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

Annotations cover the safety profile (readOnly, idempotent, non-destructive, closed-world), and the description adds substantial non-obvious behavior: prices come from a fixed table scraped from official pricing pages, each row carries a check date, source URL and verified/unverified status, and only verified rows feed the 'cheapest' result. It also discloses the important caveat that no feature requirements are applied, setting expectations about what the estimate ignores.

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?

Front-loaded with the priced quantity, unit, currency and billing basis before the sourcing caveats, and the vendor enumeration is long but serves routing. The sentence about check date/URL/status is dense and could be tightened, but nothing is truly extraneous.

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?

No output schema exists, so the description carries return-shape duty; it hints at a per-vendor price list with a 'cheapest' selection and verification status but never states the actual response structure. Combined with thorough input framing and explicit sourcing caveats, it is complete enough to call correctly, with a modest gap on what comes back.

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 is 3; the description adds real meaning on top by framing contacts/emails as the list-size inputs and clarifying SMS is optional and limited to a single destination country. It doesn't restate units or the 1000-emails-per-contact cap, leaving that to the schema.

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 (estimate) and a precisely bounded resource: monthly list price in USD, pre-tax, monthly billing, for email platforms with optional SMS to one country. The vendor list makes it unambiguous what universe of products is covered, and it is clearly a computation tool rather than the metadata-oriented sibling list_vendors.

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?

Explicitly names the triggering user intents: 'how much does an email/newsletter or SMS marketing tool cost for their list size' or wanting to compare named vendors. It does not explicitly say when NOT to use it or point at list_vendors for vendor metadata, so it stops short of full alternative routing.

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