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resend

Email Sending MCP

by resend
OverviewSchemaSourceRelated ServersDiscussionsAdmin
TypeScript
Local

Send Inbox Draft

send-inbox-draft
Destructive

Send a draft from a Resend inbox. This puts a real email in the mail the moment you call it, from the inbox's address to the recipients the draft already carries, and a sent email CANNOT be recalled or unsent.

Instructions

Send a draft from a Resend inbox. This puts a real email in the mail the moment you call it, from the inbox's address to the recipients the draft already carries, and a sent email CANNOT be recalled or unsent. Before using this tool, you MUST double-check with the user that they want to send this draft. Reference the SUBJECT and the RECIPIENTS of the draft when double-checking, and warn the user that sending cannot be undone. You may only use this tool if the user explicitly confirms they want to send the draft after you double-check. The draft must have at least one "to" recipient, a subject, and an html or text body, or the send fails. The whole email, encoded, can be at most 40 MB. A reply draft created without a subject has an empty one, so set it with update-inbox-draft first. Call get-inbox-draft first if you need to read back exactly what will go out, and update-inbox-draft to change it before sending. Get the draft ID from list-inbox-drafts. Returns the sent email's id.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
draftIdYesDraft ID. Use list-inbox-drafts to find draft IDs.
headersNoOptional custom email headers as key/value pairs (e.g. {"X-Entity-Ref-ID": "unique-id"}). Header names cannot be empty, and names and values cannot contain line breaks or null characters. In-Reply-To, References and Reply-To are reserved: Resend sets them, and passing any of them, in any letter case, fails.
inboxIdYesInbox ID
idempotencyKeyNoOptional unique key that prevents duplicate sends on retries (sent as the Idempotency-Key header). Use the same key when retrying the same logical email; use a new key for a different email. Must be 1-256 characters.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Addedv2.26.1

TDQS

A4.6/5.0
Behavior5/5

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

Annotations already declare destructiveHint=true and openWorldHint=true, and the description adds context annotations cannot: the send is irreversible ('CANNOT be recalled or unsent'), the moment-of-call side effect, hard preconditions (at least one 'to', subject, html/text body) whose violation fails the send, the 40 MB encoded limit, and the empty-subject edge case on reply drafts.

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?

Front-loaded with purpose, side effect, and the confirmation mandate before the mechanical details. The double-check/confirmation instruction is restated three times, which is slightly redundant, but the density is justified for an irreversible operation.

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 a destructive, no-output-schema tool this covers everything an agent needs: confirmation protocol, prerequisites, size limit, failure modes, sibling routing for read/modify, and the return value ('Returns the sent email's id') despite no output schema.

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

Parameters3/5

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

Schema description coverage is 100%, so draftId, headers, idempotencyKey, and inboxId are already fully documented in the schema (including header restrictions and idempotency semantics). The description largely repeats the draft-ID sourcing hint and adds no new parameter meaning, so the baseline 3 applies.

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 ('Send a draft from a Resend inbox') and immediately clarifies the scope: this sends a real email from the inbox's own address to the draft's existing recipients. An agent can distinguish this from send-new-inbox-email, send-email, and send-broadcast without opening any 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?

Explicit when-to-use and prerequisites: it names the required user double-check with subject/recipient reference, restricts use to after explicit confirmation, and routes the agent to get-inbox-draft, update-inbox-draft, and list-inbox-drafts for reading, changing, and locating the draft respectively. Nothing about invocation conditions is left to inference.

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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