The short answer
web_code_canvas is a Gemini capability name for an interactive web preview. Use Calculator Tools when that preview should become a public page on your account. Connect MCP and the assistant calls webappCreate.
The copied prompt does not list child functions under this name. The hosted URL is the part Gemini's preview does not keep for you.
What this call does
A public copy of a Gemini 3.8 prompt lists web_code_canvas as an API you load with retriever:expand_tools. The summary says it builds and previews a web app, page, or demo with HTML, CSS, JavaScript, or React.
The same copy shows the declaration as available, with an empty function list. This page does not invent a method name after the colon.
The preview stays in Gemini. It is the same job as Gemini Canvas: a session, not a calculator.tools host.
What Calculator Tools does
Calculator Tools stores the files and serves a public page. You stay the owner.
Files live in one directory. Typical files are index.html plus CSS, JavaScript, JSON, SVG, or similar static files.
Free accounts can publish 3 apps. Pro ($5.99/month) raises the limit to 25.
How to use MCP for this job
Gemini's preview and Calculator Tools MCP are different connections. Draft in Gemini if you are already there. Publish from an assistant that can call this host: Cursor, Claude, or ChatGPT.
Add https://api.calculator.tools/mcp/ as a custom MCP server. Sign in with Google. Click Approve.
The server exposes a tool named web_code_canvas. Pass html, and optional css and javascript. It calls webappCreate, writes index.html, and returns the public URL.
Ask for a hosted calculator, tracker, or game. The assistant calls webappCreate, then webappfileCreate for each static file. The reply includes the public URL.
You can skip the assistant and start on Create.
Limits
Calculator Tools does not run Gemini's web_code_canvas call. It does not replace Gemini search or Drive.
A hosted app is static files. It is not a React server and not a Google Cloud project. If the preview needs a backend, keep it in Gemini or use a full-stack host. For a page you can send, publish here.
Check generated numbers. Quota limits apply.
Next step
Open Create or connect MCP. The first 3 apps are free.
Sources
Ready to publish a public app you own?
Calculator Tools