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Prevent server errors on calls to unprovided capabilities #598

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Prevent server errors if clients call the mcp server with unprovided … 
…capabilities
  • Loading branch information
@sorin-florea
sorin-florea committed Oct 3, 2025
commit 20d52f81dad39d060dd1d52220e559dc91bca743
56 changes: 49 additions & 7 deletions mcp-core/src/main/java/io/modelcontextprotocol/server/DefaultMcpStatelessServerHandler.java
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Original file line number Diff line number Diff line change
Expand Up @@ -7,6 +7,8 @@
import io.modelcontextprotocol.common.McpTransportContext;
import io.modelcontextprotocol.spec.McpError;
import io.modelcontextprotocol.spec.McpSchema;
import io.modelcontextprotocol.spec.McpSchema.JSONRPCResponse;
import io.modelcontextprotocol.spec.McpSchema.JSONRPCResponse.JSONRPCError;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import reactor.core.publisher.Mono;
Expand All @@ -21,29 +23,41 @@ class DefaultMcpStatelessServerHandler implements McpStatelessServerHandler {

Map<String, McpStatelessNotificationHandler> notificationHandlers;

private final McpSchema.ServerCapabilities serverCapabilities;

public DefaultMcpStatelessServerHandler(Map<String, McpStatelessRequestHandler<?>> requestHandlers,
Map<String, McpStatelessNotificationHandler> notificationHandlers) {
Map<String, McpStatelessNotificationHandler> notificationHandlers,
McpSchema.ServerCapabilities serverCapabilities) {
this.requestHandlers = requestHandlers;
this.notificationHandlers = notificationHandlers;
this.serverCapabilities = serverCapabilities;
}

@Override
public Mono<McpSchema.JSONRPCResponse> handleRequest(McpTransportContext transportContext,
McpSchema.JSONRPCRequest request) {
public Mono<JSONRPCResponse> handleRequest(McpTransportContext transportContext, McpSchema.JSONRPCRequest request) {
McpStatelessRequestHandler<?> requestHandler = this.requestHandlers.get(request.method());
if (requestHandler == null) {
return Mono.error(new McpError("Missing handler for request type: " + request.method()));
// Capability is not declared, but the client is trying to call the method –
// this is an invalid request.
if (!isCapabilityDeclaredForMethod(request.method())) {
JSONRPCError error = new JSONRPCError(McpSchema.ErrorCodes.METHOD_NOT_FOUND,
"Server does not provide " + request.method() + " capability", null);
return Mono.just(new JSONRPCResponse(McpSchema.JSONRPC_VERSION, request.id(), null, error));
}
// Capability is declared, but we failed to register a handler – this is a
// server error.
return Mono.error(new McpError(new JSONRPCError(McpSchema.ErrorCodes.INTERNAL_ERROR,
"Missing handler for request type: " + request.method(), null)));
}
return requestHandler.handle(transportContext, request.params())
.map(result -> new McpSchema.JSONRPCResponse(McpSchema.JSONRPC_VERSION, request.id(), result, null))
.onErrorResume(t -> {
McpSchema.JSONRPCResponse.JSONRPCError error;
JSONRPCError error;
if (t instanceof McpError mcpError && mcpError.getJsonRpcError() != null) {
error = mcpError.getJsonRpcError();
}
else {
error = new McpSchema.JSONRPCResponse.JSONRPCError(McpSchema.ErrorCodes.INTERNAL_ERROR,
t.getMessage(), null);
error = new JSONRPCError(McpSchema.ErrorCodes.INTERNAL_ERROR, t.getMessage(), null);
}
return Mono.just(new McpSchema.JSONRPCResponse(McpSchema.JSONRPC_VERSION, request.id(), null, error));
});
Expand All @@ -60,4 +74,32 @@ public Mono<Void> handleNotification(McpTransportContext transportContext,
return notificationHandler.handle(transportContext, notification.params());
}

private boolean isCapabilityDeclaredForMethod(String method) {
if (this.serverCapabilities == null) {
return false;
}

// Ping is always supported
if (McpSchema.METHOD_PING.equals(method) || McpSchema.METHOD_INITIALIZE.equals(method))
return true;

if (McpSchema.METHOD_TOOLS_LIST.equals(method) || McpSchema.METHOD_TOOLS_CALL.equals(method)) {
return this.serverCapabilities.tools() != null;
}
if (McpSchema.METHOD_RESOURCES_LIST.equals(method) || McpSchema.METHOD_RESOURCES_READ.equals(method)
|| McpSchema.METHOD_RESOURCES_TEMPLATES_LIST.equals(method)) {
return this.serverCapabilities.resources() != null;
}
if (McpSchema.METHOD_PROMPT_LIST.equals(method) || McpSchema.METHOD_PROMPT_GET.equals(method)) {
return this.serverCapabilities.prompts() != null;
}
if (McpSchema.METHOD_LOGGING_SET_LEVEL.equals(method)) {
return this.serverCapabilities.logging() != null;
}
if (McpSchema.METHOD_COMPLETION_COMPLETE.equals(method)) {
return this.serverCapabilities.completions() != null;
}
return false;
}

}
4 changes: 2 additions & 2 deletions mcp-core/src/main/java/io/modelcontextprotocol/server/McpStatelessAsyncServer.java
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Original file line number Diff line number Diff line change
Expand Up @@ -14,7 +14,6 @@
import io.modelcontextprotocol.spec.McpSchema.CallToolResult;
import io.modelcontextprotocol.spec.McpSchema.CompleteResult.CompleteCompletion;
import io.modelcontextprotocol.spec.McpSchema.ErrorCodes;
import io.modelcontextprotocol.spec.McpSchema.JSONRPCResponse;
import io.modelcontextprotocol.spec.McpSchema.PromptReference;
import io.modelcontextprotocol.spec.McpSchema.ResourceReference;
import io.modelcontextprotocol.spec.McpSchema.Tool;
Expand Down Expand Up @@ -129,7 +128,8 @@ public class McpStatelessAsyncServer {

this.protocolVersions = new ArrayList<>(mcpTransport.protocolVersions());

McpStatelessServerHandler handler = new DefaultMcpStatelessServerHandler(requestHandlers, Map.of());
McpStatelessServerHandler handler = new DefaultMcpStatelessServerHandler(requestHandlers, Map.of(),
this.serverCapabilities);
mcpTransport.setMcpHandler(handler);
}

Expand Down
24 changes: 23 additions & 1 deletion ...re/src/test/java/io/modelcontextprotocol/server/HttpServletStatelessIntegrationTests.java
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Original file line number Diff line number Diff line change
Expand Up @@ -17,6 +17,7 @@
import io.modelcontextprotocol.server.transport.HttpServletStatelessServerTransport;
import io.modelcontextprotocol.server.transport.TomcatTestUtil;
import io.modelcontextprotocol.spec.HttpHeaders;
import io.modelcontextprotocol.spec.McpError;
import io.modelcontextprotocol.spec.McpSchema;
import io.modelcontextprotocol.spec.McpSchema.CallToolResult;
import io.modelcontextprotocol.spec.McpSchema.CompleteRequest;
Expand Down Expand Up @@ -53,6 +54,8 @@
import static net.javacrumbs.jsonunit.assertj.JsonAssertions.json;
import static org.assertj.core.api.Assertions.assertThat;
import static org.awaitility.Awaitility.await;
import static org.junit.jupiter.api.Assertions.assertThrows;
import static org.junit.jupiter.api.Assertions.assertTrue;

@Timeout(15)
class HttpServletStatelessIntegrationTests {
Expand Down Expand Up @@ -305,7 +308,7 @@ void testStructuredOutputOfObjectArrayValidationSuccess(String clientType) {
"type", "object",
"properties", Map.of(
"name", Map.of("type", "string"),
"age", Map.of("type", "number")),
"age", Map.of("type", "number")),
"required", List.of("name", "age"))); // @formatter:on

Tool calculatorTool = Tool.builder()
Expand Down Expand Up @@ -636,6 +639,25 @@ void testThrownMcpErrorAndJsonRpcError() throws Exception {
mcpServer.close();
}

@ParameterizedTest(name = "{0} : {displayName} ")
@ValueSource(strings = { "httpclient" })
void testClientAttemptsToCallUnsupportedCapabilityJsonRpcError(String clientType) {
var mcpServer = McpServer.sync(mcpStatelessServerTransport)
.serverInfo("test-server", "1.0.0")
.capabilities(ServerCapabilities.builder().build())
.build();

var clientBuilder = clientBuilders.get(clientType);
try (var mcpClient = clientBuilder.build()) {
McpSchema.JSONRPCResponse.JSONRPCError promptsError = assertThrows(McpError.class, mcpClient::listPrompts)
.getJsonRpcError();
assertThat(promptsError.code()).isEqualTo(ErrorCodes.METHOD_NOT_FOUND);
}
finally {
mcpServer.close();
}
}

private double evaluateExpression(String expression) {
// Simple expression evaluator for testing
return switch (expression) {
Expand Down