deepseek-ai/deepseek-harness · error · Error

seed event at index ${index} is not losslessly JSON-serializ

Error message

seed event at index ${index} is not losslessly JSON-serializable

What it means

Error "seed event at index ${index} is not losslessly JSON-serializable" thrown in deepseek-ai/deepseek-harness.

Source

Thrown at packages/core/session/src/index.ts:521

    mode: 'snapshot' | 'restore' = 'snapshot',
  ) {
    const restoredHeader = mode === 'restore'
      ? validateRestoredSessionHeader(id, header)
      : undefined
    if (seed !== undefined) {
      // Validate the seed to the SAME invariants `append` enforces, so a
      // replay/fork (`ctx.sessions.create(id, { seed })`) cannot construct a
      // live log that no persistence backend could store: each event's `data`
      // must be JSON-serializable, and `seq` must be contiguous from 0 (the
      // `seq = log.length` contract the whole system relies on). Without this,
      // a bad seed would surface only later as a backend rejection or a silent
      // divergence between the live log and disk.
      for (const [index, source] of seed.entries()) {
        // The seed is a persistence/replay boundary: validate and detach the
        // complete event in one lossless-JSON pass.
        const snapshot = mode === 'restore' ? source : snapshotJsonValue(source)
        if (snapshot === undefined) {
          throw new Error(`seed event at index ${index} is not losslessly JSON-serializable`)
        }
        assertSessionEventEnvelope(snapshot, index)
        assertSupportedRequestHeader(snapshot.type, snapshot.data, `seed event at index ${index}`)
        if (snapshot.seq !== index) {
          throw new Error(`seed event at index ${index} has seq ${snapshot.seq} (expected ${index}); seed must be contiguous from 0`)
        }
        // A seed is accepted incrementally through the same transition as a
        // live append and a full-log fold. The candidate is planned before it
        // enters `log`, so a failure cannot partially mutate the surface.
        try {
          this.surfaceManager.validateNext(snapshot)
        } catch (error: unknown) {
          throw new Error(`invalid seed event at index ${index}: ${error instanceof Error ? error.message : 'invalid surface metadata'}`)
        }
        this.log.push(mode === 'restore' ? freezeRestoredObject(snapshot) : deepFreeze(snapshot))
      }
    }
    this.firstLiveSeq = this.log.length

View on GitHub (pinned to b150a551b8)

Solutions

1. Ensure the seed event is losslessly JSON-serializable.

When it happens

Trigger: Thrown at packages/core/session/src/index.ts:521 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/0f0a8c81bdf7c40f. Report an issue: GitHub.