fix: align send-batch-emails with batch API tag support

[cpenned]

Summary

Aligns send-batch-emails with the Resend batch API. Only tags are supported as an extra per-email field.

Changes

• Remove scheduledAt from the batch email schema and handler (was incorrectly exposed)
• Clarify tool description: batch emails support tags only
• Align tags field description with send-email
• Add test coverage for tags being passed to resend.batch.send

  

[mayankbohradev]

Thanks for working on batch parity with send-email.

I double-checked this against current Resend docs and the official SDKs before commenting.

Public docs still say batch does not support attachments

• API reference limitations: https://resend.com/docs/api-reference/emails/send-batch-emails

  The attachments and scheduled_at fields are not supported yet.

• Batch sending guide: https://resend.com/docs/dashboard/emails/batch-sending

  The attachments field is not supported yet

• Attachments guide: https://resend.com/docs/dashboard/emails/attachments

  We currently do not support sending attachments when using our batch endpoint.

• Official skills/reference also routes attachment sends to single POST /emails, not batch: https://github.com/resend/resend-skills/blob/main/skills/resend/references/sending/overview.md

Official SDKs still omit attachments from batch types

• resend-node (CreateBatchEmailOptions):
  Omit<CreateEmailOptions, 'attachments' | 'scheduledAt'>
  with comment: “not supported in the batch API”
  https://github.com/resend/resend-node/blob/main/src/batch/interfaces/create-batch-options.interface.ts
• Installed resend@6.17.x in this repo matches that typing.
• Historical confirmation from maintainers on resend-node#409: attachments are not supported on the batch endpoint yet.

Why this matters for this PR

The PR description says this aligns with an updated batch API, and the new tests mock batch.send, so they prove MCP pass-through — not that the live API accepts attachments. If the API still rejects them, agents would get a runtime failure after the MCP accepts the args.

Questions for maintainers:

1. Has batch attachments shipped on the API already, and docs/SDKs just haven’t caught up?
2. If not, should we hold exposing attachments on send-batch-emails until the API supports it (or clearly document that it will fail upstream)?

Smaller related note: send-batch-emails already exposes scheduledAt, which has the same documented limitation. Adding attachments makes that gap more user-facing.

Happy to re-check if there’s an internal/API update I’m missing.