hash_set_if_not_exists

THE RISK

Medium Risk

Why hash_set_if_not_exists needs a policy

The tool creates or modifies data (setting a hash field) in a reversible manner. It does not execute arbitrary code, delete data irreversibly, or perform financial operations. The conditional 'if not exists' logic makes it a safe write operation with minimal blast radius, as it only affects a single field and respects existing data.

From the tool's definition Tool name and description: 'Set field in hash only if it does not exist' — this performs a conditional field assignment operation on a hash data structure, which is a reversible create/modify action.

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

POLICY

How to control hash_set_if_not_exists

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

policy.json

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

hash_set_if_not_exists 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 Prometheus 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.

LIMIT THIS TOOL →

Free to start. No card required.

EXPLORE

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All 805 Prometheus MCP Server tools →

Write tools on other servers

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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 hash_set_if_not_exists

What does the hash_set_if_not_exists tool do? +

Set field in hash only if it does not exist. It is categorised as a Write tool in the Prometheus 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 hash_set_if_not_exists? +

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

What risk level is hash_set_if_not_exists? +

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

Can I rate-limit hash_set_if_not_exists? +

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

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

hash_set_if_not_exists is provided by the Prometheus MCP Server MCP server (awslabs.prometheus-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 Prometheus MCP Server tool call.

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

CHECK YOUR STACK →