diff --git a/modules/pier/dispatch.go b/modules/pier/dispatch.go
index 6353eb6d..d741125b 100644
--- a/modules/pier/dispatch.go
+++ b/modules/pier/dispatch.go
@@ -69,7 +69,9 @@ func (d *dispatcher) call(ctx context.Context, id Identity, info callInfo, t *to
 }
 
 func (d *dispatcher) run(ctx context.Context, id Identity, info callInfo, t *tool, raw json.RawMessage) (*mcp.CallToolResult, int) {
-	if len(bytes.TrimSpace(raw)) == 0 {
+	// tools/call may omit arguments (or send null) for a tool without
+	// required input; both mean an empty object.
+	if trimmed := bytes.TrimSpace(raw); len(trimmed) == 0 || bytes.Equal(trimmed, []byte("null")) {
 		raw = json.RawMessage("{}")
 	}
 	var parts map[string]json.RawMessage