@@ -68,7 +68,7 @@ The package reads no environment variables on its own. `extract_token_from_envir
 | `max_retries` | `5` | Replays of a `429` response; reconnect attempts for `create_sse_client()` |
 | `retry_delay` | `1000` | Base backoff in milliseconds for both of the above |
 | `timeout` | `30` | Seconds, for ledger, investor and library GraphQL reads and `create_sse_client()`. Other REST calls set no client timeout |
-| `s3_endpoint_url` | `None` | Rewrites the host of file upload URLs, for example for LocalStack |
+| `s3_endpoint_url` | `None` | Where the client connects for file uploads when it reaches S3 by a different name than the API signed for (a container reaching a dev stack through `host.docker.internal`). The signed `Host` header is kept, as SigV4 requires |
 | `token_provider` | `None` | Callable returning the current credential |
 
 ## Cypher queries
 
@@ -28,7 +28,8 @@ class RoboSystemsClientConfig:
   max_retries: int = 5
   retry_delay: int = 1000
   timeout: int = 30
-  s3_endpoint_url: Optional[str] = None  # Override S3 endpoint (e.g., for LocalStack)
+  # Where the client reaches S3 when that is not the host the API signed for.
+  s3_endpoint_url: Optional[str] = None
   # Zero-arg callable returning the current auth credential, consulted
   # on every request by the GraphQL-backed facades (ledger / investor /
   # library). Wins over the static token when set — use it when the
 
@@ -109,7 +109,7 @@ def upload(
         graph_id: Graph database identifier
         table_name: Table to associate file with
         file_or_buffer: File path, Path object, BytesIO, or file-like object
-        options: Upload options (progress callback, LocalStack URL fix, auto-ingest)
+        options: Upload options (progress callback, auto-ingest)
 
     Returns:
         FileUploadResult with file metadata and status
@@ -184,13 +184,15 @@ def upload(
       upload_url = upload_data.get("upload_url")
       file_id = upload_data.get("file_id")
 
-      # Override S3 endpoint if configured (e.g., for LocalStack)
+      # The client may reach S3 by a different name than the API signed for
+      # (a container reaching a dev stack through host.docker.internal). SigV4
+      # binds the Host header, so the request keeps the signed one.
+      upload_headers = {"Content-Type": "application/x-parquet"}
       if self.s3_endpoint_url:
         from urllib.parse import urlparse, urlunparse
 
         parsed_url = urlparse(upload_url)
         override_parsed = urlparse(self.s3_endpoint_url)
-        # Replace scheme, host, and port with the override endpoint
         upload_url = urlunparse(
           (
             override_parsed.scheme or parsed_url.scheme,
@@ -201,6 +203,7 @@ def upload(
             parsed_url.fragment,
           )
         )
+        upload_headers["Host"] = parsed_url.netloc
 
       # Step 2: Upload file to S3
       if options.on_progress:
@@ -209,7 +212,7 @@ def upload(
       s3_response = self._http_client.put(
         upload_url,
         content=file_content,
-        headers={"Content-Type": "application/x-parquet"},
+        headers=upload_headers,
       )
 
       if s3_response.status_code not in [200, 204]:
 
@@ -294,14 +294,15 @@ def test_upload_with_progress_callback(
   def test_upload_with_s3_endpoint_override(
     self, mock_create, mock_update, mock_config, graph_id
   ):
-    """Test upload rewrites S3 URL when s3_endpoint_url is set."""
-    mock_config["s3_endpoint_url"] = "http://localhost:4566"
+    """The connection goes to the override; the Host header stays the signed
+    one, since SigV4 binds it and the server checks the signature against it."""
+    mock_config["s3_endpoint_url"] = "http://host.docker.internal:8333"
 
     mock_create_resp = Mock()
     mock_create_resp.status_code = 200
     mock_create_resp.parsed = Mock(
       result={
-        "upload_url": "https://s3.amazonaws.com/bucket/file.parquet?sig=abc",
+        "upload_url": "http://localhost:8333/bucket/file.parquet?X-Amz-Signature=abc",
         "file_id": "file-s3-override",
       }
     )
@@ -318,9 +319,46 @@ def test_upload_with_s3_endpoint_override(
 
     client.upload(graph_id, "Entity", BytesIO(b"data"))
 
-    # Verify S3 PUT was called with overridden URL
     put_url = client._http_client.put.call_args[0][0]
-    assert "localhost:4566" in put_url
+    put_headers = client._http_client.put.call_args.kwargs["headers"]
+    assert put_url == (
+      "http://host.docker.internal:8333/bucket/file.parquet?X-Amz-Signature=abc"
+    )
+    assert put_headers["Host"] == "localhost:8333"
+    assert put_headers["Content-Type"] == "application/x-parquet"
+
+  @patch("robosystems_client.clients.file_client.ingest_file")
+  @patch("robosystems_client.clients.file_client.create_file_upload")
+  def test_upload_without_override_sends_no_host_header(
+    self, mock_create, mock_update, mock_config, graph_id
+  ):
+    """Without an override the presigned URL is used as signed and the
+    client sets no Host header of its own."""
+    mock_create_resp = Mock()
+    mock_create_resp.status_code = 200
+    mock_create_resp.parsed = Mock(
+      result={
+        "upload_url": "https://bucket.s3.amazonaws.com/file.parquet?X-Amz-Signature=abc",
+        "file_id": "file-plain",
+      }
+    )
+    mock_create.return_value = mock_create_resp
+
+    mock_update_resp = Mock()
+    mock_update_resp.status_code = 200
+    mock_update_resp.parsed = Mock(result={"file_size_bytes": 100, "row_count": 5})
+    mock_update.return_value = mock_update_resp
+
+    client = FileClient(mock_config)
+    client._http_client = Mock()
+    client._http_client.put.return_value = Mock(status_code=200)
+
+    client.upload(graph_id, "Entity", BytesIO(b"data"))
+
+    put_url = client._http_client.put.call_args[0][0]
+    put_headers = client._http_client.put.call_args.kwargs["headers"]
+    assert put_url == "https://bucket.s3.amazonaws.com/file.parquet?X-Amz-Signature=abc"
+    assert "Host" not in put_headers
 
 
 @pytest.mark.unit