Add defs field to ToolsInputSchema (#488)

* Add  field to ToolsInputSchema

* Update MarshalJSON()

Author: ostafen

mcp/tools.go

@@ -585,6 +585,7 @@ func (t Tool) MarshalJSON() ([]byte, error) {
 }
 
 type ToolInputSchema struct {
+	Defs       map[string]any `json:"$defs,omitempty"`
 	Type       string         `json:"type"`
 	Properties map[string]any `json:"properties,omitempty"`
 	Required   []string       `json:"required,omitempty"`
@@ -595,6 +596,10 @@ func (tis ToolInputSchema) MarshalJSON() ([]byte, error) {
 	m := make(map[string]any)
 	m["type"] = tis.Type
 
+	if tis.Defs != nil {
+		m["$defs"] = tis.Defs
+	}
+
 	// Marshal Properties to '{}' rather than `nil` when its length equals zero
 	if tis.Properties != nil {
 		m["properties"] = tis.Properties