deepseek-ai/deepseek-harness · error · Error

session header is not losslessly JSON-serializable

Error message

session header is not losslessly JSON-serializable

What it means

Error "session header is not losslessly JSON-serializable" thrown in deepseek-ai/deepseek-harness.

Source

Thrown at packages/core/session/src/index.ts:155

/** Validate and freeze one exclusively owned persistence header in place. */
function validateRestoredSessionHeader(id: SessionId, input: unknown): SessionHeader {
  if (input !== null && typeof input === 'object' && !Array.isArray(input)) {
    const prototype = Reflect.getPrototypeOf(input)
    if (prototype !== Object.prototype && prototype !== null) {
      throw new Error('session header is not a plain JSON record')
    }
  }
  return validateSessionHeader(id, input)
}

/** Detach, validate, and freeze the creation metadata published by a session. */
function snapshotSessionHeader(id: SessionId, source?: SessionHeader): SessionHeader {
  const input: unknown = source === undefined
    ? { version: SESSION_FORMAT_VERSION, id, createdAt: Date.now() }
    : source
  const snapshot = snapshotJsonValue(input)
  if (snapshot === undefined) throw new Error('session header is not losslessly JSON-serializable')
  return validateSessionHeader(id, snapshot)
}

/**
 * Validate an exclusively owned event and deeply freeze its identified message
 * without copying the event. The caller transfers an object graph that no
 * producer retains and that shares no mutable children with another event.
 * Use {@link snapshotSessionEvent} when exclusive ownership is not guaranteed.
 * @param event - exclusively owned event imported across a trusted boundary.
 * @returns the same event object with a validated, deeply frozen message.
 */
export function adoptSessionEvent<T extends SessionEvent>(event: T): T {
  assertMessageEventShape(
    event,
    `session event at seq ${event.seq}`,
  )
  switch (event.type) {
    case 'user/message':

View on GitHub (pinned to b150a551b8)

Solutions

1. Ensure the session header round-trips JSON losslessly.

When it happens

Trigger: Thrown at packages/core/session/src/index.ts:155 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/71c1ec3a9e86950f. Report an issue: GitHub.