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Email Sending MCP

Email Sending MCP

by resend
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TypeScript
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Create Inbox Draft

create-inbox-draft

Draft a message in a Resend inbox without sending it, so users can compose, prepare, or review content before delivery.

Instructions

Purpose: Create a draft in a Resend inbox: a message saved for later, NOT sent. At least one of to, cc, bcc, subject, text, html, or attachments is required. Nothing leaves the account until send-inbox-draft is called on it.

NOT for: Sending mail. To send straight away, use send-new-inbox-email to start a new thread from the inbox, reply-to-inbox-thread-email to answer a thread, or forward-inbox-thread-email to pass a message along.

Returns: The draft's id.

When to use: User asks to draft, compose, or prepare a message without sending it, or wants to read the wording back before it goes out. Pass threadId and replyToEmailId together to draft a reply to a message in a thread; omit both to draft a standalone message. A reply draft without "to" is addressed to the author of the message it replies to, at its Reply-To address if it has one, or, for a message the inbox sent, to its to recipients. An unknown threadId, or a replyToEmailId that is not in that thread, returns a 404. A reply draft without a subject stores an empty one. When it is sent, it takes the thread's subject, prefixed with "Re:" when it does not already start with it. A message can have only one unsent reply draft, so drafting a second reply to it returns the ID of the existing draft. That call ignores every other field except attachments, which replace the existing draft's attachments. The id this returns is the "draftId" that get-inbox-draft, update-inbox-draft, send-inbox-draft, and remove-inbox-draft all take.

Input Schema

NameRequiredDescriptionDefault
ccNoEmail address to copy, or an array of addresses. Across to, cc, and bcc a draft can carry at most 50 recipients in total.
toNoRecipient email address (e.g. "customer@example.com"), or an array of addresses to reach several. Across to, cc, and bcc a draft can carry at most 50 recipients in total.
bccNoEmail address to blind-copy, or an array of addresses. Across to, cc, and bcc a draft can carry at most 50 recipients in total.
htmlNoHTML body of the draft.
textNoPlain text body of the draft.
inboxIdYesInbox ID, or the inbox email address (e.g. "support@example.com"). Not the Resend receiving address.
subjectNoSubject of the draft. Max 2000 characters. It cannot be only whitespace or contain a line break.
threadIdNoID of the thread this draft replies to. Must be provided together with replyToEmailId. Omit both to draft a standalone message. Get the thread ID from list-inbox-threads.
attachmentsNoFiles to store on the draft. Each attachment needs `filename` plus either `content` (base64, with `contentType`) or `url` (an http(s) URL Resend downloads; `contentType` defaults to the downloaded file's type). Together they can total at most 25 MB, downloads included, and all downloads together must finish within 20 seconds. An empty file or a blocked file type (such as .exe) is rejected. They are sent with the draft when send-inbox-draft is called.
idempotencyKeyNoOptional unique key that prevents duplicate drafts on retries (sent as the Idempotency-Key header). Use the same key when retrying the same logical draft; use a new key for a different draft. Must be 1-256 characters.
replyToEmailIdNoID of the message in that thread this draft replies to, the "Email ID" get-inbox-thread prints for it. Must be provided together with threadId.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed5 schema fields changedv2.28.0
    • changedInput schema / properties / attachments / description
      Previous value: -"Files to store on the draft. Each one needs filename, base64 content and contentType; a draft cannot attach a file by URL. Together they can total at most 25 MB. An empty file or a blocked file type (such as .exe) is rejected. They are sent with the draft when send-inbox-draft is called."New value: +"Files to store on the draft. Each attachment needs `filename` plus either `content` (base64, with `contentType`) or `url` (an http(s) URL Resend downloads; `contentType` defaults to the downloaded file's type). Together they can total at most 25 MB, downloads included, and all downloads together must finish within 20 seconds. An empty file or a blocked file type (such as .exe) is rejected. They are sent with the draft when send-inbox-draft is called."
    • changedInput schema / properties / attachments / items / properties / content / description
      Previous value: -"Base64-encoded file content."New value: +"Base64-encoded file content. Requires contentType. Provide content or url, not both."
    • changedInput schema / properties / attachments / items / properties / contentType / description
      Previous value: -"MIME type of the file (e.g. \"application/pdf\")."New value: +"MIME type of the file (e.g. \"application/pdf\"). Required with content; with url, defaults to the downloaded file's type."
    • addedInput schema / properties / attachments / items / properties / url
      Added value: +{
+  "description": "URL where the file is hosted. Resend downloads it when the draft is saved, so a URL Resend cannot download fails this call. It must start with http:// or https:// and cannot start with http://localhost:. Provide content or url, not both.",
+  "format": "uri",
+  "type": "string"
+}
    • changedInput schema / properties / attachments / items / required
      Previous value: -[
-  "filename",
-  "content",
-  "contentType"
-]New value: +[
+  "filename"
+]
  2. Changed2 schema fields changedv2.27.1
    • changedInput schema / properties / attachments / description
      Previous value: -"Files to store on the draft. Each one needs filename, base64 content and contentType; a draft cannot attach a file by URL. There is no limit on the number of files, but together they can total at most 25 MB. An empty file or a blocked file type (such as .exe) is rejected. They are sent with the draft when send-inbox-draft is called."New value: +"Files to store on the draft. Each one needs filename, base64 content and contentType; a draft cannot attach a file by URL. Together they can total at most 25 MB. An empty file or a blocked file type (such as .exe) is rejected. They are sent with the draft when send-inbox-draft is called."
    • changedInput schema / properties / inboxId / description
      Previous value: -"Inbox ID"New value: +"Inbox ID, or the inbox email address (e.g. \"support@example.com\"). Not the Resend receiving address."
  3. Addedv2.26.1

TDQS

A4.5/5.0
Behavior3/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already declare readOnlyHint=false and destructiveHint=false, and the description adds genuinely useful behavior the annotations don't cover: nothing leaves the account until send-inbox-draft, a 404 on a mismatched threadId/replyToEmailId, and the one-unsent-reply-draft rule with its ID-returning, attachment-replacing edge case. However, that leaves the annotations to carry the safety profile alone; for example, the file-download and 20-second/25 MB constraints are only in the schema, so the description can only be credited with what it actually adds, not with the full behavioral picture.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness4/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Bold labeled sections (Purpose / NOT for / Returns / When to use) front-load the essentials, and the detail on reply-draft semantics earns its space. It is long and dense, though, and the reply-draft paragraph could be tightened without losing rules an agent needs.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For an 11-parameter mutation tool with no output schema, the definition supplies the return value ('The draft's id'), the downstream consumers of that id, failure modes, and the ordering of operations relative to send-inbox-draft. Nothing an agent needs in order to call it correctly is missing.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters5/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 100%, but the description still contributes meaning the schema lacks: at least one of to/cc/bcc/subject/text/html/attachments is required, and threadId/replyToEmailId must be passed together. The reply-addressing fallback rules (author, Reply-To, or original to-recipients when 'to' is omitted) and the empty-subject-then-Re: behavior are cross-field semantics no per-parameter description supplies. That goes well beyond the 100%-coverage baseline.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb and resource ('Create a draft in a Resend inbox') and immediately disambiguates it from sending: 'a message saved for later, NOT sent.' That distinction separates it cleanly from send-inbox-draft and send-new-inbox-email without needing the schema.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines5/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

It has an explicit 'NOT for' section naming three sibling tools and the conditions that select them (send now, reply to thread, forward). It then gives a positive trigger ('User asks to draft, compose, or prepare a message without sending it') plus rules for when to pass threadId/replyToEmailId versus omitting both.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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