Github runner now pick bazel 9.0.0, instead of expected version 8.5.0

[marcalff]

Description

Image: ubuntu-24.04
Version: 20260111.209.1

According to:
https://github.com/actions/runner-images/blob/ubuntu24/20260111.209/images/ubuntu/Ubuntu2404-Readme.md

the version of bazel used should be:

Bazel 8.5.0
Bazelisk 1.26.0

Now, in the runner, invoking bazel downloads bazel 9.0.0 instead, and runs it.

2026/01/20 20:06:06 Downloading https://releases.bazel.build/9.0.0/release/bazel-9.0.0-linux-x86_64...
Extracting Bazel installation...
Starting local Bazel server (9.0.0) and connecting to it...

Note that:

• bazel 9.0.0 just released: https://github.com/bazelbuild/bazel/releases/tag/9.0.0
• bazel is actually a script (bazelisk)

Work around:

export USE_BAZEL_VERSION="8.5.0"

to force the bazelisk script to use the proper version.

This is a breaking change, and blocking (the build with bazel 9.0.0 is broken)

Platforms affected

• Azure DevOps
• GitHub Actions - Standard Runners
• GitHub Actions - Larger Runners

Runner images affected

• Ubuntu 22.04
• Ubuntu 24.04
• Ubuntu Slim
• macOS 14
• macOS 14 Arm64
• macOS 15
• macOS 15 Arm64
• macOS 26 Arm64
• Windows Server 2022
• Windows Server 2025

Image version and build link

https://github.com/open-telemetry/opentelemetry-cpp/actions/runs/21184948231/job/60936855673

Is it regression?

Regression, caused by release of bazel 9.0.0, now downloaded by default

Expected behavior

Use bazel 8.5.0 as advertised.

Actual behavior

Bazel 9.0.0 is used.

Repro steps

Run CI using bazel after the bazel 9.0.0 release.
See repo opentelemetry-cpp

[marcalff]

Note that before the release of bazel 9.0.0, runners were picking up bazel 8.5.1 instead of the advertized 8.5.0.

https://github.com/open-telemetry/opentelemetry-cpp/actions/runs/21103408495/job/60690885964

2026/01/18 00:45:38 Downloading https://releases.bazel.build/8.5.1/release/bazel-8.5.1-linux-x86_64...
Extracting Bazel installation...
Starting local Bazel server (8.5.1) and connecting to it...

Builds are not reproducible when tooling changes without notice.

[dbaileychess]

Yes, this is happening to our repo as well: https://github.com/google/flatbuffers/actions/runs/21192018596/job/60960325193?pr=8781#logs where its using bazel 9.0 instead of 8.5

[fmeum]

The general recommendation from the Bazel side is to always use Bazelisk as bazel and commit a .bazelversion to your repo.

[marcalff]

The general recommendation from the Bazel side is to always use Bazelisk as bazel and commit a .bazelversion to your repo.

On github runners, bazel is baselisk.
In the opentelemetry-cpp repo, .bazelversion exists and point to 8.5.0.

Yet, bazelisk in the github runner uses bazel 9.0.0 and ignores the .bazelversion file.

[brsm019]

Currently investigating this. Looks likely that the new Bazel update may be causing this issue, we're still yet to update the ubuntu-24.04 image, but at the moment the bazel tool downloads the latest version when called.