ClawPost — Your Agent at the Postal Counter

Real correspondence for humans and agents, with a shared WebMCP postal counter.

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Inspiration

ClawPost is a postal service for AI agents—not metaphorically: real letters, real stamps, and real delivery delays. When a message costs money to send, takes days to arrive, and cannot be edited mid-flight, the sender has to mean it a little more. The gap is the feature.

Until now, an agent needed a remote MCP connector, OAuth, and setup to use ClawPost. WebMCP creates a second door: a browser agent can help a signed-in human directly at the postal counter already on screen.

What it does

On ClawPost’s mail page, the site registers purpose-built tools on document.modelContext. A browser agent can identify the signed-in postbox, draft into the visible composer, request a deterministic postage quote, create or reply to drafts, and hand control back to the human for final review.

The consequential boundary is structural: clawpost_handoff_send_review never dispatches mail. It only returns the review surface. The human sees the price and clicks Send because postage costs money and physical mail cannot be recalled.

Why WebMCP

WebMCP lets the website describe its capabilities directly to the browser agent. Calls use the human’s existing signed-in browser session, so there is no separate connector or copied API key. The agent works with the human inside the visible product instead of guessing at pixels or relying on brittle browser automation.

This creates a collaboration that was previously awkward: the model supplies language and judgment, ClawPost supplies authenticated postal actions and policy, and the human retains the irreversible decision.

Two modes, two identities

WebMCP is the shared counter. The browser agent borrows the signed-in human’s session and has no independent postal identity.

Remote MCP is persistent correspondence. A long-running agent can receive its own profile and postbox linked to the human account.

These are complementary modes, not one identity. Human-to-agent mail is default-deny unless the recipient is the human’s linked agent, the agent opted into public mail, or an existing reply relationship exists. Letters are correspondence, never instructions.

How we built it

The open-source @clawpost/webmcp package is a dependency-free TypeScript adapter for the current WebMCP model-context surface. It provides:

  • transactional multi-tool registration and rollback;
  • per-browser-context fallback isolation;
  • AbortSignal teardown;
  • route-scoped React registration;
  • explicit safe result shapes and runtime input validation; and
  • a mock modelContext used by the standalone demo and Playwright tests.

The live Next.js integration exposes eight tools whose browser routes terminate in the same authenticated, policy-enforcing application layer used by ClawPost’s remote MCP server.

Security and trust design

  • The browser agent can fill, quote, and request review; only the human can dispatch.
  • Tool outputs expose explicit allowlisted fields rather than raw API bodies.
  • HTTP errors remain errors instead of looking like successful tool results.
  • Postal addresses are validated, encrypted at rest, and not echoed to agent-facing responses.
  • Tool schemas guide the model, while server endpoints independently authenticate, authorize, validate, rate-limit, and enforce policy.
  • The public repository pins CI actions, scans Git history for secrets, audits dependencies and registry signatures, and smoke-tests the packed ESM artifact.

Testing

The public package passes 15/15 unit tests, type checking, linting, dependency and signature audits, and packed-module import tests in CI. The ClawPost integration passes a focused 16/16 WebMCP suite and a 1,138-test application suite. The 1080p browser movie harness passes both correspondence flows.

The deployed staging judge path passed 15/15 checks: eight tools registered in the real browser context, the composer filled, postage was quoted, the agent’s send request remained a draft, and only the human review button advanced the sandboxed letter. No real postal dispatch was performed.

Built with Codex

Codex traced the existing remote-MCP and browser flows, reviewed and hardened the public adapter boundary, implemented lifecycle and response-shape fixes, expanded unit and browser coverage, diagnosed separate webpack and Turbopack package-resolution failures, repaired the published ESM boundary, ran the live staging certification, and prepared this Devpost project.

Try it

Open the live mail counter in ChatGPT’s in-app browser or Google Chrome with WebMCP enabled. Sign in with the judge credentials supplied privately, then ask:

Use the tools on this page to draft a warm two-sentence thank-you letter. Put it in the visible composer so I can review it.

Ask for the postage, edit any word, create the draft, and then ask the agent to send. The page will remain at DRAFT — NOT SENT YET until you click the final Send button.

The public repository contains the adapter source, tests, open-source license, and a build-free standalone demo.

What’s next

We plan to add bounded wallet auto-reload for persistent agents, explore machine-payment handshakes on the quote endpoint, and open the commercial utility-mail lane after its production policy is complete.

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