BabylonJS/Babylon.js · error · RuntimeError

GLTFLoaderUnexpectedMagicError

GLTFLoaderUnexpectedMagicError

Error message

Unexpected magic: 

What it means

Thrown as a RuntimeError (code GLTFLoaderUnexpectedMagicError) when the first 4 bytes of a binary glTF file do not equal the magic number 0x46546C67 ('glTF'). The loader uses this constant to confirm the stream is a binary glTF container.

Source

Thrown at packages/dev/loaders/src/glTF/glTFFileLoader.pure.ts:1263

        this._startPerformanceCounter("Parse JSON");
        this._log(`JSON length: ${json.length}`);
        const parsed = JSON.parse(json);
        this._endPerformanceCounter("Parse JSON");
        return parsed;
    }

    private _unpackBinaryAsync(dataReader: DataReader): Promise<IGLTFLoaderData> {
        this._startPerformanceCounter("Unpack Binary");

        // Read magic + version + length + json length + json format
        return dataReader.loadAsync(20).then(() => {
            const Binary = {
                Magic: 0x46546c67,
            };

            const magic = dataReader.readUint32();
            if (magic !== Binary.Magic) {
                throw new RuntimeError("Unexpected magic: " + magic, ErrorCodes.GLTFLoaderUnexpectedMagicError);
            }

            const version = dataReader.readUint32();

            if (this.loggingEnabled) {
                this._log(`Binary version: ${version}`);
            }

            const length = dataReader.readUint32();
            if (!this.useRangeRequests && length !== dataReader.buffer.byteLength) {
                Logger.Warn(`Length in header does not match actual data length: ${length} != ${dataReader.buffer.byteLength}`);
            }

            let unpacked: Promise<IGLTFLoaderData>;
            switch (version) {
                case 1: {
                    unpacked = this._unpackBinaryV1Async(dataReader, length);
                    break;

View on GitHub (pinned to 0592b347b8)

Solutions

1. Verify the URL returns an actual .glb file (check first bytes are 'glTF')
2. Check the HTTP response status/content-type and fix server routing so errors aren't served as 200
3. Re-download/re-export the file — it may be corrupted or truncated

Example fix

// before
scene.importMeshesFromMeshesAsync("model.gltf.url/that/404s.glb"); // server returns HTML 200
// after
const res = await fetch(url);
if (!res.ok || !(await res.arrayBuffer()).byteLength) throw new Error("bad asset");
await scene.importMeshesFromMeshesAsync(url);

When it happens

Trigger: Calling a load path that expects binary glTF (.glb) but the fetched bytes are JSON text, HTML (e.g. a 404 page), or a truncated file.

Common situations: Server returning an HTML error page with 200 status; wrong file extension/URL; serving .gltf JSON to a .glb path; gzip/CDN mangling the bytes.

Related errors

• Unexpected content format: ${contentFormat}
• First chunk format is not JSON
• Unexpected JSON chunk
• Expected opening { after type & name
• Expected OFFSET, but got:

AI-assisted analysis of BabylonJS/Babylon.js@0592b347b8 (2026-08-30). Data as JSON: /api/errors/e472c427cc057130. Report an issue: GitHub.