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

Annotations already establish readOnlyHint=true and openWorldHint=false, so safety is covered; the description adds the accepted-expression grammar, the explicit refusal of @reboot and six-field expressions, and the fact that no API key or account is required. What it omits is how the count interacts with sparse expressions (e.g., a yearly cron asked for 100 results) and any rate limits, which keeps it from a 5.

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 outcome and followed by accepted syntax and then the sibling exclusion. It is dense to the point of being a run-on in the first sentence, where scope, defaults, and four return fields are packed together, but no sentence is 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 compensates by naming the returned fields, and with 100% schema coverage on the four inputs the remaining burden is small. Grammar rules, refusals, and the sibling boundary are all present, so an agent can call this correctly on the first attempt.

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, but the description adds real meaning about the expression parameter that the schema's terse 'Such as 0 9 * * 1-5, or @daily' does not: lists, ranges, steps, month and weekday names, and the @hourly-to-@yearly macros. The defaults for zone (UTC) and from (now) are repeated from the schema, which is redundant but harmless.

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 opens with a specific verb and resource: it returns the next fire times of a five-field cron expression, with zone and start-point defaults. It also states the return fields (timestamp, datetime, utc_datetime, weekday), so the agent knows exactly what it gets, and it distinguishes itself from schedule_webhook without needing the sibling's schema.

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 explicitly carves out the boundary that matters most: 'This computes times only; schedule_webhook is what calls a URL on a schedule.' It also lists what is refused (@reboot, six fields), which prevents a class of failed calls. It does not contrast with convert_timestamp or list_timezones, but those are far enough apart that the gap is minor.

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