What is an MCP Tool?
An MCP tool is an executable capability exposed by an MCP server, described with a name, description, and JSON Schema parameters, that AI agents can discover and invoke through the Model Context Protocol.
WHY IT MATTERS
MCP tools are the action primitives of the protocol. They represent things an agent can do — execute a shell command, query a database, create a file, send an API request. Each tool has a schema describing its inputs and expected outputs.
What makes MCP tools powerful is standardisation. A tool defined once on an MCP server is automatically available to any compatible agent. The agent's LLM sees the tool's name, description, and parameters — enough to decide when and how to use it.
Tool design matters enormously. Well-designed tools have clear names, comprehensive descriptions (the LLM reads these to decide when to use the tool), strict input validation, and informative error messages. But even well-designed tools can be misused — which is why tool-level policy enforcement matters.
HOW POLICYLAYER USES THIS
PolicyLayer enforces policies at the individual tool level. YAML policies specify which tools are allowed or denied, with optional argument constraints. For example, you can allow the execute_sql tool but deny DROP statements, or allow write_file only to specific directories. Each tool call is evaluated against the policy before reaching the server.
IN THE CATALOGUE
PolicyLayer continuously scans the MCP ecosystem and classifies every tool it finds by risk category.