SeleniumHQ/selenium · error · Error

Params must be an instance of…

Error message

Params must be an instance of BrowsingContextPartitionDescriptor or StorageKeyPartitionDescriptor. Received:'${partition}'

What it means

Thrown by Storage.getCookies() when an optional partition argument is provided but is neither a BrowsingContextPartitionDescriptor nor a StorageKeyPartitionDescriptor. The method serializes the partition via asMap() for the storage.getCookies command, so it requires one of these two partition descriptor classes. Any other type (plain object, string) is rejected.

Solutions

1. Wrap the context id in a BrowsingContextPartitionDescriptor: getCookies(filter, new BrowsingContextPartitionDescriptor(contextId)).
2. For source-origin partitioning, use new StorageKeyPartitionDescriptor().sourceOrigin(origin).
3. Omit the partition argument if you do not need partition-scoped cookies.

Example fix

// before
const result = await storage.getCookies(filter, contextId) // raw string

// after
const { BrowsingContextPartitionDescriptor } = require('selenium-webdriver/bidi/partitionDescriptor')
const result = await storage.getCookies(filter, new BrowsingContextPartitionDescriptor(contextId))

function isPartitionDescriptor(v) {
  return v === undefined ||
    v instanceof BrowsingContextPartitionDescriptor ||
    v instanceof StorageKeyPartitionDescriptor
}
if (!isPartitionDescriptor(partition)) {
  throw new TypeError('partition must be a partition descriptor')
}

function isPartitionDescriptor(v) {
  return v === undefined ||
    v instanceof BrowsingContextPartitionDescriptor ||
    v instanceof StorageKeyPartitionDescriptor
}

When it happens

Trigger: Calling storage.getCookies(filter, 'ctx123') (raw string), getCookies(filter, {type:'context',context:'ctx123'}) (plain object), or getCookies(filter, someOtherDescriptorInstance).

Common situations: Passing the browsing context id string instead of a descriptor; building a partition from parsed JSON as a plain object; using the wrong partition descriptor class for the storage partition scheme.

Related errors

• Params must be an instance of CookieFilter. Received
• Params must be an instance of PartialCookie. Received
• Value must be an instance of BytesValue. Received
• CookieHeader must be an instance of Header. Received
• browsingContexts should be string or string array

AI-assisted analysis of SeleniumHQ/selenium@aa36b38e69 (2026-08-14). Data as JSON: /api/errors/40a6a23406f2b9cc. Report an issue: GitHub.

Appendix: source

Thrown at javascript/selenium-webdriver/bidi/storage.js:60

  /**
   * Retrieves cookies based on the provided filter and partition.
   *
   * @param {CookieFilter} [filter] - The filter to apply to the cookies.
   * @param {(BrowsingContextPartitionDescriptor|StorageKeyPartitionDescriptor)} [partition] - The partition to retrieve cookies from.
   * @returns {Promise<{ cookies: Cookie[], partitionKey: (PartitionKey|undefined) }>} - A promise that resolves to an object containing the retrieved cookies and an optional partition key.
   * @throws {Error} If the filter parameter is provided but is not an instance of CookieFilter.
   * @throws {Error} If the partition parameter is provided but is not an instance of BrowsingContextPartitionDescriptor or StorageKeyPartitionDescriptor.
   */
  async getCookies(filter = undefined, partition = undefined) {
    if (filter !== undefined && !(filter instanceof CookieFilter)) {
      throw new Error(`Params must be an instance of CookieFilter. Received:'${filter}'`)
    }

    if (
      partition !== undefined &&
      !(partition instanceof BrowsingContextPartitionDescriptor || partition instanceof StorageKeyPartitionDescriptor)
    ) {
      throw new Error(
        `Params must be an instance of BrowsingContextPartitionDescriptor or StorageKeyPartitionDescriptor. Received:'${partition}'`,
      )
    }

    const command = {
      method: 'storage.getCookies',
      params: {
        filter: filter ? Object.fromEntries(filter.asMap()) : undefined,
        partition: partition ? Object.fromEntries(partition.asMap()) : undefined,
      },
    }

    let response = await this.bidi.send(command)

    let cookies = []
    response.result.cookies.forEach((cookie) => {
      cookies.push(
        new Cookie(

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