danny-avila/LibreChat · error · AgentRunEnvelopeError

${path}[${index}] must not be an accessor property

Error message

${path}[${index}] must not be an accessor property

What it means

Thrown by cloneJsonValue when an array index property is defined by a getter/setter (accessor) rather than a plain data value. The check is Object.getOwnPropertyDescriptor lacking an own 'value' key. Accessors can execute arbitrary code or throw on read, so the envelope clone refuses to invoke them; it requires a concrete value it can copy safely.

Source

Thrown at packages/api/src/agents/envelope.ts:152

    if (Array.isArray(value)) {
      const cloned: unknown[] = new Array(value.length);
      let clonedItemCount = 0;
      for (const key of Object.getOwnPropertyNames(value)) {
        if (key === 'length') {
          continue;
        }
        const index = Number(key);
        if (
          !Number.isSafeInteger(index) ||
          index < 0 ||
          index >= value.length ||
          String(index) !== key
        ) {
          throw new AgentRunEnvelopeError(`${path} contains non-index array properties`);
        }
        const descriptor = Object.getOwnPropertyDescriptor(value, key);
        if (!descriptor || !Object.prototype.hasOwnProperty.call(descriptor, 'value')) {
          throw new AgentRunEnvelopeError(`${path}[${index}] must not be an accessor property`);
        }
        const itemValue: unknown = descriptor.value;
        cloned[index] = cloneJsonValue(itemValue, `${path}[${index}]`, ancestors, depth + 1);
        clonedItemCount++;
      }
      if (clonedItemCount !== value.length) {
        throw new AgentRunEnvelopeError(`${path} contains sparse array entries`);
      }
      return cloned;
    }

    const prototype = Object.getPrototypeOf(value);
    if (prototype !== Object.prototype && prototype !== null) {
      const typeName = value.constructor?.name ?? 'object';
      throw new AgentRunEnvelopeError(`${path} contains a non-plain ${typeName} value`);
    }

    const cloned: { [key: string]: unknown } = {};

View on GitHub (pinned to 5ff282f900)

Solutions

  1. Materialize the array into a plain Array of concrete values via Array.from(arr) or arr.map(x => x) before passing it in.
  2. Replace accessor-defined indices with normal assignments (arr[0] = value).
  3. Avoid extending Array for payload objects; use composition instead.

Example fix

// before
Object.defineProperty(results, 0, { get: () => compute(), enumerable: true });
createAgentRunEnvelope({ protocol, requestId, receivedAt, principal, payload: { results } });

// after
const results = [compute()];
createAgentRunEnvelope({ protocol, requestId, receivedAt, principal, payload: { results } });
Defensive patterns

Strategy: validation

Validate before calling

function arrayHasOnlyDataValues(arr: unknown[]): boolean {
  for (const key of Object.getOwnPropertyNames(arr)) {
    if (key === 'length') continue;
    const d = Object.getOwnPropertyDescriptor(arr, key);
    if (!d || !Object.prototype.hasOwnProperty.call(d, 'value')) return false;
  }
  return true;
}
if (Array.isArray(payload.results) && !arrayHasOnlyDataValues(payload.results)) {
  payload.results = payload.results.map((x) => x); // materialize getters into data values
}

Type guard

function isArrayDataValues(arr: unknown[]): boolean {
  return Object.getOwnPropertyNames(arr)
    .filter((k) => k !== 'length')
    .every((k) => {
      const d = Object.getOwnPropertyDescriptor(arr, k);
      return !!d && Object.prototype.hasOwnProperty.call(d, 'value');
    });
}

Try / catch

try {
  const env = createAgentRunEnvelope(input);
} catch (e) {
  if (e instanceof AgentRunEnvelopeError && /accessor property/.test(e.message)) {
    // materialize: payload = JSON.parse(JSON.stringify(payload));
  } else throw e;
}

Prevention

When it happens

Trigger: An array built with Object.defineProperty(arr, 0, { get() {...} }) or an array subclass whose indices are virtualized. Proxy-wrapped arrays whose traps surface as accessors can also reach this.

Common situations: A custom collection class extending Array that virtualizes elements; sealing/freezing with accessor descriptors; interop with libraries that wrap arrays in getters for lazy loading.

Related errors

AI-assisted analysis of danny-avila/LibreChat@5ff282f900 (2026-08-12). Data as JSON: /api/errors/ef4a995d0a10bb3d. Report an issue: GitHub.