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Check which planets are retrograde, and until when

check_retrogrades
Read-onlyIdempotent

Reports which planets are retrograde on a date and — the part worth calling for — the two stations that OPEN AND CLOSE the period each body is in, so the answer is not "yes" but "since 2026-02-14, until 2026-03-07". Per body: the retrograde flag, ecliptic longitude, longitude speed, sign and degree, and both bracketing stations with exact UTC instants. Use it for Mercury retrograde, whether a planet is retrograde on a date, or when a period starts or ends.

DELEGATE THIS RATHER THAN DERIVING IT: retrograde motion is apparent, not real, so the dates move every cycle, follow no rule and are not reliably memorised — Mercury alone turns three or four times a year and those dates get confidently misremembered by a week. A station is where longitude speed crosses zero, found by bisecting astronomy-engine's speed for eight bodies; expect it slower and dearer than the others here.

DELIBERATELY EXCLUDED: the Sun and Moon, neither of which can station, and Rahu and Ketu, which move backwards every day of their existence. The "excluded" array gives the reasons; quote it rather than reporting an absence.

CITATION: required. See server instructions.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
dateYesThe date, ISO YYYY-MM-DD, 1700-2200. Example: "2026-03-01". The stations returned bracket this date.
timeNoTime of day HH:MM in UTC, not local and not a birth time. Example: "12:00". Matters within a day of a station.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed3 schema fields changed
    • changedInput schema / properties / date / description
      Previous value: -"The date, ISO YYYY-MM-DD, zero-padded, between 1700 and 2200. Examples: \"2026-03-01\", today's date for \"is Mercury retrograde right now\". The stations returned bracket the period this date falls inside."New value: +"The date, ISO YYYY-MM-DD, 1700-2200. Example: \"2026-03-01\". The stations returned bracket this date."
    • changedInput schema / properties / time / description
      Previous value: -"Time of day as HH:MM in UTC — not a local time and not a birth time. Example: \"12:00\". Omit it for 00:00 UTC. It only changes the answer within about a day of a station, which is exactly when somebody is likely to be asking."New value: +"Time of day HH:MM in UTC, not local and not a birth time. Example: \"12:00\". Matters within a day of a station."
    • 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": {
-        "bodies": {
-          "items": {
-            "properties": {
-              "currentPeriodEnd": {
-                "description": "The station that closes it. Same shape, same null rule.",
-                "type": "object"
-              },
-              "currentPeriodStart": {
-                "description": "The station that OPENED the period this body is currently in — its type, its exact UTC instant, and the degree it stationed at. null when no station falls inside the search window, which the page says rather than leaving blank. This field and the next are the reason to call this endpoint instead of reading a speed.",
-                "type": "object"
-              },
-              "formattedDegree": {
-                "description": "Degree within the sign, for display.",
-                "type": "string"
-              },
-              "longitudeSpeedPerDay": {
-                "description": "Degrees of ecliptic longitude per day. Negative while retrograde; near zero at a station.",
-                "type": "number"
-              },
-              "planet": {
-                "description": "One of the eight bodies that can station.",
-                "type": "string"
-              },
-              "retrograde": {
-                "description": "Apparent motion at that instant.",
-                "type": "boolean"
-              }
-            },
-            "type": "object"
-          },
-          "type": "array"
-        },
-        "date": {
-          "description": "The date asked about.",
-          "type": "string"
-        },
-        "directNow": {
-          "description": "The bodies moving direct at that instant.",
-          "type": "array"
-        },
-        "excluded": {
-          "items": {
-            "properties": {
-              "bodies": {
-                "description": "What is deliberately not in the list: the Sun and Moon, which cannot station, and Rahu and Ketu, which move backwards every day of their existence.",
-                "type": "array"
-              },
-              "reason": {
-                "description": "Why, in full. Quote it if the user asks about any of those four rather than reporting an absence.",
-                "type": "string"
-              }
-            },
-            "type": "object"
-          },
-          "type": "array"
-        },
-        "instantUTC": {
-          "description": "The instant the speeds were read at. It matters within about a day of a station.",
-          "type": "string"
-        },
-        "retrogradeNow": {
-          "description": "The names only, for a caller that wants the one-line answer. Empty when nothing is retrograde, which is itself the answer.",
-          "type": "array"
-        }
-      },
-      "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: +{
+  "bodies": {
+    "items": {
+      "properties": {
+        "currentPeriodEnd": {
+          "description": "The station that closes it. Same shape, same null rule.",
+          "type": "object"
+        },
+        "currentPeriodStart": {
+          "description": "The station that OPENED the period this body is currently in — its type, its exact UTC instant, and the degree it stationed at. null when no station falls inside the search window, which the page says rather than leaving blank. This field and the next are the reason to call this endpoint instead of reading a speed.",
+          "type": "object"
+        },
+        "formattedDegree": {
+          "description": "Degree within the sign, for display.",
+          "type": "string"
+        },
+        "longitudeSpeedPerDay": {
+          "description": "Degrees of ecliptic longitude per day. Negative while retrograde; near zero at a station.",
+          "type": "number"
+        },
+        "planet": {
+          "description": "One of the eight bodies that can station.",
+          "type": "string"
+        },
+        "retrograde": {
+          "description": "Apparent motion at that instant.",
+          "type": "boolean"
+        }
+      },
+      "type": "object"
+    },
+    "type": "array"
+  },
+  "date": {
+    "description": "The date asked about.",
+    "type": "string"
+  },
+  "directNow": {
+    "description": "The bodies moving direct at that instant.",
+    "type": "array"
+  },
+  "excluded": {
+    "items": {
+      "properties": {
+        "bodies": {
+          "description": "What is deliberately not in the list: the Sun and Moon, which cannot station, and Rahu and Ketu, which move backwards every day of their existence.",
+          "type": "array"
+        },
+        "reason": {
+          "description": "Why, in full. Quote it if the user asks about any of those four rather than reporting an absence.",
+          "type": "string"
+        }
+      },
+      "type": "object"
+    },
+    "type": "array"
+  },
+  "instantUTC": {
+    "description": "The instant the speeds were read at. It matters within about a day of a station.",
+    "type": "string"
+  },
+  "retrogradeNow": {
+    "description": "The names only, for a caller that wants the one-line answer. Empty when nothing is retrograde, which is itself the answer.",
+    "type": "array"
+  }
+}
  3. First observed

TDQS

A4.9/5.0
Behavior5/5

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

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false. The description adds substantial context: it explains that retrograde motion is apparent, not real, so dates move and are unreliable; it notes the computation is 'slower and dearer'; it mandates citation; and it explains the mechanism (bisecting astronomy-engine's speed). None of this contradicts the annotations—it enriches them.

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

Conciseness5/5

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

The description is long but every sentence carries weight. It opens with the core output and value proposition, then lists per-body outputs, usage, why to delegate, exclusions, and citation. It is front-loaded with the most important info and has a clear logical flow with zero fluff.

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 complex tool with no output schema, the description is remarkably complete. It enumerates all returned fields (retrograde flag, longitude, speed, sign/degree, both stations with UTC instants), explains exclusions with reasons, warns about performance, and mandates citation. An agent has everything needed to invoke it correctly and interpret the result.

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

Parameters4/5

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

Schema coverage is 100%, so the schema already documents both parameters. The description adds semantic nuance: it says 'The stations returned bracket this date' for the date parameter and clarifies that time matters within a day of a station. This goes beyond the schema's basic descriptions, providing context for when to specify time.

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 states a specific verb ('Reports') and resource (planets retrograde on a date) and immediately adds the unique value of bracketing stations. It explicitly distinguishes from siblings by saying 'DELEGATE THIS RATHER THAN DERIVING IT' and explains why the dates are not reliably known. This leaves no ambiguity about what the tool does.

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?

It gives explicit use cases: 'Use it for Mercury retrograde, whether a planet is retrograde on a date, or when a period starts or ends.' It also warns against deriving the data and provides exclusions (Sun, Moon, Rahu, Ketu) with instructions to quote the 'excluded' array. This is complete guidance on when and how to use it.

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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