gofiber/fiber · error

min constraint requires an argument

Error message

min constraint requires an argument

What it means

The min route-parameter constraint enforces that a matched path segment, parsed as an integer, is greater than or equal to a lower bound. It is declared inline as :param<min(n)>. Its Analyze method (part of the public ConstraintAnalyzer interface) requires exactly one argument; with zero arguments it returns this error. As with the other built-ins, fiber's internal newConstraint swallows Analyze errors, so this is observed when Analyze is invoked directly or through a delegating custom-constraint wrapper.

Solutions

1. Supply one integer argument: :param<min(5)>.
2. If you want a length-based lower bound rather than a numeric one, use :param<minLen(n)>.
3. Verify the parens contain a parseable integer (strconv.Atoi is applied at constraint.go:453).

Example fix

// before
app.Get("/n/:id<min>", h)
// after
app.Get("/n/:id<min(1)>", h)

// Lint route strings so min always carries one integer argument.
var minNoArg = regexp.MustCompile(`:([^<]+)<min\(\s*\)>`)
for _, p := range routePatterns {
    if loc := minNoArg.FindStringIndex(p); loc != nil {
        log.Fatalf("route %q: min constraint missing argument", p)
    }
}

typed, err := minConstraint.Analyze(args)
if err != nil {
    return fmt.Errorf("route min constraint: %w", err)
}

When it happens

Trigger: Calling Analyze on a min constraint with no arguments, derived from a route pattern such as :id<min()> (empty parens) or :id<min> (no parens). parseConstraintArgs returns the empty slice unchanged and len(args)==0 trips the guard at constraint.go:450.

Common situations: Forgetting the numeric bound in a route template; refactoring a constraint expression and dropping the argument; whitespace-only content inside the parentheses.

Related errors

• betweenLen constraint requires two arguments
• max constraint requires an argument
• range constraint requires two arguments
• regex constraint requires a pattern argument
• Domain pattern cannot be empty

AI-assisted analysis of gofiber/fiber@a105acad6c (2026-08-11). Data as JSON: /api/errors/912850d8446d3842. Report an issue: GitHub.

Appendix: source

Thrown at constraint.go:450

	lo, ok := data[0].(int)
	if !ok {
		return false
	}
	hi, ok := data[1].(int)
	if !ok {
		return false
	}
	length := len(param)
	return length >= lo && length <= hi
}

type minConstraintType struct{}

func (minConstraintType) Name() string { return ConstraintMin }
func (minConstraintType) Analyze(args []string) ([]any, error) {
	args = parseConstraintArgs(args)
	if len(args) == 0 {
		return nil, errors.New("min constraint requires an argument")
	}
	n, err := strconv.Atoi(args[0])
	if err != nil {
		return nil, fmt.Errorf("parse constraint arg: %w", err)
	}
	return []any{n}, nil
}

func (minConstraintType) Execute(param string, data []any) bool {
	if len(data) == 0 {
		return false
	}
	limit, ok := data[0].(int)
	if !ok {
		return false
	}
	num, err := strconv.Atoi(param)
	return err == nil && num >= limit

View on GitHub (pinned to a105acad6c)