fix: handle resource not found according to spec (#567)

see: https://modelcontextprotocol.io/specification/2025-06-18/server/resources#error-handling

Resource not found should send a JSON RPC response such as:

```json
{
  "jsonrpc": "2.0",
  "id": 5,
  "error": {
    "code": -32002,
    "message": "Resource not found",
    "data": {
      "uri": "file:///nonexistent.txt"
    }
  }
}
```

This PR also changes some instances where a `McpError` was thrown instead of being passed in the reactive chain with `Mono.error`

functional style

mcp-core/src/main/java/io/modelcontextprotocol/server/McpAsyncServer.java

@@ -48,6 +48,8 @@ import org.slf4j.LoggerFactory;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
 
+import static io.modelcontextprotocol.spec.McpError.RESOURCE_NOT_FOUND;
+
 /**
  * The Model Context Protocol (MCP) server implementation that provides asynchronous
  * communication using Project Reactor's Mono and Flux types.
@@ -638,24 +640,23 @@ public class McpAsyncServer {
 	}
 
 	private McpRequestHandler<McpSchema.ReadResourceResult> resourcesReadRequestHandler() {
-		return (exchange, params) -> {
+		return (ex, params) -> {
-			McpSchema.ReadResourceRequest resourceRequest = jsonMapper.convertValue(params,
+			McpSchema.ReadResourceRequest resourceRequest = jsonMapper.convertValue(params, new TypeRef<>() {
-					new TypeRef<McpSchema.ReadResourceRequest>() {
+			});
-					});
 			var resourceUri = resourceRequest.uri();
-
+			return asyncResourceSpecification(resourceUri)
-			McpServerFeatures.AsyncResourceSpecification specification = this.resources.values()
+				.map(spec -> Mono.defer(() -> spec.readHandler().apply(ex, resourceRequest)))
-				.stream()
+				.orElseGet(() -> Mono.error(RESOURCE_NOT_FOUND.apply(resourceUri)));
-				.filter(resourceSpecification -> this.uriTemplateManagerFactory
-					.create(resourceSpecification.resource().uri())
-					.matches(resourceUri))
-				.findFirst()
-				.orElseThrow(() -> new McpError("Resource not found: " + resourceUri));
-
-			return Mono.defer(() -> specification.readHandler().apply(exchange, resourceRequest));
 		};
 	}
 
+	private Optional<McpServerFeatures.AsyncResourceSpecification> asyncResourceSpecification(String uri) {
+		return resources.values()
+			.stream()
+			.filter(spec -> uriTemplateManagerFactory.create(spec.resource().uri()).matches(uri))
+			.findFirst();
+	}
+
 	// ---------------------------------------
 	// Prompt Management
 	// ---------------------------------------
@@ -846,7 +847,7 @@ public class McpAsyncServer {
 			if (type.equals("ref/resource") && request.ref() instanceof McpSchema.ResourceReference resourceReference) {
 				McpServerFeatures.AsyncResourceSpecification resourceSpec = this.resources.get(resourceReference.uri());
 				if (resourceSpec == null) {
-					return Mono.error(new McpError("Resource not found: " + resourceReference.uri()));
+					return Mono.error(RESOURCE_NOT_FOUND.apply(resourceReference.uri()));
 				}
 				if (!uriTemplateManagerFactory.create(resourceSpec.resource().uri())
 					.getVariableNames()

mcp-core/src/main/java/io/modelcontextprotocol/server/McpStatelessAsyncServer.java

@@ -33,6 +33,8 @@ import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.CopyOnWriteArrayList;
 import java.util.function.BiFunction;
 
+import static io.modelcontextprotocol.spec.McpError.RESOURCE_NOT_FOUND;
+
 /**
  * A stateless MCP server implementation for use with Streamable HTTP transport types. It
  * allows simple horizontal scalability since it does not maintain a session and does not
@@ -478,23 +480,21 @@ public class McpStatelessAsyncServer {
 
 	private McpStatelessRequestHandler<McpSchema.ReadResourceResult> resourcesReadRequestHandler() {
 		return (ctx, params) -> {
-			McpSchema.ReadResourceRequest resourceRequest = jsonMapper.convertValue(params,
+			McpSchema.ReadResourceRequest resourceRequest = jsonMapper.convertValue(params, new TypeRef<>() {
-					new TypeRef<McpSchema.ReadResourceRequest>() {
+			});
-					});
 			var resourceUri = resourceRequest.uri();
-
+			return asyncResourceSpecification(resourceUri).map(spec -> spec.readHandler().apply(ctx, resourceRequest))
-			McpStatelessServerFeatures.AsyncResourceSpecification specification = this.resources.values()
+				.orElseGet(() -> Mono.error(RESOURCE_NOT_FOUND.apply(resourceUri)));
-				.stream()
-				.filter(resourceSpecification -> this.uriTemplateManagerFactory
-					.create(resourceSpecification.resource().uri())
-					.matches(resourceUri))
-				.findFirst()
-				.orElseThrow(() -> new McpError("Resource not found: " + resourceUri));
-
-			return specification.readHandler().apply(ctx, resourceRequest);
 		};
 	}
 
+	private Optional<McpStatelessServerFeatures.AsyncResourceSpecification> asyncResourceSpecification(String uri) {
+		return resources.values()
+			.stream()
+			.filter(spec -> uriTemplateManagerFactory.create(spec.resource().uri()).matches(uri))
+			.findFirst();
+	}
+
 	// ---------------------------------------
 	// Prompt Management
 	// ---------------------------------------
@@ -612,10 +612,10 @@ public class McpStatelessAsyncServer {
 			}
 
 			if (type.equals("ref/resource") && request.ref() instanceof McpSchema.ResourceReference resourceReference) {
-				McpStatelessServerFeatures.AsyncResourceSpecification resourceSpec = this.resources
+				McpStatelessServerFeatures.AsyncResourceSpecification resourceSpec = resources
 					.get(resourceReference.uri());
 				if (resourceSpec == null) {
-					return Mono.error(new McpError("Resource not found: " + resourceReference.uri()));
+					return Mono.error(RESOURCE_NOT_FOUND.apply(resourceReference.uri()));
 				}
 				if (!uriTemplateManagerFactory.create(resourceSpec.resource().uri())
 					.getVariableNames()

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

@@ -7,8 +7,19 @@ package io.modelcontextprotocol.spec;
 import io.modelcontextprotocol.spec.McpSchema.JSONRPCResponse.JSONRPCError;
 import io.modelcontextprotocol.util.Assert;
 
+import java.util.Map;
+import java.util.function.Function;
+
 public class McpError extends RuntimeException {
 
+	/**
+	 * <a href=
+	 * "https://modelcontextprotocol.io/specification/2025-06-18/server/resources#error-handling">Resource
+	 * Error Handling</a>
+	 */
+	public static final Function<String, McpError> RESOURCE_NOT_FOUND = resourceUri -> new McpError(new JSONRPCError(
+			McpSchema.ErrorCodes.RESOURCE_NOT_FOUND, "Resource not found", Map.of("uri", resourceUri)));
+
 	private JSONRPCError jsonRpcError;
 
 	public McpError(JSONRPCError jsonRpcError) {

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

@@ -143,6 +143,11 @@ public final class McpSchema {
 		 */
 		public static final int INTERNAL_ERROR = -32603;
 
+		/**
+		 * Resource not found.
+		 */
+		public static final int RESOURCE_NOT_FOUND = -32002;
+
 	}
 
 	public sealed interface Request

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

@@ -0,0 +1,21 @@
+package io.modelcontextprotocol.spec;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.Map;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class McpErrorTest {
+
+	@Test
+	void testNotFound() {
+		String uri = "file:///nonexistent.txt";
+		McpError mcpError = McpError.RESOURCE_NOT_FOUND.apply(uri);
+		assertNotNull(mcpError.getJsonRpcError());
+		assertEquals(-32002, mcpError.getJsonRpcError().code());
+		assertEquals("Resource not found", mcpError.getJsonRpcError().message());
+		assertEquals(Map.of("uri", uri), mcpError.getJsonRpcError().data());
+	}
+
+}