diff --git a/client.go b/client.go
index 6c826bca..8be82189 100644
--- a/client.go
+++ b/client.go
@@ -43,6 +43,12 @@ func Exchange(ctx context.Context, packet *Packet, addr string) (*Packet, error)
 
 // Exchange sends the packet to the given server and waits for a response. ctx
 // must be non-nil.
+//
+// If the request packet cannot be encoded, the returned error is a
+// *MalformedRequestError; such an error indicates a problem with the request
+// and will not be resolved by retrying against another server. Network-level
+// failures are returned as their underlying error (which usually implements
+// net.Error), and an expired or cancelled ctx is reported via ctx.Err().
 func (c *Client) Exchange(ctx context.Context, packet *Packet, addr string) (*Packet, error) {
 	if ctx == nil {
 		panic("nil context")
@@ -50,7 +56,7 @@ func (c *Client) Exchange(ctx context.Context, packet *Packet, addr string) (*Pa
 
 	wire, err := packet.Encode()
 	if err != nil {
-		return nil, err
+		return nil, &MalformedRequestError{Err: err}
 	}
 
 	connNet := c.Net
diff --git a/client_test.go b/client_test.go
index 151f1358..fd19dc32 100644
--- a/client_test.go
+++ b/client_test.go
@@ -2,6 +2,7 @@ package radius
 
 import (
 	"context"
+	"errors"
 	"strings"
 	"sync/atomic"
 	"testing"
@@ -160,3 +161,40 @@ func TestClient_Exchange_nilContext(t *testing.T) {
 	//lint:ignore SA1012 This test is specifically checking for a nil context
 	Exchange(nil, req, "")
 }
+
+func TestClient_Exchange_malformedRequest(t *testing.T) {
+	// A packet with an unknown Code cannot be encoded.
+	req := New(CodeAccessRequest, []byte(`secret`))
+	req.Code = Code(255)
+
+	client := Client{}
+	resp, err := client.Exchange(context.Background(), req, "127.0.0.1:1")
+	if resp != nil {
+		t.Fatalf("got non-nil response (%v); expected nil", resp)
+	}
+	if err == nil {
+		t.Fatal("got nil error; expected one")
+	}
+
+	var malformed *MalformedRequestError
+	if !errors.As(err, &malformed) {
+		t.Fatalf("got error %T (%v); expecting *MalformedRequestError", err, err)
+	}
+	if malformed.Unwrap() == nil {
+		t.Fatal("got nil from Unwrap; expected the underlying encode error")
+	}
+
+	// A network error must not be reported as a MalformedRequestError, so that
+	// callers can tell encoding problems apart from server/connection failures.
+	netReq := New(CodeAccessRequest, []byte(`secret`))
+	ctx, cancel := context.WithTimeout(context.Background(), time.Millisecond*50)
+	defer cancel()
+	// 203.0.113.0/24 (TEST-NET-3) is reserved and unroutable.
+	_, netErr := client.Exchange(ctx, netReq, "203.0.113.1:1")
+	if netErr == nil {
+		t.Fatal("got nil error for unreachable server; expected one")
+	}
+	if errors.As(netErr, &malformed) {
+		t.Fatalf("network error reported as *MalformedRequestError: %v", netErr)
+	}
+}
diff --git a/errors.go b/errors.go
index e6f3a5d4..8b28566a 100644
--- a/errors.go
+++ b/errors.go
@@ -8,3 +8,35 @@ type NonAuthenticResponseError struct {
 func (e *NonAuthenticResponseError) Error() string {
 	return `radius: non-authentic response`
 }
+
+// MalformedRequestError is returned by Client.Exchange when the request packet
+// could not be encoded into its wire format. It indicates a problem with the
+// request itself (for example, attributes that are too long or an unknown
+// packet Code), so retrying the same request against another server will not
+// help.
+//
+// Callers can use errors.As to distinguish this client-side error from network
+// errors (which implement net.Error) when deciding whether to fail over to
+// another server:
+//
+//	resp, err := client.Exchange(ctx, packet, addr)
+//	var malformed *radius.MalformedRequestError
+//	switch {
+//	case err == nil:
+//		// success
+//	case errors.As(err, &malformed):
+//		// the request is invalid; do not retry against other servers
+//	default:
+//		// network or server error; failover to the next server may help
+//	}
+type MalformedRequestError struct {
+	Err error
+}
+
+func (e *MalformedRequestError) Error() string {
+	return "radius: malformed request: " + e.Err.Error()
+}
+
+func (e *MalformedRequestError) Unwrap() error {
+	return e.Err
+}