Replace a value in the cache only if the key exists.
AI agents use cache_replace 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.
This tool modifies data in the cache (write operation) but is reversible and scoped to existing keys only. The impact is limited to cached data which is typically temporary and non-critical, making it low severity. The conditional check (only if key exists) further limits unintended effects.
From the tool's definition Tool description: 'Replace a value in the cache only if the key exists.' The verb 'replace' indicates modification of existing data within a cache system.
Documented attack patterns abuse exactly the kind of access cache_replace 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 cache_replace:
{
"version": "1",
"default": "deny",
"tools": {
"cache_replace": {
"limits": [
{
"counter": "cache_replace_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}cache_replace 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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Replace a value in the cache only if the key exists. 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 cache_replace: 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.
cache_replace 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 cache_replace 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 cache_replace. 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.
cache_replace 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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805 Prometheus MCP Server tools catalogued and risk-classified — across an index of 43,000+ MCP servers.