diff --git a/CHANGELOG.md b/CHANGELOG.md
index c12bb3ec..8a26a59a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -18,6 +18,8 @@ Status of the `main` branch. Changes prior to the next official version change w
   - Update prompts/instructions: Serena instructions manual, modes (editing, interactive) 
   - Allow structured tool output to be configured on a per-context basis, disabling it for Claude Code
     (which does not correctly unpack structured output) #1042
+  - Fix: Project-specific filtering of files for source files ignored the language backend. 
+    The check is really only possible for LSP. 
 
 * CLI:
   - Fix `--project-from-cwd` hijacking git worktrees nested under a Serena project. `find_project_root`
diff --git a/src/serena/project.py b/src/serena/project.py
index 87d0f969..8152a52a 100644
--- a/src/serena/project.py
+++ b/src/serena/project.py
@@ -231,17 +231,20 @@ class Project(ToStringMixin):
             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)
-        if is_file and ignore_non_source_files:
-            is_file_in_supported_language = False
-            for language in self.project_config.languages:
-                fn_matcher = language.get_source_fn_matcher()
-                if fn_matcher.is_relevant_filename(abs_path):
-                    is_file_in_supported_language = True
-                    break
-            if not is_file_in_supported_language:
-                return True
+        # check code file restriction (depending on backend)
+        if ignore_non_source_files:
+            # apply restriction only for LSP backend, which enumerates known languages
+            # and therefore can determine whether a file is a source file or not
+            if self.language_backend.is_lsp():
+                if os.path.isfile(abs_path):
+                    is_file_in_supported_language = False
+                    for language in self.project_config.languages:
+                        fn_matcher = language.get_source_fn_matcher()
+                        if fn_matcher.is_relevant_filename(abs_path):
+                            is_file_in_supported_language = True
+                            break
+                    if not is_file_in_supported_language:
+                        return True
 
         # Create normalized path for consistent handling
         rel_path = Path(relative_path)