[Changelog] Xcode 27 runner image now in public preview

[plengauer]

GitHub has announced the Xcode 27 runner image is now available in public preview for GitHub-hosted macOS runners, as described in the July 16, 2026 changelog post.

What changed

A new macOS runner image with Xcode 27 is available in public preview on GitHub-hosted runners. This typically corresponds to a new macOS version and may bring changes to:

• Pre-installed tools and their versions
• Shell environments (bash, zsh)
• Default environment variables and paths

Relevance to this project

This project tests its shell instrumentation on macOS runners (macos-15, macos-latest) in test_github.yml and experiment.yml. A new Xcode/macOS image may affect:

• Shell behavior differences (particularly in bash vs zsh as default shell on macOS)
• Pre-installed tool versions that may affect the instrumentation's auto-detection
• PATH layout changes that could affect alias resolution
• Environment variable defaults

What needs investigation / action

1. Add macOS Xcode 27 runner to the test matrix: Update test_github.yml to include the new image (e.g., macos-26 or the appropriate label) once it's generally available.
2. Verify compatibility: Ensure that shell instrumentation (otel.sh, aliases, otelapi.sh) works correctly on the new image.
3. Check default shell: Confirm the default shell on the new image and whether there are any changes that affect POSIX-compliance requirements.
4. Update runner matrix: Consider adding the new image to experiment.yml for broader coverage.

Affected areas

• .github/workflows/test_github.yml — job-macos test matrix
• .github/workflows/experiment.yml — multi-runner experiment
• Shell instrumentation compatibility on macOS/zsh environments

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[github-actions]

Analysis

This issue tracks adding macOS Xcode 27 runner support to the test matrix. The work is well-scoped and straightforward:

Action Plan

1. test_github.yml — Add macos-26 (or the appropriate label for Xcode 27) to the job-macos runner matrix alongside macos-15 / macos-latest.
2. experiment.yml — Add the new runner image to the multi-runner experiment matrix.
3. Verify no additional changes are needed for shell instrumentation compatibility (alias resolution, PATH, default shell).

Notes

• The new runner is still in public preview, so it may not be stable yet. Adding it to the matrix with continue-on-error: true initially would be prudent.
• macOS default shell is zsh but GitHub Actions uses bash by default; this should not change.
• No source code changes to otel.sh or instrumentation files are expected — purely CI matrix updates.

Assigning to Copilot agent to implement the matrix updates.

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