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

Annotations only say this is a non-read-only, idempotent, non-destructive, closed-world write. The description adds real behavioral context beyond that: the entry is key-scoped so every agent and tool using the key is affected, a Precleared key is required (and a free one suffices), there is no charge, and the stored address is retained only as a hash plus a masked copy. That is exactly the kind of side-effect and privacy detail annotations cannot convey.

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 and effect are front-loaded in the first sentence, and every subsequent sentence carries operational information. The billing note is split across two short sentences ('Needs a Precleared key (a free one is fine). Never charged.') and could be merged, but there is 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?

For a 4-parameter write tool with no output schema, the description covers prerequisites, scope of effect, storage/privacy behavior, and when to invoke. It does not say what a successful call returns (e.g., the masked address or confirmation), which is the only meaningful remaining gap.

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%, so the baseline would be 3, but the description adds meaning the schema does not: email and domain are alternatives ('Give email or domain'), and remove carries a policy condition rather than just 'true to take the entry off the list'. The reason enum's intended triggers are only loosely implied via the opt-out/bounce/complaint phrasing, keeping it off a 5.

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 ('Records that a person asked not to be emailed... on this Precleared key's opt-out list') and covers both directions (add, and remove via the remove flag). It also names the downstream consumer, contact_check, so an agent can place it relative to the sibling tools without opening the 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?

Gives concrete triggering events ('as soon as someone replies stop, unsubscribes, bounces or complains') and an explicit guard on the inverse operation ('set remove to true only when the person asks to be emailed again'). The email-or-domain choice is also stated as a calling instruction rather than left to inference.

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