Astro Agents - deterministic Western + Vedic astrology

Synastry

synastry

Read-onlyIdempotent

Western astrology synastry (relationship compatibility): compare two birth charts with cross-aspects, house overlays both ways and composite midpoints. Two natal charts compared: every cross-aspect with orbs, where each person's planets fall in the other's houses, harmonious/challenging counts, and composite (midpoint) positions and angles. Deterministic (JPL DE440, no LLM); every result carries meta.result_sha256. Price $0.12 per call via REST POST https://astro-agent.dev/v1/western/synastry (x402 or MPP); your first 3 calls are free here.

Input Schema

Name	Required	Description
`options`	No	Optional calculation settings; every default is the common Western convention.
`person_a`	Yes	First person's birth data.
`person_b`	Yes	Second person's birth data.

Output Schema

Name	Required	Description
`meta`	Yes	Provenance of the result.
`result`	Yes	The calculation result.
`endpoint`	Yes	The REST route that computed it, e.g. 'western/natal'.

Schema Changelog

Changes observed during successful MCP inspections.

Changed1 schema field changedOct 6, 2026

changedOutput schema / (root)

Previous value: -nullNew value: +{
+  "properties": {
+    "endpoint": {
+      "description": "The REST route that computed it, e.g. 'western/natal'.",
+      "type": "string"
+    },
+    "meta": {
+      "description": "Provenance of the result.",
+      "properties": {
+        "astronomy_library": {
+          "description": "Astronomy library and version.",
+          "type": "string"
+        },
+        "compute_ms": {
+          "description": "Computation time in milliseconds.",
+          "type": "number"
+        },
+        "computed_at": {
+          "description": "When it was computed (UTC, ISO 8601).",
+          "type": "string"
+        },
+        "deterministic": {
+          "description": "Always true: same input, same result.",
+          "type": "boolean"
+        },
+        "engine": {
+          "description": "Engine name and version.",
+          "type": "string"
+        },
+        "ephemeris": {
+          "description": "Ephemeris used (NASA/JPL DE440).",
+          "type": "string"
+        },
+        "frame": {
+          "description": "Reference frame of the positions.",
+          "type": "string"
+        },
+        "input_sha256": {
+          "description": "SHA-256 of the canonical input.",
+          "type": "string"
+        },
+        "llm_used": {
+          "description": "Always false: no language model is involved.",
+          "type": "boolean"
+        },
+        "payment": {
+          "description": "How this call was paid: protocol ('free' or 'none'), price and the free calls remaining.",
+          "type": "object"
+        },
+        "reproducibility": {
+          "description": "How to reproduce and check this result.",
+          "type": "string"
+        },
+        "result_sha256": {
+          "description": "SHA-256 of the canonical 'result'; check it with verify_result_hash.",
+          "type": "string"
+        }
+      },
+      "required": [
+        "engine",
+        "deterministic",
+        "llm_used",
+        "input_sha256",
+        "result_sha256"
+      ],
+      "type": "object"
+    },
+    "result": {
+      "description": "The calculation result.",
+      "properties": {
+        "aspect_counts": {
+          "description": "Number of harmonious, challenging and conjunction cross-aspects.",
+          "type": "object"
+        },
+        "composite_midpoints": {
+          "description": "Composite chart by midpoints: points, angles and the method used.",
+          "type": "object"
+        },
+        "cross_aspects": {
+          "description": "Aspects between a point of person A and a point of person B, with orb and exactness.",
+          "type": "array"
+        },
+        "house_overlays": {
+          "description": "Which house of the other chart each person's points fall in.",
+          "type": "object"
+        },
+        "person_a": {
+          "description": "First person's chart: subject, points and angles.",
+          "type": "object"
+        },
+        "person_b": {
+          "description": "Second person's chart: subject, points and angles.",
+          "type": "object"
+        },
+        "settings": {
+          "description": "Settings applied: zodiac, house system and the aspect set with its orbs.",
+          "type": "object"
+        },
+        "warnings": {
+          "description": "Notes on the input or the calculation (for example an assumed noon for an unknown birth time). Present only when there is something to say.",
+          "type": "array"
+        }
+      },
+      "required": [
+        "person_a",
+        "person_b",
+        "settings",
+        "cross_aspects",
+        "aspect_counts",
+        "house_overlays",
+        "composite_midpoints"
+      ],
+      "type": "object"
+    }
+  },
+  "required": [
+    "endpoint",
+    "result",
+    "meta"
+  ],
+  "type": "object"
+}

Changed6 schema fields changedOct 5, 2026

addedInput schema / properties / options / description

Added value: +"Optional calculation settings; every default is the common Western convention."

addedInput schema / properties / options / properties / house_system / description

Added value: +"House system for cusps and planet houses, e.g. 'placidus' (default) or 'whole_sign'."

addedInput schema / properties / options / properties / luminary_orb_bonus / description

Added value: +"Extra orb in degrees for aspects involving the Sun or the Moon. Example: 2."

addedInput schema / properties / options / properties / zodiac / description

Added value: +"'tropical' (Western, default) or 'sidereal' (pair it with 'ayanamsa')."

addedInput schema / properties / person_a / description
```
Added value: +"First person's birth data."
```
addedInput schema / properties / person_b / description
```
Added value: +"Second person's birth data."
```

First observedOct 3, 2026

TDQS

A3.9/5.0

Behavior5/5

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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, closed-world), and the description adds substantial context on top: deterministic computation (JPL DE440, no LLM), verifiability via meta.result_sha256, cost of $0.12 per call, payment mechanisms (x402 or MPP), and a free tier of three calls. That is well beyond what the structured fields provide, with no contradiction.

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?

Front-loaded with the core purpose and efficient despite being dense; pricing and determinism details earn their place. There is mild redundancy between the first sentence's 'cross-aspects, house overlays both ways and composite midpoints' and the second sentence's fuller restatement of the same outputs.

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

Completeness4/5

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

An output schema exists, so return values need not be explained, and the description still adds determinism, hashing, and cost context appropriate for a paid, nested-parameter tool. The only missing piece is guidance on when to choose this over adjacent aspect/chart tools.

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 every parameter and enum thoroughly, giving a baseline of 3. The description restates concepts (orbs, house overlays, midpoints) but adds no syntax or constraint detail not already present in the schema.

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

Purpose4/5

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

States a specific verb and resource ('compare two birth charts' for synastry relationship compatibility) and enumerates the computed artifacts: cross-aspects with orbs, bidirectional house overlays, and composite midpoints. It is clearly distinct from a single-chart tool like natal_chart or aspects by construction, though it never names a sibling to route against.

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

Usage Guidelines3/5

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

Usage is implied by the domain framing ('relationship compatibility', 'Western astrology'), which separates it from the Vedic siblings like kundli or gun_milan, plus access logistics (price, free calls, REST endpoint). There is no explicit when-to-use/when-not statement or named alternative.

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

All Astro Agents - deterministic Western + Vedic astrology tools Inspect

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