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

Annotations already declare readOnly, idempotent, non-destructive and openWorld, so the safety profile is covered. The description adds genuinely new behavioral facts: pricing ('free') and error contract ('returns isError with a message for invalid input or an upstream failure (not charged)'). It stops short of richer context like latency or output shape, but the output schema covers the latter.

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 tightly packed sentences front-loaded with purpose and payoff; the syntax-support and pricing/error details follow without filler or repetition of the schema.

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 an output schema present, return values need no explanation, and the annotations carry the safety profile. The description still supplies the error contract, pricing, and accepted syntax, so nothing an agent needs to invoke it correctly is missing.

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 meaningful semantics for the expression parameter by enumerating supported syntax: 5-field and 6-field (seconds), ranges, steps, L/W/#, and nicknames (@daily, @hourly). That tells the agent what inputs will be accepted beyond the schema's single example.

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 (explain) and resource (cron expression) plus a second concrete deliverable (next N run times in any time zone). No sibling tool does cron work, so the agent can identify it immediately from name and description alone.

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 a clear motivating use case ('catch a schedule mistake before it fires at 3 am') that tells the agent when this is the right tool. It does not name any alternative or exclusion, but no plausible sibling competes for this task.

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