Agent-Skills.md

Agent Skills: Mapbox GL JS — advanced web toolkit (v3)

Mapbox GL JS v3 (web) toolkit: custom markers, symbol/geojson layers, clustering, style expressions, 3D terrain/fill-extrusion, heatmaps, choropleths, camera animation, and performance gotchas. Use for any Mapbox GL JS work — markers, layers, style, 3D, or headless map verification.

UncategorizedID: 0xdarkmatter/claude-mods/mapbox-ops

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0xdarkmatter/claude-mods
0xDarkMatterLicense: MIT
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pnpm dlx add-skill https://github.com/0xDarkMatter/claude-mods/tree/HEAD/skills/mapbox-ops

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skills/mapbox-ops/SKILL.md

Skill Metadata

Name
mapbox-ops
Description
"Mapbox GL JS v3 (web) toolkit: custom markers, symbol/geojson layers, clustering, style expressions, 3D terrain/fill-extrusion, heatmaps, choropleths, camera animation, and performance gotchas. Use for any Mapbox GL JS work — markers, layers, style, 3D, or headless map verification."

Mapbox GL JS — advanced web toolkit (v3)

An advanced toolkit for building production Mapbox GL JS map experiences on the web: markers, thematic dataviz, 3D, terrain, cinematic camera, style composition, performance, and the hard-won gotchas that bite. Scope: mapbox-gl-js v3.x in the browser (CDN mapbox-gl-js/v3.x/) — not the native iOS/Android SDKs (different APIs). Plain GL JS, framework-agnostic. Several patterns were distilled from a production trail map; adapt the constants to your own design.

Setup invariants

  • Set mapboxgl.accessToken before new mapboxgl.Map(...).
  • The map needs 'load' before adding sources/layers/images. In a throttled or background tab 'load' can be missed — also bind 'idle' as a one-shot fallback guarded by an _inited flag (see verification.md).
  • Resolving token/style from .env: read the token FIRST (that triggers the .env load), THEN read MAPBOX_STYLE. Reading the style before the token load silently falls back to the default style. See palette.md.
  • Classic vs Standard style. Several techniques here (basemap palette recolour, the terrain boost-or-add getStyle().layers walk) assume a classic style (Streets/Outdoors/Light/Dark …-v12). The v3 default Standard style has no enumerable named layers — use slots + setConfigProperty instead. See v3-standard-style.md before porting to Standard.

Pick the technique

Read the matching reference file only when the task needs it:

| Task | Reference | |------|-----------| | Custom SVG/canvas markers, addImage/updateImage, namespacing, AA/fringing, circular image masks, anchoring | references/markers.md | | Dashed/cased trail lines, line-dasharray units, translucency over hillshade, colour-by-attribute, line-gradient/lineMetrics | references/lines-and-trails.md | | Hillshade, dense contours, 3D terrain (setTerrain), boost-or-add an existing style's terrain | references/terrain.md | | Symbol-layer text labels that never hide icons (text-optional), AllTrails-style placement | references/labels.md | | Recolour a base style's land/vegetation fills (palette shift / choropleth-style match) | references/palette.md | | Custom popups, circular photo cards, zoom-scaled offsets | references/popups.md | | Style expressions — interpolate/step/match/case, the zoom-outermost rule, feature-state in expressions | references/expressions.md | | Hover/select via feature-state (not setData), queryRenderedFeatures caveats, clustering, GeoJSON perf, event hygiene | references/interaction-and-performance.md | | Data viz & 3D — fill-extrusion buildings/extruded data, heatmap layer, data-join choropleth (feature-state/match), proportional symbols, sky/fog | references/dataviz-and-3d.md | | three.js in the map — CustomLayerInterface + shared GL context, animated 3D objects/models, the baked-matrix vs reconstructed-camera (Threebox CameraSync) fork, raycast picking, ENU-metre scene space, constant screen-size actors, terrain elevation, far-plane clipping at pitch | references/three-custom-layer.md | | Camera & animation — flyTo/easeTo/fitBounds padding, freeCameraOptions cinematics/orbit, flight/first-person camera (bearing+pitch choreography; roll is MapLibre-only), animated day–night cycle (setLights), HUD synced to camera, point-along-line, draw-in lines, paint transitions, spinning globe, the essential/reduced-motion gotcha | references/camera-and-animation.md | | Style library & composition — first-party style catalog, choosing a base by use case, custom/third-party styles, style switcher, light/dark, hand-rolled style JSON | references/styles.md (+ assets/style-catalog.json) | | setStyle wiping custom layers, the 0×0 resize() bug, SPA teardown / WebGL-context cap, token security, readiness events | references/lifecycle.md | | v3 Standard style — slots vs beforeId, setConfigProperty/lightPreset, why layer-walking (palette/terrain) breaks; localisation, RTL, globe | references/v3-standard-style.md | | Headless screenshot + pixel-accurate marker-alignment checks (Playwright, map.project) | references/verification.md |

Bundled resources

  • Starter code — assets/circular_image_marker.js: copy into a page to register a circular photo marker (canvas → premultiplied ImageBitmap, destination-in mask, contact + drop shadow). Browser-only snippet, not a CLI — adapt the frameColor/box constants to your design.

  • Verifier script — scripts/screenshot_map.py: drive headless Chromium to screenshot a served map page, assert a marker projects to its lng/lat, and surface console errors. Run it:

    python -m http.server 8777 --directory <site-dir> &          # serve the page
uv run --with playwright scripts/screenshot_map.py \
  http://localhost:8777/preview/index.html out.png --expect 146.9 -36.1
# exit 0 = no console errors; 10 = errors found; 5 = playwright missing; 7 = map never ready
uv run --with playwright scripts/screenshot_map.py URL out.png --json | jq '.data'

  • Staleness verifier — scripts/check-mapbox-facts.py: stdlib-only (no Playwright), guards the fast-moving facts this skill encodes (SKILL-RESOURCE-PROTOCOL §7). --offline (default) asserts internal consistency — the v3 Standard config enums (lightPreset/theme), terrain tileset IDs, the weather (≥3.7) version gate, the no-native-camera-roll fact (roll is MapLibre GL JS v5, not Mapbox), and every style URL/id in assets/style-catalog.json. --live resolves the third-party style URLs and probes whether Mapbox GL JS has shipped a major past v3.

    python scripts/check-mapbox-facts.py --offline            # exit 0 ok, 4 inconsistency
python scripts/check-mapbox-facts.py --live --json        # exit 7 network, 10 drift

The three highest-value gotchas (full detail in the refs)

  1. Namespace every addImage name (e.g. "rcpin-<glyph>"). Mapbox styles ship sprite icons literally named parking/toilet/etc — an un-namespaced hasImage() returns true for those and your icon is silently dropped.
  2. Register icons as premultiplied createImageBitmap(), not a raw HTMLImageElement/ImageData — straight-alpha sources make Mapbox fringe a white halo around anti-aliased edges. updateImage(name, bmp) recolours in place.
  3. Data-driven icon-offset is silently ignored in GL JS v3. Use a constant icon-offset (it scales with icon-size) or split markers into separate symbol layers, each with its own constant anchor/offset.