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

Annotations already declare readOnly/idempotent/non-destructive/openWorld, so the bar is lower, and the description adds genuinely useful non-schema context: a free tier of 10 calls/day, then paid access via x402 (MCP params._meta or HTTP) or an X-API-Key, plus the serving host. It does not say what happens on an invalid expression or how errors surface, which keeps it below 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?

Two sentences, front-loaded with what the tool returns and its input formats, then a compact billing/auth note; no filler and nothing repeated from 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?

For a low-complexity, one-required-parameter tool with full schema coverage, an output schema, and rich annotations, the description covers inputs, output flavor, and access/payment requirements. The only gap is error behavior for malformed expressions, which is minor here.

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 adds value by noting the expression may be a macro such as @daily, which the schema's plain 'cron expression' wording does not convey, and by emphasizing any IANA zone. It still does not elaborate on count bounds or defaults, which the schema already handles.

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 precisely what is computed (next N run times of a cron expression), the accepted expression dialects (5-field or @daily macros), and the time-zone scope, plus the notable extra that results come with a plain-language description. This is specific enough that an agent can distinguish it from unrelated siblings like business_days without opening the 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?

Usage is only implied — that it is for computing upcoming cron schedules — with no explicit when-to-use, when-not-to-use, or reference to an alternative (e.g. manual calculation or a date library). The remaining sentences cover billing mechanics, not selection guidance.

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