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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and a closed world, so the safety profile is covered. The description goes beyond that by disclosing the domain payload in detail — the seven day runes mapped A–G from 1 January, the two-rune dominical letter in leap years, and the sixteen Younger Futhark runes plus arlaug/tvímaðr/belgþór — and it discloses cost (10 credits, Tier 1), which the annotations do not.

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?

The core purpose is front-loaded in the first clause, followed by a single dense parenthetical that earns its length by naming the actual rune sets an agent will see in the response. The bracketed group/endpoint/cost lines are metadata rather than prose and are efficient.

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 and full parameter documentation, the description only needs to frame the domain, which it does thoroughly, and it adds cost awareness. Minor gaps remain: no mention of acceptable date ranges or behavior for out-of-range dates, and no explicit routing versus the other rune tools.

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 description coverage is 100%, with date, fields and precision all documented in the schema itself, so the baseline of 3 applies. The description adds nothing about compact mode, precision rounding, or the date format/default, but it does not need to since the schema is complete.

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 names a specific resource and scope: 'the medieval Scandinavian runic calendar for a date', and enumerates exactly what is returned (day rune, dominical/Sunday rune, golden-number rune of the 19-year cycle). This clearly separates it from the casting siblings (astroway_runes_single, _three, _five, _nine, _bind_rune), though it never explicitly states that contrast.

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?

There is no when-to-use or when-not-to-use guidance, no prerequisites, and no named alternative among the many rune siblings. The agent must infer from the phrase 'for a date' that this is a calendar lookup rather than a rune casting, and that omitting the date yields today (which only the schema states).

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