gofiber/fiber · error

failed to marshal response

Error message

failed to marshal response: %w

What it means

Returned when res.MarshalMsg(nil) fails while serializing the cached response to msgpack before storage. The response struct (status, body, headers) is msgpack-generated; marshal failures are rare and indicate a header map/value that the generated marshaler cannot encode (e.g. a non-byte-slice body or unsupported map key type introduced by a custom Bind).

Solutions

1. Regenerate msgpack code: `make generate` (or go generate ./middleware/idempotency/...).
2. Ensure response.Body stays []byte and response.Headers stays map[string][]string.
3. Run the idempotency unit tests to confirm the schema round-trips.
4. Inspect the wrapped %w for the precise encode error.

When it happens

Trigger: Constructing the response to cache and calling the msgpack-generated MarshalMsg on it returns a non-nil error — typically an unencodable value in the Headers map (non-string key, nil slice element) or a body that is not []byte after a refactor.

Common situations: A schema/codegen drift after regenerating manager_msgp.go, an upstream change that puts an unexpected type into response.Headers, or running against a msgpack version with stricter encoding rules.

Related errors

• failed to unmarshal response
• fiber: failed to encode shared state
• limiter: failed to marshal key
• limiter: failed to unmarshal key
• binder: msgpack is not configured, please check docs…

AI-assisted analysis of gofiber/fiber@a105acad6c (2026-08-11). Data as JSON: /api/errors/7521004e871cc21a. Report an issue: GitHub.

Appendix: source

Thrown at middleware/idempotency/idempotency.go:175

			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)
		if err != nil {
			return fmt.Errorf("failed to marshal response: %w", err)
		}

		// Store response
		if err := cfg.Storage.SetWithContext(c, key, bs, cfg.Lifetime); err != nil {
			return fmt.Errorf("failed to save response: %w", err)
		}

		_ = c.Locals(localsKeyWasPutToCache, true)

		return nil
	}
}

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