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

Annotations already establish read-only, idempotent, non-destructive behavior. The description adds meaningful context beyond that: ranking basis, free-tier caps, overage pricing, and importantly the provenance of prices ('curated from each provider's pricing page with a date'). This helps set expectations about data freshness and output structure.

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 sentences, no filler. The key ranking criteria are front-loaded, the data provenance is compactly stated, and the workflow hint about plan_dns_setup is a single useful clause. Every sentence earns its place.

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 tool with one optional parameter, no output schema, and read-only annotations, the description conveys what is compared, how it is ranked, relevant pricing constraints, data freshness caveat, and the logical next step. Nothing critical is missing for an agent to call it correctly.

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 single optional parameter emails_per_month is already well documented with range and default. The description's phrase 'at a monthly volume' maps to this parameter but does not add new semantic detail beyond the schema, so the baseline score is appropriate.

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 clearly identifies the resource ('services an app sends email through') and the specific action (ranked by two-year cost with free-tier caps and overage prices). It is distinct from email hosting generally, but it does not explicitly differentiate itself from the sibling 'compare_email_hosting', leaving that contrast implicit.

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 implies when to use it by focusing on transactional email like sign-in links and password resets, which gives a clear context. However, it provides no explicit exclusions or direct comparison against sibling tools, and the only alternative guidance is a follow-up recommendation to use plan_dns_setup.

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