feat: validate embedded JSON Schema documents against 2020-12 meta-schema (SEP-1613) (#949)

MCP embeds JSON Schema documents in three places — Tool.inputSchema,
Tool.outputSchema, and ElicitRequest.requestedSchema — and SEP-1613
mandates that these documents conform to JSON Schema 2020-12 by default.

Servers now reject malformed schemas at both build time (McpServer.build())
and runtime (McpAsyncServer/McpStatelessAsyncServer.addTool()), returning
an IllegalArgumentException that identifies the offending field and
references SEP-1613.  Elicitation requests whose requestedSchema violates
the meta-schema are rejected before being sent to the client.

Schemas that explicitly declare a different dialect via $schema are accepted
without meta-schema validation, since 2020-12 is the default, not the only
permitted dialect.

Refine meta-schema loading and deprecate orphan JsonSchemaValidator

Resolves #700

Signed-off-by: Dariusz Jędrzejczyk <2554306+chemicL@users.noreply.github.com>

conformance-tests/VALIDATION_RESULTS.md

@@ -2,22 +2,31 @@
 
 ## Summary
 
-**Server Tests:** 40/40 passed (100%)
+**Server Tests (active suite):** 44/44 passed (31 scenarios, 100%)
+**Server Tests (spec 2025-11-25):** 4/4 passed — SEP-1613 `json-schema-2020-12` scenario ✨
 **Client Tests:** 3/4 scenarios passed (9/10 checks passed)
 **Auth Tests:** 14/15 scenarios fully passing (196 passed, 0 failed, 1 warning, 93.3% scenarios, 99.5% checks)
 
 ## Server Test Results
 
-### Passing (40/40)
+### Active Suite — Passing (31/31 scenarios, 44/44 checks)
 
 - **Lifecycle & Utilities (4/4):** initialize, ping, logging-set-level, completion-complete
-- **Tools (11/11):** All scenarios including progress notifications ✨
+- **Tools (13/13):** All scenarios including progress notifications, sampling, elicitation ✨
 - **Elicitation (10/10):** SEP-1034 defaults (5 checks), SEP-1330 enums (5 checks)
-- **Resources (6/6):** list, read-text, read-binary, templates-read, subscribe, unsubscribe
-- **Prompts (4/4):** list, simple, with-args, embedded-resource, with-image
+- **Resources (7/7):** list, read-text, read-binary, templates-read, subscribe, unsubscribe, SEP-2164 resource-not-found
+- **Prompts (5/5):** list, simple, with-args, embedded-resource, with-image
 - **SSE Transport (2/2):** Multiple streams
 - **Security (2/2):** Localhost validation passes, DNS rebinding protection
 
+### Spec 2025-11-25 Scenarios — Passing (1/1 scenario, 4/4 checks)
+
+- **JSON Schema 2020-12 (SEP-1613) (4/4):** ✨
+  - `json_schema_2020_12_tool` found
+  - `inputSchema.$schema` field preserved
+  - `inputSchema.$defs` field preserved
+  - `inputSchema.additionalProperties` field preserved
+
 ## Client Test Results
 
 ### Passing (3/4 scenarios, 9/10 checks)
@@ -69,7 +78,7 @@ Uses the `client-spring-http-client` module with Spring Security OAuth2 and the
 
 ## Running Tests
 
-### Server
+### Server (active suite)
 ```bash
 # Start server
 ./mvnw compile -pl conformance-tests/server-servlet -am exec:java
@@ -78,6 +87,17 @@ Uses the `client-spring-http-client` module with Spring Security OAuth2 and the
 npx @modelcontextprotocol/conformance server --url http://localhost:8080/mcp --suite active
 ```
 
+### Server (spec 2025-11-25 scenarios — SEP-1613)
+```bash
+# Start server (if not already running)
+./mvnw compile -pl conformance-tests/server-servlet -am exec:java
+
+# Run json-schema-2020-12 scenario
+cd ../conformance && node --import tsx/esm src/index.ts server \
+  --url http://localhost:8080/mcp \
+  --scenario json-schema-2020-12
+```
+
 ### Client
 ```bash
 # Build

conformance-tests/server-servlet/src/main/java/io/modelcontextprotocol/conformance/server/ConformanceServlet.java

@@ -9,6 +9,7 @@ import io.modelcontextprotocol.server.McpServer;
 import io.modelcontextprotocol.server.McpServerFeatures;
 import io.modelcontextprotocol.server.transport.DefaultServerTransportSecurityValidator;
 import io.modelcontextprotocol.server.transport.HttpServletStreamableServerTransportProvider;
+import io.modelcontextprotocol.spec.McpSchema;
 import io.modelcontextprotocol.spec.McpSchema.AudioContent;
 import io.modelcontextprotocol.spec.McpSchema.BlobResourceContents;
 import io.modelcontextprotocol.spec.McpSchema.CallToolResult;
@@ -402,6 +403,29 @@ public class ConformanceServlet {
 					})
 					.build(),
 
+				// json_schema_2020_12_tool - SEP-1613 dialect/keyword preservation
+				McpServerFeatures.SyncToolSpecification.builder()
+					.tool(Tool
+						.builder("json_schema_2020_12_tool", Map.of("$schema", McpSchema.JSON_SCHEMA_DIALECT_2020_12,
+								"type", "object", "$defs",
+								Map.of("address",
+										Map.of("type", "object", "properties",
+												Map.of("street", Map.of("type", "string"), "city",
+														Map.of("type", "string")))),
+								"properties",
+								Map.of("name", Map.of("type", "string"), "address", Map.of("$ref", "#/$defs/address")),
+								"additionalProperties", false))
+						.description("Tool with JSON Schema 2020-12 features (SEP-1613)")
+						.build())
+					.callHandler((exchange, request) -> {
+						logger.info("Tool 'json_schema_2020_12_tool' called");
+						return CallToolResult.builder()
+							.content(List.of(TextContent.builder("ok").build()))
+							.isError(false)
+							.build();
+					})
+					.build(),
+
 				// test_elicitation_sep1330_enums - Tool with enum schema improvements
 				McpServerFeatures.SyncToolSpecification.builder()
 					.tool(Tool.builder("test_elicitation_sep1330_enums", EMPTY_JSON_SCHEMA)