SEP-1613: use`.catchall()` on inputSchema/outputSchema to support JSON Schema 2020-12 by felixweinberger · Pull Request #1135 · modelcontextprotocol/typescript-sdk

felixweinberger · 2025-11-18T23:18:07Z

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:

Spec compliance: The MCP spec's JSON Schema for inputSchema does NOT have additionalProperties: false, meaning additional properties are allowed by default
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
SEP-1613 backwards compatibility: Servers must be able to specify $schema to use dialects other than 2020-12
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

Add .passthrough() to inputSchema and outputSchema to accept all JSON Schema 2020-12 keywords. This is the correct approach because: - inputSchema/outputSchema are embedded external specs (JSON Schema), not MCP protocol fields that should be strictly validated - The SDK's role is to transport schemas, not validate JSON Schema structure - Enumeration approach would silently drop unrecognized keywords (data loss) Changes: - Add .passthrough() to inputSchema and outputSchema in ToolSchema - Update JSDoc to document SEP-1613/2020-12 as default dialect - Add comprehensive tests for JSON Schema 2020-12 keyword support Backwards compatible: existing typed properties (type, properties, required) remain explicitly typed for TypeScript autocomplete.

pkg-pr-new · 2025-11-18T23:20:50Z

Open in StackBlitz

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

commit: d3dfd94

KKonstantinov · 2025-11-19T06:41:59Z

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()))

More semantically correct for JSON Schema objects where additional properties are expected but not validated by Zod.

felixweinberger · 2025-11-20T14:43:19Z

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!

- Remove unused Tool type import - Remove unused JSON_SCHEMA_2020_12_INPUT_SCHEMA constant (SEP-1613 test is pending - SDK validation supports the fields via PR #1135, but tool registration doesn't yet support generating raw JSON Schema) - Remove emoji from console.log - Alphabetize devDependencies in package.json

felixweinberger · 2025-11-18T23:18:07Z

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:

Spec compliance: The MCP spec's JSON Schema for inputSchema does NOT have additionalProperties: false, meaning additional properties are allowed by default
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
SEP-1613 backwards compatibility: Servers must be able to specify $schema to use dialects other than 2020-12
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

Add .passthrough() to inputSchema and outputSchema to accept all JSON Schema 2020-12 keywords. This is the correct approach because: - inputSchema/outputSchema are embedded external specs (JSON Schema), not MCP protocol fields that should be strictly validated - The SDK's role is to transport schemas, not validate JSON Schema structure - Enumeration approach would silently drop unrecognized keywords (data loss) Changes: - Add .passthrough() to inputSchema and outputSchema in ToolSchema - Update JSDoc to document SEP-1613/2020-12 as default dialect - Add comprehensive tests for JSON Schema 2020-12 keyword support Backwards compatible: existing typed properties (type, properties, required) remain explicitly typed for TypeScript autocomplete.

pkg-pr-new · 2025-11-18T23:20:50Z

Open in StackBlitz

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

commit: d3dfd94

KKonstantinov · 2025-11-19T06:41:59Z

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()))

More semantically correct for JSON Schema objects where additional properties are expected but not validated by Zod.

felixweinberger · 2025-11-20T14:43:19Z

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!

- Remove unused Tool type import - Remove unused JSON_SCHEMA_2020_12_INPUT_SCHEMA constant (SEP-1613 test is pending - SDK validation supports the fields via PR #1135, but tool registration doesn't yet support generating raw JSON Schema) - Remove emoji from console.log - Alphabetize devDependencies in package.json

felixweinberger · 2025-11-18T23:18:07Z

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:

Spec compliance: The MCP spec's JSON Schema for inputSchema does NOT have additionalProperties: false, meaning additional properties are allowed by default
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
SEP-1613 backwards compatibility: Servers must be able to specify $schema to use dialects other than 2020-12
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

Add .passthrough() to inputSchema and outputSchema to accept all JSON Schema 2020-12 keywords. This is the correct approach because: - inputSchema/outputSchema are embedded external specs (JSON Schema), not MCP protocol fields that should be strictly validated - The SDK's role is to transport schemas, not validate JSON Schema structure - Enumeration approach would silently drop unrecognized keywords (data loss) Changes: - Add .passthrough() to inputSchema and outputSchema in ToolSchema - Update JSDoc to document SEP-1613/2020-12 as default dialect - Add comprehensive tests for JSON Schema 2020-12 keyword support Backwards compatible: existing typed properties (type, properties, required) remain explicitly typed for TypeScript autocomplete.

pkg-pr-new · 2025-11-18T23:20:50Z

Open in StackBlitz

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

commit: d3dfd94

KKonstantinov · 2025-11-19T06:41:59Z

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()))

More semantically correct for JSON Schema objects where additional properties are expected but not validated by Zod.

felixweinberger · 2025-11-20T14:43:19Z

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!

- Remove unused Tool type import - Remove unused JSON_SCHEMA_2020_12_INPUT_SCHEMA constant (SEP-1613 test is pending - SDK validation supports the fields via PR #1135, but tool registration doesn't yet support generating raw JSON Schema) - Remove emoji from console.log - Alphabetize devDependencies in package.json

felixweinberger · 2025-11-18T23:18:07Z

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:

Spec compliance: The MCP spec's JSON Schema for inputSchema does NOT have additionalProperties: false, meaning additional properties are allowed by default
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
SEP-1613 backwards compatibility: Servers must be able to specify $schema to use dialects other than 2020-12
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

Add .passthrough() to inputSchema and outputSchema to accept all JSON Schema 2020-12 keywords. This is the correct approach because: - inputSchema/outputSchema are embedded external specs (JSON Schema), not MCP protocol fields that should be strictly validated - The SDK's role is to transport schemas, not validate JSON Schema structure - Enumeration approach would silently drop unrecognized keywords (data loss) Changes: - Add .passthrough() to inputSchema and outputSchema in ToolSchema - Update JSDoc to document SEP-1613/2020-12 as default dialect - Add comprehensive tests for JSON Schema 2020-12 keyword support Backwards compatible: existing typed properties (type, properties, required) remain explicitly typed for TypeScript autocomplete.

pkg-pr-new · 2025-11-18T23:20:50Z

Open in StackBlitz

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

commit: d3dfd94

KKonstantinov · 2025-11-19T06:41:59Z

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()))

More semantically correct for JSON Schema objects where additional properties are expected but not validated by Zod.

felixweinberger · 2025-11-20T14:43:19Z

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!

- Remove unused Tool type import - Remove unused JSON_SCHEMA_2020_12_INPUT_SCHEMA constant (SEP-1613 test is pending - SDK validation supports the fields via PR #1135, but tool registration doesn't yet support generating raw JSON Schema) - Remove emoji from console.log - Alphabetize devDependencies in package.json

felixweinberger · 2025-11-18T23:18:07Z

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:

Spec compliance: The MCP spec's JSON Schema for inputSchema does NOT have additionalProperties: false, meaning additional properties are allowed by default
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
SEP-1613 backwards compatibility: Servers must be able to specify $schema to use dialects other than 2020-12
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

Add .passthrough() to inputSchema and outputSchema to accept all JSON Schema 2020-12 keywords. This is the correct approach because: - inputSchema/outputSchema are embedded external specs (JSON Schema), not MCP protocol fields that should be strictly validated - The SDK's role is to transport schemas, not validate JSON Schema structure - Enumeration approach would silently drop unrecognized keywords (data loss) Changes: - Add .passthrough() to inputSchema and outputSchema in ToolSchema - Update JSDoc to document SEP-1613/2020-12 as default dialect - Add comprehensive tests for JSON Schema 2020-12 keyword support Backwards compatible: existing typed properties (type, properties, required) remain explicitly typed for TypeScript autocomplete.

pkg-pr-new · 2025-11-18T23:20:50Z

Open in StackBlitz

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

commit: d3dfd94

KKonstantinov · 2025-11-19T06:41:59Z

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()))

More semantically correct for JSON Schema objects where additional properties are expected but not validated by Zod.

felixweinberger · 2025-11-20T14:43:19Z

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!

- Remove unused Tool type import - Remove unused JSON_SCHEMA_2020_12_INPUT_SCHEMA constant (SEP-1613 test is pending - SDK validation supports the fields via PR #1135, but tool registration doesn't yet support generating raw JSON Schema) - Remove emoji from console.log - Alphabetize devDependencies in package.json

felixweinberger · 2025-11-18T23:18:07Z

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:

Spec compliance: The MCP spec's JSON Schema for inputSchema does NOT have additionalProperties: false, meaning additional properties are allowed by default
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
SEP-1613 backwards compatibility: Servers must be able to specify $schema to use dialects other than 2020-12
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

Add .passthrough() to inputSchema and outputSchema to accept all JSON Schema 2020-12 keywords. This is the correct approach because: - inputSchema/outputSchema are embedded external specs (JSON Schema), not MCP protocol fields that should be strictly validated - The SDK's role is to transport schemas, not validate JSON Schema structure - Enumeration approach would silently drop unrecognized keywords (data loss) Changes: - Add .passthrough() to inputSchema and outputSchema in ToolSchema - Update JSDoc to document SEP-1613/2020-12 as default dialect - Add comprehensive tests for JSON Schema 2020-12 keyword support Backwards compatible: existing typed properties (type, properties, required) remain explicitly typed for TypeScript autocomplete.

pkg-pr-new · 2025-11-18T23:20:50Z

Open in StackBlitz

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

commit: d3dfd94

KKonstantinov · 2025-11-19T06:41:59Z

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()))

More semantically correct for JSON Schema objects where additional properties are expected but not validated by Zod.

felixweinberger · 2025-11-20T14:43:19Z

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!

- Remove unused Tool type import - Remove unused JSON_SCHEMA_2020_12_INPUT_SCHEMA constant (SEP-1613 test is pending - SDK validation supports the fields via PR #1135, but tool registration doesn't yet support generating raw JSON Schema) - Remove emoji from console.log - Alphabetize devDependencies in package.json

felixweinberger · 2025-11-18T23:18:07Z

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:

Spec compliance: The MCP spec's JSON Schema for inputSchema does NOT have additionalProperties: false, meaning additional properties are allowed by default
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
SEP-1613 backwards compatibility: Servers must be able to specify $schema to use dialects other than 2020-12
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

Add .passthrough() to inputSchema and outputSchema to accept all JSON Schema 2020-12 keywords. This is the correct approach because: - inputSchema/outputSchema are embedded external specs (JSON Schema), not MCP protocol fields that should be strictly validated - The SDK's role is to transport schemas, not validate JSON Schema structure - Enumeration approach would silently drop unrecognized keywords (data loss) Changes: - Add .passthrough() to inputSchema and outputSchema in ToolSchema - Update JSDoc to document SEP-1613/2020-12 as default dialect - Add comprehensive tests for JSON Schema 2020-12 keyword support Backwards compatible: existing typed properties (type, properties, required) remain explicitly typed for TypeScript autocomplete.

pkg-pr-new · 2025-11-18T23:20:50Z

Open in StackBlitz

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

commit: d3dfd94

KKonstantinov · 2025-11-19T06:41:59Z

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()))

More semantically correct for JSON Schema objects where additional properties are expected but not validated by Zod.

felixweinberger · 2025-11-20T14:43:19Z

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!

- Remove unused Tool type import - Remove unused JSON_SCHEMA_2020_12_INPUT_SCHEMA constant (SEP-1613 test is pending - SDK validation supports the fields via PR #1135, but tool registration doesn't yet support generating raw JSON Schema) - Remove emoji from console.log - Alphabetize devDependencies in package.json

felixweinberger · 2025-11-18T23:18:07Z

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:

Spec compliance: The MCP spec's JSON Schema for inputSchema does NOT have additionalProperties: false, meaning additional properties are allowed by default
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
SEP-1613 backwards compatibility: Servers must be able to specify $schema to use dialects other than 2020-12
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

Add .passthrough() to inputSchema and outputSchema to accept all JSON Schema 2020-12 keywords. This is the correct approach because: - inputSchema/outputSchema are embedded external specs (JSON Schema), not MCP protocol fields that should be strictly validated - The SDK's role is to transport schemas, not validate JSON Schema structure - Enumeration approach would silently drop unrecognized keywords (data loss) Changes: - Add .passthrough() to inputSchema and outputSchema in ToolSchema - Update JSDoc to document SEP-1613/2020-12 as default dialect - Add comprehensive tests for JSON Schema 2020-12 keyword support Backwards compatible: existing typed properties (type, properties, required) remain explicitly typed for TypeScript autocomplete.

pkg-pr-new · 2025-11-18T23:20:50Z

Open in StackBlitz

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

commit: d3dfd94

KKonstantinov · 2025-11-19T06:41:59Z

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()))

More semantically correct for JSON Schema objects where additional properties are expected but not validated by Zod.

felixweinberger · 2025-11-20T14:43:19Z

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!

- Remove unused Tool type import - Remove unused JSON_SCHEMA_2020_12_INPUT_SCHEMA constant (SEP-1613 test is pending - SDK validation supports the fields via PR #1135, but tool registration doesn't yet support generating raw JSON Schema) - Remove emoji from console.log - Alphabetize devDependencies in package.json

felixweinberger requested a review from a team as a code owner November 18, 2025 23:18

felixweinberger force-pushed the fweinberger/sep-1613 branch from 8fe3ce9 to 2d3b1a5Compare November 18, 2025 23:20

felixweinberger requested review from KKonstantinov, olaservo and pcarleton November 18, 2025 23:23

felixweinberger linked an issue Nov 18, 2025 that may be closed by this pull request
Implement SEP-1613: JSON Schema 2020-12 as Default Dialect #1057
Closed

felixweinberger mentioned this pull request Nov 18, 2025
Implement SEP-1613: JSON Schema 2020-12 as Default Dialect #1057
Closed

felixweinberger added 2 commits November 20, 2025 14:26

Address review: use .catchall(z.unknown()) instead of .passthrough()
913ca2a
More semantically correct for JSON Schema objects where additional properties are expected but not validated by Zod.

Merge main into fweinberger/sep-1613
96589bf

felixweinberger requested review from KKonstantinov and removed request for KKonstantinov November 20, 2025 14:43

felixweinberger changed the title ~~SEP-1613: Restore passthrough for inputSchema/outputSchema to support JSON Schema 2020-12~~SEP-1613: use.catchall() on inputSchema/outputSchema to support JSON Schema 2020-12Nov 20, 2025

felixweinbergerand others added 3 commits November 20, 2025 20:23

Merge branch 'main' into fweinberger/sep-1613
41c904b

Remove unnecessary comment additions from validation providers
6116af3

Merge branch 'main' into fweinberger/sep-1613
d3dfd94

pcarleton approved these changes Nov 21, 2025
View reviewed changes

pcarleton merged commit 41c6b35 into mainNov 21, 2025
10 checks passed

pcarleton deleted the fweinberger/sep-1613 branch November 21, 2025 13:41

davidmcwherter mentioned this pull request Nov 24, 2025
Gemini API rejects MCP tool schemas with $defs references google-gemini/gemini-cli#13326
Closed

This was referenced Jan 13, 2026
fix: override tools/list handler to emit raw JSON Schema 2020-12 #1378
Closed
Consider adding client test for JSON Schema 2020-12 preservation (SEP-1613) modelcontextprotocol/conformance#101
Closed

felixweinberger mentioned this pull request Mar 26, 2026
fix(core): allow additional JSON Schema properties in elicitInput requestedSchema #1768
Merged
9 tasks

rsalus mentioned this pull request May 14, 2026
[v1.x] fix(server): emit JSON Schema 2020-12 in tools/list (SEP-1613) #2085
Open
6 tasks

coderabbitaiBot mentioned this pull request Jul 3, 2026
Stabilize memory context output (roadmap Sprint 1) besfeng23/Memory#123
Merged