preview_create · Yaver MCP: Risk & Policy

WHAT IT DOES

What preview_create does on Yaver

AI agents use preview_create to create or update resources in Yaver — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Yaver environment.

PARAMETERS

Parameter	Type	Required	Description
`port`	integer	—	Port (auto-assigned if omitted)
`branch`	string	Yes
`expose`	boolean	—	Create public tunnel URL

Parameters from the server's own tool schema.

WHY IT NEEDS A POLICY

Why preview_create needs a policy

An AI agent can call preview_create faster than any human can review — one bad instruction and it creates or modifies resources in Yaver by the hundred, each call as confident as the last.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

preview_create Rate-limited

preview_create 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.

View as policy code

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "preview_create": {
      "limits": [
        {
          "counter": "preview_create_rate",
          "window": "minute",
          "max": 30,
          "scope": "grant"
        }
      ]
    }
  }
}

LIMIT THIS TOOL →

Instant setup, no code required.

PolicyLayer is an MCP gateway: every tool call is checked against your rule before it reaches Yaver, so you can run agents against it safely.

FAQ

Questions about preview_create

What does the preview_create tool do? +

Create a branch preview environment (git worktree + build + serve). It is categorised as a Write tool in the Yaver MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

What parameters does preview_create accept? +

preview_create accepts 3 parameters: port, branch, expose. Required: branch. The full parameter table on this page comes from the server's own tool schema.

How do I enforce a policy on preview_create? +

Register the Yaver MCP server in PolicyLayer and add a rule for preview_create: 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 Yaver. Nothing to install.

What risk level is preview_create? +

preview_create is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.

Can I rate-limit preview_create? +

Yes. Add a rate_limit block to the preview_create 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.

How do I block preview_create completely? +

Set action: deny in the PolicyLayer policy for preview_create. 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.

What MCP server provides preview_create? +

preview_create is provided by the Yaver MCP server (yaver-cli). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.