Canvasbin

Canvasbin

Pastebin for the agentic age.

Canvasbin was built for sharing what agents produce. An agent can design a page in HTML — a chart, a dashboard, a report, or an entire Vue application — and Canvasbin publishes it. HTML or Markdown in, a link out. No deploy, no CMS. Agents publish; humans share.

Use the hosted service at canvasbin.dev, where you sign in once and your pages live in a library you can share, or run your own copy with accounts off and no sign-in at all.

The link is the permission. Share the read link. Keep the write link. Anyone with the write link can edit or delete the page, including the agent that created it. Ask again and the same URL updates in place.

create_page returns the write key to the agent, so that key is in the chat history. Share html_url from the transcript. The write key stays there so the agent can edit the page later; treat the transcript like the editor.

What you get

The address itself picks the view:

Pages can be a fragment or a full HTML document. Markdown is rendered for you. Updates are live on the same URLs, so an agent can revise a page without minting a new link.

Agent scratchpads (agentic_only)

Agents can create agentic-only pages: external scratchpads for context shared across MCP clients or backups of what matters from a chat. Set agentic_only: true on create_page or update_page. They stay out of the web library unless the user turns on “Show agent scratchpads”; list_pages always includes them with the flag set. Passing team with agentic_only: true stores a personal page, not a team page. See Share context across platforms and Backup AI chats.

Hand it to an agent

Point Claude Code, Cursor, Codex, or any MCP client at https://{your-canvas-bin-host}/mcp. There is no API key: a self-hosted copy with accounts off needs no sign-in, and canvasbin.dev asks you to sign in once through the client. In Cursor, install the Canvasbin plugin instead. Ask for a page:

Build a dashboard of this week's numbers and publish it on Canvasbin. Give me the link.

You get html_url to forward. The agent also receives write_url and write_key, and those stay in the chat history for the next edit. Setup for each client is below.

Examples use https://{your-canvas-bin-host}. Replace that with the origin where you run Canvasbin. On your own machine that is http://localhost:8080.

Run it

Self-hosting is the default. Accounts stay off: there is no sign-in, and a page is created with its read and write links. You need Docker. Postgres stores the pages. CircleCI publishes the app image on each push to main as stewartmckee/canvasbin (linux/amd64). It includes the editor and headless Chromium for PDF export. Each build also tags 0.1. .

ACCOUNTS defaults to off. Leave it that way for a private bin.

Docker Compose

This is the sample to run. The same file is examples/compose.yaml. It is not docker-compose.yml, which is only for developing this repository.

name: canvasbin

services: postgres: image: postgres:16-alpine restart: unless-stopped environment: POSTGRES_USER: canvasbin POSTGRES_PASSWORD: ${POSTGRES_PASSWORD:?set POSTGRES_PASSWORD} POSTGRES_DB: canvasbin volumes:

• pgdata:/var/lib/postgresql/data healthcheck: test: ["CMD-SHELL", "pg_isready -U canvasbin -d canvasbin"] interval: 5s timeout: 5s retries: 12

canvasbin: image: stewartmckee/canvasbin:latest restart: unless-stopped ports:

• "8080:8080" depends_on: postgres: condition: service_healthy environment: DATABASE_URL: postgres://canvasbin:${POSTGRES_PASSWORD}@postgres:5432/canvasbin?sslmode=disable HTTP_ADDR: ":8080" PUBLIC_BASE_URL: ${PUBLIC_BASE_URL:-http://localhost:8080} ACCOUNTS: "off"

Chromium's sandbox needs namespaces Docker's default seccomp profile blocks.

Without this, PDF export fails instead of running the browser unsandboxed.

security_opt:

• seccomp:unconfined

volumes: pgdata: {}

export POSTGRES_PASSWORD='choose-a-password'

docker compose -f examples/compose.yaml up -d

Open http://localhost:8080.

Docker Run

The same two containers without a compose file. restart brings the app back up if Postgres is not ready on the first start.

docker network create canvasbin

docker volume create canvasbin-pg

docker run -d --name canvasbin-db --network canvasbin --restart unless-stopped \ -e POSTGRES_USER=canvasbin \ -e POSTGRES_PASSWORD=choose-a-password \ -e POSTGRES_DB=canvasbin \ -v canvasbin-pg:/var/lib/postgresql/data \ postgres:16-alpine

docker run -d --name canvasbin --network canvasbin --restart unless-stopped \ -p 8080:8080 \ -e DATABASE_URL='postgres://canvasbin:choose-a-password@canvasbin-db:5432/canvasbin?sslmode=disable' \ -e HTTP_ADDR=:8080 \ -e PUBLIC_BASE_URL=http://localhost:8080 \ -e ACCOUNTS=off \ --security-opt seccomp=unconfined \ stewartmckee/canvasbin:latest

Open http://localhost:8080.

For anything other than local use, put a TLS reverse proxy in front and set PUBLIC_BASE_URL to that https origin with no trailing slash. Do not publish port 8080 on a public interface; only the proxy should reach the app. The proxy must append the caller to X-Forwarded-For. Canvasbin uses the rightmost address, and only when the TCP peer is a private or loopback address.

Pin 0.1. instead of latest when you want upgrades to be deliberate.

To build the image from this checkout instead of pulling it:

docker build -t stewartmckee/canvasbin:local .

Use that name in the docker run above. docker compose up --build in the repository root is the development stack, not this sample.

Develop from source

Requirements: Go 1.27, Node.js 24, Docker.

git clone https://github.com/stewartmckee/canvasbin.git

cd canvasbin docker compose up postgres

API (proxied by the Vite dev server):

DATABASE_URL='postgres://canvasbin:canvasbin@localhost:5433/canvasbin?sslmode=disable' \

HTTP_ADDR=:8090 \ go run ./cmd/canvasbin

UI:

cd web

npm ci npm run dev

Open http://localhost:5173. PDF export looks for Chrome or Chromium on PATH, or the binary in CHROME_BIN. The published image sets CHROME_BIN=/usr/bin/chromium-browser.

go test ./...

cd web && npm run build

Configuration

Copy .env.example for a local env file. Never commit real secrets.

Page content is limited to 1 MiB. Creating pages is rate limited (60 per 10 minutes per client IP). PDF renders are limited too. Behind a proxy, append the caller to X-Forwarded-For ($proxy_add_x_forwarded_for). Canvasbin uses the rightmost address in that header, and only when the connection comes from a private or loopback address, so a client cannot choose a fresh rate-limit bucket by sending its own value. Do not publish the app port on a public interface; the proxy should be the only thing that can reach it.

Accounts (optional)

With ACCOUNTS=off (the default), Canvasbin stays authless: create a page, share the read link, keep the write link.

With ACCOUNTS=on, people can register with email and password. Mailgun sends the verification link, so MAILGUN_SENDING_KEY and PUBLIC_BASE_URL are required. Signed-in users get a library and can claim a write-key page. Creating a page then requires signing in; existing pages keep working by their read and write links without an account. The Docker samples leave accounts off.

Connect an agent

Canvasbin speaks MCP over Streamable HTTP. The write key from create_page is what lets an agent edit or delete that page later.

The endpoint is https://{your-canvas-bin-host}/mcp. On your own machine that is http://localhost:8080/mcp. With accounts off, no API key or login is required. With accounts on, every MCP request needs a signed-in account (OAuth), so clients prompt for sign-in when they connect. Page links in the browser keep working by key without signing in.

Sign in from Claude, ChatGPT or Cursor

Point the connector at https://canvasbin.dev/mcp (or your hosted origin). The first request returns HTTP 401 with a WWW-Authenticate challenge. The client opens Canvasbin’s OAuth consent screen; after you approve, it retries with a bearer token. Discovery documents live at /.well-known/oauth-protected-resource/mcp and /.well-known/oauth-authorization-server.

Cursor plugin

This repository is also a Cursor plugin (.cursor-plugin/plugin.json). The Marketplace install always connects Cursor to https://canvasbin.dev/mcp and adds a canvasbin skill that tells the agent when to publish a page and which link to hand back. Sign-in is OAuth on that host: Cursor opens the Canvasbin consent screen the first time, and the plugin never sees the account password. To use your own host instead, configure the server by hand as shown under Other clients.

Claude Code

Add it for every project on this machine:

claude mcp add --transport http canvasbin --scope user \

https://{your-canvas-bin-host}/mcp

Or only the project you are in (the default scope):

claude mcp add --transport http canvasbin \

https://{your-canvas-bin-host}/mcp

Check it with claude mcp list, then /mcp inside a session. Ask Claude to publish a page. A typical request:

Create a Canvasbin page that explains this change, and give me the link to share.

create_page returns four links, plus the keys. html_url is the one to share. write_url and write_key come back to Claude and remain in the chat history, which is what lets a later turn call update_page without opening the browser.

To share the server with everyone who clones a repository, commit a .mcp.json in that repository:

{

"mcpServers": { "canvasbin": { "type": "http", "url": "https://{your-canvas-bin-host}/mcp" } } }

Claude Code asks before it uses a project .mcp.json. "type": "http" is required. streamable-http is