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

Annotations already declare readOnly, idempotent, non-destructive and closed-world, so the safety bar is low. The description adds real substance beyond them: the exact solar/lunar thresholds per mode, the fixed set of 24 UK places, and the timezone convention (GMT/BST), which an agent needs to interpret results correctly. It does not discuss limits such as maximum month range or empty-result behavior.

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?

Three dense sentences that front-load the core return value before qualifying the photo mode and timezone. Every clause carries information, though the phrase 'with each night's window times, hours and Moon phase' is somewhat compressed and borders on listing return fields already implied by the output schema.

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, three-field query tool with an output schema and full annotation coverage, the description supplies the remaining essentials: the defining thresholds, defaults, place cardinality and timezone. Nothing an agent needs 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%, including the YYYY-MM pattern and the dark/photo enum semantics, so the schema already does the heavy lifting. The description largely restates what those parameters produce rather than adding format or edge-case detail, so the baseline 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?

States a specific verb and resource: returns nights of one UK month with a dark-sky window, at one of 24 UK places, with window times, hours and Moon phase. The astronomical thresholds (Sun >18° below horizon, Moon down) and the separate 'photo' mode make it clearly distinct from siblings like find_moon_events or check_crescent_visibility.

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 description frames the two usage contexts (dark-sky observing vs photography) and describes what each mode yields, so usage is implied. However, it never says when to prefer this tool over the listed siblings, nor any exclusions, leaving the agent to infer routing from the mode semantics alone.

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