diff --git a/Documentation/git-update-index.txt b/Documentation/git-update-index.txt
index f4bb9c5daf..1271486ae9 100644
--- a/Documentation/git-update-index.txt
+++ b/Documentation/git-update-index.txt
@@ -162,13 +162,19 @@ you will need to handle the situation manually.
 	Write the resulting index out in the named on-disk format version.
 	Supported versions are 2, 3 and 4. The current default version is 2
 	or 3, depending on whether extra features are used, such as
-	`git add -N`.
+	`git add -N`.  With `--verbose`, also report the version the index
+	file uses before and after this command.
 +
 Version 4 performs a simple pathname compression that reduces index
 size by 30%-50% on large repositories, which results in faster load
-time. Version 4 is relatively young (first released in 1.8.0 in
-October 2012). Other Git implementations such as JGit and libgit2
-may not support it yet.
+time.  Git supports it since version 1.8.0, released in October 2012,
+and support for it was added to libgit2 in 2016 and to JGit in 2020.
+Older versions of this manual page called it "relatively young", but
+it should be considered mature technology these days.
+
+--show-index-version::
+	Report the index format version used by the on-disk index file.
+	See `--index-version` above.
 
 -z::
 	Only meaningful with `--stdin` or `--index-info`; paths are
diff --git a/Makefile b/Makefile
index 5776309365..003e63b792 100644
--- a/Makefile
+++ b/Makefile
@@ -808,7 +808,6 @@ TEST_BUILTINS_OBJS += test-hash-speed.o
 TEST_BUILTINS_OBJS += test-hash.o
 TEST_BUILTINS_OBJS += test-hashmap.o
 TEST_BUILTINS_OBJS += test-hexdump.o
-TEST_BUILTINS_OBJS += test-index-version.o
 TEST_BUILTINS_OBJS += test-json-writer.o
 TEST_BUILTINS_OBJS += test-lazy-init-name-hash.o
 TEST_BUILTINS_OBJS += test-match-trees.o
diff --git a/builtin/update-index.c b/builtin/update-index.c
index 59acae3336..97617c587e 100644
--- a/builtin/update-index.c
+++ b/builtin/update-index.c
@@ -1090,6 +1090,8 @@ int cmd_update_index(int argc, const char **argv, const char *prefix)
 			resolve_undo_clear_callback),
 		OPT_INTEGER(0, "index-version", &preferred_index_format,
 			N_("write index in this format")),
+		OPT_SET_INT(0, "show-index-version", &preferred_index_format,
+			    N_("report on-disk index format version"), -1),
 		OPT_BOOL(0, "split-index", &split_index,
 			N_("enable or disable split index")),
 		OPT_BOOL(0, "untracked-cache", &untracked_cache,
@@ -1182,15 +1184,20 @@ int cmd_update_index(int argc, const char **argv, const char *prefix)
 
 	getline_fn = nul_term_line ? strbuf_getline_nul : strbuf_getline_lf;
 	if (preferred_index_format) {
-		if (preferred_index_format < INDEX_FORMAT_LB ||
-		    INDEX_FORMAT_UB < preferred_index_format)
+		if (preferred_index_format < 0) {
+			printf(_("%d\n"), the_index.version);
+		} else if (preferred_index_format < INDEX_FORMAT_LB ||
+			   INDEX_FORMAT_UB < preferred_index_format) {
 			die("index-version %d not in range: %d..%d",
 			    preferred_index_format,
 			    INDEX_FORMAT_LB, INDEX_FORMAT_UB);
-
-		if (the_index.version != preferred_index_format)
-			the_index.cache_changed |= SOMETHING_CHANGED;
-		the_index.version = preferred_index_format;
+		} else {
+			if (the_index.version != preferred_index_format)
+				the_index.cache_changed |= SOMETHING_CHANGED;
+			report(_("index-version: was %d, set to %d"),
+			       the_index.version, preferred_index_format);
+			the_index.version = preferred_index_format;
+		}
 	}
 
 	if (read_from_stdin) {
diff --git a/t/helper/test-index-version.c b/t/helper/test-index-version.c
deleted file mode 100644
index f3c2dbe0a2..0000000000
--- a/t/helper/test-index-version.c
+++ /dev/null
@@ -1,15 +0,0 @@
-#include "test-tool.h"
-#include "read-cache-ll.h"
-
-int cmd__index_version(int argc UNUSED, const char **argv UNUSED)
-{
-	struct cache_header hdr;
-	int version;
-
-	memset(&hdr,0,sizeof(hdr));
-	if (read(0, &hdr, sizeof(hdr)) != sizeof(hdr))
-		return 0;
-	version = ntohl(hdr.hdr_version);
-	printf("%d\n", version);
-	return 0;
-}
diff --git a/t/helper/test-tool.c b/t/helper/test-tool.c
index abe8a785eb..621ac3dd10 100644
--- a/t/helper/test-tool.c
+++ b/t/helper/test-tool.c
@@ -38,7 +38,6 @@ static struct test_cmd cmds[] = {
 	{ "hashmap", cmd__hashmap },
 	{ "hash-speed", cmd__hash_speed },
 	{ "hexdump", cmd__hexdump },
-	{ "index-version", cmd__index_version },
 	{ "json-writer", cmd__json_writer },
 	{ "lazy-init-name-hash", cmd__lazy_init_name_hash },
 	{ "match-trees", cmd__match_trees },
diff --git a/t/helper/test-tool.h b/t/helper/test-tool.h
index ea2672436c..a641c3a81d 100644
--- a/t/helper/test-tool.h
+++ b/t/helper/test-tool.h
@@ -32,7 +32,6 @@ int cmd__getcwd(int argc, const char **argv);
 int cmd__hashmap(int argc, const char **argv);
 int cmd__hash_speed(int argc, const char **argv);
 int cmd__hexdump(int argc, const char **argv);
-int cmd__index_version(int argc, const char **argv);
 int cmd__json_writer(int argc, const char **argv);
 int cmd__lazy_init_name_hash(int argc, const char **argv);
 int cmd__match_trees(int argc, const char **argv);
diff --git a/t/t1600-index.sh b/t/t1600-index.sh
index 9368d82f7d..62e7fd1596 100755
--- a/t/t1600-index.sh
+++ b/t/t1600-index.sh
@@ -118,7 +118,7 @@ test_index_version () {
 		fi &&
 		git add a &&
 		echo $EXPECTED_OUTPUT_VERSION >expect &&
-		test-tool index-version <.git/index >actual &&
+		git update-index --show-index-version >actual &&
 		test_cmp expect actual
 	)
 }
diff --git a/t/t1700-split-index.sh b/t/t1700-split-index.sh
index b4ab166369..a7b7263b35 100755
--- a/t/t1700-split-index.sh
+++ b/t/t1700-split-index.sh
@@ -43,7 +43,7 @@ test_expect_success 'enable split index' '
 	git config splitIndex.maxPercentChange 100 &&
 	git update-index --split-index &&
 	test-tool dump-split-index .git/index >actual &&
-	indexversion=$(test-tool index-version <.git/index) &&
+	indexversion=$(git update-index --show-index-version) &&
 
 	# NEEDSWORK: Stop hard-coding checksums.
 	if test "$indexversion" = "4"
diff --git a/t/t2104-update-index-skip-worktree.sh b/t/t2104-update-index-skip-worktree.sh
index b8686aabd3..0bab134d71 100755
--- a/t/t2104-update-index-skip-worktree.sh
+++ b/t/t2104-update-index-skip-worktree.sh
@@ -39,7 +39,7 @@ test_expect_success 'setup' '
 '
 
 test_expect_success 'index is at version 2' '
-	test "$(test-tool index-version < .git/index)" = 2
+	test "$(git update-index --show-index-version)" = 2
 '
 
 test_expect_success 'update-index --skip-worktree' '
@@ -48,7 +48,7 @@ test_expect_success 'update-index --skip-worktree' '
 '
 
 test_expect_success 'index is at version 3 after having some skip-worktree entries' '
-	test "$(test-tool index-version < .git/index)" = 3
+	test "$(git update-index --show-index-version)" = 3
 '
 
 test_expect_success 'ls-files -t' '
@@ -61,7 +61,7 @@ test_expect_success 'update-index --no-skip-worktree' '
 '
 
 test_expect_success 'index version is back to 2 when there is no skip-worktree entry' '
-	test "$(test-tool index-version < .git/index)" = 2
+	test "$(git update-index --show-index-version)" = 2
 '
 
 test_done
diff --git a/t/t2107-update-index-basic.sh b/t/t2107-update-index-basic.sh
index 89b285fa3a..22f4c92399 100755
--- a/t/t2107-update-index-basic.sh
+++ b/t/t2107-update-index-basic.sh
@@ -111,4 +111,35 @@ test_expect_success '--chmod=+x and chmod=-x in the same argument list' '
 	test_cmp expect actual
 '
 
+test_expect_success '--index-version' '
+	git commit --allow-empty -m snap &&
+	git reset --hard &&
+	git rm -f -r --cached . &&
+
+	# The default index version is 2 --- update this test
+	# when you change it in the code
+	git update-index --show-index-version >actual &&
+	echo 2 >expect &&
+	test_cmp expect actual &&
+
+	# The next test wants us to be using version 2
+	git update-index --index-version 2 &&
+
+	git update-index --index-version 4 --verbose >actual &&
+	echo "index-version: was 2, set to 4" >expect &&
+	test_cmp expect actual &&
+
+	git update-index --index-version 4 --verbose >actual &&
+	echo "index-version: was 4, set to 4" >expect &&
+	test_cmp expect actual &&
+
+	git update-index --index-version 2 --verbose >actual &&
+	echo "index-version: was 4, set to 2" >expect &&
+	test_cmp expect actual &&
+
+	# non-verbose should be silent
+	git update-index --index-version 4 >actual &&
+	test_must_be_empty actual
+'
+
 test_done