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

Annotations already declare destructiveHint=true, so the description's 'Cannot be undone after printing' adds the crucial timing detail rather than restating the flag, and it layers on the human-approval gate and the zero-price constraint. It does not cover the batch-vs-single execution path or error behavior, which keeps it off a 5.

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?

Very short and front-loaded: purpose first, then the qualification, then the alternative, then the precondition and the irreversibility warning. Every sentence carries a distinct constraint; nothing is padding.

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?

With 9 parameters, rich annotations, and an output schema, the description covers the decision-critical parts: eligibility, the bypass route, and the approval gate. Batch handling and the allow_duplicate escape hatch are only in the schema, a minor gap for an otherwise complete definition.

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 the field descriptions already explain batch_id/letter_id exclusivity, the sha256, and confirmed_price_cents. The tool description adds only the framing that the confirmed price is 0 for a resend, which the schema also states, so this is the baseline-3 case where the schema does the heavy lifting.

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 ('Mails a prepared letter by post') and immediately bounds the scope to free resends, which separates it cleanly from prepare_letter and send_card. An agent knows what this does and what it does not do without opening the 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 when-to-use (free resend via resend_of, price 0), explicit when-not ('Do not call this tool for a letter with an order_url'), an alternative action (hand the user the order_url), and a sequencing precondition (only after the user approved preview and recipient, in a later answer than prepare_letter). Nothing is left to inference.

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