Domain pattern ' ' has parameters, which exceeds the…

Error message

Domain pattern '%s' has %d parameters, which exceeds the maximum of %d

What it means

The domain pattern declares more parameters (':param' segments) than fiber/v3's internal maxParams cap. Each parameter is tracked in parallel slices (paramIdx, paramNames), and the cap bounds match-time memory and complexity. Exceeding it is treated as a malformed/abusive pattern.

Solutions

Reduce the number of ':param' segments by making low-value labels literal (e.g. keep a fixed TLD like '.com' as a constant label).
Re-design the routing so most labels are matched as a single suffix/wildcard rather than individual params.
Cap and validate the param count on the generated pattern string before passing it to the router.

Example fix

// before
app.Use(":a.:b.:c.:d.:e.:f.:g.:h.example.com", handler) // too many params
// after
app.Use(":tenant.example.com", handler) // capture only what you need

Defensive patterns

Strategy: validation

Validate before calling

const maxParams = /* mirror fiber's cap; use a conservative local ceiling */ 8

func countParams(pattern string) int {
    n := 0
    for _, lbl := range strings.Split(pattern, ".") {
        if strings.HasPrefix(lbl, ":") {
            n++
        }
    }
    return n
}

if n := countParams(domainPattern); n > maxParams {
    return fmt.Errorf("pattern has %d params, max %d", n, maxParams)
}

Try / catch

defer func() {
    if r := recover(); r != nil {
        log.Fatalf("too many domain params: %v", r)
    }
}()
app.Use(domainPattern, handler)

Prevention

Capture only the labels you actually need; make the rest literal.
Cap and validate param count in your pattern-generation helper.
Avoid auto-generated ':a.:b.:c...' patterns from unbounded user input.

When it happens

Trigger: Calling a domain-routing API with a pattern containing more ':param' tokens than maxParams, e.g. ":a.:b.:c.:d.:e..." with each label as a separate parameter beyond the configured ceiling.

Common situations: Dynamically generating a domain pattern from user input where each label becomes a param; overusing capture-everything patterns instead of matching a concrete suffix; porting a wildcard-heavy Express/Fastify route verbatim.

Related errors

AI-assisted analysis of gofiber/fiber@a105acad6c (2026-08-11). Data as JSON: /api/errors/b763513a2f9ed0c0. Report an issue: GitHub.

Appendix: source

Thrown at domain.go:128

			// Enforce RFC 1035 per-label length limit (63 characters)
			if len(part) > 63 {
				panic(fmt.Sprintf("Domain pattern '%s' has label '%s' exceeding RFC 1035 limit of 63 characters (%d chars)",
					pattern, part, len(part)))
			}
			// Validate label contains only valid ASCII domain characters (a-z, 0-9, hyphen).
			normalized := utilsstrings.ToLower(part)
			for _, ch := range normalized {
				if !isASCIIAlphanumeric(ch) && ch != '-' {
					panic(fmt.Sprintf("Domain pattern '%s' contains invalid character '%c' in label '%s'", pattern, ch, part))
				}
			}
			m.parts[i] = normalized
		}
	}

	// Check if the domain pattern has too many parameters
	if len(m.paramNames) > maxParams {
		panic(fmt.Sprintf("Domain pattern '%s' has %d parameters, which exceeds the maximum of %d",
			pattern, len(m.paramNames), maxParams))
	}

	return m
}

// match checks if a hostname matches the domain pattern.
// It returns true if matched and a slice of parameter values (parallel to paramNames).
// Uses a stack-allocated buffer to avoid heap allocation for typical domain names.
// Validates hostname to prevent DoS attacks from malicious input.
func (m *domainMatcher) match(hostname string) (bool, []string) { //nolint:gocritic // unnamedResult: named returns conflict with nonamedreturns linter
	// Trim trailing dot of a fully-qualified domain name (RFC 3986),
	// consistent with Fiber's own host normalization in Subdomains().
	hostname = utils.TrimRight(hostname, '.')

	// Validate hostname is not empty and not excessively long (DoS protection)
	// RFC 1035 limits domain names to 253 characters
	if hostname == "" || len(hostname) > 253 {

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