Resend Setup

What the source does

• FilesystemNone observedNo reads or writes in the source
• NetworkDeclared1 observation
• ShellNone observedNo commands run in the source
• DependenciesPinnedNothing floats
• SecretsNone foundNo keys in source

Five audit layers

Gates applied: no behavioural pass.

Run the store script to securely collect the API key:

callbackUrl = data.callbackUrl || data.url || data.callback_url;

assistant/CLAUDE.md

assistant/src/approvals/CLAUDE.md

assistant/src/notifications/CLAUDE.md

assistant/src/runtime/CLAUDE.md

gateway/CLAUDE.md

Audit 0.4.1 · source sha 19bc86343b63 · 14 Oct 2026Report an issue or request a re-scan

• Publishervellum-ai GitHub organizationOwner of the repository this skill is read from
• Source foldervellum-ai/vellum-assistant/skills/resend-setupThe exact folder we audited
• LicenceMITAs the repository states it
• Commit pinned for this verdict19bc86343b63A new audit runs when the source changes
• Repository statusNot archivedLast push 1 day ago
• Sandboxed runNot runSkills are read, not executed; layer L4 is skipped

Its description: Set up and send emails via a user-provided Resend account (BYO email provider)

---
name: resend-setup
description: Set up and send emails via a user-provided Resend account (BYO email provider)
compatibility: "Designed for Vellum personal assistants"
metadata:
  emoji: "📤"
  vellum:
    category: "email"
    display-name: "Resend Email Setup"
    user-invocable: true
---

## Overview

Send emails through the user's own Resend account. The user provides their Resend API key and you send via their domain.

## Setup

### Step 0: Check Existing Configuration

Before starting, check whether Resend is already configured:

```bash
bun skills/resend-setup/scripts/check-config.ts
```

The script outputs JSON: `{ "configured": boolean, "hasApiKey": boolean, "hasWebhookSecret": boolean, "details": string }`.

- If `configured` is `true` — Resend is already set up. Offer to verify the connection or reconfigure.
- If `configured` is `false` — continue to Step 1.

### Step 1: Store the API Key

Run the store script to securely collect the API key:

```bash
bun skills/resend-setup/scripts/store-api-key.ts
```

The script opens a secure credential prompt, stores the key with the correct injection templates, and exits. If it exits 0, the key is stored. Exit code **130** means the user cancelled the prompt — nothing was stored; that's a valid choice, not an error, so ask whether they'd like to try again rather than treating it as a failure. Any other non-zero exit is a real failure. **Never ask for the key in chat.**

### Step 2: Detect the Domain

After storing the API key, **automatically detect the user's domain** — don't ask them for it. Call the Resend Domains API:

```bash
curl -s https://api.resend.com/domains \
  -H "Content-Type: application/json"
```

Run this with `network_mode: "proxied"` and the resend credential so the Authorization header is injected automatically. The response contains a `data` array of domain objects with `name` and `status` fields. Pick the first domain with `"status": "verified"` (or the only domain if there's just one). If no verified domains are found, tell the user they need to verify a domain in their Resend dashboard first.

Use `hi@<domain>` as the default sender address. Remember the domain for future sends.

### Step 3: Verify the Connection

Send a test request to confirm the API key works:

```bash
curl -s https://api.resend.com/domains \
  -H "Content-Type: application/json"
```

Run with `network_mode: "proxied"` and the resend credential. A successful response (HTTP 200) with domain data confirms the connection.

### Step 4: Webhook Setup (for receiving email)

If the user also wants to **receive** emails via Resend, run the webhook setup script:

```bash
bun skills/resend-setup/scripts/setup-webhook.ts --domain "<verified domain>"
```

This script:

1. Registers a callback URL via the webhooks system
2. Creates the webhook in Resend via their API
3. Automatically stores the returned signing secret in the credential vault

If the script fails because no public base URL is configured (self-hosted only), load the `public-ingress` skill to walk the user through setting one up, then retry.

### Step 5: Report Success

Summarize with the completed checklist:

"Setup complete!
✅ API key configured
✅ Domain detected: `<domain>`
✅ Connection verified
{webhook_line}

Default sender: hi@`<domain>`"

For `{webhook_line}`:

- If webhook was set up: `✅ Webhook configured for inbound email`
- If skipped: `⬜ Webhook — run setup again to enable inbound email`

## Sending Email

Use `bash` with `curl` to call the Resend API. The credential proxy injects the `Authorization: Bearer` header automatically when using `network_mode: "proxied"` with the resend credential.

```bash
curl -X POST https://api.resend.com/emails \
  -H "Content-Type: application/json" \
  -d '{
    "from": "Name <[email protected]>",
    "to": ["[email protected]"],
    "subject": "Hello",
    "text": "Plain text body",
    "html": "<p>HTML body</p>"
  }'
```

### API Parameters

| Parameter  | Type               | Required | Description                                                     |
| ---------- | ------------------ | -------- | --------------------------------------------------------------- |
| `from`     | string             | ✅       | Sender address (`"Name <email>"` format)                        |
| `to`       | string \| string[] | ✅       | Recipient(s), max 50                                            |
| `subject`  | string             | ✅       | Email subject                                                   |
| `text`     | string             |          | Plain text body                                                 |
| `html`     | string             |          | HTML body                                                       |
| `cc`       | string \| string[] |          | CC recipients                                                   |
| `bcc`      | string \| string[] |          | BCC recipients                                                  |
| `reply_to` | string \| string[] |          | Reply-to address                                                |
| `headers`  | object             |          | Custom headers (e.g. `In-Reply-To`, `References` for threading) |

### Threading (replies)

To reply in a thread, include `In-Reply-To` and `References` headers:

```json
{
  "from": "[email protected]",
  "to": ["[email protected]"],
  "subject": "Re: Original subject",
  "text": "Reply body",
  "headers": {
    "In-Reply-To": "<original-message-id>",
    "References": "<original-message-id>"
  }
}
```

### Response

Success returns `{ "id": "email-id" }` with HTTP 200.

Errors return `{ "message": "error description" }` with 4xx/5xx status.

## Important Notes

- The `from` address must be from a domain verified in the user's Resend account.
- Default sender address is `hi@<domain>` — use this unless the user specifies otherwise.
- Always confirm with the user before sending — never send without explicit permission.
- Use `text` for plain text, `html

Excerpt, first 6,001 characters of SKILL.md, verbatim from the audited commit. It is text from someone else’s repository, quoted so you can judge it before your agent reads it.

git clone https://github.com/vellum-ai/vellum-assistant
git -C vellum-assistant checkout 19bc86343b63

Works in

1. 2026-10-14
   !CAUTION B 89 / 100

   first audit · source 19bc86343b63

Questions about Resend Setup

• github.com/vellum-ai/vellum-assistant/tree/main/skills/resend-se — the audited source at commit 19bc86343b63 and every finding aboveObserved · 14 Oct 2026

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