Tool output validation fails when outputSchema is a Zod schema other than ZodObject

[joshjg]

Describe the bug

Tool output validation fails with the following error when the tool's outputSchema is an optional, nullable, nullish, or union schema (or, most likely anything besides a plain object schema).

Cannot read properties of undefined (reading '_zod')

When validating output the SDK attempts to use normalizeObjectSchema, which returns undefined for anything besides object schemas.

To Reproduce
Steps to reproduce the behavior:

1. Define an outputSchema with something other than a plain object or z.object schema.
2. Implement the tool so that it returns structuredContent that conforms to this schema.
3. Invoke the tool.

// Works:
outputSchema: z.object({ data: z.string() })

// Fails:
outputSchema: z.object({ data: z.string() }).optional()
outputSchema: z.object({ data: z.string() }).nullable()
outputSchema: z.object({ data: z.string() }).nullish()
outputSchema: z.union([z.object({ data: z.string() }), z.object({ value: z.string() })])

Expected behavior
The SDK should be able to validate output based on Zod schemas which represent a JSON object, even if it is optional or a union of possible object schemas. Based on the spec, null is not a valid value for structuredContent, so nullable and nullish schemas should not be allowed, either through more restrictive typing of outputSchema or runtime validation with a clear error message.

Logs

    at isZ4Schema ($PROJECT/node_modules/@modelcontextprotocol/sdk/dist/cjs/server/zod-compat.js:46:21)
    at safeParseAsync ($PROJECT/node_modules/@modelcontextprotocol/sdk/dist/cjs/server/zod-compat.js:80:9)
    at McpServer.validateToolOutput ($PROJECT/node_modules/@modelcontextprotocol/sdk/dist/cjs/server/mcp.js:205:70)
    at $PROJECT/node_modules/@modelcontextprotocol/sdk/dist/cjs/server/mcp.js:135:28
    at process.processTicksAndRejections (node:internal/process/task_queues:105:5)
    at async wrappedHandler ($PROJECT/node_modules/@modelcontextprotocol/sdk/dist/cjs/server/index.js:131:32)

Additional context

"@modelcontextprotocol/sdk": "1.24.3",
"zod": "4.1.13"

I was expecting this to work based on #816 but it has possibly regressed with #1040

[KKonstantinov]

Hi, thanks for this, could you please help by providing the generated JSON schema by this?

Currently, the validation of outputSchema is:

outputSchema: z
        .object({
            type: z.literal('object'),
            properties: z.record(z.string(), AssertObjectSchema).optional(),
            required: z.array(z.string()).optional()
        })
        .catchall(z.unknown())
        .optional(),

Which matches

https://github.com/modelcontextprotocol/modelcontextprotocol/blob/11a026d882003053b86ce47ac88f296c2bc5ee80/schema/2025-11-25/schema.ts#L1277

But we'd need to check what schema is generated from .nullable, .nullish etc.