diff --git a/CHANGELOG.md b/CHANGELOG.md
index 89b8356f00..6b3194c847 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -81,7 +81,8 @@ Other changes:
   we will from now on fetch the lists of available packages on each index. The
   used package mappings will be written as facts to the `MODULE.bazel.lock` file
   on supported bazel versions and it should be done at most once. As a result,
-  per-package {obj}`experimental_index_url_overrides` is no longer needed . What
+  per-package {obj}`experimental_index_url_overrides` is no longer needed, but 
+  if specified, it needs to be provided for all packages not on the default index. What
   is more, the flags for `--index_url` and `--extra-index-url` now behave in the
   same way as in `uv` or `pip`, i.e. we default to `--index-url` if the package
   is not found in `--extra-index-url`.  Fixes
diff --git a/python/private/pypi/simpleapi_download.bzl b/python/private/pypi/simpleapi_download.bzl
index 97215d753d..63044bc14a 100644
--- a/python/private/pypi/simpleapi_download.bzl
+++ b/python/private/pypi/simpleapi_download.bzl
@@ -43,8 +43,7 @@ def simpleapi_download(
         attr: Contains the parameters for the download. They are grouped into a
           struct for better clarity. It must have attributes:
            * index_url: str, the index, or if `extra_index_urls` are passed, the default index.
-           * index_url_overrides: dict[str, str], the index overrides for
-             separate packages.
+           * index_url_overrides: dict[str, str], the index overrides for separate packages.
            * extra_index_urls: Will be looked at in the order they are defined and the first match
                 wins. This is similar to what uv does, see
                 https://docs.astral.sh/uv/concepts/indexes/#searching-across-multiple-indexes.
@@ -132,16 +131,22 @@ def simpleapi_download(
     return contents
 
 def _get_dist_urls(ctx, *, default_index, index_urls, index_url_overrides, sources, read_simpleapi, attr, block, _fail = fail, **kwargs):
+    if index_url_overrides:
+        # Let's not call the index at all and just assume that all of the overrides have been
+        # specified.
+        return {
+            pkg: _normalize_url("{}/{}/".format(
+                index_url_overrides.get(pkg, default_index),
+                pkg.replace("_", "-"),  # Use the official normalization for URLs
+            ))
+            for pkg in sources
+        }
+
     downloads = {}
     results = {}
 
     # Ensure the value is not frozen
     index_urls = [] + (index_urls or [])
-    for extra in index_url_overrides.values():
-        if extra not in index_urls:
-            index_urls.append(extra)
-
-    index_urls = index_urls or []
     if default_index not in index_urls:
         index_urls.append(default_index)
 
@@ -174,10 +179,6 @@ def _get_dist_urls(ctx, *, default_index, index_urls, index_url_overrides, sourc
                 # and in the outer function process merging of the results.
                 continue
 
-            if index_url_overrides.get(pkg, index_url) != index_url:
-                # we should not use this index for the package
-                continue
-
             found = result.output.get(pkg)
             if not found:
                 continue
diff --git a/tests/pypi/simpleapi_download/simpleapi_download_tests.bzl b/tests/pypi/simpleapi_download/simpleapi_download_tests.bzl
index 55439c2593..c515c19ae0 100644
--- a/tests/pypi/simpleapi_download/simpleapi_download_tests.bzl
+++ b/tests/pypi/simpleapi_download/simpleapi_download_tests.bzl
@@ -273,6 +273,51 @@ def _test_download_url_parallel(env):
 
 _tests.append(_test_download_url_parallel)
 
+def _test_download_url_parallel_with_overrides(env):
+    downloads = {}
+    reads = [
+        "",
+        "",
+        "",
+    ]
+
+    def download(url, output, **kwargs):
+        _ = kwargs  # buildifier: disable=unused-variable
+        downloads[url[0]] = output
+        return struct(wait = lambda: struct(success = True))
+
+    simpleapi_download(
+        ctx = struct(
+            getenv = {}.get,
+            download = download,
+            report_progress = lambda _: None,
+            # We will first add a download to the list, so this is a poor man's `next(foo)`
+            # implementation. We use 2 because we will enqueue 2 downloads in parallel.
+            read = lambda i: reads[len(downloads) - 2],
+            path = lambda i: "path/for/" + i,
+        ),
+        attr = struct(
+            index_url_overrides = {
+                "bar": "https://example.com/extra/simple/",
+            },
+            index_url = "https://example.com/default/simple/",
+            extra_index_urls = [],
+            sources = {"bar": None, "baz": None, "foo": None},
+            envsubst = [],
+        ),
+        cache = pypi_cache(),
+        parallel_download = True,
+        get_auth = lambda ctx, urls, ctx_attr: struct(),
+    )
+
+    env.expect.that_dict(downloads).contains_exactly({
+        "https://example.com/default/simple/baz/": "path/for/https___example_com_default_simple_baz.html",
+        "https://example.com/default/simple/foo/": "path/for/https___example_com_default_simple_foo.html",
+        "https://example.com/extra/simple/bar/": "path/for/https___example_com_extra_simple_bar.html",
+    })
+
+_tests.append(_test_download_url_parallel_with_overrides)
+
 def _test_download_envsubst_url(env):
     downloads = {}
     reads = [