Trim array at path to include only elements within range.
AI agents use json_arrtrim to create or update resources in Prometheus MCP Server — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Prometheus MCP Server environment.
Trimming an array modifies data by removing elements outside the specified range. This is a reversible write operation (data is modified but could potentially be restored), making it a Write category. The severity is medium as it modifies stored data and could cause data loss if misused, though the description suggests it targets a specific path and range.
From the tool's definition Trim array at path to include only elements within range
Documented attack patterns abuse exactly the kind of access json_arrtrim gives an agent:
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 json_arrtrim:
{
"version": "1",
"default": "deny",
"tools": {
"json_arrtrim": {
"limits": [
{
"counter": "json_arrtrim_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}json_arrtrim 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.
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Trim array at path to include only elements within range. 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.
Register the Prometheus MCP Server MCP server in PolicyLayer and add a rule for json_arrtrim: 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.
json_arrtrim 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 json_arrtrim 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 json_arrtrim. 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.
json_arrtrim 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.
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.
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