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

Ensure you pass a freshly built *http.Request per Test call and don't share/close internal state across tests.

failed to write: %w

Error message

failed to write: %w

What it means

Returned by App.Test (app.go:1431) when conn.r.Write(dump) fails after DumpRequest succeeds — i.e. writing the serialized request bytes into the in-memory testConn. The testConn is an internal pipe; a write failure usually means the connection was already closed or the buffer rejected the bytes. It is rare and typically indicates the testConn was pre-closed or a prior operation tore down the pipe.

Source

Thrown at app.go:1431

		}
	}

	// 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 9a4c7e57fe)

Solutions

Ensure you pass a freshly built *http.Request per Test call and don't share/close internal state across tests.
Inspect the wrapped error — it is usually io.ErrClosedPipe or a write timeout.
Reduce the request body size if the dump is pathologically large.

Defensive patterns

Strategy: try-catch

Try / catch

resp, err := app.Test(req)
if err != nil {
    if strings.Contains(err.Error(), "failed to write") {
        // testConn write failed — usually a pre-closed pipe; build a fresh request/conn
    }
    return err
}

Prevention

Pass a freshly built *http.Request per Test invocation; don't reuse closed state.
Inspect wrapped errors (often io.ErrClosedPipe) to confirm the cause.
Avoid pathologically large request bodies that stress the internal buffer.

When it happens

Trigger: Calling app.Test on a connection that has been closed, or reusing a test scenario where a previous Test closed the conn; very large dumped requests exceeding internal buffer limits under constrained conditions.

Common situations: Almost never seen in normal Test usage; surfaces in custom test harnesses that manipulate Fiber internals or in edge cases with huge request bodies.

Related errors

AI-assisted analysis of gofiber/fiber@9a4c7e57fe (2026-08-04). Data as JSON: /data/errors/bf8329e8709dbdca.json. Report an issue: GitHub.