Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

The description makes the write action explicit and discloses an authorization requirement ('finance.create') that is not present in the annotations. The annotations already indicate readOnlyHint=false and destructiveHint=false, and the description does not contradict them. It does not mention response shape or idempotency, but the annotation coverage lowers the burden.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

The description is two dense sentences with no filler. It front-loads the action and purpose, then gives the key parameter semantics and the permission requirement. Every sentence earns its place.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a create-style tool, the description covers the core business semantics, the distinguishing parameter behavior, and the required permission. The schema handles field formats and defaults for due_date and status. Minor gaps are the absence of return-value details and sort_order semantics, but an agent has enough to invoke this tool correctly.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

The description clarifies the conditional relationship between amount_type and amount_value, giving useful meaning to the core fields. However, the schema already documents most of this behavior, and the description does not explain project_id, sort_order, or description. With only 57% schema coverage, the description adds some value but does not fully compensate for the undocumented parameters.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

The description states a specific action ('Add') and a specific resource ('a fixed-fee billing milestone to a project's billing plan'), and explains its business purpose: enabling a fixed-price project to be invoiced in instalments. The verb 'Add' clearly differentiates it from sibling tools that update, delete, or retrieve billing milestones.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

The description gives clear context for when this tool is appropriate: adding instalment milestones to a fixed-fee project's billing plan. It also states a required permission ('finance.create'). It does not explicitly name alternatives or say when not to use it, so it stops short of a 5.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.