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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint=false, so the safety profile is fully covered without description help. The description adds only that three twilight phases are computed; it says nothing about timezone handling or edge cases such as polar latitudes where a phase may not occur, which is the main behavioral risk for this calculation.

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?

A single front-loaded sentence that states the computation and then its scope qualifiers, with no filler. Every clause carries information the agent can use.

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 four fully documented parameters, rich annotations, and no output schema, the description covers the essential purpose. The remaining gap is that it never indicates the shape of the result (e.g., six boundary timestamps plus the chosen timezone), which would let an agent plan output handling — but this is minor relative to the schema completeness.

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% — lat, lon, tanggal, and tz all carry their own descriptions including ranges and formats. The description's phrase 'given location and date' merely restates lat/lon/tanggal and adds no format, default, or interaction detail beyond the schema, so the baseline of 3 applies.

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 verb ('Compute'), a specific resource ('civil, nautical, and astronomical twilight boundaries'), and the two scope dimensions (location, date). It clearly distinguishes the tool from generic sun/moon/gerhana siblings by naming the three twilight phases, though it does not explicitly state how it differs from astronomi_sun.

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 parenthetical '(useful for photography & observation)' gives an implied usage context, but there is no explicit when-to-use versus when-not guidance and no named alternative (e.g., 'use astronomi_sun for sunrise/sunset instead'). The agent must infer routing from the resource name alone.

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