diff --git a/internal/sdkserver/api.go b/internal/sdkserver/api.go
index 0bda681..2176d66 100644
--- a/internal/sdkserver/api.go
+++ b/internal/sdkserver/api.go
@@ -47,6 +47,7 @@ func registerAPIRoutes(mux *http.ServeMux) {
 	mux.HandleFunc("/api/teleport", corsWrapper(apiTeleport))
 	mux.HandleFunc("/api/gachabase/fetch", corsWrapper(apiGachabaseFetch))
 	mux.HandleFunc("/api/command", corsWrapper(apiCommand))
+	mux.HandleFunc("/api/data-in-game", corsWrapper(apiDataInGame))
 }
 
 func apiRoot(w http.ResponseWriter, r *http.Request) {
@@ -340,6 +341,60 @@ func apiPersistentData(w http.ResponseWriter, r *http.Request) {
 	}
 }
 
+// apiDataInGame handles full read/write of "data-in-game.json" file for Firefly compatibility
+func apiDataInGame(w http.ResponseWriter, r *http.Request) {
+	apiMu.Lock()
+	defer apiMu.Unlock()
+
+	switch r.Method {
+	case http.MethodGet:
+		raw, err := os.ReadFile("data-in-game.json")
+		if err != nil {
+			writeJSON(w, map[string]any{"error": err.Error(), "status": 404})
+			return
+		}
+		var parsed any
+		if err := json.Unmarshal(raw, &parsed); err != nil {
+			writeJSON(w, map[string]any{"error": err.Error(), "status": 500})
+			return
+		}
+		writeJSON(w, map[string]any{"status": 200, "data": parsed})
+
+	case http.MethodPost, http.MethodPut:
+		body, err := io.ReadAll(r.Body)
+		if err != nil || len(body) == 0 {
+			writeJSON(w, map[string]any{"error": "empty body", "status": 400})
+			return
+		}
+
+		var parsed map[string]any
+		if err := json.Unmarshal(body, &parsed); err != nil {
+			writeJSON(w, map[string]any{"error": "invalid json: " + err.Error(), "status": 400})
+			return
+		}
+
+		// Merge into existing data-in-game.json if present
+		existing := make(map[string]any)
+		if raw, err := os.ReadFile("data-in-game.json"); err == nil {
+			_ = json.Unmarshal(raw, &existing)
+		}
+		for k, v := range parsed {
+			existing[k] = v
+		}
+
+		out, _ := json.MarshalIndent(existing, "", "  ")
+		if err := os.WriteFile("data-in-game.json", out, 0o644); err != nil {
+			writeJSON(w, map[string]any{"error": "failed to write data-in-game.json: " + err.Error(), "status": 500})
+			return
+		}
+
+		writeJSON(w, map[string]any{"message": "data-in-game.json saved successfully", "status": 200})
+
+	default:
+		http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
+	}
+}
+
 // apiExRankToggle toggles Ex-Rank state for characters
 func apiExRankToggle(w http.ResponseWriter, r *http.Request) {
 	if r.Method != http.MethodPost {