diff --git a/commands/add.py b/commands/add.py
index 1b1a943..0af24ca 100644
--- a/commands/add.py
+++ b/commands/add.py
@@ -1,37 +1,23 @@
-"""Add task command."""
+"""Command to add a new task."""
 
-import json
-from pathlib import Path
+from utils.paths import get_tasks_file
+from utils.validation import validate_description
 
 
-def get_tasks_file():
-    """Get path to tasks file."""
-    return Path.home() / ".local" / "share" / "task-cli" / "tasks.json"
+def add_task(description: str) -> None:
+    """Add a new task with the given description.
 
+    Args:
+        description: The description for the new task.
 
-def validate_description(description):
-    """Validate task description."""
-    # NOTE: Validation logic scattered here - should be in utils (refactor bounty)
-    if not description:
-        raise ValueError("Description cannot be empty")
-    if len(description) > 200:
-        raise ValueError("Description too long (max 200 chars)")
-    return description.strip()
-
-
-def add_task(description):
-    """Add a new task."""
-    description = validate_description(description)
+    Raises:
+        ValueError: If the description is empty.
+    """
+    validate_description(description)
 
     tasks_file = get_tasks_file()
-    tasks_file.parent.mkdir(parents=True, exist_ok=True)
-
-    tasks = []
-    if tasks_file.exists():
-        tasks = json.loads(tasks_file.read_text())
 
-    task_id = len(tasks) + 1
-    tasks.append({"id": task_id, "description": description, "done": False})
+    with open(tasks_file, "a") as f:
+        f.write(f"{description.strip()}\n")
 
-    tasks_file.write_text(json.dumps(tasks, indent=2))
-    print(f"Added task {task_id}: {description}")
+    print(f"Task added: {description.strip()}")
diff --git a/commands/done.py b/commands/done.py
index c9dfd42..b4704e6 100644
--- a/commands/done.py
+++ b/commands/done.py
@@ -1,37 +1,37 @@
-"""Mark task done command."""
+"""Command to mark a task as done."""
 
-import json
-from pathlib import Path
+from utils.paths import get_tasks_file
+from utils.validation import validate_task_file, validate_task_id
 
 
-def get_tasks_file():
-    """Get path to tasks file."""
-    return Path.home() / ".local" / "share" / "task-cli" / "tasks.json"
+def mark_done(task_id: str) -> None:
+    """Mark the task with the given ID as done (removes it from the list).
 
+    Args:
+        task_id: The 1-based index of the task to mark as done.
 
-def validate_task_id(tasks, task_id):
-    """Validate task ID exists."""
-    # NOTE: Validation logic scattered here - should be in utils (refactor bounty)
-    if task_id < 1 or task_id > len(tasks):
-        raise ValueError(f"Invalid task ID: {task_id}")
-    return task_id
+    Raises:
+        ValueError: If the task ID is not a valid positive integer or out of range.
+        FileNotFoundError: If the tasks file does not exist.
+    """
+    tid = validate_task_id(task_id)
 
-
-def mark_done(task_id):
-    """Mark a task as complete."""
     tasks_file = get_tasks_file()
-    if not tasks_file.exists():
-        print("No tasks found!")
-        return
+    validate_task_file(tasks_file)
+
+    with open(tasks_file, "r") as f:
+        lines = f.readlines()
+
+    tasks = [line for line in lines if line.strip()]
+
+    if tid > len(tasks):
+        raise ValueError(
+            f"Task ID {tid} is out of range. There are only {len(tasks)} task(s)."
+        )
 
-    tasks = json.loads(tasks_file.read_text())
-    task_id = validate_task_id(tasks, task_id)
+    completed_task = tasks.pop(tid - 1).strip()
 
-    for task in tasks:
-        if task["id"] == task_id:
-            task["done"] = True
-            tasks_file.write_text(json.dumps(tasks, indent=2))
-            print(f"Marked task {task_id} as done: {task['description']}")
-            return
+    with open(tasks_file, "w") as f:
+        f.writelines(tasks)
 
-    print(f"Task {task_id} not found")
+    print(f"Task {tid} marked as done: {completed_task}")
diff --git a/commands/list.py b/commands/list.py
index 714315d..43a9a2d 100644
--- a/commands/list.py
+++ b/commands/list.py
@@ -1,37 +1,26 @@
-"""List tasks command."""
+"""Command to list all tasks."""
 
-import json
-from pathlib import Path
+from utils.paths import get_tasks_file
+from utils.validation import validate_task_file
 
 
-def get_tasks_file():
-    """Get path to tasks file."""
-    return Path.home() / ".local" / "share" / "task-cli" / "tasks.json"
+def list_tasks() -> None:
+    """List all tasks from the tasks file.
 
-
-def validate_task_file():
-    """Validate tasks file exists."""
-    # NOTE: Validation logic scattered here - should be in utils (refactor bounty)
+    Raises:
+        FileNotFoundError: If the tasks file does not exist.
+    """
     tasks_file = get_tasks_file()
-    if not tasks_file.exists():
-        return []
-    return tasks_file
-
-
-def list_tasks():
-    """List all tasks."""
-    # NOTE: No --json flag support yet (feature bounty)
-    tasks_file = validate_task_file()
-    if not tasks_file:
-        print("No tasks yet!")
-        return
+    validate_task_file(tasks_file)
 
-    tasks = json.loads(tasks_file.read_text())
+    with open(tasks_file, "r") as f:
+        lines = f.readlines()
 
-    if not tasks:
-        print("No tasks yet!")
+    if not lines:
+        print("No tasks found.")
         return
 
-    for task in tasks:
-        status = "✓" if task["done"] else " "
-        print(f"[{status}] {task['id']}. {task['description']}")
+    for index, line in enumerate(lines, start=1):
+        task = line.strip()
+        if task:
+            print(f"{index}. {task}")
diff --git a/utils/__init__.py b/utils/__init__.py
new file mode 100644
index 0000000..11fa064
--- /dev/null
+++ b/utils/__init__.py
@@ -0,0 +1 @@
+# Utils package for shared helpers and validation logic
diff --git a/utils/paths.py b/utils/paths.py
new file mode 100644
index 0000000..39e2c5f
--- /dev/null
+++ b/utils/paths.py
@@ -0,0 +1,8 @@
+"""Shared path helpers for the task manager application."""
+
+import os
+
+
+def get_tasks_file() -> str:
+    """Return the path to the tasks file, defaulting to 'tasks.txt'."""
+    return os.environ.get("TASKS_FILE", "tasks.txt")
diff --git a/utils/validation.py b/utils/validation.py
new file mode 100644
index 0000000..f647b80
--- /dev/null
+++ b/utils/validation.py
@@ -0,0 +1,52 @@
+"""Shared validation helpers for the task manager application."""
+
+import os
+
+
+def validate_description(description: str) -> None:
+    """Validate that a task description is non-empty.
+
+    Args:
+        description: The task description string to validate.
+
+    Raises:
+        ValueError: If the description is empty or contains only whitespace.
+    """
+    if not description or not description.strip():
+        raise ValueError("Task description cannot be empty.")
+
+
+def validate_task_file(tasks_file: str) -> None:
+    """Validate that the tasks file exists.
+
+    Args:
+        tasks_file: Path to the tasks file.
+
+    Raises:
+        FileNotFoundError: If the tasks file does not exist.
+    """
+    if not os.path.exists(tasks_file):
+        raise FileNotFoundError(f"Tasks file not found: {tasks_file}")
+
+
+def validate_task_id(task_id: str) -> int:
+    """Validate that a task ID is a positive integer.
+
+    Args:
+        task_id: The task ID string to validate.
+
+    Returns:
+        The task ID as a positive integer.
+
+    Raises:
+        ValueError: If the task ID is not a valid positive integer.
+    """
+    try:
+        tid = int(task_id)
+    except (TypeError, ValueError):
+        raise ValueError(f"Invalid task ID '{task_id}': must be a positive integer.")
+
+    if tid <= 0:
+        raise ValueError(f"Invalid task ID '{task_id}': must be a positive integer.")
+
+    return tid