diff --git a/.gitignore b/.gitignore
index 1b9d52962e..1d6cefba48 100644
--- a/.gitignore
+++ b/.gitignore
@@ -111,5 +111,6 @@ doc/_sidebar.rst.inc
 *.ESMF_LogFile
 
 
-#Ignore vscode AI rules
+# AI
 .github/instructions/codacy.instructions.md
+CLAUDE.md
diff --git a/esmvalcore/_recipe/from_datasets.py b/esmvalcore/_recipe/from_datasets.py
index 996c81b37a..04cd31cb17 100644
--- a/esmvalcore/_recipe/from_datasets.py
+++ b/esmvalcore/_recipe/from_datasets.py
@@ -67,9 +67,7 @@ def _datasets_to_recipe(datasets: Iterable[Dataset]) -> Recipe:
                 f"'diagnostic' facet missing from {dataset},"
                 "unable to convert to recipe."
             )
-            raise RecipeError(
-                msg,
-            )
+            raise RecipeError(msg)
 
     recipe = _datasets_to_raw_recipe(datasets)
     diagnostics = recipe["diagnostics"].values()
diff --git a/esmvalcore/_task.py b/esmvalcore/_task.py
index 11178df503..69316038c0 100644
--- a/esmvalcore/_task.py
+++ b/esmvalcore/_task.py
@@ -177,16 +177,12 @@ def _py2ncl(value, var_name=""):
                 type_ = type(value[0])
             if any(not isinstance(v, type_) for v in value):
                 msg = f"NCL array cannot be mixed type: {value}"
-                raise ValueError(
-                    msg,
-                )
+                raise ValueError(msg)
             txt += "(/{}/)".format(", ".join(_py2ncl(v) for v in value))
     elif isinstance(value, dict):
         if not var_name:
             msg = f"NCL does not support nested dicts: {value}"
-            raise ValueError(
-                msg,
-            )
+            raise ValueError(msg)
         txt += "True\n"
         for key in value:
             txt += f"{var_name}@{key} = {_py2ncl(value[key])}\n"
@@ -272,9 +268,7 @@ def initialize_provenance(self, recipe_entity):
         """Initialize task provenance activity."""
         if self.activity is not None:
             msg = f"Provenance of {self} already initialized"
-            raise ValueError(
-                msg,
-            )
+            raise ValueError(msg)
         self.activity = get_task_provenance(self, recipe_entity)
 
     def flatten(self):
@@ -445,18 +439,14 @@ def _initialize_cmd(self):
                 msg = (
                     f"{err_msg}: program '{interpreters[ext]}' not installed."
                 )
-                raise DiagnosticError(
-                    msg,
-                )
+                raise DiagnosticError(msg)
             cmd.append(interpreter)
         elif not os.access(script_file, os.X_OK):
             msg = (
                 f"{err_msg}: non-executable file with unknown extension "
                 f"'{script_file.suffix}'."
             )
-            raise DiagnosticError(
-                msg,
-            )
+            raise DiagnosticError(msg)
 
         cmd.extend(args.get(ext, []))
         cmd.append(str(script_file))
@@ -674,9 +664,7 @@ def _run(self, input_files):
             f"Diagnostic script {self.script} failed with return code {returncode}. See the log "
             f"in {self.log}"
         )
-        raise DiagnosticError(
-            msg,
-        )
+        raise DiagnosticError(msg)
 
     def _collect_provenance(self) -> None:
         """Process provenance information provided by the diagnostic script."""
diff --git a/esmvalcore/_version.py b/esmvalcore/_version.py
index 394a7a7e23..6b9736a89c 100644
--- a/esmvalcore/_version.py
+++ b/esmvalcore/_version.py
@@ -9,6 +9,4 @@
         "ESMValCore package not found, please run `pip install -e .` before "
         "importing the package."
     )
-    raise PackageNotFoundError(
-        msg,
-    ) from exc
+    raise PackageNotFoundError(msg) from exc
diff --git a/esmvalcore/cmor/_fixes/access/_base_fix.py b/esmvalcore/cmor/_fixes/access/_base_fix.py
index bb8b2dfea7..5b7a122a88 100644
--- a/esmvalcore/cmor/_fixes/access/_base_fix.py
+++ b/esmvalcore/cmor/_fixes/access/_base_fix.py
@@ -75,9 +75,7 @@ def _get_path_from_facet(self, facet):
         path = Path(self.extra_facets[facet])
         if not path.is_file():
             msg = f"'{path}' given by facet '{facet}' does not exist"
-            raise FileNotFoundError(
-                msg,
-            )
+            raise FileNotFoundError(msg)
         return path
 
     def load_ocean_grid_data(self, facet):
diff --git a/esmvalcore/cmor/_fixes/cordex/cordex_fixes.py b/esmvalcore/cmor/_fixes/cordex/cordex_fixes.py
index d4bbcdf8cf..ed0d76190a 100644
--- a/esmvalcore/cmor/_fixes/cordex/cordex_fixes.py
+++ b/esmvalcore/cmor/_fixes/cordex/cordex_fixes.py
@@ -131,9 +131,7 @@ def _check_grid_differences(self, old_coord, new_coord):
                 "Differences between the original grid and the "
                 f"standardised grid are above 10e-4 {new_coord.units}."
             )
-            raise RecipeError(
-                msg,
-            )
+            raise RecipeError(msg)
 
     def _fix_rotated_coords(self, cube, domain, domain_info):
         """Fix rotated coordinates."""
@@ -221,8 +219,6 @@ def fix_metadata(self, cubes):
                     "not supported in CORDEX datasets. Must be "
                     "rotated_latitude_longitude or lambert_conformal_conic."
                 )
-                raise RecipeError(
-                    msg,
-                )
+                raise RecipeError(msg)
 
         return cubes
diff --git a/esmvalcore/cmor/_fixes/emac/_base_fixes.py b/esmvalcore/cmor/_fixes/emac/_base_fixes.py
index 7053b08fab..25a4f4c2dc 100644
--- a/esmvalcore/cmor/_fixes/emac/_base_fixes.py
+++ b/esmvalcore/cmor/_fixes/emac/_base_fixes.py
@@ -40,9 +40,7 @@ def get_cube(self, cubes, var_name=None):
             f"available in the input file. Please specify a valid `raw_name` "
             f"in the recipe or as extra facets."
         )
-        raise ValueError(
-            msg,
-        )
+        raise ValueError(msg)
 
 
 class NegateData(EmacFix):
diff --git a/esmvalcore/cmor/_fixes/emac/emac.py b/esmvalcore/cmor/_fixes/emac/emac.py
index 911bd46b13..6ff2e7e45e 100644
--- a/esmvalcore/cmor/_fixes/emac/emac.py
+++ b/esmvalcore/cmor/_fixes/emac/emac.py
@@ -117,9 +117,7 @@ def _fix_plev(self, cube):
             f"'{self.vardef.short_name}', searched for Z-coordinates with "
             f"units that are convertible to Pa"
         )
-        raise ValueError(
-            msg,
-        )
+        raise ValueError(msg)
 
     @staticmethod
     def _fix_alevel(cube, cubes):
diff --git a/esmvalcore/cmor/_fixes/icon/_base_fixes.py b/esmvalcore/cmor/_fixes/icon/_base_fixes.py
index 124b56de5d..4082f96bc5 100644
--- a/esmvalcore/cmor/_fixes/icon/_base_fixes.py
+++ b/esmvalcore/cmor/_fixes/icon/_base_fixes.py
@@ -162,9 +162,7 @@ def _get_grid_url(self, cube: Cube) -> tuple[str, str]:
                 f"necessary to download the ICON horizontal grid file:\n"
                 f"{cube}"
             )
-            raise ValueError(
-                msg,
-            )
+            raise ValueError(msg)
         grid_url = cube.attributes[self.GRID_FILE_ATTR]
         parsed_url = urlparse(grid_url)
         grid_name = Path(parsed_url.path).name
@@ -223,9 +221,7 @@ def _get_path_from_facet(
                     f"relative to the auxiliary_data_dir "
                     f"'{self.session['auxiliary_data_dir']}')"
                 )
-                raise FileNotFoundError(
-                    msg,
-                )
+                raise FileNotFoundError(msg)
             path = new_path
         return path
 
@@ -650,9 +646,7 @@ def _add_coord_from_grid_file(self, cube: Cube, coord_name: str) -> None:
                 f"'{coord_name}', cube does not contain a single unnamed "
                 f"dimension:\n{cube}"
             )
-            raise ValueError(
-                msg,
-            )
+            raise ValueError(msg)
         coord_dims: tuple[()] | tuple[int] = ()
         for idx in range(cube.ndim):
             if not cube.coords(dimensions=idx, dim_coords=True):
@@ -678,9 +672,7 @@ def _add_time(self, cube: Cube, cubes: CubeList) -> Cube:
             f"'{self.vardef.short_name}', cube and other cubes in file do not "
             f"contain it"
         )
-        raise ValueError(
-            msg,
-        )
+        raise ValueError(msg)
 
     def _get_z_coord(
         self,
@@ -987,9 +979,7 @@ def _get_previous_timestep(self, datetime_point: datetime) -> datetime:
                     f"got {datetime_point}. Use `shift_time=false` in the "
                     f"recipe to disable this feature"
                 )
-                raise ValueError(
-                    msg,
-                )
+                raise ValueError(msg)
 
         # Decadal data
         if "dec" in freq:
@@ -1082,9 +1072,7 @@ def _fix_invalid_time_units(time_coord: Coord) -> None:
                 f"Expected time units '{time_format}' in input file, got "
                 f"'{t_unit}'"
             )
-            raise ValueError(
-                msg,
-            )
+            raise ValueError(msg)
         new_t_units = Unit(
             "days since 1850-01-01",
             calendar="proleptic_gregorian",
diff --git a/esmvalcore/cmor/_fixes/native_datasets.py b/esmvalcore/cmor/_fixes/native_datasets.py
index 2f7f24b763..a4974c80c3 100644
--- a/esmvalcore/cmor/_fixes/native_datasets.py
+++ b/esmvalcore/cmor/_fixes/native_datasets.py
@@ -86,9 +86,7 @@ def fix_var_metadata(self, cube: Cube) -> None:
                     f"Failed to fix invalid units '{invalid_units}' for "
                     f"variable '{self.vardef.short_name}'"
                 )
-                raise ValueError(
-                    msg,
-                ) from exc
+                raise ValueError(msg) from exc
         safe_convert_units(cube, self.vardef.units)
 
         # Fix attributes
@@ -132,9 +130,7 @@ def get_cube(
                 f"Variable '{var_name}' used to extract "
                 f"'{self.vardef.short_name}' is not available in input file"
             )
-            raise ValueError(
-                msg,
-            )
+            raise ValueError(msg)
         return cubes.extract_cube(NameConstraint(var_name=var_name))
 
     def fix_regular_time(
diff --git a/esmvalcore/cmor/_fixes/shared.py b/esmvalcore/cmor/_fixes/shared.py
index 13d478ea9f..713cb7e12c 100644
--- a/esmvalcore/cmor/_fixes/shared.py
+++ b/esmvalcore/cmor/_fixes/shared.py
@@ -52,9 +52,7 @@ def add_aux_coords_from_cubes(cube, cubes, coord_dict):
                 f"Expected exactly one coordinate cube '{coord_name}' in "
                 f"list of cubes {cubes}, got {len(coord_cube):d}"
             )
-            raise ValueError(
-                msg,
-            )
+            raise ValueError(msg)
         coord_cube = coord_cube[0]
         aux_coord = cube_to_aux_coord(coord_cube)
         cube.add_aux_coord(aux_coord, coord_dims)
@@ -137,9 +135,7 @@ def add_plev_from_altitude(cube):
         "Cannot add 'air_pressure' coordinate, 'altitude' coordinate not "
         "available"
     )
-    raise ValueError(
-        msg,
-    )
+    raise ValueError(msg)
 
 
 def add_altitude_from_plev(cube):
@@ -185,9 +181,7 @@ def add_altitude_from_plev(cube):
         "Cannot add 'altitude' coordinate, 'air_pressure' coordinate not "
         "available"
     )
-    raise ValueError(
-        msg,
-    )
+    raise ValueError(msg)
 
 
 def add_scalar_depth_coord(cube, depth=0.0):
@@ -355,16 +349,12 @@ def get_bounds_cube(cubes, coord_var_name):
             return cube[0]
         if len(cube) > 1:
             msg = f"Multiple cubes with var_name '{bound_var}' found"
-            raise ValueError(
-                msg,
-            )
+            raise ValueError(msg)
     msg = (
         f"No bounds for coordinate variable '{coord_var_name}' available in "
         f"cubes\n{cubes}"
     )
-    raise ValueError(
-        msg,
-    )
+    raise ValueError(msg)
 
 
 @cache
@@ -544,16 +534,12 @@ def get_time_bounds(time: Coord, freq: str) -> np.ndarray:
                     f"For `n`-hourly data, `n` must be a divisor of 24, got "
                     f"'{freq}'"
                 )
-                raise NotImplementedError(
-                    msg,
-                )
+                raise NotImplementedError(msg)
             min_bound = date - timedelta(hours=n_hours / 2.0)
             max_bound = date + timedelta(hours=n_hours / 2.0)
         else:
             msg = f"Cannot guess time bounds for frequency '{freq}'"
-            raise NotImplementedError(
-                msg,
-            )
+            raise NotImplementedError(msg)
         bounds.append([min_bound, max_bound])
 
     return date2num(np.array(bounds), time.units, time.dtype)
diff --git a/esmvalcore/cmor/_utils.py b/esmvalcore/cmor/_utils.py
index 9728f8bcad..e2d33b762d 100644
--- a/esmvalcore/cmor/_utils.py
+++ b/esmvalcore/cmor/_utils.py
@@ -79,9 +79,7 @@ def _get_alternative_generic_lev_coord(
         f"Found no valid alternative coordinate for generic level coordinate "
         f"'{coord_name}'"
     )
-    raise ValueError(
-        msg,
-    )
+    raise ValueError(msg)
 
 
 def _get_generic_lev_coord_names(
@@ -204,9 +202,7 @@ def _get_single_cube(
             f"but none of their var_names match the expected.\nFull list of "
             f"cubes encountered: {cube_list}"
         )
-        raise ValueError(
-            msg,
-        )
+        raise ValueError(msg)
     logger.warning(
         "Found variable %s%s, but there were other present in the file. Those "
         "extra variables are usually metadata (cell area, latitude "
diff --git a/esmvalcore/experimental/recipe_metadata.py b/esmvalcore/experimental/recipe_metadata.py
index f674d33d49..6c6ee2543f 100644
--- a/esmvalcore/experimental/recipe_metadata.py
+++ b/esmvalcore/experimental/recipe_metadata.py
@@ -143,9 +143,7 @@ def __init__(self, filename: str) -> None:
                 f"{self.__class__.__name__} cannot handle bibtex files "
                 "with more than 1 entry."
             )
-            raise NotImplementedError(
-                msg,
-            )
+            raise NotImplementedError(msg)
 
         self._bib_data = bib_data
         self._key, self._entry = next(iter(bib_data.entries.items()))
@@ -202,8 +200,6 @@ def render(self, renderer: str = "html") -> str:
             rendered = formatter.text.render(backend)
         except Exception as err:  # noqa: BLE001
             msg = f"Could not render {self._key!r}: {err}"
-            raise RenderError(
-                msg,
-            ) from None
+            raise RenderError(msg) from None
 
         return rendered
diff --git a/esmvalcore/io/esgf/_download.py b/esmvalcore/io/esgf/_download.py
index abcb67ccf5..23a8e0fd69 100644
--- a/esmvalcore/io/esgf/_download.py
+++ b/esmvalcore/io/esgf/_download.py
@@ -551,9 +551,7 @@ def _download(self, local_file, url):
                     f" downloaded from {url}: expected {checksum}, but got"
                     f" {local_checksum}. Try downloading the file again."
                 )
-                raise DownloadError(
-                    msg,
-                )
+                raise DownloadError(msg)
 
         shutil.move(tmp_file, local_file)
         log_speed(url, self.size, duration.total_seconds())
diff --git a/esmvalcore/io/esgf/_search.py b/esmvalcore/io/esgf/_search.py
index 01e42a0bb4..d1e7358e77 100644
--- a/esmvalcore/io/esgf/_search.py
+++ b/esmvalcore/io/esgf/_search.py
@@ -347,9 +347,7 @@ def find_files(*, project, short_name, dataset, **facets):
             f"Unable to download from ESGF, because project {project} is not"
             " on it or is not supported by the esmvalcore.io.esgf module."
         )
-        raise ValueError(
-            msg,
-        )
+        raise ValueError(msg)
 
     # The project is required for the function to work.
     facets["project"] = project
diff --git a/esmvalcore/iris_helpers.py b/esmvalcore/iris_helpers.py
index dfba3ca503..18db216a09 100644
--- a/esmvalcore/iris_helpers.py
+++ b/esmvalcore/iris_helpers.py
@@ -522,9 +522,7 @@ def safe_convert_units(cube: Cube, units: str | Unit) -> Cube:
             f"standard_name changed from '{old_standard_name}' to "
             f"'{cube.standard_name}'"
         )
-        raise ValueError(
-            msg,
-        )
+        raise ValueError(msg)
     return cube
 
 
@@ -624,9 +622,7 @@ def dataset_to_iris(
             f"Expected type ncdata.NcData or xr.Dataset for dataset, got "
             f"type {type(dataset)}"
         )
-        raise TypeError(
-            msg,
-        )
+        raise TypeError(msg)
 
     with ignore_warnings_context(ignore_warnings):
         cubes = conversion_func(dataset)
diff --git a/esmvalcore/preprocessor/_area.py b/esmvalcore/preprocessor/_area.py
index 0c7e08ff2a..34ceacad0c 100644
--- a/esmvalcore/preprocessor/_area.py
+++ b/esmvalcore/preprocessor/_area.py
@@ -250,9 +250,7 @@ def zonal_statistics(
     """
     if cube.coord("longitude").points.ndim >= 2:
         msg = "Zonal statistics on irregular grids not yet implemented"
-        raise ValueError(
-            msg,
-        )
+        raise ValueError(msg)
     (agg, agg_kwargs) = get_iris_aggregator(operator, **operator_kwargs)
     with ignore_iris_vague_metadata_warnings():
         result = cube.collapsed("longitude", agg, **agg_kwargs)
@@ -301,9 +299,7 @@ def meridional_statistics(
     """
     if cube.coord("latitude").points.ndim >= 2:
         msg = "Meridional statistics on irregular grids not yet implemented"
-        raise ValueError(
-            msg,
-        )
+        raise ValueError(msg)
     (agg, agg_kwargs) = get_iris_aggregator(operator, **operator_kwargs)
     with ignore_iris_vague_metadata_warnings():
         result = cube.collapsed("latitude", agg, **agg_kwargs)
@@ -421,17 +417,13 @@ def extract_named_regions(cube: Cube, regions: str | Iterable[str]) -> Cube:
 
     if not isinstance(regions, (list, tuple, set)):
         msg = f'Regions "{regions}" is not an acceptable format.'
-        raise TypeError(
-            msg,
-        )
+        raise TypeError(msg)
 
     available_regions = set(cube.coord("region").points)
     invalid_regions = set(regions) - available_regions
     if invalid_regions:
         msg = f'Region(s) "{invalid_regions}" not in cube region(s): {available_regions}'
-        raise ValueError(
-            msg,
-        )
+        raise ValueError(msg)
 
     constraints = iris.Constraint(region=lambda r: r in regions)
     return cube.extract(constraint=constraints)
@@ -610,11 +602,8 @@ def _get_masks_from_geometries(
 ) -> dict[str, np.ndarray]:
     """Get cube masks from requested regions."""
     if method not in {"contains", "representative"}:
-        msg = "Invalid value for `method`. Choose from 'contains', "
-        raise ValueError(
-            msg,
-            "'representative'.",
-        )
+        msg = "Invalid value for `method`. Choose from 'contains', 'representative'."
+        raise ValueError(msg)
 
     masks = {}
     for id_, geometry in geometries.items():
diff --git a/esmvalcore/preprocessor/_cycles.py b/esmvalcore/preprocessor/_cycles.py
index 400530040a..e4b7aed3dc 100644
--- a/esmvalcore/preprocessor/_cycles.py
+++ b/esmvalcore/preprocessor/_cycles.py
@@ -70,9 +70,7 @@ def amplitude(cube, coords):
                 f"{cube.summary(shorten=True)} and cannot be added via "
                 f"iris.coord_categorisation"
             )
-            raise iris.exceptions.CoordinateNotFoundError(
-                msg,
-            )
+            raise iris.exceptions.CoordinateNotFoundError(msg)
 
     # Calculate amplitude
     with ignore_iris_vague_metadata_warnings():
diff --git a/esmvalcore/preprocessor/_derive/amoc.py b/esmvalcore/preprocessor/_derive/amoc.py
index 3607aa1d62..67b179f0dd 100644
--- a/esmvalcore/preprocessor/_derive/amoc.py
+++ b/esmvalcore/preprocessor/_derive/amoc.py
@@ -72,9 +72,7 @@ def calculate(cubes):
                 f"Amoc calculation: {cube_orig} doesn't contain"
                 f" atlantic_arctic_ocean."
             )
-            raise ValueError(
-                msg,
-            )
+            raise ValueError(msg)
 
         # 2: Remove the shallowest 500m to avoid wind driven mixed layer.
         depth_constraint = iris.Constraint(depth=lambda d: d >= 500.0)
diff --git a/esmvalcore/preprocessor/_derive/ctotal.py b/esmvalcore/preprocessor/_derive/ctotal.py
index b881f1e8ee..307ce01336 100644
--- a/esmvalcore/preprocessor/_derive/ctotal.py
+++ b/esmvalcore/preprocessor/_derive/ctotal.py
@@ -43,9 +43,7 @@ def calculate(cubes):
                     f"standard name CMIP5: soil_carbon_content "
                     f"or CMIP6: soil_mass_content_of_carbon"
                 )
-                raise ValueError(
-                    msg,
-                ) from exc
+                raise ValueError(msg) from exc
         c_veg_cube = cubes.extract_cube(
             Constraint(name="vegetation_carbon_content"),
         )
diff --git a/esmvalcore/preprocessor/_mask.py b/esmvalcore/preprocessor/_mask.py
index 790a58b3bd..916a724f5e 100644
--- a/esmvalcore/preprocessor/_mask.py
+++ b/esmvalcore/preprocessor/_mask.py
@@ -153,9 +153,7 @@ def mask_landsea(cube: Cube, mask_out: Literal["land", "sea"]) -> Cube:
             "Use of shapefiles with irregular grids not yet implemented, "
             "land-sea mask not applied."
         )
-        raise ValueError(
-            msg,
-        )
+        raise ValueError(msg)
 
     return cube
 
@@ -528,9 +526,7 @@ def _get_shape(cubes):
     shapes = {cube.shape for cube in cubes}
     if len(shapes) > 1:
         msg = f"Expected cubes with identical shapes, got shapes {shapes}"
-        raise ValueError(
-            msg,
-        )
+        raise ValueError(msg)
     return next(iter(shapes))
 
 
@@ -616,9 +612,7 @@ def mask_multimodel(products):
         f"iris.cube.Cube or esmvalcore.preprocessor.PreprocessorFile, "
         f"got {product_types}"
     )
-    raise TypeError(
-        msg,
-    )
+    raise TypeError(msg)
 
 
 def mask_fillvalues(
@@ -685,9 +679,7 @@ def mask_fillvalues(
                 valid = ~mask.all(axis=(-2, -1), keepdims=True)
             else:
                 msg = f"Unable to handle {mask.ndim} dimensional data"
-                raise NotImplementedError(
-                    msg,
-                )
+                raise NotImplementedError(msg)
             combined_mask = array_module.where(
                 valid,
                 combined_mask | mask,
@@ -730,9 +722,7 @@ def _get_fillvalues_mask(
             f"Fraction of missing values {threshold_fraction} should be "
             f"between 0 and 1.0"
         )
-        raise ValueError(
-            msg,
-        )
+        raise ValueError(msg)
     nr_time_points = len(cube.coord("time").points)
     if time_window > nr_time_points:
         msg = "Time window (in time units) larger than total time span. Stop."
diff --git a/esmvalcore/preprocessor/_multimodel.py b/esmvalcore/preprocessor/_multimodel.py
index f15193db4b..d8ce839ff7 100644
--- a/esmvalcore/preprocessor/_multimodel.py
+++ b/esmvalcore/preprocessor/_multimodel.py
@@ -453,9 +453,7 @@ def _combine(cubes):
             f"Multi-model statistics failed to merge input cubes into a "
             f"single array:\n{cubes}\n{msg}"
         )
-        raise ValueError(
-            msg,
-        ) from exc
+        raise ValueError(msg) from exc
 
     return merged_cube
 
@@ -521,9 +519,7 @@ def _compute_eager(
             f"This can happen e.g. if the calculation results in inconsistent "
             f"dtypes"
         )
-        raise ValueError(
-            msg,
-        ) from excinfo
+        raise ValueError(msg) from excinfo
 
     result_cube.data = np.ma.array(result_cube.data)
 
@@ -575,9 +571,7 @@ def _multicube_statistics(
     """
     if not cubes:
         msg = "Cannot perform multicube statistics for an empty list of cubes"
-        raise ValueError(
-            msg,
-        )
+        raise ValueError(msg)
 
     # Avoid modifying inputs
     cubes = [cube.copy() for cube in cubes]
@@ -608,9 +602,7 @@ def _multicube_statistics(
             "array: some cubes have a 'time' dimension, some do not have a "
             "'time' dimension."
         )
-        raise ValueError(
-            msg,
-        )
+        raise ValueError(msg)
 
     # Calculate statistics
     statistics_cubes = {}
@@ -681,9 +673,7 @@ def _get_operator_and_kwargs(statistic: str | dict) -> tuple[str, dict]:
                 f"`statistic` given as dictionary, but missing required key "
                 f"`operator`, got {statistic}"
             )
-            raise ValueError(
-                msg,
-            )
+            raise ValueError(msg)
         operator = statistic.pop("operator")
         kwargs = statistic
     else:
@@ -845,9 +835,7 @@ def multi_model_statistics(
         f"iris.cube.Cube or esmvalcore.preprocessor.PreprocessorFile, "
         f"got {products}"
     )
-    raise ValueError(
-        msg,
-    )
+    raise ValueError(msg)
 
 
 def ensemble_statistics(
diff --git a/esmvalcore/preprocessor/_regrid_unstructured.py b/esmvalcore/preprocessor/_regrid_unstructured.py
index d803938186..73fedc741f 100644
--- a/esmvalcore/preprocessor/_regrid_unstructured.py
+++ b/esmvalcore/preprocessor/_regrid_unstructured.py
@@ -93,17 +93,13 @@ def __init__(self, src_cube: Cube, tgt_cube: Cube) -> None:
                 f"Source cube {src_cube.summary(shorten=True)} does not have "
                 f"unstructured grid"
             )
-            raise ValueError(
-                msg,
-            )
+            raise ValueError(msg)
         if not has_regular_grid(tgt_cube):
             msg = (
                 f"Target cube {tgt_cube.summary(shorten=True)} does not have "
                 f"regular grid"
             )
-            raise ValueError(
-                msg,
-            )
+            raise ValueError(msg)
         src_lat = src_cube.coord("latitude").copy()
         src_lon = src_cube.coord("longitude").copy()
         tgt_lat = tgt_cube.coord("latitude").copy()
@@ -211,18 +207,14 @@ def __call__(self, cube: Cube) -> Cube:
                 f"Cube {cube.summary(shorten=True)} does not have "
                 f"unstructured grid"
             )
-            raise ValueError(
-                msg,
-            )
+            raise ValueError(msg)
         coords = [cube.coord("latitude"), cube.coord("longitude")]
         if coords != self.src_coords:
             msg = (
                 f"The given cube {cube.summary(shorten=True)} is not defined "
                 f"on the same source grid as this regridder"
             )
-            raise ValueError(
-                msg,
-            )
+            raise ValueError(msg)
 
         # Get coordinates of regridded cube
 
diff --git a/esmvalcore/preprocessor/_shared.py b/esmvalcore/preprocessor/_shared.py
index ecf58028e2..f5709b3653 100644
--- a/esmvalcore/preprocessor/_shared.py
+++ b/esmvalcore/preprocessor/_shared.py
@@ -173,9 +173,7 @@ def update_weights_kwargs(
     kwargs = dict(kwargs)
     if not aggregator_accept_weights(aggregator) and "weights" in kwargs:
         msg = f"Aggregator '{operator}' does not support 'weights' option"
-        raise ValueError(
-            msg,
-        )
+        raise ValueError(msg)
     if aggregator_accept_weights(aggregator) and kwargs.get("weights", True):
         kwargs["weights"] = weights
         if cube is not None and callback is not None:
@@ -232,9 +230,7 @@ def get_normalized_cube(
             f"Expected 'subtract' or 'divide' for `normalize`, got "
             f"'{normalize}'"
         )
-        raise ValueError(
-            msg,
-        )
+        raise ValueError(msg)
 
     # Keep old metadata except for units
     new_units = normalized_cube.units
@@ -274,9 +270,7 @@ def preserve_float_dtype(func: Callable) -> Callable:
             f"Cannot preserve float dtype during function '{func.__name__}', "
             f"function takes no arguments"
         )
-        raise TypeError(
-            msg,
-        )
+        raise TypeError(msg)
 
     @wraps(func)
     def wrapper(*args: Any, **kwargs: Any) -> DataType:
@@ -298,9 +292,7 @@ def wrapper(*args: Any, **kwargs: Any) -> DataType:
                 f"type {type(result)} do not have the necessary attribute "
                 f"'dtype'"
             )
-            raise TypeError(
-                msg,
-            )
+            raise TypeError(msg)
 
         return result
 
@@ -381,9 +373,7 @@ def get_weights(
                 f"`cell_area` can be given to the cube as supplementary "
                 f"variable)"
             )
-            raise CoordinateNotFoundError(
-                msg,
-            )
+            raise CoordinateNotFoundError(msg)
         try_adding_calculated_cell_area(cube)
         area_weights = cube.cell_measure("cell_area").core_data()
         if cube.has_lazy_data():
@@ -438,18 +428,14 @@ def get_coord_weights(
             f"Cannot calculate weights for coordinate '{coord.name()}' "
             f"without bounds"
         )
-        raise ValueError(
-            msg,
-        )
+        raise ValueError(msg)
     if coord.core_bounds().shape[-1] != 2:
         msg = (
             f"Cannot calculate weights for coordinate '{coord.name()}' "
             f"with {coord.core_bounds().shape[-1]} bounds per point, expected "
             f"2 bounds per point"
         )
-        raise ValueError(
-            msg,
-        )
+        raise ValueError(msg)
 
     # Calculate weights of same shape as coordinate and make sure to use
     # identical chunks as parent cube for non-scalar lazy data
@@ -573,9 +559,7 @@ def get_all_coords(
                 f"{cube.summary(shorten=True)} must not have unnamed "
                 f"dimensions"
             )
-            raise ValueError(
-                msg,
-            )
+            raise ValueError(msg)
     return coords
 
 
diff --git a/esmvalcore/preprocessor/_supplementary_vars.py b/esmvalcore/preprocessor/_supplementary_vars.py
index f7a1491d09..f1ed2915ff 100644
--- a/esmvalcore/preprocessor/_supplementary_vars.py
+++ b/esmvalcore/preprocessor/_supplementary_vars.py
@@ -82,9 +82,7 @@ def add_cell_measure(
     """
     if measure not in ["area", "volume"]:
         msg = f"measure name must be 'area' or 'volume', got {measure} instead"
-        raise ValueError(
-            msg,
-        )
+        raise ValueError(msg)
     coord_dims = tuple(
         range(cube.ndim - len(cell_measure_cube.shape), cube.ndim),
     )
diff --git a/esmvalcore/preprocessor/_units.py b/esmvalcore/preprocessor/_units.py
index 413a289066..52e80ccff6 100644
--- a/esmvalcore/preprocessor/_units.py
+++ b/esmvalcore/preprocessor/_units.py
@@ -126,15 +126,11 @@ def accumulate_coordinate(
             f"Requested coordinate {coordinate} not found in cube "
             f"{cube.summary(shorten=True)}"
         )
-        raise ValueError(
-            msg,
-        ) from err
+        raise ValueError(msg) from err
 
     if coord.ndim > 1:
         msg = f"Multidimensional coordinate {coord} not supported."
-        raise NotImplementedError(
-            msg,
-        )
+        raise NotImplementedError(msg)
 
     array_module = da if coord.has_lazy_bounds() else np
     factor = AuxCoord(
diff --git a/esmvalcore/preprocessor/_volume.py b/esmvalcore/preprocessor/_volume.py
index aefd15c8c1..e7d92f3a32 100644
--- a/esmvalcore/preprocessor/_volume.py
+++ b/esmvalcore/preprocessor/_volume.py
@@ -110,9 +110,7 @@ def extract_volume(
             'Depth extraction bounds can be set to "open", "closed", '
             f'"left_closed", or "right_closed". Got "{interval_bounds}".'
         )
-        raise ValueError(
-            msg,
-        )
+        raise ValueError(msg)
 
     z_constraint = iris.Constraint(coord_values=coord_values)
 
@@ -162,18 +160,14 @@ def calculate_volume(cube: Cube) -> np.ndarray | da.Array:
             "Bounds should be 2 in the last dimension to compute the "
             "thickness."
         )
-        raise ValueError(
-            msg,
-        )
+        raise ValueError(msg)
 
     # Convert units to get the thickness in meters
     try:
         depth.convert_units("m")
     except ValueError as err:
         msg = f"Cannot compute volume using the Z-axis. {err}"
-        raise ValueError(
-            msg,
-        ) from err
+        raise ValueError(msg) from err
 
     # Calculate Z-direction thickness
     thickness = depth.core_bounds()[..., 1] - depth.core_bounds()[..., 0]
@@ -314,9 +308,7 @@ def volume_statistics(
             "This may indicate Z axis depending on other dimension than "
             "space that could provoke invalid aggregation..."
         )
-        raise ValueError(
-            msg,
-        )
+        raise ValueError(msg)
 
     (agg, agg_kwargs) = get_iris_aggregator(operator, **operator_kwargs)
     agg_kwargs = update_weights_kwargs(
@@ -391,9 +383,7 @@ def axis_statistics(
         coord = cube.coord(axis=axis)
     except iris.exceptions.CoordinateNotFoundError as err:
         msg = f"Axis {axis} not found in cube {cube.summary(shorten=True)}"
-        raise ValueError(
-            msg,
-        ) from err
+        raise ValueError(msg) from err
 
     # Multidimensional coordinates are currently not supported
     coord_dims = cube.coord_dims(coord)
@@ -401,9 +391,7 @@ def axis_statistics(
         msg = (
             "axis_statistics not implemented for multidimensional coordinates."
         )
-        raise NotImplementedError(
-            msg,
-        )
+        raise NotImplementedError(msg)
 
     # For weighted operations, create a dummy weights coordinate using the
     # bounds of the original coordinate (this handles units properly, e.g., for
@@ -621,9 +609,7 @@ def extract_trajectory(
     """
     if len(latitudes) != len(longitudes):
         msg = "Longitude & Latitude coordinates have different lengths"
-        raise ValueError(
-            msg,
-        )
+        raise ValueError(msg)
 
     if len(latitudes) == len(longitudes) == 2:
         minlat, maxlat = np.min(latitudes), np.max(latitudes)
diff --git a/esmvalcore/preprocessor/_weighting.py b/esmvalcore/preprocessor/_weighting.py
index 9089e1d62e..dc42727616 100644
--- a/esmvalcore/preprocessor/_weighting.py
+++ b/esmvalcore/preprocessor/_weighting.py
@@ -73,18 +73,14 @@ def weighting_landsea_fraction(cube, area_type):
     """
     if area_type not in ("land", "sea"):
         msg = f"Expected 'land' or 'sea' for area_type, got '{area_type}'"
-        raise TypeError(
-            msg,
-        )
+        raise TypeError(msg)
     (land_fraction, errors) = _get_land_fraction(cube)
     if land_fraction is None:
         msg = (
             f"Weighting of '{cube.var_name}' with '{area_type}' fraction "
             f"failed because of the following errors: {' '.join(errors)}"
         )
-        raise ValueError(
-            msg,
-        )
+        raise ValueError(msg)
     core_data = cube.core_data()
     if area_type == "land":
         cube.data = core_data * land_fraction
diff --git a/tests/integration/preprocessor/_mask/test_mask.py b/tests/integration/preprocessor/_mask/test_mask.py
index 88ef081ac3..c255878941 100644
--- a/tests/integration/preprocessor/_mask/test_mask.py
+++ b/tests/integration/preprocessor/_mask/test_mask.py
@@ -3,6 +3,7 @@
 Integration tests for the :func:`esmvalcore.preprocessor._mask` module.
 """
 
+import re
 from pathlib import Path
 
 import dask.array as da
@@ -10,7 +11,8 @@
 import iris.fileformats
 import numpy as np
 import pytest
-from iris.coords import AuxCoord
+from iris.coords import AuxCoord, DimCoord
+from iris.cube import Cube, CubeList
 
 from esmvalcore.preprocessor import (
     PreprocessorFile,
@@ -19,7 +21,11 @@
     mask_landsea,
     mask_landseaice,
 )
+from esmvalcore.preprocessor._mask import _get_fillvalues_mask
 from tests import assert_array_equal
+from tests.unit.preprocessor._mask.test_mask_multimodel import (
+    PreprocessorFile as MockedPreprocessorFile,
+)
 
 
 class Test:
@@ -461,3 +467,49 @@ def test_mask_fillvalues_min_value_none(self, mocker, lazy):
             else:
                 msg = f"Invalid filename: {product.filename}"
                 raise AssertionError(msg)
+
+    def test_mask_fillvalues_unsupported_ndim_fail(self) -> None:
+        """Test ``mask_fillvalues`` with unsupported data dimensions."""
+        cube = iris.cube.Cube(
+            self.mock_data[:, 0, 0],
+            dim_coords_and_dims=[(self.times, 0)],
+        )
+        products = [MockedPreprocessorFile(CubeList([cube]), "A")]
+        msg = r"Unable to handle 0 dimensional data"
+        with pytest.raises(NotImplementedError, match=re.escape(msg)):
+            mask_fillvalues(
+                products,
+                threshold_fraction=0.5,
+                min_value=None,
+                time_window=1,
+            )
+
+    @pytest.mark.parametrize(
+        "threshold_fraction",
+        [-1000.0, -0.1, 1.1, 1000.0],
+    )
+    def test_get_fillvalues_mask_invalid_threshold_fraction_fail(
+        self,
+        threshold_fraction: float,
+    ) -> None:
+        msg = r"should be between 0 and 1.0"
+        with pytest.raises(ValueError, match=re.escape(msg)):
+            _get_fillvalues_mask(
+                Cube(0.0),
+                threshold_fraction=threshold_fraction,
+                min_value=0.0,
+                time_window=1,
+            )
+
+    def test_get_fillvalues_mask_invalid_time_window_fail(self) -> None:
+        time_coord = DimCoord([0.0], standard_name="time")
+        cube = Cube([0.0], dim_coords_and_dims=[(time_coord, 0)])
+
+        msg = r"Time window (in time units) larger than total time span. Stop."
+        with pytest.raises(ValueError, match=re.escape(msg)):
+            _get_fillvalues_mask(
+                cube,
+                threshold_fraction=0.5,
+                min_value=0.0,
+                time_window=2,
+            )
diff --git a/tests/integration/preprocessor/_supplementary_vars/test_add_supplementary_variables.py b/tests/integration/preprocessor/_supplementary_vars/test_add_supplementary_variables.py
index f1f5c93ac7..be03ddeb8d 100644
--- a/tests/integration/preprocessor/_supplementary_vars/test_add_supplementary_variables.py
+++ b/tests/integration/preprocessor/_supplementary_vars/test_add_supplementary_variables.py
@@ -4,6 +4,8 @@
 :func:`esmvalcore.preprocessor._supplementary_vars` module.
 """
 
+import re
+
 import dask.array as da
 import iris
 import iris.fileformats
@@ -356,3 +358,12 @@ def test_find_matching_coord_dims_match(self):
             self.lats,
             self.cube,
         ) == (1,)
+
+    def test_add_cell_measure_invalid_measure_fail(self) -> None:
+        msg = r"measure name must be 'area' or 'volume'"
+        with pytest.raises(ValueError, match=re.escape(msg)):
+            add_cell_measure(
+                iris.cube.Cube(0.0),
+                self.fx_area,
+                measure="invalid",  # type: ignore[arg-type]
+            )
diff --git a/tests/integration/test_task.py b/tests/integration/test_task.py
index 1f8e3db1dd..4329ec4310 100644
--- a/tests/integration/test_task.py
+++ b/tests/integration/test_task.py
@@ -1,14 +1,20 @@
+from __future__ import annotations
+
 import multiprocessing
 import os
+import re
 import shutil
 from contextlib import contextmanager
 from functools import partial
 from multiprocessing.pool import ThreadPool
+from typing import TYPE_CHECKING
+from unittest.mock import sentinel
 
 import pytest
 import yaml
 
 import esmvalcore
+import esmvalcore._task
 from esmvalcore._task import (
     BaseTask,
     DiagnosticError,
@@ -19,6 +25,11 @@
 )
 from esmvalcore.config._diagnostics import DIAGNOSTICS
 
+if TYPE_CHECKING:
+    from pathlib import Path
+
+    from pytest_mock import MockerFixture
+
 
 class MockBaseTask(BaseTask):
     def _run(self, input_files):
@@ -374,3 +385,24 @@ def _run(self, input_filesi=None):
     with pytest.raises(DiagnosticError) as err_mssg:
         task.run()
     assert diag_text[1] in str(err_mssg.value)
+
+
+def test_initialize_provenance_already_initialized_fail() -> None:
+    task = BaseTask()  # type: ignore[abstract]
+    task.activity = sentinel.activity
+
+    msg = r"Provenance of BaseTask('') already initialized"
+    with pytest.raises(ValueError, match=re.escape(msg)):
+        task.initialize_provenance(sentinel.recipe_entity)
+
+
+def test_diagnostic_task_script_not_x_fail(
+    tmp_path: Path,
+    mocker: MockerFixture,
+) -> None:
+    mock_os = mocker.patch.object(esmvalcore._task, "os", create_autospec=True)
+    mock_os.access.return_value = False
+    diag_script = tmp_path / "diag_cow.c"
+    msg = r"Cannot execute script"
+    with pytest.raises(DiagnosticError, match=re.escape(msg)):
+        _get_single_diagnostic_task(tmp_path, diag_script)
diff --git a/tests/unit/experimental/test_recipe_info.py b/tests/unit/experimental/test_recipe_info.py
index b085ace4b4..d61464fbad 100644
--- a/tests/unit/experimental/test_recipe_info.py
+++ b/tests/unit/experimental/test_recipe_info.py
@@ -1,7 +1,14 @@
+from __future__ import annotations
+
+import re
 import textwrap
 from pathlib import Path
+from typing import TYPE_CHECKING
+
+import pytest
 
 import esmvalcore
+import esmvalcore.experimental.recipe_metadata
 from esmvalcore.config._diagnostics import TAGS, Diagnostics
 from esmvalcore.experimental.recipe_info import (
     Contributor,
@@ -10,6 +17,9 @@
     Reference,
 )
 
+if TYPE_CHECKING:
+    from pytest_mock import MockerFixture
+
 DIAGNOSTICS = Diagnostics(Path(__file__).parent)
 
 
@@ -95,3 +105,53 @@ def test_recipe_info_str():
         ### Maintainers
         """).lstrip()
     assert str(recipe) == text
+
+
+def test_reference_multiple_entries_fail(tmp_path: Path) -> None:
+    bib_file = tmp_path / "bib.bib"
+    bib_file.write_text(
+        textwrap.dedent(
+            """
+            @article{a,
+                title = {a},
+                author = {a},
+                year = 2020,
+            }
+            @article{b,
+                title = {b},
+                author = {b},
+                year = 2020,
+            }
+            """,
+        ),
+    )
+    msg = r"Reference cannot handle bibtex files with more than 1 entry"
+    with pytest.raises(NotImplementedError, match=re.escape(msg)):
+        Reference(str(bib_file))
+
+
+def test_render_fail(
+    monkeypatch: pytest.MonkeyPatch,
+    mocker: MockerFixture,
+) -> None:
+    mock_pybtex = mocker.patch.object(
+        esmvalcore.experimental.recipe_metadata,
+        "pybtex",
+        create_autospec=True,
+    )
+    mock_pybtex.plugin.find_plugin.return_value.return_value.format_entry.side_effect = ValueError(
+        "err",
+    )
+    monkeypatch.setattr(
+        esmvalcore.experimental.recipe_metadata,
+        "DIAGNOSTICS",
+        DIAGNOSTICS,
+    )
+    reference = Reference.from_tag("doe2021")
+
+    msg = r"Could not render 'doe2021': err"
+    with pytest.raises(
+        esmvalcore.experimental.recipe_metadata.RenderError,
+        match=re.escape(msg),
+    ):
+        reference.render()
diff --git a/tests/unit/preprocessor/_area/test_area.py b/tests/unit/preprocessor/_area/test_area.py
index 8299d3e461..080d18c75a 100644
--- a/tests/unit/preprocessor/_area/test_area.py
+++ b/tests/unit/preprocessor/_area/test_area.py
@@ -1,5 +1,6 @@
 """Unit tests for the :func:`esmvalcore.preprocessor._area` module."""
 
+import re
 import unittest
 from pathlib import Path
 
@@ -303,8 +304,17 @@ def test_extract_named_region(self):
         # test for expected failures:
         with self.assertRaises(ValueError):
             extract_named_regions(region_cube, "reg_A")
+        with self.assertRaises(ValueError):
             extract_named_regions(region_cube, ["region1", "reg_A"])
 
+    def test_extract_named_region_invalid_regions_type_fail(self) -> None:
+        msg = r'Regions "123" is not an acceptable format.'
+        with pytest.raises(TypeError, match=re.escape(msg)):
+            extract_named_regions(Cube(0), 123)  # type: ignore[arg-type]
+        msg = r'Regions ".*" is not an acceptable format.'
+        with pytest.raises(TypeError, match=msg):
+            extract_named_regions(Cube(0), {"region1": "region2"})  # type: ignore[arg-type]
+
 
 def create_irregular_grid_cube(data, lons, lats):
     """Create test cube on irregular grid."""
diff --git a/tests/unit/preprocessor/_multimodel/test_multimodel.py b/tests/unit/preprocessor/_multimodel/test_multimodel.py
index 9d7614d100..223ff454c4 100644
--- a/tests/unit/preprocessor/_multimodel/test_multimodel.py
+++ b/tests/unit/preprocessor/_multimodel/test_multimodel.py
@@ -1,5 +1,6 @@
 """Unit test for :func:`esmvalcore.preprocessor._multimodel`."""
 
+import re
 from datetime import datetime
 from unittest import mock
 
@@ -1727,3 +1728,37 @@ def test_differing_multi_model_settings():
     msg = r"Unable to combine differing multi-dataset settings for a and b"
     with pytest.raises(ValueError, match=msg):
         _check_multi_model_settings(products)
+
+
+def test_multi_model_statistics_invalid_input_type_fail() -> None:
+    msg = r"Input type for multi_model_statistics not understood."
+    with pytest.raises(ValueError, match=re.escape(msg)):
+        mm.multi_model_statistics([0, 0], "full", [])
+
+
+def test_compute_eager_concatenate_fail() -> None:
+    """Test that ``_compute_eager`` raises on concatenation failure."""
+    # Use small cubes (no slicing) but mock concatenate_cube to fail
+    cubes = CubeList(generate_cube_from_dates("monthly") for _ in range(3))
+
+    # Create a mock result that will fail on concatenate
+    mock_result = mock.Mock()
+    mock_result.coords = mock.Mock(return_value=[])
+
+    # Test with concatenate_cube raising an exception
+    with mock.patch.object(
+        mm,
+        "_compute",
+        return_value=mock_result,
+    ):
+        with mock.patch.object(
+            iris.cube.CubeList,
+            "concatenate_cube",
+            side_effect=iris.exceptions.ConcatenateError("test error"),
+        ):
+            msg = (
+                r"Multi-model statistics failed to concatenate results into a "
+                r"single array. This happened for operator"
+            )
+            with pytest.raises(ValueError, match=re.escape(msg)):
+                mm._compute_eager(cubes, operator="mean")
diff --git a/tests/unit/preprocessor/_volume/test_volume.py b/tests/unit/preprocessor/_volume/test_volume.py
index 90a291209e..eda8682d8d 100644
--- a/tests/unit/preprocessor/_volume/test_volume.py
+++ b/tests/unit/preprocessor/_volume/test_volume.py
@@ -819,6 +819,15 @@ def test_extract_trajectory(self):
         expected = np.ones((3, 2))
         self.assert_array_equal(result.data, expected)
 
+    def test_extract_trajectory_different_lengths(self) -> None:
+        """Test that ValueError is raised when lat/lon have different lengths."""
+        with self.assertRaises(ValueError) as err:
+            extract_trajectory(self.grid_3d, [1.5, 2.5], [2.0], 2)
+        self.assertEqual(
+            "Longitude & Latitude coordinates have different lengths",
+            str(err.exception),
+        )
+
     def test__get_first_unmasked_data(self):
         """Test to get first unmasked value of an array along an axis."""
         (z_axis,) = self.grid_4d_2.coord_dims(
diff --git a/tests/unit/preprocessor/_weighting/test_weighting_landsea_fraction.py b/tests/unit/preprocessor/_weighting/test_weighting_landsea_fraction.py
index 269880c601..a2ededcedf 100644
--- a/tests/unit/preprocessor/_weighting/test_weighting_landsea_fraction.py
+++ b/tests/unit/preprocessor/_weighting/test_weighting_landsea_fraction.py
@@ -1,5 +1,7 @@
 """Unit tests for :mod:`esmvalcore.preprocessor._weighting`."""
 
+import re
+
 import iris
 import iris.fileformats
 import numpy as np
@@ -118,6 +120,12 @@ def test_get_land_fraction(cube, out, err):
 ]
 
 
+def test_weighting_landsea_fraction_invalid_area_type_fail() -> None:
+    msg = r"Expected 'land' or 'sea' for area_type"
+    with pytest.raises(TypeError, match=re.escape(msg)):
+        weighting.weighting_landsea_fraction(CUBE_3, "invalid")  # type: ignore[arg-type]
+
+
 @pytest.mark.parametrize(
     ("cube", "area_type", "out"),
     WEIGHTING_LANDSEA_FRACTION,
diff --git a/tests/unit/task/test_diagnostic_task.py b/tests/unit/task/test_diagnostic_task.py
index cb6047bc11..27cc661d48 100644
--- a/tests/unit/task/test_diagnostic_task.py
+++ b/tests/unit/task/test_diagnostic_task.py
@@ -1,7 +1,11 @@
+from __future__ import annotations
+
 import copy
 import logging
+import re
 import stat
 from pathlib import Path
+from typing import TYPE_CHECKING
 
 import pytest
 import yaml
@@ -10,6 +14,9 @@
 from esmvalcore._task import DiagnosticError, write_ncl_settings
 from esmvalcore.config._diagnostics import TagsManager
 
+if TYPE_CHECKING:
+    from pytest_mock import MockerFixture
+
 
 def test_write_ncl_settings(tmp_path):
     """Test minimally write_ncl_settings()."""
@@ -337,3 +344,55 @@ def test_collect_provenance_ancestor_hint(mocker, caplog, diagnostic_task):
             ["Valid ancestor files", "xyz.nc"],
         ],
     )
+
+
+def test_run_fails_with_nonzero_returncode(
+    mocker: MockerFixture,
+    tmp_path: Path,
+) -> None:
+    """Test that DiagnosticError is raised when script fails."""
+    mocker.patch.object(
+        esmvalcore._task.DiagnosticTask,
+        "_initialize_cmd",
+        autospec=True,
+    )
+    mocker.patch.object(
+        esmvalcore._task.DiagnosticTask,
+        "_initialize_env",
+        autospec=True,
+    )
+
+    settings = {
+        "run_dir": str(tmp_path / "run_dir"),
+        "plot_dir": str(tmp_path / "plot_dir"),
+        "work_dir": str(tmp_path / "work_dir"),
+        "profile_diagnostic": False,
+    }
+
+    task = esmvalcore._task.DiagnosticTask(
+        "test.py",
+        settings,
+        output_dir=str(tmp_path),
+        name="test-task",
+    )
+
+    # Mock subprocess.Popen to return a process with non-zero return code
+    mock_process = mocker.Mock()
+    mock_process.pid = 12345
+    mock_process.poll.return_value = (
+        1  # Non-zero return code indicates failure
+    )
+    mock_process.stdout.read.return_value = b"Error occurred\n"
+
+    mocker.patch.object(
+        esmvalcore._task.subprocess,
+        "Popen",
+        return_value=mock_process,
+    )
+
+    # Mock resource_usage_logger to avoid threading issues in tests
+    mocker.patch.object(esmvalcore._task, "resource_usage_logger")
+
+    msg = r"Diagnostic script test.py failed with return code 1"
+    with pytest.raises(DiagnosticError, match=re.escape(msg)):
+        task._run(input_files=[])
diff --git a/tests/unit/test_version.py b/tests/unit/test_version.py
index 3dca6d14ba..cb4d42cc9b 100644
--- a/tests/unit/test_version.py
+++ b/tests/unit/test_version.py
@@ -1,9 +1,30 @@
 """Test that esmvalcore.__version__ returns a version number."""
 
+from __future__ import annotations
+
+import importlib
 import re
+from importlib.metadata import PackageNotFoundError
+from typing import TYPE_CHECKING
+
+import pytest
 
 import esmvalcore
+import esmvalcore._version
+
+if TYPE_CHECKING:
+    from pytest_mock import MockerFixture
 
 
 def test_version():
     assert re.match(r"^\d+\.\d+\.\d+\S*$", esmvalcore.__version__)
+
+
+def test_version_package_not_found_fail(mocker: MockerFixture) -> None:
+    mocker.patch(
+        "importlib.metadata.version",
+        side_effect=PackageNotFoundError("ESMValCore"),
+    )
+    msg = r"ESMValCore package not found"
+    with pytest.raises(PackageNotFoundError, match=re.escape(msg)):
+        importlib.reload(esmvalcore._version)