diff --git a/.gitignore b/.gitignore
index 0780365d..8670afc6 100644
--- a/.gitignore
+++ b/.gitignore
@@ -4,5 +4,18 @@ build*/
 python-client/
 openapi-generator-cli.jar
 
-# rugnux_vs_xds.py default output dir
-rugnux_cmp/
+# rugnux_vs_xds.py output dirs (the default, and the ad-hoc ones used when comparing runs)
+rugnux_cmp*/
+
+# Processing output and the data it came from. These are USER DATA: a merged .mtz/.cif or a model
+# carries a sample's measured unit cell, and its filename usually carries the sample's name - neither
+# of which may enter this repository (see CLAUDE.md). They accumulate in the working tree while
+# testing, so keep them un-addable rather than relying on everyone typing the right paths.
+*.mtz
+*.cif
+*.pdb
+*.hkl
+*_image.dat
+
+# Test fixtures are checked in deliberately; add them with an explicit path.
+!tests/**