Add @JsonIgnoreProperties(ignoreUnknown = true) to capability sub-records

The top-level ClientCapabilities and ServerCapabilities records have
@JsonIgnoreProperties(ignoreUnknown = true) but their nested sub-records
do not. Since ObjectMapper defaults to FAIL_ON_UNKNOWN_PROPERTIES = true,
unknown fields on sub-objects cause deserialization failures.

This already caused a real breakage when elicitation gained form/url
fields (#724). The MCP spec explicitly does not close capability objects
(additionalProperties is never set to false), so SDKs must tolerate
unknown fields for forward compatibility.

Add the annotation to:
- ClientCapabilities.Sampling
- ClientCapabilities.Elicitation, Elicitation.Form, Elicitation.Url
- ServerCapabilities.CompletionCapabilities
- ServerCapabilities.LoggingCapabilities
- ServerCapabilities.PromptCapabilities
- ServerCapabilities.ResourceCapabilities
- ServerCapabilities.ToolCapabilities

Fixes #766

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: rameshreddy-adutla

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

@@ -404,6 +404,7 @@ public record RootCapabilities(@JsonProperty("listChanged") Boolean listChanged)
 		 * from MCP servers in their prompts.
 		 */
 		@JsonInclude(JsonInclude.Include.NON_ABSENT)
+		@JsonIgnoreProperties(ignoreUnknown = true)
 		public record Sampling() {
 		}
 
@@ -431,19 +432,22 @@ public record Sampling() {
 		 * @param url support for out-of-band URL-based elicitation
 		 */
 		@JsonInclude(JsonInclude.Include.NON_ABSENT)
+		@JsonIgnoreProperties(ignoreUnknown = true)
 		public record Elicitation(@JsonProperty("form") Form form, @JsonProperty("url") Url url) {
 
 			/**
 			 * Marker record indicating support for form-based elicitation mode.
 			 */
 			@JsonInclude(JsonInclude.Include.NON_ABSENT)
+			@JsonIgnoreProperties(ignoreUnknown = true)
 			public record Form() {
 			}
 
 			/**
 			 * Marker record indicating support for URL-based elicitation mode.
 			 */
 			@JsonInclude(JsonInclude.Include.NON_ABSENT)
+			@JsonIgnoreProperties(ignoreUnknown = true)
 			public record Url() {
 			}
 
@@ -542,13 +546,15 @@ public record ServerCapabilities( // @formatter:off
 		 * Present if the server supports argument autocompletion suggestions.
 		 */
 		@JsonInclude(JsonInclude.Include.NON_ABSENT)
+		@JsonIgnoreProperties(ignoreUnknown = true)
 		public record CompletionCapabilities() {
 		}
 
 		/**
 		 * Present if the server supports sending log messages to the client.
 		 */
 		@JsonInclude(JsonInclude.Include.NON_ABSENT)
+		@JsonIgnoreProperties(ignoreUnknown = true)
 		public record LoggingCapabilities() {
 		}
 
@@ -559,6 +565,7 @@ public record LoggingCapabilities() {
 		 * the prompt list
 		 */
 		@JsonInclude(JsonInclude.Include.NON_ABSENT)
+		@JsonIgnoreProperties(ignoreUnknown = true)
 		public record PromptCapabilities(@JsonProperty("listChanged") Boolean listChanged) {
 		}
 
@@ -570,6 +577,7 @@ public record PromptCapabilities(@JsonProperty("listChanged") Boolean listChange
 		 * the resource list
 		 */
 		@JsonInclude(JsonInclude.Include.NON_ABSENT)
+		@JsonIgnoreProperties(ignoreUnknown = true)
 		public record ResourceCapabilities(@JsonProperty("subscribe") Boolean subscribe,
 				@JsonProperty("listChanged") Boolean listChanged) {
 		}
@@ -581,6 +589,7 @@ public record ResourceCapabilities(@JsonProperty("subscribe") Boolean subscribe,
 		 * the tool list
 		 */
 		@JsonInclude(JsonInclude.Include.NON_ABSENT)
+		@JsonIgnoreProperties(ignoreUnknown = true)
 		public record ToolCapabilities(@JsonProperty("listChanged") Boolean listChanged) {
 		}
 

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

@@ -1760,4 +1760,75 @@ void testProgressNotificationWithoutMessage() throws Exception {
 					{"progressToken":"progress-token-789","progress":0.25}"""));
 	}
 
+	// Capability sub-records: unknown fields should be silently ignored (#766)
+
+	@Test
+	void testSamplingIgnoresUnknownFields() throws Exception {
+		McpSchema.ClientCapabilities.Sampling sampling = JSON_MAPPER.readValue("""
+				{"futureField": true}""", McpSchema.ClientCapabilities.Sampling.class);
+		assertThat(sampling).isNotNull();
+	}
+
+	@Test
+	void testElicitationIgnoresUnknownFields() throws Exception {
+		McpSchema.ClientCapabilities.Elicitation elicitation = JSON_MAPPER.readValue("""
+				{"form": {}, "url": {}, "futureField": "value"}""", McpSchema.ClientCapabilities.Elicitation.class);
+		assertThat(elicitation).isNotNull();
+	}
+
+	@Test
+	void testElicitationFormIgnoresUnknownFields() throws Exception {
+		McpSchema.ClientCapabilities.Elicitation.Form form = JSON_MAPPER.readValue("""
+				{"futureField": 42}""", McpSchema.ClientCapabilities.Elicitation.Form.class);
+		assertThat(form).isNotNull();
+	}
+
+	@Test
+	void testElicitationUrlIgnoresUnknownFields() throws Exception {
+		McpSchema.ClientCapabilities.Elicitation.Url url = JSON_MAPPER.readValue("""
+				{"futureField": "value"}""", McpSchema.ClientCapabilities.Elicitation.Url.class);
+		assertThat(url).isNotNull();
+	}
+
+	@Test
+	void testCompletionCapabilitiesIgnoresUnknownFields() throws Exception {
+		McpSchema.ServerCapabilities.CompletionCapabilities completions = JSON_MAPPER.readValue("""
+				{"futureField": true}""", McpSchema.ServerCapabilities.CompletionCapabilities.class);
+		assertThat(completions).isNotNull();
+	}
+
+	@Test
+	void testLoggingCapabilitiesIgnoresUnknownFields() throws Exception {
+		McpSchema.ServerCapabilities.LoggingCapabilities logging = JSON_MAPPER.readValue("""
+				{"futureField": "value"}""", McpSchema.ServerCapabilities.LoggingCapabilities.class);
+		assertThat(logging).isNotNull();
+	}
+
+	@Test
+	void testPromptCapabilitiesIgnoresUnknownFields() throws Exception {
+		McpSchema.ServerCapabilities.PromptCapabilities prompts = JSON_MAPPER.readValue("""
+				{"listChanged": true, "futureField": "value"}""",
+				McpSchema.ServerCapabilities.PromptCapabilities.class);
+		assertThat(prompts).isNotNull();
+		assertThat(prompts.listChanged()).isTrue();
+	}
+
+	@Test
+	void testResourceCapabilitiesIgnoresUnknownFields() throws Exception {
+		McpSchema.ServerCapabilities.ResourceCapabilities resources = JSON_MAPPER.readValue("""
+				{"subscribe": true, "listChanged": false, "futureField": 123}""",
+				McpSchema.ServerCapabilities.ResourceCapabilities.class);
+		assertThat(resources).isNotNull();
+		assertThat(resources.subscribe()).isTrue();
+		assertThat(resources.listChanged()).isFalse();
+	}
+
+	@Test
+	void testToolCapabilitiesIgnoresUnknownFields() throws Exception {
+		McpSchema.ServerCapabilities.ToolCapabilities tools = JSON_MAPPER.readValue("""
+				{"listChanged": true, "futureField": "value"}""", McpSchema.ServerCapabilities.ToolCapabilities.class);
+		assertThat(tools).isNotNull();
+		assertThat(tools.listChanged()).isTrue();
+	}
+
 }