modelcontextprotocol/java-sdk
Watch
1
0
Fork
You've already forked java-sdk
 0
Code Issues Pull requests Projects Releases Packages Wiki Activity Actions

feat: Add protocol version negotiation

Browse source 
Implement protocol version negotiation between MCP client and server to ensure
compatibility and graceful version handling.
The server will now suggest the latest supported version when an
unsupported version is requested, while the client will verify
version compatibility during initialization.

- Support multiple protocol versions in both client and server
- Negotiate compatible version during initialization
- Fall back to latest version when unsupported version requested
- Add comprehensive test coverage for version negotiation

Resolves #71

Signed-off-by: Christian Tzolov <christian.tzolov@broadcom.com>
This commit is contained in:
Christian Tzolov 2025-01-24 12:59:30 +01:00 • committed by Christian Tzolov
commit a424b36653
8 changed files with 370 additions and 13 deletions
Download patch file Download diff file Expand all files Collapse all files

23
mcp/src/main/java/org/springframework/ai/mcp/client/McpAsyncClient.java
View file

@ -146,6 +146,11 @@ public class McpAsyncClient {
*/
private final McpTransport transport;
/**
* Supported protocol versions.
*/
private List<String> protocolVersions = List.of(McpSchema.LATEST_PROTOCOL_VERSION);
/**
* Create a new McpAsyncClient with the given transport and session request-response
* timeout.
@ -257,6 +262,8 @@ public class McpAsyncClient {
Assert.notNull(requestTimeout, "Request timeout must not be null");
Assert.notNull(clientInfo, "Client info must not be null");
this.protocolVersions = List.of(McpSchema.LATEST_PROTOCOL_VERSION);
this.clientInfo = clientInfo;
this.clientCapabilities = (clientCapabilities != null) ? clientCapabilities
@ -360,8 +367,11 @@ public class McpAsyncClient {
* @return the initialize result.
*/
public Mono<McpSchema.InitializeResult> initialize() {
String latestVersion = this.protocolVersions.get(this.protocolVersions.size() - 1);
McpSchema.InitializeRequest initializeRequest = new McpSchema.InitializeRequest(// @formatter:off
McpSchema.LATEST_PROTOCOL_VERSION,
latestVersion,
this.clientCapabilities,
this.clientInfo); // @formatter:on
@ -378,7 +388,7 @@ public class McpAsyncClient {
initializeResult.protocolVersion(), initializeResult.capabilities(), initializeResult.serverInfo(),
initializeResult.instructions());
if (!McpSchema.LATEST_PROTOCOL_VERSION.equals(initializeResult.protocolVersion())) {
if (!this.protocolVersions.contains(initializeResult.protocolVersion())) {
return Mono.error(new McpError(
"Unsupported protocol version from the server: " + initializeResult.protocolVersion()));
}
@ -911,4 +921,13 @@ public class McpAsyncClient {
return this.mcpSession.sendNotification(McpSchema.METHOD_LOGGING_SET_LEVEL, params);
}
/**
* This method is package-private and used for test only. Should not be called by user
* code.
* @param protocolVersions the Client supported protocol versions.
*/
void setProtocolVersions(List<String> protocolVersions) {
this.protocolVersions = protocolVersions;
}
}

3
mcp/src/main/java/org/springframework/ai/mcp/client/McpClientFeatures.java
View file

@ -67,7 +67,6 @@ class McpClientFeatures {
/**
* Create an instance and validate the arguments.
* @param clientInfo the client implementation information.
* @param clientCapabilities the client capabilities.
* @param roots the roots.
* @param toolsChangeConsumers the tools change consumers.
@ -85,7 +84,6 @@ class McpClientFeatures {
Function<McpSchema.CreateMessageRequest, Mono<McpSchema.CreateMessageResult>> samplingHandler) {
Assert.notNull(clientInfo, "Client info must not be null");
this.clientInfo = clientInfo;
this.clientCapabilities = (clientCapabilities != null) ? clientCapabilities
: new McpSchema.ClientCapabilities(null,
@ -182,7 +180,6 @@ class McpClientFeatures {
Function<McpSchema.CreateMessageRequest, McpSchema.CreateMessageResult> samplingHandler) {
Assert.notNull(clientInfo, "Client info must not be null");
this.clientInfo = clientInfo;
this.clientCapabilities = (clientCapabilities != null) ? clientCapabilities
: new McpSchema.ClientCapabilities(null,

30
mcp/src/main/java/org/springframework/ai/mcp/server/McpAsyncServer.java
View file

@ -122,12 +122,18 @@ public class McpAsyncServer {
private LoggingLevel minLoggingLevel = LoggingLevel.DEBUG;
/**
* Supported protocol versions.
*/
private List<String> protocolVersions = List.of(McpSchema.LATEST_PROTOCOL_VERSION);
/**
* Create a new McpAsyncServer with the given transport and capabilities.
* @param mcpTransport The transport layer implementation for MCP communication.
* @param features The MCP server supported features.
*/
McpAsyncServer(ServerMcpTransport mcpTransport, McpServerFeatures.Async features) {
this.serverInfo = features.serverInfo();
this.serverCapabilities = features.serverCapabilities();
this.tools.addAll(features.tools());
@ -210,6 +216,7 @@ public class McpAsyncServer {
Map<String, ResourceRegistration> resources, List<McpSchema.ResourceTemplate> resourceTemplates,
Map<String, PromptRegistration> prompts, List<Consumer<List<McpSchema.Root>>> rootsChangeConsumers) {
this.protocolVersions = List.of(McpSchema.LATEST_PROTOCOL_VERSION);
this.serverInfo = serverInfo;
if (!Utils.isEmpty(tools)) {
this.tools.addAll(McpServer.mapDeprecatedTools(tools));
@ -299,12 +306,18 @@ public class McpAsyncServer {
initializeRequest.protocolVersion(), initializeRequest.capabilities(),
initializeRequest.clientInfo());
if (!McpSchema.LATEST_PROTOCOL_VERSION.equals(initializeRequest.protocolVersion())) {
return Mono.error(new McpError(
"Unsupported protocol version from client: " + initializeRequest.protocolVersion()));
String serverProtocolVersion = this.protocolVersions.get(this.protocolVersions.size() - 1);
if (this.protocolVersions.contains(initializeRequest.protocolVersion())) {
serverProtocolVersion = initializeRequest.protocolVersion();
}
else {
logger.warn(
"Client requested unsupported protocol version: {}, so the server will sugggest the {} version instead",
initializeRequest.protocolVersion(), serverProtocolVersion);
}
return Mono.just(new McpSchema.InitializeResult(McpSchema.LATEST_PROTOCOL_VERSION, this.serverCapabilities,
return Mono.just(new McpSchema.InitializeResult(serverProtocolVersion, this.serverCapabilities,
this.serverInfo, null));
};
}
@ -903,4 +916,13 @@ public class McpAsyncServer {
CREATE_MESSAGE_RESULT_TYPE_REF);
}
/**
* This method is package-private and used for test only. Should not be called by user
* code.
* @param protocolVersions the Client supported protocol versions.
*/
void setProtocolVersions(List<String> protocolVersions) {
this.protocolVersions = protocolVersions;
}
}

16
mcp/src/main/java/org/springframework/ai/mcp/util/Assert.java
View file

@ -16,11 +16,13 @@
package org.springframework.ai.mcp.util;
import java.util.Collection;
import reactor.util.annotation.Nullable;
/**
* Assertion utility class that assists in validating arguments.
*
*
* @author Christian Tzolov
*/
@ -29,6 +31,18 @@ import reactor.util.annotation.Nullable;
*/
public final class Assert {
/**
* Assert that the collection is not {@code null} and not empty.
* @param collection the collection to check
* @param message the exception message to use if the assertion fails
* @throws IllegalArgumentException if the collection is {@code null} or empty
*/
public static void notEmpty(@Nullable Collection<?> collection, String message) {
if (collection == null || collection.isEmpty()) {
throw new IllegalArgumentException(message);
}
}
/**
* Assert that an object is not {@code null}.
*

8
mcp/src/test/java/org/springframework/ai/mcp/MockMcpTransport.java
View file

@ -32,7 +32,10 @@ import org.springframework.ai.mcp.spec.McpSchema.JSONRPCNotification;
import org.springframework.ai.mcp.spec.McpSchema.JSONRPCRequest;
import org.springframework.ai.mcp.spec.ServerMcpTransport;
@SuppressWarnings("unused")
/**
* A mock implementation of the {@link ClientMcpTransport} and {@link ServerMcpTransport}
* interfaces.
*/
public class MockMcpTransport implements ClientMcpTransport, ServerMcpTransport {
private final AtomicInteger inboundMessageCount = new AtomicInteger(0);
@ -91,6 +94,7 @@ public class MockMcpTransport implements ClientMcpTransport, ServerMcpTransport
connected = false;
outgoing.tryEmitComplete();
inbound.tryEmitComplete();
// Wait for all subscribers to complete
return Mono.empty();
});
}
@ -100,4 +104,4 @@ public class MockMcpTransport implements ClientMcpTransport, ServerMcpTransport
return new ObjectMapper().convertValue(data, typeRef);
}
}
}

1
mcp/src/test/java/org/springframework/ai/mcp/client/McpAsyncClientResponseHandlerTests.java
View file

@ -20,7 +20,6 @@ import java.time.Duration;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.function.Consumer;
import java.util.function.Function;
import com.fasterxml.jackson.core.JsonProcessingException;

170
mcp/src/test/java/org/springframework/ai/mcp/client/McpClientProtocolVersionTests.java Normal file
View file

@ -0,0 +1,170 @@
/*
* Copyright 2024-2024 the original author or authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* https://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.springframework.ai.mcp.client;
import java.time.Duration;
import java.util.List;
import org.junit.jupiter.api.Test;
import reactor.core.publisher.Mono;
import reactor.test.StepVerifier;
import org.springframework.ai.mcp.MockMcpTransport;
import org.springframework.ai.mcp.spec.McpError;
import org.springframework.ai.mcp.spec.McpSchema;
import org.springframework.ai.mcp.spec.McpSchema.InitializeResult;
import static org.assertj.core.api.Assertions.assertThat;
/**
* Tests for MCP protocol version negotiation and compatibility.
*/
class McpClientProtocolVersionTests {
private static final Duration REQUEST_TIMEOUT = Duration.ofSeconds(30);
private static final McpSchema.Implementation CLIENT_INFO = new McpSchema.Implementation("test-client", "1.0.0");
@Test
void shouldUseLatestVersionByDefault() {
MockMcpTransport transport = new MockMcpTransport();
McpAsyncClient client = McpClient.async(transport)
.clientInfo(CLIENT_INFO)
.requestTimeout(REQUEST_TIMEOUT)
.build();
try {
Mono<InitializeResult> initializeResultMono = client.initialize();
StepVerifier.create(initializeResultMono).then(() -> {
McpSchema.JSONRPCRequest request = transport.getLastSentMessageAsRequest();
assertThat(request.params()).isInstanceOf(McpSchema.InitializeRequest.class);
McpSchema.InitializeRequest initRequest = (McpSchema.InitializeRequest) request.params();
assertThat(initRequest.protocolVersion()).isEqualTo(McpSchema.LATEST_PROTOCOL_VERSION);
transport.simulateIncomingMessage(new McpSchema.JSONRPCResponse(McpSchema.JSONRPC_VERSION, request.id(),
new McpSchema.InitializeResult(McpSchema.LATEST_PROTOCOL_VERSION, null,
new McpSchema.Implementation("test-server", "1.0.0"), null),
null));
}).assertNext(result -> {
assertThat(result.protocolVersion()).isEqualTo(McpSchema.LATEST_PROTOCOL_VERSION);
}).verifyComplete();
}
finally {
// Ensure cleanup happens even if test fails
StepVerifier.create(client.closeGracefully()).verifyComplete();
}
}
@Test
void shouldNegotiateSpecificVersion() {
String oldVersion = "0.1.0";
MockMcpTransport transport = new MockMcpTransport();
McpAsyncClient client = McpClient.async(transport)
.clientInfo(CLIENT_INFO)
.requestTimeout(REQUEST_TIMEOUT)
.build();
client.setProtocolVersions(List.of(oldVersion, McpSchema.LATEST_PROTOCOL_VERSION));
try {
Mono<InitializeResult> initializeResultMono = client.initialize();
StepVerifier.create(initializeResultMono).then(() -> {
McpSchema.JSONRPCRequest request = transport.getLastSentMessageAsRequest();
assertThat(request.params()).isInstanceOf(McpSchema.InitializeRequest.class);
McpSchema.InitializeRequest initRequest = (McpSchema.InitializeRequest) request.params();
assertThat(initRequest.protocolVersion()).isIn(List.of(oldVersion, McpSchema.LATEST_PROTOCOL_VERSION));
transport.simulateIncomingMessage(new McpSchema.JSONRPCResponse(McpSchema.JSONRPC_VERSION, request.id(),
new McpSchema.InitializeResult(oldVersion, null,
new McpSchema.Implementation("test-server", "1.0.0"), null),
null));
}).assertNext(result -> {
assertThat(result.protocolVersion()).isEqualTo(oldVersion);
}).verifyComplete();
}
finally {
StepVerifier.create(client.closeGracefully()).verifyComplete();
}
}
@Test
void shouldFailForUnsupportedVersion() {
String unsupportedVersion = "999.999.999";
MockMcpTransport transport = new MockMcpTransport();
McpAsyncClient client = McpClient.async(transport)
.clientInfo(CLIENT_INFO)
.requestTimeout(REQUEST_TIMEOUT)
.build();
try {
Mono<InitializeResult> initializeResultMono = client.initialize();
StepVerifier.create(initializeResultMono).then(() -> {
McpSchema.JSONRPCRequest request = transport.getLastSentMessageAsRequest();
assertThat(request.params()).isInstanceOf(McpSchema.InitializeRequest.class);
transport.simulateIncomingMessage(new McpSchema.JSONRPCResponse(McpSchema.JSONRPC_VERSION, request.id(),
new McpSchema.InitializeResult(unsupportedVersion, null,
new McpSchema.Implementation("test-server", "1.0.0"), null),
null));
}).expectError(McpError.class).verify();
}
finally {
StepVerifier.create(client.closeGracefully()).verifyComplete();
}
}
@Test
void shouldUseHighestVersionWhenMultipleSupported() {
String oldVersion = "0.1.0";
String middleVersion = "0.2.0";
String latestVersion = McpSchema.LATEST_PROTOCOL_VERSION;
MockMcpTransport transport = new MockMcpTransport();
McpAsyncClient client = McpClient.async(transport)
.clientInfo(CLIENT_INFO)
.requestTimeout(REQUEST_TIMEOUT)
.build();
client.setProtocolVersions(List.of(oldVersion, middleVersion, latestVersion));
try {
Mono<InitializeResult> initializeResultMono = client.initialize();
StepVerifier.create(initializeResultMono).then(() -> {
McpSchema.JSONRPCRequest request = transport.getLastSentMessageAsRequest();
McpSchema.InitializeRequest initRequest = (McpSchema.InitializeRequest) request.params();
assertThat(initRequest.protocolVersion()).isEqualTo(latestVersion);
transport.simulateIncomingMessage(new McpSchema.JSONRPCResponse(McpSchema.JSONRPC_VERSION, request.id(),
new McpSchema.InitializeResult(latestVersion, null,
new McpSchema.Implementation("test-server", "1.0.0"), null),
null));
}).assertNext(result -> {
assertThat(result.protocolVersion()).isEqualTo(latestVersion);
}).verifyComplete();
}
finally {
StepVerifier.create(client.closeGracefully()).verifyComplete();
}
}
}

132
mcp/src/test/java/org/springframework/ai/mcp/server/McpServerProtocolVersionTests.java Normal file
View file

@ -0,0 +1,132 @@
/*
* Copyright 2024-2024 the original author or authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* https://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.springframework.ai.mcp.server;
import java.util.List;
import java.util.UUID;
import org.junit.jupiter.api.Test;
import org.springframework.ai.mcp.MockMcpTransport;
import org.springframework.ai.mcp.spec.McpSchema;
import static org.assertj.core.api.Assertions.assertThat;
/**
* Tests for MCP server protocol version negotiation and compatibility.
*/
class McpServerProtocolVersionTests {
private static final McpSchema.Implementation SERVER_INFO = new McpSchema.Implementation("test-server", "1.0.0");
private static final McpSchema.Implementation CLIENT_INFO = new McpSchema.Implementation("test-client", "1.0.0");
private McpSchema.JSONRPCRequest jsonRpcInitializeRequest(String requestId, String protocolVersion) {
return new McpSchema.JSONRPCRequest(McpSchema.JSONRPC_VERSION, McpSchema.METHOD_INITIALIZE, requestId,
new McpSchema.InitializeRequest(protocolVersion, null, CLIENT_INFO));
}
@Test
void shouldUseLatestVersionByDefault() {
MockMcpTransport transport = new MockMcpTransport();
McpAsyncServer server = McpServer.async(transport).serverInfo(SERVER_INFO).build();
String requestId = UUID.randomUUID().toString();
transport.simulateIncomingMessage(jsonRpcInitializeRequest(requestId, McpSchema.LATEST_PROTOCOL_VERSION));
McpSchema.JSONRPCMessage response = transport.getLastSentMessage();
assertThat(response).isInstanceOf(McpSchema.JSONRPCResponse.class);
McpSchema.JSONRPCResponse jsonResponse = (McpSchema.JSONRPCResponse) response;
assertThat(jsonResponse.id()).isEqualTo(requestId);
assertThat(jsonResponse.result()).isInstanceOf(McpSchema.InitializeResult.class);
McpSchema.InitializeResult result = (McpSchema.InitializeResult) jsonResponse.result();
assertThat(result.protocolVersion()).isEqualTo(McpSchema.LATEST_PROTOCOL_VERSION);
server.closeGracefully().subscribe();
}
@Test
void shouldNegotiateSpecificVersion() {
String oldVersion = "0.1.0";
MockMcpTransport transport = new MockMcpTransport();
McpAsyncServer server = McpServer.async(transport).serverInfo(SERVER_INFO).build();
server.setProtocolVersions(List.of(oldVersion, McpSchema.LATEST_PROTOCOL_VERSION));
String requestId = UUID.randomUUID().toString();
transport.simulateIncomingMessage(jsonRpcInitializeRequest(requestId, oldVersion));
McpSchema.JSONRPCMessage response = transport.getLastSentMessage();
assertThat(response).isInstanceOf(McpSchema.JSONRPCResponse.class);
McpSchema.JSONRPCResponse jsonResponse = (McpSchema.JSONRPCResponse) response;
assertThat(jsonResponse.id()).isEqualTo(requestId);
assertThat(jsonResponse.result()).isInstanceOf(McpSchema.InitializeResult.class);
McpSchema.InitializeResult result = (McpSchema.InitializeResult) jsonResponse.result();
assertThat(result.protocolVersion()).isEqualTo(oldVersion);
server.closeGracefully().subscribe();
}
@Test
void shouldSuggestLatestVersionForUnsupportedVersion() {
String unsupportedVersion = "999.999.999";
MockMcpTransport transport = new MockMcpTransport();
McpAsyncServer server = McpServer.async(transport).serverInfo(SERVER_INFO).build();
String requestId = UUID.randomUUID().toString();
transport.simulateIncomingMessage(jsonRpcInitializeRequest(requestId, unsupportedVersion));
McpSchema.JSONRPCMessage response = transport.getLastSentMessage();
assertThat(response).isInstanceOf(McpSchema.JSONRPCResponse.class);
McpSchema.JSONRPCResponse jsonResponse = (McpSchema.JSONRPCResponse) response;
assertThat(jsonResponse.id()).isEqualTo(requestId);
assertThat(jsonResponse.result()).isInstanceOf(McpSchema.InitializeResult.class);
McpSchema.InitializeResult result = (McpSchema.InitializeResult) jsonResponse.result();
assertThat(result.protocolVersion()).isEqualTo(McpSchema.LATEST_PROTOCOL_VERSION);
server.closeGracefully().subscribe();
}
@Test
void shouldUseHighestVersionWhenMultipleSupported() {
String oldVersion = "0.1.0";
String middleVersion = "0.2.0";
String latestVersion = McpSchema.LATEST_PROTOCOL_VERSION;
MockMcpTransport transport = new MockMcpTransport();
McpAsyncServer server = McpServer.async(transport).serverInfo(SERVER_INFO).build();
server.setProtocolVersions(List.of(oldVersion, middleVersion, latestVersion));
String requestId = UUID.randomUUID().toString();
transport.simulateIncomingMessage(jsonRpcInitializeRequest(requestId, latestVersion));
McpSchema.JSONRPCMessage response = transport.getLastSentMessage();
assertThat(response).isInstanceOf(McpSchema.JSONRPCResponse.class);
McpSchema.JSONRPCResponse jsonResponse = (McpSchema.JSONRPCResponse) response;
assertThat(jsonResponse.id()).isEqualTo(requestId);
assertThat(jsonResponse.result()).isInstanceOf(McpSchema.InitializeResult.class);
McpSchema.InitializeResult result = (McpSchema.InitializeResult) jsonResponse.result();
assertThat(result.protocolVersion()).isEqualTo(latestVersion);
server.closeGracefully().subscribe();
}
}