diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 628cead8ab2..c1c3d15ee91 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -113,7 +113,7 @@ Those sleeps look like recoverable overhead - `fm-watch-triage.test.sh` alone is
 Sampling less often does not remove that wait, it only delays detection: raising the interval to 0.5s and charging each sample proportionally measured `fm-watch-triage.test.sh` at 435s and 440s against 390s and 393s for the unchanged script, back to back on 2026-09-03, because each of its ~40 poll-cycle waits and ~73 process-exit waits paid up to half a second more.
 Some of those loops are also catching a transient rather than waiting for a settled condition, so a coarser sample can step over the state they assert on.
 Discover tests by listing `tests/*.test.sh`: each is a self-contained bash script named `<subject>.test.sh`, and its header comment describes what it covers, so pass one to `bin/fm-test-run.sh` to focus on a subject with canonical timing output.
-Shared test helpers live in `tests/lib.sh` (reporters, temp roots, git fixtures), `tests/fixtures.sh` (fake toolchain and spawn-world builders), `tests/wake-helpers.sh`, and `tests/secondmate-helpers.sh`.
+Shared test helpers live in `tests/lib.sh` (reporters, temp roots, git fixtures), `tests/fixtures.sh` (fake toolchain and spawn-world builders), `tests/wake-helpers.sh`, `tests/secondmate-helpers.sh`, and `tests/git-config-helpers.sh` (fixture Git isolation from the host's global and system configuration, already sourced by `tests/lib.sh` and `tests/herdr-test-safety.sh`; a suite that sources neither must source it itself before its first Git operation so a direct invocation stays isolated).
 Source those instead of copying a fake toolchain into a new suite.
 A fixture may shorten a production timeout to keep a failure path prompt, but never below what the real work inside that window costs on a loaded machine: a fork, an exec, a lock acquisition, a beacon publication, or a first-poll check.
 Where a case's assertion is not about the timeout itself, give that window headroom over the measured loaded cost, and bound the test's own waiting with iteration-counted poll loops, which stretch under load where a wall-clock budget does not.
diff --git a/bin/fm-test-run.sh b/bin/fm-test-run.sh
index ee9f32e04df..8817717424b 100755
--- a/bin/fm-test-run.sh
+++ b/bin/fm-test-run.sh
@@ -111,6 +111,10 @@
 # live-capability (a live-harness guard governed by fm_live_gate, which records
 # unavailable tools and explicit policy skips; see tests/lib.sh), or none.
 #
+# Every selected script runs isolated from the host's global and system Git
+# configuration, including one that sources no test helper of its own;
+# tests/git-config-helpers.sh owns that contract and its limits.
+#
 # Family labels, the changed-file map, and production portable-shard composition
 # live in this script only (one owner). The proven-isolated candidate set remains
 # owned by bin/fm-test-isolation-proof.sh; portable parallel shards are a
@@ -1139,12 +1143,14 @@ select_family() {
   [ "$found" -eq 1 ] || die "no tests mapped to family '$want'"
 }
 
-families_for_test_reference() {
-  local needle=$1 s
+families_for_test_reference() {  # <needle>...
+  local s needle
   local found=0
+  local -a needles=()
+  for needle in "$@"; do needles+=(-e "$needle"); done
   while IFS= read -r s; do
     [ -n "$s" ] || continue
-    if grep -Fq "$needle" "$s"; then
+    if grep -Fq "${needles[@]}" "$s"; then
       family_for_basename "$(basename "$s")"
       found=1
     fi
@@ -1221,12 +1227,21 @@ families_for_changed_path() {
       # resolution in the caller; emit a marker family of __script__
       printf '%s\n' "__script__:$(basename "$path")"
       ;;
-    bin/fm-test-run.sh|bin/fm-test-isolation-proof.sh)
+    bin/fm-test-run.sh)
       # Deliberately the WHOLE family, not just the two contract tests. This
       # runner executes every pure-contract-unit script, so a change to it is
       # only proven by running them: its own contract test passing says the
       # runner's logic is right, not that the suite it drives still runs.
       printf '%s\n' pure-contract-unit
+      # Only this script wraps each suite in run_script_bounded's fixture Git
+      # isolation, and only a standalone-family script proves it.
+      printf '%s\n' "__script__:fm-test-fixtures.test.sh"
+      ;;
+    bin/fm-test-isolation-proof.sh)
+      # Same reason as the runner above: the proof drives every
+      # pure-contract-unit script. It runs each candidate directly, never
+      # through run_script_bounded, so it cannot regress fixture Git isolation.
+      printf '%s\n' pure-contract-unit
       ;;
     bin/backends/herdr*|bin/fm-herdr-lab.sh|tests/herdr-test-safety.sh)
       printf '%s\n' real-herdr-gated
@@ -1432,6 +1447,12 @@ families_for_changed_path() {
     docs/configuration.md|docs/supervision-protocols/*)
       printf '%s\n' pure-contract-unit
       ;;
+    tests/git-config-helpers.sh)
+      # The reference scan is not transitive, so match the two helpers that
+      # source this one as well: most suites inherit it only through them.
+      families_for_test_reference git-config-helpers.sh lib.sh herdr-test-safety.sh \
+        || printf '%s\n' "__unmapped__:$path"
+      ;;
     tests/lib.sh|tests/*-helpers.sh|tests/fixtures.sh)
       families_for_test_reference "$(basename "$path")" \
         || printf '%s\n' "__unmapped__:$path"
@@ -2179,6 +2200,12 @@ record_script_result() {
 # because an unbounded suite is what silently outruns its caller's budget.
 run_script_bounded() {  # <script> <out> <stream> <id>
   local script=$1 out=$2 stream=$3 id=$4
+  # Declaring the variables local first keeps the helper's export scoped to this
+  # call and its child script, so the runner's own environment is left as the
+  # caller had it.
+  local GIT_CONFIG_GLOBAL GIT_CONFIG_NOSYSTEM
+  # shellcheck source=tests/git-config-helpers.sh
+  . "$ROOT/tests/git-config-helpers.sh" || return
   local rc
   : "$id"
   set +e
diff --git a/docs/fm-test-isolation-proof.md b/docs/fm-test-isolation-proof.md
index e8df1878041..6f0766a16bc 100644
--- a/docs/fm-test-isolation-proof.md
+++ b/docs/fm-test-isolation-proof.md
@@ -119,10 +119,10 @@ Both `bin/fm-test-run.sh` and the current proof harness therefore order concurre
 | 1 | `FM_ISOLATION_SUMMARY total=32 failed=0 concurrency=4 duration_ms=161837` |
 | 2 | `FM_ISOLATION_SUMMARY total=32 failed=0 concurrency=4 duration_ms=156462` |
 
-This family is what a change to `bin/fm-test-run.sh` itself selects, so it decides that selection's wall clock.
-Before admission, 14 of its scripts fell to the serial tail and the 33-script selection measured 327.3s against a 300s budget: the concurrent group was 19 scripts totalling 273.4s while the tail alone was 215.7s, dominated by `fm-calm-pi-extension` (77.5s), `fm-vendor-auth-probe` (51.0s), and `fm-muse-harness` (39.7s).
+The current runner-change selection is owned by [`bin/fm-test-run.sh`](../bin/fm-test-run.sh)'s changed-file map.
+Before admission, 14 of the family's scripts fell to the serial tail and the 33-script selection measured 327.3s against a 300s budget: the concurrent group was 19 scripts totalling 273.4s while the tail alone was 215.7s, dominated by `fm-calm-pi-extension` (77.5s), `fm-vendor-auth-probe` (51.0s), and `fm-muse-harness` (39.7s).
 Admitting the family moves that tail into the bounded concurrent group.
-Current runner-file selection was verified on 2026-08-28 with the runner and its tests bound to each measured Bash version.
+The then-current runner-file selection was verified on 2026-08-28 with the runner and its tests bound to each measured Bash version.
 Because the runner uses `#!/usr/bin/env bash` and invokes each test with `bash` from `PATH`, the stock macOS measurement used `PATH=/bin:$PATH bin/fm-test-run.sh --changed --max-wall-ms 300000` so both resolved to `/bin/bash` 3.2.57.
 Two runs selected all 33 scripts, passed the five-minute result check in 153.5s and 166.8s, and reported the same two failures as `main`: `tests/fm-muse-harness.test.sh` and `tests/fm-composer-lib.test.sh`.
 With Bash 5.3.9 on `PATH`, three runs of `bin/fm-test-run.sh --changed --max-wall-ms 300000` selected the same 33 scripts, completed with 0 failures, and reported 163.8s, 172.0s, and 166.9s.
diff --git a/tests/fm-gitignore-config.test.sh b/tests/fm-gitignore-config.test.sh
index a221b4176d2..9c6c4c7ad6f 100755
--- a/tests/fm-gitignore-config.test.sh
+++ b/tests/fm-gitignore-config.test.sh
@@ -8,6 +8,8 @@
 set -u
 
 ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+# shellcheck source=tests/git-config-helpers.sh
+. "$ROOT/tests/git-config-helpers.sh"
 
 fail() {
   printf 'not ok - %s\n' "$1" >&2
diff --git a/tests/fm-test-fixtures.test.sh b/tests/fm-test-fixtures.test.sh
index 9176771dd5d..5d9c67c8b1a 100755
--- a/tests/fm-test-fixtures.test.sh
+++ b/tests/fm-test-fixtures.test.sh
@@ -6,6 +6,12 @@
 # filesystem effects - never on helper source text. Migrated spawn suites cover
 # fm_test_run_spawn through the real fm-spawn.sh; this file pins the shared
 # primitives and stubs those suites use.
+#
+# It is also the fixture Git-config isolation regression, with host signing
+# armed on a scratch config file: it drives every entry point that must reach
+# tests/git-config-helpers.sh - the shared helpers, bin/fm-test-run.sh's
+# per-suite wrapper, and the standalone scripts runnable without a live vendor.
+# That helper's header owns the contract and the layers it leaves in force.
 set -u
 
 # shellcheck source=tests/fixtures.sh
@@ -13,6 +19,140 @@ set -u
 
 TMP_ROOT=$(fm_test_tmproot fm-test-fixtures)
 
+test_git_config_isolation() (
+  local dir="$TMP_ROOT/git-config" helper jobs timeout fakebin rc
+  mkdir -p "$dir/runner/bin" "$dir/runner/tests"
+  git init -q "$dir/caller"
+  git -C "$dir/caller" config commit.gpgsign false
+  cd "$dir/caller" || exit 1
+  cp "$ROOT/bin/fm-test-run.sh" "$ROOT/bin/fm-timeout-lib.sh" "$dir/runner/bin/"
+  cp "$ROOT/tests/git-config-helpers.sh" "$dir/runner/tests/"
+  fakebin=$(fm_fakebin "$dir/standalone")
+  fm_fake_exit0 "$fakebin" pi
+  cat > "$fakebin/tmux" <<'SH'
+#!/usr/bin/env bash
+set -eu
+while [ "$#" -gt 0 ]; do
+  if [ "$1" = -c ]; then
+    git -C "$2" log -1 --format=%s > "${FM_TEST_STANDALONE_COMMIT:?}"
+    exit 1
+  fi
+  shift
+done
+SH
+  chmod +x "$fakebin/tmux"
+  cat > "$dir/runner/tests/fm-test-run.test.sh" <<'SH'
+#!/usr/bin/env bash
+set -eu
+repo=$(mktemp -d "${TMPDIR:-/tmp}/fm-git-runner.XXXXXX")
+trap 'rm -rf "$repo"' EXIT
+git init -q "$repo"
+git -C "$repo" config user.name 'Runner Fixture'
+git -C "$repo" config user.email runner@example.invalid
+git -C "$repo" commit -q --allow-empty -m initial
+[ "$(git -C "$repo" log -1 --format='%s:%an:%ae')" = 'initial:Runner Fixture:runner@example.invalid' ]
+[ "$(git -C "$repo" config --get fixture.input)" = preserved ]
+[ "$(GIT_CONFIG_GLOBAL="$FM_TEST_GIT_CONFIG" git config --global --get commit.gpgsign)" = true ]
+SH
+  chmod +x "$dir/runner/tests/fm-test-run.test.sh"
+  export GIT_CONFIG_GLOBAL="$dir/global" GIT_CONFIG_SYSTEM="$dir/system"
+  export GIT_CONFIG_NOSYSTEM=0
+  unset GIT_CONFIG_COUNT GIT_CONFIG_PARAMETERS
+
+  # A failing signer exposes inherited config without requiring GPG or keys.
+  arm_host_signing() {  # <scope>: only this layer carries the failing signer
+    : > "$dir/global"
+    : > "$dir/system"
+    git config --file "$dir/$1" commit.gpgsign true
+    git config --file "$dir/$1" gpg.format openpgp
+    git config --file "$dir/$1" gpg.program /usr/bin/false
+    cp "$dir/$1" "$dir/expected"
+  }
+
+  assert_helper_isolates() {  # <helper> <scope>
+    bash -eus -- "$ROOT/tests/$1.sh" "$dir/$2-$1" "$dir/$2" <<'SH' || exit 1
+. "$1"
+fm_git_init_commit "$2"
+[ "$(git -C "$2" log -1 --format=%s)" = initial ] || fail "fixture has no initial commit"
+fm_git_identity
+# Child Git processes and direct commits inherit the same isolation.
+bash -eu -c 'git -C "$1" commit -q --allow-empty -m child' _ "$2"
+# Repository-local config and explicit command inputs remain authoritative.
+git -C "$2" config commit.gpgsign true
+git -C "$2" config gpg.program /usr/bin/false
+if git -C "$2" commit -q --allow-empty -m signed > "$2/signing.log" 2>&1; then
+  fail "repository-local signing config was ignored"
+fi
+assert_grep 'gpg failed to sign' "$2/signing.log" "local signing was not attempted"
+git -C "$2" -c commit.gpgsign=false commit -q --allow-empty -m explicit
+GIT_CONFIG_COUNT=1 GIT_CONFIG_KEY_0=commit.gpgsign GIT_CONFIG_VALUE_0=false \
+  git -C "$2" commit -q --allow-empty -m environment
+# A config test can deliberately supply its own global file after sourcing.
+[ "$(GIT_CONFIG_GLOBAL="$3" git config --global --get commit.gpgsign)" = true ] || fail "explicit global config was ignored"
+SH
+  }
+
+  assert_host_config_still_governs() {  # <scope>
+    # Sourcing in test subprocesses cannot change the caller or its config files.
+    [ "$(git config --"$1" --get commit.gpgsign)" = true ] || fail "caller lost signing preference"
+    cmp -s "$dir/$1" "$dir/expected" || fail "host config file was changed"
+    git init -q "$dir/$1-outside"
+    if git -C "$dir/$1-outside" -c user.name=test -c user.email=test@example.invalid \
+      commit -q --allow-empty -m outside > "$dir/outside.log" 2>&1; then
+      fail "commit outside fixtures bypassed signing"
+    fi
+    assert_grep 'gpg failed to sign' "$dir/outside.log" "outside commit did not attempt signing"
+  }
+
+  # Every fixture entry point, once. Each only has to reach the shared helper;
+  # which layers that helper neutralizes is the helper's own property, settled
+  # by the system-layer case below.
+  arm_host_signing global
+  for helper in lib fixtures secondmate-helpers wake-helpers; do
+    assert_helper_isolates "$helper" global
+  done
+  bash -eus -- "$ROOT/tests/herdr-test-safety.sh" "$dir/global-herdr" <<'SH' || exit 1
+. "$1"
+git init -q "$2"
+git -C "$2" -c user.name=test -c user.email=test@example.invalid \
+  commit -q --allow-empty -m initial
+[ "$(git -C "$2" log -1 --format=%s)" = initial ]
+SH
+  for jobs in 1 2; do
+    for timeout in 0 30; do
+      GIT_CONFIG_COUNT=1 GIT_CONFIG_KEY_0=fixture.input GIT_CONFIG_VALUE_0=preserved \
+        FM_TEST_GIT_CONFIG="$dir/global" \
+        "$dir/runner/bin/fm-test-run.sh" --jobs "$jobs" --per-script-timeout-secs "$timeout" \
+        tests/fm-test-run.test.sh > "$dir/runner.log" 2>&1 \
+        || fail "runner inherited global config (jobs=$jobs, timeout=$timeout): $(cat "$dir/runner.log")"
+      assert_grep 'FM_TEST_SUMMARY total=1 failed=0 skipped_gate=0' "$dir/runner.log" \
+        "runner did not execute the Git fixture"
+    done
+  done
+  rc=0
+  FM_SESSIONSTART_INSTRUCTION_REFRESH_LIVE_E2E=1 FM_SESSIONSTART_INSTRUCTION_REFRESH_REF=HEAD \
+    FM_SESSIONSTART_INSTRUCTION_REFRESH_EXPECT=updated \
+    FM_TEST_STANDALONE_COMMIT="$dir/global-standalone-commit" PATH="$fakebin:$PATH" \
+    bash "$ROOT/tests/fm-sessionstart-instruction-refresh-live-e2e.test.sh" \
+    > "$dir/standalone.log" 2>&1 || rc=$?
+  [ "$rc" = 1 ] || fail "standalone fixture did not stop at the tmux launch"
+  assert_grep 'could not start isolated Pi session' "$dir/standalone.log" \
+    "standalone fixture failed before the tmux launch: $(cat "$dir/standalone.log")"
+  [ "$(cat "$dir/global-standalone-commit")" = 'test: initial instruction contract' ] \
+    || fail "standalone fixture did not create its initial commit"
+  bash "$ROOT/tests/fm-gitignore-config.test.sh" > "$dir/gitignore.log" 2>&1 \
+    || fail "standalone gitignore fixture inherited global config: $(cat "$dir/gitignore.log")"
+  assert_host_config_still_governs global
+
+  # The system layer is the shared helper's other half: one entry point settles
+  # it, and the caller still signing proves the layer was genuinely armed.
+  arm_host_signing system
+  assert_helper_isolates lib system
+  assert_host_config_still_governs system
+
+  pass "runner and shared helpers isolate host Git config and preserve explicit config and outside commits"
+)
+
 test_touch_epoch_preserves_repeated_dst_hour() {
   local TZ=Europe/Paris epoch path actual
   export TZ
@@ -139,6 +279,7 @@ test_spawn_home_layout() {
   pass "spawn-home layout writes harness pin, beat, and brief"
 }
 
+test_git_config_isolation || fail "Git fixture config isolation"
 test_touch_epoch_preserves_repeated_dst_hour
 test_no_mistakes_version_constant
 test_no_mistakes_init_doctor_markers
diff --git a/tests/fm-test-run.test.sh b/tests/fm-test-run.test.sh
index b90dfa2129d..767838126d5 100755
--- a/tests/fm-test-run.test.sh
+++ b/tests/fm-test-run.test.sh
@@ -92,6 +92,7 @@ init_changed_fixture_repo() {
   local repo=$1 script
   mkdir -p "$repo/bin" "$repo/tests"
   cp "$RUNNER" "$repo/bin/fm-test-run.sh"
+  cp "$ROOT/tests/git-config-helpers.sh" "$repo/tests/"
   chmod +x "$repo/bin/fm-test-run.sh"
   for script in \
     fm-brief.test.sh \
@@ -99,6 +100,7 @@ init_changed_fixture_repo() {
     fm-documentation-audiences.test.sh \
     fm-test-isolation-proof.test.sh \
     fm-test-run.test.sh \
+    fm-test-fixtures.test.sh \
     fm-cd-pretool-check.test.sh \
     fm-daemon.test.sh \
     fm-harness-adapter-instructions-live-e2e.test.sh \
@@ -174,6 +176,7 @@ init_primary_and_linked_worktree() {
   for tree in "$repo" "$linked"; do
     mkdir -p "$tree/bin" "$tree/tests"
     cp "$RUNNER" "$tree/bin/fm-test-run.sh"
+    cp "$ROOT/tests/git-config-helpers.sh" "$tree/tests/"
     chmod +x "$tree/bin/fm-test-run.sh"
     cat >"$tree/tests/probe.test.sh" <<PROBE
 #!/usr/bin/env bash
@@ -252,6 +255,12 @@ test_changed_runner_surfaces_select_their_family() {
     *tests/fm-ask-user-authority.test.sh*) ;;
     *) fail "runner change did not select its pure-contract-unit family: $listed" ;;
   esac
+  # The suite that proves the runner's per-suite fixture Git isolation lives in
+  # the standalone family, which pure-contract-unit never reaches.
+  case "$listed" in
+    *tests/fm-test-fixtures.test.sh*) ;;
+    *) fail "runner change did not select its fixture-isolation regression: $listed" ;;
+  esac
   git -C "$repo" add bin/fm-test-run.sh
   git -C "$repo" -c user.name=test -c user.email=test@example.invalid commit -qm runner-change
 
@@ -299,6 +308,14 @@ test_changed_dependency_selection_and_unmapped_failure() {
   git -C "$repo" add tests/lib.sh
   git -C "$repo" -c user.name=test -c user.email=test@example.invalid commit -qm helper-change
 
+  printf '\n' >>"$repo/tests/git-config-helpers.sh"
+  listed=$(cd "$repo" && bin/fm-test-run.sh --list --changed --base HEAD)
+  assert_contains "$listed" "tests/fm-pr-merge.test.sh" "git-config helper selects lib.sh dependents"
+  assert_contains "$listed" "tests/fm-secondmate-safety.test.sh" "git-config helper selects secondmate dependents"
+  assert_contains "$listed" "tests/fm-bearings-snapshot.test.sh" "git-config helper selects snapshot dependents"
+  git -C "$repo" add tests/git-config-helpers.sh
+  git -C "$repo" -c user.name=test -c user.email=test@example.invalid commit -qm git-config-helper-change
+
   printf '\n' >>"$repo/tests/fm-backend-herdr-eventwait.test.py"
   listed=$(cd "$repo" && bin/fm-test-run.sh --list --changed --base HEAD)
   assert_contains "$listed" "tests/fm-backend-herdr-smoke.test.sh" "eventwait test selects Herdr coverage"
@@ -484,6 +501,7 @@ PY
   timeout_script=tests/fm-calm-pi-extension.test.sh
   mkdir -p "$timeout_repo/bin" "$timeout_repo/tests"
   cp "$RUNNER" "$timeout_repo/bin/fm-test-run.sh"
+  cp "$ROOT/tests/git-config-helpers.sh" "$timeout_repo/tests/"
   cat >"$timeout_repo/bin/fm-timeout-lib.sh" <<'SH'
 fm_run_timed() {
   [ "$1" -eq 900 ] || return 99
@@ -635,6 +653,7 @@ test_family_proofs_run_in_separate_concurrent_phases() {
   repo="$tmp/repo"
   mkdir -p "$repo/bin" "$repo/tests"
   cp "$RUNNER" "$repo/bin/fm-test-run.sh"
+  cp "$ROOT/tests/git-config-helpers.sh" "$repo/tests/"
   cp "$ROOT/bin/fm-timeout-lib.sh" "$repo/bin/fm-timeout-lib.sh"
   chmod +x "$repo/bin/fm-test-run.sh"
   for script in \
@@ -1180,6 +1199,7 @@ test_unmapped_new_test_never_inherits_family_concurrency() {
   repo="$tmp/repo"
   mkdir -p "$repo/bin" "$repo/tests"
   cp "$RUNNER" "$repo/bin/fm-test-run.sh"
+  cp "$ROOT/tests/git-config-helpers.sh" "$repo/tests/"
   chmod +x "$repo/bin/fm-test-run.sh"
   # Two members of the proven residual family, plus a test basename the family
   # map has never seen - the shape of any test added tomorrow.
@@ -1257,6 +1277,7 @@ test_per_script_timeout_bounds_a_hang() {
   hang=tests/fm-hang-fixture.test.sh
   mkdir -p "$repo/bin" "$repo/tests"
   cp "$RUNNER" "$runner"
+  cp "$ROOT/tests/git-config-helpers.sh" "$repo/tests/"
   cp "$ROOT/bin/fm-timeout-lib.sh" "$repo/bin/fm-timeout-lib.sh"
   grandchild_pid="$tmp/grandchild.pid"
   cat >"$repo/$hang" <<'SH'
@@ -1320,6 +1341,7 @@ test_max_wall_ms_is_a_result_not_advice() {
   fast=tests/fm-budget-fixture.test.sh
   mkdir -p "$repo/bin" "$repo/tests"
   cp "$RUNNER" "$runner"
+  cp "$ROOT/tests/git-config-helpers.sh" "$repo/tests/"
   cat >"$repo/$fast" <<'SH'
 #!/usr/bin/env bash
 sleep 1
@@ -1384,6 +1406,7 @@ test_jobs_parallel_scheduler_and_failure_propagation() {
   d=tests/fm-supervision-instructions.test.sh
   mkdir -p "$repo/bin" "$repo/tests" "$evidence" "$fake_bin"
   cp "$RUNNER" "$runner"
+  cp "$ROOT/tests/git-config-helpers.sh" "$repo/tests/"
   cat >"$fake_bin/stat" <<'SH'
 #!/usr/bin/env bash
 if [ "$1" = "-c" ] && [ "$2" = "%a" ]; then
diff --git a/tests/git-config-helpers.sh b/tests/git-config-helpers.sh
new file mode 100644
index 00000000000..8fde3e7f85e
--- /dev/null
+++ b/tests/git-config-helpers.sh
@@ -0,0 +1,26 @@
+#!/usr/bin/env bash
+# tests/git-config-helpers.sh - fixture Git isolation from the host's global and
+# system configuration.
+#
+# Source this before a fixture's first Git operation:
+#   # shellcheck source=tests/git-config-helpers.sh
+#   . "$(dirname "${BASH_SOURCE[0]}")/git-config-helpers.sh"
+#
+# Fixture Git processes must not inherit host signing, hooks, or other global and
+# system preferences: with commit.gpgsign=true set globally and no secret key for
+# the fixture identities, every fixture commit fails before its assertion.
+# Isolation is limited to those two layers on purpose, so repository-local config,
+# inline `git -c`, GIT_CONFIG_COUNT, and a GIT_CONFIG_GLOBAL the caller supplies
+# after sourcing all stay authoritative - the Git-config suites assert on them.
+# The export reaches only the sourcing shell and its children, so the developer's
+# own config files are never written and real project commits made outside the
+# fixtures keep their configuration and signing.
+#
+# tests/lib.sh and tests/herdr-test-safety.sh source this for every suite that
+# uses them, bin/fm-test-run.sh sources it per suite in run_script_bounded, and a
+# suite reaching none of those sources it directly so a hand-run invocation is
+# isolated too. tests/fm-test-fixtures.test.sh is the regression - it drives the
+# shared helpers, the runner, and the standalone entry points that run without a
+# live vendor - and the changed-file map selects it for a change to this file.
+
+export GIT_CONFIG_GLOBAL=/dev/null GIT_CONFIG_NOSYSTEM=1
diff --git a/tests/herdr-test-safety.sh b/tests/herdr-test-safety.sh
index 59a2bb46cc2..810b4a518fe 100644
--- a/tests/herdr-test-safety.sh
+++ b/tests/herdr-test-safety.sh
@@ -4,6 +4,9 @@
 # fleet-state tripwire contract is bin/fm-herdr-lab.sh.
 set -u
 
+# shellcheck source=tests/git-config-helpers.sh
+. "$(dirname "${BASH_SOURCE[0]}")/git-config-helpers.sh"
+
 # Herdr backend tests drive the real fm-spawn/fm-teardown but do not source
 # tests/lib.sh, so exempt them from the gate-lifecycle refusal here too (see
 # tests/lib.sh and bin/fm-gate-refuse-lib.sh for why firstmate's own suite,
diff --git a/tests/lib.sh b/tests/lib.sh
index 5942e711623..c10f87fe9b1 100644
--- a/tests/lib.sh
+++ b/tests/lib.sh
@@ -33,6 +33,11 @@ FM_TEST_LIB_SOURCED=1
 # suite's fixtures were written against.
 umask 022
 
+# Fixture Git isolation for every suite that reaches this library; the helper's
+# header owns the invariant and the layers it deliberately leaves in force.
+# shellcheck source=tests/git-config-helpers.sh
+. "$(dirname "${BASH_SOURCE[0]}")/git-config-helpers.sh"
+
 # Exempt firstmate's own test suite from the gate-lifecycle refusal
 # (bin/fm-gate-refuse-lib.sh). The no-mistakes gate runs this suite FROM a gate
 # worktree - the exact environment that guard refuses - so without this every