diff --git a/backend/packages/harness/deerflow/uploads/manager.py b/backend/packages/harness/deerflow/uploads/manager.py
index 58d6b3ad2..875dc90e4 100644
--- a/backend/packages/harness/deerflow/uploads/manager.py
+++ b/backend/packages/harness/deerflow/uploads/manager.py
@@ -29,6 +29,8 @@ logger = logging.getLogger(__name__)
 UPLOAD_STAGING_PREFIX = ".upload-"
 UPLOAD_STAGING_SUFFIX = ".part"
 
+_MAX_FILENAME_BYTES = 255
+
 
 def get_uploads_dir(thread_id: str, *, user_id: str | None = None) -> Path:
     """Return the uploads directory path for a thread (no side effects)."""
@@ -66,16 +68,31 @@ def normalize_filename(filename: str) -> str:
     # but they indicate a Windows-style path that should be stripped or rejected.
     if "\\" in safe:
         raise ValueError(f"Filename contains backslash: {filename!r}")
-    if len(safe.encode("utf-8")) > 255:
+    if len(safe.encode("utf-8")) > _MAX_FILENAME_BYTES:
         raise ValueError(f"Filename too long: {len(safe)} chars")
     return safe
 
 
+def _fit_utf8_bytes(text: str, budget: int) -> str:
+    """Truncate *text* to at most *budget* UTF-8 bytes without splitting a code point."""
+    encoded = text.encode("utf-8")
+    if len(encoded) <= budget:
+        return text
+    return encoded[:budget].decode("utf-8", errors="ignore")
+
+
 def claim_unique_filename(name: str, seen: set[str]) -> str:
     """Generate a unique filename by appending ``_N`` suffix on collision.
 
     Automatically adds the returned name to *seen* so callers don't need to.
 
+    The deduplicated name stays within the 255-byte filename limit that
+    :func:`normalize_filename` enforces: when appending ``_N`` (plus the
+    preserved extension) would exceed it, the stem is truncated on a UTF-8
+    boundary to make room. Otherwise a maximum-length upload that collides
+    would produce a name the filesystem (and a later ``normalize_filename``
+    call on the write path) rejects.
+
     Args:
         name: Candidate filename.
         seen: Set of filenames already claimed (mutated in place).
@@ -88,10 +105,18 @@ def claim_unique_filename(name: str, seen: set[str]) -> str:
         return name
     stem, suffix = Path(name).stem, Path(name).suffix
     counter = 1
-    candidate = f"{stem}_{counter}{suffix}"
-    while candidate in seen:
+    while True:
+        tag = f"_{counter}"
+        budget = _MAX_FILENAME_BYTES - len(tag.encode("utf-8")) - len(suffix.encode("utf-8"))
+        if budget < 1:
+            # Pathological suffix that leaves no room for a stem; keep the
+            # unique tag and fit the rest (stem + suffix tail) around it.
+            candidate = _fit_utf8_bytes(stem + suffix, _MAX_FILENAME_BYTES - len(tag.encode("utf-8"))) + tag
+        else:
+            candidate = f"{_fit_utf8_bytes(stem, budget)}{tag}{suffix}"
+        if candidate not in seen:
+            break
         counter += 1
-        candidate = f"{stem}_{counter}{suffix}"
     seen.add(candidate)
     return candidate
 
diff --git a/backend/tests/test_uploads_manager.py b/backend/tests/test_uploads_manager.py
index 28364e951..0b5ec0eda 100644
--- a/backend/tests/test_uploads_manager.py
+++ b/backend/tests/test_uploads_manager.py
@@ -73,6 +73,40 @@ class TestDeduplicateFilename:
         claim_unique_filename("a.txt", seen)
         assert seen == {"a.txt", "a_1.txt"}
 
+    def test_max_length_name_stays_within_filename_limit(self):
+        # A 255-byte name passes normalize_filename; the deduplicated name
+        # must not exceed that limit, or the write path rejects it.
+        name = "a" * 251 + ".txt"
+        seen = {name}
+        deduped = claim_unique_filename(name, seen)
+        assert deduped != name
+        assert deduped.endswith("_1.txt")
+        assert len(deduped.encode("utf-8")) <= 255
+        # The truncated result must round-trip through normalize_filename.
+        assert normalize_filename(deduped) == deduped
+
+    def test_max_length_collisions_stay_unique_across_truncation(self):
+        name = "a" * 251 + ".txt"
+        seen = {name}
+        first = claim_unique_filename(name, seen)
+        second = claim_unique_filename(name, seen)
+        assert first != second
+        assert len(second.encode("utf-8")) <= 255
+
+    def test_multibyte_stem_is_truncated_on_a_codepoint_boundary(self):
+        # 85 CJK chars × 3 bytes = 255 bytes.
+        name = "深" * 85
+        seen = {name}
+        deduped = claim_unique_filename(name, seen)
+        assert len(deduped.encode("utf-8")) <= 255
+        assert deduped.endswith("_1")
+        # No replacement characters / decode artifacts.
+        deduped.encode("utf-8").decode("utf-8")
+
+    def test_short_names_keep_existing_dedupe_shape(self):
+        seen = {"data.txt"}
+        assert claim_unique_filename("data.txt", seen) == "data_1.txt"
+
 
 # ---------------------------------------------------------------------------
 # validate_path_traversal
diff --git a/backend/tests/test_uploads_router.py b/backend/tests/test_uploads_router.py
index 6f72bd0bd..fc8b81bcf 100644
--- a/backend/tests/test_uploads_router.py
+++ b/backend/tests/test_uploads_router.py
@@ -132,6 +132,50 @@ def test_upload_files_auto_renames_duplicate_form_filenames(tmp_path):
     assert (thread_uploads_dir / "data_1.txt").read_bytes() == b"second"
 
 
+def test_upload_files_deduplicates_max_length_filenames_without_failing_the_batch(tmp_path):
+    # A 255-byte filename is the longest normalize_filename accepts. Before
+    # the byte-budget truncation in claim_unique_filename, deduplicating a
+    # duplicate at that length produced a 257-byte name that the write path
+    # rejected, failing the whole request with a 500 and rolling back files
+    # that had already been written.
+    thread_uploads_dir = tmp_path / "uploads"
+    thread_uploads_dir.mkdir(parents=True)
+
+    provider = MagicMock()
+    provider.uses_thread_data_mounts = True
+
+    max_length_name = "a" * 251 + ".txt"
+
+    with (
+        patch.object(uploads, "get_uploads_dir", return_value=thread_uploads_dir),
+        patch.object(uploads, "ensure_uploads_dir", return_value=thread_uploads_dir),
+        patch.object(uploads, "get_sandbox_provider", return_value=provider),
+    ):
+        result = asyncio.run(
+            call_unwrapped(
+                uploads.upload_files,
+                "thread-local",
+                request=MagicMock(),
+                files=[
+                    UploadFile(filename="innocent.txt", file=BytesIO(b"kept")),
+                    UploadFile(filename=max_length_name, file=BytesIO(b"first")),
+                    UploadFile(filename=max_length_name, file=BytesIO(b"second")),
+                ],
+                config=SimpleNamespace(),
+            )
+        )
+
+    assert result.success is True
+    assert len(result.files) == 3
+    deduped_name = result.files[2].filename
+    assert deduped_name != max_length_name
+    assert deduped_name.endswith("_1.txt")
+    assert len(deduped_name.encode("utf-8")) <= 255
+    assert (thread_uploads_dir / "innocent.txt").read_bytes() == b"kept"
+    assert (thread_uploads_dir / max_length_name).read_bytes() == b"first"
+    assert (thread_uploads_dir / deduped_name).read_bytes() == b"second"
+
+
 def test_upload_files_skips_acquire_when_thread_data_is_mounted(tmp_path):
     thread_uploads_dir = tmp_path / "uploads"
     thread_uploads_dir.mkdir(parents=True)