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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"NullPointerException in completion handler when Prompt has null arguments","articleBody":"Bug description\n\nThe 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.\n\nThis 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.\n\nAdditionally, `MIGRATION-2.0.md` references a `Prompt.withDefaults()` factory method as the migration path, but this method was never implemented.\n\n### Environment\n\n- MCP Java SDK: `main` branch, commit `4c85963`\n- Java: 17+\n- Transport: stdio (also affects servlet-based transports)\n\n### Steps to reproduce\n\n1. Create a server with a prompt that has null arguments:\n```java\nPrompt prompt = new Prompt(\"summarize\", \"Summarize a document\", (List\u003cPromptArgument\u003e) null);\n```\n2. Register a completion handler for that prompt\n3. Send a `completion/complete` request:\n```json\n{\"jsonrpc\":\"2.0\",\"id\":2,\"method\":\"completion/complete\",\"params\":{\"ref\":{\"type\":\"ref/prompt\",\"name\":\"summarize\"},\"argument\":{\"name\":\"query\",\"value\":\"sum\"}}}\n```\n\n### Expected behavior\n\nAn empty completion result or a proper error response.\n\n### Actual behavior\n\n```json\n{\"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\"}}\n```\n\n### Minimal Complete Reproducible example\nhttps://github.com/sainathreddyb/mcp-test-servers/tree/main/bug-repro-server\n\n### Root cause\n\n`McpAsyncServer.java` ~line 1002 and `McpStatelessAsyncServer.java` ~line 747:\n```java\nif (!promptSpec.prompt()\n    .arguments()   // returns null since 2.0\n    .stream()      // NPE\n    ...\n```\n\n### Fix\n- Add null guard before `.stream()` in both server classes\n- Implement the `Prompt.withDefaults()` factory referenced in `MIGRATION-2.0.md`\n---","author":{"url":"https://github.com/sainathreddyb","@type":"Person","name":"sainathreddyb"},"datePublished":"2026-04-23T23:22:17.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":0},"url":"https://github.com/932/java-sdk/issues/932"}