diff --git a/mcphttp/client.go b/mcphttp/client.go
index fa6658b..25cbb61 100644
--- a/mcphttp/client.go
+++ b/mcphttp/client.go
@@ -28,7 +28,7 @@ import (
 	actool "github.com/Autumn-27/norma/tool"
 )
 
-const protocolVersion = "2025-06-18"
+const protocolVersion = "2025-11-25"
 const legacySSEProtocolVersion = "2024-11-05"
 
 // ToolName mirrors mcp.ToolName so remote tools share the mcp__server__tool scheme.
@@ -66,6 +66,11 @@ type Client struct {
 	mu        sync.Mutex
 	nextID    int
 	sessionID string
+	// initialized is set once initialize succeeds. Until then the
+	// MCP-Protocol-Version header is omitted: the spec requires it on requests
+	// after initialization, and the offered version already travels in the
+	// initialize body.
+	initialized bool
 
 	// legacySSE is the pre-2025 MCP SSE transport used by servers such as GSL5.
 	legacySSE    bool
@@ -247,18 +252,47 @@ func (c *Client) readLegacySSE(r *bufio.Reader) {
 }
 
 func (c *Client) initialize(ctx context.Context) error {
-	version := c.protocol
-	if version == "" {
-		version = protocolVersion
+	versions := []string{protocolVersion, "2025-06-18"}
+	if c.legacySSE {
+		versions = []string{legacySSEProtocolVersion}
 	}
-	if _, err := c.call(ctx, "initialize", map[string]any{
-		"protocolVersion": version,
-		"capabilities":    map[string]any{},
-		"clientInfo":      map[string]any{"name": "artex", "version": "0.2"},
-	}); err != nil {
-		return err
+	for i, version := range versions {
+		c.mu.Lock()
+		c.protocol = version
+		c.mu.Unlock()
+		result, err := c.call(ctx, "initialize", map[string]any{
+			"protocolVersion": version,
+			"capabilities":    map[string]any{},
+			"clientInfo":      map[string]any{"name": "artex", "version": "0.2"},
+		})
+		if err != nil {
+			if i+1 < len(versions) && isUnsupportedVersion(err) {
+				continue
+			}
+			return err
+		}
+		var negotiated struct {
+			ProtocolVersion string `json:"protocolVersion"`
+		}
+		c.mu.Lock()
+		if json.Unmarshal(result, &negotiated) == nil && negotiated.ProtocolVersion != "" {
+			c.protocol = negotiated.ProtocolVersion
+		}
+		c.initialized = true
+		c.mu.Unlock()
+		return c.notify(ctx, "notifications/initialized", map[string]any{})
 	}
-	return c.notify(ctx, "notifications/initialized", map[string]any{})
+	return fmt.Errorf("mcp: no supported protocol version")
+}
+
+// isUnsupportedVersion reports whether err is a server rejecting the offered
+// protocol version: the spec's "Unsupported protocol version" (JSON-RPC -32602,
+// or an HTTP 400 on the MCP-Protocol-Version header) or the
+// UNSUPPORTED_PROTOCOL_VERSION code some servers return instead.
+func isUnsupportedVersion(err error) bool {
+	msg := strings.ToLower(err.Error())
+	return strings.Contains(msg, "unsupported protocol version") ||
+		strings.Contains(msg, "unsupported_protocol_version")
 }
 
 // Tools lists the server's tools and adapts them to CoreTools.
@@ -438,17 +472,16 @@ func (c *Client) roundTrip(ctx context.Context, body rpcRequest, expectResp bool
 	}
 	req.Header.Set("Content-Type", "application/json")
 	req.Header.Set("Accept", "application/json, text/event-stream")
-	version := c.protocol
-	if version == "" {
-		version = protocolVersion
+	c.mu.Lock()
+	sid := c.sessionID
+	initialized, version := c.initialized, c.protocol
+	c.mu.Unlock()
+	if initialized && version != "" {
+		req.Header.Set("MCP-Protocol-Version", version)
 	}
-	req.Header.Set("MCP-Protocol-Version", version)
 	for k, v := range c.headers {
 		req.Header.Set(k, v)
 	}
-	c.mu.Lock()
-	sid := c.sessionID
-	c.mu.Unlock()
 	if sid != "" {
 		req.Header.Set("Mcp-Session-Id", sid)
 	}
diff --git a/mcphttp/client_test.go b/mcphttp/client_test.go
index cc6c50a..12e7810 100644
--- a/mcphttp/client_test.go
+++ b/mcphttp/client_test.go
@@ -73,3 +73,163 @@ func TestStreamableClientRequestTimeout(t *testing.T) {
 	}
 	_ = c3.Close()
 }
+
+// initRequest captures what the client sent for one MCP request.
+type initRequest struct {
+	method, header, bodyVersion string
+}
+
+func recordRequest(r *http.Request) (rpcRequest, initRequest) {
+	var req rpcRequest
+	_ = json.NewDecoder(r.Body).Decode(&req)
+	rec := initRequest{method: req.Method, header: r.Header.Get("MCP-Protocol-Version")}
+	if p, ok := req.Params.(map[string]any); ok {
+		rec.bodyVersion, _ = p["protocolVersion"].(string)
+	}
+	return req, rec
+}
+
+// TestStreamableClientProtocolVersions uses spec-shaped servers that each support
+// one version: they answer initialize with their own version, accept a request
+// without MCP-Protocol-Version, and reject any other header value. The client
+// must send no version header on initialize and the negotiated one afterwards.
+func TestStreamableClientProtocolVersions(t *testing.T) {
+	for _, supported := range []string{"2025-11-25", "2025-06-18", "2025-03-26"} {
+		t.Run(supported, func(t *testing.T) {
+			var seen []initRequest
+			srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				req, rec := recordRequest(r)
+				seen = append(seen, rec)
+				if rec.header != "" && rec.header != supported {
+					w.WriteHeader(http.StatusBadRequest)
+					fmt.Fprint(w, `{"code":"UNSUPPORTED_PROTOCOL_VERSION"}`)
+					return
+				}
+				w.Header().Set("Content-Type", "application/json")
+				switch req.Method {
+				case "initialize":
+					fmt.Fprintf(w, `{"jsonrpc":"2.0","id":%d,"result":{"protocolVersion":%q,"capabilities":{},"serverInfo":{"name":"t","version":"0"}}}`, req.ID, supported)
+				case "notifications/initialized":
+					w.WriteHeader(http.StatusAccepted)
+				case "tools/list":
+					fmt.Fprintf(w, `{"jsonrpc":"2.0","id":%d,"result":{"tools":[]}}`, req.ID)
+				default:
+					http.NotFound(w, r)
+				}
+			}))
+			defer srv.Close()
+
+			c, err := New(context.Background(), "t", srv.URL, nil, false, 0)
+			if err != nil {
+				t.Fatalf("connect: %v", err)
+			}
+			defer c.Close()
+			if _, err := c.Tools(context.Background()); err != nil {
+				t.Fatalf("tools/list: %v", err)
+			}
+			if c.protocol != supported {
+				t.Fatalf("negotiated protocol = %q", c.protocol)
+			}
+			var inits int
+			for _, rec := range seen {
+				if rec.method == "initialize" {
+					inits++
+					if rec.header != "" {
+						t.Fatalf("initialize carried MCP-Protocol-Version %q", rec.header)
+					}
+					if rec.bodyVersion != protocolVersion {
+						t.Fatalf("initialize offered %q, want %q", rec.bodyVersion, protocolVersion)
+					}
+					continue
+				}
+				if rec.header != supported {
+					t.Fatalf("%s carried MCP-Protocol-Version %q, want %q", rec.method, rec.header, supported)
+				}
+			}
+			if inits != 1 {
+				t.Fatalf("initialize attempts = %d, want 1", inits)
+			}
+		})
+	}
+}
+
+// TestStreamableClientVersionRejectionForms keeps the fallback loop honest for
+// servers that reject the initialize body version instead of negotiating: an
+// HTTP 400 "Unsupported protocol version", the JSON-RPC -32602 error from the
+// spec's example, and an unrelated failure that must not trigger a retry.
+func TestStreamableClientVersionRejectionForms(t *testing.T) {
+	cases := []struct {
+		name     string
+		reject   func(w http.ResponseWriter, id int)
+		fallback bool
+	}{
+		{"http 400", func(w http.ResponseWriter, _ int) {
+			http.Error(w, "Bad Request: Unsupported protocol version", http.StatusBadRequest)
+		}, true},
+		{"json-rpc -32602", func(w http.ResponseWriter, id int) {
+			w.Header().Set("Content-Type", "application/json")
+			fmt.Fprintf(w, `{"jsonrpc":"2.0","id":%d,"error":{"code":-32602,"message":"Unsupported protocol version","data":{"supported":["2025-06-18"],"requested":"2025-11-25"}}}`, id)
+		}, true},
+		{"unrelated 401", func(w http.ResponseWriter, _ int) {
+			http.Error(w, "unauthorized", http.StatusUnauthorized)
+		}, false},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			var inits []initRequest
+			var after []initRequest
+			srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				req, rec := recordRequest(r)
+				if req.Method == "initialize" {
+					inits = append(inits, rec)
+					if rec.bodyVersion != "2025-06-18" {
+						tc.reject(w, req.ID)
+						return
+					}
+				} else {
+					after = append(after, rec)
+				}
+				w.Header().Set("Content-Type", "application/json")
+				switch req.Method {
+				case "initialize":
+					fmt.Fprintf(w, `{"jsonrpc":"2.0","id":%d,"result":{"protocolVersion":"2025-06-18","capabilities":{},"serverInfo":{"name":"t","version":"0"}}}`, req.ID)
+				case "notifications/initialized":
+					w.WriteHeader(http.StatusAccepted)
+				default:
+					http.NotFound(w, r)
+				}
+			}))
+			defer srv.Close()
+
+			c, err := New(context.Background(), "t", srv.URL, nil, false, 0)
+			for _, rec := range inits {
+				if rec.header != "" {
+					t.Fatalf("initialize carried MCP-Protocol-Version %q", rec.header)
+				}
+			}
+			if !tc.fallback {
+				if err == nil {
+					_ = c.Close()
+					t.Fatal("expected connect error")
+				}
+				if len(inits) != 1 {
+					t.Fatalf("unrelated error retried: %d initialize attempts", len(inits))
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("connect: %v", err)
+			}
+			defer c.Close()
+			if len(inits) != 2 || inits[0].bodyVersion != protocolVersion || inits[1].bodyVersion != "2025-06-18" {
+				t.Fatalf("fallback attempts = %+v", inits)
+			}
+			if c.protocol != "2025-06-18" {
+				t.Fatalf("negotiated protocol = %q", c.protocol)
+			}
+			if len(after) == 0 || after[0].method != "notifications/initialized" || after[0].header != "2025-06-18" {
+				t.Fatalf("requests after initialize = %+v", after)
+			}
+		})
+	}
+}