What causes "Path must start with a /" in BabylonJS/Babylon.js?

Ensure the path starts with '/', e.g. "/nodes/0/translation".

Path must start with a /

Error message

Path must start with a /

What it means

gltfPathToObjectConverter.convert() walks a glTF JSON Pointer by splitting on '/'; a path not beginning with '/' would produce a bogus first segment, so the converter requires the standard leading slash and throws otherwise. This is the generic (non-Babylon) path-to-object converter used for extension targets like KHR_animation_pointer.

Source

Thrown at packages/dev/loaders/src/glTF/2.0/Extensions/gltfPathToObjectConverter.ts:77

     * <digit> := D+
     *
     * Examples:
     *  - "/nodes/0/rotation"
     * - "/nodes.length"
     *  - "/materials/2/emissiveFactor"
     *  - "/materials/2/pbrMetallicRoughness/baseColorFactor"
     *  - "/materials/2/extensions/KHR_materials_emissive_strength/emissiveStrength"
     *
     * @param path The path to convert
     * @returns The object and info associated with the path
     */
    public convert(path: string): IObjectInfo<IObjectAccessor<T, BabylonType, BabylonValue>> {
        let objectTree: any = this._gltf;
        let infoTree: any = this._infoTree;
        let target: any = undefined;

        if (!path.startsWith("/")) {
            throw new Error("Path must start with a /");
        }
        const parts = path.split("/");
        parts.shift();

        //if the last part has ".length" in it, separate that as an extra part
        if (parts[parts.length - 1].includes(".length")) {
            const lastPart = parts[parts.length - 1];
            const split = lastPart.split(".");
            parts.pop();
            parts.push(...split);
        }

        let ignoreObjectTree = false;

        for (const part of parts) {
            const isLength = part === "length";
            if (isLength) {
                // For .length, check if the current level has a 'length' accessor

View on GitHub (pinned to 0592b347b8)

Solutions

Ensure the path starts with '/', e.g. "/nodes/0/translation".
When building from segments, prefix the join: `/${parts.join("/")}`.
If input comes from config or an external source, normalize it (trim and prepend '/') before calling convert().

Example fix

// before
converter.convert("nodes/0/translation");
// after
converter.convert("/nodes/0/translation");

Defensive patterns

Strategy: validation

Validate before calling

if (typeof path !== "string" || !path.startsWith("/")) {
  throw new Error(`glTF pointer must start with '/': ${path}`);
}
converter.convert(path);

Type guard

function isJsonPointer(path: unknown): path is string {
  return typeof path === "string" && path.startsWith("/");
}

Try / catch

try {
  const info = converter.convert(path);
} catch (e) {
  if (String(e).includes("must start with a /")) {
    path = "/" + String(path).replace(/^\/+/, "");
  }
}

Prevention

Build pointers as `/${segments.join("/")}` so the leading slash is never lost
Normalize externally supplied pointers (trim, ensure leading '/') before conversion
Add a JSON Pointer format check at config-load time

When it happens

Trigger: convert() called with "nodes/0/translation", "extensions/KHR_materials_emissive_strength/emissiveStrength", or any pointer missing the leading '/', including empty strings and paths built by joining segments with '/' but omitting the initial one.

Common situations: Building paths with ["nodes","0","translation"].join("/") instead of "/" + ...join("/"); trimming whitespace or a leading slash accidentally (e.g. .replace(/^\//, "")); receiving a relative reference from user config or an external tool that stores pointers without the leading slash.

Related errors

AI-assisted analysis of BabylonJS/Babylon.js@0592b347b8 (2026-08-30). Data as JSON: /api/errors/db08e1dc0835cbdc. Report an issue: GitHub.