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

With no annotations, the description carries the full burden and does so well: it explains that active_count means contacts receiving sends vs total_count being all memberships, names the source (list_sizes_v1, same as the app), discloses the size_ok=false null-count edge case, and flags contacts_count as a deprecated alias. This is rich semantic disclosure beyond the empty schema.

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?

Purpose is front-loaded in the first clause, followed by field semantics and caveats. Dense but every sentence adds meaning; only mild run-on tendency across the long first sentence.

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 correctly fills that gap by defining active_count, total_count, size_ok, smart-list behavior, and the deprecated alias. It is nearly complete for a zero-parameter read tool; the only missing piece is explicit usage guidance.

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?

The tool takes zero parameters, so per the rubric the baseline is 4. The description appropriately spends its budget on return-value semantics rather than parameter explanation.

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 ('Lista todas las listas de contactos') with an immediately clear scope: enumerate contact lists with their sizes. An agent can distinguish this from sibling write tools like create_list or the per-resource listers (list_campaigns, list_forms) without ambiguity.

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 description never says when to call this tool versus alternatives, nor states any prerequisites or exclusions. It mentions that smart lists aren't currently sent to, but that is a data caveat, not usage guidance. No when/when-not framing is present.

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