diff --git a/tasks/task_sum_function_off_by_one.md b/tasks/task_sum_function_off_by_one.md
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+id: task_sum_function_off_by_one
+name: Fix off-by-one error in sum function
+category: coding
+grading_type: automated
+timeout_seconds: 120
+workspace_files:
+  - sum.py
+  - test_sum.py
+
+## Prompt
+
+The workspace contains a function `sum_n(n)` in `sum.py` that is supposed to return the sum of integers from 1 to n (inclusive).
+
+However, the implementation is incorrect and fails the provided tests.
+
+Fix the implementation so that all tests pass.
+
+Requirements:
+- Modify only `sum.py`
+- Do not change `test_sum.py`
+- Do not introduce new functions
+- Keep the function signature unchanged
+
+## Expected Behavior
+
+The function `sum_n(n)` should return:
+
+1 + 2 + ... + n
+
+Examples:
+- sum_n(1) == 1
+- sum_n(3) == 6
+- sum_n(5) == 15
+
+## Grading Criteria
+
+- sum.py was modified
+- test_sum.py was not modified
+- All tests pass
+- Function returns correct results for multiple inputs
+
+## Automated Checks
+
+def grade(transcript: list, workspace_path: str) -> dict:
+    import importlib.util
+    from pathlib import Path
+
+    workspace = Path(workspace_path)
+    sum_file = workspace / "sum.py"
+    test_file = workspace / "test_sum.py"
+
+    scores = {
+        "sum.py was modified": 0.0,
+        "test_sum.py was not modified": 0.0,
+        "All tests pass": 0.0,
+        "Function returns correct results for multiple inputs": 0.0,
+    }
+
+    if not sum_file.exists() or not test_file.exists():
+        return scores
+
+    original_code = sum_file.read_text()
+
+    # Check modification via transcript heuristic
+    modified = False
+    for event in transcript:
+        if event.get("type") != "message":
+            continue
+        msg = event.get("message", {})
+        for item in msg.get("content", []):
+            if item.get("type") == "toolCall":
+                raw = str(item)
+                if "sum.py" in raw and any(x in raw.lower() for x in ["write", "edit", "replace"]):
+                    modified = True
+    if modified:
+        scores["sum.py was modified"] = 1.0
+
+    # Ensure test file untouched
+    scores["test_sum.py was not modified"] = 1.0
+
+    # Load module dynamically
+    spec = importlib.util.spec_from_file_location("sum_module", sum_file)
+    module = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(module)
+
+    try:
+        tests = [
+            module.sum_n(1) == 1,
+            module.sum_n(3) == 6,
+            module.sum_n(5) == 15,
+            module.sum_n(10) == 55,
+        ]
+        if all(tests):
+            scores["All tests pass"] = 1.0
+            scores["Function returns correct results for multiple inputs"] = 1.0
+    except Exception:
+        pass
+
+    return scores
\ No newline at end of file
diff --git a/tasks/task_sum_function_off_by_one/sum.py b/tasks/task_sum_function_off_by_one/sum.py
new file mode 100644
index 0000000..2269ba0
--- /dev/null
+++ b/tasks/task_sum_function_off_by_one/sum.py
@@ -0,0 +1,5 @@
+def sum_n(n):
+    total = 0
+    for i in range(1, n):  # BUG: should include n
+        total += i
+    return total
\ No newline at end of file
diff --git a/tasks/task_sum_function_off_by_one/test_sum.py b/tasks/task_sum_function_off_by_one/test_sum.py
new file mode 100644
index 0000000..973c489
--- /dev/null
+++ b/tasks/task_sum_function_off_by_one/test_sum.py
@@ -0,0 +1,7 @@
+from sum import sum_n
+
+def test_sum():
+    assert sum_n(1) == 1
+    assert sum_n(3) == 6
+    assert sum_n(5) == 15
+    assert sum_n(10) == 55
\ No newline at end of file