---

name: migrate-to-ubuntu-slim description: This skill should be used when the user asks to "migrate to ubuntu-slim". allowed-tools: Bash, Read, Edit, Glob, Grep user-invocable: true

Migrate GitHub Actions to ubuntu-slim

Analyze GitHub Actions workflows and migrate eligible jobs from ubuntu-latest to ubuntu-slim for cost optimization.

About ubuntu-slim

ubuntu-slim is a lightweight, single-CPU GitHub-hosted runner optimized for lightweight operations:

Best suited for: automation tasks, issue operations, short-running jobs, linting, simple tests.

Migration Workflow

Phase 1: Create Plan

First, discover all workflows and create a task list.

1. List workflow files:

Copyls .github/workflows/*.yml .github/workflows/*.yaml 2>/dev/null || true

1. Create a todo list with tasks:
   • "Analyze workflow: " for each workflow file found
   • "Present migration plan"
   • "Apply changes (after user confirmation)"
   • "Show diff and prompt commit"

Phase 2: Analyze Each Workflow

For each workflow, perform TWO checks:

Check 1: Runtime Statistics

Run the analysis script:

Copy${CLAUDE_PLUGIN_ROOT}/skills/migrate-to-ubuntu-slim/scripts/analyze-workflow-duration.sh <workflow-file>

Output: workflow n mean stddev mean+2σ migration reason

• migration: yes, no, or - (unknown)
• reason: Human-readable explanation of the decision

Criteria: mean + 2σ < 420 seconds (7 minutes). This allows 2x slowdown on ubuntu-slim (1 CPU vs 4 CPU) while staying under 15-minute timeout.

Check 2: Implementation Patterns

Read the workflow YAML and check for incompatible patterns.

Run a quick keyword scan to surface suspicious commands:

Copy${CLAUDE_PLUGIN_ROOT}/skills/migrate-to-ubuntu-slim/scripts/check-slim-compat.sh <workflow-file>

Review matched lines and check if the workflow installs the tool before using it. If not, keep ubuntu-latest or add explicit install steps. Note: this script only checks for common CLI tools. Manually verify the patterns listed below (e.g., services:, container:).

Reject if ANY found:

Good candidates:

• Linting (eslint, ruff, golangci-lint)
• Formatting checks (prettier, black)
• Simple unit tests
• Notifications, issue/PR operations
• Deploy polling/wait jobs - mostly I/O wait, low CPU usage

Result

Mark workflow as:

• Eligible: Runtime OK AND no blocking patterns
• Not eligible: Explain reason (runtime or pattern)

Update todo: mark "Analyze workflow: " as completed.

Phase 3: Present Migration Plan

After analyzing all workflows, present summary:

Copy## Migration Plan

### Eligible:
| Workflow | mean+2σ | Note |
|----------|---------|------|
| test.yml | 198s | Simple pytest |
| lint.yml | 51s | ruff check |

### Not eligible:
| Workflow | Reason |
|----------|--------|
| build.yml | Uses services: postgres |
| e2e.yml | mean+2σ=920s exceeds limit |

Ask user: "Proceed with migration?"

Phase 4: Apply Changes

After user confirmation, edit each eligible workflow file:

Copy# Before
runs-on: ubuntu-latest

# After
runs-on: ubuntu-slim

Update todo as each file is edited.

Phase 5: Show Diff and Prompt Commit

1. Show diff:

Copygit diff .github/workflows/

1. Suggest commit message:

Copychore(ci): migrate eligible jobs to ubuntu-slim runner

1. Do NOT commit - let user decide.

Important Notes

• Always verify run time data before recommending migration
• Consider peak run times, not just averages - jobs with high variance may exceed 15-minute limit
• Some actions may not work correctly on ubuntu-slim due to reduced resources
• ubuntu-slim has fewer preinstalled tools - workflows may fail due to missing commands even if analysis passes. Actual execution is required to confirm compatibility
• Recommend starting with non-critical workflows (linting, notifications) before migrating build/test jobs

Reference

• GitHub Docs: GitHub-hosted runners
• ubuntu-slim preinstalled software

Note: ubuntu-slim runs as a container (not a full VM) and is available for both public and private repositories.