AI agents call nix_versions to retrieve information from Mcp Nixos without modifying anything — typically the context-gathering step in research, monitoring, and reporting workflows, before the agent takes action elsewhere.
This tool performs a read-only query operation to fetch historical version information about NixOS packages. It has no capability to modify, delete, or execute code. The blast radius of misuse is minimal—an AI agent could only retrieve potentially incorrect version information, which would not cause system harm or data loss.
From the tool's definition Tool retrieves package version history from NixHub.io. The description uses 'Get' which indicates data retrieval with no modification or execution. No mention of side effects, deletion, financial operations, or code execution.
Documented attack patterns abuse exactly the kind of access nix_versions gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and Mcp Nixos, and nothing reaches the server without passing your rules. This is the rule we recommend for nix_versions:
{
"version": "1",
"default": "deny",
"tools": {
"nix_versions": {}
}
}nix_versions is read-only, so it stays allowed — but everything else on the server is denied unless you say otherwise.
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Get package version history from NixHub.io. It is categorised as a Read tool in the Mcp Nixos MCP Server, which means it retrieves data without modifying state.
Register the Mcp Nixos MCP server in PolicyLayer and add a rule for nix_versions: 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 Mcp Nixos. Nothing to install.
nix_versions is a Read tool with low risk. Read-only tools are generally safe to allow by default.
Yes. Add a rate_limit block to the nix_versions 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 nix_versions. 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.
nix_versions is provided by the Mcp Nixos MCP server (utensils/mcp-nixos). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Deterministic rules across all 3 Mcp Nixos tools. Per-identity grants. Full audit log. Live in minutes. Nothing to install.
Free to start. No card required.
3 Mcp Nixos tools catalogued and risk-classified — across an index of 42,500+ MCP servers.