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

Substantially exceeds the annotations: it discloses the credit/per-verdict billing model, refund of unspent credits, a hard spend cap, and idempotent retry with no double charge. The annotations already declare idempotentHint=true, so part of that is redundant. It still never describes the response shape or how partial verdicts are returned, and there is no output schema to compensate.

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-loads the action and the cost model, then preconditions, then the REST mapping; each block has a purpose. Some redundancy with the schema ('the most credits this call may spend, never exceeded' appears in both) costs it a point. The REST endpoint sentence is optional but cheap and useful.

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 paid, mutating, credit-consuming call with no output schema, the description covers cost, safety gating and retry but says nothing about what comes back — no result shape, no indication of how to read per-address verdicts or what to do when some addresses are skipped. An agent could call it correctly but not interpret the result from the definition alone.

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 baseline is 3, but the description adds genuine meaning by tying maxCredits to a concrete upstream value (progress.emailCheck.unverified of get_list) — something the schema does not say. The idempotencyKey retry contract is also restated in operational terms. This is meaningful value beyond the schema, though some phrasing is repeated verbatim.

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 ('Verify the email addresses of a ready list') and enumerates the outcome space (deliverable, risky, undeliverable), which tells an agent what it produces. It also maps itself to a concrete REST endpoint. It does not explicitly contrast itself with siblings like complete_list or get_list, so differentiation is only implied by the 'ready list' precondition.

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?

Gives real preconditions and routing: only for a 'ready list', derive maxCredits from progress.emailCheck.unverified of get_list, and 'Ask the user before calling it' — an explicit gating instruction rare in tool docs. Retry behavior is spelled out too. No alternative tool is named for the case where the list is not ready, but the context is clear.

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