feat(schema): add support for JSON Schema $defs and definitions (#146)

Added support for $defs and definitions properties in JsonSchema record to handle JSON Schema references properly. Added tests to verify both formats work correctly.

The JsonSchema test approach uses serialization/deserialization round-trip validation instead of property-by-property assertions. This makes tests more maintainable and less likely to break when new properties are added.

🤖 Generated with [Claude Code](https://claude.ai/code)

---------

Co-authored-by: Claude <noreply@anthropic.com>

mcp/src/main/java/io/modelcontextprotocol/spec/McpSchema.java

@@ -703,7 +703,9 @@ public final class McpSchema {
 		@JsonProperty("type") String type,
 		@JsonProperty("properties") Map<String, Object> properties,
 		@JsonProperty("required") List<String> required,
-		@JsonProperty("additionalProperties") Boolean additionalProperties) {
+		@JsonProperty("additionalProperties") Boolean additionalProperties,
+		@JsonProperty("$defs") Map<String, Object> defs,
+		@JsonProperty("definitions") Map<String, Object> definitions) {
 	} // @formatter:on
 
 	/**

mcp/src/test/java/io/modelcontextprotocol/spec/McpSchemaTests.java

@@ -9,6 +9,7 @@ import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 
+import com.fasterxml.jackson.core.type.TypeReference;
 import com.fasterxml.jackson.databind.ObjectMapper;
 import com.fasterxml.jackson.databind.exc.InvalidTypeIdException;
 import io.modelcontextprotocol.spec.McpSchema.TextResourceContents;
@@ -449,6 +450,92 @@ public class McpSchemaTests {
 
 	// Tool Tests
 
+	@Test
+	void testJsonSchema() throws Exception {
+		String schemaJson = """
+				{
+					"type": "object",
+					"properties": {
+						"name": {
+							"type": "string"
+						},
+						"address": {
+							"$ref": "#/$defs/Address"
+						}
+					},
+					"required": ["name"],
+					"$defs": {
+						"Address": {
+							"type": "object",
+							"properties": {
+								"street": {"type": "string"},
+								"city": {"type": "string"}
+							},
+							"required": ["street", "city"]
+						}
+					}
+				}
+				""";
+
+		// Deserialize the original string to a JsonSchema object
+		McpSchema.JsonSchema schema = mapper.readValue(schemaJson, McpSchema.JsonSchema.class);
+
+		// Serialize the object back to a string
+		String serialized = mapper.writeValueAsString(schema);
+
+		// Deserialize again
+		McpSchema.JsonSchema deserialized = mapper.readValue(serialized, McpSchema.JsonSchema.class);
+
+		// Serialize one more time and compare with the first serialization
+		String serializedAgain = mapper.writeValueAsString(deserialized);
+
+		// The two serialized strings should be the same
+		assertThatJson(serializedAgain).when(Option.IGNORING_ARRAY_ORDER).isEqualTo(json(serialized));
+	}
+
+	@Test
+	void testJsonSchemaWithDefinitions() throws Exception {
+		String schemaJson = """
+				{
+					"type": "object",
+					"properties": {
+						"name": {
+							"type": "string"
+						},
+						"address": {
+							"$ref": "#/definitions/Address"
+						}
+					},
+					"required": ["name"],
+					"definitions": {
+						"Address": {
+							"type": "object",
+							"properties": {
+								"street": {"type": "string"},
+								"city": {"type": "string"}
+							},
+							"required": ["street", "city"]
+						}
+					}
+				}
+				""";
+
+		// Deserialize the original string to a JsonSchema object
+		McpSchema.JsonSchema schema = mapper.readValue(schemaJson, McpSchema.JsonSchema.class);
+
+		// Serialize the object back to a string
+		String serialized = mapper.writeValueAsString(schema);
+
+		// Deserialize again
+		McpSchema.JsonSchema deserialized = mapper.readValue(serialized, McpSchema.JsonSchema.class);
+
+		// Serialize one more time and compare with the first serialization
+		String serializedAgain = mapper.writeValueAsString(deserialized);
+
+		// The two serialized strings should be the same
+		assertThatJson(serializedAgain).when(Option.IGNORING_ARRAY_ORDER).isEqualTo(json(serialized));
+	}
+
 	@Test
 	void testTool() throws Exception {
 		String schemaJson = """
@@ -477,6 +564,48 @@ public class McpSchemaTests {
 							{"name":"test-tool","description":"A test tool","inputSchema":{"type":"object","properties":{"name":{"type":"string"},"value":{"type":"number"}},"required":["name"]}}"""));
 	}
 
+	@Test
+	void testToolWithComplexSchema() throws Exception {
+		String complexSchemaJson = """
+				{
+					"type": "object",
+					"$defs": {
+						"Address": {
+							"type": "object",
+							"properties": {
+								"street": {"type": "string"},
+								"city": {"type": "string"}
+							},
+							"required": ["street", "city"]
+						}
+					},
+					"properties": {
+						"name": {"type": "string"},
+						"shippingAddress": {"$ref": "#/$defs/Address"}
+					},
+					"required": ["name", "shippingAddress"]
+				}
+				""";
+
+		McpSchema.Tool tool = new McpSchema.Tool("addressTool", "Handles addresses", complexSchemaJson);
+
+		// Serialize the tool to a string
+		String serialized = mapper.writeValueAsString(tool);
+
+		// Deserialize back to a Tool object
+		McpSchema.Tool deserializedTool = mapper.readValue(serialized, McpSchema.Tool.class);
+
+		// Serialize again and compare with first serialization
+		String serializedAgain = mapper.writeValueAsString(deserializedTool);
+
+		// The two serialized strings should be the same
+		assertThatJson(serializedAgain).when(Option.IGNORING_ARRAY_ORDER).isEqualTo(json(serialized));
+
+		// Just verify the basic structure was preserved
+		assertThat(deserializedTool.inputSchema().defs()).isNotNull();
+		assertThat(deserializedTool.inputSchema().defs()).containsKey("Address");
+	}
+
 	@Test
 	void testCallToolRequest() throws Exception {
 		Map<String, Object> arguments = new HashMap<>();