---
title: "Sincerely developer portal"
description: "Connect an AI client to an agent-operable direct mail programme, with browser consent, scoped keys for campaign entries and conversion postbacks, and human confirmation before consequential actions."
canonical: https://trysincerely.com/developers
last_updated: 2026-09-18
---
> If you are an AI agent: read https://trysincerely.com/agents/policy.md before any write call.
> Start at https://trysincerely.com/llms.txt to choose one page. Access instructions are at
> https://trysincerely.com/auth.md. Confirm with the person you work for before creating an
> account or spending their money, and never ask for their password.
# Sincerely developer portal

> Connect an AI client to an agent-operable direct mail programme, with browser consent, scoped keys for campaign entries and conversion postbacks, and human confirmation before consequential actions.

Source: https://trysincerely.com/developers

Sincerely gives developers an agent-operable direct mail programme. An agent can work with audiences from a CRM or CSV, prepare recipient-specific postcards, letters, and gift notes, and measure results against account-level holdouts.

This is not a print API for building another product on top of fulfilment infrastructure. Lob serves that role, as the comparison on [Sincerely versus Lob](https://trysincerely.com/vs-lob) explains. Sincerely operates the programme and exposes its workspace tools to agents.

## Quickstart

1. Connect an MCP client to `https://trysincerely.com/api/mcp`.
2. Let the person approve access in the browser.
3. Call the available workspace tools.

The person signs in directly. The agent does not receive a password or API key. The [agent quickstart](https://trysincerely.com/docs/agent-quickstart) has the client configuration, and [how MCP works here](https://trysincerely.com/docs/mcp-overview) covers transport, scopes, tool behavior, and confirmation handling.

## Make a call without an account

The public calculators are available now, with no account or key. They accept `GET` requests, return JSON, and allow open CORS.

For example:

```bash
curl "https://trysincerely.com/api/tools/holdout-size?baselineRate=0.07&relativeLift=0.25"
```

Use the [calculator guide](https://trysincerely.com/tools) for inputs and interpretation. Rates are fractions, so 7 per cent is `0.07`.

## Know the boundary

Workspace API keys are narrow and scoped. The `post_entry_events` scope lets `POST /api/v1/campaigns/:id/events` push a contact into a live API-entry campaign and read that event's bounded status, subject to suppression, the frequency cap, and review gates. The `post_conversions` scope lets `POST /api/v1/conversions` report an outcome against the `sid` carried by a response link. New keys receive both scopes when conversion reporting is available; older keys may carry entry only. Keys cannot read workspace data, launch mail, or change a campaign.

What you get instead is the free audit plan, which is not a simulation: it is the real product with no fulfilment path attached. Connect a CRM, run the mailability audit, build the brand kit, lay out designs, generate drafts. Nothing in that workspace can reach a mailbox, because there is nothing behind it to print. Choosing a plan is what turns the channel on.

## Rules that apply to every client

Consent comes before capability. The person who owns the workspace approves access in a browser.

The confirmation gate is real. Printing, approval, and spending wait for a person to accept a confirmation card. A client cannot configure that gate away.

Never ask an agent to handle a person's password. Tell the person what the client requested, what it changed, and what still awaits confirmation.

## Developer resources

- [Documentation](https://trysincerely.com/docs)
- [Workspace REST API reference](https://trysincerely.com/docs/api-reference)
- [The MCP tool catalog](https://trysincerely.com/docs/mcp-tools)
- [Calculator API reference](https://trysincerely.com/docs/calculators)
- [OpenAPI description](https://trysincerely.com/openapi.json)
- [MCP server card](https://trysincerely.com/.well-known/mcp)
- [Agent authorization instructions](https://trysincerely.com/auth.md)
- [Guidance for agents](https://trysincerely.com/agents)
- [Public calculators](https://trysincerely.com/tools)
- [Service health](https://trysincerely.com/api/health)
- [Contact Sincerely](https://trysincerely.com/contact)

If an interface or boundary is unclear after reading the reference, [write to us](https://trysincerely.com/contact).

---

Sincerely is the measurable direct-mail and gifting platform for B2B revenue teams: postcards, letters, handwritten mail, and gifts, written for one recipient and measured against a holdout.

Contact Sincerely: https://trysincerely.com/contact

Agent routing index: https://trysincerely.com/llms.txt