Fix MCP protocol version negotiation and null prompts array

Two issues prevented Claude Code CLI from discovering MCP prompts:

1. Protocol version mismatch: Server always returned "2025-06-18" but
   Claude Code sends "2025-11-25". Per MCP spec, server must echo
   back the client's version if supported. Now supports all known
   versions (2024-11-05 through 2025-11-25).

2. Null prompts array: Go nil slices marshal as JSON null, but Claude
   Code expects an empty array []. Initialize prompts slice with make()
   to ensure [] in JSON output.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

3 files changed, 52 insertions, 7 deletions

diff --git a/internal/mcp/server.go b/internal/mcp/server.go
index 4bc66dd..6ac7537 100644
--- a/internal/mcp/server.go
+++ b/internal/mcp/server.go
@@ -107,13 +107,13 @@ func (s *Server) handleInitialize(req Request) {
 	s.logger.Printf("initialize from client: %s %s (protocol: %s)",
 		params.ClientInfo.Name, params.ClientInfo.Version, params.ProtocolVersion)
 
-	// Validate protocol version (accept both old and new versions for compatibility)
-	if params.ProtocolVersion != "2024-11-05" && params.ProtocolVersion != "2025-06-18" {
-		s.logger.Printf("warning: unsupported protocol version: %s", params.ProtocolVersion)
-	}
+	// Negotiate protocol version: echo client's version if valid, otherwise use latest.
+	// This follows the MCP spec where the server responds with a version it supports.
+	negotiatedVersion := negotiateProtocolVersion(params.ProtocolVersion)
+	s.logger.Printf("negotiated protocol version: %s", negotiatedVersion)
 
 	result := InitializeResult{
-		ProtocolVersion: "2025-06-18",
+		ProtocolVersion: negotiatedVersion,
 		Capabilities: ServerCapabilities{
 			Prompts: &PromptsCapability{
 				ListChanged: false,
@@ -133,6 +133,17 @@ func (s *Server) handleInitialize(req Request) {
 	s.sendResponse(req.ID, result)
 }
 
+// negotiateProtocolVersion returns the client's version if supported,
+// otherwise returns the latest version this server supports.
+func negotiateProtocolVersion(clientVersion string) string {
+	for _, v := range ValidProtocolVersions {
+		if v == clientVersion {
+			return clientVersion
+		}
+	}
+	return LatestProtocolVersion
+}
+
 // handleInitialized processes the initialized notification.
 // This is sent by the client after receiving initialize response.
 func (s *Server) handleInitialized(_ Request) {
@@ -166,8 +177,8 @@ func (s *Server) handlePromptsList(req Request) {
 		return
 	}
 
-	// Convert to PromptInfo
-	var infos []PromptInfo
+	// Convert to PromptInfo (initialize as empty slice so JSON marshals as [] not null)
+	infos := make([]PromptInfo, 0, len(prompts))
 	for _, p := range prompts {
 		args := make([]PromptArgument, len(p.Arguments))
 		for i, a := range p.Arguments {
diff --git a/internal/mcp/server_test.go b/internal/mcp/server_test.go
index 0c767f2..0944f35 100644
--- a/internal/mcp/server_test.go
+++ b/internal/mcp/server_test.go
@@ -500,6 +500,28 @@ func TestServer_ReadMessage(t *testing.T) {
 	})
 }
 
+func TestNegotiateProtocolVersion(t *testing.T) {
+	tests := []struct {
+		name     string
+		client   string
+		expected string
+	}{
+		{"echoes supported version", "2025-11-25", "2025-11-25"},
+		{"echoes older version", "2025-06-18", "2025-06-18"},
+		{"echoes oldest version", "2024-11-05", "2024-11-05"},
+		{"returns latest for unknown", "9999-01-01", LatestProtocolVersion},
+		{"returns latest for empty", "", LatestProtocolVersion},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := negotiateProtocolVersion(tt.client)
+			if got != tt.expected {
+				t.Errorf("negotiateProtocolVersion(%q) = %q, want %q", tt.client, got, tt.expected)
+			}
+		})
+	}
+}
+
 func TestServer_HandleInitialized(t *testing.T) {
 	store := &mockPromptStore{prompts: make(map[string]*promptstore.Prompt)}
 	server, _, _ := createTestServer(t, store)
diff --git a/internal/mcp/types.go b/internal/mcp/types.go
index e165972..0cd843a 100644
--- a/internal/mcp/types.go
+++ b/internal/mcp/types.go
@@ -38,6 +38,18 @@ const (
 	ErrCodeInternalError  = -32603 // Server internal error
 )
 
+// LatestProtocolVersion is the newest MCP protocol version this server supports.
+const LatestProtocolVersion = "2025-11-25"
+
+// ValidProtocolVersions lists all MCP protocol versions this server accepts.
+// Server echoes back the client's version if valid; otherwise responds with latest.
+var ValidProtocolVersions = []string{
+	"2025-11-25",
+	"2025-06-18",
+	"2025-03-26",
+	"2024-11-05",
+}
+
 // InitializeRequest is the first message from client to establish connection.
 // Client sends capabilities and protocol version; server responds with its capabilities.
 type InitializeRequest struct {