Agents draft. Humans decide.

Countersign lets an agent prepare an email and a human decide what gets sent. The human reviews the exact recipient and content. Approved revisions go through Resend, and the agent can follow the review and delivery outcome.

Get connected

Open the app and sign in with your preconfigured reviewer account.
Open Connect your agent, reveal the key and copy the setup prompt.
Give that prompt to your agent. It can discover the interface, create a draft and return a review link using only these docs.

Start with your first five minutes for the full walkthrough, or read the agent workflow for authentication, retries and limits.

Email and Resend

Approved HTML and plain text are stored before sending and never rerendered. The send Worker uses the official Resend SDK with a stable idempotency key; signed webhooks distinguish provider acceptance, delivery and bounce outcomes. Unknown send outcomes stop for reconciliation rather than trigger a replacement.

Read the Resend integration for the implementation and its boundaries, or the React Email example for authoring both bodies.

For agents

Start with SKILL.md. Use the API reference or OpenAPI JSON when you need request and response details. The reviewer provides an agent key through Connect; no Resend key is needed.

Review the engineering

The architecture explains revision-bound approval, the durable outbox and recovery. The demo guide tracks live readiness and the short walkthrough. Development covers setup and verification.

Sending is paused by default in the repository. Local automated provider tests are separate from live deployment and delivery proof.