deepseek-ai/deepseek-harness · error · WorkflowError

RESULT_UNSERIALIZABLE

RESULT_UNSERIALIZABLE

Error message

the workflow's return value is not plain JSON data — ${error.message}. Return only JSON-serializable objects/arrays/scalars.

What it means

Error "the workflow's return value is not plain JSON data — ${error.message}. Return only JSON-serializable objects/arrays/scalars." thrown in deepseek-ai/deepseek-harness.

Source

Thrown at packages/workflow/workflow-worker-thread/src/runtime.ts:214

    promise.catch(() => { /* consumed: see method contract — a dropped hook promise must not surface an unhandled rejection */ })
    return promise
  }

  private cancelledError(): WorkflowError {
    // cancel() arms cancelError before any caller can observe isCancelled()
    // === true; the fallback guards the type, not a reachable path.
    /* v8 ignore next */
    return this.cancelError ?? new WorkflowError('workflow run cancelled', 'CANCELLED')
  }

  /** Materialize the script's return value; violations become RESULT_UNSERIALIZABLE. */
  private materializeResult(raw: unknown): unknown {
    try {
      return materializeFromRealm(raw, 'workflow result')
    } catch (error: unknown) {
      /* v8 ignore next -- defensive rethrow arm: materializeFromRealm only throws MaterializeError */
      if (!(error instanceof MaterializeError)) throw error
      throw new WorkflowError(
        `the workflow's return value is not plain JSON data — ${error.message}. Return only JSON-serializable objects/arrays/scalars.`,
        'RESULT_UNSERIALIZABLE',
        { cause: error },
      )
    }
  }

  /**
   * Acquire one concurrency slot (FIFO). Cancellation rejects QUEUED waiters
   * (see {@link cancel}); the callers guard their own entry and post-acquire
   * windows, so no cancelled-precheck is duplicated here.
   */
  private acquireSlot(): Promise<void> {
    if (this.activeSlots < this.limits.maxConcurrentAgents) {
      this.activeSlots += 1
      return Promise.resolve()
    }
    return new Promise<void>((resolve, reject) => {

View on GitHub (pinned to b150a551b8)

Solutions

1. Return only JSON-serializable objects, arrays, and scalars from the workflow; apply the inner error's guidance for the offending value.

When it happens

Trigger: Thrown at packages/workflow/workflow-worker-thread/src/runtime.ts:214 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/a79ce1db543cf3de. Report an issue: GitHub.