diff --git a/internal/handlers/sidecar.go b/internal/handlers/sidecar.go
index 3bd0f77..449c2d0 100644
--- a/internal/handlers/sidecar.go
+++ b/internal/handlers/sidecar.go
@@ -134,6 +134,21 @@ func (h *SidecarHandler) GetSidecarConfig(c *echo.Context) error {
 			SHA256:           item.FileSha256.String,
 			FilePath:         item.FilePath,
 		}
+
+		// Also key the book by each per-format hash (KEPUB/PDF/...), so a device
+		// holding a converted format resolves via the sidecar the same way it
+		// would via BookResolver on the server.
+		formats, ferr := h.db.GetMediaItemFormats(ctx, item.ID)
+		if ferr == nil {
+			entry := books[key]
+			for _, f := range formats {
+				if f.FileSha256.Valid && f.FileSha256.String != "" {
+					if _, exists := books[f.FileSha256.String]; !exists {
+						books[f.FileSha256.String] = entry
+					}
+				}
+			}
+		}
 	}
 
 	// Get collections
@@ -269,6 +284,21 @@ func (h *SidecarHandler) DownloadSidecarConfig(c *echo.Context) error {
 			SHA256:           item.FileSha256.String,
 			FilePath:         item.FilePath,
 		}
+
+		// Also key the book by each per-format hash (KEPUB/PDF/...), so a device
+		// holding a converted format resolves via the sidecar the same way it
+		// would via BookResolver on the server.
+		formats, ferr := h.db.GetMediaItemFormats(ctx, item.ID)
+		if ferr == nil {
+			entry := books[key]
+			for _, f := range formats {
+				if f.FileSha256.Valid && f.FileSha256.String != "" {
+					if _, exists := books[f.FileSha256.String]; !exists {
+						books[f.FileSha256.String] = entry
+					}
+				}
+			}
+		}
 	}
 
 	// Get collections