Alertmanager not supported other slack compatibility config (Mattermost)

What did you do?

Not work update_message for mattermost
I create issue https://github.com/prometheus-operator/prometheus-operator/issues/8808#issuecomment-5633648340 but reported ALSO here

Steps to Reproduce

slack_configs:
  - channel: '#alerts-staging'
    send_resolved: true
    title: '[{{ .Status | toUpper }}{{ if eq .Status "firing" }}:{{ .Alerts.Firing | len }}{{ end }}] Monitoring Event Notification'
    api_url: https://mymat.mycompany.us/api/chat.postMessage
    #app_url: https://mymat.mycompany.us/api/chat.postMessage
    http_config:
      authorization:
          credentials: "mypasswd"
    update_message: true

I found hard code

https://github.com/search?q=repo%3Aprometheus%2Falertmanager+https%3A%2F%2Fslack.com%2Fapi%2Fchat.postMessage&type=code

What did you expect to see?

all work no checked hard code api_url

What did you see instead? Under which circumstances?

api_url must be set to https://slack.com/api/chat.postMessage

System information

Linux 6.12.0-124.45.1.el10_1.x86_64 x86_64

Alertmanager version

alertmanager, version 0.33.1 (branch: HEAD, revision: 2c8da51e03f3dbbed24f9711ca2d76aab4eef9c5)
  build user:       root@34d840a05dd5
  build date:       20260704-19:26:21
  go version:       go1.26.4
  platform:         linux/amd64
  tags:             netgo

Alertmanager configuration file

.....
slack_configs:
  - channel: '#alerts-staging'
    send_resolved: true
    title: '[{{ .Status | toUpper }}{{ if eq .Status "firing" }}:{{ .Alerts.Firing | len }}{{ end }}] Monitoring Event Notification'
    api_url: https://mymat.mycompany.us/api/chat.postMessage
    #app_url: https://mymat.mycompany.us/api/chat.postMessage
    http_config:
      authorization:
          credentials: "mypasswd"
    update_message: true

Prometheus version

prometheus, version 3.11.3 (branch: HEAD, revision: eb173f5256d4022afba1e9bc3d19740a76859fae)
  build user:       root@83aad33dd38e
  build date:       20260427-14:45:32
  go version:       go1.26.2
  platform:         linux/amd64
  tags:             netgo,builtinassets

Prometheus configuration file

It doesn't matter if the problem is in Alertmanager.

Logs

api_url must be set to https://slack.com/api/chat.postMessage

CPU profiles

It doesn't matter

Memory profiles

It doesn't matter

Number of alerts

No response

Number of silences

No response

prometheus-operator checklist

• I have confirmed this issue is not caused by a missing feature in prometheus-operator.
• I have reproduced this issue when running Alertmanager directly (without prometheus-operator).

Start by tracing the hard-coded https://slack.com/api/chat.postMessage check used by slack_configs when update_message is enabled. Reproduce the provided Mattermost configuration and verify that a non-Slack api_url works with update_message, including the existing error path and relevant tests if found.