fix(schema): remove AddNullable from draft2020_12 settings (#664)

* fix(schema): remove AddNullable from draft2020_12 settings

The `nullable` keyword is an OpenAPI 3.0 extension, not part of
JSON Schema 2020-12. Using AddNullable with draft2020_12 settings
causes validation failures with strict JSON Schema validators.

JSON Schema 2020-12 represents nullable types using:
- {"type": ["string", "null"]} (type array with null)
- {"anyOf": [{"type": "string"}, {"type": "null"}]}

Fixes #663

* test(schema): update complex schema nullable expectation

* test(schema): align macro optional-field expectations with draft2020

Author: mwotton

crates/rmcp/src/handler/server/common.rs

@@ -23,8 +23,10 @@ pub fn schema_for_type<T: JsonSchema + std::any::Any>() -> Arc<JsonObject> {
         } else {
             // explicitly to align json schema version to official specifications.
             // refer to https://github.com/modelcontextprotocol/modelcontextprotocol/pull/655 for details.
-            let mut settings = SchemaSettings::draft2020_12();
-            settings.transforms = vec![Box::new(schemars::transform::AddNullable::default())];
+            let settings = SchemaSettings::draft2020_12();
+            // Note: AddNullable is intentionally NOT used here because the `nullable` keyword
+            // is an OpenAPI 3.0 extension, not part of JSON Schema 2020-12. Using it would
+            // cause validation failures with strict JSON Schema validators.
             let generator = settings.into_generator();
             let schema = generator.into_root_schema_for::<T>();
             let object = serde_json::to_value(schema).expect("failed to serialize schema");

crates/rmcp/tests/test_complex_schema.rs

@@ -80,8 +80,10 @@ fn expected_schema() -> serde_json::Value {
           "type": "array"
         },
         "system": {
-          "nullable": true,
-          "type": "string"
+          "type": [
+            "string",
+            "null"
+          ]
         }
       },
       "required": [

crates/rmcp/tests/test_tool_macros.rs

@@ -238,7 +238,7 @@ fn test_optional_field_schema_generation_via_macro() {
     );
 
     // Verify the schema generated for the aggregated OptionalFieldTestSchema
-    // by the macro infrastructure (which should now use OpenAPI 3 settings)
+    // by the macro infrastructure using JSON Schema 2020-12 settings.
     let input_schema_map = &*tool_attr.input_schema; // Dereference Arc<JsonObject>
 
     // Check the schema for the 'description' property within the input schema
@@ -253,18 +253,21 @@ fn test_optional_field_schema_generation_via_macro() {
         .as_object()
         .unwrap();
 
-    // Assert that the format is now `type: "string", nullable: true`
+    // Assert nullable Option<T> is represented in JSON Schema 2020-12 form.
+    let type_value = description_schema
+        .get("type")
+        .expect("Schema for Option<String> should include a type field");
+    let type_array = type_value
+        .as_array()
+        .expect("Schema for Option<String> should use a type array [T, null]");
     assert_eq!(
-        description_schema.get("type").map(|v| v.as_str().unwrap()),
-        Some("string"),
-        "Schema for Option<String> generated by macro should be type: \"string\""
+        type_array,
+        &vec![serde_json::json!("string"), serde_json::json!("null")],
+        "Schema for Option<String> should be type: [\"string\", \"null\"]"
     );
-    assert_eq!(
-        description_schema
-            .get("nullable")
-            .map(|v| v.as_bool().unwrap()),
-        Some(true),
-        "Schema for Option<String> generated by macro should have nullable: true"
+    assert!(
+        description_schema.get("nullable").is_none(),
+        "Schema for Option<String> should not use OpenAPI nullable in JSON Schema 2020-12"
     );
     // We still check the description is correct
     assert_eq!(
@@ -274,12 +277,8 @@ fn test_optional_field_schema_generation_via_macro() {
         Some("An optional description field")
     );
 
-    // Ensure the old 'type: [T, null]' format is NOT used
-    let type_value = description_schema.get("type").unwrap();
-    assert!(
-        !type_value.is_array(),
-        "Schema type should not be an array [T, null]"
-    );
+    // Ensure no OpenAPI-only nullable extension was emitted.
+    assert!(description_schema.get("nullable").is_none());
 }
 
 // Define a dummy client handler