Fixes #400

Motivation and Context

When a tool has all optional parameters, some LLM models call the tool without providing an
arguments field. This causes Zod validation to fail with:

MCP error -32602: Invalid arguments for tool [name]: Invalid input: expected object, received
undefined

Even though the schema allows all fields to be optional, Zod's z.object() expects the input
itself to be an object, not undefined or null.

How Has This Been Tested?

• Added unit test: should accept undefined arguments when all tool params are optional
• Verified Zod behavior with direct testing:
  • undefined → fails with "expected object, received undefined"
  • null → fails with "expected object, received null"
  • {} → passes validation
• All existing tests pass (1,392 tests)

Breaking Changes

None. This is a backward-compatible fix that only affects edge cases where arguments is
omitted.

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

The fix is a single line change in packages/server/src/server/mcp.ts:

// Before                                                                                       
const parseResult = await safeParseAsync(schemaToParse, args);                                  
                                                                                                
// After                                                                                        
const parseResult = await safeParseAsync(schemaToParse, args ?? {});                            
                                                                                                
Using ?? handles both undefined and null cases.