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

Annotations already declare destructive, non-idempotent and open-world, but the description adds value beyond them by disclosing that the action is irreversible and that it charges the user's account – cost context the annotations do not convey. It stops short of stating atomicity (partial-batch failure) or volume/rate implications.

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?

Three short sentences, front-loaded with the action, then the risk (irreversible, charges account), then the prerequisites. No filler; every clause carries information an agent needs.

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?

Covers the essentials for a destructive, cost-incurring send: prerequisite token, approval gate, irreversibility and billing impact. Minor gaps remain around failure/partial-send behavior and whether the token expires, but there is no output schema to explain.

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% and both parameters carry detailed descriptions (including the 'never assume approval' constraint on confirm), so the description's mention of confirmation_token and confirm=true largely restates the schema. Baseline 3 is appropriate.

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 ('Mail the letters') and pins the scope to letters already approved in spur_preview_letter_campaign, which cleanly separates it from the preview tool and from the sibling senders spur_send_postcards and spur_send_sms.

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 clear preconditions: a confirmation_token from the preview step and explicit user approval with confirm=true. It does not explicitly name the sibling alternatives for postcards/SMS or say what to do if the preview is stale, so it stops short of full when/when-not guidance.

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