that use z.union(...) and z.intersection(...). Historically, some creators have had to wrap existing schemas in some wrapper object that has fixed keys, e.g. {bodyParams: z.union(...)} to work around this limitation.

How Has This Been Tested?

Added automated tests in mcp.test.ts and confirmed they pass locally with npm run test.

Also manually tried changing the test locally to use inputSchemas for types that aren't objects at the top level -- e.g. z.number() and z.union([z.number(), z.object({})]), and confirmed typechecking fails (which is good, since I'm guessing we still want guardrails to push toward objects for params and responses. If this ever changes in the future it's easy enough to change all ZodType<object> to ZodTypeAny!)

Breaking Changes

Should be backwards-compatible in the SDK with existing server code. This is purely additive since we can easily dynamically distinguish between existing ZodRawShapes being passed in vs. direct ZodType<object> and branch accordingly. ZodType<object> is added as a union type variant for server.registerTool, and isn't mandatory.

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

N/A