diff --git a/Docs/Tasks.md b/Docs/Tasks.md
index af05129..551b745 100644
--- a/Docs/Tasks.md
+++ b/Docs/Tasks.md
@@ -253,7 +253,7 @@ fail2ban ships with many action definitions in `action.d/` (iptables, firewalld,
 
 ---
 
-### Task 3.3 — Frontend: Actions Tab with Active/Inactive Display and Activation
+### Task 3.3 — Frontend: Actions Tab with Active/Inactive Display and Activation ✅ DONE
 
 **Goal:** Enhance the Actions tab in the Configuration page to show all actions with active/inactive status and allow editing and assignment.
 
@@ -300,7 +300,7 @@ fail2ban ships with many action definitions in `action.d/` (iptables, firewalld,
 
 ## Stage 4 — Unified Config File Service and Shared Utilities
 
-### Task 4.1 — Config File Parser Utility
+### Task 4.1 — Config File Parser Utility ✅ DONE
 
 **Goal:** Build a robust, reusable parser for fail2ban INI-style config files that all config-related features share.
 
@@ -324,7 +324,7 @@ fail2ban ships with many action definitions in `action.d/` (iptables, firewalld,
 
 ---
 
-### Task 4.2 — Config File Writer Utility
+### Task 4.2 — Config File Writer Utility ✅ DONE
 
 **Goal:** Build a safe writer utility for creating and updating `.local` override files.
 
diff --git a/backend/app/utils/config_parser.py b/backend/app/utils/config_parser.py
new file mode 100644
index 0000000..4202917
--- /dev/null
+++ b/backend/app/utils/config_parser.py
@@ -0,0 +1,358 @@
+"""Fail2ban INI-style config parser with include and interpolation support.
+
+Provides a :class:`Fail2BanConfigParser` class that wraps Python's
+:class:`configparser.RawConfigParser` with fail2ban-specific behaviour:
+
+- **Merge order**: ``.conf`` file first, then ``.local`` overlay, then ``*.d/``
+  directory overrides — each subsequent layer overwrites earlier values.
+- **Include directives**: ``[INCLUDES]`` sections can specify ``before`` and
+  ``after`` filenames.  ``before`` is loaded at lower priority (loaded first),
+  ``after`` at higher priority (loaded last).  Both are resolved relative to
+  the directory of the including file.  Circular includes and runaway recursion
+  are detected and logged.
+- **Variable interpolation**: :meth:`interpolate` resolves ``%(variable)s``
+  references using the ``[DEFAULT]`` section, the ``[Init]`` section, and any
+  caller-supplied variables.  Multiple passes handle nested references.
+- **Multi-line values**: Handled transparently by ``configparser``; the
+  :meth:`split_multiline` helper further strips blank lines and ``#`` comments.
+- **Comments**: ``configparser`` strips full-line ``#``/``;`` comments; inline
+  comments inside multi-line values are stripped by :meth:`split_multiline`.
+
+All methods are synchronous.  Call from async contexts via
+:func:`asyncio.get_event_loop().run_in_executor`.
+"""
+
+from __future__ import annotations
+
+import configparser
+import re
+from typing import TYPE_CHECKING
+
+import structlog
+
+if TYPE_CHECKING:
+    from pathlib import Path
+
+log: structlog.stdlib.BoundLogger = structlog.get_logger()
+
+# Compiled pattern that matches fail2ban-style %(variable_name)s references.
+_INTERPOLATE_RE: re.Pattern[str] = re.compile(r"%\((\w+)\)s")
+
+# Guard against infinite interpolation loops.
+_MAX_INTERPOLATION_PASSES: int = 10
+
+
+class Fail2BanConfigParser:
+    """Parse fail2ban INI config files with include resolution and interpolation.
+
+    Typical usage for a ``filter.d/`` file::
+
+        parser = Fail2BanConfigParser(config_dir=Path("/etc/fail2ban"))
+        parser.read_with_overrides(Path("/etc/fail2ban/filter.d/sshd.conf"))
+        section = parser.section_dict("Definition")
+        failregex = parser.split_multiline(section.get("failregex", ""))
+
+    Args:
+        config_dir: Optional fail2ban configuration root directory.  Used only
+            by :meth:`ordered_conf_files`; pass ``None`` if not needed.
+        max_include_depth: Maximum ``[INCLUDES]`` nesting depth before giving up.
+    """
+
+    def __init__(
+        self,
+        config_dir: Path | None = None,
+        max_include_depth: int = 10,
+    ) -> None:
+        self._config_dir = config_dir
+        self._max_include_depth = max_include_depth
+        self._parser: configparser.RawConfigParser = self._make_parser()
+        # Tracks resolved absolute paths to detect include cycles.
+        self._read_paths: set[Path] = set()
+
+    # ------------------------------------------------------------------
+    # Internal helpers
+    # ------------------------------------------------------------------
+
+    @staticmethod
+    def _make_parser() -> configparser.RawConfigParser:
+        """Return a case-sensitive :class:`configparser.RawConfigParser`."""
+        parser = configparser.RawConfigParser(interpolation=None, strict=False)
+        # Keep original key casing (fail2ban is case-sensitive in option names).
+        parser.optionxform = str  # type: ignore[assignment]
+        return parser
+
+    def _get_include(
+        self,
+        include_dir: Path,
+        tmp_parser: configparser.RawConfigParser,
+        key: str,
+    ) -> Path | None:
+        """Return the resolved path for an include directive, or ``None``."""
+        if not tmp_parser.has_section("INCLUDES"):
+            return None
+        if not tmp_parser.has_option("INCLUDES", key):
+            return None
+        raw = tmp_parser.get("INCLUDES", key).strip()
+        if not raw:
+            return None
+        return include_dir / raw
+
+    # ------------------------------------------------------------------
+    # Public interface — reading files
+    # ------------------------------------------------------------------
+
+    def read_file(self, path: Path, _depth: int = 0) -> None:
+        """Read *path*, following ``[INCLUDES]`` ``before``/``after`` directives.
+
+        ``before`` references are loaded before the current file (lower
+        priority); ``after`` references are loaded after (higher priority).
+        Circular includes are detected by tracking resolved absolute paths.
+
+        Args:
+            path: Config file to read.
+            _depth: Current include nesting depth.  Internal parameter.
+        """
+        if _depth > self._max_include_depth:
+            log.warning(
+                "include_depth_exceeded",
+                path=str(path),
+                max_depth=self._max_include_depth,
+            )
+            return
+
+        resolved = path.resolve()
+        if resolved in self._read_paths:
+            log.debug("include_cycle_detected", path=str(path))
+            return
+
+        try:
+            content = path.read_text(encoding="utf-8")
+        except OSError as exc:
+            log.warning("config_read_error", path=str(path), error=str(exc))
+            return
+
+        # Pre-scan for includes without yet committing to the main parser.
+        tmp = self._make_parser()
+        try:
+            tmp.read_string(content)
+        except configparser.Error as exc:
+            log.warning("config_parse_error", path=str(path), error=str(exc))
+            return
+
+        include_dir = path.parent
+        before_path = self._get_include(include_dir, tmp, "before")
+        after_path = self._get_include(include_dir, tmp, "after")
+
+        # Load ``before`` first (lower priority than current file).
+        if before_path is not None:
+            self.read_file(before_path, _depth=_depth + 1)
+
+        # Mark this path visited *before* merging to guard against cycles
+        # introduced by the ``after`` include referencing the same file.
+        self._read_paths.add(resolved)
+
+        # Merge current file into the accumulating parser.
+        try:
+            self._parser.read_string(content, source=str(path))
+        except configparser.Error as exc:
+            log.warning(
+                "config_parse_string_error", path=str(path), error=str(exc)
+            )
+
+        # Load ``after`` last (highest priority).
+        if after_path is not None:
+            self.read_file(after_path, _depth=_depth + 1)
+
+    def read_with_overrides(self, conf_path: Path) -> None:
+        """Read *conf_path* and its ``.local`` override if it exists.
+
+        The ``.local`` file is read after the ``.conf`` file so its values
+        take precedence.  Include directives inside each file are still honoured.
+
+        Args:
+            conf_path: Path to the ``.conf`` file.  The corresponding
+                ``.local`` is derived by replacing the suffix with ``.local``.
+        """
+        self.read_file(conf_path)
+        local_path = conf_path.with_suffix(".local")
+        if local_path.is_file():
+            self.read_file(local_path)
+
+    # ------------------------------------------------------------------
+    # Public interface — querying parsed data
+    # ------------------------------------------------------------------
+
+    def sections(self) -> list[str]:
+        """Return all section names (excludes the ``[DEFAULT]`` pseudo-section).
+
+        Returns:
+            Sorted list of section names present in the parsed files.
+        """
+        return list(self._parser.sections())
+
+    def has_section(self, section: str) -> bool:
+        """Return whether *section* exists in the parsed configuration.
+
+        Args:
+            section: Section name to check.
+        """
+        return self._parser.has_section(section)
+
+    def get(self, section: str, key: str) -> str | None:
+        """Return the raw value for *key* in *section*, or ``None``.
+
+        Args:
+            section: Section name.
+            key: Option name.
+
+        Returns:
+            Raw option value string, or ``None`` if not present.
+        """
+        if self._parser.has_section(section) and self._parser.has_option(
+            section, key
+        ):
+            return self._parser.get(section, key)
+        return None
+
+    def section_dict(
+        self,
+        section: str,
+        *,
+        skip: frozenset[str] | None = None,
+    ) -> dict[str, str]:
+        """Return all key-value pairs from *section* as a plain :class:`dict`.
+
+        Keys whose names start with ``__`` (configparser internals from
+        ``DEFAULT`` inheritance) are always excluded.
+
+        Args:
+            section: Section name to read.
+            skip: Additional key names to exclude.
+
+        Returns:
+            Mapping of option name → raw value.  Empty dict if section absent.
+        """
+        if not self._parser.has_section(section):
+            return {}
+        drop: frozenset[str] = skip or frozenset()
+        return {
+            k: v
+            for k, v in self._parser.items(section)
+            if not k.startswith("__") and k not in drop
+        }
+
+    def defaults(self) -> dict[str, str]:
+        """Return all ``[DEFAULT]`` section key-value pairs.
+
+        Returns:
+            Dict of default keys and their values.
+        """
+        return dict(self._parser.defaults())
+
+    # ------------------------------------------------------------------
+    # Public interface — interpolation and helpers
+    # ------------------------------------------------------------------
+
+    def interpolate(
+        self,
+        value: str,
+        extra_vars: dict[str, str] | None = None,
+    ) -> str:
+        """Resolve ``%(variable)s`` references in *value*.
+
+        Variables are resolved in the following priority order (low → high):
+
+        1. ``[DEFAULT]`` section values.
+        2. ``[Init]`` section values (fail2ban action parameters).
+        3. *extra_vars* provided by the caller.
+
+        Multiple passes are performed to handle nested references (up to
+        :data:`_MAX_INTERPOLATION_PASSES` iterations).  Unresolvable references
+        are left unchanged.
+
+        Args:
+            value: Raw string possibly containing ``%(name)s`` placeholders.
+            extra_vars: Optional caller-supplied variables (highest priority).
+
+        Returns:
+            String with ``%(name)s`` references substituted where possible.
+        """
+        vars_: dict[str, str] = {}
+        vars_.update(self.defaults())
+        vars_.update(self.section_dict("Init"))
+        if extra_vars:
+            vars_.update(extra_vars)
+
+        def _sub(m: re.Match[str]) -> str:
+            return vars_.get(m.group(1), m.group(0))
+
+        result = value
+        for _ in range(_MAX_INTERPOLATION_PASSES):
+            new = _INTERPOLATE_RE.sub(_sub, result)
+            if new == result:
+                break
+            result = new
+        return result
+
+    @staticmethod
+    def split_multiline(raw: str) -> list[str]:
+        """Split a multi-line INI value into individual non-blank lines.
+
+        Each line is stripped of surrounding whitespace.  Lines that are empty
+        or that start with ``#`` (comments) are discarded.
+
+        Used for ``failregex``, ``ignoreregex``, ``action``, and ``logpath``
+        values which fail2ban allows to span multiple lines.
+
+        Args:
+            raw: Raw multi-line string from configparser.
+
+        Returns:
+            List of stripped, non-empty, non-comment strings.
+        """
+        result: list[str] = []
+        for line in raw.splitlines():
+            stripped = line.strip()
+            if stripped and not stripped.startswith("#"):
+                result.append(stripped)
+        return result
+
+    # ------------------------------------------------------------------
+    # Class-level utility — file ordering
+    # ------------------------------------------------------------------
+
+    @classmethod
+    def ordered_conf_files(cls, config_dir: Path, base_name: str) -> list[Path]:
+        """Return config files for *base_name* in fail2ban merge order.
+
+        Merge order (ascending priority — later entries override earlier):
+
+        1. ``{config_dir}/{base_name}.conf``
+        2. ``{config_dir}/{base_name}.local``
+        3. ``{config_dir}/{base_name}.d/*.conf`` (sorted alphabetically)
+        4. ``{config_dir}/{base_name}.d/*.local`` (sorted alphabetically)
+
+        Args:
+            config_dir: Fail2ban configuration root directory.
+            base_name: Config base name without extension (e.g. ``"jail"``).
+
+        Returns:
+            List of existing :class:`~pathlib.Path` objects in ascending
+            priority order (only files that actually exist are included).
+        """
+        files: list[Path] = []
+
+        conf = config_dir / f"{base_name}.conf"
+        if conf.is_file():
+            files.append(conf)
+
+        local = config_dir / f"{base_name}.local"
+        if local.is_file():
+            files.append(local)
+
+        d_dir = config_dir / f"{base_name}.d"
+        if d_dir.is_dir():
+            files.extend(sorted(d_dir.glob("*.conf")))
+            files.extend(sorted(d_dir.glob("*.local")))
+
+        return files
diff --git a/backend/app/utils/config_writer.py b/backend/app/utils/config_writer.py
new file mode 100644
index 0000000..112f4e6
--- /dev/null
+++ b/backend/app/utils/config_writer.py
@@ -0,0 +1,303 @@
+"""Atomic config file writer for fail2ban ``.local`` override files.
+
+All write operations are atomic: content is first written to a temporary file
+in the same directory as the target, then :func:`os.replace` is used to rename
+it into place.  This guarantees that a crash or power failure during the write
+never leaves a partially-written file behind.
+
+A per-file :class:`threading.Lock` prevents concurrent writes from the same
+process from racing.
+
+Security constraints
+--------------------
+- Every write function asserts that the target path **ends in ``.local``**.
+  This prevents accidentally writing to ``.conf`` files (which belong to the
+  fail2ban package and should never be modified by BanGUI).
+
+Public functions
+----------------
+- :func:`write_local_override` — create or update keys inside a ``.local`` file.
+- :func:`remove_local_key` — remove a single key from a ``.local`` file.
+- :func:`delete_local_file` — delete an entire ``.local`` file.
+"""
+
+from __future__ import annotations
+
+import configparser
+import contextlib
+import io
+import os
+import tempfile
+import threading
+from typing import TYPE_CHECKING
+
+import structlog
+
+if TYPE_CHECKING:
+    from pathlib import Path
+
+log: structlog.stdlib.BoundLogger = structlog.get_logger()
+
+# ---------------------------------------------------------------------------
+# Per-file lock registry
+# ---------------------------------------------------------------------------
+
+# Maps resolved absolute path strings → threading.Lock instances.
+_locks: dict[str, threading.Lock] = {}
+# Guards the _locks dict itself.
+_registry_lock: threading.Lock = threading.Lock()
+
+
+def _get_file_lock(path: Path) -> threading.Lock:
+    """Return the per-file :class:`threading.Lock` for *path*.
+
+    The lock is created on first access and reused on subsequent calls.
+
+    Args:
+        path: Target file path (need not exist yet).
+
+    Returns:
+        :class:`threading.Lock` bound to the resolved absolute path of *path*.
+    """
+    key = str(path.resolve())
+    with _registry_lock:
+        if key not in _locks:
+            _locks[key] = threading.Lock()
+        return _locks[key]
+
+
+# ---------------------------------------------------------------------------
+# Internal helpers
+# ---------------------------------------------------------------------------
+
+
+def _assert_local_file(path: Path) -> None:
+    """Raise :class:`ValueError` if *path* does not end with ``.local``.
+
+    This is a safety guard against accidentally modifying ``.conf`` files.
+
+    Args:
+        path: Path to validate.
+
+    Raises:
+        ValueError: When *path* does not have a ``.local`` suffix.
+    """
+    if path.suffix != ".local":
+        raise ValueError(
+            f"Refusing to write to non-.local file: {path!r}. "
+            "Only .local override files may be modified by BanGUI."
+        )
+
+
+def _make_parser() -> configparser.RawConfigParser:
+    """Return a case-sensitive :class:`configparser.RawConfigParser`."""
+    parser = configparser.RawConfigParser(interpolation=None, strict=False)
+    parser.optionxform = str  # type: ignore[assignment]
+    return parser
+
+
+def _read_or_new_parser(path: Path) -> configparser.RawConfigParser:
+    """Read *path* into a parser, or return a fresh empty parser.
+
+    If the file does not exist or cannot be read, a fresh parser is returned.
+    Any parse errors are logged as warnings (not re-raised).
+
+    Args:
+        path: Path to the ``.local`` file to read.
+
+    Returns:
+        Populated (or empty) :class:`configparser.RawConfigParser`.
+    """
+    parser = _make_parser()
+    if path.is_file():
+        try:
+            content = path.read_text(encoding="utf-8")
+            parser.read_string(content)
+        except (OSError, configparser.Error) as exc:
+            log.warning("local_file_read_error", path=str(path), error=str(exc))
+    return parser
+
+
+def _write_parser_atomic(
+    parser: configparser.RawConfigParser,
+    path: Path,
+) -> None:
+    """Write *parser* contents to *path* atomically.
+
+    Writes to a temporary file in the same directory as *path*, then renames
+    the temporary file over *path* using :func:`os.replace`.  The temporary
+    file is cleaned up on failure.
+
+    Args:
+        parser: Populated parser whose contents should be written.
+        path: Destination ``.local`` file path.
+
+    Raises:
+        OSError: On filesystem errors (propagated to caller).
+    """
+    buf = io.StringIO()
+    parser.write(buf)
+    content = buf.getvalue()
+
+    path.parent.mkdir(parents=True, exist_ok=True)
+
+    fd, tmp_path_str = tempfile.mkstemp(
+        dir=str(path.parent),
+        prefix=f".{path.name}.tmp",
+        suffix="",
+    )
+    try:
+        with os.fdopen(fd, "w", encoding="utf-8") as f:
+            f.write(content)
+        os.replace(tmp_path_str, str(path))
+    except Exception:
+        with contextlib.suppress(OSError):
+            os.unlink(tmp_path_str)
+        raise
+
+
+# ---------------------------------------------------------------------------
+# Public API
+# ---------------------------------------------------------------------------
+
+
+def write_local_override(
+    base_path: Path,
+    section: str,
+    key_values: dict[str, str],
+) -> None:
+    """Create or update keys in a ``.local`` override file.
+
+    If the file already exists, only the specified *key_values* are written
+    under *section*; all other sections and keys are preserved.
+
+    If the file does not exist, it is created with the given *section* and
+    *key_values*.
+
+    The write is **atomic**: a temporary file is written and renamed into place.
+
+    Args:
+        base_path: Absolute path to the ``.local`` file  (e.g.
+            ``filter.d/sshd.local``).  The parent directory is created if it
+            does not already exist.
+        section: INI section name (e.g. ``"Definition"``, ``"Init"``).
+        key_values: Mapping of option name → value to write/update.
+
+    Raises:
+        ValueError: If *base_path* does not end with ``.local``.
+    """
+    _assert_local_file(base_path)
+
+    lock = _get_file_lock(base_path)
+    with lock:
+        parser = _read_or_new_parser(base_path)
+
+        if not parser.has_section(section):
+            parser.add_section(section)
+
+        for key, value in key_values.items():
+            parser.set(section, key, value)
+
+        log.info(
+            "local_override_written",
+            path=str(base_path),
+            section=section,
+            keys=sorted(key_values),
+        )
+        _write_parser_atomic(parser, base_path)
+
+
+def remove_local_key(base_path: Path, section: str, key: str) -> None:
+    """Remove a single key from a ``.local`` override file.
+
+    Post-removal cleanup:
+
+    - If the section becomes empty after key removal, the section is also
+      removed.
+    - If no sections remain after section removal, the file is deleted.
+
+    This function is a no-op when the file, section, or key does not exist.
+
+    Args:
+        base_path: Path to the ``.local`` file to update.
+        section: INI section containing the key.
+        key: Option name to remove.
+
+    Raises:
+        ValueError: If *base_path* does not end with ``.local``.
+    """
+    _assert_local_file(base_path)
+
+    if not base_path.is_file():
+        return
+
+    lock = _get_file_lock(base_path)
+    with lock:
+        parser = _read_or_new_parser(base_path)
+
+        if not parser.has_section(section) or not parser.has_option(section, key):
+            return  # Nothing to remove.
+
+        parser.remove_option(section, key)
+
+        # Remove the section if it has no remaining options.
+        if not parser.options(section):
+            parser.remove_section(section)
+
+        # Delete the file entirely if it has no remaining sections.
+        if not parser.sections():
+            with contextlib.suppress(OSError):
+                base_path.unlink()
+            log.info("local_file_deleted_empty", path=str(base_path))
+            return
+
+        log.info(
+            "local_key_removed",
+            path=str(base_path),
+            section=section,
+            key=key,
+        )
+        _write_parser_atomic(parser, base_path)
+
+
+def delete_local_file(path: Path, *, allow_orphan: bool = False) -> None:
+    """Delete a ``.local`` override file.
+
+    By default, refuses to delete a ``.local`` file that has no corresponding
+    ``.conf`` file (an *orphan* ``.local``), because it may be the only copy of
+    a user-defined config.  Pass ``allow_orphan=True`` to override this guard.
+
+    Args:
+        path: Path to the ``.local`` file to delete.
+        allow_orphan: When ``True``, delete even if no corresponding ``.conf``
+            exists alongside *path*.
+
+    Raises:
+        ValueError: If *path* does not end with ``.local``.
+        FileNotFoundError: If *path* does not exist.
+        OSError: If no corresponding ``.conf`` exists and *allow_orphan* is
+            ``False``.
+    """
+    _assert_local_file(path)
+
+    if not path.is_file():
+        raise FileNotFoundError(f"Local file not found: {path!r}")
+
+    if not allow_orphan:
+        conf_path = path.with_suffix(".conf")
+        if not conf_path.is_file():
+            raise OSError(
+                f"No corresponding .conf file found for {path!r}. "
+                "Pass allow_orphan=True to delete a local-only file."
+            )
+
+    lock = _get_file_lock(path)
+    with lock:
+        try:
+            path.unlink()
+            log.info("local_file_deleted", path=str(path))
+        except OSError as exc:
+            log.error(
+                "local_file_delete_failed", path=str(path), error=str(exc)
+            )
+            raise
diff --git a/backend/tests/test_utils/__init__.py b/backend/tests/test_utils/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/backend/tests/test_utils/test_config_parser.py b/backend/tests/test_utils/test_config_parser.py
new file mode 100644
index 0000000..3ae2343
--- /dev/null
+++ b/backend/tests/test_utils/test_config_parser.py
@@ -0,0 +1,473 @@
+"""Tests for app.utils.config_parser.Fail2BanConfigParser."""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+from app.utils.config_parser import Fail2BanConfigParser
+
+# ---------------------------------------------------------------------------
+# Fixtures and helpers
+# ---------------------------------------------------------------------------
+
+_FILTER_CONF = """\
+[INCLUDES]
+before = common.conf
+
+[Definition]
+failregex = ^%(host)s .*$
+ignoreregex =
+"""
+
+_COMMON_CONF = """\
+[DEFAULT]
+host = <HOST>
+"""
+
+_FILTER_LOCAL = """\
+[Definition]
+failregex = ^OVERRIDE %(host)s$
+"""
+
+_ACTION_CONF = """\
+[Definition]
+actionstart = iptables -N f2b-<name>
+actionstop  = iptables -X f2b-<name>
+actionban   = iptables -I INPUT -s <ip> -j DROP
+actionunban = iptables -D INPUT -s <ip> -j DROP
+
+[Init]
+name = default
+ip   = 1.2.3.4
+"""
+
+
+def _write(tmp_path: Path, name: str, content: str) -> Path:
+    """Write *content* to *tmp_path/name* and return the path."""
+    p = tmp_path / name
+    p.write_text(content, encoding="utf-8")
+    return p
+
+
+# ---------------------------------------------------------------------------
+# TestOrderedConfFiles
+# ---------------------------------------------------------------------------
+
+
+class TestOrderedConfFiles:
+    def test_empty_dir_returns_empty(self, tmp_path: Path) -> None:
+        result = Fail2BanConfigParser.ordered_conf_files(tmp_path, "jail")
+        assert result == []
+
+    def test_conf_only(self, tmp_path: Path) -> None:
+        conf = _write(tmp_path, "jail.conf", "[DEFAULT]\n")
+        result = Fail2BanConfigParser.ordered_conf_files(tmp_path, "jail")
+        assert result == [conf]
+
+    def test_conf_then_local(self, tmp_path: Path) -> None:
+        conf = _write(tmp_path, "jail.conf", "[DEFAULT]\n")
+        local = _write(tmp_path, "jail.local", "[DEFAULT]\n")
+        result = Fail2BanConfigParser.ordered_conf_files(tmp_path, "jail")
+        assert result == [conf, local]
+
+    def test_d_dir_overrides_appended(self, tmp_path: Path) -> None:
+        conf = _write(tmp_path, "jail.conf", "[DEFAULT]\n")
+        local = _write(tmp_path, "jail.local", "[DEFAULT]\n")
+        d_dir = tmp_path / "jail.d"
+        d_dir.mkdir()
+        d_conf = _write(d_dir, "extra.conf", "[DEFAULT]\n")
+        d_local = _write(d_dir, "extra.local", "[DEFAULT]\n")
+        result = Fail2BanConfigParser.ordered_conf_files(tmp_path, "jail")
+        assert result == [conf, local, d_conf, d_local]
+
+    def test_missing_local_skipped(self, tmp_path: Path) -> None:
+        conf = _write(tmp_path, "jail.conf", "[DEFAULT]\n")
+        result = Fail2BanConfigParser.ordered_conf_files(tmp_path, "jail")
+        assert conf in result
+        assert len(result) == 1
+
+    def test_d_dir_sorted(self, tmp_path: Path) -> None:
+        d_dir = tmp_path / "jail.d"
+        d_dir.mkdir()
+        _write(d_dir, "zzz.conf", "[DEFAULT]\n")
+        _write(d_dir, "aaa.conf", "[DEFAULT]\n")
+        result = Fail2BanConfigParser.ordered_conf_files(tmp_path, "jail")
+        names = [p.name for p in result]
+        assert names == ["aaa.conf", "zzz.conf"]
+
+
+# ---------------------------------------------------------------------------
+# TestReadFile
+# ---------------------------------------------------------------------------
+
+
+class TestReadFile:
+    def test_reads_single_file(self, tmp_path: Path) -> None:
+        _write(tmp_path, "common.conf", _COMMON_CONF)
+        p = _write(tmp_path, "filter.conf", _FILTER_CONF)
+        parser = Fail2BanConfigParser()
+        parser.read_file(p)
+        assert parser.has_section("Definition")
+
+    def test_before_include_loaded(self, tmp_path: Path) -> None:
+        _write(tmp_path, "common.conf", _COMMON_CONF)
+        p = _write(tmp_path, "filter.conf", _FILTER_CONF)
+        parser = Fail2BanConfigParser()
+        parser.read_file(p)
+        # DEFAULT from common.conf should be merged.
+        defaults = parser.defaults()
+        assert "host" in defaults
+
+    def test_missing_file_is_silent(self, tmp_path: Path) -> None:
+        parser = Fail2BanConfigParser()
+        parser.read_file(tmp_path / "nonexistent.conf")
+        assert parser.sections() == []
+
+    def test_after_include_overrides(self, tmp_path: Path) -> None:
+        after_content = """\
+[Definition]
+key = after_value
+"""
+        after = _write(tmp_path, "after.conf", after_content)
+        _ = after  # used via [INCLUDES]
+        main_content = """\
+[INCLUDES]
+after = after.conf
+
+[Definition]
+key = main_value
+"""
+        p = _write(tmp_path, "main.conf", main_content)
+        parser = Fail2BanConfigParser()
+        parser.read_file(p)
+        # 'after' was loaded last → highest priority.
+        assert parser.get("Definition", "key") == "after_value"
+
+    def test_cycle_detection(self, tmp_path: Path) -> None:
+        # A includes B, B includes A.
+        a_content = """\
+[INCLUDES]
+before = b.conf
+
+[Definition]
+key = from_a
+"""
+        b_content = """\
+[INCLUDES]
+before = a.conf
+
+[Definition]
+key = from_b
+"""
+        _write(tmp_path, "a.conf", a_content)
+        _write(tmp_path, "b.conf", b_content)
+        parser = Fail2BanConfigParser()
+        # Should not infinite-loop; terminates via cycle detection.
+        parser.read_file(tmp_path / "a.conf")
+        assert parser.has_section("Definition")
+
+    def test_max_depth_guard(self, tmp_path: Path) -> None:
+        # Create a chain: 0→1→2→…→max+1
+        max_depth = 3
+        for i in range(max_depth + 2):
+            content = f"[INCLUDES]\nbefore = {i + 1}.conf\n\n[s{i}]\nk = v\n"
+            _write(tmp_path, f"{i}.conf", content)
+        parser = Fail2BanConfigParser(max_include_depth=max_depth)
+        parser.read_file(tmp_path / "0.conf")
+        # Should complete without recursion error; some sections will be missing.
+        assert isinstance(parser.sections(), list)
+
+    def test_invalid_ini_is_ignored(self, tmp_path: Path) -> None:
+        bad = _write(tmp_path, "bad.conf", "this is not valid [[[ini\nstuff")
+        parser = Fail2BanConfigParser()
+        # Should not raise; parser logs and continues.
+        parser.read_file(bad)
+
+
+# ---------------------------------------------------------------------------
+# TestReadWithOverrides
+# ---------------------------------------------------------------------------
+
+
+class TestReadWithOverrides:
+    def test_local_overrides_conf(self, tmp_path: Path) -> None:
+        _write(tmp_path, "sshd.conf", "[Definition]\nfailregex = original\n")
+        _write(tmp_path, "sshd.local", "[Definition]\nfailregex = overridden\n")
+        parser = Fail2BanConfigParser()
+        parser.read_with_overrides(tmp_path / "sshd.conf")
+        assert parser.get("Definition", "failregex") == "overridden"
+
+    def test_no_local_just_reads_conf(self, tmp_path: Path) -> None:
+        _write(tmp_path, "sshd.conf", "[Definition]\nfailregex = only_conf\n")
+        parser = Fail2BanConfigParser()
+        parser.read_with_overrides(tmp_path / "sshd.conf")
+        assert parser.get("Definition", "failregex") == "only_conf"
+
+    def test_local_adds_new_key(self, tmp_path: Path) -> None:
+        _write(tmp_path, "sshd.conf", "[Definition]\nfailregex = orig\n")
+        _write(tmp_path, "sshd.local", "[Definition]\nextrakey = newval\n")
+        parser = Fail2BanConfigParser()
+        parser.read_with_overrides(tmp_path / "sshd.conf")
+        assert parser.get("Definition", "extrakey") == "newval"
+
+    def test_conf_keys_preserved_when_local_overrides_other(
+        self, tmp_path: Path
+    ) -> None:
+        _write(
+            tmp_path,
+            "sshd.conf",
+            "[Definition]\nfailregex = orig\nignoreregex = keep_me\n",
+        )
+        _write(tmp_path, "sshd.local", "[Definition]\nfailregex = new\n")
+        parser = Fail2BanConfigParser()
+        parser.read_with_overrides(tmp_path / "sshd.conf")
+        assert parser.get("Definition", "ignoreregex") == "keep_me"
+        assert parser.get("Definition", "failregex") == "new"
+
+
+# ---------------------------------------------------------------------------
+# TestSections
+# ---------------------------------------------------------------------------
+
+
+class TestSections:
+    def test_sections_excludes_default(self, tmp_path: Path) -> None:
+        content = "[DEFAULT]\nfoo = bar\n\n[Definition]\nbaz = qux\n"
+        p = _write(tmp_path, "x.conf", content)
+        parser = Fail2BanConfigParser()
+        parser.read_file(p)
+        secs = parser.sections()
+        assert "DEFAULT" not in secs
+        assert "Definition" in secs
+
+    def test_has_section_true(self, tmp_path: Path) -> None:
+        p = _write(tmp_path, "x.conf", "[Init]\nname = test\n")
+        parser = Fail2BanConfigParser()
+        parser.read_file(p)
+        assert parser.has_section("Init") is True
+
+    def test_has_section_false(self, tmp_path: Path) -> None:
+        p = _write(tmp_path, "x.conf", "[Init]\nname = test\n")
+        parser = Fail2BanConfigParser()
+        parser.read_file(p)
+        assert parser.has_section("Nonexistent") is False
+
+    def test_get_returns_none_for_missing_section(self, tmp_path: Path) -> None:
+        p = _write(tmp_path, "x.conf", "[Init]\nname = test\n")
+        parser = Fail2BanConfigParser()
+        parser.read_file(p)
+        assert parser.get("NoSection", "key") is None
+
+    def test_get_returns_none_for_missing_key(self, tmp_path: Path) -> None:
+        p = _write(tmp_path, "x.conf", "[Init]\nname = test\n")
+        parser = Fail2BanConfigParser()
+        parser.read_file(p)
+        assert parser.get("Init", "nokey") is None
+
+
+# ---------------------------------------------------------------------------
+# TestSectionDict
+# ---------------------------------------------------------------------------
+
+
+class TestSectionDict:
+    def test_returns_all_keys(self, tmp_path: Path) -> None:
+        p = _write(tmp_path, "x.conf", "[Definition]\na = 1\nb = 2\n")
+        parser = Fail2BanConfigParser()
+        parser.read_file(p)
+        d = parser.section_dict("Definition")
+        assert d == {"a": "1", "b": "2"}
+
+    def test_empty_for_missing_section(self, tmp_path: Path) -> None:
+        p = _write(tmp_path, "x.conf", "[Definition]\na = 1\n")
+        parser = Fail2BanConfigParser()
+        parser.read_file(p)
+        assert parser.section_dict("Init") == {}
+
+    def test_skip_excludes_keys(self, tmp_path: Path) -> None:
+        p = _write(tmp_path, "x.conf", "[Definition]\na = 1\nb = 2\nc = 3\n")
+        parser = Fail2BanConfigParser()
+        parser.read_file(p)
+        d = parser.section_dict("Definition", skip=frozenset({"b"}))
+        assert "b" not in d
+        assert d["a"] == "1"
+
+    def test_dunder_keys_excluded(self, tmp_path: Path) -> None:
+        # configparser can inject __name__, __add__ etc. from DEFAULT.
+        content = "[DEFAULT]\n__name__ = foo\n\n[Definition]\nreal = val\n"
+        p = _write(tmp_path, "x.conf", content)
+        parser = Fail2BanConfigParser()
+        parser.read_file(p)
+        d = parser.section_dict("Definition")
+        assert "__name__" not in d
+        assert "real" in d
+
+
+# ---------------------------------------------------------------------------
+# TestDefaults
+# ---------------------------------------------------------------------------
+
+
+class TestDefaults:
+    def test_defaults_from_default_section(self, tmp_path: Path) -> None:
+        content = "[DEFAULT]\nhost = <HOST>\n\n[Definition]\nfailregex = ^\n"
+        p = _write(tmp_path, "x.conf", content)
+        parser = Fail2BanConfigParser()
+        parser.read_file(p)
+        assert parser.defaults().get("host") == "<HOST>"
+
+    def test_defaults_empty_when_no_default_section(self, tmp_path: Path) -> None:
+        p = _write(tmp_path, "x.conf", "[Definition]\nfailregex = ^\n")
+        parser = Fail2BanConfigParser()
+        parser.read_file(p)
+        assert parser.defaults() == {}
+
+
+# ---------------------------------------------------------------------------
+# TestInterpolate
+# ---------------------------------------------------------------------------
+
+
+class TestInterpolate:
+    def _parser_with(self, tmp_path: Path, content: str) -> Fail2BanConfigParser:
+        p = _write(tmp_path, "x.conf", content)
+        parser = Fail2BanConfigParser()
+        parser.read_file(p)
+        return parser
+
+    def test_substitutes_default_var(self, tmp_path: Path) -> None:
+        parser = self._parser_with(
+            tmp_path,
+            "[DEFAULT]\nhost = <HOST>\n\n[Definition]\nrule = match %(host)s\n",
+        )
+        assert parser.interpolate("match %(host)s") == "match <HOST>"
+
+    def test_substitutes_init_var(self, tmp_path: Path) -> None:
+        parser = self._parser_with(
+            tmp_path,
+            "[Init]\nname = sshd\n",
+        )
+        assert parser.interpolate("f2b-%(name)s") == "f2b-sshd"
+
+    def test_extra_vars_highest_priority(self, tmp_path: Path) -> None:
+        parser = self._parser_with(
+            tmp_path,
+            "[DEFAULT]\nname = default_name\n",
+        )
+        result = parser.interpolate("%(name)s", extra_vars={"name": "override"})
+        assert result == "override"
+
+    def test_unresolvable_left_unchanged(self, tmp_path: Path) -> None:
+        parser = Fail2BanConfigParser()
+        result = parser.interpolate("value %(unknown)s end")
+        assert result == "value %(unknown)s end"
+
+    def test_nested_interpolation(self, tmp_path: Path) -> None:
+        # %(outer)s → %(inner)s → final
+        parser = self._parser_with(
+            tmp_path,
+            "[DEFAULT]\ninner = final\nouter = %(inner)s\n",
+        )
+        assert parser.interpolate("%(outer)s") == "final"
+
+    def test_no_references_returned_unchanged(self, tmp_path: Path) -> None:
+        parser = Fail2BanConfigParser()
+        assert parser.interpolate("plain value") == "plain value"
+
+    def test_empty_string(self, tmp_path: Path) -> None:
+        parser = Fail2BanConfigParser()
+        assert parser.interpolate("") == ""
+
+
+# ---------------------------------------------------------------------------
+# TestSplitMultiline
+# ---------------------------------------------------------------------------
+
+
+class TestSplitMultiline:
+    def test_strips_blank_lines(self) -> None:
+        raw = "line1\n\nline2\n\n"
+        assert Fail2BanConfigParser.split_multiline(raw) == ["line1", "line2"]
+
+    def test_strips_comment_lines(self) -> None:
+        raw = "line1\n# comment\nline2"
+        assert Fail2BanConfigParser.split_multiline(raw) == ["line1", "line2"]
+
+    def test_strips_leading_whitespace(self) -> None:
+        raw = "  line1\n  line2"
+        assert Fail2BanConfigParser.split_multiline(raw) == ["line1", "line2"]
+
+    def test_empty_input(self) -> None:
+        assert Fail2BanConfigParser.split_multiline("") == []
+
+    def test_all_comments(self) -> None:
+        raw = "# first\n# second"
+        assert Fail2BanConfigParser.split_multiline(raw) == []
+
+    def test_single_line(self) -> None:
+        assert Fail2BanConfigParser.split_multiline("single") == ["single"]
+
+    def test_preserves_internal_spaces(self) -> None:
+        raw = "iptables -I INPUT -s <ip> -j DROP"
+        assert Fail2BanConfigParser.split_multiline(raw) == [
+            "iptables -I INPUT -s <ip> -j DROP"
+        ]
+
+    def test_multiline_regex_list(self) -> None:
+        raw = (
+            "\n"
+            "  ^%(__prefix_line)s Authentication failure for .* from <HOST>\n"
+            "  # inline comment, skip\n"
+            "  ^%(__prefix_line)s BREAK-IN ATTEMPT by <HOST>\n"
+        )
+        result = Fail2BanConfigParser.split_multiline(raw)
+        assert len(result) == 2
+        assert all("HOST" in r for r in result)
+
+
+# ---------------------------------------------------------------------------
+# TestMultipleFiles (integration-style tests)
+# ---------------------------------------------------------------------------
+
+
+class TestMultipleFilesIntegration:
+    """Tests that combine several files to verify merge order."""
+
+    def test_local_only_override(self, tmp_path: Path) -> None:
+        _write(tmp_path, "test.conf", "[Definition]\nfailregex = base\n")
+        _write(tmp_path, "test.local", "[Definition]\nfailregex = local\n")
+        parser = Fail2BanConfigParser()
+        parser.read_with_overrides(tmp_path / "test.conf")
+        assert parser.get("Definition", "failregex") == "local"
+
+    def test_before_then_conf_then_local(self, tmp_path: Path) -> None:
+        # before.conf → test.conf → test.local (ascending priority)
+        _write(tmp_path, "before.conf", "[Definition]\nsource = before\n")
+        _write(
+            tmp_path,
+            "test.conf",
+            "[INCLUDES]\nbefore = before.conf\n\n[Definition]\nsource = conf\n",
+        )
+        _write(tmp_path, "test.local", "[Definition]\nsource = local\n")
+        parser = Fail2BanConfigParser()
+        parser.read_with_overrides(tmp_path / "test.conf")
+        assert parser.get("Definition", "source") == "local"
+
+    def test_before_key_preserved_if_not_overridden(self, tmp_path: Path) -> None:
+        _write(tmp_path, "common.conf", "[DEFAULT]\nhost = <HOST>\n")
+        _write(
+            tmp_path,
+            "filter.conf",
+            "[INCLUDES]\nbefore = common.conf\n\n[Definition]\nfailregex = ^%(host)s\n",
+        )
+        parser = Fail2BanConfigParser()
+        parser.read_file(tmp_path / "filter.conf")
+        assert parser.defaults().get("host") == "<HOST>"
+        assert parser.get("Definition", "failregex") == "^%(host)s"
+
+    def test_fresh_parser_has_no_state(self) -> None:
+        p1 = Fail2BanConfigParser()
+        p2 = Fail2BanConfigParser()
+        assert p1.sections() == []
+        assert p2.sections() == []
+        assert p1 is not p2
diff --git a/backend/tests/test_utils/test_config_writer.py b/backend/tests/test_utils/test_config_writer.py
new file mode 100644
index 0000000..f749d21
--- /dev/null
+++ b/backend/tests/test_utils/test_config_writer.py
@@ -0,0 +1,290 @@
+"""Tests for app.utils.config_writer."""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+import pytest  # noqa: F401 — used by pytest.raises
+
+from app.utils.config_writer import (
+    _get_file_lock,
+    delete_local_file,
+    remove_local_key,
+    write_local_override,
+)
+
+# ---------------------------------------------------------------------------
+# Helpers
+# ---------------------------------------------------------------------------
+
+
+def _write(tmp_path: Path, name: str, content: str) -> Path:
+    p = tmp_path / name
+    p.write_text(content, encoding="utf-8")
+    return p
+
+
+def _read(path: Path) -> str:
+    return path.read_text(encoding="utf-8")
+
+
+# ---------------------------------------------------------------------------
+# TestGetFileLock
+# ---------------------------------------------------------------------------
+
+
+class TestGetFileLock:
+    def test_same_path_returns_same_lock(self, tmp_path: Path) -> None:
+        path = tmp_path / "test.local"
+        lock_a = _get_file_lock(path)
+        lock_b = _get_file_lock(path)
+        assert lock_a is lock_b
+
+    def test_different_paths_return_different_locks(self, tmp_path: Path) -> None:
+        lock_a = _get_file_lock(tmp_path / "a.local")
+        lock_b = _get_file_lock(tmp_path / "b.local")
+        assert lock_a is not lock_b
+
+
+# ---------------------------------------------------------------------------
+# TestWriteLocalOverride
+# ---------------------------------------------------------------------------
+
+
+class TestWriteLocalOverride:
+    def test_creates_new_file(self, tmp_path: Path) -> None:
+        path = tmp_path / "sshd.local"
+        write_local_override(path, "Definition", {"failregex": "^bad$"})
+        assert path.is_file()
+
+    def test_file_contains_written_key(self, tmp_path: Path) -> None:
+        path = tmp_path / "sshd.local"
+        write_local_override(path, "Definition", {"failregex": "^bad$"})
+        content = _read(path)
+        assert "failregex" in content
+        assert "^bad$" in content
+
+    def test_creates_parent_directory(self, tmp_path: Path) -> None:
+        path = tmp_path / "subdir" / "sshd.local"
+        write_local_override(path, "Definition", {"key": "val"})
+        assert path.is_file()
+
+    def test_updates_existing_key(self, tmp_path: Path) -> None:
+        path = tmp_path / "sshd.local"
+        write_local_override(path, "Definition", {"failregex": "original"})
+        write_local_override(path, "Definition", {"failregex": "updated"})
+        content = _read(path)
+        assert "updated" in content
+        assert "original" not in content
+
+    def test_preserves_other_sections(self, tmp_path: Path) -> None:
+        existing = "[Init]\nname = sshd\n\n[Definition]\nfailregex = orig\n"
+        path = _write(tmp_path, "sshd.local", existing)
+        write_local_override(path, "Definition", {"failregex": "new"})
+        content = _read(path)
+        assert "Init" in content
+        assert "name" in content
+
+    def test_preserves_other_keys_in_section(self, tmp_path: Path) -> None:
+        existing = "[Definition]\nfailregex = orig\nignoreregex = keep\n"
+        path = _write(tmp_path, "sshd.local", existing)
+        write_local_override(path, "Definition", {"failregex": "new"})
+        content = _read(path)
+        assert "ignoreregex" in content
+        assert "keep" in content
+
+    def test_adds_new_section(self, tmp_path: Path) -> None:
+        existing = "[Definition]\nfailregex = orig\n"
+        path = _write(tmp_path, "sshd.local", existing)
+        write_local_override(path, "Init", {"name": "sshd"})
+        content = _read(path)
+        assert "[Init]" in content
+        assert "name" in content
+
+    def test_writes_multiple_keys(self, tmp_path: Path) -> None:
+        path = tmp_path / "sshd.local"
+        write_local_override(path, "Definition", {"a": "1", "b": "2", "c": "3"})
+        content = _read(path)
+        assert "a" in content
+        assert "b" in content
+        assert "c" in content
+
+    def test_raises_for_conf_path(self, tmp_path: Path) -> None:
+        bad = tmp_path / "sshd.conf"
+        with pytest.raises(ValueError, match=r"\.local"):
+            write_local_override(bad, "Definition", {"key": "val"})
+
+    def test_raises_for_non_local_extension(self, tmp_path: Path) -> None:
+        bad = tmp_path / "sshd.ini"
+        with pytest.raises(ValueError):
+            write_local_override(bad, "Definition", {"key": "val"})
+
+
+# ---------------------------------------------------------------------------
+# TestRemoveLocalKey
+# ---------------------------------------------------------------------------
+
+
+class TestRemoveLocalKey:
+    def test_removes_existing_key(self, tmp_path: Path) -> None:
+        path = _write(
+            tmp_path,
+            "sshd.local",
+            "[Definition]\nfailregex = bad\nignoreregex = keep\n",
+        )
+        remove_local_key(path, "Definition", "failregex")
+        content = _read(path)
+        assert "failregex" not in content
+        assert "ignoreregex" in content
+
+    def test_noop_for_missing_key(self, tmp_path: Path) -> None:
+        path = _write(tmp_path, "sshd.local", "[Definition]\nother = val\n")
+        # Should not raise.
+        remove_local_key(path, "Definition", "nonexistent")
+        assert path.is_file()
+
+    def test_noop_for_missing_section(self, tmp_path: Path) -> None:
+        path = _write(tmp_path, "sshd.local", "[Definition]\nother = val\n")
+        remove_local_key(path, "Init", "name")
+        assert path.is_file()
+
+    def test_noop_for_missing_file(self, tmp_path: Path) -> None:
+        path = tmp_path / "missing.local"
+        # Should not raise even if file doesn't exist.
+        remove_local_key(path, "Definition", "key")
+
+    def test_removes_empty_section(self, tmp_path: Path) -> None:
+        # [Definition] will become empty and be removed; [Init] keeps the file.
+        path = _write(
+            tmp_path,
+            "sshd.local",
+            "[Definition]\nonly_key = val\n\n[Init]\nname = sshd\n",
+        )
+        remove_local_key(path, "Definition", "only_key")
+        content = _read(path)
+        assert "[Definition]" not in content
+        assert "[Init]" in content
+
+    def test_deletes_file_when_no_sections_remain(self, tmp_path: Path) -> None:
+        path = _write(tmp_path, "sshd.local", "[Definition]\nonly_key = val\n")
+        remove_local_key(path, "Definition", "only_key")
+        assert not path.exists()
+
+    def test_preserves_other_sections_after_removal(self, tmp_path: Path) -> None:
+        path = _write(
+            tmp_path,
+            "sshd.local",
+            "[Definition]\nkey = val\n\n[Init]\nname = sshd\n",
+        )
+        remove_local_key(path, "Definition", "key")
+        content = _read(path)
+        assert "[Init]" in content
+        assert "name" in content
+
+    def test_raises_for_conf_path(self, tmp_path: Path) -> None:
+        bad = tmp_path / "sshd.conf"
+        with pytest.raises(ValueError, match=r"\.local"):
+            remove_local_key(bad, "Definition", "key")
+
+
+# ---------------------------------------------------------------------------
+# TestDeleteLocalFile
+# ---------------------------------------------------------------------------
+
+
+class TestDeleteLocalFile:
+    def test_deletes_existing_local_with_conf(self, tmp_path: Path) -> None:
+        _write(tmp_path, "sshd.conf", "[Definition]\n")
+        path = _write(tmp_path, "sshd.local", "[Definition]\nkey = val\n")
+        delete_local_file(path)
+        assert not path.exists()
+
+    def test_raises_file_not_found(self, tmp_path: Path) -> None:
+        _write(tmp_path, "sshd.conf", "[Definition]\n")
+        missing = tmp_path / "sshd.local"
+        with pytest.raises(FileNotFoundError):
+            delete_local_file(missing)
+
+    def test_raises_oserror_for_orphan_without_flag(self, tmp_path: Path) -> None:
+        path = _write(tmp_path, "orphan.local", "[Definition]\nkey = val\n")
+        with pytest.raises(OSError, match="No corresponding .conf"):
+            delete_local_file(path)
+
+    def test_allow_orphan_deletes_local_only_file(self, tmp_path: Path) -> None:
+        path = _write(tmp_path, "orphan.local", "[Definition]\nkey = val\n")
+        delete_local_file(path, allow_orphan=True)
+        assert not path.exists()
+
+    def test_raises_for_conf_path(self, tmp_path: Path) -> None:
+        bad = _write(tmp_path, "sshd.conf", "[Definition]\n")
+        with pytest.raises(ValueError, match=r"\.local"):
+            delete_local_file(bad)
+
+    def test_raises_for_non_local_extension(self, tmp_path: Path) -> None:
+        bad = tmp_path / "sshd.ini"
+        bad.write_text("x", encoding="utf-8")
+        with pytest.raises(ValueError):
+            delete_local_file(bad)
+
+
+# ---------------------------------------------------------------------------
+# TestAtomicWrite (integration)
+# ---------------------------------------------------------------------------
+
+
+class TestAtomicWrite:
+    def test_no_temp_files_left_after_write(self, tmp_path: Path) -> None:
+        path = tmp_path / "sshd.local"
+        write_local_override(path, "Definition", {"key": "val"})
+        files = list(tmp_path.iterdir())
+        # Only the target file should exist.
+        assert len(files) == 1
+        assert files[0].name == "sshd.local"
+
+    def test_write_is_idempotent(self, tmp_path: Path) -> None:
+        path = tmp_path / "sshd.local"
+        for _ in range(5):
+            write_local_override(path, "Definition", {"key": "val"})
+        content = _read(path)
+        # 'key' should appear exactly once.
+        assert content.count("key") == 1
+
+
+# ---------------------------------------------------------------------------
+# TestEdgeCases
+# ---------------------------------------------------------------------------
+
+
+class TestEdgeCases:
+    def test_write_empty_key_values_creates_empty_section(
+        self, tmp_path: Path
+    ) -> None:
+        path = tmp_path / "sshd.local"
+        write_local_override(path, "Definition", {})
+        content = _read(path)
+        assert "[Definition]" in content
+
+    def test_remove_key_with_unicode_value(self, tmp_path: Path) -> None:
+        path = _write(
+            tmp_path,
+            "sshd.local",
+            "[Definition]\nkey = 日本語\nother = keep\n",
+        )
+        remove_local_key(path, "Definition", "key")
+        content = _read(path)
+        assert "日本語" not in content
+        assert "other" in content
+
+    def test_write_value_with_newlines(self, tmp_path: Path) -> None:
+        path = tmp_path / "sshd.local"
+        # configparser stores multi-line values with continuation indent.
+        multiline = "line1\n  line2\n  line3"
+        write_local_override(path, "Definition", {"failregex": multiline})
+        assert path.is_file()
+
+    def test_remove_last_key_of_last_section_deletes_file(
+        self, tmp_path: Path
+    ) -> None:
+        path = _write(tmp_path, "sshd.local", "[Definition]\nlast_key = val\n")
+        remove_local_key(path, "Definition", "last_key")
+        assert not path.exists()