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

Annotations already cover the read-only, non-destructive safety profile, and the description adds context beyond them: it discloses the comparison dimensions returned and the important caveat that results are 'figures only — which is better depends on what the client wants.' It doesn't mention rate limits, ordering, or failure behavior on bad ids, but the interpretive caveat is real added value.

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?

Front-loaded with the core action and constraint in the first clause, with the caveat closing the description. The long enumeration of compared attributes is dense but each item is substantive rather than filler.

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 must carry the return-value burden, and it does: it lists the compared fields plus the derived 'cheapest/fastest/highest authority/most visited' answers. Only minor gaps remain around error handling for invalid or duplicate ids.

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% and the single `ids` param is documented in-schema as coming from search_placements; the description only repeats that origin. The 2-4 cardinality bound is also already in the schema's minItems/maxItems, so no new parameter meaning is added.

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 (compare) plus resource (placements) and enumerates exactly what is compared — price, DA, rating, traffic, link type, turnaround, region, and derived winner picks. An agent can distinguish this from get_placement or quote_placements without opening any schema.

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 says to source ids from search_placements and constrains to two-to-four publications, giving clear operating context. It does not name when to prefer this over quote_placements or plan_placements, so the alternative-routing is left implied.

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