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Calculate synastry - Relationship compatibility analysis API

post_astrology_synastry

Read-only

Calculate synastry compatibility between two natal charts. Returns a 0-100 score, inter-chart aspects with strength ratings, and relationship dynamics for dating or counseling apps.

Instructions

Calculate complete synastry (relationship compatibility) between two natal charts using Western tropical astrology. Analyzes inter-chart aspects between all planets to determine romantic, friendship, and karmic compatibility. Returns compatibility score (0-100), detailed inter-aspects with strength ratings, harmonious vs challenging aspect counts, and relationship dynamics analysis. Perfect for dating apps, matrimonial sites, relationship counseling tools, and astrology compatibility features. Based on professional astrological techniques.

Input Schema

Name	Required	Description	Default
`lang`	No	Response language (BCP 47). Supported: en, tr, de, es, hi, pt, fr, ru, zh-Hans, zh-Hant. Defaults to en. Coverage varies by domain, and a field with no translation in the requested language returns English.	en
`compact`	No	Set true for the same data in a compact shape: arrays of same-shaped objects arrive columnar as {"__cols":[names],"__rows":[[values]]}. Lossless, typically 40 to 52 percent fewer tokens.
`person1`	Yes
`person2`	Yes
`houseSystem`	No	House system for both natal charts. Placidus (default), Whole Sign, Equal, or Koch.	placidus

Output Schema

Name	Required	Description	Default
`person1`	Yes
`person2`	Yes
`summary`	Yes
`analysis`	Yes
`interAspects`	Yes
`compatibilityScore`	Yes

Schema Changelog

Changes observed during successful MCP inspections.

Changed3 schema fields changedv1.1.2Oct 5, 2026

changedInput schema / examples

Previous value: -[
-  {
-    "person1": {
-      "date": "1990-07-15",
-      "latitude": 40.7128,
-      "longitude": -74.006,
-      "time": "14:30:00",
-      "timezone": -5
-    },
-    "person2": {
-      "date": "1990-07-15",
-      "latitude": 40.7128,
-      "longitude": -74.006,
-      "time": "14:30:00",
-      "timezone": -5
-    }
-  }
-]New value: +[
+  {
+    "person1": {
+      "date": "1990-07-15",
+      "latitude": 40.7128,
+      "longitude": -74.006,
+      "time": "14:30:00",
+      "timezone": "America/New_York"
+    },
+    "person2": {
+      "date": "1990-07-15",
+      "latitude": 40.7128,
+      "longitude": -74.006,
+      "time": "14:30:00",
+      "timezone": "America/New_York"
+    }
+  }
+]

changedInput schema / properties / person1 / properties / timezone / example
```
Previous value: --5New value: +"America/New_York"
```
changedInput schema / properties / person2 / properties / timezone / example
```
Previous value: --5New value: +"America/New_York"
```

First observedv0.1.0Oct 4, 2026

TDQS

C2.9/5.0

Behavior3/5

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

Annotations already declare readOnlyHint=true and destructiveHint=false, so the safety profile is covered. The description adds that it returns a compatibility score and inter-aspect details, but since an output schema exists, this return-value summary is mostly redundant and it does not disclose auth, rate limits, or error behavior.

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

Conciseness3/5

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

The first three sentences are informative and front-loaded with the core calculation and return contents. However, the final two sentences ('Perfect for dating apps...' and 'Based on professional astrological techniques.') are promotional filler that does not help an agent select or invoke the tool.

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

Completeness3/5

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

The tool is complex, with nested objects, five top-level parameters, and an output schema. The description covers the computation and output at a high level, but it omits usage guidance relative to siblings and adds no parameter semantics, so it is only minimally complete for correct selection.

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

Parameters2/5

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

Schema description coverage is 60%, and the description mentions no parameters at all. It does not explain the required person1/person2 objects, the optional lang/compact controls, or the houseSystem/nodeType choices beyond what the schema already provides, leaving a compensation gap for the uncovered parameter surface.

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?

The description states a specific verb and resource: 'Calculate complete synastry (relationship compatibility) between two natal charts using Western tropical astrology.' It is clear what the tool computes, but it does not distinguish itself from sibling tools like post_astrology_compatibility_score or post_astrology_composite_chart, which likely overlap in relationship-compatibility scope.

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

Usage Guidelines2/5

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

The description gives marketing-oriented use cases ('Perfect for dating apps, matrimonial sites...') but no operational guidance about when an agent should call this tool versus sibling compatibility or chart tools. There are no exclusions, prerequisites, or named alternatives.

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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