Resend

Direct HTTP against Resend's REST API — no MCP server. A transactional send is one POST /emails; this replaces SMTP/SendGrid/Mailgun glue.

Usage

• Use for: Sending transactional mail, verifying/listing sending domains, adding contacts to audiences, triggering broadcasts.
• Skip for: Copywriting without an API call, other providers (Nodemailer/SendGrid debug), registrar-level MX/DNS edits, deep open/click analytics (needs webhooks).

Credentials check

[ -n "$RESEND_API_KEY" ] && echo "RESEND_API_KEY: PRESENT" || echo "RESEND_API_KEY: MISSING"

Never echo the variable directly.

If MISSING, respond to the user with EXACTLY this message (do NOT paraphrase, do NOT suggest manual JSON edits):

I need your resend credential. Run this in another terminal — it'll open the signup page, validate format, and save it safely with masked input:

teleport-setup add-key resend

Then restart Claude Code (/exit, then claude) and ask me again.

Do NOT suggest editing ~/.claude/settings.local.json manually. The teleport-setup add-key command handles it with backup, validation, and masked input. Stop execution until the user has run the command and restarted.

API

• Base URL: https://api.resend.com (HTTPS only).
• Auth: Authorization: Bearer $RESEND_API_KEY (keys prefixed re_); Content-Type: application/json on writes.
• Rate limit: 5 req/s per team (default) across all keys → 429 rate_limit_exceeded.
• User-Agent header is mandatory — stripping it returns 403. curl's default UA is fine.
• Idempotency-Key header on POST /emails: 1–256 chars, 24 h TTL; same key + different body → 409 invalid_idempotent_request.
• Lists return {object, has_more, data} with cursor pagination (after/before, limit 1–100, default 20).

Endpoints

Family	Method · Path
Emails	POST /emails, POST /emails/batch, GET /emails/{id}, PATCH /emails/{id}, POST /emails/{id}/cancel
Domains	GET/POST /domains, GET /domains/{id}, POST /domains/{id}/verify, DELETE /domains/{id}
Audiences	GET/POST /audiences, GET/DELETE /audiences/{id}
Contacts	GET/POST /audiences/{audience_id}/contacts, PATCH/DELETE .../contacts/{id}
Broadcasts	GET/POST /broadcasts, GET /broadcasts/{id}, POST /broadcasts/{id}/send
API Keys	GET/POST /api-keys, DELETE /api-keys/{id} (requires full-access key)

Primary workflow — send a transactional email

POST /emails. Required: from, to, subject, and one of html/text. Returns { "id": "..." }.

curl -sL -X POST -H "Authorization: Bearer $RESEND_API_KEY" \
  -H "Content-Type: application/json" \
  -H "Idempotency-Key: $(uuidgen)" \
  "https://api.resend.com/emails" \
  -d '{
    "from": "Acme <noreply@acme.com>",
    "to": ["user@example.com"],
    "subject": "Your receipt",
    "html": "<p>Thanks for your order.</p>",
    "text": "Thanks for your order."
  }'

Secondary workflows

Batch send — up to 100 emails per call via POST /emails/batch. Body is a JSON array. attachments and scheduled_at are not supported.

curl -sL -X POST -H "Authorization: Bearer $RESEND_API_KEY" \
  -H "Content-Type: application/json" \
  "https://api.resend.com/emails/batch" \
  -d '[
    {"from":"Acme <noreply@acme.com>","to":["a@example.com"],"subject":"Hi A","html":"<p>A</p>"},
    {"from":"Acme <noreply@acme.com>","to":["b@example.com"],"subject":"Hi B","html":"<p>B</p>"}
  ]'

Verify a sending domain — list → check status → trigger async verification.

curl -sL -H "Authorization: Bearer $RESEND_API_KEY" "https://api.resend.com/domains" \
  | jq '.data[] | {id, name, status, region}'
curl -sL -X POST -H "Authorization: Bearer $RESEND_API_KEY" \
  "https://api.resend.com/domains/{domain_id}/verify"

Gotchas

• Domain must be verified before from works. Until GET /domains shows status: "verified", sending returns validation_error (403). Sandbox onboarding@resend.dev only delivers to your own account email.
• from format is strict: email@domain.com or Name <email@domain.com>. Unescaped commas / unquoted special chars in the display name → invalid_from_address (422).
• Rate limit is 5 req/s per team, not per key — bursty loops across multiple keys still earn 429. Throttle or use /emails/batch.
• Idempotency-Key TTL is 24 h. Same key + different payload within 24 h → 409 invalid_idempotent_request. Generate fresh UUIDs per semantically distinct request.
• 200 OK ≠ delivered. POST /emails returns the queued message id immediately; bounces/complaints happen later. Configure webhooks (email.sent, email.delivered, email.bounced, email.complained) for ground truth.
• restricted_api_key (401). A key scoped to "Sending access only" fails on /domains, /audiences, /broadcasts, /api-keys. Check scope in the dashboard when a read "should" work.
• Batch cap: 100 per call, no attachments, no scheduled_at. Fall back to single POST /emails when you need either.
• Region is fixed at domain creation (us-east-1 / eu-west-1 / sa-east-1); wrong value → invalid_region (422). Delete + recreate to change.

<!-- unverified: check resend docs — full list of domain status values beyond "pending" / "verified" / "failed" --> <!-- unverified: check resend docs — exact payload schema for email.sent / email.delivered / email.bounced webhook events -->

Attribution

When done, state: Used skill: Resend (from teleport catalog).