diff --git a/.agents/skills/graphile-postgis/SKILL.md b/.agents/skills/graphile-postgis/SKILL.md
new file mode 100644
index 000000000..967a0c080
--- /dev/null
+++ b/.agents/skills/graphile-postgis/SKILL.md
@@ -0,0 +1,121 @@
+# graphile-postgis Development
+
+## Overview
+
+The `graphile-postgis` package lives at `graphile/graphile-postgis/` in the constructive monorepo. It provides PostGIS support for PostGraphile v5 — GeoJSON types, spatial filter operators, measurement fields, transformation fields, and aggregate function definitions.
+
+## Architecture
+
+### Plugin Structure
+
+All plugins are in `src/plugins/` and follow the `GraphileConfig.Plugin` pattern:
+
+- **codec.ts** — GeoJSON scalar type codec
+- **inflection.ts** — Type naming for subtypes, Z/M variants (e.g., `GeometryPointZ`)
+- **detect-extension.ts** — Auto-detects PostGIS extension in the database
+- **register-types.ts** — Registers 56 concrete GraphQL types (geometry/geography × 7 subtypes × 4 dimension variants)
+- **geometry-fields.ts** — Coordinate fields (x, y, z, points, exterior/interiors)
+- **measurement-fields.ts** — `area`, `length`, `perimeter` (client-side geodesic calculations)
+- **transformation-functions.ts** — `centroid`, `bbox`, `numPoints` (client-side from GeoJSON)
+- **aggregate-functions.ts** — Definitions for ST_Extent, ST_Union, ST_Collect, ST_ConvexHull
+- **connection-filter-operators.ts** — 26 spatial filter operators via declarative factory API
+- **within-distance-operator.ts** — `withinDistance` (ST_DWithin) compound input type operator
+
+### Connection Filter Operator Factory Pattern
+
+Custom operators are registered via `connectionFilterOperatorFactories` in the preset's schema options. Each factory is a function that receives the `build` object and returns an array of operator registrations:
+
+```typescript
+export function createMyOperatorFactory(): ConnectionFilterOperatorFactory {
+  return (build) => {
+    if (!build.pgGISExtensionInfo) return [];
+    // Return array of { typeNames, operatorName, spec } registrations
+  };
+}
+```
+
+The `connectionFilterOperatorFactories` type is augmented by `graphile-connection-filter` (not a direct dependency of graphile-postgis), so preset.ts uses a type assertion: `as GraphileConfig.Preset['schema'] & Record<string, unknown>`.
+
+### Key Build Object Properties
+
+- `build.pgGISExtensionInfo` — Set by detect-extension plugin: `{ schemaName, geometryCodec, geographyCodec }`
+- `build.inflection.gisType(...)` / `build.inflection.gisInterfaceName(...)` — Generate type names
+- `build.getTypeByName('GeoJSON')` — Get the GeoJSON scalar type
+
+## Testing
+
+### Prerequisites
+
+```bash
+# Start Docker DB with PostGIS support
+pgpm docker start --image docker.io/constructiveio/postgres-plus:18 --recreate
+eval "$(pgpm env)"
+pgpm admin-users bootstrap --yes
+pgpm admin-users add --test --yes
+```
+
+### Running Tests
+
+```bash
+cd graphile/graphile-postgis
+
+# Run all tests
+pnpm test
+
+# Watch mode
+pnpm test:watch
+```
+
+### Test Status Notes
+
+- 4 test suites (preset, index, connection-filter-operators, within-distance-operator) import `connection-filter-operators.ts` which depends on `graphile-connection-filter`. Since that's not a direct dependency, these suites show TS compilation errors in the test runner but still pass because the runtime JS works fine.
+- The `sql.raw` warning in test output is expected from connection-filter-operators.ts test mocking.
+
+## Building
+
+```bash
+cd graphile/graphile-postgis
+pnpm build  # Uses makage — outputs CJS + ESM to dist/
+```
+
+Or from the monorepo root:
+```bash
+cd ~/repos/constructive && pnpm build
+```
+
+## Key Patterns
+
+### Adding a New Object Field Plugin
+
+1. Create `src/plugins/my-fields.ts`
+2. Use `schema.hooks.GraphQLObjectType_fields` hook
+3. Check `context.scope.isPgGISType` and `context.scope.pgGISTypeDetails.subtype` to filter applicable types
+4. Use `build.extend(fields, newFields, 'reason')` to add fields
+5. Add to `src/preset.ts` plugins array and export from `src/index.ts`
+6. Update `__tests__/index.test.ts` expected exports list and `__tests__/preset.test.ts` plugin count
+
+### Adding a New Filter Operator
+
+1. Create a factory function in `src/plugins/my-operator.ts`
+2. For simple operators (2 args): follow `connection-filter-operators.ts` pattern
+3. For compound operators (3+ args): follow `within-distance-operator.ts` pattern with custom `resolveType` hook
+4. Add factory to `connectionFilterOperatorFactories` array in `src/preset.ts`
+5. Export from `src/index.ts`
+
+### GisFieldValue Type
+
+All geometry object field resolvers receive `GisFieldValue` as the source:
+```typescript
+interface GisFieldValue {
+  __gisType: string;   // e.g., 'Point', 'Polygon'
+  __srid: number;      // e.g., 4326
+  __geojson: Geometry;  // GeoJSON geometry object
+}
+```
+
+## Constants
+
+- `CONCRETE_SUBTYPES` — Array of 7 PostGIS subtypes (Point, LineString, Polygon, MultiPoint, MultiLineString, MultiPolygon, GeometryCollection)
+- `GisSubtype` — Enum mapping subtype names to numeric codes (0-7)
+- 26 total spatial filter operators (13 function-based ST_* + 13 bbox operators)
+- 58 type registrations per operator (29 geometry + 29 geography type names)