Accept missing required fields on deserialization

Some clients or servers fail to provide required JSON fields so instead of
failing we replace them by default values and log with WARN level and allow the
application to make progress.

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

Author: chemicL

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

@@ -1599,6 +1599,10 @@ public CallToolRequest build() {
 	 * @param structuredContent An optional JSON object that represents the structured
 	 * result of the tool call.
 	 * @param meta See specification for notes on _meta usage
+	 * <p>
+	 * Note: {@code content} is required by the MCP specification. Deserialization accepts
+	 * a missing value and substitutes an empty list to avoid breaking existing
+	 * integrations that may omit the field.
 	 */
 	@JsonInclude(JsonInclude.Include.NON_ABSENT)
 	@JsonIgnoreProperties(ignoreUnknown = true)
@@ -1612,6 +1616,17 @@ public record CallToolResult( // @formatter:off
 			Assert.notNull(content, "content must not be null");
 		}
 
+		@JsonCreator
+		static CallToolResult fromJson(@JsonProperty("content") List<Content> content,
+				@JsonProperty("isError") Boolean isError, @JsonProperty("structuredContent") Object structuredContent,
+				@JsonProperty("_meta") Map<String, Object> meta) {
+			if (content == null) {
+				logger.warn("CallToolResult: missing required fields during deserialization: content -> []");
+				content = List.of();
+			}
+			return new CallToolResult(content, isError, structuredContent, meta);
+		}
+
 		/**
 		 * Creates a builder for {@link CallToolResult}.
 		 * @return a new builder instance
@@ -1839,6 +1854,10 @@ public static ModelHint of(String name) {
 	 *
 	 * @param role The sender or recipient of messages and data in a conversation
 	 * @param content The content of the message
+	 * <p>
+	 * Note: {@code role} and {@code content} are required by the MCP specification.
+	 * Deserialization accepts missing values and substitutes defaults to avoid breaking
+	 * existing integrations that may omit these fields.
 	 */
 	@JsonInclude(JsonInclude.Include.NON_ABSENT)
 	@JsonIgnoreProperties(ignoreUnknown = true)
@@ -1850,6 +1869,24 @@ public record SamplingMessage( // @formatter:off
 			Assert.notNull(role, "role must not be null");
 			Assert.notNull(content, "content must not be null");
 		}
+
+		@JsonCreator
+		static SamplingMessage fromJson(@JsonProperty("role") Role role, @JsonProperty("content") Content content) {
+			if (role == null || content == null) {
+				List<String> missing = new ArrayList<>();
+				if (role == null) {
+					missing.add("role -> 'user'");
+					role = Role.USER;
+				}
+				if (content == null) {
+					missing.add("content -> ''");
+					content = new TextContent("");
+				}
+				logger.warn("SamplingMessage: missing required fields during deserialization: {}",
+						String.join(", ", missing));
+			}
+			return new SamplingMessage(role, content);
+		}
 	}
 
 	/**
@@ -1873,6 +1910,10 @@ public record SamplingMessage( // @formatter:off
 	 * @param metadata Optional metadata to pass through to the LLM provider. The format
 	 * of this metadata is provider-specific
 	 * @param meta See specification for notes on _meta usage
+	 * <p>
+	 * Note: {@code messages} and {@code maxTokens} are required by the MCP specification.
+	 * Deserialization accepts missing values and substitutes defaults to avoid breaking
+	 * existing integrations that may omit these fields.
 	 */
 	@JsonInclude(JsonInclude.Include.NON_ABSENT)
 	@JsonIgnoreProperties(ignoreUnknown = true)
@@ -1892,6 +1933,32 @@ public record CreateMessageRequest( // @formatter:off
 			Assert.notNull(maxTokens, "maxTokens must not be null");
 		}
 
+		@JsonCreator
+		static CreateMessageRequest fromJson(@JsonProperty("messages") List<SamplingMessage> messages,
+				@JsonProperty("modelPreferences") ModelPreferences modelPreferences,
+				@JsonProperty("systemPrompt") String systemPrompt,
+				@JsonProperty("includeContext") ContextInclusionStrategy includeContext,
+				@JsonProperty("temperature") Double temperature, @JsonProperty("maxTokens") Integer maxTokens,
+				@JsonProperty("stopSequences") List<String> stopSequences,
+				@JsonProperty("metadata") Map<String, Object> metadata,
+				@JsonProperty("_meta") Map<String, Object> meta) {
+			if (messages == null || maxTokens == null) {
+				List<String> missing = new ArrayList<>();
+				if (messages == null) {
+					missing.add("messages -> []");
+					messages = List.of();
+				}
+				if (maxTokens == null) {
+					missing.add("maxTokens -> 0");
+					maxTokens = 0;
+				}
+				logger.warn("CreateMessageRequest: missing required fields during deserialization: {}",
+						String.join(", ", missing));
+			}
+			return new CreateMessageRequest(messages, modelPreferences, systemPrompt, includeContext, temperature,
+					maxTokens, stopSequences, metadata, meta);
+		}
+
 		// backwards compatibility constructor
 		public CreateMessageRequest(List<SamplingMessage> messages, ModelPreferences modelPreferences,
 				String systemPrompt, ContextInclusionStrategy includeContext, Double temperature, Integer maxTokens,
@@ -2136,6 +2203,10 @@ public CreateMessageResult build() {
 	 * @param requestedSchema A restricted subset of JSON Schema. Only top-level
 	 * properties are allowed, without nesting
 	 * @param meta See specification for notes on _meta usage
+	 * <p>
+	 * Note: {@code message} and {@code requestedSchema} are required by the MCP
+	 * specification. Deserialization accepts missing values and substitutes defaults to
+	 * avoid breaking existing integrations that may omit these fields.
 	 */
 	@JsonInclude(JsonInclude.Include.NON_ABSENT)
 	@JsonIgnoreProperties(ignoreUnknown = true)
@@ -2149,6 +2220,26 @@ public record ElicitRequest( // @formatter:off
 			Assert.notNull(requestedSchema, "requestedSchema must not be null");
 		}
 
+		@JsonCreator
+		static ElicitRequest fromJson(@JsonProperty("message") String message,
+				@JsonProperty("requestedSchema") Map<String, Object> requestedSchema,
+				@JsonProperty("_meta") Map<String, Object> meta) {
+			if (message == null || requestedSchema == null) {
+				List<String> missing = new ArrayList<>();
+				if (message == null) {
+					missing.add("message -> ''");
+					message = "";
+				}
+				if (requestedSchema == null) {
+					missing.add("requestedSchema -> {}");
+					requestedSchema = Map.of();
+				}
+				logger.warn("ElicitRequest: missing required fields during deserialization: {}",
+						String.join(", ", missing));
+			}
+			return new ElicitRequest(message, requestedSchema, meta);
+		}
+
 		// backwards compatibility constructor
 		public ElicitRequest(String message, Map<String, Object> requestedSchema) {
 			this(message, requestedSchema, null);
@@ -2340,6 +2431,10 @@ public record PaginatedResult(@JsonProperty("nextCursor") String nextCursor) {
 	 * @param total An optional total amount of work to be done, if known.
 	 * @param message An optional message providing additional context about the progress.
 	 * @param meta See specification for notes on _meta usage
+	 * <p>
+	 * Note: {@code progressToken} and {@code progress} are required by the MCP
+	 * specification. Deserialization accepts missing values and substitutes defaults to
+	 * avoid breaking existing integrations that may omit these fields.
 	 */
 	@JsonInclude(JsonInclude.Include.NON_ABSENT)
 	@JsonIgnoreProperties(ignoreUnknown = true)
@@ -2355,6 +2450,26 @@ public record ProgressNotification( // @formatter:off
 			Assert.notNull(progress, "progress must not be null");
 		}
 
+		@JsonCreator
+		static ProgressNotification fromJson(@JsonProperty("progressToken") Object progressToken,
+				@JsonProperty("progress") Double progress, @JsonProperty("total") Double total,
+				@JsonProperty("message") String message, @JsonProperty("_meta") Map<String, Object> meta) {
+			if (progressToken == null || progress == null) {
+				List<String> missing = new ArrayList<>();
+				if (progressToken == null) {
+					missing.add("progressToken -> ''");
+					progressToken = "";
+				}
+				if (progress == null) {
+					missing.add("progress -> 0.0");
+					progress = 0.0;
+				}
+				logger.warn("ProgressNotification: missing required fields during deserialization: {}",
+						String.join(", ", missing));
+			}
+			return new ProgressNotification(progressToken, progress, total, message, meta);
+		}
+
 		public ProgressNotification(Object progressToken, double progress, Double total, String message) {
 			this(progressToken, progress, total, message, null);
 		}
@@ -2432,6 +2547,10 @@ public ResourcesUpdatedNotification(String uri) {
 	 * @param logger The logger that generated the message.
 	 * @param data JSON-serializable logging data.
 	 * @param meta See specification for notes on _meta usage
+	 * <p>
+	 * Note: {@code level} and {@code data} are required by the MCP specification.
+	 * Deserialization accepts missing values and substitutes defaults to avoid breaking
+	 * existing integrations that may omit these fields.
 	 */
 	@JsonInclude(JsonInclude.Include.NON_ABSENT)
 	@JsonIgnoreProperties(ignoreUnknown = true)
@@ -2446,6 +2565,26 @@ public record LoggingMessageNotification( // @formatter:off
 			Assert.notNull(data, "data must not be null");
 		}
 
+		@JsonCreator
+		static LoggingMessageNotification fromJson(@JsonProperty("level") LoggingLevel level,
+				@JsonProperty("logger") String loggerName, @JsonProperty("data") String data,
+				@JsonProperty("_meta") Map<String, Object> meta) {
+			if (level == null || data == null) {
+				List<String> missing = new ArrayList<>();
+				if (level == null) {
+					missing.add("level -> INFO");
+					level = LoggingLevel.INFO;
+				}
+				if (data == null) {
+					missing.add("data -> ''");
+					data = "";
+				}
+				McpSchema.logger.warn("LoggingMessageNotification: missing required fields during deserialization: {}",
+						String.join(", ", missing));
+			}
+			return new LoggingMessageNotification(level, loggerName, data, meta);
+		}
+
 		// backwards compatibility constructor
 		public LoggingMessageNotification(LoggingLevel level, String logger, String data) {
 			this(level, logger, data, null);

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

@@ -1346,6 +1346,16 @@ void testCallToolResultBuilderWithErrorResult() throws Exception {
 					{"content":[{"type":"text","text":"Error: Operation failed"}],"isError":true}"""));
 	}
 
+	@Test
+	void testCallToolResultDeserializationWithMissingContent() throws Exception {
+		McpSchema.CallToolResult result = JSON_MAPPER.readValue("""
+				{"isError":false}""", McpSchema.CallToolResult.class);
+
+		assertThat(result).isNotNull();
+		assertThat(result.content()).isEmpty();
+		assertThat(result.isError()).isFalse();
+	}
+
 	// Sampling Tests
 
 	@Test
@@ -1382,6 +1392,26 @@ void testCreateMessageRequest() throws Exception {
 							{"messages":[{"role":"user","content":{"type":"text","text":"User message"}}],"modelPreferences":{"hints":[{"name":"gpt-4"}],"costPriority":0.3,"speedPriority":0.7,"intelligencePriority":0.9},"systemPrompt":"You are a helpful assistant","includeContext":"thisServer","temperature":0.7,"maxTokens":1000,"stopSequences":["STOP","END"],"metadata":{"session":"test-session"}}"""));
 	}
 
+	@Test
+	void testSamplingMessageDeserializationWithMissingFields() throws Exception {
+		McpSchema.SamplingMessage message = JSON_MAPPER.readValue("{}", McpSchema.SamplingMessage.class);
+
+		assertThat(message).isNotNull();
+		assertThat(message.role()).isEqualTo(McpSchema.Role.USER);
+		assertThat(message.content()).isInstanceOf(McpSchema.TextContent.class);
+	}
+
+	@Test
+	void testCreateMessageRequestDeserializationWithMissingRequiredFields() throws Exception {
+		McpSchema.CreateMessageRequest request = JSON_MAPPER.readValue("""
+				{"systemPrompt":"hello"}""", McpSchema.CreateMessageRequest.class);
+
+		assertThat(request).isNotNull();
+		assertThat(request.messages()).isEmpty();
+		assertThat(request.maxTokens()).isZero();
+		assertThat(request.systemPrompt()).isEqualTo("hello");
+	}
+
 	@Test
 	void testCreateMessageResult() throws Exception {
 		McpSchema.TextContent content = new McpSchema.TextContent("Assistant response");
@@ -1455,6 +1485,15 @@ void testCreateElicitationResult() throws Exception {
 					{"action":"accept","content":{"foo":"bar"}}"""));
 	}
 
+	@Test
+	void testElicitRequestDeserializationWithMissingRequiredFields() throws Exception {
+		McpSchema.ElicitRequest request = JSON_MAPPER.readValue("{}", McpSchema.ElicitRequest.class);
+
+		assertThat(request).isNotNull();
+		assertThat(request.message()).isEmpty();
+		assertThat(request.requestedSchema()).isEmpty();
+	}
+
 	@Test
 	void testElicitRequestWithMeta() throws Exception {
 		Map<String, Object> requestedSchema = Map.of("type", "object", "required", List.of("name"), "properties",
@@ -1752,6 +1791,17 @@ void testProgressNotificationDeserialization() throws Exception {
 		assertThat(notification.meta()).containsEntry("key", "value");
 	}
 
+	@Test
+	void testProgressNotificationDeserializationWithMissingRequiredFields() throws Exception {
+		McpSchema.ProgressNotification notification = JSON_MAPPER.readValue("""
+				{"total":1.0}""", McpSchema.ProgressNotification.class);
+
+		assertThat(notification).isNotNull();
+		assertThat(notification.progressToken()).isEqualTo("");
+		assertThat(notification.progress()).isZero();
+		assertThat(notification.total()).isEqualTo(1.0);
+	}
+
 	@Test
 	void testProgressNotificationWithoutMessage() throws Exception {
 		McpSchema.ProgressNotification notification = new McpSchema.ProgressNotification("progress-token-789", 0.25,
@@ -1765,4 +1815,15 @@ void testProgressNotificationWithoutMessage() throws Exception {
 					{"progressToken":"progress-token-789","progress":0.25}"""));
 	}
 
+	@Test
+	void testLoggingMessageNotificationDeserializationWithMissingRequiredFields() throws Exception {
+		McpSchema.LoggingMessageNotification notification = JSON_MAPPER.readValue("""
+				{"logger":"my-logger"}""", McpSchema.LoggingMessageNotification.class);
+
+		assertThat(notification).isNotNull();
+		assertThat(notification.level()).isEqualTo(McpSchema.LoggingLevel.INFO);
+		assertThat(notification.logger()).isEqualTo("my-logger");
+		assertThat(notification.data()).isEmpty();
+	}
+
 }