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

With no annotations, the description carries the full behavioral burden. It usefully implies this is a side-effect-free estimate that precedes billing, but it never states whether the estimate call itself consumes credits, what permissions are needed, or how the result should be interpreted. This leaves real gaps for a billing-adjacent tool.

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, front-loaded with the purpose and closing on the action directive, with no filler. Minor redundancy in re-listing the input modes that the schema already enumerates keeps it out of the top band.

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?

The description covers the three input modes and the when-to-use trigger, which is solid for a parameterized tool. However, with no output schema and no annotations, it never says what the estimate returns (a number? a boolean affordability flag?) or its units beyond the schema's credit hints, so an agent lacks a complete picture of the response.

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 description coverage is 100%, so the schema already documents emails (1 credit each), list_id, and the operation enum with its per-type credit rates. The description's restatement of the three mutually exclusive input modes adds only selection guidance, not new parameter meaning. Baseline 3 applies.

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 ('estimate the credit cost of a verification') plus a secondary outcome ('whether the user can afford it'). The word 'before running it' implicitly contrasts this with the sibling tools that actually perform verification (verify_email, verify_batch), letting an agent tell them apart without opening schemas.

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?

'Always quote this before proposing anything billable' is an explicit, actionable trigger that tells the agent exactly when to reach for this tool. It stops short of a 5 because it never names a specific alternative or a when-not condition (e.g., don't call this before a free read like get_credit_balance).

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