vercel/next.js · error

Failed to parse source map for ${filename}.

Error message

Failed to parse source map for ${filename}.

What it means

Thrown by getSourceMapFromFile when an inline data: URI source map has the correct application/json MIME type but its body fails JSON.parse. This means the decoded bytes are present but are not valid JSON, so Next.js cannot build a RawSourceMap. The underlying JSON.parse error is attached as cause. It only occurs in dev when the overlay/stack-trace resolver needs the original source for a file whose source map is inlined.

Source

Thrown at packages/next/src/server/dev/get-source-map-from-file.ts:66

    try {
      buffer = dataUriToBuffer(sourceUrl)
    } catch (error) {
      throw new Error(`Failed to parse source map URL for ${filename}.`, {
        cause: error,
      })
    }

    if (buffer.type !== 'application/json') {
      throw new Error(
        `Unknown source map type for ${filename}: ${buffer.typeFull}.`
      )
    }

    try {
      return JSON.parse(buffer.toString())
    } catch (error) {
      throw new Error(`Failed to parse source map for ${filename}.`, {
        cause: error,
      })
    }
  }

  const sourceMapFilename = path.resolve(
    path.dirname(filename),
    decodeURIComponent(sourceUrl)
  )

  try {
    const sourceMapContents = await fs.readFile(sourceMapFilename, 'utf-8')

    return JSON.parse(sourceMapContents.toString())
  } catch (error) {
    throw new Error(`Failed to parse source map ${sourceMapFilename}.`, {
      cause: error,
    })

View on GitHub (pinned to 0ae8c72462)

Solutions

1. Delete .next and rebuild to regenerate clean inline source maps.
2. Inspect the file named in the error; decode its sourceMappingURL data URI and run it through a JSON validator to find the syntax error.
3. Fix or remove the loader/plugin producing the truncated inline source map.
4. Fall back to external .map files (sourceMap emission) so the inline body is not the source of corruption.

Example fix

// before (truncated base64 body that fails JSON.parse)
//# sourceMappingURL=data:application/json;base64,eyJ2ZXJzaW9uIj

// after (regenerated, complete, valid source map)
//# sourceMappingURL=data:application/json;base64,eyJ2ZXJzaW9uIjozfQ==

// Decode + validate an inline JSON source map body before use
function validateInlineJsonMap(dataUri: string) {
  const idx = dataUri.indexOf(',')
  const encoded = dataUri.slice(idx + 1)
  const isBase64 = /;base64/.test(dataUri.slice(0, idx))
  const text = isBase64 ? Buffer.from(encoded, 'base64').toString('utf8') : decodeURIComponent(encoded)
  return JSON.parse(text) // throws on truncated/corrupt JSON, surfaced before Next.js wraps it
}

try {
  return JSON.parse(buffer.toString())
} catch (cause) {
  throw new Error(`Failed to parse source map for ${filename}.`, { cause })
}

When it happens

Trigger: A file's sourceMappingURL=data:application/json;... comment decodes to bytes that are not parseable JSON (truncated, base64-mangled, or hand-edited). JSON.parse throws and is wrapped into this error.

Common situations: A base64 inline source map was truncated by a build step, editor, or version-control line-ending normalization. A minifier emitted a malformed inline source map. Manual edits to a compiled file corrupted the trailing data URI body.

Understand the failure class

• Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

Related errors

• Failed to read file contents of ${filename}.
• Failed to parse source map URL for ${filename}.
• Unknown source map type for ${filename}: ${buffer.typeFull}.
• Failed to parse source map ${sourceMapFilename}.
• ${sourceURL}: Invalid source map. Only conformant source map

AI-assisted analysis of vercel/next.js@0ae8c72462 (2026-08-06). Data as JSON: /api/errors/7ba850b2d584fa95. Report an issue: GitHub.