AI agents use string_increment_float 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.
The tool modifies a numeric (float) value by incrementing it, which is a write/update operation. However, the description is very sparse and does not clarify what data store or resource is being modified, lowering confidence. In the context of a Prometheus MCP server, this likely increments a metric counter or gauge value in AWS Managed Prometheus, which is a reversible write operation.
From the tool's definition Tool name: 'string_increment_float', description: 'Increment float value.'
Documented attack patterns abuse exactly the kind of access string_increment_float 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 string_increment_float:
{
"version": "1",
"default": "deny",
"tools": {
"string_increment_float": {
"limits": [
{
"counter": "string_increment_float_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}string_increment_float 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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Increment float value. 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 string_increment_float: 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.
string_increment_float 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 string_increment_float 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 string_increment_float. 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.
string_increment_float 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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