Gen Devcontainer

gen-devcontainer

Wraps bin/gen-devcontainer.sh. Emits a devcontainer.json; preview- by-default; idempotent on apply.

When to trigger

• User wants a Codespaces-ready repo and there's no .devcontainer/ directory yet.
• User wants to refresh an existing devcontainer.json after upgrading nyann (base image tags + extension lists shift with the snapshot_version comment in the file).
• User wants to add forwarded ports / cpus / memory to an existing devcontainer (use --port 3000 --cpus 4 --memory 8gb).

When NOT to trigger

• User wants a production Dockerfile — that's an entirely different concern; devcontainer.json is dev-environment only.
• User wants to debug a broken devcontainer build — open the existing file with them; this skill generates fresh, not diagnoses.
• User is on a non-Codespaces, non-VS Code editor — the file still works under JetBrains' Dev Containers plugin and devcontainer open (CLI), but flag the broader VS Code-first defaults.

Picking the language

Read StackDescriptor.primary_language from bin/detect-stack.sh:

For polyglot monorepos (workspaces[] present), pick the language of the most-important workspace OR ask the user — running multiple devcontainers from a single repo is uncommon and confusing.

Preview, then apply

Always preview first. Show the rendered JSON; wait for confirmation:

bin/gen-devcontainer.sh --language python --name my-app

Then on confirmation:

bin/gen-devcontainer.sh --language python --name my-app \
  --target . --apply

If the destination already matches: log "unchanged" and exit. If it differs: print the unified diff to stderr, exit 3, prompt the user before re-running with --force-overwrite.

Output shape

.devcontainer/devcontainer.json containing:

• $schema pointing at the official devcontainer spec
• $comment tagging the nyann snapshot version
• name (display name in 'Open in Container' picker)
• image (per-language, pinned to a major:minor tag)
• features (gh CLI, git-lfs, common-utils — version-pinned)
• customizations.vscode.extensions (per-language pack + gitlens + git-graph)
• postCreateCommand (per-language dep install — npm ci, uv sync, cargo fetch, etc.)
• forwardPorts (when --port flag(s) supplied)
• hostRequirements (when --cpus / --memory / --storage supplied)

Defaults explained

• Base image: Microsoft devcontainers/ images where they exist (well-maintained, security-scanned, multi-arch). Vendor images for Dart / Swift / Elixir where no MS variant exists.
• Image tag: pinned to major:minor, NOT a digest. Operators who need byte-reproducible builds should pin to a digest by hand; the Docker ecosystem of gen-dependency-updater can roll the tag.
• Extensions: per-language language pack + gitlens + git-graph baseline. Conservative pick; extend via --extension.
• Features: github-cli:1 + git-lfs:1 + common-utils:2, always with a version pin so floating tags can't quietly re-broadcast.
• postCreateCommand: best-effort install that no-ops when the expected manifest is missing (so the file works in a partially- scaffolded repo). Override with --post-create-command for non- standard package managers (e.g. pnpm rather than npm).

Reading the output back

When previewing:

• Surface the base image (the operator may want to swap to an org-internal mirror).
• Surface the postCreateCommand (so they can spot install commands that don't fit their workflow).
• Suggest --port flags if the user mentioned a dev-server port in the conversation.

When applying:

• Confirm the dest path didn't already exist (or that the diff was intentional if --force-overwrite was used).
• Suggest a follow-up gh codespace create invocation if the user asked for Codespaces specifically.