API endpoint error returns error text with bad content type

[sysadmind]

What did you do?

While trying to build the API client for the new Mantine UI, I started getting error responses from the API with an incorrect content type. The errors come back as raw text, but the content-type header is set to application/json.

I tried to dig in, but most of the code is generated and it's hard to follow.

What did you expect to see?

I expected to see either a plain text content-type OR a JSON response body with some sort of error key.

What did you see instead? Under which circumstances?

When trying to get the alert group endpoint to work with filters, I created the incorrect URL /api/v2/alerts/groups?silenced=false&inhibited=false&filter=a%3D%22b%22&c%3D%22d%22 and this produces a response "bad matcher format: a%3D%22b%22&c%3D%22d%22", but again, the content-type header is application/json.

System information

No response

Alertmanager version

alertmanager, version  (branch: main , revision: unknown)
  build user:       
  build date:       
  go version:       go1.25.6
  platform:         linux/amd64
  tags:             unknown

Alertmanager configuration file

Prometheus version

Prometheus configuration file

Logs

[mehrdadbn9]

@mehrdadbn9 I would like to work on this issue. Please assign it to me.