fix: answer completion/complete without ref or argument with -32602

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

@@ -1028,8 +1028,18 @@ private McpRequestHandler<Object> setLoggerRequestHandler() {
 	private McpRequestHandler<McpSchema.CompleteResult> completionCompleteRequestHandler() {
 		return (exchange, params) -> {
 
-			McpSchema.CompleteRequest request = jsonMapper.convertValue(params, new TypeRef<>() {
-			});
+			McpSchema.CompleteRequest request;
+			try {
+				request = jsonMapper.convertValue(params, new TypeRef<>() {
+				});
+			}
+			catch (IllegalArgumentException e) {
+				// ref and argument are required and have no default to fall back on
+				logger.debug("Malformed completion request", e);
+				return Mono.error(McpError.builder(ErrorCodes.INVALID_PARAMS)
+					.message("Invalid completion request: ref and argument are required")
+					.build());
+			}
 
 			if (request.ref() == null) {
 				return Mono.error(

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

@@ -764,8 +764,18 @@ private McpStatelessRequestHandler<McpSchema.GetPromptResult> promptsGetRequestH
 
 	private McpStatelessRequestHandler<McpSchema.CompleteResult> completionCompleteRequestHandler() {
 		return (ctx, params) -> {
-			McpSchema.CompleteRequest request = jsonMapper.convertValue(params, new TypeRef<>() {
-			});
+			McpSchema.CompleteRequest request;
+			try {
+				request = jsonMapper.convertValue(params, new TypeRef<>() {
+				});
+			}
+			catch (IllegalArgumentException e) {
+				// ref and argument are required and have no default to fall back on
+				logger.debug("Malformed completion request", e);
+				return Mono.error(McpError.builder(ErrorCodes.INVALID_PARAMS)
+					.message("Invalid completion request: ref and argument are required")
+					.build());
+			}
 
 			if (request.ref() == null) {
 				return Mono.error(

mcp-test/src/test/java/io/modelcontextprotocol/server/CompletionRequestValidationTests.java

@@ -0,0 +1,94 @@
+/*
+ * Copyright 2026-2026 the original author or authors.
+ */
+
+package io.modelcontextprotocol.server;
+
+import java.time.Duration;
+import java.util.Map;
+import java.util.concurrent.atomic.AtomicReference;
+
+import io.modelcontextprotocol.MockMcpServerTransport;
+import io.modelcontextprotocol.MockMcpServerTransportProvider;
+import io.modelcontextprotocol.common.McpTransportContext;
+import io.modelcontextprotocol.spec.McpSchema;
+import io.modelcontextprotocol.spec.McpSchema.ErrorCodes;
+import io.modelcontextprotocol.spec.McpSchema.ServerCapabilities;
+import io.modelcontextprotocol.spec.McpStatelessServerTransport;
+import io.modelcontextprotocol.spec.ProtocolVersions;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.MethodSource;
+import reactor.core.publisher.Mono;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * A {@code completion/complete} request without its spec-required {@code ref} or
+ * {@code argument} is answered with Invalid params.
+ */
+class CompletionRequestValidationTests {
+
+	static Map<String, Object>[] incompleteParams() {
+		@SuppressWarnings("unchecked")
+		Map<String, Object>[] params = new Map[] { Map.of("argument", Map.of("name", "a", "value", "b")),
+				Map.of("ref", Map.of("type", "ref/prompt", "name", "p")) };
+		return params;
+	}
+
+	@ParameterizedTest
+	@MethodSource("incompleteParams")
+	void incompleteRequestIsInvalidParams(Map<String, Object> params) {
+		MockMcpServerTransport transport = new MockMcpServerTransport();
+		MockMcpServerTransportProvider transportProvider = new MockMcpServerTransportProvider(transport);
+		McpAsyncServer server = McpServer.async(transportProvider)
+			.serverInfo("test-server", "1.0.0")
+			.capabilities(ServerCapabilities.builder().completions().build())
+			.build();
+		transportProvider.simulateIncomingMessage(new McpSchema.JSONRPCRequest(McpSchema.METHOD_INITIALIZE, "init",
+				McpSchema.InitializeRequest
+					.builder(ProtocolVersions.MCP_2025_11_25, McpSchema.ClientCapabilities.builder().build(),
+							McpSchema.Implementation.builder("test-client", "1.0.0").build())
+					.build()));
+		transportProvider
+			.simulateIncomingMessage(new McpSchema.JSONRPCNotification(McpSchema.METHOD_NOTIFICATION_INITIALIZED));
+
+		transportProvider.simulateIncomingMessage(
+				new McpSchema.JSONRPCRequest(McpSchema.METHOD_COMPLETION_COMPLETE, "complete", params));
+
+		McpSchema.JSONRPCResponse response = (McpSchema.JSONRPCResponse) transport.getLastSentMessage();
+		assertThat(response.error()).isNotNull();
+		assertThat(response.error().code()).isEqualTo(ErrorCodes.INVALID_PARAMS);
+		server.closeGracefully().block(Duration.ofSeconds(10));
+	}
+
+	@ParameterizedTest
+	@MethodSource("incompleteParams")
+	void incompleteStatelessRequestIsInvalidParams(Map<String, Object> params) {
+		AtomicReference<McpStatelessServerHandler> handler = new AtomicReference<>();
+		McpStatelessServerTransport transport = new McpStatelessServerTransport() {
+			@Override
+			public void setMcpHandler(McpStatelessServerHandler mcpHandler) {
+				handler.set(mcpHandler);
+			}
+
+			@Override
+			public Mono<Void> closeGracefully() {
+				return Mono.empty();
+			}
+		};
+		McpStatelessAsyncServer server = McpServer.async(transport)
+			.serverInfo("test-server", "1.0.0")
+			.capabilities(ServerCapabilities.builder().completions().build())
+			.build();
+
+		McpSchema.JSONRPCResponse response = handler.get()
+			.handleRequest(McpTransportContext.EMPTY,
+					new McpSchema.JSONRPCRequest(McpSchema.METHOD_COMPLETION_COMPLETE, "complete", params))
+			.block(Duration.ofSeconds(10));
+
+		assertThat(response.error()).isNotNull();
+		assertThat(response.error().code()).isEqualTo(ErrorCodes.INVALID_PARAMS);
+		server.closeGracefully().block(Duration.ofSeconds(10));
+	}
+
+}