diff --git a/mcp/client.go b/mcp/client.go
index 3a935040..512be2cb 100644
--- a/mcp/client.go
+++ b/mcp/client.go
@@ -6,6 +6,7 @@ package mcp
 
 import (
 	"context"
+	"fmt"
 	"iter"
 	"slices"
 	"sync"
@@ -86,6 +87,15 @@ func (c *Client) disconnect(cs *ClientSession) {
 	})
 }
 
+// TODO: Consider exporting this type and its field.
+type unsupportedProtocolVersionError struct {
+	version string
+}
+
+func (e unsupportedProtocolVersionError) Error() string {
+	return fmt.Sprintf("unsupported protocol version: %q", e.version)
+}
+
 // Connect begins an MCP session by connecting to a server over the given
 // transport, and initializing the session.
 //
@@ -106,19 +116,22 @@ func (c *Client) Connect(ctx context.Context, t Transport) (cs *ClientSession, e
 	}
 
 	params := &InitializeParams{
+		ProtocolVersion: latestProtocolVersion,
 		ClientInfo:      &implementation{Name: c.name, Version: c.version},
 		Capabilities:    caps,
-		ProtocolVersion: "2025-03-26",
 	}
-	// TODO(rfindley): handle protocol negotiation gracefully. If the server
-	// responds with 2024-11-05, surface that failure to the caller of connect,
-	// so that they can choose a different transport.
 	res, err := handleSend[*InitializeResult](ctx, cs, methodInitialize, params)
 	if err != nil {
 		_ = cs.Close()
 		return nil, err
 	}
+	if !slices.Contains(supportedProtocolVersions, res.ProtocolVersion) {
+		return nil, unsupportedProtocolVersionError{res.ProtocolVersion}
+	}
 	cs.initializeResult = res
+	if hc, ok := cs.mcpConn.(httpConnection); ok {
+		hc.setProtocolVersion(res.ProtocolVersion)
+	}
 	if err := handleNotify(ctx, cs, notificationInitialized, &InitializedParams{}); err != nil {
 		_ = cs.Close()
 		return nil, err
diff --git a/mcp/server.go b/mcp/server.go
index cd8f808b..31b4226d 100644
--- a/mcp/server.go
+++ b/mcp/server.go
@@ -634,10 +634,11 @@ func (ss *ServerSession) initialize(ctx context.Context, params *InitializeParam
 		ss.mu.Unlock()
 	}()
 
-	version := "2025-03-26" // preferred version
-	switch v := params.ProtocolVersion; v {
-	case "2024-11-05", "2025-03-26":
-		version = v
+	// If we support the client's version, reply with it. Otherwise, reply with our
+	// latest version.
+	version := params.ProtocolVersion
+	if !slices.Contains(supportedProtocolVersions, params.ProtocolVersion) {
+		version = latestProtocolVersion
 	}
 
 	return &InitializeResult{
diff --git a/mcp/shared.go b/mcp/shared.go
index db871ca8..8a38777e 100644
--- a/mcp/shared.go
+++ b/mcp/shared.go
@@ -22,6 +22,16 @@ import (
 	"github.com/modelcontextprotocol/go-sdk/internal/jsonrpc2"
 )
 
+// latestProtocolVersion is the latest protocol version that this version of the SDK supports.
+// It is the version that the client sends in the initialization request.
+const latestProtocolVersion = "2025-06-18"
+
+var supportedProtocolVersions = []string{
+	latestProtocolVersion,
+	"2025-03-26",
+	"2024-11-05",
+}
+
 // A MethodHandler handles MCP messages.
 // For methods, exactly one of the return values must be nil.
 // For notifications, both must be nil.
diff --git a/mcp/streamable.go b/mcp/streamable.go
index db3add85..52208948 100644
--- a/mcp/streamable.go
+++ b/mcp/streamable.go
@@ -18,6 +18,11 @@ import (
 	"github.com/modelcontextprotocol/go-sdk/internal/jsonrpc2"
 )
 
+const (
+	protocolVersionHeader = "Mcp-Protocol-Version"
+	sessionIDHeader       = "Mcp-Session-Id"
+)
+
 // A StreamableHTTPHandler is an http.Handler that serves streamable MCP
 // sessions, as defined by the [MCP spec].
 //
@@ -88,7 +93,7 @@ func (h *StreamableHTTPHandler) ServeHTTP(w http.ResponseWriter, req *http.Reque
 	}
 
 	var session *StreamableServerTransport
-	if id := req.Header.Get("Mcp-Session-Id"); id != "" {
+	if id := req.Header.Get(sessionIDHeader); id != "" {
 		h.sessionsMu.Lock()
 		session, _ = h.sessions[id]
 		h.sessionsMu.Unlock()
@@ -386,7 +391,7 @@ func (t *StreamableServerTransport) streamResponse(w http.ResponseWriter, req *h
 		t.mu.Unlock()
 	}
 
-	w.Header().Set("Mcp-Session-Id", t.id)
+	w.Header().Set(sessionIDHeader, t.id)
 	w.Header().Set("Content-Type", "text/event-stream") // Accept checked in [StreamableHTTPHandler]
 	w.Header().Set("Cache-Control", "no-cache, no-transform")
 	w.Header().Set("Connection", "keep-alive")
@@ -636,12 +641,19 @@ type streamableClientConn struct {
 	closeOnce sync.Once
 	closeErr  error
 
-	mu         sync.Mutex
-	_sessionID string
+	mu              sync.Mutex
+	protocolVersion string
+	_sessionID      string
 	// bodies map[*http.Response]io.Closer
 	err error
 }
 
+func (c *streamableClientConn) setProtocolVersion(s string) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.protocolVersion = s
+}
+
 func (c *streamableClientConn) SessionID() string {
 	c.mu.Lock()
 	defer c.mu.Unlock()
@@ -707,8 +719,11 @@ func (s *streamableClientConn) postMessage(ctx context.Context, sessionID string
 	if err != nil {
 		return "", err
 	}
+	if s.protocolVersion != "" {
+		req.Header.Set(protocolVersionHeader, s.protocolVersion)
+	}
 	if sessionID != "" {
-		req.Header.Set("Mcp-Session-Id", sessionID)
+		req.Header.Set(sessionIDHeader, sessionID)
 	}
 	req.Header.Set("Content-Type", "application/json")
 	req.Header.Set("Accept", "application/json, text/event-stream")
@@ -724,7 +739,7 @@ func (s *streamableClientConn) postMessage(ctx context.Context, sessionID string
 		return "", fmt.Errorf("broken session: %v", resp.Status)
 	}
 
-	sessionID = resp.Header.Get("Mcp-Session-Id")
+	sessionID = resp.Header.Get(sessionIDHeader)
 	if resp.Header.Get("Content-Type") == "text/event-stream" {
 		go s.handleSSE(resp)
 	} else {
@@ -763,7 +778,11 @@ func (s *streamableClientConn) Close() error {
 		if err != nil {
 			s.closeErr = err
 		} else {
-			req.Header.Set("Mcp-Session-Id", s._sessionID)
+			// TODO(jba): confirm that we don't need a lock here, or add locking.
+			if s.protocolVersion != "" {
+				req.Header.Set(protocolVersionHeader, s.protocolVersion)
+			}
+			req.Header.Set(sessionIDHeader, s._sessionID)
 			if _, err := s.client.Do(req); err != nil {
 				s.closeErr = err
 			}
diff --git a/mcp/streamable_test.go b/mcp/streamable_test.go
index 8925b3da..3329caea 100644
--- a/mcp/streamable_test.go
+++ b/mcp/streamable_test.go
@@ -36,7 +36,9 @@ func TestStreamableTransports(t *testing.T) {
 	// 2. Start an httptest.Server with the StreamableHTTPHandler, wrapped in a
 	// cookie-checking middleware.
 	handler := NewStreamableHTTPHandler(func(req *http.Request) *Server { return server }, nil)
+	var header http.Header
 	httpServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		header = r.Header
 		cookie, err := r.Cookie("test-cookie")
 		if err != nil {
 			t.Errorf("missing cookie: %v", err)
@@ -72,6 +74,9 @@ func TestStreamableTransports(t *testing.T) {
 	if sid == "" {
 		t.Error("empty session ID")
 	}
+	if g, w := session.mcpConn.(*streamableClientConn).protocolVersion, latestProtocolVersion; g != w {
+		t.Fatalf("got protocol version %q, want %q", g, w)
+	}
 	// 4. The client calls the "greet" tool.
 	params := &CallToolParams{
 		Name:      "greet",
@@ -84,6 +89,9 @@ func TestStreamableTransports(t *testing.T) {
 	if g := session.ID(); g != sid {
 		t.Errorf("session ID: got %q, want %q", g, sid)
 	}
+	if g, w := header.Get(protocolVersionHeader), latestProtocolVersion; g != w {
+		t.Errorf("got protocol version header %q, want %q", g, w)
+	}
 
 	// 5. Verify that the correct response is received.
 	want := &CallToolResult{
@@ -154,7 +162,7 @@ func TestStreamableServerTransport(t *testing.T) {
 			Resources:   &resourceCapabilities{ListChanged: true},
 			Tools:       &toolCapabilities{ListChanged: true},
 		},
-		ProtocolVersion: "2025-03-26",
+		ProtocolVersion: latestProtocolVersion,
 		ServerInfo:      &implementation{Name: "testServer", Version: "v1.0.0"},
 	}, nil)
 	initializedMsg := req(0, "initialized", &InitializedParams{})
diff --git a/mcp/transport.go b/mcp/transport.go
index 85bfaf65..f0b81650 100644
--- a/mcp/transport.go
+++ b/mcp/transport.go
@@ -53,6 +53,12 @@ type Connection interface {
 	SessionID() string
 }
 
+// An httpConnection is a [Connection] that runs over HTTP.
+type httpConnection interface {
+	Connection
+	setProtocolVersion(string)
+}
+
 // A StdioTransport is a [Transport] that communicates over stdin/stdout using
 // newline-delimited JSON.
 type StdioTransport struct {