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

Annotations already declare readOnlyHint, idempotentHint, non-destructive and closed-world, so the safety profile is covered. The description adds genuinely useful behavioral detail beyond that: values are sampled at a fixed 12:00 UK time and all times are UK-based, which tells the agent how to interpret the output. It says nothing about error behavior or edge cases at the 1900/2100 boundaries, keeping it short of 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?

One sentence, packed but front-loaded with the core resource and scope, followed by the exact payload. There is no filler, no restatement of the title, and every clause carries information.

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 two-parameter optional-input, read-only tool with a full output schema and complete annotations, the description covers scope, sampling time, timezone convention, and returned fields. Nothing an agent needs in order to call 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 description coverage is 100% and both parameters already document their ranges and their defaults ('The current UK year/month without it'), so the schema carries the semantics. The description adds nothing about year/month beyond confirming the 'one UK calendar month' granularity, which makes the baseline 3 correct.

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 gives a specific verb and resource ('Returns every day of one UK calendar month with its Moon phase and percent lit') and pins the scope to a whole month, which cleanly separates it from single-moment siblings like get_moon and event-oriented ones like find_moon_events. It also states the two payloads (daily phase values plus the four exact quarter-Moon times), so an agent knows exactly what it gets.

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?

The usage context is inferable from the stated scope - an agent can deduce this is the tool for a monthly overview rather than a single instant or a zodiac sign. However, it never explicitly says when to prefer it over get_moon or find_moon_events, nor does it state any exclusions, so the routing decision is left to inference.

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