deepseek-ai/deepseek-harness · error · MaterializeError

only plain objects and arrays are JSON data (exotic prototyp

Error message

only plain objects and arrays are JSON data (exotic prototype)

What it means

Error "only plain objects and arrays are JSON data (exotic prototype)" thrown in deepseek-ai/deepseek-harness.

Source

Thrown at packages/workflow/workflow-worker-thread/src/realm.ts:132

    out.push(materialize(value[index], `${path}[${index}]`, seen))
  }
  // Own enumerable props beyond the indices (e.g. `arr.total = 3`) would be
  // silently dropped by JSON — reject them instead.
  for (const key of Object.keys(value)) {
    const index = Number(key)
    if (!Number.isInteger(index) || index < 0 || index >= value.length) {
      throw new MaterializeError(`${path}.${key}`, 'arrays with non-index properties are not JSON data')
    }
  }
  if (Object.getOwnPropertySymbols(value).length > 0) {
    throw new MaterializeError(path, 'symbol-keyed properties are not plain JSON data')
  }
  return out
}

function materializeObject(value: object, path: string, seen: Set<object>): Record<string, unknown> {
  if (!hasPlainPrototype(value)) {
    throw new MaterializeError(path, 'only plain objects and arrays are JSON data (exotic prototype)')
  }
  if (Object.getOwnPropertySymbols(value).length > 0) {
    throw new MaterializeError(path, 'symbol-keyed properties are not plain JSON data')
  }
  const out: Record<string, unknown> = {}
  // Object.keys = own enumerable string keys, matching JSON.stringify's
  // property selection exactly (non-enumerable props never reach JSON output).
  for (const key of Object.keys(value)) {
    // defineProperty, never assignment: a "__proto__" key must become an OWN
    // data property of the copy, not a prototype mutation.
    Object.defineProperty(out, key, {
      value: materialize((value as Record<string, unknown>)[key], `${path}.${key}`, seen),
      enumerable: true,
      writable: true,
      configurable: true,
    })
  }
  return out

View on GitHub (pinned to b150a551b8)

Solutions

1. Return plain objects and arrays created with {} / [] or Object.create(null); class instances and exotic prototypes are not JSON data.

When it happens

Trigger: Thrown at packages/workflow/workflow-worker-thread/src/realm.ts:132 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of deepseek-ai/deepseek-harness@b150a551b8 (2026-08-24). Data as JSON: /api/errors/b5fb987b7bc37f76. Report an issue: GitHub.