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

Annotations declare destructive=true and non-idempotent, and the description backs that up with concrete consequences the annotations cannot express: immediate switch, prorated charge or credit to the saved card, and a required two-call confirm handshake. It also explains what the free-plan path does differently (billing-portal link rather than an in-place change).

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 verb-and-resource sentence followed by three bullets, one per branch, each of which carries a distinct decision. No restatement of the title and 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 billing mutation with no output schema, the description covers everything an agent needs: where ids come from, what is returned in each case, what is charged, and when confirmation is required. Nothing material is left to inference.

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 would be 3; the description earns extra by explaining the two-phase use of 'confirm' (call once without to see the exact proration, then again with confirm=true after user approval) and by naming valid plan ids. That is workflow meaning the schema's per-field text does not convey.

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 ('Moves the user to another Spur plan') and immediately names the sibling that supplies plan ids (spur_show_billing_options). An agent can distinguish this from spur_manage_billing or spur_show_billing_options 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 explicit branching conditions covering the three real states (no paid subscription, already subscribed, moving to free Solo) and the exact confirm protocol for the destructive path. It also routes to the correct prerequisite tool for obtaining plan ids.

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