diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 1a97e20f9..66788c490 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -26,6 +26,8 @@ env:
   # Tell `anyhow` *not* to collect backtraces for errors.
   # <https://users.rust-lang.org/t/anyhow-backtraces/107437>
   RUST_LIB_BACKTRACE: 0
+  # This will cause a test to fail if it has a dependence on a system/user config file.
+  AIC_DO_NOT_USE_CONFIG_FILES_IN_TESTS: 1
 
 jobs:
   build:
diff --git a/.github/workflows/mutants.yml b/.github/workflows/mutants.yml
index 59ca2b0c5..9ce072ebf 100644
--- a/.github/workflows/mutants.yml
+++ b/.github/workflows/mutants.yml
@@ -8,6 +8,8 @@ on: [workflow_dispatch]
 
 env:
   CARGO_TERM_COLOR: always
+  # This will cause a test to fail if it has a dependence on a system/user config file.
+  AIC_DO_NOT_USE_CONFIG_FILES_IN_TESTS: 1
 
 jobs:
   mutants:
diff --git a/.github/workflows/unstable-ci.yml b/.github/workflows/unstable-ci.yml
index 9fb2b2e51..e0ac4649e 100644
--- a/.github/workflows/unstable-ci.yml
+++ b/.github/workflows/unstable-ci.yml
@@ -35,6 +35,8 @@ env:
   # artifacts, so they won't be useful for anything (other than maybe the exhaustive
   # builds with different features).
   CARGO_INCREMENTAL: 0
+  # This will cause a test to fail if it has a dependence on a system/user config file.
+  AIC_DO_NOT_USE_CONFIG_FILES_IN_TESTS: 1
 
 jobs:
   build:
diff --git a/all-is-cubes-desktop/src/config_files.rs b/all-is-cubes-desktop/src/config_files.rs
index b14a30c26..efc613ee0 100644
--- a/all-is-cubes-desktop/src/config_files.rs
+++ b/all-is-cubes-desktop/src/config_files.rs
@@ -12,6 +12,12 @@ use all_is_cubes_render::camera::GraphicsOptions;
 pub fn load_config() -> Result<GraphicsOptions, anyhow::Error> {
     // TODO: make testable
     // TODO: allow users of this library function to pick their own config dir
+
+    if std::env::var("AIC_DO_NOT_USE_CONFIG_FILES_IN_TESTS").is_ok() {
+        panic!("tests should be hermetic and not touch user config files \
+        (environment variable AIC_DO_NOT_USE_CONFIG_FILES_IN_TESTS set but --no-config-files not passed)");
+    }
+
     let project_dirs = ProjectDirs::from("org.switchb", "", "all-is-cubes")
         .ok_or_else(|| anyhow::anyhow!("could not find configuration directory"))?;
     create_dir_all(project_dirs.config_dir())?;
diff --git a/all-is-cubes-desktop/tests/end-to-end.rs b/all-is-cubes-desktop/tests/end-to-end.rs
index 6b26acd61..fdb59957b 100644
--- a/all-is-cubes-desktop/tests/end-to-end.rs
+++ b/all-is-cubes-desktop/tests/end-to-end.rs
@@ -4,6 +4,10 @@
 fn trycmd_tests() {
     let tc = trycmd::TestCases::new();
 
+    // Extra protection against depending on config now or in the future.
+    // This will cause a test to fail if it has a dependence on a system/user config file.
+    tc.env("AIC_DO_NOT_USE_CONFIG_FILES_IN_TESTS", "1");
+
     // Prevent `anyhow` from printing backtraces if it otherwise would due to env having
     // "RUST_BACKTRACE=1", because they'd cause our error output tests to fail.
     // Docs: https://docs.rs/anyhow/1.0.75/anyhow/index.html#details
diff --git a/all-is-cubes-desktop/tests/gltf-validation.rs b/all-is-cubes-desktop/tests/gltf-validation.rs
index 2bdb227df..e2a97fbfb 100644
--- a/all-is-cubes-desktop/tests/gltf-validation.rs
+++ b/all-is-cubes-desktop/tests/gltf-validation.rs
@@ -10,6 +10,7 @@ fn gltf_round_trip_test(args: &[&str]) {
     let gltf_path = temp_dir.path().join("output.gltf");
 
     let output = process::Command::new(env!("CARGO_BIN_EXE_all-is-cubes"))
+        .env("AIC_DO_NOT_USE_CONFIG_FILES_IN_TESTS", "1")
         .arg("--no-config-files")
         .arg("--graphics=record")
         .arg("--output")