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

Annotations already declare readOnly, idempotent, non-destructive, closed-world, but the description still adds real context beyond them: the call is free and needs no account, prices are in GBP excluding VAT, and one credit equals one complete UK record at parity across app, API and site. It does not discuss rate limits or pagination, but with an output schema present that omission is minor.

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?

The first two sentences are dense but front-loaded and well-organized around purpose, trigger and alternatives. The closing sentence on UK GDPR Article 6(1)(f) and PECR subscriber defaults is domain boilerplate that is not needed to decide whether to call get_plans, and it dilutes an otherwise tight definition.

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 zero-parameter read with a declared output schema and full annotation coverage, the description supplies everything an agent needs: what comes back, that it is free and unauthenticated, currency and tax treatment, and the sibling to use for related questions. The regulatory tail is extra rather than missing.

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 there is nothing for the description to disambiguate and the baseline applies. It does not need to explain inputs at all.

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?

The description names a specific verb (returns) and resource (plans and prices of MyDataHub by Emailmovers) and enumerates what is included: credit packs, credit rules, licence terms, bounce warranty, free trial. It also distinguishes itself from nearby tools by naming get_account and count_audience for adjacent questions.

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?

It gives an explicit trigger ('when the user asks what a list costs or which plan fits') and routes two closely related intents to named alternatives: get_account for the caller's balance, count_audience for sizing a list. Nothing about when to pick this tool is left to inference.

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