Preamble

Title: Establish JSON Schema 2020-12 as Default Dialect for MCP
Author: Ola Hungerford
Status: Accepted
Type: Standards Track
Created: 2025-10-06 (original PR created 2025-06-05 before formalized SEP process)
Historical Context: Originally proposed in PR #655, later incorporated into PR #881/SEP-834. Re-formatting as an independent SEP while other changes in SEP-834 are still under review. Note: This proposal was already reviewed as part of SEP-834 and now needs a sponsor from the MCP steering group to move from proposal to draft status.

Abstract

This SEP establishes JSON Schema 2020-12 as the default dialect for embedded schemas within MCP messages (tool inputSchema/outputSchema and elicitation requestedSchema fields). Schemas may explicitly declare alternative dialects via the $schema field. This resolves ambiguity that has caused compatibility issues between implementations.

Motivation

The MCP specification does not explicitly state which JSON Schema version to use for embedded schemas. This has caused:

• Validation failures between clients and servers assuming different versions
• Implementation divergence across SDK ecosystems
• Developer uncertainty requiring arbitrary version choices

Community discussion (GitHub Discussion #366, PR #655) revealed that implementations were split between draft-07 and 2020-12, with multiple maintainers and community members expressing strong preference for 2020-12 as the default.

Specification

1. Default Dialect

Embedded JSON schemas within MCP messages MUST conform to JSON Schema 2020-12 when no $schema field is present.

2. Explicit Dialect Declaration

Schemas MAY include an explicit $schema field to declare a different dialect:

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "type": "object",
  "properties": {
    "name": { "type": "string" }
  }
}

3. Schema Validation Requirements

• Schemas MUST be valid according to their declared or default dialect
• The inputSchema field MUST NOT be null

For tools with no parameters, use one of these valid approaches:

• true - accepts any input (most permissive)
• {} - equivalent to true, accepts any input
• { "type": "object" } - accepts any object with any properties
• { "type": "object", "additionalProperties": false } - accepts only empty objects {}

Example for a tool with no parameters:

{
  "name": "get_current_time",
  "description": "Returns the current server time",
  "inputSchema": {
    "type": "object",
    "additionalProperties": false
  }
}

4. Scope of Application

This specification applies to:

• tools/list response: inputSchema and outputSchema
• prompts/elicit request: requestedSchema
• Future MCP features embedding JSON Schema definitions

5. Implementation Requirements

Servers MUST:

• Generate schemas conforming to 2020-12 by default
• Include explicit $schema when using non-default dialects

Clients MUST:

• Validate schemas according to declared or default dialect
• Support at least JSON Schema 2020-12

Rationale

Why 2020-12?

1. Ecosystem alignment: Python SDK (via Pydantic) and Go SDK implementations prefer/use 2020-12
2. Modern features: Better validation capabilities and composition support
3. Community preference: Multiple maintainers and community members in PR SEP-1613: Establish JSON Schema 2020-12 as Default Dialect for MCP #655 discussion advocated for 2020-12 over draft-07
4. Current standard: 2020-12 is the stable version as of 2025

Why allow explicit declaration?

• Supports migration paths for existing schemas
• Provides flexibility without protocol changes
• Follows JSON Schema best practices

Alternatives considered

• Draft-07 as default: Rejected after community feedback; older version with less capability
• No default: Rejected as unnecessarily verbose; adds boilerplate
• Multiple equal versions: Rejected; creates unpredictability and fragmentation

Backward Compatibility

This is technically a clarification, and not a breaking change:

• Existing schemas without $schema default to 2020-12
• Servers can add explicit $schema during transition
• Basic schemas (type, properties, required) work across versions

Migration may be needed for schemas assuming draft-07 by default:

• Schemas using dependencies (→ dependentSchemas + dependentRequired)
• Positional array validation (→ prefixItems)

Migration strategy: Add explicit $schema: "http://json-schema.org/draft-07/schema#" during transition, then update to 2020-12 features.

Reference Implementation

SDK Implementations

Python SDK - Already compatible:

• Uses Pydantic for schema generation
• Pydantic defaults to 2020-12 via .model_json_schema()

Go SDK - Implemented 2020-12:

• Explicit 2020-12 implementation completed
• Confirmed by @samthanawalla in PR SEP-1613: Establish JSON Schema 2020-12 as Default Dialect for MCP #655 discussion

Other SDKs:

• May require updates but based on other examples, there should be straightforward or out-of-the-box options to support this. I can add more examples here or we can create issues to follow up on these after acceptance.

Security Implications

No specific security implications have been identified from establishing 2020-12 as the default dialect. The clarification reduces ambiguity that could lead to validation mismatches between implementations, which is a minor security improvement through increased predictability.

Implementations should use well-maintained JSON Schema validator libraries and keep them updated, as with any dependency.

Related Work

SEP-1330: Elicitation Enum Schema Improvements

SEP-1330 proposes deprecating the non-standard enumNames property in favor of JSON Schema 2020-12 compliant patterns. This work is directly enabled by establishing 2020-12 as the default dialect.

Implementation Consideration:
As noted in SEP-1330 discussion, there is some concern about parsing complexity with advanced JSON Schema features like oneOf and anyOf. However, these features are part of the JSON Schema standard and well-supported by mature validator libraries. Implementations can balance standards compliance with their parsing needs by using well-tested JSON Schema validation libraries.

SEP-834: Full JSON Schema 2020-12 Support

This SEP establishes the foundation (default dialect) while SEP-834 addresses comprehensive support for 2020-12 features.

Open Questions

The schema for the spec itself references draft-07 and the typescript-json-schema package we use to generate it only supports draft-07.

Options:

1. Update schema generation script to patch to 2020-12 after generation (this is what I did in the current PR)
2. Switch to a different schema generator that supports 2020-12
3. Leave as-is since it doesn't actually conflict with the spec?

Personally I'd prefer (1) in the short term and then (2) as a follow-up.