diff --git a/problems/medium/group-anagrams/solution.py b/problems/medium/group-anagrams/solution.py
new file mode 100644
index 0000000..1b0a1d0
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+++ b/problems/medium/group-anagrams/solution.py
@@ -0,0 +1,38 @@
+"""
+Group Anagrams
+
+You're building a word game assistant that groups words which are
+anagrams of each other. Two words are anagrams if they contain
+the same letters in any order.
+
+Given a list of strings, group the anagrams together. Each group
+should be a sorted list of words, and the groups themselves should
+be sorted by their first element.
+
+Example 1:
+    Input: words = ["eat", "tea", "tan", "ate", "nat", "bat"]
+    Output: [["ate", "eat", "tea"], ["bat"], ["nat", "tan"]]
+    Explanation: "eat", "tea", and "ate" are all anagrams of each other
+
+Example 2:
+    Input: words = ["hello", "world"]
+    Output: [["hello"], ["world"]]
+    Explanation: No words are anagrams of each other
+
+Example 3:
+    Input: words = ["", ""]
+    Output: [["", ""]]
+    Explanation: Empty strings are anagrams of each other
+
+Constraints:
+    - 0 <= len(words) <= 10^4
+    - 0 <= len(words[i]) <= 100
+    - Words consist of lowercase English letters
+    - Each group is sorted alphabetically
+    - Groups are sorted by their first element
+"""
+
+
+def group_anagrams(words: list[str]) -> list[list[str]]:
+    """Return a list of anagram groups, each sorted, groups sorted by first element."""
+    pass  # Your implementation here
diff --git a/problems/medium/group-anagrams/tests.py b/problems/medium/group-anagrams/tests.py
new file mode 100644
index 0000000..4ec0c84
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+++ b/problems/medium/group-anagrams/tests.py
@@ -0,0 +1,45 @@
+"""Tests for group-anagrams."""
+import pytest
+from solution import group_anagrams
+
+
+class TestBasicCases:
+    """Test basic functionality with typical inputs."""
+
+    def test_example_one(self):
+        """Test first example from problem description."""
+        assert group_anagrams(["eat", "tea", "tan", "ate", "nat", "bat"]) == [
+            ["ate", "eat", "tea"], ["bat"], ["nat", "tan"]
+        ]
+
+    def test_example_two(self):
+        """Test second example with no anagrams."""
+        assert group_anagrams(["hello", "world"]) == [["hello"], ["world"]]
+
+    def test_example_three(self):
+        """Test third example with empty strings."""
+        assert group_anagrams(["", ""]) == [["", ""]]
+
+    def test_single_word(self):
+        """Test with a single word."""
+        assert group_anagrams(["abc"]) == [["abc"]]
+
+
+class TestEdgeCases:
+    """Test edge cases and boundary conditions."""
+
+    def test_empty_list(self):
+        """Test with no words."""
+        assert group_anagrams([]) == []
+
+    def test_all_same_word(self):
+        """Test with identical words."""
+        assert group_anagrams(["abc", "abc", "abc"]) == [["abc", "abc", "abc"]]
+
+    def test_single_character_words(self):
+        """Test with single character strings."""
+        assert group_anagrams(["a", "b", "a"]) == [["a", "a"], ["b"]]
+
+    def test_different_lengths_not_grouped(self):
+        """Test that words of different lengths are never grouped."""
+        assert group_anagrams(["ab", "abc", "ba"]) == [["ab", "ba"], ["abc"]]
diff --git a/problems/medium/merge-intervals/solution.py b/problems/medium/merge-intervals/solution.py
new file mode 100644
index 0000000..84a6f12
--- /dev/null
+++ b/problems/medium/merge-intervals/solution.py
@@ -0,0 +1,35 @@
+"""
+Merge Intervals
+
+You're building a calendar app that needs to consolidate overlapping
+meetings. Given a list of time intervals represented as [start, end]
+pairs, merge all overlapping intervals and return the resulting list
+of non-overlapping intervals sorted by start time.
+
+Two intervals overlap if one starts before the other ends.
+
+Example 1:
+    Input: intervals = [[1, 3], [2, 6], [8, 10], [15, 18]]
+    Output: [[1, 6], [8, 10], [15, 18]]
+    Explanation: [1, 3] and [2, 6] overlap, merged into [1, 6]
+
+Example 2:
+    Input: intervals = [[1, 4], [4, 5]]
+    Output: [[1, 5]]
+    Explanation: Intervals sharing an endpoint are considered overlapping
+
+Example 3:
+    Input: intervals = [[1, 4], [0, 4]]
+    Output: [[0, 4]]
+    Explanation: Input is not necessarily sorted
+
+Constraints:
+    - 0 <= len(intervals) <= 10^4
+    - Each interval is [start, end] where start <= end
+    - Start and end are non-negative integers
+"""
+
+
+def merge(intervals: list[list[int]]) -> list[list[int]]:
+    """Return a list of merged non-overlapping intervals sorted by start."""
+    pass  # Your implementation here
diff --git a/problems/medium/merge-intervals/tests.py b/problems/medium/merge-intervals/tests.py
new file mode 100644
index 0000000..6f41609
--- /dev/null
+++ b/problems/medium/merge-intervals/tests.py
@@ -0,0 +1,47 @@
+"""Tests for merge-intervals."""
+import pytest
+from solution import merge
+
+
+class TestBasicCases:
+    """Test basic functionality with typical inputs."""
+
+    def test_example_one(self):
+        """Test first example from problem description."""
+        assert merge([[1, 3], [2, 6], [8, 10], [15, 18]]) == [[1, 6], [8, 10], [15, 18]]
+
+    def test_example_two(self):
+        """Test second example from problem description."""
+        assert merge([[1, 4], [4, 5]]) == [[1, 5]]
+
+    def test_example_three(self):
+        """Test third example with unsorted input."""
+        assert merge([[1, 4], [0, 4]]) == [[0, 4]]
+
+    def test_no_overlaps(self):
+        """Test intervals with no overlaps."""
+        assert merge([[1, 2], [5, 6], [9, 10]]) == [[1, 2], [5, 6], [9, 10]]
+
+
+class TestEdgeCases:
+    """Test edge cases and boundary conditions."""
+
+    def test_empty_list(self):
+        """Test with no intervals."""
+        assert merge([]) == []
+
+    def test_single_interval(self):
+        """Test with exactly one interval."""
+        assert merge([[1, 5]]) == [[1, 5]]
+
+    def test_all_overlapping(self):
+        """Test when all intervals merge into one."""
+        assert merge([[1, 4], [2, 5], [3, 6]]) == [[1, 6]]
+
+    def test_contained_interval(self):
+        """Test when one interval is fully inside another."""
+        assert merge([[1, 10], [3, 5]]) == [[1, 10]]
+
+    def test_unsorted_input(self):
+        """Test with input not sorted by start time."""
+        assert merge([[5, 6], [1, 3], [2, 4]]) == [[1, 4], [5, 6]]
diff --git a/problems/medium/valid-parentheses/solution.py b/problems/medium/valid-parentheses/solution.py
new file mode 100644
index 0000000..5814a85
--- /dev/null
+++ b/problems/medium/valid-parentheses/solution.py
@@ -0,0 +1,37 @@
+"""
+Valid Parentheses
+
+You're building a syntax checker for a code editor. Before running
+any code, the editor needs to verify that all brackets are properly
+matched and nested. Given a string containing only parentheses,
+square brackets, and curly braces, determine if the input is valid.
+
+A string is valid if:
+    - Open brackets are closed by the same type of bracket
+    - Open brackets are closed in the correct order
+    - Every close bracket has a corresponding open bracket
+
+Example 1:
+    Input: s = "([]){}"
+    Output: True
+    Explanation: All brackets are properly matched and nested
+
+Example 2:
+    Input: s = "([)]"
+    Output: False
+    Explanation: The square bracket closes before the parenthesis
+
+Example 3:
+    Input: s = "{{"
+    Output: False
+    Explanation: Opening braces are never closed
+
+Constraints:
+    - 0 <= len(s) <= 10^4
+    - s consists only of characters '(){}[]'
+"""
+
+
+def is_valid(s: str) -> bool:
+    """Return True if the bracket string is valid, False otherwise."""
+    pass  # Your implementation here
diff --git a/problems/medium/valid-parentheses/tests.py b/problems/medium/valid-parentheses/tests.py
new file mode 100644
index 0000000..8d43c64
--- /dev/null
+++ b/problems/medium/valid-parentheses/tests.py
@@ -0,0 +1,51 @@
+"""Tests for valid-parentheses."""
+import pytest
+from solution import is_valid
+
+
+class TestBasicCases:
+    """Test basic functionality with typical inputs."""
+
+    def test_example_one(self):
+        """Test first example from problem description."""
+        assert is_valid("([]){}") is True
+
+    def test_example_two(self):
+        """Test second example from problem description."""
+        assert is_valid("([)]") is False
+
+    def test_example_three(self):
+        """Test third example from problem description."""
+        assert is_valid("{{") is False
+
+    def test_simple_pairs(self):
+        """Test simple matched pairs."""
+        assert is_valid("()") is True
+        assert is_valid("[]") is True
+        assert is_valid("{}") is True
+
+
+class TestEdgeCases:
+    """Test edge cases and boundary conditions."""
+
+    def test_empty_string(self):
+        """Test with empty input."""
+        assert is_valid("") is True
+
+    def test_single_bracket(self):
+        """Test with a single unmatched bracket."""
+        assert is_valid("(") is False
+        assert is_valid("]") is False
+
+    def test_deeply_nested(self):
+        """Test with deeply nested brackets."""
+        assert is_valid("{[({[]})]}") is True
+
+    def test_close_before_open(self):
+        """Test closing bracket appearing before any opener."""
+        assert is_valid(")(") is False
+
+    def test_mismatched_types(self):
+        """Test opening one type and closing another."""
+        assert is_valid("{)") is False
+        assert is_valid("[}") is False