diff --git a/backend/packages/harness/deerflow/uploads/companions.py b/backend/packages/harness/deerflow/uploads/companions.py
index 325c3cc977..cbfc783482 100644
--- a/backend/packages/harness/deerflow/uploads/companions.py
+++ b/backend/packages/harness/deerflow/uploads/companions.py
@@ -87,7 +87,7 @@ def resolve_companion(original: Path) -> Path | None:
             return None
         data = json.loads(record.read_text(encoding="utf-8"))
         name = data["markdown"]
-        if data["original"] != original.name or not isinstance(name, str) or Path(name).name != name or not name.endswith(".md"):
+        if data["original"] != original.name or not isinstance(name, str) or Path(name).name != name or Path(name).suffix.lower() != ".md":
             return None
         source_stat = original.stat(follow_symlinks=False)
         # Legacy three-field records cannot establish whether an equal-size
diff --git a/backend/tests/test_upload_companions.py b/backend/tests/test_upload_companions.py
index 837eb9112a..0c3fa5ca4c 100644
--- a/backend/tests/test_upload_companions.py
+++ b/backend/tests/test_upload_companions.py
@@ -32,6 +32,21 @@ def test_unchanged_source_keeps_companion_after_read(converted_upload):
     assert extract_outline_for_file(original)[0] == [{"title": "Original A", "line": 1}]
 
 
+def test_companion_record_accepts_uppercase_markdown_suffix(tmp_path):
+    uploads = tmp_path / "thread" / "user-data" / "uploads"
+    uploads.mkdir(parents=True)
+    original = uploads / "report.pdf"
+    markdown = uploads / "report.MD"
+    original.write_bytes(b"%PDF ORIGINAL A")
+    markdown.write_text("# Uppercase suffix\n", encoding="utf-8")
+
+    register_companion(original, markdown)
+
+    assert resolve_companion(original) == markdown
+    assert companion_names(original.parent) == {markdown.name}
+    assert extract_outline_for_file(original)[0] == [{"title": "Uppercase suffix", "line": 1}]
+
+
 def test_same_size_sandbox_rewrite_rejects_stale_outline(converted_upload):
     original, markdown = converted_upload
     before = original.stat()