Manual fallback for creating canvas and adding elements. ⚠️ IMPORTANT: Call get_guide first and follow its instructions! Use this tool only when the user's request is not a good fit for Mermaid. If the request can be represented in Mermaid, use add_elements_from_mermaid instead. If room_id is NOT...
Risk signalsHigh parameter count (28 properties)
Part of the Canvs server.
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AI agents use add_elements to create or modify resources in Canvs. Write operations carry medium risk because an autonomous agent could trigger bulk unintended modifications. Rate limits prevent a single agent session from making hundreds of changes in rapid succession. Argument validation ensures the agent passes expected values.
Without a policy, an AI agent could call add_elements repeatedly, creating or modifying resources faster than any human could review. PolicyLayer's rate limiting ensures write operations happen at a controlled pace, and argument validation catches malformed or unexpected inputs before they reach Canvs.
Write tools can modify data. A rate limit prevents runaway bulk operations from AI agents.
{
"version": "1",
"default": "deny",
"tools": {
"add_elements": {
"limits": [
{
"counter": "add_elements_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}See the full Canvs policy for all 6 tools.
These attack patterns abuse exactly the kind of access add_elements gives an agent. Each links to the full case and the policy that stops it:
Other write tools across the catalogue. The same approach applies to each: rate-limit and validate the arguments.
Manual fallback for creating canvas and adding elements. ⚠️ IMPORTANT: Call get_guide first and follow its instructions! Use this tool only when the user's request is not a good fit for Mermaid. If the request can be represented in Mermaid, use add_elements_from_mermaid instead. If room_id is NOT provided - creates a NEW canvas and returns url plus room_id. If the user did not explicitly mention an existing board/canvas/room, do NOT ask for a room_id; create a new canvas instead. If a previous Canvs tool result or assistant message in the same conversation contains a room_id, reuse it for follow-up requests like 'add to it' or 'same board'. If you only have a room URL, extract room_id from https://[host]/?room=[room_id] or https://[host]/gdrive?id=[room_id]. If the user refers to a previous board but no usable room_id is available, create a new canvas instead of asking for the URL by default. If room_id IS provided - adds elements to that canvas. If the canvas is displayed as an inline widget in the interface, do NOT include the url in your reply. If no widget is shown, share the url so the user can open the canvas.Use cases: wireframes, UI mockups, freeform illustrations, standalone shapes, and manual layouts that Mermaid does not cover well. Do not use this tool for diagrams that Mermaid can express. Element types: rectangle, ellipse, diamond, arrow, line, freedraw, text, image. Rich text: the text field supports limited HTML for text elements and rectangle/ellipse/diamond shape labels. Supports the rich-text plugin HTML subset: <b>/<strong>, <i>/<em>, <u>, <br>, <div>/<p>, <ul>/<ol>/<li>, <a href>, <font color>, and <span style="color, background/background-color, font-size, font-family">. Use textAlign for left/center/right alignment; avoid unsupported HTML/CSS. TEXT IN SHAPES: use containerId on text element pointing to shape id. ARROWS: Position at EDGE of source shape. Auto-bound within 30px. Colors: strokeColor, backgroundColor (hex).. It is categorised as a Write tool in the Canvs MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the Canvs MCP server in PolicyLayer and add a rule for add_elements: allow, deny, rate-limit, or require approval. Point your MCP client at the PolicyLayer proxy URL and the rule is enforced on every call, before it reaches Canvs. Nothing to install.
add_elements is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.
Yes. Add a rate_limit block to the add_elements rule in your PolicyLayer policy. For example, setting max: 10 and window: 60 limits the tool to 10 calls per minute. Rate limits are tracked per agent session and reset automatically.
Set action: deny in the PolicyLayer policy for add_elements. The AI agent will receive a policy violation error and cannot call the tool. You can also include a reason field to explain why the tool is blocked.
add_elements is provided by the Canvs MCP server (https://app.canvs.io/mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Deterministic rules across all 6 Canvs tools. Per-identity grants. Full audit log. Live in minutes. Nothing to install.
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