SEP-1613: use.catchall() on inputSchema/outputSchema to support JSON Schema 2020-12

[felixweinberger]

Summary

Restores .catchall() on inputSchema and outputSchema in ToolSchema to properly support JSON Schema 2020-12 keywords as required by SEP-1613.

Motivation and Context

SEP-1613 establishes JSON Schema 2020-12 as the default dialect for MCP embedded schemas. This requires the SDK to preserve all JSON Schema properties (like $schema, additionalProperties, $defs, allOf, etc.) rather than stripping them.

The Problem: PR #1086 removed .passthrough() from ToolSchema, causing the SDK to strip valid JSON Schema properties. This was reported in #1057 where $schema was being stripped, breaking SEP-1613's backwards compatibility mechanism (servers need $schema to specify alternative dialects).

Why passthrough is correct here:

1. Spec compliance: The MCP spec's JSON Schema for inputSchema does NOT have additionalProperties: false, meaning additional properties are allowed by default
2. inputSchema is an external spec: Unlike MCP protocol fields, inputSchema/outputSchema are JSON Schema objects - the SDK should transport them, not validate their internal structure
3. SEP-1613 backwards compatibility: Servers must be able to specify $schema to use dialects other than 2020-12
4. Restores original behavior: The SDK had passthrough before PR SEP-1319: Decouple Request Payloads, Remove passthrough iteration, Typecheck fixes #1086

Changes:

• Add .catchall() to inputSchema and outputSchema in ToolSchema
• Preserve existing typed properties (type, properties, required) for TypeScript backwards compatibility
• Add JSDoc documenting SEP-1613/2020-12 default dialect
• Add comprehensive tests for JSON Schema 2020-12 keyword acceptance

How Has This Been Tested?

• Added unit tests ensuring required fields are passed through.

Breaking Changes

None. This restores the ability to use JSON Schema properties that were being silently stripped since PR #1125.

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

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/sdk@1135

commit: 6116af3

[KKonstantinov]

Hello,

There should be a way to achieve it without using .passthrough() and without necessarily listing all the possible fields.

.catchall(z.unknown) should achieve the same effect:

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

or alternatively an intersection with a Record<string, unknown>

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

[felixweinberger]

Hello,

There should be a way to achieve it without using .passthrough() and without necessarily listing all the possible fields.

.catchall(z.unknown) should achieve the same effect:

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

or alternatively an intersection with a Record<string, unknown>

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

Done!