diff --git a/src/git.rs b/src/git.rs
index 34a7801f..b4cca10c 100644
--- a/src/git.rs
+++ b/src/git.rs
@@ -2,7 +2,7 @@
 
 use std::fs::{File, TryLockError};
 use std::path::{Path, PathBuf};
-use std::process::{Command, Output};
+use std::process::Output;
 use std::time::{Duration, Instant};
 
 use anyhow::{Context, Result, bail};
@@ -304,8 +304,12 @@ impl Git {
     }
 
     fn output(&self, args: &[&str]) -> Result<Output> {
+        #[cfg(not(test))]
+        let mut command = std::process::Command::new("git");
+        #[cfg(test)]
+        let mut command = crate::test_support::git_command();
         process::output(
-            Command::new("git").args(args).current_dir(&self.dir),
+            command.args(args).current_dir(&self.dir),
             None,
             Control {
                 name: &format!("git {}", args.join(" ")),
@@ -419,11 +423,9 @@ mod tests {
     pub(super) fn repo_with_origin() -> (tempfile::TempDir, Git) {
         let temp = tempfile::TempDir::new().unwrap();
         let isolated = |dir: &Path, args: &[&str]| {
-            let status = Command::new("git")
+            let status = crate::test_support::git_command()
                 .args(args)
                 .current_dir(dir)
-                .env("GIT_CONFIG_GLOBAL", "/dev/null")
-                .env("GIT_CONFIG_NOSYSTEM", "1")
                 .status()
                 .unwrap();
             assert!(status.success(), "git {args:?}");
@@ -434,6 +436,8 @@ mod tests {
         for (key, value) in [
             ("user.name", "Test Runner"),
             ("user.email", "runner@example.com"),
+            ("commit.gpgsign", "false"),
+            ("tag.gpgsign", "false"),
             ("remote.origin.url", "../origin.git"),
         ] {
             isolated(&work, &["config", key, value]);
diff --git a/src/launch.rs b/src/launch.rs
index efa9a2d2..f135f579 100644
--- a/src/launch.rs
+++ b/src/launch.rs
@@ -456,6 +456,8 @@ mod tests {
         for (key, value) in [
             ("user.name", "Test Runner"),
             ("user.email", "runner@example.com"),
+            ("commit.gpgsign", "false"),
+            ("tag.gpgsign", "false"),
             ("remote.origin.url", "https://github.com/acme/widgets.git"),
             ("remote.origin.fetch", "+refs/heads/*:refs/remotes/origin/*"),
             (
@@ -471,11 +473,9 @@ mod tests {
     }
 
     fn isolated(dir: &Path, args: &[&str]) -> String {
-        let output = Command::new("git")
+        let output = crate::test_support::git_command()
             .args(args)
             .current_dir(dir)
-            .env("GIT_CONFIG_GLOBAL", "/dev/null")
-            .env("GIT_CONFIG_NOSYSTEM", "1")
             .output()
             .unwrap();
         assert!(output.status.success(), "git {args:?}");
diff --git a/src/session/execution_tests.rs b/src/session/execution_tests.rs
index 8a4af382..a5c212e0 100644
--- a/src/session/execution_tests.rs
+++ b/src/session/execution_tests.rs
@@ -64,6 +64,7 @@ echo '{{"type":"step_start","sessionID":"s1","part":{{}}}}'
             .args(["--exact", test, "--nocapture"])
             .env(FIXTURE, self.0.path())
             .env("PATH", path);
+        crate::test_support::isolate_git_config(&mut command);
         command
     }
 
@@ -105,7 +106,7 @@ fn execute_session(root: &Path) -> anyhow::Error {
     let worktree = root.join("worktree");
     fs::create_dir(&worktree).unwrap();
     assert!(
-        Command::new("git")
+        crate::test_support::git_command()
             .args(["init", "-q"])
             .arg(&worktree)
             .status()
@@ -368,7 +369,7 @@ fn with_review_sessions(
     let worktree = root.join("worktree");
     fs::create_dir(&worktree).unwrap();
     assert!(
-        Command::new("git")
+        crate::test_support::git_command()
             .args(["init", "-q"])
             .arg(&worktree)
             .status()
@@ -440,6 +441,31 @@ fn keeps_review_reports_beside_each_harness_session_log() {
     }
 }
 
+#[test]
+fn shell_git_fixtures_ignore_global_and_system_config() {
+    let temp = tempfile::tempdir().unwrap();
+    let config = temp.path().join("host.gitconfig");
+    fs::write(&config, "[core]\n ignoreCase = invalid\n").unwrap();
+    let output = Command::new(std::env::current_exe().unwrap())
+        .args([
+            "--exact",
+            "session::execution_tests::keeps_review_reports_beside_each_harness_session_log",
+            "--nocapture",
+        ])
+        .env_remove(FIXTURE)
+        .env("GIT_CONFIG_GLOBAL", &config)
+        .env("GIT_CONFIG_SYSTEM", &config)
+        .env_remove("GIT_CONFIG_NOSYSTEM")
+        .output()
+        .unwrap();
+    assert!(
+        output.status.success(),
+        "{}\n{}",
+        String::from_utf8_lossy(&output.stdout),
+        String::from_utf8_lossy(&output.stderr)
+    );
+}
+
 #[test]
 fn missing_review_reports_get_a_progress_line_and_the_run_carries_on() {
     if let Some(root) = std::env::var_os(FIXTURE) {
diff --git a/src/test_support.rs b/src/test_support.rs
index c8d0d059..fb9e5dd5 100644
--- a/src/test_support.rs
+++ b/src/test_support.rs
@@ -11,6 +11,22 @@ use std::process::Command;
 
 use crate::interrupt;
 
+/// Git for unit-test fixtures, including commands made after repository setup.
+/// Keep this on each child Command so parallel tests never change the process
+/// environment and new Git views of a fixture inherit the same isolation.
+pub(crate) fn git_command() -> Command {
+    let mut command = Command::new("git");
+    isolate_git_config(&mut command);
+    command
+}
+
+/// Also cover Git invoked by shell scripts in a fixture subprocess.
+pub(crate) fn isolate_git_config(command: &mut Command) {
+    command
+        .env("GIT_CONFIG_GLOBAL", "/dev/null")
+        .env("GIT_CONFIG_NOSYSTEM", "1");
+}
+
 /// A child that announces Session s1 after writing a native Codex quota record.
 /// The record may deliberately name another Session/directory or a stale time;
 /// `current` writes a fresh timestamp. HOME is set only on the child Command.
@@ -44,7 +60,7 @@ touch "$HOME/../continued"
 pub(crate) fn scratch_worktree() -> tempfile::TempDir {
     let worktree = tempfile::tempdir().unwrap();
     assert!(
-        Command::new("git")
+        git_command()
             .args(["init", "-q"])
             .arg(worktree.path())
             .status()
diff --git a/src/worktree.rs b/src/worktree.rs
index 2cee57a7..e49a86f0 100644
--- a/src/worktree.rs
+++ b/src/worktree.rs
@@ -529,7 +529,6 @@ fn lock_launch(launch: &Git) -> Result<File> {
 
 #[cfg(test)]
 mod tests {
-    use std::process::Command;
     use std::time::Duration;
 
     use super::*;
@@ -539,11 +538,9 @@ mod tests {
     pub(super) fn launch_directory() -> (tempfile::TempDir, Git) {
         let temp = tempfile::TempDir::new().unwrap();
         let git = |dir: &Path, args: &[&str]| {
-            let status = Command::new("git")
+            let status = crate::test_support::git_command()
                 .args(args)
                 .current_dir(dir)
-                .env("GIT_CONFIG_GLOBAL", "/dev/null")
-                .env("GIT_CONFIG_NOSYSTEM", "1")
                 .status()
                 .unwrap();
             assert!(status.success(), "git {args:?}");
@@ -568,6 +565,7 @@ mod tests {
             ("user.name", "Test Runner"),
             ("user.email", "runner@example.com"),
             ("commit.gpgSign", "false"),
+            ("tag.gpgsign", "false"),
             ("core.hooksPath", hooks.to_str().unwrap()),
         ] {
             git(&work, &["config", key, value]);
diff --git a/src/worktree/synchronization_tests.rs b/src/worktree/synchronization_tests.rs
index 34ed995c..107faffc 100644
--- a/src/worktree/synchronization_tests.rs
+++ b/src/worktree/synchronization_tests.rs
@@ -1060,6 +1060,63 @@ fn base_merges_ignore_shadowing_branch_and_tag_names() {
     }
 }
 
+#[test]
+fn git_fixtures_ignore_global_and_system_config() {
+    const CHILD: &str = "THIRDSHIFT_TEST_HOST_GIT_CONFIG";
+    if std::env::var_os(CHILD).is_some() {
+        base_merges_ignore_shadowing_branch_and_tag_names();
+        foreign_observation_and_merge_ignore_shadowing_branch_and_tag_names();
+        spec_catch_up_ignores_shadowing_branch_and_tag_names();
+
+        let (_temp, launch, worktree) = fixture();
+        for git in [launch, Git::new(worktree.path()).completion()] {
+            for scope in ["global", "system"] {
+                assert!(
+                    !git.succeeds(&["config", "--get", &format!("fixture.{scope}")])
+                        .unwrap(),
+                    "fixture read {scope} config"
+                );
+            }
+            git.run(&["config", "fixture.local", "visible"]).unwrap();
+            assert_eq!(
+                git.run(&["config", "--get", "fixture.local"]).unwrap(),
+                "visible"
+            );
+        }
+        return;
+    }
+
+    let temp = tempfile::tempdir().unwrap();
+    for scope in ["global", "system"] {
+        fs::write(
+            temp.path().join(scope),
+            format!(
+                "[tag]\n gpgsign = true\n[commit]\n gpgsign = true\n[fixture]\n {scope} = visible\n"
+            ),
+        )
+        .unwrap();
+    }
+    let output = std::process::Command::new(std::env::current_exe().unwrap())
+        .args([
+            "--exact",
+            "worktree::synchronization_tests::git_fixtures_ignore_global_and_system_config",
+            "--nocapture",
+        ])
+        .env(CHILD, "1")
+        .env("GIT_CONFIG_GLOBAL", temp.path().join("global"))
+        .env("GIT_CONFIG_SYSTEM", temp.path().join("system"))
+        .env_remove("GIT_CONFIG_NOSYSTEM")
+        .env("GIT_EDITOR", "true")
+        .output()
+        .unwrap();
+    assert!(
+        output.status.success(),
+        "{}\n{}",
+        String::from_utf8_lossy(&output.stdout),
+        String::from_utf8_lossy(&output.stderr)
+    );
+}
+
 #[test]
 fn foreign_observation_and_merge_ignore_shadowing_branch_and_tag_names() {
     for kind in ["branch", "tag"] {