diff --git a/src/bhcli/api/__init__.py b/src/bhcli/api/__init__.py
index bc631ef..69b6eae 100644
--- a/src/bhcli/api/__init__.py
+++ b/src/bhcli/api/__init__.py
@@ -66,6 +66,8 @@ def _send(self, method, endpoint, data=None, content_type="application/json"):
             digester = hmac.new(digester.digest(), None, hashlib.sha256)
             if data is not None:
                 if isinstance(data, io.BufferedIOBase):
+                    # Seek to the start *before hashing, so the first attempt and any retry are retry-safe
+                    data.seek(0)
                     for chunk in iter(lambda: data.read(65536), b""):
                         digester.update(chunk)
                     data.seek(0)