Issue Report: Infinite Loop in MCP Client When Handling Empty nextCursor During Tool Listing #628

1. Title

Infinite Loop in MCP Client When Processing Empty nextCursor During listTools() Call

2. Description

The MCP Java SDK client enters an infinite loop when processing the /tools response from Alibaba Cloud DashScope MCP applications. The client incorrectly interprets an empty nextCursor value ("") as a signal to continue pagination, resulting in repeated requests that eventually time out (10s default timeout).

Affected SDK Version: io.modelcontextprotocol.client:client-java:0.14.1 (and likely all prior versions)

3. Steps to Reproduce

1. Configure MCP client to connect to Alibaba Cloud DashScope MCP application with market-cmapi00035184 (Global IP Geolocation)
2. Call client.listTools()
3. Observe repeated HTTP requests to /tools endpoint

java
编辑
McpClientConfig config = McpClientConfig.builder()
    .addHeader("Authorization", "Bearer YOUR_API_KEY")
    .build();
McpSyncClient client = McpClient.connect(

    "https://dashscope.aliyuncs.com/api/v1/mcps/market-cmapi00035184/mcp",

    config

);
List<Tool> tools = client.listTools(); // Triggers infinite loop

4. Expected Behavior

The client should:

1. Receive a single /tools response
2. Detect that nextCursor is empty ("")
3. Exit pagination loop immediately
4. Return the tool list without timeout

5. Actual Behavior

The client:

1. Receives valid tool list response with nextCursor: ""
2. Incorrectly interprets empty string as needing pagination
3. Sends repeated /tools requests (e.g., 2e9e1828-904, 2e9e1828-905, 2e9e1828-906)
4. Eventually fails with TimeoutException after 10s

Error Log:

log
编辑
[DEBUG] Sending message for method tools/list (ID: 2e9e1828-904)
[DEBUG] Received JSON message: {"jsonrpc":"2.0","id":"2e9e1828-904","result":{"nextCursor":"","tools":[...]}}
[DEBUG] Sending message for method tools/list (ID: 2e9e1828-905)
[DEBUG] Received JSON message: {"jsonrpc":"2.0","id":"2e9e1828-905","result":{"nextCursor":"","tools":[...]}}
[DEBUG] Sending message for method tools/list (ID: 2e9e1828-906)
...
[ERROR] TimeoutException: Did not observe any item within 10000ms

6. Key Evidence

12. Resolution Plan

1. Patch: Implement the fix in McpClientSession.java (as shown in Section 8)
2. Test: Add unit tests for empty string handling (see Test Plan)
3. Release: New SDK version 1.0.1
4. Documentation: Update README with pagination best practices

Note: The fix does not affect applications that return nextCursor: null

Reported By: moyuyanli
Date: October 20, 2025
SDK Version: io.modelcontextprotocol.client:client-java:0.14.1
Platform: Java 11+
Environment: Alibaba Cloud DashScope MCP (v1.0)