diff --git a/tested/configs.py b/tested/configs.py
index 0cb0cc03a..b1be1f5da 100644
--- a/tested/configs.py
+++ b/tested/configs.py
@@ -190,14 +190,16 @@ def _get_language(config: DodonaConfig) -> tuple[SupportedLanguage, int]:
     bang = _consume_shebang(config.source)
     if bang and langs.language_exists(bang):
         _logger.debug(
-            f"Found shebang language and it exists, using {bang} instead "
-            f"of config language {config.programming_language}."
+            "Found shebang language and it exists, using %s instead of config language %s.",
+            bang,
+            config.programming_language,
         )
         return bang, 1
     else:
         _logger.debug(
-            f"No shebang found or it doesn't exist: {bang}. Using "
-            f"configuration language {config.programming_language}."
+            "No shebang found or it doesn't exist: %s. Using configuration language %s.",
+            bang,
+            config.programming_language,
         )
         return config.programming_language, 0
 
diff --git a/tested/internationalization/__init__.py b/tested/internationalization/__init__.py
index 918e7854e..a13943240 100644
--- a/tested/internationalization/__init__.py
+++ b/tested/internationalization/__init__.py
@@ -14,7 +14,7 @@
 
 
 def set_locale(locale: str):
-    _logger.debug(f"Change locale to {locale}")
+    _logger.debug("Change locale to %s", locale)
     i18n.set("locale", locale)
 
 
diff --git a/tested/judge/collector.py b/tested/judge/collector.py
index 35752fabf..10658532d 100644
--- a/tested/judge/collector.py
+++ b/tested/judge/collector.py
@@ -67,8 +67,8 @@ def add(self, command: Update, index: int | None = None):
         """
         assert not self.finalized, "OutputManager already finished!"
         action, type_ = command.command.split("-")
-        _logger.debug(f"Adding {command}")
-        _logger.debug(f"Stack is {self.open_stack}")
+        _logger.debug("Adding %s", command)
+        _logger.debug("Stack is %s", self.open_stack)
         if action == "start":
             self.open_stack.append(type_)
         elif action == "close":
@@ -86,7 +86,7 @@ def add(self, command: Update, index: int | None = None):
                 tabs, contexts, _ = self.currently_open
                 self.currently_open = (tabs, contexts, index + 1)
 
-        _logger.debug(f"After adding, stack is {self.open_stack}")
+        _logger.debug("After adding, stack is %s", self.open_stack)
         report_update(self.out, command)
 
     def terminate(
diff --git a/tested/judge/compilation.py b/tested/judge/compilation.py
index b079285eb..e6538208f 100644
--- a/tested/judge/compilation.py
+++ b/tested/judge/compilation.py
@@ -64,7 +64,7 @@ def run_compilation(
         "Generating files with command %s in directory %s", command, directory
     )
     result = run_command(directory, remaining, command)
-    _logger.debug(f"Compilation dependencies are: {files}")
+    _logger.debug("Compilation dependencies are: %s", files)
     return result, files
 
 
diff --git a/tested/judge/core.py b/tested/judge/core.py
index 24bdd3b31..6c54c9fb9 100644
--- a/tested/judge/core.py
+++ b/tested/judge/core.py
@@ -171,7 +171,7 @@ def _process_one_unit(
     else:
         max_workers = 1
 
-    _logger.debug(f"Executing with {max_workers} workers")
+    _logger.debug("Executing with %s workers", max_workers)
 
     with ThreadPoolExecutor(max_workers=max_workers) as executor:
         remaining_time = plan.remaining_time()
@@ -186,7 +186,7 @@ def _process_one_unit(
                 execution_dir,
             ) in enumerate(results):
                 planned_unit = plan.units[i]
-                _logger.debug(f"Processing results for execution unit {i}")
+                _logger.debug("Processing results for execution unit %s", i)
                 result_status, currently_open_tab = _process_results(
                     bundle=bundle,
                     unit=planned_unit,
@@ -259,7 +259,7 @@ def _generate_files(
     common_dir = Path(bundle.config.workdir, f"common")
     common_dir.mkdir()
 
-    _logger.debug(f"Generating files in common directory {common_dir}")
+    _logger.debug("Generating files in common directory %s", common_dir)
 
     # Copy dependencies
     dependency_paths = bundle.language.path_to_dependencies()
@@ -278,7 +278,7 @@ def _generate_files(
     execution_names = []
     # Generate the files for each execution.
     for execution_unit in execution_plan:
-        _logger.debug(f"Generating file for execution {execution_unit.name}")
+        _logger.debug("Generating file for execution %s", execution_unit.name)
         generated, evaluators = generate_execution(
             bundle=bundle, destination=common_dir, execution_unit=execution_unit
         )
@@ -286,7 +286,7 @@ def _generate_files(
         # Copy functions to the directory.
         for evaluator in evaluators:
             source = Path(bundle.config.resources) / evaluator
-            _logger.debug(f"Copying oracle from {source} to {common_dir}")
+            _logger.debug("Copying oracle from %s to %s", source, common_dir)
             shutil.copy2(source, common_dir)
 
         dependencies.extend(evaluators)
diff --git a/tested/judge/evaluation.py b/tested/judge/evaluation.py
index 44b2571ab..59b74ba36 100644
--- a/tested/judge/evaluation.py
+++ b/tested/judge/evaluation.py
@@ -246,7 +246,7 @@ def evaluate_context_results(
 
     # Begin processing the normal testcases.
     for i, testcase in enumerate(context.testcases):
-        _logger.debug(f"Evaluating testcase {i}")
+        _logger.debug("Evaluating testcase %s", i)
 
         readable_input, seen = get_readable_input(bundle, testcase)
         all_files = all_files - seen
diff --git a/tested/judge/execution.py b/tested/judge/execution.py
index c5ee6f42c..eb2c591d0 100644
--- a/tested/judge/execution.py
+++ b/tested/judge/execution.py
@@ -133,7 +133,7 @@ def execute_file(
         file=executable_name,
         arguments=[argument] if argument else [],
     )
-    _logger.debug(f"Executing {command} in directory {working_directory}")
+    _logger.debug("Executing %s in directory %s", command, working_directory)
 
     result = run_command(working_directory, remaining, command, stdin)
 
@@ -163,7 +163,7 @@ def set_up_unit(
     # Filter dependencies of the global compilation results.
     dependencies = filter_files(plan.files, plan.common_directory)
     dependencies = bundle.language.filter_dependencies(dependencies, unit.name)
-    _logger.debug(f"Dependencies are {dependencies}")
+    _logger.debug("Dependencies are %s", dependencies)
     copy_workdir_files(bundle, execution_dir, True)
 
     # Copy files from the common directory to the context directory.
@@ -172,7 +172,7 @@ def set_up_unit(
         destination = execution_dir / file
         # Ensure we preserve subdirectories.
         destination.parent.mkdir(parents=True, exist_ok=True)
-        _logger.debug(f"Copying {origin} to {destination}")
+        _logger.debug("Copying %s to %s", origin, destination)
         if origin == destination:
             continue  # Don't copy the file to itself
 
@@ -235,7 +235,7 @@ def execute_unit(
         argument = None
 
     executable_or_status = bundle.language.find_main_file(files, main_file_name)
-    _logger.debug(f"Searched for main file: {executable_or_status}")
+    _logger.debug("Searched for main file: %s", executable_or_status)
     if isinstance(executable_or_status, Status):
         return executable_or_status
 
diff --git a/tested/judge/utils.py b/tested/judge/utils.py
index c50a00b25..384bbdd39 100644
--- a/tested/judge/utils.py
+++ b/tested/judge/utils.py
@@ -123,14 +123,14 @@ def recursive_copy(src: Path, dst: Path):
                 all_files or bundle.language.is_source_file(origin)
             ):
                 source_files.append(str(dst / origin.name))
-                _logger.debug(f"Copying {origin} to {dst}")
+                _logger.debug("Copying %s to %s", origin, dst)
                 shutil.copy2(origin, dst)
             elif (
                 origin.is_dir()
                 and not file.startswith(EXECUTION_PREFIX)
                 and file != "common"
             ):
-                _logger.debug(f"Iterate subdir {dst / file}")
+                _logger.debug("Iterate subdir %s", dst / file)
                 shutil.copytree(origin, dst / file)
 
     recursive_copy(bundle.config.workdir, destination)
diff --git a/tested/oracles/programmed.py b/tested/oracles/programmed.py
index 5b76a6600..39ef669a5 100644
--- a/tested/oracles/programmed.py
+++ b/tested/oracles/programmed.py
@@ -219,7 +219,7 @@ def evaluate(
     assert isinstance(channel, OracleOutputChannel)
     assert isinstance(channel.oracle, CustomCheckOracle)
 
-    _logger.debug(f"Programmed oracle for output {actual_str}")
+    _logger.debug("Programmed oracle for output %s", actual_str)
 
     # Convert the expected item to a Value, which is then passed to the
     # oracle for evaluation.
@@ -241,9 +241,9 @@ def evaluate(
         )
 
     _logger.debug(
-        f"Calling programmed evaluation with params:\n"
-        f"expected: {expected}\n"
-        f"actual: {actual}"
+        "Calling programmed evaluation with params:\nexpected: %s\nactual: %s",
+        expected,
+        actual,
     )
     context = OracleContext(
         expected=expected,
diff --git a/tested/parsing.py b/tested/parsing.py
index 3b7f3d64f..9ecca27b9 100644
--- a/tested/parsing.py
+++ b/tested/parsing.py
@@ -38,12 +38,12 @@ def unstructure_decimal(obj: Decimal) -> str:
 def structure_every_union(to_convert: Any, the_type: type) -> Any:
     from tested.serialisation import Identifier
 
-    _logger.debug(f"=== Finding type for {to_convert}, from {the_type}...")
+    _logger.debug("=== Finding type for %s, from %s...", to_convert, the_type)
     if to_convert is None and type(None) in get_args(the_type):
         _logger.debug("Yes: found None.")
         return None
     if isinstance(to_convert, bool) and bool in get_args(the_type):
-        _logger.debug(f"Yes: found boolean: {to_convert}.")
+        _logger.debug("Yes: found boolean: %s.", to_convert)
         return to_convert
 
     for possible_class in get_args(the_type):
@@ -61,10 +61,10 @@ def structure_every_union(to_convert: Any, the_type: type) -> Any:
             if possible_class is Identifier:
                 assert isinstance(to_convert, str)
             result = _suite_converter.structure(to_convert, possible_class)
-            _logger.debug(f"{debug_message} accepted.")
+            _logger.debug("%s accepted.", debug_message)
             return result
         except Exception:
-            _logger.debug(f"{debug_message} rejected.")
+            _logger.debug("%s rejected.", debug_message)
     raise TypeError(
         f"{to_convert} could not be converted into a {the_type}. Check the syntax or file a bug."
     )