# 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.