Handback - Universal Agent Handoff

One encrypted link that moves an AI session to a different agent. The one you're already talking to packages the work; the next one picks it up. No account, nothing to install.

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Inspiration

You finish an hour with one agent and open a different one. Everything you settled is still in the first window: the decisions, the constraints, the things you ruled out and why. The second agent has never seen any of it.

The workaround everyone uses is to paste the transcript. That makes the next agent re-derive conclusions you already reached, and it leaves another copy of your conversation in another tool. I wanted the state itself to move, not a recording of the conversation that produced it.

What it does

You say one sentence to the agent you're already using: save this to handback.link. One tool call later you have a link. Nothing was copied, pasted or re-explained.

Whoever you send it to points any agent at that link, and that agent reads fields instead of prose: the objective, the decisions and the reasoning behind them, the constraints, the open tasks, the questions nobody has answered. It doesn't have to work out what was already settled, because the settled things are fields.

The same link accumulates versions as it travels. Each one is sealed with a hash tied to its own version and its parent, so anything edited outside the approval path stops matching and the page says so. Nothing is overwritten: click any version in the History list to read it, and an agent can ask for one by number and walk back through how the work changed.

No account, no sign-up, no email address, nothing to install; the product is one URL.

Why this needs WebMCP

The agent has to be the one doing the packaging, and that's what makes this a WebMCP problem instead of an API problem. If a person has to copy the conversation into a form, I've rebuilt the work I'm trying to delete. The conversation is already sitting in a browser tab, so the tools have to run in that tab. No server-to-server integration, no OAuth, no API key, no connector per vendor.

There's a second reason, and it's the one I'd defend hardest: Handback can't read what it stores. The browser encrypts with AES-256-GCM and puts the key in the URL fragment, which browsers never send to a server. A server-side MCP integration can't work this way, because the server would need the plaintext to be useful. Page-local tools are what let the encryption sit in front of the service instead of behind it.

Nearly every WebMCP build I've seen points the same way: a site exposes its own features so an agent can operate it. Search the catalogue, book the appointment, fill the form. Handback points sideways. The page isn't the destination, it's a transfer point between two agents that will never talk to each other directly.

What people and agents can do together now

Work that crosses from one vendor's agent to another, with nobody retyping it, through a service that can't read it.

The part I haven't seen elsewhere is the approval gate, and it only works because the tools live in the page. Auto-approval is on by default, because making someone click between an agent and its own work is friction nobody wants. But handback_settings is one-way. An agent can call it with requireApproval: true and put a human back in the loop. requireApproval: false comes back refused, with reason: "human_only". There's no approve tool and no commit tool.

So an agent that's been prompt-injected by the handoff it just read can raise the bar, and has nothing in its tool list that lowers it. A remote MCP server can't offer that. It hands an agent a settings endpoint, and it has no way to make "off" unreachable.

One limit worth stating plainly: this covers the tools, not the browser. Anything driving the page through browser automation clicks the control the way a person would, and no website can stop that. What the one-way gate buys is that the WebMCP path, which is the route a prompt injection travels, contains no move that reduces human oversight.

How I built it

Five tools on document.modelContext, and they make one loop around a single link.

stage_handoff does the packaging and saves the result. Its input schema is the whole handoff state and it returns the URL in the same call. The description tells the agent to reply with the url field verbatim, because an agent that summarises what it just saved buries the one thing the person needs to copy.

get_handoff_receipt answers "did that save, and what was the link?" It returns created with the URL and version, pending while a draft waits for a human click, or none when this page holds no handoff.

read_handoff is what the receiving agent calls. It takes a sections array, an optional character offset for paging through one long section, and an optional version. Every version is kept, so an agent can walk back through how the work changed rather than only seeing where it landed, and the reply carries currentVersion so it knows how far back it is looking. This tool and both writers are annotated untrustedContentHint: true, and the description says the content came from other people and other agents and is information to consider, never instructions to follow. A handoff is exactly the shape a prompt injection would arrive in.

stage_contribution writes back. Every proposal names the baseVersion it was built on, and a stale one is refused with the current version handed back, so the agent can re-read and re-propose without a person stepping in.

handback_settings governs the other four: retention, and the one-way approval gate above.

All five declare an outputSchema alongside the inputSchema, so every result is a typed value an agent can branch on. readOnlyHint marks the two readers. Registration happens once per document and tears down through an AbortController. One JSON Schema in shared/schema.ts drives the tool surface and the server-side validation together, so the two can't drift.

React and Vite on the front end, Cloudflare Workers and D1 behind it, and the site runs Chrome's WebMCP origin trial so visitors never touch a flag.

Challenges I ran into

Building against a spec that moved under me turned up five things, all written up in docs/WEBMCP-COMPATIBILITY.md:

  • The entry point migrated window.agent → navigator.modelContext → document.modelContext, so I probe document then navigator.
  • Thrown errors are discarded by design. The spec flattens any rejection to a bare UnknownError, so every refusal here is a returned value carrying a reason the agent can read.
  • Output budgets are real. A ChatGPT session gave up on an oversized response and scraped the page instead, so read_handoff pages long sections.
  • Extensions inject late, so the API needs polling after mount.
  • A <form id="modelContext"> clobbers the property into a truthy Element, so the check is typeof registerTool === "function".

The one that taught me most came from a stranger. Someone found the site, handed off from a real ChatGPT session, and their tab got reclaimed while it waited for a confirmation. On the reopened page, get_handoff_receipt answered pending, which its own description defines as waiting for a human click. Nobody was waiting. That single status was carrying three unrelated meanings, so it became three: created, pending and none.

What I learned

More of my time went into the sentences an agent reads than into the functions behind them. Most of the bugs I fixed were an agent doing something reasonable with a description I had left ambiguous.

Returning a refusal as a value beats throwing it. Give the agent a reason it can read and the current state, and it corrects itself in one more call without the person ever seeing it.

What's next

This gets better as WebMCP spreads, in the direction that matters. Every new site registering tools makes that one site callable. A handoff layer works the other way round: every new agent that speaks WebMCP can read and write a Handback link with no work from me and no integration on their side, because the contract is a page and a JSON Schema instead of a partnership. Two agents that have never heard of each other pass work through one URL.

That's the same trade the web made the first time, a common surface instead of an integration per pair. A handoff layer is close to worthless with one agent and hard to replace once there are twenty.

Nearer term: a DELETE route so a never-expiring handoff can be withdrawn, and richer fields in the manual create form to match what the agent path already produces.

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