deepseek-ai/deepseek-harness · error · TypeError

tool result must be losslessly JSON-serializable

Error message

tool result must be losslessly JSON-serializable

What it means

Error "tool result must be losslessly JSON-serializable" thrown in deepseek-ai/deepseek-harness.

Source

Thrown at packages/core/tools/src/index.ts:636

    // coercion. Error normalization is the outermost safety boundary, so its
    // fallback must itself be total.
    return '<unprintable thrown value>'
  }
}

/** Derive one failure message from policy feedback without changing its rendered blocks. */
function failureMessageFromContent(content: ContentBlock[]): string {
  const text = content
    .map(block => block.type === 'text' ? block.text : `[${block.type} content]`)
    .join('\n')
  return text.length > 0 ? text : 'tool result blocked by post-execute policy'
}

/** Snapshot and freeze one durable tool-result projection or reject lossy data. */
function materializePresentation<T>(candidate: T): T {
  const detached = snapshotJsonValue(candidate)
  if (detached === undefined) {
    throw new TypeError('tool result must be losslessly JSON-serializable')
  }
  return deepFreeze(detached)
}

/** Structured `{ name, code }` for a thrown HarnessError, else undefined. */
function errorInfo(error: unknown): ToolErrorInfo | undefined {
  try {
    return error instanceof HarnessError ? { name: error.name, code: error.code } : undefined
  } catch {
    return undefined
  }
}

/** How the registry presents its tools to the model (see {@link Config.mode}). */
export type ToolPresentationMode = 'native' | 'code' | 'both'

/** Plugin config: how the registered tools are presented to the model. */
export interface Config {

View on GitHub (pinned to b150a551b8)

When it happens

Trigger: Thrown at packages/core/tools/src/index.ts:636 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/9ef9bd6bf3753997. Report an issue: GitHub.