diff --git a/.github/workflows/ci-shimforge-config.yml b/.github/workflows/ci-shimforge-config.yml
new file mode 100644
index 00000000..3a036917
--- /dev/null
+++ b/.github/workflows/ci-shimforge-config.yml
@@ -0,0 +1,29 @@
+name: Fork shimforge config tests
+on:
+  push:
+    branches: ['shimforge-config-tests']
+  workflow_dispatch:
+permissions:
+  contents: read
+jobs:
+  verify:
+    if: github.repository == 'tanglearncode/zoxide'
+    runs-on: ubuntu-24.04
+    timeout-minutes: 20
+    steps:
+      - uses: actions/checkout@v5
+        with:
+          persist-credentials: false
+      - uses: dtolnay/rust-toolchain@stable
+        with:
+          toolchain: 1.88.0
+      - name: Build the config tests at zoxide's MSRV, with the lockfile verified
+        run: cargo test --locked --bins config -- --list
+      - name: Run the config tests 20 times at 8 threads, nothing serialized
+        run: |
+          for attempt in $(seq 1 20); do
+            cargo test --locked --bins config -- --test-threads=8
+          done
+          echo 'config tests: 20 runs x 8 threads, no env mutation.' >> "$GITHUB_STEP_SUMMARY"
+      - name: Run the full test suite with the mocks present
+        run: cargo test --locked
diff --git a/Cargo.lock b/Cargo.lock
index 16edf6e8..47d40d12 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -808,6 +808,27 @@ dependencies = [
  "zmij",
 ]
 
+[[package]]
+name = "shimforge"
+version = "0.1.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2ebf7ede656bc1cbb829e0ce8e9abf91fa3d1862b7862be358bc08a5b3064b63"
+dependencies = [
+ "libc",
+ "shimforge-macros",
+]
+
+[[package]]
+name = "shimforge-macros"
+version = "0.1.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "30e52f9314e5203ce9e4d956d789be7457d07c0afc65448ec5e0e1d9c92cf614"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+]
+
 [[package]]
 name = "static_assertions"
 version = "1.1.0"
@@ -1171,6 +1192,7 @@ dependencies = [
  "rstest",
  "rstest_reuse",
  "serde",
+ "shimforge",
  "tempfile",
  "time",
  "which",
diff --git a/Cargo.toml b/Cargo.toml
index bfcf4086..d93aac27 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -44,6 +44,7 @@ color-print = "0.3.4"
 
 [dev-dependencies]
 assert_cmd = "2.0.0"
+shimforge = "=0.1.1"
 rstest = { version = "0.26.0", default-features = false }
 rstest_reuse = "0.7.0"
 tempfile = "3.15.0"
diff --git a/src/config.rs b/src/config.rs
index 0aeda5c5..caf6e185 100644
--- a/src/config.rs
+++ b/src/config.rs
@@ -60,3 +60,58 @@ pub fn maxage() -> Result<Rank> {
 pub fn resolve_symlinks() -> bool {
     env::var_os("_ZO_RESOLVE_SYMLINKS").is_some_and(|var| var == "1")
 }
+
+#[cfg(test)]
+mod tests {
+    use std::ffi::OsString;
+
+    use shimforge::{Session, mock};
+
+    use super::*;
+
+    // Each test gets its own value for one variable, on its own thread. The
+    // process environment is never written, so these run in parallel with
+    // each other and with every other test in the suite.
+    fn env_var(session: &mut Session, name: &'static str, value: Option<&str>) {
+        let var_os = mock!(session, env::var_os::<&str>, fn(&str) -> Option<OsString>);
+        let value = value.map(OsString::from);
+        var_os.expect().with(move |var| *var == name).returns(value);
+    }
+
+    #[test]
+    fn maxage_defaults_when_unset() {
+        let mut session = Session::new();
+        env_var(&mut session, "_ZO_MAXAGE", None);
+        assert_eq!(maxage().unwrap(), 10_000.0);
+    }
+
+    #[test]
+    fn maxage_reads_the_variable() {
+        let mut session = Session::new();
+        env_var(&mut session, "_ZO_MAXAGE", Some("500"));
+        assert_eq!(maxage().unwrap(), 500.0);
+    }
+
+    #[test]
+    fn maxage_rejects_a_value_that_is_not_a_number() {
+        let mut session = Session::new();
+        env_var(&mut session, "_ZO_MAXAGE", Some("soon"));
+        let error = maxage().unwrap_err().to_string();
+        assert!(error.contains("unable to parse _ZO_MAXAGE"), "{error}");
+    }
+
+    #[test]
+    fn a_relative_data_dir_is_rejected() {
+        let mut session = Session::new();
+        env_var(&mut session, "_ZO_DATA_DIR", Some("relative/zoxide"));
+        let error = data_dir().unwrap_err().to_string();
+        assert!(error.contains("must be an absolute path"), "{error}");
+    }
+
+    #[test]
+    fn resolve_symlinks_is_opt_in() {
+        let mut session = Session::new();
+        env_var(&mut session, "_ZO_RESOLVE_SYMLINKS", Some("1"));
+        assert!(resolve_symlinks());
+    }
+}