diff --git a/scripts/visual_evidence.py b/scripts/visual_evidence.py
index 7188f19..53a470a 100644
--- a/scripts/visual_evidence.py
+++ b/scripts/visual_evidence.py
@@ -31,7 +31,19 @@ def load_json(path: Path, label: str) -> dict[str, Any]:
     return value
 
 
+def comparison_commit(root: Path, reference: str) -> str:
+    result = subprocess.run(
+        ["git", "rev-parse", "--verify", "--end-of-options", f"{reference}^{{commit}}"],
+        cwd=root, check=False, capture_output=True, text=True,
+    )
+    commit = result.stdout.strip()
+    if result.returncode or re.fullmatch(r"(?:[0-9a-f]{40}|[0-9a-f]{64})", commit) is None:
+        raise VisualEvidenceError("The comparison base must resolve to one Git commit")
+    return commit
+
+
 def changed_paths(root: Path, base_ref: str) -> list[str]:
+    commit = comparison_commit(root, base_ref)
     result = subprocess.run(
         [
             "git",
@@ -40,7 +52,7 @@ def changed_paths(root: Path, base_ref: str) -> list[str]:
             "--no-renames",
             "--diff-filter=ACDMRTUXB",
             "-z",
-            f"{base_ref}...HEAD",
+            f"{commit}...HEAD",
         ],
         cwd=root,
         check=True,
@@ -55,8 +67,9 @@ def file_content(root: Path, path: str, base_ref: str | None = None) -> str:
     if candidate.is_file():
         contents.append(candidate.read_text(encoding="utf-8", errors="replace"))
     if base_ref:
+        commit = comparison_commit(root, base_ref)
         result = subprocess.run(
-            ["git", "show", f"{base_ref}:{path}"],
+            ["git", "show", f"{commit}:{path}"],
             cwd=root,
             check=False,
             capture_output=True,
diff --git a/tests/test_visual_evidence.py b/tests/test_visual_evidence.py
index cd850a5..e7646f6 100644
--- a/tests/test_visual_evidence.py
+++ b/tests/test_visual_evidence.py
@@ -15,6 +15,7 @@
     VisualEvidenceError,
     changed_paths,
     classify_changes,
+    file_content,
     load_json,
     validate_manifest,
     validate_report,
@@ -25,6 +26,56 @@
 SCHEMA_PATH = ROOT / "visual-evidence" / "schema.json"
 
 
+class VisualEvidenceGitBoundaryTest(unittest.TestCase):
+    def setUp(self):
+        temporary = tempfile.TemporaryDirectory()
+        self.addCleanup(temporary.cleanup)
+        self.root = Path(temporary.name) / "repository"
+        self.output = Path(temporary.name) / "outside"
+        self.root.mkdir()
+        self.output.mkdir()
+        self.git("init", "-q")
+        (self.root / "source.css").write_text("old selector\n")
+        self.git("add", "source.css")
+        self.git("commit", "-qm", "baseline")
+        self.base = self.git("rev-parse", "HEAD").stdout.strip()
+        self.git("branch", "comparison-base")
+        (self.root / "source.css").unlink()
+        self.git("add", "source.css")
+        self.git("commit", "-qm", "remove source")
+
+    def git(self, *arguments):
+        return subprocess.run(
+            ["git", "-c", "user.name=Visual Evidence Test", "-c",
+             "user.email=visual-evidence@example.invalid", "-c", "commit.gpgsign=false", *arguments],
+            cwd=self.root, check=True, capture_output=True, text=True,
+        )
+
+    def test_sha_branch_and_relative_commit_keep_diff_and_deleted_content(self):
+        for reference in (self.base, "comparison-base", "HEAD~1"):
+            with self.subTest(reference=reference):
+                self.assertEqual(["source.css"], changed_paths(self.root, reference))
+                self.assertEqual("old selector\n", file_content(self.root, "source.css", reference))
+
+    def test_option_shaped_refs_refuse_without_writing_outside_repository(self):
+        reference = "--output=" + str(self.output / "injected")
+        calls = ((changed_paths, (self.root, reference)),
+                 (file_content, (self.root, "source.css", reference)))
+        for function, arguments in calls:
+            with self.subTest(function=function.__name__):
+                with self.assertRaises(VisualEvidenceError):
+                    function(*arguments)
+        self.assertEqual([], list(self.output.iterdir()))
+
+    def test_missing_and_noncommit_refs_refuse(self):
+        for reference in ("missing-reference", self.base + ":source.css"):
+            for function, arguments in ((changed_paths, (self.root, reference)),
+                                        (file_content, (self.root, "source.css", reference))):
+                with self.subTest(reference=reference, function=function.__name__):
+                    with self.assertRaises(VisualEvidenceError):
+                        function(*arguments)
+
+
 class VisualEvidencePolicyTest(unittest.TestCase):
     def setUp(self) -> None:
         self.policy = load_json(POLICY_PATH, "visual evidence policy")