fix: Support form and url fields in Elicitation capability per 2025-11-25 spec

Update the ClientCapabilities.Elicitation record to accept optional "form"
and "url" fields as defined in the MCP 2025-11-25 specification.

Previously, deserializing an InitializeRequest with
`{"capabilities":{"elicitation":{"form":{}}}}` would fail with
UnrecognizedPropertyException because the Elicitation record was empty.

Changes:
- Add nested Form and Url marker records to Elicitation
- Add no-arg constructor for backward compatibility (serializes to {})
- Add elicitation(boolean form, boolean url) builder method
- Add comprehensive tests for deserialization and serialization

Fixes #724

Author: rhtnr

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

@@ -417,10 +417,50 @@ public record Sampling() {
 		 * maintain control over user interactions and data sharing while enabling servers
 		 * to gather necessary information dynamically. Servers can request structured
 		 * data from users with optional JSON schemas to validate responses.
+		 *
+		 * <p>
+		 * Per the 2025-11-25 spec, clients can declare support for specific elicitation
+		 * modes:
+		 * <ul>
+		 * <li>{@code form} - In-band structured data collection with optional schema
+		 * validation</li>
+		 * <li>{@code url} - Out-of-band interaction via URL navigation</li>
+		 * </ul>
+		 *
+		 * <p>
+		 * For backward compatibility, an empty elicitation object {@code {}} is
+		 * equivalent to declaring support for form mode only.
+		 *
+		 * @param form support for in-band form-based elicitation
+		 * @param url support for out-of-band URL-based elicitation
 		 */
 		@JsonInclude(JsonInclude.Include.NON_ABSENT)
 		@JsonIgnoreProperties(ignoreUnknown = true)
-		public record Elicitation() {
+		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() {
+			}
+
+			/**
+			 * Creates an Elicitation with default settings (backward compatible, produces
+			 * empty JSON object).
+			 */
+			public Elicitation() {
+				this(null, null);
+			}
 		}
 
 		public static Builder builder() {
@@ -452,11 +492,28 @@ public Builder sampling() {
 				return this;
 			}
 
+			/**
+			 * Enables elicitation capability with default settings (backward compatible,
+			 * produces empty JSON object).
+			 * @return this builder
+			 */
 			public Builder elicitation() {
 				this.elicitation = new Elicitation();
 				return this;
 			}
 
+			/**
+			 * Enables elicitation capability with explicit form and/or url mode support.
+			 * @param form whether to support form-based elicitation
+			 * @param url whether to support URL-based elicitation
+			 * @return this builder
+			 */
+			public Builder elicitation(boolean form, boolean url) {
+				this.elicitation = new Elicitation(form ? new Elicitation.Form() : null,
+						url ? new Elicitation.Url() : null);
+				return this;
+			}
+
 			public ClientCapabilities build() {
 				return new ClientCapabilities(experimental, roots, sampling, elicitation);
 			}

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

@@ -363,8 +363,9 @@ void testParseInitializeRequest() throws IOException {
 
 		McpSchema.InitializeRequest deserialized = JSON_MAPPER.readValue(serialized, McpSchema.InitializeRequest.class);
 
+		// The JSON includes form:{} so we need to use elicitation(true, false) to match
 		McpSchema.ClientCapabilities capabilities = McpSchema.ClientCapabilities.builder()
-			.elicitation()
+			.elicitation(true, false)
 			.sampling()
 			.build();
 		McpSchema.Implementation clientInfo = new McpSchema.Implementation("test-client", "1.0.0");
@@ -1639,6 +1640,107 @@ void testListRootsResult() throws Exception {
 
 	}
 
+	// Elicitation Capability Tests (Issue #724)
+
+	@Test
+	void testElicitationCapabilityWithFormField() throws Exception {
+		// Test that elicitation with "form" field can be deserialized (2025-11-25 spec)
+		String json = """
+				{"protocolVersion":"2024-11-05","capabilities":{"elicitation":{"form":{}}},"clientInfo":{"name":"test-client","version":"1.0.0"}}
+				""";
+
+		McpSchema.InitializeRequest request = JSON_MAPPER.readValue(json, McpSchema.InitializeRequest.class);
+
+		assertThat(request).isNotNull();
+		assertThat(request.capabilities()).isNotNull();
+		assertThat(request.capabilities().elicitation()).isNotNull();
+	}
+
+	@Test
+	void testElicitationCapabilityWithFormAndUrlFields() throws Exception {
+		// Test that elicitation with both "form" and "url" fields can be deserialized
+		String json = """
+				{"protocolVersion":"2024-11-05","capabilities":{"elicitation":{"form":{},"url":{}}},"clientInfo":{"name":"test-client","version":"1.0.0"}}
+				""";
+
+		McpSchema.InitializeRequest request = JSON_MAPPER.readValue(json, McpSchema.InitializeRequest.class);
+
+		assertThat(request).isNotNull();
+		assertThat(request.capabilities()).isNotNull();
+		assertThat(request.capabilities().elicitation()).isNotNull();
+	}
+
+	@Test
+	void testElicitationCapabilityBackwardCompatibilityEmptyObject() throws Exception {
+		// Test backward compatibility: empty elicitation {} should still work
+		String json = """
+				{"protocolVersion":"2024-11-05","capabilities":{"elicitation":{}},"clientInfo":{"name":"test-client","version":"1.0.0"}}
+				""";
+
+		McpSchema.InitializeRequest request = JSON_MAPPER.readValue(json, McpSchema.InitializeRequest.class);
+
+		assertThat(request).isNotNull();
+		assertThat(request.capabilities()).isNotNull();
+		assertThat(request.capabilities().elicitation()).isNotNull();
+	}
+
+	@Test
+	void testElicitationCapabilityBuilderBackwardCompatibility() throws Exception {
+		// Test that the existing builder API still works and produces valid JSON
+		McpSchema.ClientCapabilities capabilities = McpSchema.ClientCapabilities.builder().elicitation().build();
+
+		assertThat(capabilities.elicitation()).isNotNull();
+
+		// Serialize and verify it produces valid JSON (should be {} for backward compat)
+		String json = JSON_MAPPER.writeValueAsString(capabilities);
+		assertThat(json).contains("\"elicitation\"");
+	}
+
+	@Test
+	void testElicitationCapabilitySerializationRoundTrip() throws Exception {
+		// Test that serialization and deserialization round-trip works
+		McpSchema.ClientCapabilities original = McpSchema.ClientCapabilities.builder().elicitation().build();
+
+		String json = JSON_MAPPER.writeValueAsString(original);
+		McpSchema.ClientCapabilities deserialized = JSON_MAPPER.readValue(json, McpSchema.ClientCapabilities.class);
+
+		assertThat(deserialized.elicitation()).isNotNull();
+	}
+
+	@Test
+	void testElicitationCapabilityBuilderWithFormAndUrl() throws Exception {
+		// Test the new builder method that explicitly sets form and url support
+		McpSchema.ClientCapabilities capabilities = McpSchema.ClientCapabilities.builder()
+			.elicitation(true, true)
+			.build();
+
+		assertThat(capabilities.elicitation()).isNotNull();
+		assertThat(capabilities.elicitation().form()).isNotNull();
+		assertThat(capabilities.elicitation().url()).isNotNull();
+
+		// Verify serialization produces the expected JSON
+		String json = JSON_MAPPER.writeValueAsString(capabilities);
+		assertThatJson(json).when(Option.IGNORING_ARRAY_ORDER).isObject().containsKey("elicitation");
+		assertThat(json).contains("\"form\"");
+		assertThat(json).contains("\"url\"");
+	}
+
+	@Test
+	void testElicitationCapabilityBuilderFormOnly() throws Exception {
+		// Test builder with form only
+		McpSchema.ClientCapabilities capabilities = McpSchema.ClientCapabilities.builder()
+			.elicitation(true, false)
+			.build();
+
+		assertThat(capabilities.elicitation()).isNotNull();
+		assertThat(capabilities.elicitation().form()).isNotNull();
+		assertThat(capabilities.elicitation().url()).isNull();
+
+		String json = JSON_MAPPER.writeValueAsString(capabilities);
+		assertThat(json).contains("\"form\"");
+		assertThat(json).doesNotContain("\"url\"");
+	}
+
 	// Progress Notification Tests
 
 	@Test