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

Annotations declare destructiveHint=true and idempotentHint=true, but the description adds material context those hints cannot express: it charges the saved card, prorated, owner-only authorization, and that a 3D Secure challenge or decline returns a payment link while leaving the account unchanged. That last point is the key behavioral guarantee for a money-moving tool.

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?

Roughly four dense sentences, front-loaded with the core action and the money warning, with the procedural flow following. The telegraphic style ('MONEY:', 'Owner only.') maximizes signal per token with no 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, money-charging, multi-step tool with no output schema, the description covers the return artifacts (quote, preview_id, payment link), authorization, and the failure path well. It stops short of explaining workspace_id scoping or the interval/expected_interval distinction, so it is strong but not fully complete.

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 description coverage is 0%, so the description carries the burden and does well on the critical params: confirm, preview_id, and the expected_ guard fields are all given usage semantics (quote-first, then confirm with matching preview). However, interval and workspace_id receive no mention at all, leaving two of seven parameters unexplained anywhere.

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+resource with a hard constraint: 'Upgrade to a strictly higher plan.' This immediately distinguishes it from the sibling downgrade_subscription, cancel_subscription, pause_subscription, and resume_subscription without opening any schema.

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?

Gives an explicit two-phase protocol: call without confirm to get a quote and preview_id, then after user agreement call with confirm=true, that preview_id and the expected_ fields. The when-to-use and when-not-to-confirm conditions are spelled out, not inferred.

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