GitHub Codespaces Efficiency
Use this skill as a lean entrypoint for GitHub Codespaces efficiency work. Inspect the repo, identify waste, and load only needed references.
If no .devcontainer/ exists yet, load references/codespaces.md and define a baseline before proceeding with the steps below.
Use This Skill When
- The user wants faster Codespaces startup or lower Codespaces spend.
- The repo has a
.devcontainer/or explicit Codespaces configuration questions. - The user asks for devcontainer optimization, machine sizing, prebuild strategy, or idle-timeout guidance.
- The user is setting up Codespaces for the first time or needs help creating a new
.devcontainer/from scratch.
Load Only What You Need
references/codespaces.md— devcontainer, machine-sizing, prebuild, idle-timeout guidance, and reporting.references/review-rubric.md— load only for review passes.
Core Workflow
1. Measure first
find .devcontainer -maxdepth 2 -type f
gh codespace list
repo=$(gh repo view --json nameWithOwner --jq .nameWithOwner)
gh api "/repos/$repo/codespaces/machines"
If gh auth fails or the user lacks repo admin scope, proceed with static analysis of .devcontainer/ files; mark machine-type and prebuild recommendations as unverified.
Look for: devcontainer image >2 GB or more than 10 features, machine type larger than usage data supports, missing devcontainer-lock.json (recommend adding — many repos predate lock-file support), prebuilds scoped too broadly, and idle timeout mismatched to usage patterns.
2. Apply guardrails
Check each proposed fix against these rules before recommending it:
- Does not remove tools the team uses every day — drop any fix that strips required development tools or extensions.
- Does not assume smaller is always better — balance machine cost against developer experience and throughput.
- Does not turn the devcontainer into a production image — drop any fix that adds production-only dependencies unless the team explicitly requires it.
- Incremental changes preferred — a greenfield baseline is appropriate only when no
.devcontainer/exists; flag (do not drop) changes that restructure an existing config. - Repo changes stay separate from org settings — split any fix that mixes repo-editable files with org-level or user-level Codespaces settings into two distinct recommendations.
3. Select the top 3 fixes
From the six candidates below, keep only those supported by audit evidence from step 1 and passing all guardrails from step 2. Rank survivors by estimated monthly cost savings (USD). Select all candidates that meet both criteria, up to a maximum of 3.
- Trim devcontainer — remove features, packages, or extensions not needed for everyday development work; target image <2 GB and fewer than 10 features
- Right-size machine type — match to observed usage patterns; if data is unavailable, state assumptions explicitly
- Scope prebuilds — enable for the default branch,
release/*branches active in the last 14 days, and branches with more than 5 Codespaces per week; disable for all others - Tune idle timeout — 30 min default; 15 min if most sessions end before 30 min; 60 min if most sessions run longer
- Remove unused extensions or port-forwarding rules
- Reduce devcontainer image size and improve layer caching
4. Verify
- Start a test Codespace to confirm devcontainer changes build and start as expected.
- Validate machine sizing against observed usage when telemetry is available; otherwise mark as unverified.
- Treat unexpected build or startup failures as real bugs even when the configuration looks correct.
Required Output
Waste sources: [top cost or startup-time drivers]
Proposed fixes: [top 3 changes supported by audit evidence and passing guardrails]
Validation: [proven live / static-only / remaining risk]
Impact:
- Startup time: [expected] / [measured if available]
- Monthly spend: [expected] / [measured if available]
- Resource utilization: [expected] / [measured if available]
References
references/codespaces.mdreferences/review-rubric.md— load when reviewing completed efficiency work