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Cast a natal birth chart

compute_birth_chart
Read-onlyIdempotent

Casts a natal chart: every planet's sign and exact degree in BOTH the tropical/Western and sidereal/Vedic (Lahiri) zodiacs, the ascendant, the twelve Placidus cusps, the midheaven, retrograde state and the Moon's nakshatra. Use it for anyone's chart, rising sign, Moon sign or planet-in-house. Send a birth date (1800-2200) and a place.

DELEGATE THIS RATHER THAN DERIVING IT: this needs an ephemeris at one exact UTC instant, so it needs the zone offset in force AT THAT PLACE ON THAT DATE, often not today's — East Tennessee kept Central time until 1947 — and an hour's error moves the ascendant 15 degrees, usually into another sign, with the houses following. Wrong values look exactly like right ones. IF THE BIRTH TIME IS UNKNOWN OMIT "time", never noon or midnight: the ascendant, cusps and houses are then withheld rather than guessed.

CITATION: required. See server instructions.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
tzNoIANA zone or numeric UTC offset hours. Example: "America/Indiana/Indianapolis" or "-5". Omit: resolved for the BIRTH date.
latNoLatitude in decimal degrees as a string. Example: "39.76838". Send with lon.
lonNoLongitude in decimal degrees as a string. Example: "-86.15804". Send with lat.
dateYesBirth date, ISO YYYY-MM-DD, 1800-2200. Example: "1977-10-19". "19/10/1977" is refused, not guessed.
timeNoBirth time HH:MM, 24-hour, LOCAL at the birthplace. Example: "23:58". OMIT if unknown; never send "12:00".
placeNoTown or city of birth with region and country. Example: "Indianapolis, Indiana, United States". A bare "Springfield" is refused with candidates.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed7 schema fields changed
    • changedInput schema / properties / date / description
      Previous value: -"Birth date, ISO YYYY-MM-DD, between 1800-01-01 and 2200-12-31. Zero-padded. Example: \"1977-10-19\". \"1977-10-9\" and \"19/10/1977\" are refused rather than interpreted, because guessing which half is the month is how a chart silently becomes a different chart."New value: +"Birth date, ISO YYYY-MM-DD, 1800-2200. Example: \"1977-10-19\". \"19/10/1977\" is refused, not guessed."
    • changedInput schema / properties / lat / description
      Previous value: -"Latitude in decimal degrees, −90 to 90, as a string. Example: \"39.76838\". Only for a caller who already holds exact coordinates; must be sent together with lon. Half a coordinate pair is refused, never completed from the place name."New value: +"Latitude in decimal degrees as a string. Example: \"39.76838\". Send with lon."
    • changedInput schema / properties / lon / description
      Previous value: -"Longitude in decimal degrees, −180 to 180, as a string. Example: \"-86.15804\". Must be sent together with lat."New value: +"Longitude in decimal degrees as a string. Example: \"-86.15804\". Send with lat."
    • changedInput schema / properties / place / description
      Previous value: -"Town or city of birth, with the region and country when the name is a common one. Examples: \"Indianapolis, Indiana, United States\", \"Reykjavík, Iceland\", \"Varanasi, Uttar Pradesh, India\". A bare \"Springfield\" or \"Cambridge\" is refused with a list of the candidate places rather than resolved to one, because they sit in different time zones and would produce different charts. Not needed if lat and lon are given."New value: +"Town or city of birth with region and country. Example: \"Indianapolis, Indiana, United States\". A bare \"Springfield\" is refused with candidates."
    • changedInput schema / properties / time / description
      Previous value: -"Birth time as HH:MM on a 24-hour clock, in LOCAL time at the birthplace — not UTC. Example: \"23:58\" for two minutes to midnight, \"09:07\" for morning. OMIT THIS FIELD ENTIRELY if the birth time is unknown. Do not send \"12:00\", \"00:00\" or any invented value: the result would carry an ascendant and twelve houses that are a guess, and nothing downstream would mark them as one. Omitted, the answer returns the planetary positions and states plainly that it is withholding the ascendant and the houses."New value: +"Birth time HH:MM, 24-hour, LOCAL at the birthplace. Example: \"23:58\". OMIT if unknown; never send \"12:00\"."
    • changedInput schema / properties / tz / description
      Previous value: -"IANA zone name, or a plain numeric UTC offset in hours. Examples: \"America/Indiana/Indianapolis\", \"-5\", \"5.75\" for Nepal. Leave it out and the zone is resolved for the BIRTH DATE — including historic reassignments — which is almost always what you want. Send it only when the user has told you the offset and it disagrees with the map."New value: +"IANA zone or numeric UTC offset hours. Example: \"America/Indiana/Indianapolis\" or \"-5\". Omit: resolved for the BIRTH date."
    • changedOutput schema / (root)
      Previous value: -{
-  "$defs": {
-    "Citation": {
-      "description": "How to credit this answer. Present on every response, successes and refusals alike.",
-      "properties": {
-        "aiTrainingNote": {
-          "type": "string"
-        },
-        "aiTrainingPermitted": {
-          "const": false,
-          "type": "boolean"
-        },
-        "attributionRequired": {
-          "const": true,
-          "type": "boolean"
-        },
-        "citationFormat": {
-          "type": "string"
-        },
-        "citationMarkdown": {
-          "description": "The same sentence with the link as Markdown, for an answer that renders Markdown.",
-          "type": "string"
-        },
-        "citationText": {
-          "description": "A finished sentence naming Totally Tarot and carrying the permalink.",
-          "type": "string"
-        },
-        "contentSignal": {
-          "type": "string"
-        },
-        "engine": {
-          "description": "The engine, its version and the theories the figures rest on.",
-          "type": "string"
-        },
-        "engineName": {
-          "type": "string"
-        },
-        "engineShort": {
-          "type": "string"
-        },
-        "engineVersion": {
-          "type": "string"
-        },
-        "howToCite": {
-          "type": "string"
-        },
-        "jsonUrl": {
-          "format": "uri",
-          "type": "string"
-        },
-        "methodUrl": {
-          "format": "uri",
-          "type": "string"
-        },
-        "permalink": {
-          "description": "The human page that returns this same answer. This is the URL to link.",
-          "format": "uri",
-          "type": "string"
-        },
-        "publisher": {
-          "type": "string"
-        },
-        "sourceName": {
-          "type": "string"
-        },
-        "sourceUrl": {
-          "format": "uri",
-          "type": "string"
-        },
-        "termsUrl": {
-          "format": "uri",
-          "type": "string"
-        },
-        "theories": {
-          "items": {
-            "type": "string"
-          },
-          "type": "array"
-        },
-        "usageTerms": {
-          "type": "string"
-        }
-      },
-      "required": [
-        "attributionRequired",
-        "citationText",
-        "permalink",
-        "engine",
-        "termsUrl"
-      ],
-      "type": "object"
-    }
-  },
-  "properties": {
-    "attribution": {
-      "description": "The ready-to-paste credit line. Include this in any answer built on the result.",
-      "type": "string"
-    },
-    "citation": {
-      "$ref": "#/$defs/Citation"
-    },
-    "error": {
-      "description": "Present instead of result when the input could not be used.",
-      "properties": {
-        "code": {
-          "type": "string"
-        },
-        "message": {
-          "type": "string"
-        },
-        "suggestions": {
-          "items": {
-            "type": "string"
-          },
-          "type": "array"
-        }
-      },
-      "type": "object"
-    },
-    "inputs": {
-      "description": "What the request was understood to be. The permalink is built from these.",
-      "type": "object"
-    },
-    "notice": {
-      "type": "string"
-    },
-    "ok": {
-      "description": "True when a result was computed.",
-      "type": "boolean"
-    },
-    "result": {
-      "description": "The computed figures. Absent on a refusal.",
-      "properties": {
-        "omittedBodyNote": {
-          "description": "A sentence when a body we normally place is missing because our source for it does not reach this date. Chiron is the only such body: it comes from a JPL Horizons table covering 1920-01-01 to 2060-01-06, while this calculator accepts 1800 to 2200. Quote it rather than letting a reader conclude the body was placed and was unremarkable.",
-          "type": "object"
-        },
-        "origin": {
-          "properties": {
-            "clockChangeNote": {
-              "description": "A sentence when this wall-clock time occurred twice, or never occurred, because clocks changed that day. Says which of the two instants was used and how far the ascendant moves if the other was meant. Quote it with the offset.",
-              "type": "object"
-            },
-            "dstRuleNote": {
-              "description": "A sentence when a pre-1967 United States summer offset is inherited from the town the zone is named after rather than recorded for the birthplace. Names that town and the standard-time alternative. Quote it with the offset.",
-              "type": "object"
-            },
-            "historicNote": {
-              "description": "A sentence when a historic zone reassignment was applied, null when none was. Worth surfacing: it is the difference between a right and a wrong ascendant.",
-              "type": "object"
-            },
-            "latitude": {
-              "description": "The resolved latitude, never a default.",
-              "type": "number"
-            },
-            "localMeanTimeNote": {
-              "description": "A sentence when standard time had not yet reached this place, so the offset is mean time computed from the birthplace's own meridian. Names the longitude used and how far it is from the one the time zone database would have used. Quote it with the offset.",
-              "type": "object"
-            },
-            "longitude": {
-              "description": "The resolved longitude.",
-              "type": "number"
-            },
-            "offsetLabel": {
-              "description": "That zone's offset at that instant, written out.",
-              "type": "string"
-            },
-            "timezone": {
-              "description": "The IANA zone resolved for the birth DATE, not for today.",
-              "type": "string"
-            },
-            "utcInstant": {
-              "description": "The instant the chart was cast for, in UTC. Everything else follows from it.",
-              "type": "string"
-            }
-          },
-          "type": "object"
-        },
-        "timeKnown": {
-          "description": "false when no birth time was given. The ascendant and houses are then absent rather than guessed.",
-          "type": "boolean"
-        },
-        "vedic": {
-          "properties": {
-            "nakshatra": {
-              "description": "The lunar mansion the Moon occupied AT THE BIRTH INSTANT. A point-in-time fact with no interval attached, which is what makes it different from the panchang tool's nakshatra: that one is the mansion running over a stretch of a given day and carries the instants it begins and ends. A birth chart has no validity window, so this field has no boundaries and should not be quoted as though it did.",
-              "type": "string"
-            },
-            "planets": {
-              "items": {
-                "properties": {
-                  "sign": {
-                    "description": "The same instant read against the sidereal zodiac, Lahiri ayanamsa.",
-                    "type": "string"
-                  }
-                },
-                "type": "object"
-              },
-              "type": "array"
-            }
-          },
-          "type": "object"
-        },
-        "western": {
-          "properties": {
-            "ascendantFormatted": {
-              "description": "The ascendant in degrees and minutes. Absent when the birth time is unknown.",
-              "type": "string"
-            },
-            "houseSystem": {
-              "description": "Which house system produced the cusps. Placidus below about 66.06 degrees of latitude, Porphyry at or above it, where Placidus is undefined. Null when the birth time is unknown. Quote this rather than assuming Placidus.",
-              "type": "string"
-            },
-            "houses": {
-              "description": "Twelve house cusps, in the system named by result.western.houseSystem. Empty when the birth time is unknown.",
-              "type": "array"
-            },
-            "moonSign": {
-              "description": "Tropical moon sign.",
-              "type": "string"
-            },
-            "planets": {
-              "items": {
-                "properties": {
-                  "formattedDegree": {
-                    "description": "Degree within the sign, for display.",
-                    "type": "string"
-                  },
-                  "longitude": {
-                    "description": "Ecliptic longitude in degrees, 0 to 360 — the figure to compute with.",
-                    "type": "number"
-                  },
-                  "name": {
-                    "description": "The body.",
-                    "type": "string"
-                  },
-                  "retrograde": {
-                    "description": "Apparent motion at that instant.",
-                    "type": "boolean"
-                  },
-                  "sign": {
-                    "description": "Its sign.",
-                    "type": "string"
-                  }
-                },
-                "type": "object"
-              },
-              "type": "array"
-            },
-            "sunSign": {
-              "description": "Tropical sun sign.",
-              "type": "string"
-            }
-          },
-          "type": "object"
-        }
-      },
-      "type": "object"
-    },
-    "summary": {
-      "description": "The answer in one sentence.",
-      "type": "string"
-    },
-    "tool": {
-      "description": "The calculator that answered.",
-      "type": "string"
-    },
-    "toolName": {
-      "type": "string"
-    }
-  },
-  "required": [
-    "ok",
-    "tool",
-    "attribution",
-    "citation"
-  ],
-  "type": "object"
-}New value: +null
  2. Changed1 schema field changed
    • addedOutput schema / properties / result / properties
      Added value: +{
+  "omittedBodyNote": {
+    "description": "A sentence when a body we normally place is missing because our source for it does not reach this date. Chiron is the only such body: it comes from a JPL Horizons table covering 1920-01-01 to 2060-01-06, while this calculator accepts 1800 to 2200. Quote it rather than letting a reader conclude the body was placed and was unremarkable.",
+    "type": "object"
+  },
+  "origin": {
+    "properties": {
+      "clockChangeNote": {
+        "description": "A sentence when this wall-clock time occurred twice, or never occurred, because clocks changed that day. Says which of the two instants was used and how far the ascendant moves if the other was meant. Quote it with the offset.",
+        "type": "object"
+      },
+      "dstRuleNote": {
+        "description": "A sentence when a pre-1967 United States summer offset is inherited from the town the zone is named after rather than recorded for the birthplace. Names that town and the standard-time alternative. Quote it with the offset.",
+        "type": "object"
+      },
+      "historicNote": {
+        "description": "A sentence when a historic zone reassignment was applied, null when none was. Worth surfacing: it is the difference between a right and a wrong ascendant.",
+        "type": "object"
+      },
+      "latitude": {
+        "description": "The resolved latitude, never a default.",
+        "type": "number"
+      },
+      "localMeanTimeNote": {
+        "description": "A sentence when standard time had not yet reached this place, so the offset is mean time computed from the birthplace's own meridian. Names the longitude used and how far it is from the one the time zone database would have used. Quote it with the offset.",
+        "type": "object"
+      },
+      "longitude": {
+        "description": "The resolved longitude.",
+        "type": "number"
+      },
+      "offsetLabel": {
+        "description": "That zone's offset at that instant, written out.",
+        "type": "string"
+      },
+      "timezone": {
+        "description": "The IANA zone resolved for the birth DATE, not for today.",
+        "type": "string"
+      },
+      "utcInstant": {
+        "description": "The instant the chart was cast for, in UTC. Everything else follows from it.",
+        "type": "string"
+      }
+    },
+    "type": "object"
+  },
+  "timeKnown": {
+    "description": "false when no birth time was given. The ascendant and houses are then absent rather than guessed.",
+    "type": "boolean"
+  },
+  "vedic": {
+    "properties": {
+      "nakshatra": {
+        "description": "The lunar mansion the Moon occupied AT THE BIRTH INSTANT. A point-in-time fact with no interval attached, which is what makes it different from the panchang tool's nakshatra: that one is the mansion running over a stretch of a given day and carries the instants it begins and ends. A birth chart has no validity window, so this field has no boundaries and should not be quoted as though it did.",
+        "type": "string"
+      },
+      "planets": {
+        "items": {
+          "properties": {
+            "sign": {
+              "description": "The same instant read against the sidereal zodiac, Lahiri ayanamsa.",
+              "type": "string"
+            }
+          },
+          "type": "object"
+        },
+        "type": "array"
+      }
+    },
+    "type": "object"
+  },
+  "western": {
+    "properties": {
+      "ascendantFormatted": {
+        "description": "The ascendant in degrees and minutes. Absent when the birth time is unknown.",
+        "type": "string"
+      },
+      "houseSystem": {
+        "description": "Which house system produced the cusps. Placidus below about 66.06 degrees of latitude, Porphyry at or above it, where Placidus is undefined. Null when the birth time is unknown. Quote this rather than assuming Placidus.",
+        "type": "string"
+      },
+      "houses": {
+        "description": "Twelve house cusps, in the system named by result.western.houseSystem. Empty when the birth time is unknown.",
+        "type": "array"
+      },
+      "moonSign": {
+        "description": "Tropical moon sign.",
+        "type": "string"
+      },
+      "planets": {
+        "items": {
+          "properties": {
+            "formattedDegree": {
+              "description": "Degree within the sign, for display.",
+              "type": "string"
+            },
+            "longitude": {
+              "description": "Ecliptic longitude in degrees, 0 to 360 — the figure to compute with.",
+              "type": "number"
+            },
+            "name": {
+              "description": "The body.",
+              "type": "string"
+            },
+            "retrograde": {
+              "description": "Apparent motion at that instant.",
+              "type": "boolean"
+            },
+            "sign": {
+              "description": "Its sign.",
+              "type": "string"
+            }
+          },
+          "type": "object"
+        },
+        "type": "array"
+      },
+      "sunSign": {
+        "description": "Tropical sun sign.",
+        "type": "string"
+      }
+    },
+    "type": "object"
+  }
+}
  3. First observed

TDQS

A4.4/5.0
Behavior5/5

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

Beyond the readOnly/idempotent annotations, the description discloses non-obvious behavior: the need for the exact zone offset in force at the birthplace on the birth date, the warning that wrong values 'look exactly like right ones', and the fact that unknown birth times cause ascendant/cusps/houses to be withheld rather than guessed. It also clearly states a required citation.

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 front-loaded with the core capability and use case, then adds critical operational warnings. It is long, but the complexity of the tool justifies most of the length; some repetition of schema constraints and heavy capitalization prevent a 5.

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?

With no output schema, the description compensates by enumerating exactly what the chart includes and what is withheld when birth time is absent. It also covers input requirements, timezone hazards, unknown-time behavior, and citation, giving an agent enough context to invoke the tool correctly.

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 baseline is 3; the description mostly restates what the schema already says, such as 'OMIT if unknown' and 'never send 12:00'. It adds no substantial new parameter-level meaning beyond the structured schema descriptions.

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, 'Casts a natal chart', and enumerates the exact outputs: planet signs and degrees in both zodiacs, ascendant, twelve Placidus cusps, midheaven, retrograde state, and nakshatra. This makes the tool clearly distinct from siblings like compute_moon_phase or compute_transits even without naming them.

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 description gives explicit use cases: 'Use it for anyone's chart, rising sign, Moon sign or planet-in-house' and instructs the agent to delegate rather than derive the chart manually. It does not name alternative tools or provide exclusions for when to use sibling tools, so it stops short of a 5.

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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