fix: allow non-existent paths in ignore check

The _is_ignored_relative_path method used to raise FileNotFoundError
when checking a path that does not exist on disk. This prevented
editing tools (replace_content, insert_after_symbol, etc.) from
operating on newly created files.

Since a non-existent file cannot be matched by ignore patterns,
returning False (not ignored) is semantically correct and safe.

Changes:
- _is_ignored_relative_path: return False instead of raising for
  non-existent paths, with a debug log message
- validate_relative_path docstring: updated to reflect new behavior
- Added 3 test cases covering non-existent path scenarios

CHANGELOG.md

@@ -2,6 +2,9 @@
 
 Status of the `main` branch. Changes prior to the next official version change will appear here.
 
+* General:
+  - Not existing paths return `False` on is ignored checks (instead of raising an error)
+
 # v1.5.1 (2026-05-18)
 
 * General:

src/serena/project.py

@@ -191,8 +191,8 @@ class Project(ToStringMixin):
 
     def _is_ignored_relative_path(self, relative_path: str | Path, ignore_non_source_files: bool = True) -> bool:
         """
-        Determine whether an existing path should be ignored based on file type and ignore patterns.
+        Determine whether a path should be ignored based on file type and ignore patterns.
-        Raises `FileNotFoundError` if the path does not exist.
+        Returns False for non-existent paths since they cannot be matched by ignore patterns.
 
         :param relative_path: Relative path to check
         :param ignore_non_source_files: whether files that are not source files (according to the file masks
@@ -208,7 +208,8 @@ class Project(ToStringMixin):
 
         abs_path = os.path.join(self.project_root, relative_path)
         if not os.path.exists(abs_path):
-            raise FileNotFoundError(f"File {abs_path} not found, the ignore check cannot be performed")
+            log.debug(f"Path {abs_path} does not exist, skipping ignore check")
+            return False
 
         # Check file extension if it's a file
         is_file = os.path.isfile(abs_path)
@@ -284,10 +285,11 @@ class Project(ToStringMixin):
 
     def validate_relative_path(self, relative_path: str, require_not_ignored: bool = False) -> None:
         """
-        Validates that the given relative path to an existing file/dir is safe to read or edit,
+        Validates that the given relative path is safe to read or edit,
-        meaning it's inside the project directory.
+        meaning it's inside the project directory and not ignored.
 
-        Passing a path to a non-existing file will lead to a `FileNotFoundError`.
+        Non-existent paths are allowed (not considered ignored) to support
+        editing tools that create new files.
 
         :param relative_path: the path to validate, relative to the project root
         :param require_not_ignored: if True, the path must not be ignored according to the project's ignore settings

test/serena/config/test_global_ignored_paths.py

@@ -102,6 +102,31 @@ class TestGlobalIgnoredPaths:
         assert project.is_ignored_path(str(self.project_path / "debug.log"))
         assert not project.is_ignored_path(str(self.project_path / "main.py"))
 
+    def test_nonexistent_path_not_ignored(self) -> None:
+        """Non-existent paths should return False (not ignored), not raise FileNotFoundError."""
+        project = _create_test_project(self.project_path)
+        nonexistent = str(self.project_path / "src" / "new_file.py")
+        assert not os.path.exists(nonexistent)
+        assert not project.is_ignored_path(nonexistent)
+
+    def test_nonexistent_path_with_ignore_patterns(self) -> None:
+        """Non-existent paths matching ignore patterns still return False (cannot check)."""
+        project = _create_test_project(
+            self.project_path,
+            global_ignored_paths=["*.log"],
+        )
+        nonexistent = str(self.project_path / "nonexistent.log")
+        assert not os.path.exists(nonexistent)
+        # Should not raise, should not match the *.log pattern
+        assert not project.is_ignored_path(nonexistent)
+
+    def test_validate_relative_path_allows_nonexistent(self) -> None:
+        """validate_relative_path with require_not_ignored should not raise for non-existent paths."""
+        project = _create_test_project(self.project_path)
+        nonexistent = "src/new_file.py"
+        # Should not raise
+        project.validate_relative_path(nonexistent, require_not_ignored=True)
+
 
 class TestRegisteredProjectGlobalIgnoredPaths:
     """RegisteredProject.get_project_instance() correctly passes global patterns to Project."""