External Service in a Managed Package Doesn't Work in Subscriber Orgs

Question

I'm developing an api integration feature from Salesforce to an external web service in a managed package using External Service, Named Credential and Flow.

The feature works in a packaging org but doesn't work in a subscriber org. The error title is "Callout failed: The callout couldn't access the endpoint."

It looks like a namespace issue when a system refers to a named credential like this: named credential problem

However, I can only choose a Named Credential declaratively in the External Service setting screen, so I have no idea if it refers namespace or not.

Do you guys have any idea to solve this issue or to know an External Service refers a namespace?

Additional Info:

My Schema

 {
  "swagger": "2.0",
  "info": {
    "title": "Slack + Salesforce",
    "description": "Instantly Send Messages to Slack from Salesforce",
    "version": "1.0.0"
  },
  "host": "hooks.slack.com",
  "schemes": [ "https" ],
  "paths": {
    "/workflows/myworkflowid": {
      "post": {
        "consumes": [ "application/json" ],
        "parameters": [
          {
            "in": "body",
            "name": "slackMessageRequest",
            "type": "object",
            "required": true,
            "schema" : {
              "$ref" : "#/definitions/SlackMessageRequest"
            }
          }
        ],
        "responses": {
          "200": {
            "description": "Successful Operation"
          }
        }
      }
    }
  },
  "definitions": {
    "SlackMessageRequest": {
      "type": "object",
      "properties": {
        "message": {
          "type": "string"
        },
        "organization":{
           "type": "string"
        },
       "error_name":{
           "type": "string"
        },
      "log_number":{
           "type": "string"
        },
      "record_url":{
           "type":"string"
        },
      "skill":{
         "type": "string"
       },
      "error_stack":{
         "type": "string"
       }
      }
    }
  }
}

My named credential

Answer 1

The issue you are facing is already filed as a bug in Salesforce internally(#W-11058954). According to this, users are unable to make callout for Namespace'd managed package External Services.

When we make the External Service callout, we don't use the full NamedCredential name with the namespace. So it is unable to find the named credential causing the error.

There is no public-facing known issue for this bug at the moment. The ETA for the bug fix is Winter'23(Safe Harbour)

Hope this helps!