fix(streamable): emit explicit SSE event name for JSON-RPC responses

pkg/transport/proxy/streamable/streamable_proxy.go

@@ -46,6 +46,10 @@ const (
 	// does not affect responses, so SSE response streams are unaffected. Matches
 	// the vMCP server default.
 	defaultReadTimeout = 30 * time.Second
+
+	// sseMessageEvent is the SSE event name used for JSON-RPC responses, matching
+	// the reference MCP server transports and ToolHive's own SSE transport.
+	sseMessageEvent = "message"
 )
 
 // HTTPProxy implements a proxy for streamable HTTP transport.
@@ -530,7 +534,7 @@ func (p *HTTPProxy) handleSingleRequestSSE(
 			},
 		}
 		if data, mErr := json.Marshal(errObj); mErr == nil {
-			if _, err := fmt.Fprintf(w, "data: %s\n\n", data); err != nil {
+			if _, err := fmt.Fprintf(w, "event: %s\ndata: %s\n\n", sseMessageEvent, data); err != nil {
 				slog.Debug("failed to write error message", "error", err)
 				return
 			}
@@ -546,7 +550,8 @@ func (p *HTTPProxy) handleSingleRequestSSE(
 		return
 	}
 	// Write SSE event with the JSON-RPC response and flush
-	if _, err := fmt.Fprintf(w, "data: %s\n\n", data); err != nil { //nolint:gosec // G705: SSE data from MCP protocol
+	//nolint:gosec // G705: SSE data from MCP protocol
+	if _, err := fmt.Fprintf(w, "event: %s\ndata: %s\n\n", sseMessageEvent, data); err != nil {
 		slog.Debug("failed to write response", "error", err)
 		return
 	}

pkg/transport/proxy/streamable/streamable_proxy_spec_test.go

@@ -53,6 +53,39 @@ func startProxyWithBackend(t *testing.T, port int) (*HTTPProxy, context.Context,
 	return proxy, ctx, cancel
 }
 
+// TestSSEResponseIncludesEventName ensures SSE responses include an explicit
+// event: message line, matching reference MCP server transports. Clients such
+// as @ai-sdk/mcp only dispatch frames with event === "message".
+func TestSSEResponseIncludesEventName(t *testing.T) {
+	t.Parallel()
+
+	const port = 8110
+	proxy, ctx, cancel := startProxyWithBackend(t, port)
+	defer cancel()
+	defer func() { _ = proxy.Stop(ctx) }()
+
+	url := "http://127.0.0.1:8110" + StreamableHTTPEndpoint
+
+	initJSON := `{"jsonrpc":"2.0","id":"1","method":"initialize","params":{"protocolVersion":"2025-03-26","clientInfo":{"name":"spec-test","version":"0.0.0"},"capabilities":{}}}`
+	req, err := http.NewRequest(http.MethodPost, url, bytes.NewReader([]byte(initJSON)))
+	require.NoError(t, err)
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("Accept", "application/json, text/event-stream")
+
+	resp, err := http.DefaultClient.Do(req)
+	require.NoError(t, err)
+	defer resp.Body.Close()
+
+	assert.Equal(t, http.StatusOK, resp.StatusCode)
+	assert.Equal(t, "text/event-stream", resp.Header.Get("Content-Type"))
+
+	body, err := io.ReadAll(resp.Body)
+	require.NoError(t, err)
+	sseContent := string(body)
+	assert.Contains(t, sseContent, "event: message\n", "SSE frame should include explicit event name")
+	assert.Contains(t, sseContent, "data: ", "SSE frame should include JSON-RPC data line")
+}
+
 // TestGETReturns405 validates that GET on MCP endpoint returns 405 (server does not offer SSE here).
 func TestGETReturns405(t *testing.T) {
 	t.Parallel()