diff --git a/scripts/test.sh b/scripts/test.sh
index 8b9cade28..cdc6d3e1d 100755
--- a/scripts/test.sh
+++ b/scripts/test.sh
@@ -357,6 +357,12 @@ else
     make -f Makefile.cbm test-par ${MAKE_ARGS[@]+"${MAKE_ARGS[@]}"}
 fi
 
+# Step 3a: the runner must ignore an inherited git repository environment
+# (#2003: the pre-commit hook exports GIT_DIR, which overrides `git -C` and
+# pointed fixture git commands at the committer's real repository).
+echo "=== Step 3a: inherited git environment isolation (#2003) ==="
+bash "$ROOT/tests/test_git_env_isolation_contract.sh" "$ROOT/$BUILD_DIR/test-runner"
+
 # Step 4: C++ large-TU index-hang regression guard (#410). Runs the PROD binary
 # in a subprocess with a wall-clock timeout — a hang must fail, not block the run.
 # Opt-in via CBM_RUN_HANG_TEST=1 (it needs the prod binary, which the ASan unit
diff --git a/tests/repro/repro_main.c b/tests/repro/repro_main.c
index 7fc1cc20b..b7c478ac7 100644
--- a/tests/repro/repro_main.c
+++ b/tests/repro/repro_main.c
@@ -26,6 +26,7 @@ int tf_fail_count = 0;
 int tf_skip_count = 0;
 
 #include "test_framework.h"
+#include "test_helpers.h"
 #include "repro_runner.h"
 #include "foundation/compat.h" /* cbm_setenv — #845 supervisor kill switch */
 #include "daemon/bootstrap.h"  /* refuse the developer's default daemon rendezvous */
@@ -139,6 +140,8 @@ int main(void) {
      * loudly instead of reaching it. */
     cbm_daemon_bootstrap_forbid_default_runtime_for_test(true);
 #endif
+    /* #2003: fixture git commands must never reach the caller's repository. */
+    th_clear_git_repo_env();
 
     /* Unbuffered: a reproduction may crash/_exit (or a sanitizer may _exit on a
      * leak) before stdio flushes — keep every printed line so the summary and the
diff --git a/tests/test_git_env_isolation_contract.sh b/tests/test_git_env_isolation_contract.sh
new file mode 100644
index 000000000..17a2b185d
--- /dev/null
+++ b/tests/test_git_env_isolation_contract.sh
@@ -0,0 +1,124 @@
+#!/usr/bin/env bash
+# Contract (#2003): the test runner must be immune to an inherited git
+# repository environment.
+#
+# Git exports GIT_DIR, GIT_INDEX_FILE, ... into hooks, and those variables take
+# precedence over `git -C <dir>`. The pre-commit hook runs the suite, so every
+# fixture git command in the runner used to act on the committer's REAL
+# repository: fixture assertions failed, and branches, commits and linked
+# worktrees were written into the contributor's clone. The runner now clears
+# git's local-env list at startup (th_clear_git_repo_env in tests/test_helpers.h).
+#
+# This drives the real runner with those variables aimed at a throwaway decoy
+# repository and asserts both halves: the git fixture suite passes, and the
+# decoy is left exactly as it was (no refs, no linked worktrees).
+#
+# Usage: tests/test_git_env_isolation_contract.sh [path/to/test-runner]
+set -euo pipefail
+
+ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+RUNNER="${1:-$ROOT/build/c/test-runner}"
+SUITE="git_context"
+
+if [ ! -x "$RUNNER" ]; then
+    echo "FAIL: test runner not found at '$RUNNER'; build it first (make -f Makefile.cbm build/c/test-runner)." >&2
+    exit 1
+fi
+if ! command -v git >/dev/null 2>&1; then
+    echo "SKIP: git is not on PATH, so there is no git environment to leak."
+    exit 0
+fi
+
+# This script's own git calls must address the decoy, not whatever repository
+# the caller (e.g. the pre-commit hook) exported.
+while IFS= read -r variable; do
+    unset "$variable"
+done < <(git rev-parse --local-env-vars)
+
+WORK="$(mktemp -d "${TMPDIR:-/tmp}/cbm-git-env-XXXXXX")"
+cleanup() { rm -rf "$WORK"; }
+trap cleanup EXIT
+
+decoy_state() {
+    printf 'refs:\n'
+    git -C "$1" for-each-ref
+    printf 'worktrees:\n'
+    git -C "$1" worktree list --porcelain
+}
+
+# Baseline: the same suite in a clean environment. Comparing summaries (not just
+# exit codes) matters: with GIT_WORK_TREE inherited, fixture commits fail and
+# the suite SKIPs as "git not available" instead of failing -- a silent green.
+BASE_LOG="$WORK/baseline.log"
+"$RUNNER" "$SUITE" >"$BASE_LOG" 2>&1 || {
+    echo "FAIL: '$SUITE' is red even in a clean environment; fix that first." >&2
+    grep -E 'FAIL|passed' "$BASE_LOG" >&2 || true
+    exit 1
+}
+
+summary() { grep -E '^ *[0-9]+ passed' "$1" | tail -1; }
+BASE_SUMMARY="$(summary "$BASE_LOG")"
+FAILED=0
+
+# Two inherited shapes: GIT_DIR alone (the reported `GIT_DIR=... scripts/test.sh`
+# case: fixture git commands write into the decoy) and the full set a hook
+# exports (fixture commits fail, so the suite degrades to SKIPs).
+check_variant() {
+    local name="$1"
+    local decoy="$WORK/decoy-$name"
+    shift
+    git init -q "$decoy"
+    local before
+    before="$(decoy_state "$decoy")"
+    local -a assignments=()
+    local var
+    for var in "$@"; do
+        assignments+=("$var=$decoy$(git_env_suffix "$var")")
+    done
+    local log="$WORK/runner-$name.log"
+    local rc
+    set +e
+    env "${assignments[@]}" "$RUNNER" "$SUITE" >"$log" 2>&1
+    rc=$?
+    set -e
+    local env_summary after
+    env_summary="$(summary "$log")"
+    after="$(decoy_state "$decoy")"
+    if [ "$rc" -ne 0 ]; then
+        echo "FAIL[$name]: '$SUITE' exited $rc with an inherited git environment." >&2
+        grep -E 'FAIL|passed' "$log" >&2 || true
+        FAILED=1
+    fi
+    if [ -z "$BASE_SUMMARY" ] || [ "$BASE_SUMMARY" != "$env_summary" ]; then
+        echo "FAIL[$name]: '$SUITE' behaves differently with an inherited git environment." >&2
+        echo "  clean env:     ${BASE_SUMMARY:-<no summary>}" >&2
+        echo "  inherited env: ${env_summary:-<no summary>}" >&2
+        grep -E 'FAIL|SKIP' "$log" >&2 || true
+        FAILED=1
+    fi
+    if [ "$before" != "$after" ]; then
+        echo "FAIL[$name]: the test runner modified the repository named by the inherited GIT_DIR." >&2
+        echo "--- before ---" >&2
+        echo "$before" >&2
+        echo "--- after ---" >&2
+        echo "$after" >&2
+        FAILED=1
+    fi
+}
+
+# Where each exported variable points inside a repository, as a hook sees it.
+git_env_suffix() {
+    case "$1" in
+        GIT_WORK_TREE) printf '' ;;
+        GIT_INDEX_FILE) printf '/.git/index' ;;
+        *) printf '/.git' ;;
+    esac
+}
+
+check_variant git-dir GIT_DIR
+check_variant hook-env GIT_DIR GIT_WORK_TREE GIT_INDEX_FILE GIT_COMMON_DIR
+
+if [ "$FAILED" -ne 0 ]; then
+    exit 1
+fi
+echo "PASS: test runner ignores an inherited git repository environment (#2003)."
diff --git a/tests/test_helpers.h b/tests/test_helpers.h
index 2559bafb5..91ea56f50 100644
--- a/tests/test_helpers.h
+++ b/tests/test_helpers.h
@@ -226,4 +226,37 @@ static inline void th_cleanup(const char *path) {
     }
 }
 
+/* ── Inherited git repository environment (#2003) ─────────────── */
+
+/* Git exports repository-local variables into hooks (GIT_DIR, GIT_INDEX_FILE,
+ * ...), and they take precedence over `git -C <dir>`. A test runner started
+ * from a pre-commit hook (or any shell with GIT_DIR set) would otherwise point
+ * every fixture's git command at the caller's real repository: branches and
+ * worktrees get created there and fixture assertions fail. This is the list
+ * `git rev-parse --local-env-vars` prints; runners clear it once at startup so
+ * every spawned git (and every re-exec'd child) scopes to its fixture. */
+static const char *const th_git_repo_env_vars[] = {
+    "GIT_ALTERNATE_OBJECT_DIRECTORIES",
+    "GIT_CONFIG",
+    "GIT_CONFIG_PARAMETERS",
+    "GIT_CONFIG_COUNT",
+    "GIT_OBJECT_DIRECTORY",
+    "GIT_DIR",
+    "GIT_WORK_TREE",
+    "GIT_IMPLICIT_WORK_TREE",
+    "GIT_GRAFT_FILE",
+    "GIT_INDEX_FILE",
+    "GIT_NO_REPLACE_OBJECTS",
+    "GIT_REPLACE_REF_BASE",
+    "GIT_PREFIX",
+    "GIT_SHALLOW_FILE",
+    "GIT_COMMON_DIR",
+};
+
+static inline void th_clear_git_repo_env(void) {
+    for (size_t i = 0U; i < sizeof(th_git_repo_env_vars) / sizeof(th_git_repo_env_vars[0]); i++) {
+        (void)cbm_unsetenv(th_git_repo_env_vars[i]);
+    }
+}
+
 #endif /* TEST_HELPERS_H */
diff --git a/tests/test_main.c b/tests/test_main.c
index 5c3dcf067..08552160a 100644
--- a/tests/test_main.c
+++ b/tests/test_main.c
@@ -1028,6 +1028,9 @@ extern void suite_dump_verify_io(void);
 extern void cbm_kind_in_set_free_cache(void);
 
 int main(int argc, char **argv) {
+    /* #2003: never let a caller's GIT_DIR/GIT_INDEX_FILE/... redirect fixture
+     * git commands at the caller's real repository. */
+    th_clear_git_repo_env();
     int memory_limit_probe_rc = tf_maybe_run_windows_memory_limit_probe(argc, argv);
     if (memory_limit_probe_rc >= 0) {
         return memory_limit_probe_rc;