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

Annotations already declare readOnlyHint=true, destructiveHint=false and openWorldHint=true, so the safety profile is covered. The description adds genuinely useful operational context beyond that: roughly 24 listings per page, cursor must be followed as-is, dedupe by listing id, sort order is not strictly newest-first, and last_24_hours uses calendar days. However, it contains an internal contradiction on cost ('cost 1 credit' versus 'Completed lookups cost at least 10 Nexscope credits'), which can mislead an agent reasoning about spend.

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?

The core paragraph is efficient and front-loaded, but the entire description is then repeated verbatim a second time, followed by marketing boilerplate ('Facebook Marketplace Marketplace Search API View Pricing...'), roughly doubling length with zero added information and introducing the pricing inconsistency.

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?

For a 13-parameter, paginated, credit-metered tool with no output schema, the description covers pagination discipline, cost, filter applicability and field nullability well. It never sketches the response shape (listing fields an agent can rely on) beyond mentioning creation_time and listing_date_text, which is the main remaining gap given there is no output schema.

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, but the description adds cross-parameter semantics the schema cannot express: which filters are only valid in category-only mode, that condition is keyword-dependent, and that cursors are mode-bound. Those are real invocation constraints beyond per-parameter text.

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 opening sentence gives a specific verb and resource ('Search Facebook Marketplace') and clarifies the two operating modes: keyword search or keyword-free category browsing. It does not explicitly name siblings such as social_facebook_marketplace_item or social_facebook_marketplace_location_search, so an agent must infer the boundary from the 'listings' framing rather than being routed by name.

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?

Offers concrete usage conditions: lat and lng are mandatory, category_id enables query-free browsing, and condition requires a keyword. It also warns that keyword and category-only cursors cannot be mixed and that category-only filters (date_listed, availability, delivery_method) only apply to listing cards. It stops short of naming an alternative tool for retrieving a single listing or resolving a location.

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