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

Annotations already cover the safe-read and open-world profile, but the description adds genuinely useful context the agent could not get from them: seller identity is never returned because Facebook hides it from logged-out visitors, only public listings are accessible, and each call costs 1 credit. That is real disclosure beyond structured data, though scope/rate behavior is otherwise unspecified.

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 short sentences, front-loaded with the identity of the tool and followed immediately by the returned fields, then the seller caveat and cost. No filler and no repetition of the title.

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?

There is no output schema, so enumerating the returned fields is necessary and done well, and the seller-identity exclusion plus credit cost round out the picture. The one hole is 'cache_max_age', undocumented anywhere, which leaves an agent guessing about a supported parameter.

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 50%: the 'url' parameter is documented in the schema, while 'cache_max_age' is undocumented in both schema and description. The description adds no parameter-level detail (accepted ID formats, cache semantics) to compensate for that gap, so it sits at the baseline.

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 resource (a single public Facebook Marketplace listing) and enumerates exactly what is returned: title, description, price, location, delivery options, photos. It is distinguishable from facebook_marketplaceSearch by the singular 'one listing' framing, though it never names the sibling explicitly.

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 phrasing 'One public Facebook Marketplace listing' implies this is a single-item detail lookup versus a search call, but there is no explicit statement of when to use this instead of facebook_marketplaceSearch or what input forms are acceptable beyond the schema. Usage is inferable, not stated.

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