Hopi

Moon phase calculator

hopi_moon_phase_calculator

Read-onlyIdempotent

Work out the moon phase for a date, based on the synodic month since a known new moon. Dates use the YYYY-MM-DD format and default to today. Returns the phase name, illuminated percentage, moon age in days, and the next full and new moons. Source: https://hopi.co.uk/moon-phase-calculator/

Input Schema

TableJSON Schema

Name	Required	Description	Default
`date`	No	Optional date in YYYY-MM-DD format (defaults to today)

Output Schema

TableJSON Schema

Name	Required	Description
`date`	No
`phase`	No
`summary`	Yes
`source_url`	Yes	The tool's page on hopi.co.uk (cite or open to check the figure)
`moonAgeDays`	No
`nextNewMoon`	No
`nextFullMoon`	No
`illuminationPercent`	No

Schema Changelog

Changes observed during successful MCP inspections.

Changed1 schema field changedSep 20, 2026
- addedOutput schema / required
```
Added value: +[
+  "source_url",
+  "summary"
+]
```
First observedSep 20, 2026

TDQS

A4.2/5.0

Behavior4/5

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

Annotations already declare readOnly, idempotent, and non-destructive behavior. The description adds valuable context beyond that: the synodic-month basis, date format and default, and the specific outputs returned (phase name, illuminated percentage, moon age, next full/new moons).

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness4/5

Is the description appropriately sized, front-loaded, and free of redundancy?

The description is compact and front-loaded: purpose, input format/default, then returned values. The source URL is a minor extra but does not bloat the definition or obscure the essentials.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a one-parameter, read-only calculator with full input-schema coverage and an output schema, the description supplies the algorithm basis, input expectations, and return content. Nothing essential is missing for correct invocation.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

The input schema fully documents the only optional parameter `date`, including format and default. The description merely restates that information, so it adds no new semantic value; the high schema coverage baseline of 3 applies.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description opens with a specific verb and resource ('Work out the moon phase for a date') and clarifies the calculation basis and output fields. It is clearly distinguishable from the large sibling set, where no other tool computes moon phases.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines4/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

The use case is unambiguous: given a date (defaulting to today) the tool returns moon-phase data. While no explicit alternatives or exclusions are named, none are necessary because no sibling overlaps with this functionality.

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

All Hopi tools Inspect

Try in Browser

One endpoint for every MCP toolRayrun runs every MCP server behind one URL and holds the credentials. Each call is decided per client, tool, and argument, then recorded.ray.runAd

Glama MCP Gateway

Add one secure layer between your agents and this server.

Resources

Need Help?