deepseek-ai/deepseek-harness · error · Error

session event "${type}" carries non-JSON-serializable data

Error message

session event "${type}" carries non-JSON-serializable data

What it means

Error "session event "${type}" carries non-JSON-serializable data" thrown in deepseek-ai/deepseek-harness.

Source

Thrown at packages/core/session/src/index.ts:616

   *   to validation and another to storage. The event log is the durable source
   *   of truth, so a bad event fails at the append site rather than later during
   *   a backend flush. A synchronous internal dispatch validation failure or an
   *   append reentered while this acceptance/publication boundary is open also
   *   rejects before the log changes.
   */
  append<T extends SessionEventType>(
    type: T,
    data: SessionEventMap[T],
    ...opts: T extends SurfaceEventType ? [opts: SurfaceIntent] : []
  ): SessionEvent<T> {
    const surfaceOpts: SurfaceIntent | undefined = opts[0]
    const surfaceMetadata = {
      ...surfaceOpts?.sourceEventSeqs === undefined ? {} : { sourceEventSeqs: surfaceOpts.sourceEventSeqs },
      ...surfaceOpts?.surfaceOp === undefined ? {} : { surfaceOp: surfaceOpts.surfaceOp },
    }
    const dataSnapshot = snapshotJsonValue(data)
    if (dataSnapshot === undefined) {
      throw new Error(`session event "${type}" carries non-JSON-serializable data`)
    }
    assertSupportedRequestHeader(type, dataSnapshot, `session event "${type}"`)
    const surfaceMetadataSnapshot = snapshotJsonValue(surfaceMetadata)
    if (surfaceMetadataSnapshot === undefined) {
      throw new Error(`session event "${type}" carries non-JSON-serializable surface metadata`)
    }
    const entry = attachments.get(this)
    if (entry?.appending) {
      throw new Error('session append cannot reenter while another append is being published')
    }
    const event = deepFreeze({
      type,
      seq: this.log.length,
      time: Date.now(),
      data: dataSnapshot,
      ...(surfaceMetadataSnapshot as { surfaceOp?: unknown; sourceEventSeqs?: unknown }),
    } as unknown as SessionEvent<T>)
    this.surfaceManager.validateNext(event as SessionEvent)

View on GitHub (pinned to b150a551b8)

Solutions

1. Ensure the session event payload is JSON-serializable before emitting it.

When it happens

Trigger: Thrown at packages/core/session/src/index.ts:616 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/dc06abf373ff37da. Report an issue: GitHub.