Go (stdlib-first) — Claude Code rules

Module hygiene

• Go 1.22+ — use the standard library net/http.ServeMux (method + pattern routing). No gin/echo/chi unless the requirements clearly justify it.
• go vet ./... and staticcheck ./... must pass.
• gofmt -s + goimports formatting is mandatory.

Errors

• Always wrap with fmt.Errorf("...: %w", err) so callers can errors.Is / errors.As.
• Sentinel errors are package-level var ErrFoo = errors.New("..."). Typed errors implement Error() and may implement Unwrap().
• Don't ignore errors. If genuinely OK to drop, name the variable _ with a comment.

Concurrency

• Pass context.Context as the first param of any blocking function. Never store it in a struct.
• Use errgroup.Group (from golang.org/x/sync/errgroup) for fan-out + first-error semantics.
• Goroutines must have a clear shutdown path. No "fire and forget" without context.Cancel plumbing.

HTTP

• http.ServeMux with method-aware patterns (r.HandleFunc("GET /users/{id}", h)).
• Handlers take (w http.ResponseWriter, r *http.Request) and read deadlines from r.Context().
• JSON via encoding/json with explicit struct tags. No reflection-heavy frameworks.

What not to do

• Don't introduce a third-party router unless there's a hard requirement.
• Don't use panic for control flow.
• Don't hand-roll struct tag-less JSON.