feat(mcp): add support for custom MIME types in tool responses

auto-configurations/spring-ai-mcp-server/src/main/java/org/springframework/ai/autoconfigure/mcp/server/McpServerAutoConfiguration.java

@@ -54,6 +54,7 @@
 import org.springframework.context.annotation.Bean;
 import org.springframework.core.log.LogAccessor;
 import org.springframework.util.CollectionUtils;
+import org.springframework.util.MimeType;
 
 /**
  * {@link EnableAutoConfiguration Auto-configuration} for the Model Context Protocol (MCP)
@@ -127,9 +128,21 @@ public McpSchema.ServerCapabilities.Builder capabilitiesBuilder() {
 	@Bean
 	@ConditionalOnProperty(prefix = McpServerProperties.CONFIG_PREFIX, name = "type", havingValue = "SYNC",
 			matchIfMissing = true)
-	public List<McpServerFeatures.SyncToolRegistration> syncTools(ObjectProvider<List<ToolCallback>> toolCalls) {
-		var tools = toolCalls.stream().flatMap(List::stream).toList();
-		return McpToolUtils.toSyncToolRegistration(tools);
+	public List<McpServerFeatures.SyncToolRegistration> syncTools(ObjectProvider<List<ToolCallback>> toolCalls,
+			McpServerProperties serverProperties) {
+		List<ToolCallback> tools = toolCalls.stream().flatMap(List::stream).toList();
+
+		return this.toSyncToolRegistration(tools, serverProperties);
+	}
+
+	private List<McpServerFeatures.SyncToolRegistration> toSyncToolRegistration(List<ToolCallback> tools,
+			McpServerProperties serverProperties) {
+		return tools.stream().map(tool -> {
+			String toolName = tool.getToolDefinition().name();
+			MimeType mimeType = (serverProperties.getToolResponseMimeType().containsKey(toolName))
+					? MimeType.valueOf(serverProperties.getToolResponseMimeType().get(toolName)) : null;
+			return McpToolUtils.toSyncToolRegistration(tool, mimeType);
+		}).toList();
 	}
 
 	@Bean
@@ -149,13 +162,16 @@ public McpSyncServer mcpSyncServer(ServerMcpTransport transport,
 		SyncSpec serverBuilder = McpServer.sync(transport).serverInfo(serverInfo);
 
 		List<SyncToolRegistration> toolResgistrations = new ArrayList<>(tools.stream().flatMap(List::stream).toList());
+
 		List<ToolCallback> providerToolCallbacks = toolCallbackProvider.stream()
 			.map(pr -> List.of(pr.getToolCallbacks()))
 			.flatMap(List::stream)
 			.filter(fc -> fc instanceof ToolCallback)
 			.map(fc -> (ToolCallback) fc)
 			.toList();
-		toolResgistrations.addAll(McpToolUtils.toSyncToolRegistration(providerToolCallbacks));
+
+		toolResgistrations.addAll(this.toSyncToolRegistration(providerToolCallbacks, serverProperties));
+
 		if (!CollectionUtils.isEmpty(toolResgistrations)) {
 			serverBuilder.tools(toolResgistrations);
 			capabilitiesBuilder.tools(serverProperties.isToolChangeNotification());
@@ -191,9 +207,21 @@ public McpSyncServer mcpSyncServer(ServerMcpTransport transport,
 
 	@Bean
 	@ConditionalOnProperty(prefix = McpServerProperties.CONFIG_PREFIX, name = "type", havingValue = "ASYNC")
-	public List<McpServerFeatures.AsyncToolRegistration> asyncTools(ObjectProvider<List<ToolCallback>> toolCalls) {
+	public List<McpServerFeatures.AsyncToolRegistration> asyncTools(ObjectProvider<List<ToolCallback>> toolCalls,
+			McpServerProperties serverProperties) {
 		var tools = toolCalls.stream().flatMap(List::stream).toList();
-		return McpToolUtils.toAsyncToolRegistration(tools);
+
+		return this.toAsyncToolRegistration(tools, serverProperties);
+	}
+
+	private List<McpServerFeatures.AsyncToolRegistration> toAsyncToolRegistration(List<ToolCallback> tools,
+			McpServerProperties serverProperties) {
+		return tools.stream().map(tool -> {
+			String toolName = tool.getToolDefinition().name();
+			MimeType mimeType = (serverProperties.getToolResponseMimeType().containsKey(toolName))
+					? MimeType.valueOf(serverProperties.getToolResponseMimeType().get(toolName)) : null;
+			return McpToolUtils.toAsyncToolRegistration(tool, mimeType);
+		}).toList();
 	}
 
 	@Bean
@@ -219,7 +247,9 @@ public McpAsyncServer mcpAsyncServer(ServerMcpTransport transport,
 			.filter(fc -> fc instanceof ToolCallback)
 			.map(fc -> (ToolCallback) fc)
 			.toList();
-		toolResgistrations.addAll(McpToolUtils.toAsyncToolRegistration(providerToolCallbacks));
+
+		toolResgistrations.addAll(this.toAsyncToolRegistration(providerToolCallbacks, serverProperties));
+
 		if (!CollectionUtils.isEmpty(toolResgistrations)) {
 			serverBilder.tools(toolResgistrations);
 			capabilitiesBuilder.tools(serverProperties.isToolChangeNotification());

auto-configurations/spring-ai-mcp-server/src/main/java/org/springframework/ai/autoconfigure/mcp/server/McpServerProperties.java

@@ -16,6 +16,9 @@
 
 package org.springframework.ai.autoconfigure.mcp.server;
 
+import java.util.HashMap;
+import java.util.Map;
+
 import org.springframework.boot.context.properties.ConfigurationProperties;
 import org.springframework.util.Assert;
 
@@ -130,6 +133,11 @@ public enum ServerType {
 
 	}
 
+	/**
+	 * (Optinal) response MIME type per tool name.
+	 */
+	private Map<String, String> toolResponseMimeType = new HashMap<>();
+
 	public boolean isStdio() {
 		return this.stdio;
 	}
@@ -206,4 +214,8 @@ public void setType(ServerType serverType) {
 		this.type = serverType;
 	}
 
+	public Map<String, String> getToolResponseMimeType() {
+		return this.toolResponseMimeType;
+	}
+
 }

mcp/common/src/main/java/org/springframework/ai/mcp/McpToolUtils.java

@@ -22,12 +22,14 @@
 import io.modelcontextprotocol.server.McpServerFeatures;
 import io.modelcontextprotocol.server.McpServerFeatures.AsyncToolRegistration;
 import io.modelcontextprotocol.spec.McpSchema;
+import io.modelcontextprotocol.spec.McpSchema.Role;
 import reactor.core.publisher.Mono;
 import reactor.core.scheduler.Schedulers;
 
 import org.springframework.ai.model.ModelOptionsUtils;
 import org.springframework.ai.tool.ToolCallback;
 import org.springframework.util.CollectionUtils;
+import org.springframework.util.MimeType;
 
 /**
  * Utility class that provides helper methods for working with Model Context Protocol
@@ -105,12 +107,44 @@ public static List<McpServerFeatures.SyncToolRegistration> toSyncToolRegistratio
 	 * @throws RuntimeException if there's an error during the function execution
 	 */
 	public static McpServerFeatures.SyncToolRegistration toSyncToolRegistration(ToolCallback toolCallback) {
+		return toSyncToolRegistration(toolCallback, null);
+	}
+
+	/**
+	 * Converts a Spring AI FunctionCallback to an MCP SyncToolRegistration. This enables
+	 * Spring AI functions to be exposed as MCP tools that can be discovered and invoked
+	 * by language models.
+	 *
+	 * <p>
+	 * The conversion process:
+	 * <ul>
+	 * <li>Creates an MCP Tool with the function's name and input schema</li>
+	 * <li>Wraps the function's execution in a SyncToolRegistration that handles the MCP
+	 * protocol</li>
+	 * <li>Provides error handling and result formatting according to MCP
+	 * specifications</li>
+	 * </ul>
+	 *
+	 * You can use the FunctionCallback builder to create a new instance of
+	 * FunctionCallback using either java.util.function.Function or Method reference.
+	 * @param toolCallback the Spring AI function callback to convert
+	 * @param mimeType the MIME type of the output content
+	 * @return an MCP SyncToolRegistration that wraps the function callback
+	 * @throws RuntimeException if there's an error during the function execution
+	 */
+	public static McpServerFeatures.SyncToolRegistration toSyncToolRegistration(ToolCallback toolCallback,
+			MimeType mimeType) {
+
 		var tool = new McpSchema.Tool(toolCallback.getToolDefinition().name(),
 				toolCallback.getToolDefinition().description(), toolCallback.getToolDefinition().inputSchema());
 
 		return new McpServerFeatures.SyncToolRegistration(tool, request -> {
 			try {
 				String callResult = toolCallback.call(ModelOptionsUtils.toJsonString(request));
+				if (mimeType != null && mimeType.toString().startsWith("image")) {
+					return new McpSchema.CallToolResult(List.of(new McpSchema.ImageContent(List.of(Role.ASSISTANT),
+							null, "image", callResult, mimeType.toString())), false);
+				}
 				return new McpSchema.CallToolResult(List.of(new McpSchema.TextContent(callResult)), false);
 			}
 			catch (Exception e) {
@@ -174,8 +208,39 @@ public static List<McpServerFeatures.AsyncToolRegistration> toAsyncToolRegistrat
 	 * @see Schedulers#boundedElastic()
 	 */
 	public static McpServerFeatures.AsyncToolRegistration toAsyncToolRegistration(ToolCallback toolCallback) {
+		return toAsyncToolRegistration(toolCallback, null);
+	}
+
+	/**
+	 * Converts a Spring AI tool callback to an MCP asynchronous tool registration.
+	 * <p>
+	 * This method enables Spring AI tools to be exposed as asynchronous MCP tools that
+	 * can be discovered and invoked by language models. The conversion process:
+	 * <ul>
+	 * <li>First converts the callback to a synchronous registration</li>
+	 * <li>Wraps the synchronous execution in a reactive Mono</li>
+	 * <li>Configures execution on a bounded elastic scheduler for non-blocking
+	 * operation</li>
+	 * </ul>
+	 * <p>
+	 * The resulting async registration will:
+	 * <ul>
+	 * <li>Execute the tool without blocking the calling thread</li>
+	 * <li>Handle errors and results asynchronously</li>
+	 * <li>Provide backpressure through Project Reactor</li>
+	 * </ul>
+	 * @param toolCallback the Spring AI tool callback to convert
+	 * @param mimeType the MIME type of the output content
+	 * @return an MCP asynchronous tool registration that wraps the tool callback
+	 * @see McpServerFeatures.AsyncToolRegistration
+	 * @see Mono
+	 * @see Schedulers#boundedElastic()
+	 */
+	public static McpServerFeatures.AsyncToolRegistration toAsyncToolRegistration(ToolCallback toolCallback,
+			MimeType mimeType) {
+
+		McpServerFeatures.SyncToolRegistration syncToolRegistration = toSyncToolRegistration(toolCallback, mimeType);
 
-		McpServerFeatures.SyncToolRegistration syncToolRegistration = toSyncToolRegistration(toolCallback);
 		return new AsyncToolRegistration(syncToolRegistration.tool(),
 				map -> Mono.fromCallable(() -> syncToolRegistration.call().apply(map))
 					.subscribeOn(Schedulers.boundedElastic()));

spring-ai-docs/src/main/antora/modules/ROOT/pages/api/mcp/mcp-server-boot-starter-docs.adoc

@@ -87,8 +87,9 @@ All properties are prefixed with `spring.ai.mcp.server`:
 |`version` |Server version |`1.0.0`
 |`type` |Server type (SYNC/ASYNC) |`SYNC`
 |`resource-change-notification` |Enable resource change notifications |`true`
-|`tool-change-notification` |Enable tool change notifications |`true`
 |`prompt-change-notification` |Enable prompt change notifications |`true`
+|`tool-change-notification` |Enable tool change notifications |`true`
+|`tool-response-mime-type` |(optinal) response MIME type per tool name. For example `spring.ai.mcp.server.tool-response-mime-type.generateImage=image/png` will assosiate the `image/png` mime type with the `generateImage()` tool name |`-`
 |`sse-message-endpoint` |SSE endpoint path for web transport |`/mcp/message`
 |===