What causes "failed to bind to response headers: %w" in gofiber/fiber?

Audit headers set by your handlers and any middleware upstream of idempotency.

failed to bind to response headers

Error message

failed to bind to response headers: %w

What it means

Returned when c.Bind().RespHeader(headers) fails while collecting response headers to cache. Binding response headers into a map[string][]string fails if the binder encounters malformed header bytes or a header map type it cannot populate. It is rare — Fiber's binder is generally robust — but surfaces bad header content set by upstream handlers.

Solutions

Audit headers set by your handlers and any middleware upstream of idempotency.
Sanitize header values (strip control bytes) before setting them.
Use KeepResponseHeaders to restrict caching to a known-good header subset.
Update Fiber and check the bind release notes for stricter validation.

Example fix

// before
app.Use(idempotency.New()) // caches all headers
// after — only cache safe headers
app.Use(idempotency.New(idempotency.Config{
    KeepResponseHeaders: []string{"Content-Type", "X-Request-Id"},
}))

Defensive patterns

Strategy: validation

Prevention

Sanitize header values before setting them (strip control bytes).
Use KeepResponseHeaders to cache only known-safe headers.
Audit upstream middleware that mutates response headers.

When it happens

Trigger: After the request handler runs, the idempotency middleware binds the response headers into a map; if any header contains invalid bytes (control chars, non-ASCII without encoding) or the target type is wrong, Bind returns an error wrapped here.

Common situations: An upstream handler sets a header with raw binary/control characters, a Content-Type with bad encoding metadata, or a custom binder configuration that rejects certain headers; also possible after a Fiber version upgrade that tightened header validation.

Related errors

AI-assisted analysis of gofiber/fiber@a105acad6c (2026-08-11). Data as JSON: /api/errors/b9778f4e6389ed8c. Report an issue: GitHub.

Appendix: source

Thrown at middleware/idempotency/idempotency.go:155

		} else if ok {
			return nil
		}

		// Execute the request handler
		if err := c.Next(); err != nil {
			// If the request handler returned an error, return it and skip idempotency
			return err
		}

		// Construct response
		res := &response{
			StatusCode: c.Response().StatusCode(),
			Body:       c.Response().Body(),
		}
		{
			headers := make(map[string][]string)
			if err := c.Bind().RespHeader(headers); err != nil {
				return fmt.Errorf("failed to bind to response headers: %w", err)
			}

			if keepResponseHeaders == nil {
				// Keep all
				res.Headers = headers
			} else {
				// Filter
				res.Headers = make(map[string][]string)
				for h, vals := range headers {
					if shouldKeepHeader(h) {
						res.Headers[h] = vals
					}
				}
			}
		}

		// Marshal response
		bs, err := res.MarshalMsg(nil)

View on GitHub (pinned to a105acad6c)