Countersign

Agents prepare emails, humans review them, and Resend sends the approved revision. Approval covers the exact recipients and content.

Live app · Agent docs

Try it

Reviewer credentials are shared privately. A private seven-day access link can also sign you in; keep it separate from the agent setup prompt.

1. Sign in, open Connect your agent and copy the setup prompt into your agent.
2. Ask: “Prepare a short onboarding follow-up email to delivered@resend.dev, submit it for approval and return the review link.”
3. Review the recipient and content. Reject with feedback, or use Edit email to save your own revision.
4. Approve it and follow Sending status and Delivery events. These refresh every five seconds while the page is visible and you are not editing.

Edits need approval before sending. Sent means Resend accepted the request; delivered means the receiving mail server accepted it. delivered@resend.dev is a Resend test address.

Agent API

The setup prompt includes a scoped bearer key. Agents can create, retrieve, revise and submit emails, then check review and sending status. They cannot approve or send directly.

Endpoint	Purpose
/SKILL.md	Workflow, authentication and retry guidance
/openapi.json	API schemas and examples
/onboarding.md	First email walkthrough
/v1/me	Authenticated identity, sender and allowed recipients

Use curl or an identified user agent such as CountersignAgent/1.0; the deployed zone rejects Python urllib’s default user agent. No MCP installation is required.

Architecture

TypeScript, Hono, Drizzle and Cloudflare Workers, D1 and Queues. The review UI uses React, shadcn/ReUI and React Email Editor.

flowchart LR
    Agent -->|create, revise, submit| Web[API and review app]
    Human -->|approve revision and hash| Web
    Web --> DB[(D1: revisions and outbox)]
    Web --> Queue
    Queue --> Sender[Send Worker]
    Sender --> DB
    Sender --> Resend
    Resend -->|signed webhooks| Web

Each edit creates an immutable revision. Approval checks its ID and a canonical hash of the sender, recipient, subject, HTML and text. Stale reviews fail, and approved revisions are locked.

draft → in_review → approved → sending → sent → delivered

Rejected emails can be revised and resubmitted. Bounce and failure outcomes remain visible. The HTML preview is sandboxed; the editor saves a new revision, and the sender never rerenders approved content.

Resend and retries

• Only the sender Worker holds the domain-restricted, sending-only Resend key.
• Approval freezes the request and creates one send intent per revision. The official Resend SDK sends that stored payload with a stable idempotency key across retries.
• Database constraints prevent duplicate intents, leases coordinate workers, and Cron recovers missed queue work. D1 is the source of truth.
• Retries use bounded backoff within a 23-hour window. Unresolved outcomes stop for reconciliation before risking a send beyond the provider’s idempotency window.
• Signed webhooks are deduplicated and correlated with send intents. Late events cannot overwrite stronger delivery evidence; complaints and delays are recorded separately.

Run locally

Requires Node 22.22+ and pnpm 9.

pnpm install --frozen-lockfile
cp apps/web/.dev.vars.example .dev.vars
cp apps/send/.dev.vars.example apps/send/.dev.vars

Set AGENT_KEY_ENCRYPTION_KEY in .dev.vars to a private random 32-byte hex value. Leave SEND_ENABLED=false for development. For live testing, set RESEND_WEBHOOK_SECRET in the root .dev.vars and RESEND_API_KEY in the sender’s .dev.vars.

pnpm types
pnpm db:migrate
pnpm accounts create < /path/to/private-account.json
pnpm dev

The provisioning JSON requires email, name, password, recipient and agentName. Use a strong password and delivered@resend.dev for the test recipient. Sign in to reveal the linked agent’s setup prompt. Keep provisioning files and credentials out of Git.

App: http://localhost:8787. Docs: http://localhost:4321.

For local access links, run pnpm accounts link < /path/to/private-link-input.json with reviewer_id, origin and an output path inside .wrangler/. The command saves a private file containing the link, expiry and token hash. pnpm accounts revoke-link accepts reviewer_id and token_hash on stdin and revokes the link’s sessions too. Password resets also revoke access links.

Deploy your own instance

Configure your Cloudflare account, D1 database ID, queues, domains and sender in the Wrangler configs. Apply migrations and register /webhooks/resend in Resend.

Worker	Secrets
Web	AGENT_KEY_ENCRYPTION_KEY, RESEND_WEBHOOK_SECRET
Sender	RESEND_API_KEY
Docs	None

Use the same sender in web and sender configs. Set DOCS_URL, COUNTERSIGN_APP_URL and COUNTERSIGN_DOCS_URL to your origins. Enable dispatch after domain verification, recipient policy and webhooks are configured.

Tests

pnpm check          # formatting, contracts, types, tests and builds
pnpm test:browser   # review, editing, preview isolation, polling and docs

Tests cover ownership, stale revisions, concurrent approvals, duplicate requests, retries, uncertain provider outcomes and webhook handling. Automated provider tests use simulated responses. Separate live tests verified the rejection/revision flow, approval and Resend delivery using an agent given only the setup prompt and public docs.

Scope

One reviewer and linked agent per account, one recipient per email, a fixed sender and preconfigured credentials. Campaigns, segments, attachments, scheduling, inbound mail and account management are outside scope. Five-second polling keeps status updates simple and pauses while hidden or editing.