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

Annotations cover the safety profile (readOnly, idempotent, closed-world), and the description adds substantial extra context: return shape (local wall-clock plus UTC instants), the DST gap flag, the evaluation ceiling that causes rejection, and a per-call price paid via x402. These are behavioral traits an agent cannot infer from annotations or schema.

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?

Five dense sentences, front-loaded with purpose before inputs, limits, and cost. Every sentence carries information, though the enumeration of RRULE parts is slightly list-heavy rather than prose.

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 describing the return format (local + UTC pairs, gap flag) and the failure mode (evaluation limit). Combined with the pricing disclosure, an agent has everything needed to call this correctly.

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 baseline is 3, but the description adds meaning beyond the schema by enumerating the supported RRULE components (FREQ, INTERVAL, COUNT, UNTIL, BYDAY, BYMONTHDAY, BYMONTH, BYSETPOS, WKST) and by noting the 200-occurrence cap on count. It omits any explanation of the 'after' parameter, keeping it out of 5 territory.

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 precise verb+resource: 'Expand an RFC 5545 recurrence rule into concrete occurrence timestamps.' This clearly separates it from siblings like time_calendar, time_dst, and time_business-days, which handle different temporal concerns.

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 states inputs (RRULE + DTSTART + zone + optional EXDATE/RDATE) and operational limits (max 200 occurrences, rejection above 50,000 candidate evaluations), which tells the agent when the call will succeed. It never names an alternative sibling or an explicit when-not-to-use condition, so it falls short of a 5.

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