set_active_fields

THE RISK

Medium Risk

Why set_active_fields needs a policy

This tool modifies the visibility configuration of fields in the FiftyOne application interface. While it doesn't alter underlying data permanently, it changes application state in a reversible manner (visibility settings can be toggled back). This is a Write operation rather than Read (which would only query visibility), and less severe than Destructive/Execute since it affects UI state only.

From the tool's definition Tool name 'set_active_fields' and description 'Control which fields are visible in the App' indicates modification of application state/configuration regarding field visibility.

Documented attack patterns abuse exactly the kind of access set_active_fields gives an agent:

POLICY

How to control set_active_fields

PolicyLayer is an MCP gateway — it sits between your AI agents and FiftyOne MCP Server, and nothing reaches the server without passing your rules. This is the rule we recommend for set_active_fields:

policy.json

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

set_active_fields 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.

Create a free account and register FiftyOne MCP Server — nothing to install.
Add this policy — paste it, or build it visually.
Point your MCP client (Claude, Cursor, anything) at your gateway URL.

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EXPLORE

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Go deeper

The MCP Attack Database → Documented attack patterns against MCP deployments — and the policies that stop them.
Argument validation → Checking tool call arguments against schema and policy before they run.
Scoped token → Per-person credentials that grant a defined subset of servers and tools.
Rate Limiting MCP Tool Calls: A Practical Guide →
MCP Security: Why Prompt Guardrails Aren't Enough →

FAQ

Questions about set_active_fields

What does the set_active_fields tool do? +

Control which fields are visible in the App. It is categorised as a Write tool in the FiftyOne MCP Server MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on set_active_fields? +

Register the FiftyOne MCP Server MCP server in PolicyLayer and add a rule for set_active_fields: 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 FiftyOne MCP Server. Nothing to install.

What risk level is set_active_fields? +

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

Can I rate-limit set_active_fields? +

Yes. Add a rate_limit block to the set_active_fields 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 set_active_fields completely? +

Set action: deny in the PolicyLayer policy for set_active_fields. 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 set_active_fields? +

set_active_fields is provided by the FiftyOne MCP Server MCP server (voxel51/fiftyone-mcp-server). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.

Enforce policy on every FiftyOne MCP Server tool call.

Start from FiftyOne MCP Server, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.

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