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

Reduce test payload size, or test large bodies against a real started server with http.Client.

failed to write

Error message

failed to write: %w

What it means

After DumpRequest, the TestClient writes the dumped bytes into an in-memory testConn via conn.r.Write(dump). If that write fails (almost always indicates the dump is empty/oversized or the conn is in a bad state), the wrapped error is returned.

Solutions

Reduce test payload size, or test large bodies against a real started server with http.Client.
Inspect the wrapped error to identify the buffer/conn failure.
Run each app.Test() in isolation to rule out shared-state corruption.

Example fix

// before
body := strings.Repeat("x", 1<<30)
resp, _ := app.Test(httptest.NewRequest("POST", "/", strings.NewReader(body)))

// after (use a real server for huge bodies)
body := strings.Repeat("x", 1<<20)
resp, _ := app.Test(httptest.NewRequest("POST", "/", strings.NewReader(body)))

Defensive patterns

Strategy: validation

Validate before calling

const maxTestBody = 1 << 20 // 1 MiB
if req.ContentLength > maxTestBody {
    return errors.New("use a real server for large bodies")
}

Try / catch

if _, err := app.Test(req); err != nil {
    return fmt.Errorf("test conn write failed: %w", err)
}

Prevention

Keep test payloads small; route huge bodies through a real server.
Run app.Test() in isolation to avoid shared-state issues.
Inspect wrapped errors before retrying.

When it happens

Trigger: Calling app.Test() with an extremely large request body, or in a test setup where the testConn has been closed/corrupted.

Common situations: Very large upload tests exceeding in-memory buffer limits; concurrent test runs sharing state; custom test harnesses reusing testConn incorrectly.

Related errors

AI-assisted analysis of gofiber/fiber@a105acad6c (2026-08-11). Data as JSON: /api/errors/bf8329e8709dbdca. Report an issue: GitHub.

Appendix: source

Thrown at app.go:1432

		}
	}

	// Clear RequestURI so DumpRequest writes origin-form request line with
	// Host header instead of absolute-form URI without Host header.
	req.RequestURI = ""

	// Dump raw http request
	dump, err := httputil.DumpRequest(req, true)
	if err != nil {
		return nil, fmt.Errorf("failed to dump request: %w", err)
	}

	// Create test connection
	conn := new(testConn)

	// Write raw http request
	if _, err = conn.r.Write(dump); err != nil {
		return nil, fmt.Errorf("failed to write: %w", err)
	}
	// prepare the server for the start
	app.startupProcess()

	// Serve conn to server
	channel := make(chan error, 1)
	go func() {
		var returned bool
		defer func() {
			if !returned {
				channel <- ErrHandlerExited
			}
		}()

		channel <- app.server.ServeConn(conn)
		returned = true
	}()

View on GitHub (pinned to a105acad6c)