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

Annotations already cover readOnly, openWorld, idempotent, and non-destructive, so the bar is lower. The description adds genuinely useful context beyond them: the calculation engine (SunCalc), the free/not-charged-on-error pricing model, and explicit error behavior (returns isError for invalid input or upstream failure). It does not describe pagination or output shape, but the output schema covers that.

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 core deliverable, followed by engine, timezone note, pricing, and error semantics. Dense and mostly waste-free, though the pricing/error tail is slightly list-like; every clause still 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?

With an output schema present, return values need not be explained, and the description covers engine, timezone handling, and failure semantics. Minor gaps around the required 'place' format are handled by the schema, so an agent has what it needs to call it 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 description coverage is 100%, so both tz and date are already documented ('IANA zone for local times', date pattern with default). The description's 'Pass tz for local times' restates the schema rather than adding new meaning, so the baseline 3 is 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 names a specific computational resource (sunrise/sunset/golden hour/twilight/day length/solar noon plus moon phase, illumination, moonrise/moonset) for any place and date. This is clearly distinguishable from siblings like weather, time_convert, or geocode, and no schema needs to be opened to know what it returns.

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 implied by the data the tool produces ('for any place and date') and the tz hint, but there is no explicit when-to-use guidance and no named alternative (e.g., weather for conditions vs this for astronomical times). Adequate but leaves routing to inference.

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