diff --git a/client.go b/client.go
index c57a87c..cafd780 100644
--- a/client.go
+++ b/client.go
@@ -27,8 +27,6 @@ type Client struct {
 	cbctx    context.Context // terminates when the client is closed
 	cbcancel func()          // cancels cbctx
 
-	allow1 bool // tolerate v1 replies with no version marker
-
 	mu      sync.Mutex           // protects the fields below
 	ch      channel.Channel      // channel to the server
 	err     error                // error from a previous operation
@@ -40,13 +38,12 @@ type Client struct {
 func NewClient(ch channel.Channel, opts *ClientOptions) *Client {
 	cbctx, cbcancel := context.WithCancel(context.Background())
 	c := &Client{
-		done:   new(sync.WaitGroup),
-		log:    opts.logFunc(),
-		allow1: opts.allowV1(),
-		enctx:  opts.encodeContext(),
-		snote:  opts.handleNotification(),
-		scall:  opts.handleCallback(),
-		chook:  opts.handleCancel(),
+		done:  new(sync.WaitGroup),
+		log:   opts.logFunc(),
+		enctx: opts.encodeContext(),
+		snote: opts.handleNotification(),
+		scall: opts.handleCallback(),
+		chook: opts.handleCancel(),
 
 		cbctx:    cbctx,
 		cbcancel: cbcancel,
@@ -423,12 +420,7 @@ func (c *Client) stop(err error) {
 	c.ch = nil
 }
 
-func (c *Client) versionOK(v string) bool {
-	if v == "" {
-		return c.allow1
-	}
-	return v == Version
-}
+func (c *Client) versionOK(v string) bool { return v == Version }
 
 // marshalParams validates and marshals params to JSON for a request.  The
 // value of params must be either nil or encodable as a JSON object or array.
diff --git a/jrpc2_test.go b/jrpc2_test.go
index b78c69e..f10b920 100644
--- a/jrpc2_test.go
+++ b/jrpc2_test.go
@@ -134,10 +134,7 @@ func TestClient_Call(t *testing.T) {
 	loc := server.NewLocal(handler.ServiceMap{
 		"Test": testService,
 	}, &server.LocalOptions{
-		Server: &jrpc2.ServerOptions{
-			AllowV1:     true,
-			Concurrency: 16,
-		},
+		Server: &jrpc2.ServerOptions{Concurrency: 16},
 	})
 	defer loc.Close()
 	c := loc.Client
@@ -191,10 +188,7 @@ func TestClient_Batch(t *testing.T) {
 	loc := server.NewLocal(handler.ServiceMap{
 		"Test": testService,
 	}, &server.LocalOptions{
-		Server: &jrpc2.ServerOptions{
-			AllowV1:     true,
-			Concurrency: 16,
-		},
+		Server: &jrpc2.ServerOptions{Concurrency: 16},
 	})
 	defer loc.Close()
 	c := loc.Client
diff --git a/opts.go b/opts.go
index 062bdda..d9cf062 100644
--- a/opts.go
+++ b/opts.go
@@ -23,10 +23,6 @@ type ServerOptions struct {
 	// received and each response or error returned.
 	RPCLog RPCLogger
 
-	// Instructs the server to tolerate requests that do not include the
-	// required "jsonrpc" version marker.
-	AllowV1 bool
-
 	// Instructs the server to allow server callbacks and notifications, a
 	// non-standard extension to the JSON-RPC protocol. If AllowPush is false,
 	// the Notify and Callback methods of the server report errors if called.
@@ -79,7 +75,6 @@ func (s *ServerOptions) logFunc() func(string, ...interface{}) {
 	return s.Logger.Printf
 }
 
-func (s *ServerOptions) allowV1() bool      { return s != nil && s.AllowV1 }
 func (s *ServerOptions) allowPush() bool    { return s != nil && s.AllowPush }
 func (s *ServerOptions) allowBuiltin() bool { return s == nil || !s.DisableBuiltin }
 
@@ -144,10 +139,6 @@ type ClientOptions struct {
 	// If not nil, send debug text logs here.
 	Logger Logger
 
-	// Instructs the client to tolerate responses that do not include the
-	// required "jsonrpc" version marker.
-	AllowV1 bool
-
 	// If set, this function is called with the context, method name, and
 	// encoded request parameters before the request is sent to the server.
 	// Its return value replaces the request parameters. This allows the client
@@ -191,8 +182,6 @@ func (c *ClientOptions) logFunc() func(string, ...interface{}) {
 	return c.Logger.Printf
 }
 
-func (c *ClientOptions) allowV1() bool { return c != nil && c.AllowV1 }
-
 type encoder = func(context.Context, string, json.RawMessage) (json.RawMessage, error)
 
 func (c *ClientOptions) encodeContext() encoder {
diff --git a/server.go b/server.go
index 1d82d6d..61a3c9f 100644
--- a/server.go
+++ b/server.go
@@ -26,7 +26,6 @@ type Server struct {
 	sem *semaphore.Weighted // bounds concurrent execution (default 1)
 
 	// Configurable settings
-	allow1  bool                         // allow v1 requests with no version marker
 	allowP  bool                         // allow server notifications to the client
 	log     func(string, ...interface{}) // write debug logs here
 	rpcLog  RPCLogger                    // log RPC requests and responses here
@@ -71,7 +70,6 @@ func NewServer(mux Assigner, opts *ServerOptions) *Server {
 	s := &Server{
 		mux:     mux,
 		sem:     semaphore.NewWeighted(opts.concurrency()),
-		allow1:  opts.allowV1(),
 		allowP:  opts.allowPush(),
 		log:     opts.logFunc(),
 		rpcLog:  opts.rpcLog(),
@@ -693,12 +691,7 @@ func (s *Server) cancel(id string) bool {
 	return ok
 }
 
-func (s *Server) versionOK(v string) bool {
-	if v == "" {
-		return s.allow1 // an empty version is OK if the server allows it
-	}
-	return v == Version // ... otherwise it must match the spec
-}
+func (s *Server) versionOK(v string) bool { return v == Version }
 
 // A task represents a pending method invocation received by the server.
 type task struct {