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

Beyond the readOnly/idempotent annotations, it discloses that results are published list prices rather than live checkout quotes, that existing supplier credits can be counted, and enumerates the returned fields (ranked packs within comparable groups, cost for this job, unconfirmed items, price-check date, seller page). The date-staleness caveat is exactly the kind of behavioral context annotations cannot carry.

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 the when-to-use triggers are front-loaded in the first two sentences, and every later sentence carries distinct information (input constraint, credit handling, return shape, caveats). It is on the long side and could tighten the returns enumeration, but there is no redundant 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?

With no output schema, the description takes on the burden of describing return values and does so concretely, including ranking groups and unconfirmed fields. Combined with the schema's full parameter documentation and the safety annotations, an agent has everything needed to invoke and interpret this tool.

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 earns extra credit by clarifying intent behind two parameters: the count-only rule for 'emails' and the fact that pre-held credits ('existing_credits') are factored into the comparison. It does not restate the goal/lang enum semantics, which the schema already handles well.

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 (compare) and resource (email verification prices), lists the exact suppliers covered, and scopes the input to a count of addresses. This clearly separates it from siblings like compare_font_licenses and compare_sound_effects_licenses.

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 explicit triggers ('when the user asks how much it costs to verify a number of emails, which service is cheapest for their list size, or how many credit packs to buy') plus an operational constraint ('Give only the number of emails, never the addresses'). It also states what the tool does not do (verify emails, buy anything), which routes the agent away from a purchase/verification sibling.

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