diff --git a/crates/effects/src/ocio_config/tests.rs b/crates/effects/src/ocio_config/tests.rs
index ec34b6723..31cac56c2 100644
--- a/crates/effects/src/ocio_config/tests.rs
+++ b/crates/effects/src/ocio_config/tests.rs
@@ -465,9 +465,10 @@ fn check_lut_files_notices_a_configs_lut() {
     let config = dir.join("config.ocio");
     std::fs::write(&config, "ocio_profile_version: 2\ncolorspaces:\n  - !<ColorSpace>\n    name: lin\n  - !<ColorSpace>\n    name: graded\n    to_reference: !<FileTransform> {src: watched.cube}\n").unwrap();
     let src = config.to_string_lossy().into_owned();
-    assert!(load_config(&src).is_ok());
+    // A config not read yet is read: its files name its LUT from the first render on.
     let files = crate::lut_files(&src);
     assert!(files.contains(&lut.to_string_lossy().into_owned()), "{files:?}");
+    assert!(load_config(&src).is_ok());
     let before = crate::check_lut_files();
     assert!(cached_files(&src).is_some(), "an unchanged config stays");
     std::fs::write(&lut, "LUT_1D_SIZE 3\n0 0 0\n0.5 0.5 0.5\n1 1 1\n").unwrap();
diff --git a/crates/effects/src/utility.rs b/crates/effects/src/utility.rs
index d735dc286..afb430e1d 100644
--- a/crates/effects/src/utility.rs
+++ b/crates/effects/src/utility.rs
@@ -359,9 +359,15 @@ pub fn lut_file(src: &str) -> Option<&str> {
 }
 
 /// The files a [`lut_params`] parameter set to `src` reads: the ones its cached result was read
-/// from (an OCIO config's also the LUTs it names, and the places it looked for missing ones), else
-/// the file it names.
+/// from (an OCIO config's also the LUTs it names, and the places it looked for missing ones; an
+/// `.ocio` config not read yet is read), else the file it names.
 pub fn lut_files(src: &str) -> Vec<String> {
+    // A config not read yet is read now: the LUTs it names are its files from the first render.
+    if crate::ocio_config::cached_files(src).is_none()
+        && lut_file(src).is_some_and(|p| std::path::Path::new(p).extension().is_some_and(|e| e.eq_ignore_ascii_case("ocio")))
+    {
+        let _ = crate::ocio_config::load_config(src);
+    }
     crate::ocio_config::cached_files(src)
         .or_else(|| crate::ocio::cached_files(src))
         .or_else(|| file_cache::lock(LUTS.get()).files(src))
diff --git a/crates/engine/src/tests/lut_files.rs b/crates/engine/src/tests/lut_files.rs
index 0153a9997..98c95652a 100644
--- a/crates/engine/src/tests/lut_files.rs
+++ b/crates/engine/src/tests/lut_files.rs
@@ -205,20 +205,19 @@ fn ocio_files_render_again_from_warm_caches() {
         "OCIO Color Space Transform",
         &[("config", json!(1)), ("configFile", json!(config.to_string_lossy())), ("sourceName", json!("graded")), ("destinationName", json!("lin"))],
     );
-    assert_all(colours(&s, file), 0.0);
-    assert_all(colours(&s, custom), 0.0);
-    // (the first key of a config's layer can't know the LUTs it names before it is read)
-    assert_all(colours(&s, custom), 0.0);
-    let misses = s.layer_cache.stats().misses;
-    assert_all(colours(&s, file), 0.0);
-    assert_all(colours(&s, custom), 0.0);
-    assert_eq!(s.layer_cache.stats().misses, misses, "unchanged files keep the cached layers");
+    // A render notes the LUT the config names (a frame on disk is made of it too).
     let log = ReadLog::new();
     let mut r = s.renderer(RenderOpts::default());
     r.reads = Some(&log);
     let _ = r.comp_frame(custom.1, Tick::ZERO);
     let files = log.into_files();
     assert!(files.get(&grade.to_string_lossy()).is_some(), "the render notes the config's LUT: {files:?}");
+    assert_all(colours(&s, file), 0.0);
+    assert_all(colours(&s, custom), 0.0);
+    let misses = s.layer_cache.stats().misses;
+    assert_all(colours(&s, file), 0.0);
+    assert_all(colours(&s, custom), 0.0);
+    assert_eq!(s.layer_cache.stats().misses, misses, "unchanged files keep the cached layers");
     let generation = effectcraft_effects::lut_generation();
     write(&lut, WHITE, 2_000);
     assert_all(colours(&s, file), 1.0);