fix: accept omitted prompt arguments

[pragnyanramtha]

Summary

• default omitted prompts/get arguments to {} before prompt argument schema validation
• add a regression test for a prompt whose schema fields are all optional
• add a server patch changeset

Root cause

prompts/get passes request.params.arguments through to the registered prompt handler. When arguments is omitted, createPromptHandler validated undefined against the prompt's object schema, so all-optional object schemas still failed at the top level. The tools path already normalizes omitted arguments with args ?? {}; this applies the same behavior to prompts.

Addresses the main-branch prompt portion of #1869. I did not include v1.x backport changes in this PR.

Related prior work: this revives the same main-branch behavior targeted by closed, unmerged #1921.

Validation

• Added regression test and verified it failed before the handler fix.
• pnpm --dir test/integration exec vitest run test/server/mcp.test.ts -t "should accept omitted prompt arguments"
• pnpm --dir test/integration exec vitest run test/server/mcp.test.ts
• pnpm --filter @modelcontextprotocol/server typecheck
• pnpm --filter @modelcontextprotocol/server lint
• pnpm --filter @modelcontextprotocol/test-integration lint
• Full pre-push hook: typecheck passed; repo-wide lint/build did not complete due unrelated existing issues:
  • @modelcontextprotocol/test-helpers lint reports existing vitest extraneous dependency errors in test/helpers/src/helpers/http.ts and test/helpers/src/helpers/oauth.ts.
  • @modelcontextprotocol/node build OOMs with the default Node heap, but passes with NODE_OPTIONS=--max-old-space-size=4096 pnpm --filter @modelcontextprotocol/node build.

CI note: Node 20 test (20) currently fails in unrelated test/server/cloudflareWorkers.test.ts with a Miniflare Network connection lost; the changed test/server/mcp.test.ts suite passed in that same run. I do not have permission to rerun the failed job.

[changeset-bot]

🦋 Changeset detected

Latest commit: a1dba81

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 5 packages

Name	Type
@modelcontextprotocol/server	Patch
@modelcontextprotocol/express	Patch
@modelcontextprotocol/fastify	Patch
@modelcontextprotocol/hono	Patch
@modelcontextprotocol/node	Patch

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commit: a1dba81

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