deepseek-ai/deepseek-harness · error · Error

the Host half returned `undefined` — did you forget `return`

Error message

the Host half returned `undefined` — did you forget `return`?

What it means

Error "the Host half returned `undefined` — did you forget `return`?" thrown in deepseek-ai/deepseek-harness.

Source

Thrown at packages/extensions/cordis-host-runner/src/index.ts:901

  private async startHost(
    plugin: DynamicCordisPlugin,
    hostCode: string,
    run: DynamicCordisRun,
  ): Promise<CordisErrorDetails | undefined> {
    const handle = (method: unknown, fn: unknown): (() => void) => {
      const normalized = normalizeHandler(method, fn)
      run.handlers.set(normalized.method, normalized.handler)
      const dispose = (): void => {
        if (run.handlers.get(normalized.method) === normalized.handler) run.handlers.delete(normalized.method)
      }
      run.handlerDisposers.push(dispose)
      return dispose
    }
    try {
      const sandbox = createSandbox(plugin.pluginId, { handle })
      const evaluated = await evaluateHostCode(sandbox, hostCode, plugin.pluginId, this.resolved.vmTimeoutMs)
      if (!isPlugin(evaluated)) {
        throw new Error(evaluated === undefined
          ? 'the Host half returned `undefined` — did you forget `return`?'
          : 'the Host half must return a Plugin function or an object with apply(ctx)')
      }
      run.fiber = await startHostHalf(
        this.requireGroup(),
        evaluated,
        (error) => { this.steerGuardFailure(plugin, run, 'Host', errorDetails(error)) },
      )
      return undefined
    } catch (error) {
      for (const dispose of run.handlerDisposers.splice(0)) dispose()
      return errorDetails(error)
    }
  }

  private async settleActivation(
    plugin: DynamicCordisPlugin | undefined,
    resolution: DynamicCordisRunResolution,

View on GitHub (pinned to b150a551b8)

When it happens

Trigger: Thrown at packages/extensions/cordis-host-runner/src/index.ts:901 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/1d593f9717e658a3. Report an issue: GitHub.