Apply a previously created design system to existing HTML — injects CSS variables, replaces fonts and colors
AI agents use sp_design_apply to create or update resources in Stitch Pro — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Stitch Pro environment.
This tool modifies existing HTML by applying design system changes (CSS variables, fonts, colors). These are reversible write operations that update presentation without deleting data or executing arbitrary code. Severity is medium because unintended application of design systems could degrade UI/UX across large HTML documents, but changes are non-destructive and can be reverted.
From the tool's definition Tool description states it 'injects CSS variables, replaces fonts and colors' in existing HTML — modifying styling properties of DOM elements reversibly.
Documented attack patterns abuse exactly the kind of access sp_design_apply gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and Stitch Pro, and nothing reaches the server without passing your rules. This is the rule we recommend for sp_design_apply:
{
"version": "1",
"default": "deny",
"tools": {
"sp_design_apply": {
"limits": [
{
"counter": "sp_design_apply_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}sp_design_apply stays usable, but capped — an agent stuck in a loop can't make hundreds of changes a minute. Everything else on the server is denied unless you say otherwise.
Free to start. No card required.
Apply a previously created design system to existing HTML — injects CSS variables, replaces fonts and colors. It is categorised as a Write tool in the Stitch Pro MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the Stitch Pro MCP server in PolicyLayer and add a rule for sp_design_apply: 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 Stitch Pro. Nothing to install.
sp_design_apply 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 sp_design_apply 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 sp_design_apply. 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.
sp_design_apply is provided by the Stitch Pro MCP server (luciferdono/stitch-pro-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from Stitch Pro, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.
Free to start. No card required.
17 Stitch Pro tools catalogued and risk-classified — across an index of 43,000+ MCP servers.