diff --git a/tests/fixture_git.py b/tests/fixture_git.py
new file mode 100644
index 0000000000..e0f6bec0b3
--- /dev/null
+++ b/tests/fixture_git.py
@@ -0,0 +1,24 @@
+"""The one git identity every test repository commits under, given to each command.
+
+No test writes identity or signing into a repository's config. When GIT_DIR leaks in
+(``git bisect run`` from a linked worktree, a hook), ``git config`` writes the caller's
+repository instead of the fixture's: on 10-23 two fixtures stamped "Citation fixture" and
+"Ratchet test" into the shared agent-comms config, and 106 real commits carried the second.
+A setting given per command persists nothing, whichever repository git resolves."""
+
+SETTINGS = (
+    ("user.name", "Test fixture"),
+    ("user.email", "fixture@example.invalid"),
+    ("commit.gpgsign", "false"),
+)
+
+# Global options for one git command: ("git", *IDENTITY, "-C", path, ...).
+IDENTITY = tuple(part for key, value in SETTINGS for part in ("-c", f"{key}={value}"))
+
+# The same settings for a git that the code under test starts, through git's own
+# GIT_CONFIG_COUNT channel; set with monkeypatch so they end with the test.
+ENVIRONMENT = {
+    "GIT_CONFIG_COUNT": str(len(SETTINGS)),
+    **{f"GIT_CONFIG_KEY_{index}": key for index, (key, _) in enumerate(SETTINGS)},
+    **{f"GIT_CONFIG_VALUE_{index}": value for index, (_, value) in enumerate(SETTINGS)},
+}
diff --git a/tests/guards/test_debt_ratchet.py b/tests/guards/test_debt_ratchet.py
index 13a9c89234..c07467a696 100644
--- a/tests/guards/test_debt_ratchet.py
+++ b/tests/guards/test_debt_ratchet.py
@@ -15,6 +15,7 @@
 
 from agent_comms.child_process import ObservedProcess, SpawnedProcess
 from agent_comms.field_codec import FieldCodec
+from fixture_git import IDENTITY
 from process_tree_fixture import TREE, identities
 
 from tools.debt_ratchet import (
@@ -130,12 +131,10 @@ def __init__(self, path: Path, root: str, ratchet: RatchetServer):
         self.root = root
         self.ratchet = ratchet
         self.git("init", "-q")
-        self.git("config", "user.email", "ratchet@example.invalid")
-        self.git("config", "user.name", "Ratchet test")
 
     def git(self, *args: str) -> str:
         return subprocess.check_output(
-            ["git", "-C", str(self.path), *args], text=True, timeout=10
+            ["git", *IDENTITY, "-C", str(self.path), *args], text=True, timeout=10
         ).strip()
 
     def commit(self, files: dict[str, str | None]) -> str:
@@ -147,7 +146,7 @@ def commit(self, files: dict[str, str | None]) -> str:
                 path.parent.mkdir(parents=True, exist_ok=True)
                 path.write_text(source)
         self.git("add", "--all")
-        self.git("-c", "commit.gpgsign=false", "commit", "-qm", "test", "--allow-empty")
+        self.git("commit", "-qm", "test", "--allow-empty")
         return self.git("rev-parse", "HEAD")
 
     def arguments(self, base: str, head: str) -> tuple[str, ...]:
diff --git a/tests/source_revision_fixture.py b/tests/source_revision_fixture.py
index 457274def5..9ca9dab132 100644
--- a/tests/source_revision_fixture.py
+++ b/tests/source_revision_fixture.py
@@ -9,6 +9,7 @@
 from agent_comms.repository_name import RepositoryName
 from agent_comms.change_report import FileLine
 from agent_comms.todos import FixScope, FixShape, PullRequestLink
+from fixture_git import IDENTITY
 
 
 class SourceRepository(Repo):
@@ -36,10 +37,16 @@ def scope(cls) -> FixScope:
         parent = next(node for node in ast.parse(cls.main_source).body if isinstance(node, ast.ClassDef))
         return FixScope(FixShape(parents=(FileLine('owner.py', parent.lineno),), members=()))
 
+    @staticmethod
+    def _run(checkout: Path | str, *args: str, timeout: float = 60,
+             input: bytes = b"") -> subprocess.CompletedProcess[bytes]:
+        """Repo's one git subprocess, every command carrying the fixture identity."""
+        return Repo._run(checkout, *IDENTITY, *args, timeout=timeout, input=input)
+
     @staticmethod
     def git_input(repository: Path, text: str, *arguments: str) -> str:
         return subprocess.run(
-            ('git', '-C', str(repository), *arguments), input=text, text=True,
+            ('git', *IDENTITY, '-C', str(repository), *arguments), input=text, text=True,
             capture_output=True, check=True, timeout=30,
         ).stdout.strip()
 
@@ -58,9 +65,6 @@ def create(cls, root: Path, name: RepositoryName = RepositoryName.from_wire('exa
         repository.mkdir(parents=True)
         remote = cls.remote_for(root, name)
         cls.git(repository, 'init', '--initial-branch=main')
-        cls.git(repository, 'config', 'user.name', 'Citation fixture')
-        cls.git(repository, 'config', 'user.email', 'citation@example.invalid')
-        cls.git(repository, 'config', 'commit.gpgsign', 'false')
         cls.git(repository, 'remote', 'add', 'origin', str(remote))
         main_blob = cls.git_input(repository, cls.main_source,
                                   'hash-object', '-w', '--stdin')
diff --git a/tests/test_carry.py b/tests/test_carry.py
index 8c7ac13b9b..441ac3f15b 100644
--- a/tests/test_carry.py
+++ b/tests/test_carry.py
@@ -48,6 +48,7 @@
 from cutover.carry_commander_project import CommanderProjectCarry
 from cutover.carry_drop_connectivity import DropConnectivity
 from cutover.carry_parked_threads import ParkedThreads
+from fixture_git import IDENTITY
 
 REPO = Path(__file__).resolve().parents[1]
 
@@ -65,11 +66,9 @@ class Repository:
     def __init__(self, path: Path):
         self.path = path
         self.git("init", "-q")
-        self.git("config", "user.email", "carry@example.invalid")
-        self.git("config", "user.name", "Carry test")
 
     def git(self, *args: str) -> str:
-        return subprocess.check_output(["git", "-C", str(self.path), *args], text=True, timeout=10).strip()
+        return subprocess.check_output(["git", *IDENTITY, "-C", str(self.path), *args], text=True, timeout=10).strip()
 
     def commit(self, steps: dict[str, str | None]) -> str:
         """One commit per step, in order, as the main line adds steps one at a time."""
diff --git a/tests/test_checkout_submodules.py b/tests/test_checkout_submodules.py
index 9db3097a4c..18cd68b0c3 100644
--- a/tests/test_checkout_submodules.py
+++ b/tests/test_checkout_submodules.py
@@ -8,11 +8,12 @@
 import pytest
 
 from agent_comms.repo import Checkout, CheckoutHoldsWorkError, ScratchCheckout
+from fixture_git import IDENTITY
 
 
 def git(path: Path, *args: str) -> str:
     return subprocess.run(
-        ("git", "-C", str(path), "-c", "user.name=t", "-c", "user.email=t@t", *args),
+        ("git", *IDENTITY, "-C", str(path), *args),
         check=True,
         capture_output=True,
         text=True,
diff --git a/tests/test_checkpoint_command.py b/tests/test_checkpoint_command.py
index c29797a385..beaf61d425 100644
--- a/tests/test_checkpoint_command.py
+++ b/tests/test_checkpoint_command.py
@@ -9,6 +9,15 @@
     CheckpointCliCommand, CleanCheckpoint, PushUnknownCheckpoint, PushedCheckpoint, RefusedCheckpoint,
 )
 from bus_record_fixture import context_comms
+from fixture_git import ENVIRONMENT
+
+
+@pytest.fixture(autouse=True)
+def _fixture_identity(monkeypatch):
+    """The checkpoint command commits through its own git, so the identity rides the
+    environment that git inherits, never a repository's config."""
+    for name, value in ENVIRONMENT.items():
+        monkeypatch.setenv(name, value)
 
 
 def _git(worktree: Path, *arguments: str):
@@ -23,8 +32,6 @@ def _repo(tmp_path: Path, name: str, *, remote: bool = True) -> Path:
     worktree = tmp_path / name
     worktree.mkdir()
     _git(worktree, "init", "-q", "-b", "main")
-    _git(worktree, "config", "user.name", "agent")
-    _git(worktree, "config", "user.email", "agent@example.com")
     if remote:
         _git(tmp_path, "init", "-q", "--bare", f"{name}.origin.git")
         _git(worktree, "remote", "add", "origin", str(tmp_path / f"{name}.origin.git"))
diff --git a/tests/test_installed_merge.py b/tests/test_installed_merge.py
index 29d38f7e62..629c4b382e 100644
--- a/tests/test_installed_merge.py
+++ b/tests/test_installed_merge.py
@@ -16,9 +16,9 @@ def test_a_merge_made_after_the_last_fetch_is_fetched_then_answered(tmp_path):
     origin, installed = tmp_path / 'origin', tmp_path / 'installed'
     origin.mkdir()
     git(origin, 'init', '-q', '-b', 'main')
-    git(origin, '-c', 'user.name=t', '-c', 'user.email=t@t', 'commit', '-q', '--allow-empty', '-m', 'base')
+    git(origin, 'commit', '-q', '--allow-empty', '-m', 'base')
     git(tmp_path, 'clone', '-q', str(origin), str(installed))
-    git(origin, '-c', 'user.name=t', '-c', 'user.email=t@t', 'commit', '-q', '--allow-empty', '-m', 'merged later')
+    git(origin, 'commit', '-q', '--allow-empty', '-m', 'merged later')
     sha = git(origin, 'rev-parse', 'HEAD')
     repo = Repo(checkout=installed, github=RepositoryName.from_wire('test/repo'))
 
diff --git a/tests/test_order_progress.py b/tests/test_order_progress.py
index f1b04ee4ca..285c020d3f 100644
--- a/tests/test_order_progress.py
+++ b/tests/test_order_progress.py
@@ -12,6 +12,7 @@
 import pytest
 from test_todos import order
 from source_revision_fixture import SourceRepository
+from fixture_git import IDENTITY
 
 from agent_comms.comms import wire
 from agent_comms.goal_actions import CreateProblemGoalAction
@@ -28,7 +29,7 @@
 
 
 def git(checkout, *args):
-    return subprocess.run(['git', '-C', str(checkout), *args], check=True, capture_output=True, text=True).stdout.strip()
+    return subprocess.run(['git', *IDENTITY, '-C', str(checkout), *args], check=True, capture_output=True, text=True).stdout.strip()
 
 
 def test_prior_declarations_do_not_enter_native_request_schemas():
@@ -58,7 +59,7 @@ def unit_checkout(path):
     git(path, 'init', '-q')
     (path / 'owner.py').write_text('one\n')
     git(path, 'add', 'owner.py')
-    git(path, '-c', 'user.name=t', '-c', 'user.email=t@t', 'commit', '-q', '-m', 'base')
+    git(path, 'commit', '-q', '-m', 'base')
     return path
 
 
@@ -103,7 +104,7 @@ def test_a_submodule_with_a_missing_git_directory_does_not_stop_reading_edits(tm
     package = unit_checkout(tmp_path / 'package')
     worktree = unit_checkout(tmp_path / 'worker--P1')
     git(worktree, '-c', 'protocol.file.allow=always', 'submodule', 'add', '-q', str(package), 'pinned')
-    git(worktree, '-c', 'user.name=t', '-c', 'user.email=t@t', 'commit', '-q', '-m', 'pin')
+    git(worktree, 'commit', '-q', '-m', 'pin')
     (worktree / 'pinned' / '.git').write_text('gitdir: ../.git/modules/removed\n')
     assert WorkBranches(worktree).uncommitted() == ()
     edited(worktree, 'owner.py', 700.0)
diff --git a/tests/test_phase_evidence.py b/tests/test_phase_evidence.py
index e8029a1d0d..dec0195224 100644
--- a/tests/test_phase_evidence.py
+++ b/tests/test_phase_evidence.py
@@ -29,10 +29,11 @@
 from agent_comms.plan_phases import BusDeletionPhase, FLAG, ExternalBoundariesPhase, PlanPhase, RostersPhase
 from agent_comms.repo import Repo
 from agent_comms.repository_name import RepositoryName
+from fixture_git import IDENTITY
 
 
 def git(checkout, *args):
-    return subprocess.run(("git", "-C", str(checkout), *args), check=True,
+    return subprocess.run(("git", *IDENTITY, "-C", str(checkout), *args), check=True,
                           capture_output=True, text=True, timeout=30).stdout.strip()
 
 
@@ -41,7 +42,7 @@ def test_evidence_reads_origin_main_and_never_the_worktree(tmp_path):
     (tmp_path / "src").mkdir()
     (tmp_path / "src/kept.py").write_text("flag = True\n")
     git(tmp_path, "add", ".")
-    git(tmp_path, "-c", "user.name=t", "-c", "user.email=t@t", "commit", "-qm", "main")
+    git(tmp_path, "commit", "-qm", "main")
     sha = git(tmp_path, "rev-parse", "HEAD")
     git(tmp_path, "update-ref", "refs/remotes/origin/main", sha)
     (tmp_path / "src/kept.py").unlink()
@@ -66,7 +67,7 @@ def test_boundary_evidence_ignores_its_declaration_and_exception_groups(tmp_path
         "try:\n    pass\nexcept ExceptionGroup:\n    raise\n"
     )
     git(tmp_path, "add", ".")
-    git(tmp_path, "-c", "user.name=t", "-c", "user.email=t@t", "commit", "-qm", "boundary")
+    git(tmp_path, "commit", "-qm", "boundary")
     sha = git(tmp_path, "rev-parse", "HEAD")
     git(tmp_path, "update-ref", "refs/remotes/origin/main", sha)
     reading = PlanReading((), MainTree.of(
@@ -75,7 +76,7 @@ def test_boundary_evidence_ignores_its_declaration_and_exception_groups(tmp_path
 
     source.write_text("try:\n    pass\nexcept Exception:\n    raise\n")
     git(tmp_path, "add", ".")
-    git(tmp_path, "-c", "user.name=t", "-c", "user.email=t@t", "commit", "-qm", "broad")
+    git(tmp_path, "commit", "-qm", "broad")
     git(tmp_path, "update-ref", "refs/remotes/origin/main", git(tmp_path, "rev-parse", "HEAD"))
     changed = PlanReading((), MainTree.of(reading.main.repo))
     assert not changed.holds(ExternalBoundariesPhase.evidence)
@@ -93,7 +94,7 @@ def main_of(tmp_path, files):
         (tmp_path / path).parent.mkdir(parents=True, exist_ok=True)
         (tmp_path / path).write_text(text)
     git(tmp_path, "add", ".")
-    git(tmp_path, "-c", "user.name=t", "-c", "user.email=t@t", "commit", "-qm", "main")
+    git(tmp_path, "commit", "-qm", "main")
     git(tmp_path, "update-ref", "refs/remotes/origin/main", git(tmp_path, "rev-parse", "HEAD"))
     return PlanReading((), MainTree.of(
         Repo(checkout=tmp_path, github=RepositoryName.from_wire("OpenHCSDev/agent-comms"))))
@@ -123,7 +124,7 @@ def test_roster_evidence_keeps_external_abi_and_each_remaining_internal_registry
     ):
         (tmp_path / "src/remaining.py").write_text(remaining)
         git(tmp_path, "add", ".")
-        git(tmp_path, "-c", "user.name=t", "-c", "user.email=t@t", "commit", "-qm", "registry")
+        git(tmp_path, "commit", "-qm", "registry")
         git(tmp_path, "update-ref", "refs/remotes/origin/main", git(tmp_path, "rev-parse", "HEAD"))
         changed = PlanReading((), MainTree.of(reading.main.repo))
         assert not changed.holds(RostersPhase.evidence), remaining
@@ -138,7 +139,7 @@ def test_main_tree_grep_uses_the_same_repository_tree_as_its_module_reader(tmp_p
     reading = main_of(checkout, {"src/owner.py": "class OtherOwner:\n    members = {}\n"})
     git(checkout, "-c", "protocol.file.allow=always", "submodule", "add", "-q",
         str(dependency), "src/dependency")
-    git(checkout, "-c", "user.name=t", "-c", "user.email=t@t", "commit", "-qam", "dependency")
+    git(checkout, "commit", "-qam", "dependency")
     git(checkout, "update-ref", "refs/remotes/origin/main", git(checkout, "rev-parse", "HEAD"))
     git(checkout, "config", "submodule.recurse", "true")
     assert "src/dependency" in git(checkout, "grep", "--recurse-submodules", "-n",
@@ -172,7 +173,7 @@ def test_bus_deletion_evidence_reads_uses_without_matching_its_own_declaration(t
     ):
         (tmp_path / "src/uses.py").write_text(source)
         git(tmp_path, "add", ".")
-        git(tmp_path, "-c", "user.name=t", "-c", "user.email=t@t", "commit", "-qm", "use")
+        git(tmp_path, "commit", "-qm", "use")
         git(tmp_path, "update-ref", "refs/remotes/origin/main", git(tmp_path, "rev-parse", "HEAD"))
         changed = PlanReading((), MainTree.of(reading.main.repo))
         assert not changed.holds(BusDeletionPhase.evidence), source
diff --git a/tests/test_proof_build.py b/tests/test_proof_build.py
index 3a1cf3cadc..b0ed3fcf61 100644
--- a/tests/test_proof_build.py
+++ b/tests/test_proof_build.py
@@ -48,6 +48,7 @@
 )
 from agent_comms.tool_caller import ThreadCaller
 from agent_comms.tools import CommsOrderDoneTool, CommsProblemCloseTool, CommsTodoCreateTool
+from fixture_git import IDENTITY
 from order_scope_fixture import FIX_SCOPE
 from source_revision_fixture import SourceRepository
 
@@ -78,12 +79,7 @@ def proof_build_checkouts(
             subprocess.run(
                 (
                     "git",
-                    "-c",
-                    "user.name=Proof Build",
-                    "-c",
-                    "user.email=proof@example.invalid",
-                    "-c",
-                    "commit.gpgsign=false",
+                    *IDENTITY,
                     "commit",
                     "--allow-empty",
                     "-m",