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

With no annotations, the description carries the safety burden and does so: 'Read-only, free' covers both the read-only and non-destructive/non-billing traits. It also discloses the pricing model (2 credits per contact) needed to interpret the result, though it says nothing about behavior on an unknown slug.

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 sentences, front-loaded with what is returned, then the pricing rationale, then the safety note. No filler, though the 'marketplace set' vs 'audience set' wording is a slight terminology wobble.

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?

No output schema exists, so the description usefully previews the return shape (size, description, freshness). For a single-parameter read tool this is nearly complete; only error behavior for a bad slug is unaddressed.

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% and the schema itself clarifies 'Human-readable slug, not a UUID'. The description only restates 'by slug' and adds no format, casing, or lookup semantics 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?

States a specific verb and resource ('Details of one marketplace set by slug') and enumerates the returned fields (size, description, freshness). The word 'one' plus the slug key distinguishes it cleanly from list_audience_sets and get_audience_set_contacts.

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?

Gives a concrete use case — 'Use the contact count here to work out the purchase cost' — which implicitly situates it as the pre-purchase inspection step ahead of buy_audience_set, and 'Read-only, free' signals when it is safe to call. It never names the alternative (preview_buy_audience_set, buy_audience_set) explicitly, so it stays short of a full when/when-not statement.

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