Input Validation Bug?

AgentTools

Hey guys! Running into what looks like a bug with tool input validation when using nested objects with .partial().

The issue: When I create a tool with a Zod schema that has a nested object field marked as .partial(), the schema validates correctly when I test it directly, but Mastra's internal validation is still failing with "expected string, received undefined" errors for all the partial fields.

Versions:
@mastra/core: 0.24.8
zod: 4.1.13

Minimal repro:

import { createTool } from '@mastra/core';
import { z } from 'zod';

// Schema with nested partial object
const inputSchema = z.object({
  eventId: z.string(),
  request: z.object({
    City: z.string(),
    Name: z.string(),
    Slug: z.string(),
  }).partial().passthrough(),
  eventImageFile: z.any().optional()
});

// This validates successfully:
const testData = {
  eventId: '123',
  request: { Name: 'Test' }, // Only providing one field
  eventImageFile: null
};
console.log(inputSchema.safeParse(testData).success); // :white_check_mark: true

// But when passed to Mastra:
const tool = createTool({
  id: 'updateEvent',
  description: 'Update event',
  inputSchema,
  execute: async (input) => {
    return { success: true };
  }
});

// LLM calls with: { eventId: '123', request: { Name: 'Test' }, eventImageFile: null }
// :x: Mastra validation fails with:
// Tool input validation failed for 'updateEvent'
// errors: {
//   request: {
//     City: { _errors: ['Invalid input: expected string, received undefined'] },
//     Slug: { _errors: ['Invalid input: expected string, received undefined'] },
//   }
// }

import { createTool } from '@mastra/core';
import { z } from 'zod';

// Schema with nested partial object
const inputSchema = z.object({
  eventId: z.string(),
  request: z.object({
    City: z.string(),
    Name: z.string(),
    Slug: z.string(),
  }).partial().passthrough(),
  eventImageFile: z.any().optional()
});

// This validates successfully:
const testData = {
  eventId: '123',
  request: { Name: 'Test' }, // Only providing one field
  eventImageFile: null
};
console.log(inputSchema.safeParse(testData).success); // :white_check_mark: true

// But when passed to Mastra:
const tool = createTool({
  id: 'updateEvent',
  description: 'Update event',
  inputSchema,
  execute: async (input) => {
    return { success: true };
  }
});

// LLM calls with: { eventId: '123', request: { Name: 'Test' }, eventImageFile: null }
// :x: Mastra validation fails with:
// Tool input validation failed for 'updateEvent'
// errors: {
//   request: {
//     City: { _errors: ['Invalid input: expected string, received undefined'] },
//     Slug: { _errors: ['Invalid input: expected string, received undefined'] },
//   }
// }

What I've confirmed:
- The nested request.shape.City is correctly a ZodOptional after .partial()
- Direct validation with inputSchema.safeParse() succeeds
- But... Mastra's internal validation (in builder.ts createExecute) fails

Input Validation Bug?

AgentTools

import { createTool } from '@mastra/core';
import { z } from 'zod';

// Schema with nested partial object
const inputSchema = z.object({
  eventId: z.string(),
  request: z.object({
    City: z.string(),
    Name: z.string(),
    Slug: z.string(),
  }).partial().passthrough(),
  eventImageFile: z.any().optional()
});

// This validates successfully:
const testData = {
  eventId: '123',
  request: { Name: 'Test' }, // Only providing one field
  eventImageFile: null
};
console.log(inputSchema.safeParse(testData).success); // :white_check_mark: true

// But when passed to Mastra:
const tool = createTool({
  id: 'updateEvent',
  description: 'Update event',
  inputSchema,
  execute: async (input) => {
    return { success: true };
  }
});

// LLM calls with: { eventId: '123', request: { Name: 'Test' }, eventImageFile: null }
// :x: Mastra validation fails with:
// Tool input validation failed for 'updateEvent'
// errors: {
//   request: {
//     City: { _errors: ['Invalid input: expected string, received undefined'] },
//     Slug: { _errors: ['Invalid input: expected string, received undefined'] },
//   }
// }

import { createTool } from '@mastra/core';
import { z } from 'zod';

// Schema with nested partial object
const inputSchema = z.object({
  eventId: z.string(),
  request: z.object({
    City: z.string(),
    Name: z.string(),
    Slug: z.string(),
  }).partial().passthrough(),
  eventImageFile: z.any().optional()
});

// This validates successfully:
const testData = {
  eventId: '123',
  request: { Name: 'Test' }, // Only providing one field
  eventImageFile: null
};
console.log(inputSchema.safeParse(testData).success); // :white_check_mark: true

// But when passed to Mastra:
const tool = createTool({
  id: 'updateEvent',
  description: 'Update event',
  inputSchema,
  execute: async (input) => {
    return { success: true };
  }
});

// LLM calls with: { eventId: '123', request: { Name: 'Test' }, eventImageFile: null }
// :x: Mastra validation fails with:
// Tool input validation failed for 'updateEvent'
// errors: {
//   request: {
//     City: { _errors: ['Invalid input: expected string, received undefined'] },
//     Slug: { _errors: ['Invalid input: expected string, received undefined'] },
//   }
// }

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