400 Bad request on trying to create pipeline with `glab ci run`
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### Checklist
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- [x] I'm using the latest version of the extension (Run `glab --version`)
- Extension version: _Put your extension version here_
- [x] Operating system and version: macOS 15.6
- [x] Gitlab.com or self-managed instance? self-managed instance (Omnibus installation)
- [x] GitLab version (if self-managed) 18.0.1-ee
(Use the `version` endpoint, like this: gitlab.my-company.com/api/v4/version)
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### Summary
I'm trying to create a new pipeline with `glab ci run -b main`, however I only get `400 Bad request` errors. See logs below. I get no useful error message, like mentioned in the [docs](https://docs.gitlab.com/api/rest/troubleshooting/#status-code-400).
### Environment
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- OS: Darwin 24.6.0 arm64
- SHELL: /opt/homebrew/bin/fish
- TERM: xterm-256color
- GLAB: glab 1.67.0 (1e957280)
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Other:
### Steps to reproduce
<!-- How one can reproduce the issue - this is very important -->
Run `glab ci run` on any project.
### What is the current _bug_ behavior?
```
$ GLAB_DEBUG_HTTP=true GITLAB_REPO=https://gitlab.mycompany.com/my-org/my-project.git glab ci run -b main
REQUEST:
POST /api/v4/projects/my-org%2Fmy-project/pipeline HTTP/1.1
Host: gitlab.mycompany.com
User-Agent: glab/v1.67.0 (darwin, arm64)
Content-Length: 29
Accept: application/json
Content-Type: application/json
Private-Token: [REDACTED]
Accept-Encoding: gzip
{"ref":"main","variables":[]}
RESPONSE:
HTTP/2.0 400 Bad Request
Content-Length: 150
Content-Type: text/html
Date: Wed, 20 Aug 2025 18:15:39 GMT
Server: nginx
Strict-Transport-Security: max-age=63072000
<html>
<head><title>400 Bad Request</title></head>
<body>
<center><h1>400 Bad Request</h1></center>
<hr><center>nginx</center>
</body>
</html>
ERROR: could not create branch pipeline for branch main: POST https://gitlab.mycompany.com/api/v4/projects/my-org%2Fmy-project/pipeline: 400 failed to parse unknown error format: <html>
<head><title>400 Bad Request</title></head>
<body>
<center><h1>400 Bad Request</h1></center>
<hr><center>nginx</center>
</body>
</html>
```
### What is the expected _correct_ behavior?
Start the pipeline and return pipeline response just like curl:
```
$ curl -v --http1.1 --request POST \
--header "Content-Type: application/json" \
--header "PRIVATE-TOKEN: [redacted]" \
"https://gitlab.mycompany.com/api/v4/projects/my-org%2Fmy-project/pipeline?ref=main"
...
> POST /api/v4/projects/my-org%2Fmy-project/pipeline?ref=main HTTP/1.1
> Host: gitlab.mycompany.com
> User-Agent: curl/8.7.1
> Accept: */*
> Content-Type: application/json
> PRIVATE-TOKEN: [redacted]
>
* Request completely sent off
< HTTP/1.1 201 Created
...
{"id":320225,"iid":109,...
...
```
### Relevant logs and/or screenshots
See above.
### Possible fixes
<!-- If you can, link to the line of code that might be responsible for the problem -->
issue