diff --git a/mcp-server/src/legal_mcp/services/storage.py b/mcp-server/src/legal_mcp/services/storage.py
index 38d577b..ec146b8 100644
--- a/mcp-server/src/legal_mcp/services/storage.py
+++ b/mcp-server/src/legal_mcp/services/storage.py
@@ -88,6 +88,22 @@ def normalize_key(key: str | Path) -> str:
     return posix.as_posix().lstrip("/")
 
 
+def _ascii_metadata(value) -> str:
+    """Coerce an S3 user-metadata value to ASCII.
+
+    S3/MinIO object metadata must be ASCII (botocore raises ParamValidationError
+    otherwise). The only non-ASCII value we attach is the original Hebrew
+    filename (``ingest._stage_file`` → ``metadata={"filename": ...}``), so a
+    Hebrew name like ``"יומון 5167 - 11.6.26.pdf"`` would 500 every s3-only
+    upload. Percent-encode non-ASCII losslessly (recover with
+    ``urllib.parse.unquote``) while leaving plain-ASCII values readable."""
+    s = str(value)
+    if s.isascii():
+        return s
+    from urllib.parse import quote
+    return quote(s)
+
+
 class StorageBackend:
     """Abstract backend. All methods are async except the cheap path helpers."""
 
@@ -247,7 +263,7 @@ class S3Backend(StorageBackend):
         if content_type:
             extra["ContentType"] = content_type
         if metadata:
-            extra["Metadata"] = {kk: str(vv) for kk, vv in metadata.items()}
+            extra["Metadata"] = {kk: _ascii_metadata(vv) for kk, vv in metadata.items()}
         async with self._client() as s3:
             await s3.put_object(Bucket=_bucket_name(bucket), Key=k, Body=data, **extra)
         return f"s3://{_bucket_name(bucket)}/{k}"
diff --git a/mcp-server/tests/test_storage_staging.py b/mcp-server/tests/test_storage_staging.py
index 7768d1a..d347468 100644
--- a/mcp-server/tests/test_storage_staging.py
+++ b/mcp-server/tests/test_storage_staging.py
@@ -79,3 +79,48 @@ def test_put_bytes_sync_roundtrip(_tmp_datadir):
     src_key = "cases/1/exports/x.docx"
     storage.put_bytes_sync(src_key, b"PK\x03\x04zip", bucket=storage.Bucket.DOCUMENTS)
     assert (_tmp_datadir / src_key).read_bytes() == b"PK\x03\x04zip"
+
+
+def test_ascii_metadata_encodes_hebrew():
+    """S3 object metadata must be ASCII (botocore enforces). A Hebrew original
+    filename — the value ingest._stage_file attaches — must come back ASCII and
+    losslessly recoverable (regression: s3-only digest/document upload 500)."""
+    from urllib.parse import unquote
+    name = "יומון 5167 - 11.6.26.pdf"
+    out = storage._ascii_metadata(name)
+    assert out.isascii()
+    assert unquote(out) == name
+    # plain ASCII passes through untouched (readability)
+    assert storage._ascii_metadata("digest_ab12.pdf") == "digest_ab12.pdf"
+
+
+def test_s3_put_bytes_sends_ascii_metadata(monkeypatch):
+    """Reproduce the failure path: S3Backend.put_bytes with a Hebrew filename in
+    metadata must hand put_object an ASCII-only Metadata mapping (no
+    ParamValidationError)."""
+    captured = {}
+
+    class _FakeS3:
+        async def put_object(self, **kwargs):
+            captured.update(kwargs)
+            return {}
+
+    class _FakeClientCtx:
+        async def __aenter__(self):
+            return _FakeS3()
+        async def __aexit__(self, *exc):
+            return False
+
+    backend = storage.S3Backend()
+    monkeypatch.setattr(backend, "_client", lambda **kw: _FakeClientCtx())
+    monkeypatch.setattr(storage, "_bucket_name", lambda b: "documents")
+
+    run(backend.put_bytes(
+        "digests/incoming/ab12_x.pdf", b"%PDF-1.4",
+        bucket=storage.Bucket.DOCUMENTS,
+        metadata={"filename": "יומון 5167 - 11.6.26.pdf"},
+    ))
+    meta = captured["Metadata"]
+    assert all(v.isascii() for v in meta.values())
+    from urllib.parse import unquote
+    assert unquote(meta["filename"]) == "יומון 5167 - 11.6.26.pdf"