diff --git a/dandi/files/zarr.py b/dandi/files/zarr.py
index efb1efd2d..943e36288 100644
--- a/dandi/files/zarr.py
+++ b/dandi/files/zarr.py
@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 from base64 import b64encode
+from collections import Counter
 from collections.abc import Generator, Iterator
 from concurrent.futures import Future, ThreadPoolExecutor, as_completed
 from contextlib import closing
@@ -8,12 +9,14 @@
 from datetime import datetime
 from enum import Enum
 import json
+import math
 import os
 import os.path
-import urllib.parse
 from pathlib import Path
+import random
 from time import sleep
 from typing import Any, Optional
+import urllib.parse
 
 from dandischema.models import BareAsset, DigestType
 from pydantic import BaseModel, ConfigDict, ValidationError
@@ -745,6 +748,7 @@ def mkzarr() -> str:
                     items_to_upload = list(items)
                     max_retries = 5
                     retry_count = 0
+                    current_jobs = jobs or 5
                     # Add all items to checksum tree (only done once)
                     for it in items_to_upload:
                         zcc.add_leaf(Path(it.entry_path), it.size, it.digest)
@@ -774,7 +778,7 @@ def mkzarr() -> str:
                         r = client.post(f"/zarr/{zarr_id}/files/", json=uploading)
 
                         # Upload files in parallel
-                        with ThreadPoolExecutor(max_workers=jobs or 5) as executor:
+                        with ThreadPoolExecutor(max_workers=current_jobs) as executor:
                             futures = [
                                 executor.submit(
                                     _upload_zarr_file,
@@ -817,14 +821,22 @@ def mkzarr() -> str:
                             # Prepare for next iteration with retry items
                             if items_to_upload := retry_items:
                                 retry_count += 1
+                                current_jobs = max(1, math.ceil(current_jobs / 2))
                                 if retry_count <= max_retries:
                                     lgr.info(
-                                        "%s: %s got 403 errors, requesting new URLs",
+                                        "%s: %s got 403 errors, requesting new URLs"
+                                        " (attempt %d/%d, workers: %d)",
                                         asset_path,
                                         pluralize(len(items_to_upload), "file"),
+                                        retry_count,
+                                        max_retries,
+                                        current_jobs,
+                                    )
+                                    # Exponential backoff with jitter before retry
+                                    sleep(
+                                        min(2**retry_count * 5, 120)
+                                        + random.uniform(0, 5)
                                     )
-                                    # Small delay before retry
-                                    sleep(1 * retry_count)
 
                     # Check if we exhausted retries
                     if items_to_upload:
@@ -899,6 +911,18 @@ def _handle_failed_items_and_raise(
     # Log all failures
     for item, error in failed_items:
         lgr.error("Failed to upload %s: %s", item.filepath, error)
+
+    # Summary diagnostics
+    exc_counts = Counter(type(error).__name__ for _, error in failed_items)
+    exc_summary = ", ".join(f"{k}: {v}" for k, v in exc_counts.most_common())
+    lgr.error(
+        "Upload failure summary: %d/%d files failed; exception types: {%s}%s",
+        len(failed_items),
+        len(futures),
+        exc_summary,
+        " (systematic — all same exception type)" if len(exc_counts) == 1 else "",
+    )
+
     # Raise the first error
     raise failed_items[0][1]
 
@@ -945,6 +969,7 @@ def _upload_zarr_file(
                 json_resp=False,
                 retry_if=_retry_zarr_file,
                 headers=headers,
+                timeout=(60, 7200),
             )
     except requests.HTTPError as e:
         post_upload_size_check(item.filepath, item.size, True)