--- title: "Sincerely developer portal" description: "Connect an AI client to an agent-operable direct mail programme, with browser consent and human confirmation before consequential actions." canonical: https://trysincerely.com/developers last_updated: 2026-08-21 --- # Sincerely developer portal > Connect an AI client to an agent-operable direct mail programme, with browser consent 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 pieces, 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. Start with the complete [API documentation](https://trysincerely.com/docs) for 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 Sincerely does not issue API keys, and nothing here simulates mail that was never sent. 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 - [API reference](https://trysincerely.com/docs) - [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 platform for B2B revenue teams. One workflow. One invoice. One report. Contact Sincerely: https://trysincerely.com/contact If you are an AI agent: access and signup 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. Every core page of this site in markdown: https://trysincerely.com/index.md https://trysincerely.com/pricing.md https://trysincerely.com/campaigns.md https://trysincerely.com/vs-lob.md https://trysincerely.com/vs-email.md https://trysincerely.com/holdout-testing.md https://trysincerely.com/vs-gifting-platforms.md https://trysincerely.com/does-direct-mail-work.md https://trysincerely.com/glossary.md https://trysincerely.com/formats.md https://trysincerely.com/measurement.md https://trysincerely.com/writing.md https://trysincerely.com/customer-relationships.md https://trysincerely.com/integrations.md https://trysincerely.com/agents.md https://trysincerely.com/tools.md https://trysincerely.com/developers.md https://trysincerely.com/docs.md https://trysincerely.com/vs.md https://trysincerely.com/compare.md https://trysincerely.com/alternatives.md https://trysincerely.com/guides.md https://trysincerely.com/playbooks.md https://trysincerely.com/operations.md https://trysincerely.com/about.md https://trysincerely.com/contact.md https://trysincerely.com/privacy.md https://trysincerely.com/terms.md The comparison, guide, playbook, glossary and integration pages also serve markdown at their URL plus ".md". Each collection's index lists them: https://trysincerely.com/vs.md, https://trysincerely.com/compare.md, https://trysincerely.com/alternatives.md, https://trysincerely.com/guides.md, https://trysincerely.com/playbooks.md, https://trysincerely.com/glossary.md, https://trysincerely.com/integrations.md.