gofiber/fiber · error

failed to marshal response: %w

Error message

failed to marshal response: %w

What it means

Returned by the middleware (idempotency.go:174-176) when res.MarshalMsg(nil) fails for the response struct. msgpack marshaling of a fixed struct of (int, []byte, map[string][]string) does not fail under normal conditions.

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
	}
}

View on GitHub (pinned to 9a4c7e57fe)

Solutions

1. Run `make generate` (msgp codegen) after any change to the response struct.
2. Verify manager_msgp.go / response_msgp.go are present and not hand-edited.
3. Treat occurrence as a build/codegen bug.

Example fix

# before — edited response struct, forgot regen
$ go build ./...

# after
$ make generate
$ go build ./...

// CI step: regenerate msgp codegen and fail if diff
// make generate && git diff --exit-code middleware/idempotency/

When it happens

Trigger: Unreachable for the current response struct shape. Would require an embedded type that implements MarshalMsg incorrectly, or a msgp codegen mismatch (regenerated msgp.go out of sync with the struct).

Common situations: Editing the response struct without re-running `go generate` (msgp), leaving the MarshalMsg method out of sync with the field set; very rare.

Related errors

• failed to unmarshal response: %w
• failed to bind to response headers: %w
• invalid idempotency key
• cache: failed to unmarshal key %q: %w
• cache: failed to marshal key %q: %w

AI-assisted analysis of gofiber/fiber@9a4c7e57fe (2026-08-04). Data as JSON: /data/errors/7521004e871cc21a.json. Report an issue: GitHub.