schema_for_type uses AddNullable which is non-compliant with JSON Schema 2020-12

[mwotton]

Problem

The schema_for_type() function in crates/rmcp/src/handler/server/common.rs uses AddNullable::default() as a transform when generating JSON Schema 2020-12 schemas:

let mut settings = SchemaSettings::draft2020_12();
settings.transforms = vec![Box::new(schemars::transform::AddNullable::default())];

However, the nullable keyword is an OpenAPI 3.0 extension, not part of JSON Schema 2020-12. This causes validation failures with strict JSON Schema validators.

Background

PR #549 updated schema_for_type() to use SchemaSettings::draft2020_12() to align with the MCP spec update (modelcontextprotocol/modelcontextprotocol#655), but it retained the AddNullable transform from the previous Draft 7 implementation.

The nullable keyword originated from OpenAPI 3.0 and was never part of any JSON Schema draft. In JSON Schema 2020-12, nullable types should be represented using:

• {"type": ["string", "null"]} (type array with null)
• {"anyOf": [{"type": "string"}, {"type": "null"}]}

Impact

• Strict JSON Schema 2020-12 validators (like mcporter) reject schemas containing nullable
• Generated tool schemas may fail validation in environments with strict schema checking
• The `` declaration claims 2020-12 compliance but includes non-2020-12 keywords

Example

With the current implementation, an optional string field produces:

{
  "nullable": true,
  "type": "string"
}

This should instead be:

{
  "type": ["string", "null"]
}

Suggested Fix

Remove or conditionally disable the AddNullable transform when using SchemaSettings::draft2020_12():

let mut settings = SchemaSettings::draft2020_12();
// Do not add AddNullable - nullable is OpenAPI 3.0, not JSON Schema 2020-12
let generator = settings.into_generator();

References

• PR fix(shemars): use JSON Schema 2020-12 as Default Dialect #549: Switched to JSON Schema 2020-12 but retained AddNullable
• JSON Schema 2020-12 Specification
• OpenAPI 3.0 nullable extension
• schemars AddNullable documentation

[mwotton]

(this was lodged by my agent but I believe it to be correct: i hit the issue while trying to use mcporter on a mcp server using rmcp)