deepseek-ai/deepseek-harness · error · Error

${at} must be lossless JSON data (objects, arrays, strings,

Error message

${at} must be lossless JSON data (objects, arrays, strings, numbers, booleans, null) — not a class instance, function, Map/Set, Date, or undefined. Return a plain object built from the values you need, or `return null` when the caller needs no value back.

What it means

Error "${at} must be lossless JSON data (objects, arrays, strings, numbers, booleans, null) — not a class instance, function, Map/Set, Date, or undefined. Return a plain object built from the values you need, or `return null` when the caller needs no value back." thrown in deepseek-ai/deepseek-harness.

Source

Thrown at packages/extensions/cordis-host-runner/src/guard.ts:122

      root = item
      return
    }
    if (destination.kind === 'array') {
      destination.target[destination.index] = item
      return
    }
    Object.defineProperty(destination.target, destination.key, {
      value: item,
      enumerable: true,
      configurable: true,
      writable: true,
    })
  }
  const reject = (at: string): never => {
    // Naming the executable next step matters more than naming the rule: the
    // usual cause is a handler that returns whatever its last call produced,
    // and the fix is one keyword.
    throw new Error(`${at} must be lossless JSON data (objects, arrays, strings, numbers, booleans, null) — `
      + 'not a class instance, function, Map/Set, Date, or undefined. Return a plain object built from the '
      + 'values you need, or `return null` when the caller needs no value back.')
  }

  const tasks: CloneTask[] = [{ kind: 'visit', value, path, destination: { kind: 'root' } }]
  for (let task = tasks.pop(); task !== undefined; task = tasks.pop()) {
    if (task.kind === 'leave') {
      ancestors.delete(task.source)
      continue
    }
    if (task.kind === 'array-item') {
      if (!Object.hasOwn(task.source, task.index)) reject(task.path)
      tasks.push({
        kind: 'visit',
        value: task.source[task.index],
        path: `${task.path}[${task.index}]`,
        destination: { kind: 'array', target: task.target, index: task.index },
      })

View on GitHub (pinned to b150a551b8)

When it happens

Trigger: Thrown at packages/extensions/cordis-host-runner/src/guard.ts:122 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/c73949d69f2dfcf6. Report an issue: GitHub.