Get a post by its slug or URL. Args: identifier: the slug, URL, or path fragment to search for
AI agents call get_by_slug_or_url to retrieve information from Library MCP without modifying anything — typically the context-gathering step in research, monitoring, and reporting workflows, before the agent takes action elsewhere.
This is a query/retrieval operation with no side effects. It searches and returns existing markdown content based on an identifier parameter. The blast radius of misuse is minimal—an attacker could only exfiltrate information already stored in the knowledge base, not modify or destroy it.
From the tool's definition Tool retrieves content by slug or URL from markdown knowledge base. Description explicitly states "Get a post" (retrieval), with no modification, deletion, or execution capabilities.
Documented attack patterns abuse exactly the kind of access get_by_slug_or_url gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and Library MCP, and nothing reaches the server without passing your rules. This is the rule we recommend for get_by_slug_or_url:
{
"version": "1",
"default": "deny",
"tools": {
"get_by_slug_or_url": {}
}
}get_by_slug_or_url is read-only, so it stays allowed — but everything else on the server is denied unless you say otherwise.
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Get a post by its slug or URL. Args: identifier: the slug, URL, or path fragment to search for. It is categorised as a Read tool in the Library MCP MCP Server, which means it retrieves data without modifying state.
Register the Library MCP server in PolicyLayer and add a rule for get_by_slug_or_url: 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 Library MCP. Nothing to install.
get_by_slug_or_url 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 get_by_slug_or_url 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 get_by_slug_or_url. 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.
get_by_slug_or_url is provided by the Library MCP server (lethain/library-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from Library MCP, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.
Free to start. No card required.
7 Library MCP tools catalogued and risk-classified — across an index of 43,000+ MCP servers.