pydantic/monty · error · TypeError

ClassType attrs must be an array of [name, value] pairs

Error message

ClassType attrs must be an array of [name, value] pairs

What it means

pushClassType requires a ClassType marker's `attrs` to be an array of [name, value] pairs. A missing or non-array attrs is treated as a forged or malformed marker (not an empty class) and rejected with this TypeError, matching the native binding's contract.

Source

Thrown at crates/monty-js/ts/worker/value.ts:217

    val: {
      classType: classTypeIndex,
      instanceId: uuidString(object.instanceId, 'ClassInstance instanceId'),
      attrs: pushPairs(pairs, nodes),
    },
  }
}

/** Builds a class-type node from the plain `classType` marker object,
 *  appending its eager attr nodes to the arena. */
function pushClassType(
  object: Record<string, unknown>,
  nodes: ValueNode[],
): Extract<ValueNode, { tag: 'class-type' }>['val'] {
  // Require an array like the native binding does, so both transports
  // enforce the same marker contract (a missing `attrs` is a forged or
  // malformed marker, not an empty attribute list).
  if (!Array.isArray(object.attrs)) {
    throw new TypeError('ClassType attrs must be an array of [name, value] pairs')
  }
  const attrPairs: [unknown, unknown][] = []
  for (const pair of object.attrs as unknown[]) {
    if (!Array.isArray(pair)) throw new TypeError('ClassType attrs entries must be [name, value] pairs')
    if (typeof pair[0] !== 'string') throw new TypeError('ClassType attr name must be a string')
    if (!(1 in pair)) throw new TypeError('ClassType attr value missing')
    attrPairs.push([pair[0], pair[1]])
  }
  return {
    name: String(object.name),
    id: uuidString(object.id, 'ClassType id'),
    hostDefined: object.hostDefined === true,
    isDataclass: object.isDataclass === true,
    attrs: pushPairs(attrPairs, nodes),
  }
}

/** A canonical uuid string is required for identities crossing the wire. */

View on GitHub (pinned to adc986b362)

Solutions

1. Include attrs: [] explicitly even when the class has no attributes
2. Ensure attrs is an array of [stringName, value] pairs
3. Validate with Array.isArray(marker.attrs) before passing the marker

Example fix

// before
{ marker: 'ClassType', name: 'Empty' } // attrs missing
// after
{ marker: 'ClassType', name: 'Empty', attrs: [] }

const assertClassType = (o: Record<string, unknown>): void => {
  if (!Array.isArray(o.attrs)) throw new Error('ClassType.attrs must be an array of [name, value] pairs')
  if (!o.attrs.every(p => Array.isArray(p) && typeof p[0] === 'string' && 1 in p)) throw new Error('Bad ClassType attrs pair')
}

const isClassType = (v: unknown): v is { name: string; attrs: [string, unknown][] } =>
  typeof v === 'object' && v !== null && typeof (v as any).name === 'string' && Array.isArray((v as any).attrs)

try {
  return pushClassType(object, nodes)
} catch (e) {
  if (e instanceof TypeError && e.message.startsWith('ClassType attrs must be an array')) {
    throw new Error('Forged or incomplete ClassType marker: attrs array required')
  }
  throw e
}

When it happens

Trigger: Passing a ClassType marker without an attrs key; attrs as an object map instead of an array of pairs; markers reconstructed from partial data where class attributes were dropped.

Common situations: Hand-building class type descriptors for typed inputs; trimming marker payloads and accidentally removing attrs; converting markers from a format where empty attrs is null.

Understand the failure class

Background: Schema validation failed / invalid input schema: payload rejected because its shape doesn't match the expected schema — this error's family across 28 libraries.

Related errors

• Object property 'type' type mismatch. Expect value to be Obj
• Max input depth exceeded
• raw Type markers are not accepted — pass the class through C
• ClassInstance marker instanceId must be a uuid string
• ${field} must be 'all', undefined or a list/Set of names, go

AI-assisted analysis of pydantic/monty@adc986b362 (2026-09-13). Data as JSON: /api/errors/9ab134d445c9877d. Report an issue: GitHub.