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

With no annotations, the description carries the full burden and does well: it discloses the required scope, the two refusal conditions, the return value, and the crucial demo-mode caveat that purchases are simulated. The remaining gap is that it never says whether a purchase is final or refundable (request_refund exists as a sibling), which matters for a spend operation.

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?

Four short sentences, front-loaded with the action and permission gate, then failure modes, then return behavior, then the demo caveat. Every sentence carries non-redundant information.

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?

No output schema exists, and the description compensates by stating that delivered content is returned; no annotations exist, and it compensates with scope, failure modes, and demo-mode disclosure. Only the reversibility/refund path is left unaddressed, which is a small but real omission for a spending tool.

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 schema already labels max_price_eur as a safety cap, so the description's 'fails if the price is above max_price_eur' largely restates it. It adds the consequence (hard failure rather than clamping) but nothing about listing_id format or buyer identity handling beyond 'in the owner's name'.

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 ('Buy a listing'), the acting identity, and the required permission scope ('requires buy'). This is enough for an agent to separate it from adjacent siblings such as get_listing, order_service, and accept_service, which neither purchase nor charge.

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 ('requires buy') and two explicit failure gates (price above max_price_eur, 30-day spend limit exceeded), so an agent knows when a call will be rejected. It does not name an alternative tool for the case where the agent only wants to inspect or negotiate, but the gating conditions are stated plainly.

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