What is a Required Argument?
A policy rule enforcing that a specific argument must be present in a tool call. Required argument constraints prevent tools from being called with missing critical parameters that the MCP server might otherwise silently default or ignore.
WHY IT MATTERS
MCP tool schemas define which arguments are optional and which are required. But schema-level validation and policy-level requirements are different things. A tool might technically accept a call without a reason field, but the organisation's governance policy might mandate that every write operation includes a justification.
Required argument constraints at the policy layer enforce organisational rules that go beyond schema validation. They ensure that audit-critical metadata is always present — who requested the action, why, what ticket it relates to. This metadata is essential for compliance, debugging, and forensic analysis.
For AI agents, required argument constraints also catch a common LLM failure mode: dropping optional parameters. When an LLM generates a tool call, it sometimes omits parameters it considers unimportant. A policy-level required constraint catches these omissions before they reach the server, ensuring consistent, well-formed tool calls.
HOW POLICYLAYER USES THIS
Intercept supports required argument rules in YAML policies. The required: true flag on an argument constraint ensures the field is present and non-empty in the tool call. If the argument is missing, Intercept denies the call with a clear message stating which required argument was absent. This enforcement is independent of the MCP tool's own schema validation.