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

Annotations already cover the safety profile (readOnly, idempotent, open-world), and the description adds substantial behavior on top: 15-minute cache, a request-cost model including cache hits, free failed searches, and a caveat that status is seller-set rather than a verified sale. These are billing and data-trust facts an agent cannot get from the schema or 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?

Two dense sentences front-load the purpose, then move through return fields, the status caveat, caching, and cost before the docs link. All content earns its place, though the cost and cache clauses make the sentence long and slightly crowded for a one-parameter tool.

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 the description carries the burden and does so by naming the returned fields (description, condition, posted date, coordinates, delivery, photos). Combined with the caching, cost, and status caveats, an agent has everything needed to call and interpret this 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 description coverage is 100% for the single parameter, so the schema already documents the ID-or-URL format and length bounds. The description's 'by ID or URL' merely restates that, adding no format, validation, or resolution detail beyond the schema. 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 states a specific verb and resource ('Fetch one Facebook Marketplace listing') with an explicit cardinality ('one'), which cleanly separates it from the plural sibling facebook_listings. It also enumerates the returned fields, so an agent knows exactly what it gets. No ambiguity about what the tool does.

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 identifier source is only implied: the schema says the ID comes from 'search results', and the description says to fetch 'by ID or URL', which hints at a search-then-detail flow. However, it never names facebook_listings as the discovery counterpart or states when not to use this tool. Usage is inferable but not stated.

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