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

Inspect the wrapped error to locate the parse failure (header line, chunk size, body EOF).

failed to read response: %w

Error message

failed to read response: %w

What it means

Returned by App.Test (app.go:1491) when http.ReadResponse (httpReadResponse) fails to parse the response the handler wrote, excluding the io.ErrUnexpectedEOF case (which maps to ErrTestGotEmptyResponse). It means the bytes on the testConn do not form a valid HTTP response — the handler produced malformed output or the connection was reset mid-response.

Source

Thrown at app.go:1491

	}

	// Check for errors
	if err != nil && !errors.Is(err, fasthttp.ErrGetOnly) && !errors.Is(err, errTestConnClosed) {
		return nil, err
	}

	// Read response(s)
	buffer := bufio.NewReader(&conn.w)

	var res *http.Response
	for {
		// Convert raw http response to *http.Response
		res, err = httpReadResponse(buffer, req)
		if err != nil {
			if errors.Is(err, io.ErrUnexpectedEOF) {
				return nil, ErrTestGotEmptyResponse
			}
			return nil, fmt.Errorf("failed to read response: %w", err)
		}

		// Break if this response is non-1xx or there are no more responses
		if res.StatusCode >= http.StatusOK || buffer.Buffered() == 0 {
			break
		}

		// Discard interim response body before reading the next one
		if res.Body != nil {
			if _, errCopy := io.Copy(io.Discard, res.Body); errCopy != nil {
				return nil, fmt.Errorf("failed to discard interim response body: %w", errCopy)
			}
			if errClose := res.Body.Close(); errClose != nil {
				return nil, fmt.Errorf("failed to close interim response body: %w", errClose)
			}
		}
	}

View on GitHub (pinned to 9a4c7e57fe)

Solutions

Inspect the wrapped error to locate the parse failure (header line, chunk size, body EOF).
Increase TestConfig.Timeout (or set FailOnTimeout:false) if the handler is legitimately slow.
Test the handler in isolation to confirm it emits a well-formed HTTP response.
Verify Content-Length / Transfer-Encoding headers match the body the handler writes.

Example fix

// before
resp, err := app.Test(req) // 1s default timeout

// after
resp, err := app.Test(req, fiber.TestConfig{Timeout: 5 * time.Second})

Defensive patterns

Strategy: try-catch

Try / catch

resp, err := app.Test(req, fiber.TestConfig{Timeout: 5 * time.Second})
if err != nil {
    if errors.Is(err, fiber.ErrTestGotEmptyResponse) {
        // handler wrote nothing
    } else if strings.Contains(err.Error(), "failed to read response") {
        // malformed/truncated response from handler
    }
    return err
}

Prevention

Raise TestConfig.Timeout for slow handlers (default is 1s).
Verify the handler emits a well-formed HTTP response (valid status, headers, body framing).
Confirm Content-Length / Transfer-Encoding match the body written.

When it happens

Trigger: A handler under test writing an invalid status line, missing/oversized headers, or a chunked body that breaks framing; the handler panicking after partial writes; the response exceeding the testConn buffer; a timeout closing the conn mid-response (FailOnTimeout interactions).

Common situations: Handlers that write raw bytes via fasthttp in unusual ways; custom serializers emitting non-RFC-compliant responses; tests that hit the 1s default timeout on slow handlers; gzip/compression framing bugs.

Related errors

AI-assisted analysis of gofiber/fiber@9a4c7e57fe (2026-08-04). Data as JSON: /data/errors/5b68a930b6aaccbd.json. Report an issue: GitHub.