diff --git a/aegis/core/manual_updater.py b/aegis/core/manual_updater.py
index 66398bec..d31087e7 100644
--- a/aegis/core/manual_updater.py
+++ b/aegis/core/manual_updater.py
@@ -986,9 +986,12 @@ def _regenerate_shared_files(
                             verbose_print(f"      • {var_name}={var_value}")
 
             elif policy.get("warn"):
-                # Only warn if file actually has changes that need manual merge
-                existing_content = output_path.read_text()
-                if content != existing_content:
+                if self._shared_file_is_pristine(output_path, template_file):
+                    self._write_rendered(output_path, content)
+                    verbose_print(f"   Updated: {shared_file}")
+                    shared_files_updated.append(shared_file)
+                # Only warn if a diverged file has changes that need manual merge.
+                elif content != output_path.read_text():
                     print(f"   Manual merge required: {shared_file}")
                     shared_files_need_manual_merge.append(shared_file)
 
diff --git a/tests/cli/test_issue_870_warn_only_regeneration.py b/tests/cli/test_issue_870_warn_only_regeneration.py
new file mode 100644
index 00000000..94b10b28
--- /dev/null
+++ b/tests/cli/test_issue_870_warn_only_regeneration.py
@@ -0,0 +1,81 @@
+"""Regression tests for issue #870 — stale warn-only Dockerfiles."""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+import pytest
+
+from aegis.core import manual_updater
+from aegis.core.manual_updater import ManualUpdater
+
+
+@pytest.mark.parametrize(
+    ("before", "after", "expected"),
+    [(False, True, "css-build"), (True, False, "FROM python")],
+)
+def test_htmx_change_regenerates_pristine_warn_only_dockerfile(
+    tmp_path: Path,
+    monkeypatch: pytest.MonkeyPatch,
+    before: bool,
+    after: bool,
+    expected: str,
+) -> None:
+    dockerfile = tmp_path / "Dockerfile"
+    renders = {
+        False: "FROM python:3.13\n",
+        True: "FROM node:22-alpine AS css-build\nFROM python:3.13\n",
+    }
+    dockerfile.write_text(renders[before])
+    updater = ManualUpdater.__new__(ManualUpdater)
+    updater.project_path = tmp_path
+    updater.answers = {"include_htmx": before}
+    monkeypatch.setattr(
+        updater,
+        "_render_template_file",
+        lambda _template, answers: renders[answers["include_htmx"]],
+    )
+    monkeypatch.setattr(
+        manual_updater,
+        "SHARED_TEMPLATE_FILES",
+        {"Dockerfile": {"overwrite": False, "backup": False, "warn": True}},
+    )
+
+    updated, _, need_merge = updater._regenerate_shared_files({"include_htmx": after})
+
+    assert expected in dockerfile.read_text()
+    assert dockerfile.read_text() == renders[after]
+    assert "Dockerfile" in updated
+    assert "Dockerfile" not in need_merge
+
+
+def test_htmx_change_preserves_custom_dockerfile(
+    tmp_path: Path,
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    dockerfile = tmp_path / "Dockerfile"
+    dockerfile.write_text("FROM python:3.13\nRUN custom-build-step\n")
+    before = dockerfile.read_text()
+    updater = ManualUpdater.__new__(ManualUpdater)
+    updater.project_path = tmp_path
+    updater.answers = {"include_htmx": False}
+    monkeypatch.setattr(
+        updater,
+        "_render_template_file",
+        lambda _template, answers: (
+            "FROM node:22-alpine AS css-build\nFROM python:3.13\n"
+            if answers["include_htmx"]
+            else "FROM python:3.13\n"
+        ),
+    )
+    monkeypatch.setattr(
+        manual_updater,
+        "SHARED_TEMPLATE_FILES",
+        {"Dockerfile": {"overwrite": False, "backup": False, "warn": True}},
+    )
+
+    updated, _, need_merge = updater._regenerate_shared_files({"include_htmx": True})
+
+    assert dockerfile.read_text() == before
+    assert "Dockerfile" not in updated
+    assert "Dockerfile" in need_merge