fix: answer -32602 when a prompt is requested without a required argument - #390

Problem

prompts/get for a prompt that declares a required argument, called without that argument, rejects with -32600 Invalid request:

server.addPrompt({
  name: "git-commit",
  arguments: [{ name: "changes", required: true }],
  load: async ({ changes }) => `Commit: ${changes}`,
});
await client.getPrompt({ name: "git-commit" }); // McpError code -32600

The MCP specification's error handling section for prompts says:

Missing required arguments: -32602 (Invalid params)

The request is well-formed; its params are what's wrong. tools/call already answers ErrorCode.InvalidParams for argument validation, so this also makes the two consistent.

Fix

Use ErrorCode.InvalidParams for the missing-required-argument check in the prompts/get handler. The message is unchanged.

Tests

src/FastMCP.prompt-arguments.test.ts (in-memory transport): asserts the error code and message for the missing argument, and that the same prompt still loads when the argument is present. It fails on main and passes with this change. prettier, eslint and tsc --noEmit are clean.