fix: allow None results for Optional task types (#1232)

* fix: allow None results for Optional task types

Tasks with Optional result types (e.g., `str | None`) were incorrectly
raising a ValueError when returning None. This fix checks if None is an
allowed type in the union before raising the error.

Without this fix, tasks like:
```python
@task
def maybe_value() -> str | None:
    return None
```

Would fail validation with "Result is None but result type is not None"
even though None is explicitly allowed in the type annotation.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

* test: add unit test for optional result types

Verify that tasks with Optional result types (e.g., str | None) can
successfully return None without raising validation errors.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

* test: expand coverage for Optional types (Union and complex)

* test: simplify tests, add end-to-end execution

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: zzstoatzz

src/marvin/tasks/task.py

@@ -20,6 +20,7 @@
     Sequence,
     TypeAlias,
     TypeVar,
+    get_args,
 )
 
 from pydantic import TypeAdapter
@@ -452,7 +453,10 @@ def validate_result(self, raw_result: Any) -> T:
             else:
                 raise ValueError("Result type is None but result is not None")
         elif raw_result is None:
-            raise ValueError("Result is None but result type is not None")
+            # Check if None is allowed in the type (e.g., Optional[str] = str | None)
+            type_args = get_args(result_type)
+            if type(None) not in type_args:
+                raise ValueError("Result is None but result type is not None")
 
         type_adapter = get_type_adapter(result_type)
         return type_adapter.validate_python(raw_result)

tests/ai/tasks/test_tasks.py

@@ -12,3 +12,23 @@ def get_prompt(self) -> str:
         prompt = task.get_prompt()
         assert prompt == 'Say the word "hello"'
         assert task.run() == "hello"
+
+
+class TestOptionalResults:
+    async def test_optional_result_returns_none(self):
+        """Test that tasks with optional result types can actually return None from AI."""
+        task = Task(
+            instructions="return null",
+            result_type=str | None,
+        )
+        result = await task.run_async()
+        assert result is None
+
+    async def test_optional_result_returns_value(self):
+        """Test that optional tasks can also return actual values."""
+        task = Task(
+            instructions='say "hello"',
+            result_type=str | None,
+        )
+        result = await task.run_async()
+        assert result == "hello"

tests/basic/tasks/test_tasks.py

@@ -258,6 +258,13 @@ async def test_task_mark_successful():
     assert task.result == "test result"
 
 
+async def test_task_optional_result_type():
+    """Test that tasks with Optional result types can validate None."""
+    task = Task(instructions="test", result_type=str | None)
+    validated = task.validate_result(None)
+    assert validated is None
+
+
 async def test_task_mark_failed():
     """Test marking task as failed."""
     task = Task(instructions="Test failure")