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

Annotations already cover read-only, idempotent, non-destructive and closed-world behavior, and the description adds genuinely useful context beyond them: what is returned by default (the period running now or the next one, plus shadow dates, signs, and subsequent periods), that it needs no birth details or place, and a reassurance that this is calendrical rather than a warning. No output schema exists, so describing the payload here is valuable; batch limits or date-range bounds are not mentioned.

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-loaded with the trigger conditions and example queries, then the return description. Slightly padded by the rhetorical closer 'Say it without fear: it is a stretch of the calendar, not a warning,' which costs space but does counter a common misreading.

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 zero-required-parameter read tool with strong annotations and no output schema, the description covers the when, the two output modes, and the return contents. Only the year bounds (2024-2031) visible in the schema are left unmentioned in prose, which is a minor omission.

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 goes further by contrasting default behavior (current/next period) with the year parameter's effect (list every retrograde of that year), clarifying the mode switch rather than just repeating the schema text.

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?

Names a specific resource (Mercury retrograde periods) and scope (current, next, or all dates for a year) with concrete example queries, so an agent can distinguish it from the chart-based siblings like vedic_birth_chart or sade_sati_check even though no sibling is named.

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 explicit trigger conditions ('when someone asks whether Mercury is retrograde now, when the next starts or ends, or ... dates of a year') backed by three quoted user phrasings. It does not name alternative tools or state when not to use it, but the niche is narrow enough that routing is unambiguous.

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