API reference

Base URL https://api.sendslice.com. JSON in, JSON out.

Quickstart
Authentication
Sending
Sending domains
Errors
Endpoints
Plans

Quickstart

POST /signup with your email. You get a starter key.
Click the confirmation link. This ends the starter key.
Sign in with the same address and issue a new API key.
Design the email in the builder and save it. You get a code like bld_x7Qp2mZc.
POST /send with that code.

New accounts send from the shared address, to confirmed addresses only (up to 5), with a test banner, 50 a month. That is why the example sends to you. See Plans.

shell

# 1. create an account (returns a starter key)
curl -X POST https://api.sendslice.com/signup \
  -H 'content-type: application/json' \
  -d '{ "email": "you@example.com" }'

# 2. click the link in the confirmation email
#    (this ends the starter key)

# 3. sign in with the same address, issue a new key

# 4. save a design in the builder, copy its code

# 5. send
curl -X POST https://api.sendslice.com/send \
  -H 'authorization: Bearer ss_live_...' \
  -H 'content-type: application/json' \
  -d '{ "to": "you@example.com", "subject": "Hello",
        "code": "bld_x7Qp2mZc", "data": { "first_name": "Sam" } }'

Authentication

Send your API key on every authenticated call, in either header.

Only a hash of the key is stored, so it cannot be shown again. If you lose it, sign in and issue a new one. If it leaks, call POST /keys/rotate.

headers

authorization: Bearer ss_live_...
or
x-api-key: ss_live_...

Sending

POST /send takes the message body in one of three ways. Give exactly one.

1. `code`: a saved build

Design the email in the builder and save it, or call POST /builds. Each build has a code like bld_x7Qp2mZc that fixes the template and the look. Pass the code and the template fields in data. Editing the build changes every later send that uses its code.

curl

curl -X POST https://api.sendslice.com/send \
  -H 'authorization: Bearer ss_live_...' \
  -H 'content-type: application/json' \
  -d '{
    "to": "sam@example.com",
    "subject": "Welcome to Acme",
    "code": "bld_x7Qp2mZc",
    "data": { "first_name": "Sam" }
  }'

fetch

const res = await fetch('https://api.sendslice.com/send', {
  method: 'POST',
  headers: {
    authorization: `Bearer ${process.env.SENDSLICE_API_KEY}`,
    'content-type': 'application/json',
  },
  body: JSON.stringify({
    "to": "sam@example.com",
    "subject": "Welcome to Acme",
    "code": "bld_x7Qp2mZc",
    "data": { "first_name": "Sam" }
  }),
})
const result = await res.json()

2. `template_id`: a template with your brand kit

Name a template (welcome password-reset alert generic-message) and it is styled with the account brand kit from POST /brand-kit. Defaults apply if no brand kit is set.

curl

curl -X POST https://api.sendslice.com/send \
  -H 'authorization: Bearer ss_live_...' \
  -H 'content-type: application/json' \
  -d '{
    "to": "sam@example.com",
    "subject": "Reset your password",
    "template_id": "password-reset",
    "data": { "reset_url": "https://your.app/r/abc" }
  }'

3. `html`: your own markup

Sent as given, with a plain-text part generated from it. No template and no brand kit. data is not allowed. Accounts that carry the test banner (see Plans) get it on html sends too.

curl

curl -X POST https://api.sendslice.com/send \
  -H 'authorization: Bearer ss_live_...' \
  -H 'content-type: application/json' \
  -d '{
    "to": "sam@example.com",
    "subject": "Receipt #1042",
    "html": "<h1>Thanks</h1><p>Your order shipped.</p>"
  }'

Sending domains

Until a domain verifies, mail goes from the shared address.

Confirm your account address. Registering a domain before that returns email_not_confirmed.
POST /domains with the domain. The response lists the records.
Publish the ownership TXT record: name _sendslice.<domain>, value sendslice-verify=<token>. The token is in the response.
Publish the DKIM CNAME records from the same response.
Poll GET /domains until the status is verified. Both the TXT and the CNAMEs must resolve.

On a paid plan, a verified domain is what lets you send to anyone and removes the test banner. Without one, a paid plan has its full monthly allowance but the shared-address rules: confirmed addresses only, up to 5, with the banner. On the free plan, a verified domain changes the From address only.

Up to 3 domains per account. If a verified domain stops verifying, sends fail with domain_not_verified rather than falling back to the shared address.

dns

_sendslice.mail.your.app   TXT    "sendslice-verify=3f9c0e..."
abc123._domainkey.mail.your.app  CNAME  abc123.dkim.amazonses.com
def456._domainkey.mail.your.app  CNAME  def456.dkim.amazonses.com
ghi789._domainkey.mail.your.app  CNAME  ghi789.dkim.amazonses.com

Errors

Every error has the same shape: a stable code and a request_id to quote to support.

Quota refusals share the top-level forbidden code; branch on details.code for those. Suppression refusals have their own top-level codes.

response403

{
  "error": {
    "code": "forbidden",
    "message": "Sandbox accounts can send 50 emails a month, and the allowance resets on the 1st. ...",
    "details": { "code": "send_cap", "plan": "sandbox", "sends_allowed": 50, "window": "month" },
    "request_id": "req_8c1f..."
  }
}

Code	Status	Meaning
invalid_request	400	The body did not match the schema. `details` names the field.
unauthorized	401	Missing or unrecognised API key.
forbidden details.code: send_cap	403	The month’s allowance is spent. It resets on the 1st.
forbidden details.code: unverified_recipient	403	This account can only send to confirmed addresses, and that one is not confirmed.
forbidden details.code: recipient_cap	403	At the limit of confirmed addresses.
forbidden details.code: build_limit_reached	403	The account already has 250 saved builds.
email_not_confirmed	403	Confirm your own address before registering a domain.
domain_not_verified	403	Your sending domain is no longer verified. Nothing is sent until it is.
recipient_suppressed	403	That address bounced or complained on this account. `GET /suppressions` lists it.
recipient_suppressed_platform	403	That address bounced or complained on the shared address. Send from your own verified domain to reach it.
not_found	404	No such route or resource, including a deleted build code.
conflict	409	That resource already exists, or something else changed it first.
domain_taken	409	Another account has registered that domain.
conflict details.code: idempotency_key_in_progress	409	A request with that `Idempotency-Key` is still running. Retry with the same key after it finishes.
rate_limited	429	Too many requests. Back off and retry.
internal_error	500	Failure on our side. Quote the `request_id`.
upstream_failure	502	A service we depend on failed. Safe to retry.
service_unavailable	503	A service we depend on is unavailable, or sending is turned off. Right now this is what every `POST /send` returns while billing is being set up. Building, saving and previewing are unaffected. Do not retry in a loop; this lasts until billing opens.

Endpoints

Account

POST /signup no key

Create an account. The response carries a starter API key, shown once. Only a hash of it is stored.

The starter key stops working when you click the confirmation link. After confirming, sign in with the same address and issue a new key.

Field	Type	Description
`email`	string	Required. The account address. A confirmation link is sent to it.
`name`	string	Optional. Shown as the sender name once you send from your own domain.
`brand_kit`	object	Optional. Same fields as `POST /brand-kit`.

curl

curl -X POST https://api.sendslice.com/signup \
  -H 'content-type: application/json' \
  -d '{ "email": "you@example.com", "name": "Acme" }'

response200

{
  "customer_id": "cus_9f2c...",
  "email": "you@example.com",
  "plan": "sandbox",
  "api_key": "ss_live_...",
  "api_key_prefix": "ss_live_...a1b2",
  "sandbox": {
    "monthly_sends": 50,
    "verified_recipients": [],
    "note": "Check you@example.com and click the confirmation link. ..."
  },
  "confirmation_email_sent": true
}

GET /confirm/:customer_id/:token no key

The link in the confirmation email. The token is single-use. Opened in a browser it renders a page; requested with an Accept header that does not rank HTML first, it returns JSON.

Confirming ends every API key and sign-in issued before it, including the starter key from POST /signup. The response says so with credentials_reset: true. Sign in with the same address and issue a new key.

response200

{
  "confirmed": "you@example.com",
  "customer_id": "cus_9f2c...",
  "verified_recipients": ["you@example.com"],
  "note": "You can now send to you@example.com. Every API key and sign-in issued before this confirmation has stopped working: sign in with you@example.com and issue a new key. ...",
  "credentials_reset": true
}

POST /confirm/resend key

Send a new confirmation link to the account address. No body. Use the starter key from signup. Rate limited.

curl

curl -X POST https://api.sendslice.com/confirm/resend \
  -H 'authorization: Bearer ss_live_...'

POST /confirm/recipient key

Send a confirmation link to another address. Free accounts, and paid accounts without a verified domain, may only mail confirmed addresses, up to 5 of them. Confirm your own address first. Three requests an hour.

curl

curl -X POST https://api.sendslice.com/confirm/recipient \
  -H 'authorization: Bearer ss_live_...' \
  -H 'content-type: application/json' \
  -d '{ "email": "colleague@yourcompany.com" }'

POST /keys/rotate key

Replace your API key. Authenticate with the key being replaced; it stops working as soon as the new one is issued. No body.

curl

curl -X POST https://api.sendslice.com/keys/rotate \
  -H 'authorization: Bearer ss_live_...'

Sending

POST /send key

Send one email. Give exactly one of code, template_id or html. More than one, or none, is a 400. See Sending for how the three differ.

Field	Type	Description
`to`	string	Required. One recipient address.
`subject`	string	Required. One line, up to 998 characters.
`code`	string	A saved build code, `bld_...`. Uses that build’s template and look.
`template_id`	string	A template id, styled with your brand kit: `welcome` `password-reset` `alert` `generic-message`.
`html`	string	Your own HTML. No template, no brand kit.
`data`	object	Template fields. Only with `code` or `template_id`.
`reply_to`	string	Optional reply-to address.

Optional header: Idempotency-Key, 1-128 characters of letters, digits, ., _ or -. A retry with the same key within 24 hours returns the original result instead of sending again. A second call with the same key while the first is still running gets a 409.

curl

curl -X POST https://api.sendslice.com/send \
  -H 'authorization: Bearer ss_live_...' \
  -H 'idempotency-key: welcome-sam-1' \
  -H 'content-type: application/json' \
  -d '{
    "to": "sam@example.com",
    "subject": "Welcome to Acme",
    "code": "bld_x7Qp2mZc",
    "data": { "first_name": "Sam" }
  }'

fetch

const res = await fetch('https://api.sendslice.com/send', {
  method: 'POST',
  headers: {
    authorization: `Bearer ${process.env.SENDSLICE_API_KEY}`,
    'idempotency-key': 'welcome-sam-1',
    'content-type': 'application/json',
  },
  body: JSON.stringify({
    "to": "sam@example.com",
    "subject": "Welcome to Acme",
    "code": "bld_x7Qp2mZc",
    "data": { "first_name": "Sam" }
  }),
})
const result = await res.json()

response202

{
  "send_id": "snd_4a81...",
  "message_id": "0100018f...",
  "status": "sent",
  "to": "sam@example.com",
  "subject": "Welcome to Acme",
  "template_used": "bld_x7Qp2mZc",
  "sent_at": "2026-10-12T14:03:22.418Z"
}

GET /sends key

Your send log, newest first: recipient, subject, template or code, time, status and any error. Refused sends are listed as failed with the refusal code. Message bodies are not stored.

Query: limit (1-200, default 50), since (an ISO time), cursor (the next_cursor from the previous page).

curl

curl -X GET https://api.sendslice.com/sends?limit=50 \
  -H 'authorization: Bearer ss_live_...'

Builds

POST /builds key

Save a design and get its code. The builder calls this when you press save; you can call it directly. template_id is required; the look fields are the same as POST /brand-kit.

Up to 250 builds per account.

curl

curl -X POST https://api.sendslice.com/builds \
  -H 'authorization: Bearer ss_live_...' \
  -H 'content-type: application/json' \
  -d '{
    "name": "Welcome, pecan",
    "template_id": "welcome",
    "flavor": "pecan",
    "logo_url": "https://your.app/logo.png"
  }'

response201

{
  "build": {
    "code": "bld_x7Qp2mZc",
    "name": "Welcome, pecan",
    "template_id": "welcome",
    "brand_kit": { "flavor": "pecan", "logo_url": "https://your.app/logo.png" },
    "created_at": "2026-10-12T14:00:00.000Z",
    "updated_at": "2026-10-12T14:00:00.000Z"
  }
}

GET /builds key

Every saved build on the account, newest first.

curl

curl -X GET https://api.sendslice.com/builds \
  -H 'authorization: Bearer ss_live_...'

GET /builds/:code key

One saved build.

curl

curl -X GET https://api.sendslice.com/builds/bld_x7Qp2mZc \
  -H 'authorization: Bearer ss_live_...'

PUT /builds/:code key

Replace a build. The code stays the same, so sends using it pick up the new look straight away. Every field is replaced; nothing is carried over.

curl

curl -X PUT https://api.sendslice.com/builds/bld_x7Qp2mZc \
  -H 'authorization: Bearer ss_live_...' \
  -H 'content-type: application/json' \
  -d '{ "template_id": "welcome", "flavor": "cherry" }'

DELETE /builds/:code key

Delete a build. Returns 204. Sends using its code fail with 404 from then on; a deleted code is never reused.

curl

curl -X DELETE https://api.sendslice.com/builds/bld_x7Qp2mZc \
  -H 'authorization: Bearer ss_live_...'

Brand kit

POST /brand-kit key

Set the account’s brand kit, used by sends with template_id. Sends with code use the build’s own look instead.

A flavour sets every field at once; any field you also pass overrides it. Flavours: blueberry apple cherry pecan key-lime lemon-meringue pumpkin blackberry grasshopper coconut-cream rhubarb chocolate-cream. Or try them in the builder.

curl

curl -X POST https://api.sendslice.com/brand-kit \
  -H 'authorization: Bearer ss_live_...' \
  -H 'content-type: application/json' \
  -d '{ "flavor": "pecan", "logo_url": "https://your.app/logo.png" }'

GET /brand-kit key

The saved brand kit, or the defaults if none was set.

Domains

POST /domains key

Register a sending domain and get the DNS records to publish. Your own address must be confirmed first (email_not_confirmed otherwise). 201 for a new registration, 200 when the domain is already on your account. Up to 3 domains per account.

The first record is the ownership TXT; the rest are DKIM CNAMEs. Publish all of them, then poll GET /domains. See Sending domains.

curl

curl -X POST https://api.sendslice.com/domains \
  -H 'authorization: Bearer ss_live_...' \
  -H 'content-type: application/json' \
  -d '{ "domain": "mail.your.app" }'

fetch

const res = await fetch('https://api.sendslice.com/domains', {
  method: 'POST',
  headers: {
    authorization: `Bearer ${process.env.SENDSLICE_API_KEY}`,
    'content-type': 'application/json',
  },
  body: JSON.stringify({ domain: 'mail.your.app' }),
})
const result = await res.json()

response201

{
  "domain": {
    "domain": "mail.your.app",
    "status": "pending",
    "sends_from": "no-reply@mail.your.app",
    "created_at": "2026-10-12T14:00:00.000Z",
    "dns_records": [
      { "type": "TXT",
        "name": "_sendslice.mail.your.app",
        "value": "sendslice-verify=3f9c0e..." },
      { "type": "CNAME",
        "name": "abc123._domainkey.mail.your.app",
        "value": "abc123.dkim.amazonses.com" },
      ...two more CNAMEs
    ]
  },
  "next": "Publish these records (the TXT and the CNAMEs), then poll GET /domains. ..."
}

GET /domains key

Your domains, each re-checked on read. A domain is verified once the DKIM records resolve and the ownership TXT carries its token.

curl

curl -X GET https://api.sendslice.com/domains \
  -H 'authorization: Bearer ss_live_...'

DELETE /domains/:domain key

Release a domain and free its slot. Returns 204; a repeat call returns 404. If it was your sending domain, sends go from the shared address again.

curl

curl -X DELETE https://api.sendslice.com/domains/mail.your.app \
  -H 'authorization: Bearer ss_live_...'

Suppressions

GET /suppressions key

Addresses that hard-bounced or marked your mail as spam. Sends to them are refused with recipient_suppressed.

Reference data

GET /templates no key

The templates and the fields each takes: welcome password-reset alert generic-message.

GET /brand-options no key

Every value a brand kit accepts: flavours, corner styles, border styles, font pairings.

GET /health no key

Liveness and the deployed revision. /ready also checks storage.

Plans

Every plan is metered by calendar month and resets on the 1st. A send past the allowance is refused, not billed.

Plan	Per month	Sends	No verified domain	Verified domain
Whiff	Free	50	From the shared address, to confirmed addresses only (max 5), test banner	From your domain, to confirmed addresses only (max 5), test banner
Taste	$9	5,000	From the shared address, to confirmed addresses only (max 5), test banner	From your domain, to anyone, no banner
Slice	$19	25,000	From the shared address, to confirmed addresses only (max 5), test banner	From your domain, to anyone, no banner
Whole pie	$99	250,000	From the shared address, to confirmed addresses only (max 5), test banner	From your domain, to anyone, no banner

API reference

Quickstart

Authentication

Sending

1. code: a saved build

2. template_id: a template with your brand kit

3. html: your own markup

Sending domains

Errors

Endpoints

Account

Sending

Builds

Brand kit

Domains

Suppressions

Reference data

Plans

1. `code`: a saved build

2. `template_id`: a template with your brand kit

3. `html`: your own markup