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geo_nearest_point_on_line

The closest position on a polyline to a given coordinate, plus the geodesic distance to it in metres. The answer may lie between vertices, not only on them. Useful for 'how far is this address from the route?'. Local computation: no network call, no quota.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
lineYesThe polyline's coordinates, 2 or more.
pointYesThe coordinate to measure from.

Output Schema

TableJSON Schema
NameRequiredDescriptionDefault
pointYesThe closest position on the line, which may lie between vertices.
distance_mYesGeodesic distance from the input point to that position, metres.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed10 schema fields changed
    • addedInput schema / $defs / LatLon / anyOf
      Added value: +[
+  {
+    "properties": {
+      "lat": {
+        "description": "Latitude in decimal degrees (−90 to 90).",
+        "format": "double",
+        "type": "number"
+      },
+      "lon": {
+        "description": "Longitude in decimal degrees (−180 to 180).",
+        "format": "double",
+        "type": "number"
+      }
+    },
+    "required": [
+      "lat",
+      "lon"
+    ],
+    "type": "object"
+  },
+  {
+    "description": "GeoJSON position [lon, lat]: longitude FIRST.",
+    "items": {
+      "format": "double",
+      "type": "number"
+    },
+    "maxItems": 2,
+    "minItems": 2,
+    "type": "array"
+  }
+]
    • changedInput schema / $defs / LatLon / description
      Previous value: -"A WGS84 coordinate pair in decimal degrees."New value: +"A WGS84 coordinate in decimal degrees: a {lat, lon} object (preferred), or a GeoJSON [lon, lat] array with LONGITUDE FIRST, the same order as every polygon field on this server."
    • removedInput schema / $defs / LatLon / properties
      Removed value: -{
-  "lat": {
-    "description": "Latitude in decimal degrees (−90 to 90).",
-    "format": "double",
-    "type": "number"
-  },
-  "lon": {
-    "description": "Longitude in decimal degrees (−180 to 180).",
-    "format": "double",
-    "type": "number"
-  }
-}
    • removedInput schema / $defs / LatLon / required
      Removed value: -[
-  "lat",
-  "lon"
-]
    • removedInput schema / $defs / LatLon / type
      Removed value: -"object"
    • addedOutput schema / $defs / LatLon / anyOf
      Added value: +[
+  {
+    "properties": {
+      "lat": {
+        "description": "Latitude in decimal degrees (−90 to 90).",
+        "format": "double",
+        "type": "number"
+      },
+      "lon": {
+        "description": "Longitude in decimal degrees (−180 to 180).",
+        "format": "double",
+        "type": "number"
+      }
+    },
+    "required": [
+      "lat",
+      "lon"
+    ],
+    "type": "object"
+  },
+  {
+    "description": "GeoJSON position [lon, lat]: longitude FIRST.",
+    "items": {
+      "format": "double",
+      "type": "number"
+    },
+    "maxItems": 2,
+    "minItems": 2,
+    "type": "array"
+  }
+]
    • changedOutput schema / $defs / LatLon / description
      Previous value: -"A WGS84 coordinate pair in decimal degrees."New value: +"A WGS84 coordinate in decimal degrees: a {lat, lon} object (preferred), or a GeoJSON [lon, lat] array with LONGITUDE FIRST, the same order as every polygon field on this server."
    • removedOutput schema / $defs / LatLon / properties
      Removed value: -{
-  "lat": {
-    "description": "Latitude in decimal degrees (−90 to 90).",
-    "format": "double",
-    "type": "number"
-  },
-  "lon": {
-    "description": "Longitude in decimal degrees (−180 to 180).",
-    "format": "double",
-    "type": "number"
-  }
-}
    • removedOutput schema / $defs / LatLon / required
      Removed value: -[
-  "lat",
-  "lon"
-]
    • removedOutput schema / $defs / LatLon / type
      Removed value: -"object"
  2. First observed

TDQS

A4.3/5.0
Behavior4/5

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

With no annotations, the description carries the behavioral burden and handles it well: it discloses local computation, no network/quota, geodesic distance, and interpolation between vertices. It implies a read-only calculation without side effects, though it does not explicitly state that or describe edge-case behavior.

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?

Four sentences, each earning its place: result, interpolation caveat, use case, and execution traits. The most important information is front-loaded and there is no redundant boilerplate.

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 two-parameter local calculation with a rich input schema and an output schema present, the description gives all needed call-time context: result meaning, units, interpolation behavior, and cost model. Nothing critical for invoking the tool correctly is missing.

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 adds no parameter-level syntax beyond the schema. It correctly maps the two inputs to 'coordinate' and 'polyline', but the schema already documents the {lat, lon} object vs [lon, lat] array distinction and the 2+ vertex requirement.

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 precise verb+resource statement: it returns the closest point on a polyline to a coordinate plus the geodesic distance in metres. It clearly distinguishes itself from generic geo_distance/geo_length by noting the answer may lie between vertices, and the use case 'how far is this address from the route?' reinforces its identity.

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?

It provides an explicit use case and a practical execution trait: local computation with no network call and no quota. It does not name sibling alternatives or state when not to use it, but the context is clear enough to guide an agent toward this tool over generic distance or geometry helpers.

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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