fix(shemars): use JSON Schema 2020-12 as Default Dialect

Author: jokemanfire

README.md

@@ -28,7 +28,8 @@ rmcp = { git = "https://github.com/modelcontextprotocol/rust-sdk", branch = "mai
 Basic dependencies:
 - [tokio required](https://github.com/tokio-rs/tokio)
 - [serde required](https://github.com/serde-rs/serde)
-
+Json Schema generation(Must follow the 2020-12 version):
+- [shemars required](https://github.com/GREsau/schemars)
 
 
 ### Build a Client

crates/rmcp/Cargo.toml

@@ -143,7 +143,7 @@ schemars = ["dep:schemars"]
 
 [dev-dependencies]
 tokio = { version = "1", features = ["full"] }
-schemars = { version = "1.0", features = ["chrono04"] }
+schemars = { version = "1.1.0", features = ["chrono04"] }
 
 anyhow = "1.0"
 tracing-subscriber = { version = "0.3", features = [

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

@@ -11,10 +11,8 @@ use crate::{
 /// A shortcut for generating a JSON schema for a type.
 pub fn schema_for_type<T: JsonSchema>() -> JsonObject {
     // explicitly to align json schema version to official specifications.
-    // https://github.com/modelcontextprotocol/modelcontextprotocol/blob/main/schema/2025-03-26/schema.json
-    // TODO: update to 2020-12 waiting for the mcp spec update
-    let mut settings = SchemaSettings::draft07();
-    settings.transforms = vec![Box::new(schemars::transform::AddNullable::default())];
+    // refer to https://github.com/modelcontextprotocol/modelcontextprotocol/pull/655 for details.
+    let settings = SchemaSettings::draft2020_12();
     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

@@ -3,6 +3,7 @@ use rmcp::{
     tool_router,
 };
 use serde::{Deserialize, Serialize};
+use serde_json; // ensure json macro path
 
 #[derive(Debug, Clone, Serialize, Deserialize, schemars::JsonSchema)]
 pub enum ChatRole {
@@ -43,24 +44,62 @@ impl Demo {
     }
 }
 
+fn expected_schema() -> serde_json::Value {
+    serde_json::json!({
+    "$defs": {
+        "ChatMessage": {
+            "properties": {
+                "content": {
+                    "type": "string"
+                },
+                "role": {
+                    "$ref": "#/$defs/ChatRole"
+                }
+            },
+            "required": [
+                "role",
+                "content"
+            ],
+            "type": "object"
+        },
+        "ChatRole": {
+            "enum": [
+                "System",
+                "User",
+                "Assistant",
+                "Tool"
+            ],
+            "type": "string"
+        }
+    },
+    "$schema": "https://json-schema.org/draft/2020-12/schema",
+    "properties": {
+        "messages": {
+            "items": {
+                "$ref": "#/$defs/ChatMessage"
+            },
+            "type": "array"
+        },
+        "system": {
+            "type": [
+                "string",
+                "null"
+            ]
+        }
+    },
+    "required": [
+        "messages"
+    ],
+    "title": "ChatRequest",
+    "type": "object"
+    })
+}
+
 #[test]
 fn test_complex_schema() {
     let attr = Demo::chat_tool_attr();
     let input_schema = attr.input_schema;
-    let enum_number = input_schema
-        .get("definitions")
-        .unwrap()
-        .as_object()
-        .unwrap()
-        .get("ChatRole")
-        .unwrap()
-        .as_object()
-        .unwrap()
-        .get("enum")
-        .unwrap()
-        .as_array()
-        .unwrap()
-        .len();
-    assert_eq!(enum_number, 4);
-    println!("{}", serde_json::to_string_pretty(&input_schema).unwrap());
+    let expected = expected_schema();
+    let produced = serde_json::Value::Object(input_schema.as_ref().clone());
+    assert_eq!(produced, expected, "schema mismatch");
 }