Asterwise

Western Moon Phase

asterwise_get_western_moon_phase

Read-onlyIdempotent

Calculates the tropical lunar phase for any date using Swiss Ephemeris. Returns the phase name, phase angle, illumination percentage, Moon age in days, and the next major phase transition.

WORKFLOW: BEFORE: None — standalone. AFTER: asterwise_get_western_moon_calendar — get the full month's phase data.

INPUT CONTRACT: date (optional string YYYY-MM-DD, years 1-3000) — Date to compute phase for. Defaults to today (UTC). An invalid or out-of-range date → 422 validation_error, surfaces as MCP INTERNAL_ERROR with the API message. Example: '2026-05-01'

DO NOT CONFUSE WITH: asterwise_get_western_moon_calendar — full monthly day-by-day phase table. asterwise_get_panchanga — Vedic tithi system (lunar day based on 12° arc increments).

Full output and error contract: https://docs.asterwise.com/mcp/tools/get-western-moon-phase/

Tool call template

This tool declares no required parameters. Add optional parameters from the input schema as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {},
    "name": "asterwise_get_western_moon_phase"
  }
}

Input Schema

Name	Required	Description	Default
`date`	No	Date for the moon phase, YYYY-MM-DD (years 1-3000). Defaults to today (UTC).
`response_format`	No	Output format: 'markdown' (default) for a readable report, or 'json' for the raw structured payload.	markdown

Output Schema

Name	Required	Description	Default
`result`	Yes

Schema Changelog

Changes observed during successful MCP inspections.

Changed1 schema field changedOct 8, 2026

changedInput schema / properties / date / description

Previous value: -"Date for the moon phase, YYYY-MM-DD. Defaults to today."New value: +"Date for the moon phase, YYYY-MM-DD (years 1-3000). Defaults to today (UTC)."

Changed2 schema fields changedSep 5, 2026

addedInput schema / properties / date / description

Added value: +"Date for the moon phase, YYYY-MM-DD. Defaults to today."

addedInput schema / properties / response_format / description

Added value: +"Output format: 'markdown' (default) for a readable report, or 'json' for the raw structured payload."

Changed2 schema fields changedSep 4, 2026
- addedInput schema / properties / response_format / default
```
Added value: +"markdown"
```
- removedInput schema / required
```
Removed value: -[
-  "response_format"
-]
```
AddedMay 1, 2026

TDQS

A4.4/5.0

Behavior4/5

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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive), and the description goes further by disclosing the error contract: an invalid or out-of-range date yields 422 validation_error that surfaces as MCP INTERNAL_ERROR. It also states the default (today UTC) and supported year range, which is real behavioral context. It does not discuss rate limits or latency, so it falls 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.

Conciseness4/5

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

The labeled blocks (WORKFLOW, INPUT CONTRACT, DO NOT CONFUSE WITH) are front-loaded and scannable, and the key disambiguation is stated before the trailing docs URL. It is slightly longer than strictly necessary — the enumerated return fields partly duplicate the output schema — but little is wasted.

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 zero-required-parameter lookup with an output schema, the description covers everything the agent needs: defaults, valid range, failure mode and how the failure is surfaced, plus routing to the follow-up tool. Return-value detail is correctly delegated to the output schema.

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?

Schema description coverage is 100%, so the schema already documents both parameters, including the enum for response_format. The description restates the date format, range and default and adds only a sample value ('2026-05-01'); it says nothing about response_format. Baseline 3 is appropriate when the schema carries the semantics.

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 opening sentence states a specific verb and resource ('Calculates the tropical lunar phase for any date') and names the engine (Swiss Ephemeris). It also enumerates the returned values, so an agent knows exactly what artifact it gets, and the DO NOT CONFUSE section distinguishes it from the two nearest siblings.

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

Usage Guidelines5/5

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

The WORKFLOW block states explicitly that no prior tool is needed and routes the agent onward to asterwise_get_western_moon_calendar. The DO NOT CONFUSE WITH block names two alternatives and gives the discriminating condition for each (monthly day-by-day table vs. Vedic tithi based on 12° arcs).

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

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