From 4cda4e5e705a1672dcbec4b81ba927dcdbcc7570 Mon Sep 17 00:00:00 2001
From: Twirre Meulenbelt <43213592+TwirreM@users.noreply.github.com>
Date: Thu, 15 Jan 2026 17:07:49 +0100
Subject: [PATCH 1/4] feat: experiment log stream, to file and UI

Adds a few new logging utility classes. One to save to files with a date, one to support optional fields in formats, last to filter partial log messages.

ref: N25B-401
---
 .gitignore                                    |   3 +
 .logging_config.yaml                          |  37 ++-
 src/control_backend/agents/base.py            |   3 +-
 src/control_backend/core/config.py            |  14 ++
 src/control_backend/logging/__init__.py       |   3 +
 .../logging/dated_file_handler.py             |  29 +++
 .../logging/optional_field_formatter.py       |  67 ++++++
 src/control_backend/logging/partial_filter.py |  10 +
 src/control_backend/logging/setup_logging.py  |   4 +-
 test/unit/logging/test_file_handler.py        |  18 ++
 .../logging/test_optional_field_formatter.py  | 218 ++++++++++++++++++
 test/unit/logging/test_partial_filter.py      |  83 +++++++
 12 files changed, 479 insertions(+), 10 deletions(-)
 create mode 100644 src/control_backend/logging/dated_file_handler.py
 create mode 100644 src/control_backend/logging/optional_field_formatter.py
 create mode 100644 src/control_backend/logging/partial_filter.py
 create mode 100644 test/unit/logging/test_file_handler.py
 create mode 100644 test/unit/logging/test_optional_field_formatter.py
 create mode 100644 test/unit/logging/test_partial_filter.py

diff --git a/.gitignore b/.gitignore
index f58719a..47ef46d 100644
--- a/.gitignore
+++ b/.gitignore
@@ -222,6 +222,9 @@ __marimo__/
 docs/*
 !docs/conf.py
 
+# Generated files
+experiment-*.log
+
 
 
 
diff --git a/.logging_config.yaml b/.logging_config.yaml
index 4af5d56..bf4c8c5 100644
--- a/.logging_config.yaml
+++ b/.logging_config.yaml
@@ -1,36 +1,56 @@
 version: 1
 
 custom_levels:
-  OBSERVATION: 25
-  ACTION: 26
+  OBSERVATION: 24
+  ACTION: 25
+  CHAT: 26
   LLM: 9
 
 formatters:
   # Console output
   colored:
-    (): "colorlog.ColoredFormatter"
+    class: colorlog.ColoredFormatter
     format: "{log_color}{asctime}.{msecs:03.0f} | {levelname:11} | {name:70} | {message}"
     style: "{"
     datefmt: "%H:%M:%S"
 
   # User-facing UI (structured JSON)
-  json_experiment:
-    (): "pythonjsonlogger.jsonlogger.JsonFormatter"
+  json:
+    class: pythonjsonlogger.jsonlogger.JsonFormatter
     format: "{name} {levelname} {levelno} {message} {created} {relativeCreated}"
     style: "{"
 
+  # Experiment stream for console and file output, with optional `role` field
+  experiment:
+    class: control_backend.logging.OptionalFieldFormatter
+    format: "%(asctime)s %(levelname)s %(role?)s %(message)s"
+    defaults:
+      role: "-"
+
+filters:
+  # Filter out any log records that have the extra field "partial" set to True, indicating that they
+  #  will be replaced later.
+  partial:
+    (): control_backend.logging.PartialFilter
+
 handlers:
   console:
     class: logging.StreamHandler
     level: DEBUG
     formatter: colored
+    filters: [partial]
     stream: ext://sys.stdout
   ui:
     class: zmq.log.handlers.PUBHandler
     level: LLM
-    formatter: json_experiment
+    formatter: json
+  file:
+    class: control_backend.logging.DatedFileHandler
+    formatter: experiment
+    filters: [partial]
+    file_prefix: experiment_logs/experiment
 
-# Level of external libraries
+# Level for external libraries
 root:
   level: WARN
   handlers: [console]
@@ -39,3 +59,6 @@ loggers:
   control_backend:
     level: LLM
     handlers: [ui]
+  experiment:
+    level: DEBUG
+    handlers: [ui, file]
diff --git a/src/control_backend/agents/base.py b/src/control_backend/agents/base.py
index ec50af5..beccdaa 100644
--- a/src/control_backend/agents/base.py
+++ b/src/control_backend/agents/base.py
@@ -1,9 +1,10 @@
 import logging
+from abc import ABC
 
 from control_backend.core.agent_system import BaseAgent as CoreBaseAgent
 
 
-class BaseAgent(CoreBaseAgent):
+class BaseAgent(CoreBaseAgent, ABC):
     """
     The primary base class for all implementation agents.
 
diff --git a/src/control_backend/core/config.py b/src/control_backend/core/config.py
index c4a4db7..c8af094 100644
--- a/src/control_backend/core/config.py
+++ b/src/control_backend/core/config.py
@@ -142,6 +142,18 @@ class SpeechModelSettings(BaseModel):
     openai_model_name: str = "small.en"
 
 
+class LoggingSettings(BaseModel):
+    """
+    Configuration for logging.
+
+    :ivar logging_config_file: Path to the logging configuration file.
+    :ivar experiment_logger_name: Name of the experiment logger, should match the logging config.
+    """
+
+    logging_config_file: str = ".logging_config.yaml"
+    experiment_logger_name: str = "experiment"
+
+
 class Settings(BaseSettings):
     """
     Global application settings.
@@ -163,6 +175,8 @@ class Settings(BaseSettings):
 
     ri_host: str = "localhost"
 
+    logging_settings: LoggingSettings = LoggingSettings()
+
     zmq_settings: ZMQSettings = ZMQSettings()
 
     agent_settings: AgentSettings = AgentSettings()
diff --git a/src/control_backend/logging/__init__.py b/src/control_backend/logging/__init__.py
index c97af40..a08e9b8 100644
--- a/src/control_backend/logging/__init__.py
+++ b/src/control_backend/logging/__init__.py
@@ -1 +1,4 @@
+from .dated_file_handler import DatedFileHandler as DatedFileHandler
+from .optional_field_formatter import OptionalFieldFormatter as OptionalFieldFormatter
+from .partial_filter import PartialFilter as PartialFilter
 from .setup_logging import setup_logging as setup_logging
diff --git a/src/control_backend/logging/dated_file_handler.py b/src/control_backend/logging/dated_file_handler.py
new file mode 100644
index 0000000..b927f9d
--- /dev/null
+++ b/src/control_backend/logging/dated_file_handler.py
@@ -0,0 +1,29 @@
+from datetime import datetime
+from logging import FileHandler
+from pathlib import Path
+
+
+class DatedFileHandler(FileHandler):
+    def __init__(self, file_prefix: str, **kwargs):
+        if not file_prefix:
+            raise ValueError("`file_prefix` argument cannot be empty.")
+        self._file_prefix = file_prefix
+        kwargs["filename"] = self._make_filename()
+        super().__init__(**kwargs)
+
+    def _make_filename(self) -> str:
+        filepath = Path(f"{self._file_prefix}-{datetime.now():%Y%m%d-%H%M%S}.log")
+        if not filepath.parent.is_dir():
+            filepath.parent.mkdir(parents=True, exist_ok=True)
+        return str(filepath)
+
+    def do_rollover(self):
+        self.acquire()
+        try:
+            if self.stream:
+                self.stream.close()
+
+            self.baseFilename = self._make_filename()
+            self.stream = self._open()
+        finally:
+            self.release()
diff --git a/src/control_backend/logging/optional_field_formatter.py b/src/control_backend/logging/optional_field_formatter.py
new file mode 100644
index 0000000..886e9a4
--- /dev/null
+++ b/src/control_backend/logging/optional_field_formatter.py
@@ -0,0 +1,67 @@
+import logging
+import re
+
+
+class OptionalFieldFormatter(logging.Formatter):
+    """
+    Logging formatter that supports optional fields marked by `?`.
+
+    Optional fields are denoted by placing a `?` after the field name inside
+    the parentheses, e.g., `%(role?)s`. If the field is not provided in the
+    log call's `extra` dict, it will use the default value from `defaults`
+    or `None` if no default is specified.
+
+    :param fmt: Format string with optional `%(name?)s` style fields.
+    :type fmt: str or None
+    :param datefmt: Date format string, passed to parent :class:`logging.Formatter`.
+    :type datefmt: str or None
+    :param style: Formatting style, must be '%'. Passed to parent.
+    :type style: str
+    :param defaults: Default values for optional fields when not provided.
+    :type defaults: dict or None
+
+    :example:
+
+        >>> formatter = OptionalFieldFormatter(
+        ...     fmt="%(asctime)s %(levelname)s %(role?)s %(message)s",
+        ...     defaults={"role": ""-""}
+        ... )
+        >>> handler = logging.StreamHandler()
+        >>> handler.setFormatter(formatter)
+        >>> logger = logging.getLogger(__name__)
+        >>> logger.addHandler(handler)
+        >>>
+        >>> logger.chat("Hello there!", extra={"role": "USER"})
+        2025-01-15 10:30:00 CHAT USER Hello there!
+        >>>
+        >>> logger.info("A logging message")
+        2025-01-15 10:30:01 INFO - A logging message
+
+    .. note::
+        Only `%`-style formatting is supported. The `{` and `$` styles are not
+        compatible with this formatter.
+
+    .. seealso::
+        :class:`logging.Formatter` for base formatter documentation.
+    """
+
+    # Match %(name?)s or %(name?)d etc.
+    OPTIONAL_PATTERN = re.compile(r"%\((\w+)\?\)([sdifFeEgGxXocrba%])")
+
+    def __init__(self, fmt=None, datefmt=None, style="%", defaults=None):
+        self.defaults = defaults or {}
+
+        self.optional_fields = set(self.OPTIONAL_PATTERN.findall(fmt or ""))
+
+        # Convert %(name?)s to %(name)s for standard formatting
+        normalized_fmt = self.OPTIONAL_PATTERN.sub(r"%(\1)\2", fmt or "")
+
+        super().__init__(normalized_fmt, datefmt, style)
+
+    def format(self, record):
+        for field, _ in self.optional_fields:
+            if not hasattr(record, field):
+                default = self.defaults.get(field, None)
+                setattr(record, field, default)
+
+        return super().format(record)
diff --git a/src/control_backend/logging/partial_filter.py b/src/control_backend/logging/partial_filter.py
new file mode 100644
index 0000000..1b121cb
--- /dev/null
+++ b/src/control_backend/logging/partial_filter.py
@@ -0,0 +1,10 @@
+import logging
+
+
+class PartialFilter(logging.Filter):
+    """
+    Class to filter any log records that have the "partial" attribute set to ``True``.
+    """
+
+    def filter(self, record):
+        return getattr(record, "partial", False) is not True
diff --git a/src/control_backend/logging/setup_logging.py b/src/control_backend/logging/setup_logging.py
index 05ae85a..7147dcc 100644
--- a/src/control_backend/logging/setup_logging.py
+++ b/src/control_backend/logging/setup_logging.py
@@ -37,7 +37,7 @@ def add_logging_level(level_name: str, level_num: int, method_name: str | None =
     setattr(logging, method_name, log_to_root)
 
 
-def setup_logging(path: str = ".logging_config.yaml") -> None:
+def setup_logging(path: str = settings.logging_settings.logging_config_file) -> None:
     """
     Setup logging configuration of the CB. Tries to load the logging configuration from a file,
     in which we specify custom loggers, formatters, handlers, etc.
@@ -65,7 +65,7 @@ def setup_logging(path: str = ".logging_config.yaml") -> None:
 
             # Patch ZMQ PUBHandler to know about custom levels
             if custom_levels:
-                for logger_name in ("control_backend",):
+                for logger_name in config.get("loggers", {}):
                     logger = logging.getLogger(logger_name)
                     for handler in logger.handlers:
                         if isinstance(handler, PUBHandler):
diff --git a/test/unit/logging/test_file_handler.py b/test/unit/logging/test_file_handler.py
new file mode 100644
index 0000000..9d1ee90
--- /dev/null
+++ b/test/unit/logging/test_file_handler.py
@@ -0,0 +1,18 @@
+from unittest.mock import MagicMock, patch
+
+from control_backend.logging.dated_file_handler import DatedFileHandler
+
+
+@patch("control_backend.logging.file_handler.DatedFileHandler._open")
+def test_reset(open_):
+    stream = MagicMock()
+    open_.return_value = stream
+
+    # A file should be opened when the logger is created
+    handler = DatedFileHandler(prefix="anything")
+    assert open_.call_count == 1
+
+    # Upon reset, the current file should be closed, and a new one should be opened
+    handler.do_rollover()
+    assert stream.close.call_count == 1
+    assert open_.call_count == 2
diff --git a/test/unit/logging/test_optional_field_formatter.py b/test/unit/logging/test_optional_field_formatter.py
new file mode 100644
index 0000000..ae75bd9
--- /dev/null
+++ b/test/unit/logging/test_optional_field_formatter.py
@@ -0,0 +1,218 @@
+import logging
+
+import pytest
+
+from control_backend.logging.optional_field_formatter import OptionalFieldFormatter
+
+
+@pytest.fixture
+def logger():
+    """Create a fresh logger for each test."""
+    logger = logging.getLogger(f"test_{id(object())}")
+    logger.setLevel(logging.DEBUG)
+    logger.handlers = []
+    return logger
+
+
+@pytest.fixture
+def log_output(logger):
+    """Capture log output and return a function to get it."""
+
+    class ListHandler(logging.Handler):
+        def __init__(self):
+            super().__init__()
+            self.records = []
+
+        def emit(self, record):
+            self.records.append(self.format(record))
+
+    handler = ListHandler()
+    logger.addHandler(handler)
+
+    def get_output():
+        return handler.records
+
+    return get_output
+
+
+def test_optional_field_present(logger, log_output):
+    """Optional field should appear when provided in extra."""
+    formatter = OptionalFieldFormatter("%(levelname)s - %(role?)s - %(message)s")
+    logger.handlers[0].setFormatter(formatter)
+
+    logger.info("test message", extra={"role": "user"})
+
+    assert log_output() == ["INFO - user - test message"]
+
+
+def test_optional_field_missing_no_default(logger, log_output):
+    """Missing optional field with no default should be None."""
+    formatter = OptionalFieldFormatter("%(levelname)s - %(role?)s - %(message)s")
+    logger.handlers[0].setFormatter(formatter)
+
+    logger.info("test message")
+
+    assert log_output() == ["INFO - None - test message"]
+
+
+def test_optional_field_missing_with_default(logger, log_output):
+    """Missing optional field should use provided default."""
+    formatter = OptionalFieldFormatter(
+        "%(levelname)s - %(role?)s - %(message)s", defaults={"role": "assistant"}
+    )
+    logger.handlers[0].setFormatter(formatter)
+
+    logger.info("test message")
+
+    assert log_output() == ["INFO - assistant - test message"]
+
+
+def test_optional_field_overrides_default(logger, log_output):
+    """Provided extra value should override default."""
+    formatter = OptionalFieldFormatter(
+        "%(levelname)s - %(role?)s - %(message)s", defaults={"role": "assistant"}
+    )
+    logger.handlers[0].setFormatter(formatter)
+
+    logger.info("test message", extra={"role": "user"})
+
+    assert log_output() == ["INFO - user - test message"]
+
+
+def test_multiple_optional_fields(logger, log_output):
+    """Multiple optional fields should work independently."""
+    formatter = OptionalFieldFormatter(
+        "%(levelname)s - %(role?)s - %(request_id?)s - %(message)s", defaults={"role": "assistant"}
+    )
+    logger.handlers[0].setFormatter(formatter)
+
+    logger.info("test", extra={"request_id": "123"})
+
+    assert log_output() == ["INFO - assistant - 123 - test"]
+
+
+def test_mixed_optional_and_required_fields(logger, log_output):
+    """Standard fields should work alongside optional fields."""
+    formatter = OptionalFieldFormatter("%(levelname)s %(name)s %(role?)s %(message)s")
+    logger.handlers[0].setFormatter(formatter)
+
+    logger.info("test", extra={"role": "user"})
+
+    output = log_output()[0]
+    assert "INFO" in output
+    assert "user" in output
+    assert "test" in output
+
+
+def test_no_optional_fields(logger, log_output):
+    """Formatter should work normally with no optional fields."""
+    formatter = OptionalFieldFormatter("%(levelname)s %(message)s")
+    logger.handlers[0].setFormatter(formatter)
+
+    logger.info("test message")
+
+    assert log_output() == ["INFO test message"]
+
+
+def test_integer_format_specifier(logger, log_output):
+    """Optional fields with %d specifier should work."""
+    formatter = OptionalFieldFormatter(
+        "%(levelname)s %(count?)d %(message)s", defaults={"count": 0}
+    )
+    logger.handlers[0].setFormatter(formatter)
+
+    logger.info("test", extra={"count": 42})
+
+    assert log_output() == ["INFO 42 test"]
+
+
+def test_float_format_specifier(logger, log_output):
+    """Optional fields with %f specifier should work."""
+    formatter = OptionalFieldFormatter(
+        "%(levelname)s %(duration?)f %(message)s", defaults={"duration": 0.0}
+    )
+    logger.handlers[0].setFormatter(formatter)
+
+    logger.info("test", extra={"duration": 1.5})
+
+    assert "1.5" in log_output()[0]
+
+
+def test_empty_string_default(logger, log_output):
+    """Empty string default should work."""
+    formatter = OptionalFieldFormatter("%(levelname)s %(role?)s %(message)s", defaults={"role": ""})
+    logger.handlers[0].setFormatter(formatter)
+
+    logger.info("test")
+
+    assert log_output() == ["INFO  test"]
+
+
+def test_none_format_string():
+    """None format string should not raise."""
+    formatter = OptionalFieldFormatter(fmt=None)
+    assert formatter.optional_fields == set()
+
+
+def test_optional_fields_parsed_correctly():
+    """Check that optional fields are correctly identified."""
+    formatter = OptionalFieldFormatter("%(asctime)s %(role?)s %(level?)d %(name)s")
+
+    assert formatter.optional_fields == {("role", "s"), ("level", "d")}
+
+
+def test_format_string_normalized():
+    """Check that ? is removed from format string."""
+    formatter = OptionalFieldFormatter("%(role?)s %(message)s")
+
+    assert "?" not in formatter._style._fmt
+    assert "%(role)s" in formatter._style._fmt
+
+
+def test_field_with_underscore(logger, log_output):
+    """Field names with underscores should work."""
+    formatter = OptionalFieldFormatter("%(levelname)s %(user_id?)s %(message)s")
+    logger.handlers[0].setFormatter(formatter)
+
+    logger.info("test", extra={"user_id": "abc123"})
+
+    assert log_output() == ["INFO abc123 test"]
+
+
+def test_field_with_numbers(logger, log_output):
+    """Field names with numbers should work."""
+    formatter = OptionalFieldFormatter("%(levelname)s %(field2?)s %(message)s")
+    logger.handlers[0].setFormatter(formatter)
+
+    logger.info("test", extra={"field2": "value"})
+
+    assert log_output() == ["INFO value test"]
+
+
+def test_multiple_log_calls(logger, log_output):
+    """Formatter should work correctly across multiple log calls."""
+    formatter = OptionalFieldFormatter(
+        "%(levelname)s %(role?)s %(message)s", defaults={"role": "other"}
+    )
+    logger.handlers[0].setFormatter(formatter)
+
+    logger.info("first", extra={"role": "assistant"})
+    logger.info("second")
+    logger.info("third", extra={"role": "user"})
+
+    assert log_output() == [
+        "INFO assistant first",
+        "INFO other second",
+        "INFO user third",
+    ]
+
+
+def test_default_not_mutated(logger, log_output):
+    """Original defaults dict should not be mutated."""
+    defaults = {"role": "other"}
+    formatter = OptionalFieldFormatter("%(levelname)s %(role?)s %(message)s", defaults=defaults)
+    logger.handlers[0].setFormatter(formatter)
+
+    logger.info("test")
+
+    assert defaults == {"role": "other"}
diff --git a/test/unit/logging/test_partial_filter.py b/test/unit/logging/test_partial_filter.py
new file mode 100644
index 0000000..bd5ef10
--- /dev/null
+++ b/test/unit/logging/test_partial_filter.py
@@ -0,0 +1,83 @@
+import logging
+
+import pytest
+
+from control_backend.logging import PartialFilter
+
+
+@pytest.fixture
+def logger():
+    """Create a fresh logger for each test."""
+    logger = logging.getLogger(f"test_{id(object())}")
+    logger.setLevel(logging.DEBUG)
+    logger.handlers = []
+    return logger
+
+
+@pytest.fixture
+def log_output(logger):
+    """Capture log output and return a function to get it."""
+
+    class ListHandler(logging.Handler):
+        def __init__(self):
+            super().__init__()
+            self.records = []
+
+        def emit(self, record):
+            self.records.append(self.format(record))
+
+    handler = ListHandler()
+    handler.addFilter(PartialFilter())
+    handler.setFormatter(logging.Formatter("%(message)s"))
+    logger.addHandler(handler)
+
+    return lambda: handler.records
+
+
+def test_no_partial_attribute(logger, log_output):
+    """Records without partial attribute should pass through."""
+    logger.info("normal message")
+
+    assert log_output() == ["normal message"]
+
+
+def test_partial_true_filtered(logger, log_output):
+    """Records with partial=True should be filtered out."""
+    logger.info("partial message", extra={"partial": True})
+
+    assert log_output() == []
+
+
+def test_partial_false_passes(logger, log_output):
+    """Records with partial=False should pass through."""
+    logger.info("complete message", extra={"partial": False})
+
+    assert log_output() == ["complete message"]
+
+
+def test_partial_none_passes(logger, log_output):
+    """Records with partial=None should pass through."""
+    logger.info("message", extra={"partial": None})
+
+    assert log_output() == ["message"]
+
+
+def test_partial_truthy_value_passes(logger, log_output):
+    """
+    Records with truthy but non-True partial should pass through, that is, only when it's exactly
+    ``True`` should it pass.
+    """
+    logger.info("message", extra={"partial": "yes"})
+
+    assert log_output() == ["message"]
+
+
+def test_multiple_records_mixed(logger, log_output):
+    """Filter should handle mixed records correctly."""
+    logger.info("first")
+    logger.info("second", extra={"partial": True})
+    logger.info("third", extra={"partial": False})
+    logger.info("fourth", extra={"partial": True})
+    logger.info("fifth")
+
+    assert log_output() == ["first", "third", "fifth"]

From ba79d09c5d2ca3a449ba56c9aa0293029e91d6f2 Mon Sep 17 00:00:00 2001
From: Twirre Meulenbelt <43213592+TwirreM@users.noreply.github.com>
Date: Fri, 16 Jan 2026 16:32:51 +0100
Subject: [PATCH 2/4] feat: log download endpoints

ref: N25B-401
---
 .logging_config.yaml                         |  3 +-
 src/control_backend/api/v1/endpoints/logs.py | 31 ++++++++++++++++++--
 src/control_backend/core/config.py           |  4 ++-
 3 files changed, 34 insertions(+), 4 deletions(-)

diff --git a/.logging_config.yaml b/.logging_config.yaml
index bf4c8c5..f7cccf9 100644
--- a/.logging_config.yaml
+++ b/.logging_config.yaml
@@ -48,6 +48,7 @@ handlers:
     class: control_backend.logging.DatedFileHandler
     formatter: experiment
     filters: [partial]
+    # Directory must match config.logging_settings.experiment_log_directory
     file_prefix: experiment_logs/experiment
 
 # Level for external libraries
@@ -59,6 +60,6 @@ loggers:
   control_backend:
     level: LLM
     handlers: [ui]
-  experiment:
+  experiment:  # This name must match config.logging_settings.experiment_logger_name
     level: DEBUG
     handlers: [ui, file]
diff --git a/src/control_backend/api/v1/endpoints/logs.py b/src/control_backend/api/v1/endpoints/logs.py
index ccccf44..0e2dff9 100644
--- a/src/control_backend/api/v1/endpoints/logs.py
+++ b/src/control_backend/api/v1/endpoints/logs.py
@@ -1,8 +1,9 @@
 import logging
+from pathlib import Path
 
 import zmq
-from fastapi import APIRouter
-from fastapi.responses import StreamingResponse
+from fastapi import APIRouter, HTTPException
+from fastapi.responses import FileResponse, StreamingResponse
 from zmq.asyncio import Context
 
 from control_backend.core.config import settings
@@ -38,3 +39,29 @@ async def log_stream():
             yield f"data: {message}\n\n"
 
     return StreamingResponse(gen(), media_type="text/event-stream")
+
+
+LOGGING_DIR = Path(settings.logging_settings.experiment_log_directory).resolve()
+
+
+@router.get("/logs/files")
+@router.get("/api/logs/files")
+async def log_directory():
+    """
+    Get a list of all log files stored in the experiment log file directory.
+    """
+    return [f.name for f in LOGGING_DIR.glob("*.log")]
+
+
+@router.get("/logs/files/{filename}")
+@router.get("/api/logs/files/{filename}")
+async def log_file(filename: str):
+    # Prevent path-traversal
+    file_path = (LOGGING_DIR / filename).resolve()  # This .resolve() is important
+    if not file_path.is_relative_to(LOGGING_DIR):
+        raise HTTPException(status_code=400, detail="Invalid filename.")
+
+    if not file_path.is_file():
+        raise HTTPException(status_code=404, detail="File not found.")
+
+    return FileResponse(file_path, filename=file_path.name)
diff --git a/src/control_backend/core/config.py b/src/control_backend/core/config.py
index c8af094..2dbde02 100644
--- a/src/control_backend/core/config.py
+++ b/src/control_backend/core/config.py
@@ -147,10 +147,12 @@ class LoggingSettings(BaseModel):
     Configuration for logging.
 
     :ivar logging_config_file: Path to the logging configuration file.
-    :ivar experiment_logger_name: Name of the experiment logger, should match the logging config.
+    :ivar experiment_log_directory: Location of the experiment logs. Must match the logging config.
+    :ivar experiment_logger_name: Name of the experiment logger. Must match the logging config.
     """
 
     logging_config_file: str = ".logging_config.yaml"
+    experiment_log_directory: str = "experiment_logs"
     experiment_logger_name: str = "experiment"
 
 

From 58881b5914fb7bbf6d5e7caccb4172e81888c3c6 Mon Sep 17 00:00:00 2001
From: Twirre Meulenbelt <43213592+TwirreM@users.noreply.github.com>
Date: Mon, 19 Jan 2026 12:47:59 +0100
Subject: [PATCH 3/4] test: add test cases

ref: N25B-401
---
 .../api/v1/endpoints/test_logs_endpoint.py    | 68 ++++++++++++++++++-
 test/unit/logging/test_dated_file_handler.py  | 45 ++++++++++++
 test/unit/logging/test_file_handler.py        | 18 -----
 3 files changed, 111 insertions(+), 20 deletions(-)
 create mode 100644 test/unit/logging/test_dated_file_handler.py
 delete mode 100644 test/unit/logging/test_file_handler.py

diff --git a/test/unit/api/v1/endpoints/test_logs_endpoint.py b/test/unit/api/v1/endpoints/test_logs_endpoint.py
index 50ee740..4aaa90e 100644
--- a/test/unit/api/v1/endpoints/test_logs_endpoint.py
+++ b/test/unit/api/v1/endpoints/test_logs_endpoint.py
@@ -1,7 +1,7 @@
-from unittest.mock import patch
+from unittest.mock import MagicMock, patch
 
 import pytest
-from fastapi import FastAPI
+from fastapi import FastAPI, HTTPException
 from fastapi.testclient import TestClient
 from starlette.responses import StreamingResponse
 
@@ -61,3 +61,67 @@ async def test_log_stream_endpoint_lines(client):
         # Optional: assert subscribe/connect were called
         assert dummy_socket.subscribed  # at least some log levels subscribed
         assert dummy_socket.connected  # connect was called
+
+
+@patch("control_backend.api.v1.endpoints.logs.LOGGING_DIR")
+def test_files_endpoint(LOGGING_DIR, client):
+    file_1, file_2 = MagicMock(), MagicMock()
+    file_1.name = "file_1"
+    file_2.name = "file_2"
+    LOGGING_DIR.glob.return_value = [file_1, file_2]
+    result = client.get("/api/logs/files")
+
+    assert result.status_code == 200
+    assert result.json() == ["file_1", "file_2"]
+
+
+@patch("control_backend.api.v1.endpoints.logs.FileResponse")
+@patch("control_backend.api.v1.endpoints.logs.LOGGING_DIR")
+def test_log_file_endpoint_success(LOGGING_DIR, MockFileResponse, client):
+    mock_file_path = MagicMock()
+    mock_file_path.is_relative_to.return_value = True
+    mock_file_path.is_file.return_value = True
+    mock_file_path.name = "test.log"
+
+    LOGGING_DIR.__truediv__ = MagicMock(return_value=mock_file_path)
+    mock_file_path.resolve.return_value = mock_file_path
+
+    MockFileResponse.return_value = MagicMock()
+
+    result = client.get("/api/logs/files/test.log")
+
+    assert result.status_code == 200
+    MockFileResponse.assert_called_once_with(mock_file_path, filename="test.log")
+
+
+@pytest.mark.asyncio
+@patch("control_backend.api.v1.endpoints.logs.LOGGING_DIR")
+async def test_log_file_endpoint_path_traversal(LOGGING_DIR):
+    from control_backend.api.v1.endpoints.logs import log_file
+
+    mock_file_path = MagicMock()
+    mock_file_path.is_relative_to.return_value = False
+
+    LOGGING_DIR.__truediv__ = MagicMock(return_value=mock_file_path)
+    mock_file_path.resolve.return_value = mock_file_path
+
+    with pytest.raises(HTTPException) as exc_info:
+        await log_file("../secret.txt")
+
+    assert exc_info.value.status_code == 400
+    assert exc_info.value.detail == "Invalid filename."
+
+
+@patch("control_backend.api.v1.endpoints.logs.LOGGING_DIR")
+def test_log_file_endpoint_file_not_found(LOGGING_DIR, client):
+    mock_file_path = MagicMock()
+    mock_file_path.is_relative_to.return_value = True
+    mock_file_path.is_file.return_value = False
+
+    LOGGING_DIR.__truediv__ = MagicMock(return_value=mock_file_path)
+    mock_file_path.resolve.return_value = mock_file_path
+
+    result = client.get("/api/logs/files/nonexistent.log")
+
+    assert result.status_code == 404
+    assert result.json()["detail"] == "File not found."
diff --git a/test/unit/logging/test_dated_file_handler.py b/test/unit/logging/test_dated_file_handler.py
new file mode 100644
index 0000000..14809fb
--- /dev/null
+++ b/test/unit/logging/test_dated_file_handler.py
@@ -0,0 +1,45 @@
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+from control_backend.logging.dated_file_handler import DatedFileHandler
+
+
+@patch("control_backend.logging.dated_file_handler.DatedFileHandler._open")
+def test_reset(open_):
+    stream = MagicMock()
+    open_.return_value = stream
+
+    # A file should be opened when the logger is created
+    handler = DatedFileHandler(file_prefix="anything")
+    assert open_.call_count == 1
+
+    # Upon reset, the current file should be closed, and a new one should be opened
+    handler.do_rollover()
+    assert stream.close.call_count == 1
+    assert open_.call_count == 2
+
+
+@patch("control_backend.logging.dated_file_handler.Path")
+@patch("control_backend.logging.dated_file_handler.DatedFileHandler._open")
+def test_creates_dir(open_, Path_):
+    stream = MagicMock()
+    open_.return_value = stream
+
+    test_path = MagicMock()
+    test_path.parent.is_dir.return_value = False
+    Path_.return_value = test_path
+
+    DatedFileHandler(file_prefix="anything")
+
+    # The directory should've been created
+    test_path.parent.mkdir.assert_called_once()
+
+
+@patch("control_backend.logging.dated_file_handler.DatedFileHandler._open")
+def test_invalid_constructor(_):
+    with pytest.raises(ValueError):
+        DatedFileHandler(file_prefix=None)
+
+    with pytest.raises(ValueError):
+        DatedFileHandler(file_prefix="")
diff --git a/test/unit/logging/test_file_handler.py b/test/unit/logging/test_file_handler.py
deleted file mode 100644
index 9d1ee90..0000000
--- a/test/unit/logging/test_file_handler.py
+++ /dev/null
@@ -1,18 +0,0 @@
-from unittest.mock import MagicMock, patch
-
-from control_backend.logging.dated_file_handler import DatedFileHandler
-
-
-@patch("control_backend.logging.file_handler.DatedFileHandler._open")
-def test_reset(open_):
-    stream = MagicMock()
-    open_.return_value = stream
-
-    # A file should be opened when the logger is created
-    handler = DatedFileHandler(prefix="anything")
-    assert open_.call_count == 1
-
-    # Upon reset, the current file should be closed, and a new one should be opened
-    handler.do_rollover()
-    assert stream.close.call_count == 1
-    assert open_.call_count == 2

From a74ecc6c4549a645402303a3e5ea9f6787211e78 Mon Sep 17 00:00:00 2001
From: Twirre Meulenbelt <43213592+TwirreM@users.noreply.github.com>
Date: Thu, 22 Jan 2026 11:48:02 +0100
Subject: [PATCH 4/4] docs: add docstrings to dated file handler

ref: N25B-401
---
 src/control_backend/logging/dated_file_handler.py | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/src/control_backend/logging/dated_file_handler.py b/src/control_backend/logging/dated_file_handler.py
index b927f9d..3a405bb 100644
--- a/src/control_backend/logging/dated_file_handler.py
+++ b/src/control_backend/logging/dated_file_handler.py
@@ -12,12 +12,21 @@ class DatedFileHandler(FileHandler):
         super().__init__(**kwargs)
 
     def _make_filename(self) -> str:
+        """
+        Create the filename for the current logfile, using the configured file prefix and the
+        current date and time. If the directory does not exist, it gets created.
+
+        :return: A filepath.
+        """
         filepath = Path(f"{self._file_prefix}-{datetime.now():%Y%m%d-%H%M%S}.log")
         if not filepath.parent.is_dir():
             filepath.parent.mkdir(parents=True, exist_ok=True)
         return str(filepath)
 
     def do_rollover(self):
+        """
+        Close the current logfile and create a new one with the current date and time.
+        """
         self.acquire()
         try:
             if self.stream: