Hi there,

Can you confirm that you’re using the client-side Subscribe endpoint? And if so, can you share the entire URL that you are calling? That should include the company ID, which is likely where the issue is.

​@jrdgdfg I echo Christian that my hunch is that whatever your passing for company_id to the URL eg https://a.klaviyo.com/client/subscriptions?company_id=ddddd is invalid.

Assuming that you are calling this method too

You have to enter the Public API key as specified here

Hi ​@whereisjad ​@ChristianPaget - Issue is resolved. Under company_id i was using API Key instead of Company Id. 

​@jrdgdfg what do you mean using API Key instead of Company ID? aren’t they the same thing (assuming public API key)

I am having the same issue and I am sure I am appending the six-letters company ID in the request endpoint URL as per https://developers.klaviyo.com/en/reference/create_client_subscription

I am receiving the same error  as OP: “Invalid Company ID”.

I’m guessing OP was using a Private API key vs. the Public API key/company ID. The private API key should not be used with the client APIs.

​@beka-kestrel, one thing to double-check- you’re positive the company id you’re using is valid? And are you positive the list ID you’re using belongs to the account referenced by that company? 

​@cadence yeah I am pretty certain, I also double checked the list ID (although I’d expect a different error if the list ID was the problem anyway). I have obtained by public API key from  Settings > API Key under Public API Key / site ID.  I made sure it’s enabled for all domains  and “Allow empty domains” is ticked. 

Where else the problem could be? 

I am issuing a POST request to https://a.klaviyo.com/client/subscriptions?company_id=XXXXXX  - I triple checked the request body and even reduced it to a bare minimum with just the required fields (again, I would expect a different error if something went wrong there). 

I used the following headers:

"Accept" => "application/vnd.api+json",
"Content-Type" => "application/vnd.api+json",
"revision" => "2025-10-15"

the error received is 

{
  "errors": [
    {
      "id": "a8aff291-8ba9-487f-bffc-f284233d2fbc",
      "status": 400,
      "code": "invalid",
      "title": "Invalid input.",
      "detail": "Invalid Company ID",
      "source": {
        "pointer": "/data"
      },
      "links": {},
      "meta": {}
    }
  ]
}

what else could I try?

​@beka-kestrel, Do you mind sharing a bit more about how you're issuing the request? Is this in postman or somewhere else? If postman, can you confirm that your company_id is being sent over in the console?

I have a feeling your company_id may not be getting propagated correctly in your request URL. 

If you want to share more details, happy to reply over dm as well. 

Thank you and sorry for the late reply -- this turned out to be a bug in the URL query arg which didn’t show up transparently in our logs. All is good now!