diff --git a/odb/source-files.c b/odb/source-files.c
index 455f409db0..9268f5aa9a 100644
--- a/odb/source-files.c
+++ b/odb/source-files.c
@@ -426,6 +426,7 @@ static enum odb_read_status odb_source_files_read_object_info(struct odb_source
 {
 	struct odb_source_files *files = odb_source_files_downcast(source);
 	enum odb_read_status status = ODB_READ_NOT_FOUND;
+	enum odb_read_status ret;
 
 	/*
 	 * Reading an object may fail even though the object exists, for
@@ -434,15 +435,15 @@ static enum odb_read_status odb_source_files_read_object_info(struct odb_source
 	 * prefer the first such error in case multiple reads have failed.
 	 */
 	for (struct odb_files_dir *dir = files->dirs; dir; dir = dir->next) {
-		enum odb_read_status ret;
-
 		ret = odb_source_read_object_info(&dir->packed->base, oid, oi, flags,
 						  status == ODB_READ_NOT_FOUND ? errmsg : NULL);
 		if (!ret)
 			return 0;
 		if (ret != ODB_READ_NOT_FOUND && status == ODB_READ_NOT_FOUND)
 			status = ret;
+	}
 
+	for (struct odb_files_dir *dir = files->dirs; dir; dir = dir->next) {
 		ret = odb_source_read_object_info(&dir->loose->base, oid, oi, flags,
 						  status == ODB_READ_NOT_FOUND ? errmsg : NULL);
 		if (!ret)
diff --git a/t/t5615-alternate-env.sh b/t/t5615-alternate-env.sh
index 9d6aa2187f..3259852a0f 100755
--- a/t/t5615-alternate-env.sh
+++ b/t/t5615-alternate-env.sh
@@ -88,4 +88,47 @@ test_expect_success !MINGW 'broken quoting falls back to interpreting raw' '
 	EOF
 '
 
+test_expect_success 'packs across sources are checked before loose objects' '
+	# Regression test for a performance issue in which an object that
+	# resides in an alternate as a packed object caused a spurious loose
+	# object lookup (a filesystem stat) on the main object store before the
+	# alternate packfile was consulted. Reading such an object must resolve
+	# to the alternate packfile, never to a loose copy in the main store.
+	#
+	# Build an alternate whose object "B" is stored as a delta in a
+	# packfile. git deltifies successive versions of a tracked file, so the
+	# older, shorter blob "B" becomes a delta against the newer, longer
+	# blob "O". A loose object has no delta base, so %(deltabase) tells us
+	# which store answered the read: the alternate pack (O) or a loose copy
+	# (the zero oid).
+	git init alt-src &&
+	test_seq 1 200 >alt-src/file &&
+	git -C alt-src add file &&
+	git -C alt-src commit -q -m base &&
+	B=$(git -C alt-src rev-parse HEAD:file) &&
+	git -C alt-src cat-file blob "$B" >b-content &&
+	test_seq 1 210 >alt-src/file &&
+	git -C alt-src add file &&
+	git -C alt-src commit -q -m more &&
+	O=$(git -C alt-src rev-parse HEAD:file) &&
+	git -C alt-src repack -adf --window=10 --depth=50 &&
+
+	# Precondition: in the alternate, B is a delta based on O.
+	echo "$B" >in &&
+	echo "$O" >expect &&
+	git -C alt-src cat-file --batch-check="%(deltabase)" <in >actual &&
+	test_cmp expect actual &&
+
+	# Main repo: write B as a loose object, before any alternate is active.
+	git init main &&
+	git -C main hash-object -w --stdin <b-content >/dev/null &&
+	test_path_is_file "main/.git/objects/$(test_oid_to_path "$B")" &&
+
+	# With the alternate active, B must resolve to the alternate packfile
+	# (deltabase O), not to the main store loose copy (deltabase zero oid).
+	GIT_ALTERNATE_OBJECT_DIRECTORIES="$PWD/alt-src/.git/objects" \
+		git -C main cat-file --batch-check="%(deltabase)" <in >actual &&
+	test_cmp expect actual
+'
+
 test_done