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

Beyond the annotations (readOnlyHint=false, destructiveHint=false, openWorldHint=false), it discloses that the operation mutates the user's library, that it costs nothing, and that it hard-refuses paid listings. It also reveals the divergent return behavior by type even though there is no output schema. It does not say what happens if the item is already owned or what errors look like, keeping it short of 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?

Two compact clauses, front-loaded with the primary action, and the constraint plus alternative are packed into a trailing clause with zero filler. The parenthetical tool list is terse but each token conveys routing 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?

With only 3 well-documented parameters, no output schema, and sparse annotations, the description covers cost, mutation effect, refusal behavior, and return-value divergence by type. The remaining gap is edge-case behavior (already-owned items, failure modes), which is minor for this tool's complexity.

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%, so the baseline is 3, and the description earns an extra point by explaining the behavioral consequence of the 'type' parameter: avatar adds to library, wake_word returns a link. It adds no detail on listing_id or language beyond the schema, so it is not a 5.

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 ('Adds a FREE marketplace avatar to the user's library') and covers the second mode ('returns the download link of a free wake word'). The dual behavior tied to the type enum is clear, and the tool is distinguished from buy_listing by the paid/free split.

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?

Explicitly states when to use it (free listings only), when not to (paid listings), and names the alternative: 'refuses paid listings (use buy_listing)'. It also names the follow-up tools (my_avatars / show_my_avatar / download_avatar) so the agent knows the next step in the flow.

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