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

Annotations only declare readOnlyHint=false, idempotentHint=false, destructiveHint=false; the description adds substantially more: the server never signs or holds keys, the exact return payload (Tips contract, selector, hex calldata, HBAR value), live-rate price conversion, and the atomic 98/2 seller/treasury split with no escrow. That is real behavioral context an agent needs and cannot get from annotations.

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 with the core purpose, then mechanics, then the verification hand-off; every sentence carries information. It is slightly dense with parenthetical contract IDs and rate details, but no sentence is wasted.

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 no output schema, the description compensates by enumerating what is returned (contract address, selector, hex calldata, exact HBAR value) and the signing/submission model, which is everything an agent needs to call it correctly and chain into verify_purchase.

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?

Only one parameter with 100% schema coverage, so the schema already documents listing_id and its example. The description adds peripheral meaning (the listing price is what gets converted into the HBAR value to attach) but no syntax or constraints beyond the schema, so the baseline 3 applies.

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?

The description names a specific verb and artifact ('Build the UNSIGNED buyListing calldata for a marketplace listing') and clearly separates this tool from siblings like my_purchases and verify_purchase, which are reads/confirmations rather than preparation steps.

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?

It states the workflow context clearly — prepare here, sign with your own key, then 'Verify afterward with verify_purchase' — giving both a prerequisite (you must sign and submit yourself) and the follow-up tool. It stops short of an explicit when-not-to-use or a direct comparison against other prepare_* siblings.

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