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Fix: McpAsyncClient#listTools prevent infinite recursion #631

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Merged
merged 2 commits into from
Oct 30, 2025
+90 −22
Merged

Fix: McpAsyncClient#listTools prevent infinite recursion #631

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c39f515
Fix: prevent infinite recursion in listTools() when nextCursor is emp…
leehaut Oct 23, 2025
e6559ca
fix McpAsyncClient#listTools prevent infinite recursion
leehaut Oct 28, 2025
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16 changes: 9 additions & 7 deletions mcp-core/src/main/java/io/modelcontextprotocol/client/McpAsyncClient.java
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Original file line number Diff line number Diff line change
Expand Up @@ -402,6 +402,7 @@ public Mono<Void> closeGracefully() {
// --------------------------
// Initialization
// --------------------------

/**
* The initialization phase should be the first interaction between client and server.
* The client will ensure it happens in case it has not been explicitly called and in
Expand Down Expand Up @@ -448,6 +449,7 @@ public Mono<Object> ping() {
// --------------------------
// Roots
// --------------------------

/**
* Adds a new root to the client's root list.
* @param root The root to add.
Expand Down Expand Up @@ -625,13 +627,13 @@ private McpSchema.CallToolResult validateToolResult(String toolName, McpSchema.C
* @return A Mono that emits the list of all tools result
*/
public Mono<McpSchema.ListToolsResult> listTools() {
return this.listTools(McpSchema.FIRST_PAGE)
.expand(result -> (result.nextCursor() != null) ? this.listTools(result.nextCursor()) : Mono.empty())
.reduce(new McpSchema.ListToolsResult(new ArrayList<>(), null), (allToolsResult, result) -> {
allToolsResult.tools().addAll(result.tools());
return allToolsResult;
})
.map(result -> new McpSchema.ListToolsResult(Collections.unmodifiableList(result.tools()), null));
return this.listTools(McpSchema.FIRST_PAGE).expand(result -> {
String next = result.nextCursor();
return (next != null && !next.isEmpty()) ? this.listTools(next) : Mono.empty();
}).reduce(new McpSchema.ListToolsResult(new ArrayList<>(), null), (allToolsResult, result) -> {
allToolsResult.tools().addAll(result.tools());
return allToolsResult;
}).map(result -> new McpSchema.ListToolsResult(Collections.unmodifiableList(result.tools()), null));
}

/**
Expand Down
96 changes: 81 additions & 15 deletions mcp-core/src/test/java/io/modelcontextprotocol/client/McpAsyncClientTests.java
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Original file line number Diff line number Diff line change
Expand Up @@ -4,24 +4,22 @@

package io.modelcontextprotocol.client;

import java.util.List;
import java.util.Map;
import java.util.Objects;
import java.util.Set;
import java.util.concurrent.atomic.AtomicReference;
import java.util.function.Function;
import java.util.stream.Collectors;

import io.modelcontextprotocol.json.TypeRef;
import io.modelcontextprotocol.spec.McpClientTransport;
import io.modelcontextprotocol.spec.McpSchema;
import io.modelcontextprotocol.spec.ProtocolVersions;

import org.junit.jupiter.api.Test;

import com.fasterxml.jackson.core.JsonProcessingException;

import reactor.core.publisher.Mono;
import reactor.test.StepVerifier;

import java.util.List;
import java.util.Map;
import java.util.Objects;
import java.util.concurrent.atomic.AtomicReference;
import java.util.function.Function;

import static io.modelcontextprotocol.util.McpJsonMapperUtils.JSON_MAPPER;
import static org.assertj.core.api.Assertions.assertThat;
import static org.assertj.core.api.Assertions.assertThatCode;
Expand All @@ -40,8 +38,7 @@ class McpAsyncClientTests {

private static final String CONTEXT_KEY = "context.key";

private McpClientTransport createMockTransportForToolValidation(boolean hasOutputSchema, boolean invalidOutput)
throws JsonProcessingException {
private McpClientTransport createMockTransportForToolValidation(boolean hasOutputSchema, boolean invalidOutput) {

// Create tool with or without output schema
Map<String, Object> inputSchemaMap = Map.of("type", "object", "properties",
Expand Down Expand Up @@ -182,7 +179,7 @@ public java.lang.reflect.Type getType() {
}

@Test
void testCallToolWithOutputSchemaValidationSuccess() throws JsonProcessingException {
void testCallToolWithOutputSchemaValidationSuccess() {
McpClientTransport transport = createMockTransportForToolValidation(true, false);

McpAsyncClient client = McpClient.async(transport).enableCallToolSchemaCaching(true).build();
Expand All @@ -204,7 +201,7 @@ void testCallToolWithOutputSchemaValidationSuccess() throws JsonProcessingExcept
}

@Test
void testCallToolWithNoOutputSchemaSuccess() throws JsonProcessingException {
void testCallToolWithNoOutputSchemaSuccess() {
McpClientTransport transport = createMockTransportForToolValidation(false, false);

McpAsyncClient client = McpClient.async(transport).enableCallToolSchemaCaching(true).build();
Expand All @@ -226,7 +223,7 @@ void testCallToolWithNoOutputSchemaSuccess() throws JsonProcessingException {
}

@Test
void testCallToolWithOutputSchemaValidationFailure() throws JsonProcessingException {
void testCallToolWithOutputSchemaValidationFailure() {
McpClientTransport transport = createMockTransportForToolValidation(true, true);

McpAsyncClient client = McpClient.async(transport).enableCallToolSchemaCaching(true).build();
Expand All @@ -241,4 +238,73 @@ void testCallToolWithOutputSchemaValidationFailure() throws JsonProcessingExcept
StepVerifier.create(client.closeGracefully()).verifyComplete();
}

@Test
void testListToolsWithEmptyCursor() {
McpSchema.Tool addTool = McpSchema.Tool.builder().name("add").description("calculate add").build();
McpSchema.Tool subtractTool = McpSchema.Tool.builder()
.name("subtract")
.description("calculate subtract")
.build();
McpSchema.ListToolsResult mockToolsResult = new McpSchema.ListToolsResult(List.of(addTool, subtractTool), "");

McpClientTransport transport = new McpClientTransport() {
Function<Mono<McpSchema.JSONRPCMessage>, Mono<McpSchema.JSONRPCMessage>> handler;

@Override
public Mono<Void> connect(
Function<Mono<McpSchema.JSONRPCMessage>, Mono<McpSchema.JSONRPCMessage>> handler) {
return Mono.deferContextual(ctx -> {
this.handler = handler;
return Mono.empty();
});
}

@Override
public Mono<Void> closeGracefully() {
return Mono.empty();
}

@Override
public Mono<Void> sendMessage(McpSchema.JSONRPCMessage message) {
if (!(message instanceof McpSchema.JSONRPCRequest request)) {
return Mono.empty();
}

McpSchema.JSONRPCResponse response;
if (McpSchema.METHOD_INITIALIZE.equals(request.method())) {
response = new McpSchema.JSONRPCResponse(McpSchema.JSONRPC_VERSION, request.id(), MOCK_INIT_RESULT,
null);
}
else if (McpSchema.METHOD_TOOLS_LIST.equals(request.method())) {
response = new McpSchema.JSONRPCResponse(McpSchema.JSONRPC_VERSION, request.id(), mockToolsResult,
null);
}
else {
return Mono.empty();
}

return handler.apply(Mono.just(response)).then();
}

@Override
public <T> T unmarshalFrom(Object data, TypeRef<T> typeRef) {
return JSON_MAPPER.convertValue(data, new TypeRef<>() {
@Override
public java.lang.reflect.Type getType() {
return typeRef.getType();
}
});
}
};

McpAsyncClient client = McpClient.async(transport).enableCallToolSchemaCaching(true).build();

Mono<McpSchema.ListToolsResult> mono = client.listTools();
McpSchema.ListToolsResult toolsResult = mono.block();
assertThat(toolsResult).isNotNull();

Set<String> names = toolsResult.tools().stream().map(McpSchema.Tool::name).collect(Collectors.toSet());
assertThat(names).containsExactlyInAnyOrder("subtract", "add");
}

}