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

Adds meaningful behavior beyond the annotations: server-side refusal with no charge when a pass is already active, retry semantics via the durable client_purchase_key, and the required auth scope. Safety is not described beyond what the annotations and schema already imply, and it does not state what is returned on success. The idempotency claim is explicit-key-based rather than natural, so it does not contradict idempotentHint=false.

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 dense sentences, each carrying a distinct fact: what is bought, the refusal condition, the retry mechanism, and the required scope. Front-loaded with the purpose, with zero 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?

Covers preconditions, cost-safety via max_price_cents in the schema, auth scope, and retry behavior for a 7-parameter purchase tool with no output schema. The remaining gap is what a successful call returns (a pass record? expiry? purchase id?) and what happens when credit is insufficient, which the agent must discover by calling.

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 only 57%: public, idempotency_key, max_price_cents, and client_purchase_key are documented in the schema, but tier, authorization_id, and publication_notice_version carry no explanation anywhere. The description mentions the prepaid-credit authorization and the role of client_purchase_key but leaves the tier choice and the version constant opaque, so it does not compensate for the gaps.

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?

Opening clause names a specific verb and a precisely scoped resource ('Buy one 30-day posting pass with prepaid credit'), which is enough to distinguish it from buy_plan_period or create_credit_purchase. It does not, however, explicitly differentiate itself from the closely named sibling buy_board_post_time, which an agent could easily confuse with this tool.

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?

The description gives a real precondition ('Refused without charge while a pass is active') and a hard scope requirement ('boards:publish'), so an agent knows one condition under which the call is safe to attempt. It offers no routing guidance versus sibling tools such as get_pricing, get_offers, quote_board_post_time, or authorize_board_purchases, leaving the choice to inference.

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