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

With readOnlyHint/openWorldHint already declaring the safety profile, the description adds two genuinely useful facts not in the annotations: how to interpret a null result ("A rank of 0 means the app is not in the results") and the cost ("Costs 9 credits"). It omits failure behavior and whether cost scales with keyword count, which keeps it from a 5.

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, each carrying distinct load: what it does, when to prefer it, and the two facts an agent would otherwise get wrong (zero-rank semantics, credit cost). No 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, and the description covers the single most important return-value ambiguity (rank 0 = absent) plus cost. It stops short of describing the per-keyword result shape or ordering, a minor gap for a batch 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% and every parameter (store enum, ISO country, package id, keyword array with 50-item cap) is already documented in the schema. The description adds no syntax or constraint detail beyond restating that keywords come as a list, 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+resource+scope ("One app's rank for a list of keywords in a single call") and explicitly frames itself as the batch counterpart to keyword_app_metrics_rankings. An agent can separate it from the single-keyword, history, and worldwide siblings without opening a 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?

"Use it to check a whole keyword set at once instead of calling keyword_app_metrics_rankings repeatedly" gives an explicit selection rule against the closest alternative. It does not address the other relevant siblings (keyword_app_metrics_rankings_history, _worldwide), so the guidance is clear but not exhaustive.

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