probo / rules

---
description: Go URL construction — never use fmt.Sprintf or concatenation for URLs
globs: "**/*.go"
alwaysApply: false
---

# Go URL and query parameter construction

**Never** build URLs with `fmt.Sprintf`, string concatenation, or any string formatting.

Use `net/url` package or `pkg/baseurl.URLBuilder`.

**Always** wrap user-supplied path segments with `url.PathEscape` before passing them to `url.JoinPath`. `url.JoinPath` does **not** percent-encode slashes or reserved characters in its arguments — a value like `parent/child` silently adds an extra path segment.

```go
// BAD
endpoint := fmt.Sprintf("https://api.example.com/users/%s?active=%t", userID, active)
endpoint := "https://api.example.com/orgs/" + orgID + "/members"
raw := baseEndpoint + "?domain=" + domain + "&limit=100"

// BAD — user-supplied value without PathEscape
u, err := url.JoinPath("https://api.example.com", "groups", groupID, "members")

// GOOD — url.JoinPath with PathEscape + url.Values
u, err := url.JoinPath("https://api.example.com", "groups", url.PathEscape(groupID), "members")
if err != nil {
	return fmt.Errorf("cannot build URL: %w", err)
}

parsed, err := url.Parse(u)
if err != nil {
	return fmt.Errorf("cannot parse URL: %w", err)
}

q := parsed.Query()
q.Set("active", strconv.FormatBool(active))
parsed.RawQuery = q.Encode()

// GOOD — URLBuilder from pkg/baseurl
u, err := baseURL.URL("/users", userID).
	Query("active", strconv.FormatBool(active)).
	Build()
```

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