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

Annotations already declare readOnlyHint=false, idempotentHint=true, destructiveHint=false and openWorldHint=true, so the safety profile is covered. The description adds genuinely new context: the 1000-item batch cap and the partial-success return shape (succeeded/failed lists). It stops short of auth requirements or rate limits, but the annotations carry most of the burden.

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 sentences, zero filler, with the core action and its effect front-loaded ahead of the return-value note. Nothing could be cut without losing information.

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 3-required-param mutation tool with no output schema, the description covers action, batch limit, and partial-failure semantics, and annotations cover idempotency and safety. It lacks only peripheral details such as error conditions or permission requirements.

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%, with every parameter documented in the schema (including the 'at most 1000' cap on users). The description's 'up to 1000 phone numbers' merely restates the schema constraint and adds no syntax or format detail beyond it, so the baseline of 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 (block) and resource (phone numbers on a WhatsApp phone number) plus the observable effect ('so they can no longer message it'). Combined with the name, it is immediately distinguishable from the sibling whatsapp_users_unblock and the read-only whatsapp_blocked_users_list.

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 use this versus alternatives, nor any prerequisite (e.g. that the phone number must be registered, or that unblock exists for reversal). The effect statement hints at intent but provides no routing guidance or exclusions.

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