01
coss.emailTransactional email from your own domains, tracked to every recipient.
- Single, batch and scheduled sends
- Per-recipient delivery events
- Open and click tracking
The foundation the world's apps are built on.
Email, SMS, inboxes, scheduling and more, behind one SDK and one API key. Give your coding agent one prompt and every part is wired in, so you ship the product only you can build.
Catalog
Every product runs on the same key, the same SDK and the same platform, so the next one you add needs no new setup.
02
SMS
coss.smsText messages worldwide, on the same key, events and logs as email.
- Send and receive
- Delivery events on your webhooks
- Suppressions and logs
03
Inboxes
coss.inboxesEmail addresses for your AI agents, with mail kept as threads and replies that thread.
- An address per agent, created with one call
- A signed webhook for every message
- Your own domain or coss.com's
04
Scheduling
coss.schedulingAvailability and bookings, from the team behind Cal.com.
- Availability across calendars
- Bookings and reschedules
- Booking events on your webhooks
Also on the same key
Signatures, one-time codes, forms, webhooks for your own users, risk decisions and rate limits: the same key, SDK, events and logs. Every product, and what teams build with them.
- SignE-signatures, embedded in your app or sent by email.
- VerifyOne-time codes by SMS or email, behind a fraud guard.
- FormsForm endpoints and hosted forms, screened for spam.
- HooksWebhooks you send to your own customers, with a portal.
- RiskAllow, review or block sign-ups, logins and payments.
- Rate limitingA hosted rate limiter for your own endpoints.
One vendor, not four
Every channel you add usually means another account, another SDK, another dashboard and another invoice. With coss.com, the next one is already set up. See how coss.com compares.
Four vendors
4 integrations- Four accounts and four sets of keys
- Four SDKs to learn and keep updated
- Four webhook formats to verify
- Four dashboards to search when something breaks
- Four invoices
- Start over for every new channel
With coss.com
1 integration- One account, one key per environment
- One SDK for every product
- One webhook format and signature
- One log across every product
- One bill
- The next product is already set up
Assembly takes one prompt
coss.com is written for the agent doing the work as much as for you: plain-text docs, typed results, and keys that can't send by accident.
Add coss.com to this project: read https://coss.com/llms.txt, get a development key with `npx coss-cli init --agents`, and build and test with it.
- 1
Copy one prompt
One line for Claude Code, Cursor or Codex. It points your agent at the manual and at a key you approve.
- 2
Your agent reads the manual
/llms.txt indexes every docs page, each one Markdown at its URL plus .md, the API reference with every field.
- 3
It tests with a sandbox key
coss_test_ keys record every email and event and deliver nothing, so the agent can run its code for real.
agent session
- Add coss.com to this project: read https://coss.com/llms.txt, get a development key…
- Fetch coss.com/llms.txt
- Bash npx coss-cli init
- ✔ Saved a development key to .env.local as COSS_API_KEY.
- Fetch coss.com/docs/email/sending.md
- Fetch coss.com/docs/sdk/webhooks.md
- Write src/lib/coss.ts
- Write src/app/api/webhooks/coss/route.ts
- Bash npx tsx scripts/send-test.ts
- email_01k7c8m4q2f3e9v6t5r1n0p8yz queued
- Email and signed delivery webhooks are in. The test send was recorded with your sandbox key; nothing was delivered.
Interfaces for agents
/llms.txtHow to start, and every docs page as Markdown./llms-full.txtThe whole manual in one file.OpenAPI 3.1The source of the API reference, its samples and playground.{ data, error }No thrown API errors: every failure is a value with a type and a code.Idempotency-KeySent with every POST and reused across retries, so a retried send never sends twice.Coss-VersionDated versions: what works today keeps working until you upgrade.
Know what happened to every message
Every message keeps its events. Look one up in the dashboard, ask your agent, or show it in your own admin panel.
Look it up in the dashboard
Search by address and see every event, from sent to opened.
coss.com/acme/…/emails
Welcome to Acme, JohnTo john@example.com
- sent09:41:08
- delivered09:41:12
- opened09:43:27
Ask your agent with the MCP server
coss-mcp
Did john@example.com get the onboarding email?
Yes, delivered at 09:41 and opened two minutes later.
Show it in your admin panel with the API
admin.ts
const { data } = await diagnose(coss, emailId);
// data.summary: "Delivered to john@example.com."Also from coss.com
More from the team behind coss.com, each with its own pages here.
Shipyard
Shipyard runs Claude Code, Codex and other coding agents on your own machines, each in its own worktree. Close the laptop and they keep going.
kloq
A React calendar obsessed with micro-perfection: hand-tuned drag physics, a design mode that annotates its own craft, and a command palette that reaches everything. One component, on coss ui.
Questions
- What is coss.com?
- coss.com is a developer platform: one API key and one SDK for transactional and marketing email, inboxes for AI agents, SMS, one-time codes, forms, scheduling, webhooks for your own users, and risk checks with rate limiting. It is built by the team behind Cal.com.
- How much does coss.com cost?
- You can start free with 3,000 emails a month, and sandbox keys are never billed. Paid plans turn their monthly fee into credit for usage of any coss.com product.
- Is coss.com open source?
- Yes. The platform is open source under the AGPL, and the SDK, MCP server and CLI are MIT licensed.
- Which languages and frameworks does coss.com support?
- There are SDKs for TypeScript, Python, Go, Ruby, PHP, Java and .NET, and drop-in integrations for Nodemailer, Better Auth, Auth.js, Laravel, Rails and Django. Apps that can't call an API can send email through the SMTP relay.
- Can I test without sending real messages?
- Yes. Development keys (coss_test_) work with every API and record every email and SMS with their events and webhooks, and deliver nothing.
- Can I move to coss.com from Resend, SendGrid or Postmark?
- Yes. An import brings over your domains, templates, contacts, suppressions and webhooks from any of the three.
- Does coss.com work with coding agents?
- Yes. Every docs page is Markdown at its URL plus .md, /llms.txt indexes them, npx coss-cli init gets a development key approved in the browser, and an MCP server lets agents search the docs and explain deliveries.
Fit your first part before your coffee cools.
Start free with 3,000 emails a month. Sandbox keys are never billed.