diff --git a/backend/app.py b/backend/app.py
index c9fbd5b..237f8a5 100755
--- a/backend/app.py
+++ b/backend/app.py
@@ -1,31 +1,59 @@
 from flask import Flask, request, jsonify
 
-# declare constants
+comments = []
+comment_id_counter = 1
+
 HOST = '0.0.0.0'
 PORT = 5000
 
-# initialize flask application
 app = Flask(__name__)
 
-# sample hello world page
 @app.route('/')
 def hello():
     return "<h1>Hello World</h1>"
 
-# sample api endpoint
 @app.route('/api/test', methods=['GET', 'POST'])
 def test():
     if request.method == 'POST':
-        # get parameters from post request
         parameters = request.get_json()
-        if 'test' in parameters:
+        if parameters and 'test' in parameters:
             return jsonify({'value': parameters['test']})
-        return jsonify({'error'})
-    else:
-        return jsonify({'test': 'success'})
+        return jsonify({'error': 'Invalid request'}), 400
+    return jsonify({'test': 'success'})
+
+@app.route('/comments', methods=['POST'])
+def add_comment():
+    global comment_id_counter
+    data = request.get_json()
+    if not data or 'task_id' not in data or 'text' not in data:
+        return jsonify({'error': 'task_id and text are required'}), 400
+    comment = {
+        'id': comment_id_counter,
+        'task_id': data['task_id'],
+        'text': data['text']
+    }
+    comments.append(comment)
+    comment_id_counter += 1
+    return jsonify(comment), 201
+
+@app.route('/comments/<int:comment_id>', methods=['PUT'])
+def update_comment(comment_id):
+    data = request.get_json()
+    if not data or 'text' not in data:
+        return jsonify({'error': 'text is required'}), 400
+    for comment in comments:
+        if comment['id'] == comment_id:
+            comment['text'] = data['text']
+            return jsonify(comment), 200
+    return jsonify({'error': 'Comment not found'}), 404
 
+@app.route('/comments/<int:comment_id>', methods=['DELETE'])
+def delete_comment(comment_id):
+    for comment in comments:
+        if comment['id'] == comment_id:
+            comments.remove(comment)
+            return jsonify({'message': 'Comment deleted'}), 200
+    return jsonify({'error': 'Comment not found'}), 404
 
 if __name__ == '__main__':
-    app.run(host=HOST,
-            debug=True,
-            port=PORT)
+    app.run(host=HOST, port=PORT, debug=True)
diff --git a/backend/tests/test_comments.py b/backend/tests/test_comments.py
new file mode 100644
index 0000000..832bec6
--- /dev/null
+++ b/backend/tests/test_comments.py
@@ -0,0 +1,304 @@
+import json
+import pytest
+import sys
+import os
+
+sys.path.append(os.path.dirname(os.path.dirname(__file__)))
+
+from app import app
+
+
+@pytest.fixture
+def client():
+    app.testing = True
+    with app.test_client() as client:
+        yield client
+
+
+# =====================================================
+# POST /comments — POSITIVE TEST CASES (4)
+# =====================================================
+
+def test_post_comment_success(client):
+    res = client.post(
+        "/comments",
+        data=json.dumps({"task_id": 1, "text": "Valid comment"}),
+        content_type="application/json"
+    )
+    assert res.status_code == 201
+    assert "id" in res.get_json()
+
+
+def test_post_comment_different_task_id(client):
+    res = client.post(
+        "/comments",
+        data=json.dumps({"task_id": 99, "text": "Another comment"}),
+        content_type="application/json"
+    )
+    assert res.status_code == 201
+
+
+def test_post_comment_long_text(client):
+    res = client.post(
+        "/comments",
+        data=json.dumps({"task_id": 1, "text": "A" * 200}),
+        content_type="application/json"
+    )
+    assert res.status_code == 201
+
+
+def test_post_multiple_comments(client):
+    for i in range(3):
+        res = client.post(
+            "/comments",
+            data=json.dumps({"task_id": 1, "text": f"Comment {i}"}),
+            content_type="application/json"
+        )
+        assert res.status_code == 201
+
+
+# =====================================================
+# POST /comments — NEGATIVE TEST CASES (4)
+# =====================================================
+
+def test_post_comment_missing_text(client):
+    res = client.post(
+        "/comments",
+        data=json.dumps({"task_id": 1}),
+        content_type="application/json"
+    )
+    assert res.status_code == 400
+
+
+def test_post_comment_missing_task_id(client):
+    res = client.post(
+        "/comments",
+        data=json.dumps({"text": "No task"}),
+        content_type="application/json"
+    )
+    assert res.status_code == 400
+
+
+def test_post_comment_empty_body(client):
+    res = client.post(
+        "/comments",
+        data=json.dumps({}),
+        content_type="application/json"
+    )
+    assert res.status_code == 400
+
+
+def test_post_comment_no_json_header(client):
+    res = client.post(
+        "/comments",
+        data=json.dumps({"task_id": 1, "text": "No header"})
+    )
+    assert res.status_code == 415
+
+
+# =====================================================
+# PUT /comments/<id> — POSITIVE TEST CASES (4)
+# =====================================================
+
+def test_put_comment_success(client):
+    c = client.post(
+        "/comments",
+        data=json.dumps({"task_id": 1, "text": "Old"}),
+        content_type="application/json"
+    )
+    cid = c.get_json()["id"]
+
+    res = client.put(
+        f"/comments/{cid}",
+        data=json.dumps({"text": "Updated"}),
+        content_type="application/json"
+    )
+    assert res.status_code == 200
+
+
+def test_put_comment_multiple_updates(client):
+    c = client.post(
+        "/comments",
+        data=json.dumps({"task_id": 1, "text": "Init"}),
+        content_type="application/json"
+    )
+    cid = c.get_json()["id"]
+
+    for i in range(3):
+        res = client.put(
+            f"/comments/{cid}",
+            data=json.dumps({"text": f"Update {i}"}),
+            content_type="application/json"
+        )
+        assert res.status_code == 200
+
+
+def test_put_comment_long_text(client):
+    c = client.post(
+        "/comments",
+        data=json.dumps({"task_id": 1, "text": "Short"}),
+        content_type="application/json"
+    )
+    cid = c.get_json()["id"]
+
+    res = client.put(
+        f"/comments/{cid}",
+        data=json.dumps({"text": "B" * 300}),
+        content_type="application/json"
+    )
+    assert res.status_code == 200
+
+
+def test_put_comment_special_characters(client):
+    c = client.post(
+        "/comments",
+        data=json.dumps({"task_id": 1, "text": "Text"}),
+        content_type="application/json"
+    )
+    cid = c.get_json()["id"]
+
+    res = client.put(
+        f"/comments/{cid}",
+        data=json.dumps({"text": "@#$%^&*()"}),
+        content_type="application/json"
+    )
+    assert res.status_code == 200
+
+
+# =====================================================
+# PUT /comments/<id> — NEGATIVE TEST CASES (4)
+# =====================================================
+
+def test_put_comment_not_found(client):
+    res = client.put(
+        "/comments/999",
+        data=json.dumps({"text": "No comment"}),
+        content_type="application/json"
+    )
+    assert res.status_code == 404
+
+
+def test_put_comment_missing_text(client):
+    c = client.post(
+        "/comments",
+        data=json.dumps({"task_id": 1, "text": "Valid"}),
+        content_type="application/json"
+    )
+    cid = c.get_json()["id"]
+
+    res = client.put(
+        f"/comments/{cid}",
+        data=json.dumps({}),
+        content_type="application/json"
+    )
+    assert res.status_code == 400
+
+
+def test_put_comment_no_json_header(client):
+    c = client.post(
+        "/comments",
+        data=json.dumps({"task_id": 1, "text": "Valid"}),
+        content_type="application/json"
+    )
+    cid = c.get_json()["id"]
+
+    res = client.put(
+        f"/comments/{cid}",
+        data=json.dumps({"text": "Updated"})
+    )
+    assert res.status_code == 415
+
+
+def test_put_comment_invalid_id(client):
+    res = client.put(
+        "/comments/abc",
+        data=json.dumps({"text": "Invalid"}),
+        content_type="application/json"
+    )
+    assert res.status_code == 404
+
+
+# =====================================================
+# DELETE /comments/<id> — POSITIVE TEST CASES (4)
+# =====================================================
+
+def test_delete_comment_success(client):
+    c = client.post(
+        "/comments",
+        data=json.dumps({"task_id": 2, "text": "Delete"}),
+        content_type="application/json"
+    )
+    cid = c.get_json()["id"]
+
+    res = client.delete(f"/comments/{cid}")
+    assert res.status_code == 200
+
+
+def test_delete_multiple_comments(client):
+    ids = []
+    for i in range(3):
+        c = client.post(
+            "/comments",
+            data=json.dumps({"task_id": 1, "text": f"Del {i}"}),
+            content_type="application/json"
+        )
+        ids.append(c.get_json()["id"])
+
+    for cid in ids:
+        res = client.delete(f"/comments/{cid}")
+        assert res.status_code == 200
+
+
+def test_delete_comment_after_update(client):
+    c = client.post(
+        "/comments",
+        data=json.dumps({"task_id": 1, "text": "Temp"}),
+        content_type="application/json"
+    )
+    cid = c.get_json()["id"]
+
+    client.put(
+        f"/comments/{cid}",
+        data=json.dumps({"text": "Updated"}),
+        content_type="application/json"
+    )
+
+    res = client.delete(f"/comments/{cid}")
+    assert res.status_code == 200
+
+
+def test_delete_comment_high_id(client):
+    res = client.delete("/comments/100000")
+    assert res.status_code == 404
+
+
+# =====================================================
+# DELETE /comments/<id> — NEGATIVE TEST CASES (4)
+# =====================================================
+
+def test_delete_comment_twice(client):
+    c = client.post(
+        "/comments",
+        data=json.dumps({"task_id": 1, "text": "Once"}),
+        content_type="application/json"
+    )
+    cid = c.get_json()["id"]
+
+    client.delete(f"/comments/{cid}")
+    res = client.delete(f"/comments/{cid}")
+    assert res.status_code == 404
+
+
+def test_delete_comment_not_found(client):
+    res = client.delete("/comments/999")
+    assert res.status_code == 404
+
+
+def test_delete_comment_invalid_id(client):
+    res = client.delete("/comments/abc")
+    assert res.status_code == 404
+
+
+def test_delete_comment_negative_id(client):
+    res = client.delete("/comments/-1")
+    assert res.status_code == 404