NullPointerException in completion handler when Prompt has null arguments #932

Bug description

The completion validation code in McpAsyncServer and McpStatelessAsyncServer throws a NullPointerException when a Prompt is registered with null arguments and a completion/complete request is received for that prompt.

This was introduced in commit 4c85963 (#972, the 2.0 forward-compat refactor), which changed Prompt constructors to stop coercing null arguments to an empty list — but did not update the SDK's own completion handler code that calls .arguments().stream() without a null guard.

Additionally, MIGRATION-2.0.md references a Prompt.withDefaults() factory method as the migration path, but this method was never implemented.

Environment

• MCP Java SDK: main branch, commit 4c85963
• Java: 17+
• Transport: stdio (also affects servlet-based transports)

Steps to reproduce

1. Create a server with a prompt that has null arguments:

Prompt prompt = new Prompt("summarize", "Summarize a document", (List<PromptArgument>) null);

2. Register a completion handler for that prompt
3. Send a completion/complete request:

{"jsonrpc":"2.0","id":2,"method":"completion/complete","params":{"ref":{"type":"ref/prompt","name":"summarize"},"argument":{"name":"query","value":"sum"}}}

Expected behavior

An empty completion result or a proper error response.

Actual behavior

{"jsonrpc":"2.0","id":2,"error":{"code":-32603,"message":"Cannot invoke \"java.util.List.stream()\" because the return value of \"io.modelcontextprotocol.spec.McpSchema$Prompt.arguments()\" is null"}}

Minimal Complete Reproducible example

https://github.com/sainathreddyb/mcp-test-servers/tree/main/bug-repro-server

Root cause

McpAsyncServer.java ~line 1002 and McpStatelessAsyncServer.java ~line 747:

if (!promptSpec.prompt()
    .arguments()   // returns null since 2.0
    .stream()      // NPE
    ...

Fix

• Add null guard before .stream() in both server classes
• Implement the Prompt.withDefaults() factory referenced in MIGRATION-2.0.md