deepseek-ai/deepseek-harness · error · Error

${location} uses unsupported legacy request/header reason "f

Error message

${location} uses unsupported legacy request/header reason "fallback"

What it means

Error "${location} uses unsupported legacy request/header reason "fallback"" thrown in deepseek-ai/deepseek-harness.

Source

Thrown at packages/core/session/src/index.ts:370

}

/** Whether an unknown value carries the current provider/model pair. */
function hasProviderModel(value: unknown): boolean {
  if (typeof value !== 'object' || value === null) return false
  const pair = value as Record<string, unknown>
  return typeof pair['provider'] === 'string' && pair['provider'].length > 0
    && typeof pair['model'] === 'string' && pair['model'].length > 0
}

/** Reject request-header vocabulary removed with the legacy delta codec. */
function assertSupportedRequestHeader(type: string, data: unknown, location: string): void {
  if (type === 'request/header-delta') {
    throw new Error(`${location} uses unsupported legacy request/header-delta format`)
  }
  if (type === 'request/header'
    && data !== null && typeof data === 'object' && !Array.isArray(data)
    && (data as Record<string, unknown>)['reason'] === 'fallback') {
    throw new Error(`${location} uses unsupported legacy request/header reason "fallback"`)
  }
}

type SessionCallback = (...args: unknown[]) => unknown

/** Resolve one listener snapshot, including Cordis's internal dispatch checks. */
function collectSessionCallbacks(ctx: Context, args: unknown[]): SessionCallback[] {
  return [...ctx.events.dispatch('emit', args)] as SessionCallback[]
}

/** Invoke one resolved observe-only listener snapshot with per-listener containment. */
function invokeContainedSessionObservers(
  ctx: Context,
  name: 'session/event' | 'session/disposed',
  id: SessionId,
  args: unknown[],
  callbacks: SessionCallback[],
): void {

View on GitHub (pinned to b150a551b8)

Solutions

1. Remove the legacy request/header fallback reason usage.

When it happens

Trigger: Thrown at packages/core/session/src/index.ts:370 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/3e89396c8083e57c. Report an issue: GitHub.