fix(bump): Preserve existing changelog header when changelog_merge_prerelease is used with cz bump --changelog

commitizen/commands/changelog.py

@@ -227,16 +227,32 @@ def __call__(self) -> None:
             latest_full_release_info = self.changelog_format.get_latest_full_release(
                 self.file_name
             )
-            if latest_full_release_info.index:
-                changelog_meta.unreleased_start = 0
+            # Determine if there are prereleases to merge:
+            # - Only prereleases in changelog (no full release found), OR
+            # - First version in changelog is before first full release (prereleases exist)
+            if latest_full_release_info.index is not None and (
+                latest_full_release_info.name is None
+                or (
+                    changelog_meta.latest_version_position is not None
+                    and changelog_meta.latest_version_position
+                    < latest_full_release_info.index
+                )
+            ):
+                # Use the existing unreleased_start if available (from get_metadata()).
+                # Otherwise, use the position of the first version entry (prerelease)
+                # to preserve the changelog header.
+                if changelog_meta.unreleased_start is None:
+                    changelog_meta.unreleased_start = (
+                        changelog_meta.latest_version_position
+                    )
                 changelog_meta.latest_version_position = latest_full_release_info.index
                 changelog_meta.unreleased_end = latest_full_release_info.index - 1
 
-            start_rev = latest_full_release_info.name or ""
-            if not start_rev and latest_full_release_info.index:
-                # Only pre-releases in changelog
-                changelog_meta.latest_version_position = None
-                changelog_meta.unreleased_end = latest_full_release_info.index + 1
+                start_rev = latest_full_release_info.name or ""
+                if not start_rev:
+                    # Only pre-releases in changelog
+                    changelog_meta.latest_version_position = None
+                    changelog_meta.unreleased_end = latest_full_release_info.index + 1
 
         commits = git.get_commits(start=start_rev, end=end_rev, args="--topo-order")
         if not commits and (

tests/commands/test_bump_command.py

@@ -1494,6 +1494,84 @@ def test_changelog_config_flag_merge_prerelease_only_prerelease_present(
     file_regression.check(out, extension=".md")
 
 
+@pytest.mark.usefixtures("tmp_commitizen_project")
+@pytest.mark.freeze_time("2025-01-01")
+def test_changelog_merge_prerelease_preserves_header(
+    mocker: MockFixture,
+    util: UtilFixture,
+    changelog_path: Path,
+    config_path: Path,
+    file_regression: FileRegressionFixture,
+):
+    """Test that merge_prerelease preserves existing changelog header."""
+    with config_path.open("a") as f:
+        f.write("changelog_merge_prerelease = true\n")
+        f.write("update_changelog_on_bump = true\n")
+        f.write("annotated_tag = true\n")
+
+    # Create initial version with changelog that has a header
+    util.create_file_and_commit("irrelevant commit")
+    mocker.patch("commitizen.git.GitTag.date", "1970-01-01")
+    git.tag("0.1.0")
+
+    # Create a changelog with a header manually
+    changelog_path.write_text(
+        "# Changelog\n\nAll notable changes to this project will be documented here.\n\n## 0.1.0 (1970-01-01)\n"
+    )
+
+    util.create_file_and_commit("feat: add new output")
+    util.create_file_and_commit("fix: output glitch")
+    util.run_cli("bump", "--prerelease", "alpha", "--yes")
+
+    util.run_cli("bump", "--changelog")
+
+    with changelog_path.open() as f:
+        out = f.read()
+
+    file_regression.check(out, extension=".md")
+
+
+@pytest.mark.usefixtures("tmp_commitizen_project")
+@pytest.mark.freeze_time("2025-01-01")
+def test_changelog_merge_prerelease_no_prereleases_to_merge(
+    mocker: MockFixture,
+    util: UtilFixture,
+    changelog_path: Path,
+    config_path: Path,
+    file_regression: FileRegressionFixture,
+):
+    """Test that merge_prerelease works correctly when there are no prereleases.
+
+    When changelog_merge_prerelease is enabled but there are no prereleases to merge,
+    the normal incremental changelog behavior should apply and the existing changelog
+    content should be preserved.
+    """
+    with config_path.open("a") as f:
+        f.write("changelog_merge_prerelease = true\n")
+        f.write("update_changelog_on_bump = true\n")
+        f.write("annotated_tag = true\n")
+
+    # Create initial version with changelog that has a header
+    util.create_file_and_commit("feat: initial feature")
+    mocker.patch("commitizen.git.GitTag.date", "1970-01-01")
+    git.tag("0.1.0")
+
+    # Create a changelog with a header manually
+    changelog_path.write_text(
+        "# Changelog\n\nAll notable changes.\n\n## 0.1.0 (1970-01-01)\n\n### Feat\n\n- initial feature\n"
+    )
+
+    # Add new commits and do a regular bump (no prerelease)
+    util.create_file_and_commit("feat: add new output")
+    util.create_file_and_commit("fix: output glitch")
+    util.run_cli("bump", "--changelog")
+
+    with changelog_path.open() as f:
+        out = f.read()
+
+    file_regression.check(out, extension=".md")
+
+
 @pytest.mark.usefixtures("tmp_commitizen_project")
 def test_bump_deprecate_files_only(util: UtilFixture):
     util.create_file_and_commit("feat: new file")

tests/commands/test_bump_command/test_changelog_merge_prerelease_no_prereleases_to_merge.md

@@ -0,0 +1,19 @@
+# Changelog
+
+All notable changes.
+
+## 0.2.0 (2025-01-01)
+
+### Feat
+
+- add new output
+
+### Fix
+
+- output glitch
+
+## 0.1.0 (1970-01-01)
+
+### Feat
+
+- initial feature

tests/commands/test_bump_command/test_changelog_merge_prerelease_preserves_header.md

@@ -0,0 +1,15 @@
+# Changelog
+
+All notable changes to this project will be documented here.
+
+## 0.2.0 (2025-01-01)
+
+### Feat
+
+- add new output
+
+### Fix
+
+- output glitch
+
+## 0.1.0 (1970-01-01)