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

Adds substantial behavior beyond the annotations: discloses the required billing:write scope, the default period-end semantics for active/trialing plans, and the important edge case that past_due/unpaid plans end immediately regardless of immediate=false (unpaid renewal dropped, retries stop). This is exactly the kind of consequence detail an agent cannot infer from destructiveHint alone.

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?

Front-loaded with the action and the DESTRUCTIVE/scope warning, then the required workflow, then the edge-case semantics. It is fairly long but nearly every sentence carries distinct operational information; minor tightening is possible but nothing is filler.

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 destructive mutation with no output schema, the description covers permissions, prerequisites, default vs immediate behavior, and the anomalous past_due/unpaid path. An agent has everything needed to call it correctly and warn the user appropriately.

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?

Schema coverage is 100%, so baseline is 3, but the description adds real meaning: it explains that reason must be captured from the user, that immediate is ignored for past_due/unpaid, and that confirm gates the call. It exceeds the schema rather than restating it.

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?

States a specific verb and resource: 'Cancel the authenticated user's posterly subscription.' An agent immediately understands the scope, and it is clearly differentiable from pause_subscription and downgrade_subscription by the plain 'cancel' semantics.

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?

Extensive when/when-not guidance: ask the reason FIRST, only pass immediate=true when the user explicitly wants an active plan cancelled now, only call after explicit confirmation. It does not name sibling alternatives (pause_subscription, downgrade_subscription) as substitutes, so it falls just 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.