diff --git a/internal/runtime/memory/vector/index.go b/internal/runtime/memory/vector/index.go
index 0fdd8c1..508181d 100644
--- a/internal/runtime/memory/vector/index.go
+++ b/internal/runtime/memory/vector/index.go
@@ -98,6 +98,15 @@ func New(cfg config.DBConfig, embedder embedding.Embedder, log *slog.Logger) (In
 		return nil, ErrNoEmbedder
 	}
 	dim := embedder.Dim()
+	if dim <= 0 {
+		// API-backed embedders do not know their vector dimension until the first
+		// successful response. Probe once so the index is built with the real
+		// dimension instead of erroring out and falling back to in-memory.
+		if _, err := embedder.Embed(context.Background(), []string{"probe"}); err != nil {
+			return nil, fmt.Errorf("memory/vector: probe embedder dim: %w", err)
+		}
+		dim = embedder.Dim()
+	}
 	if dim <= 0 {
 		return nil, fmt.Errorf("memory/vector: %w (got %d)", ErrInvalidDim, dim)
 	}