diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index 5a689628b6..d714435001 100644
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c
@@ -266,7 +266,7 @@ struct configured_exclusion {
 static struct oidmap configured_exclusions;
 
 static struct oidset excluded_by_config;
-static int use_full_name_hash;
+static int use_full_name_hash = -1;
 
 static inline uint32_t pack_name_hash_fn(const char *name)
 {
@@ -4586,8 +4586,10 @@ int cmd_pack_objects(int argc,
 	if (pack_to_stdout || !rev_list_all)
 		write_bitmap_index = 0;
 
-	if (write_bitmap_index && use_full_name_hash)
+	if (write_bitmap_index && use_full_name_hash > 0)
 		die(_("currently, the --full-name-hash option is incompatible with --write-bitmap-index"));
+	if (use_full_name_hash < 0)
+		use_full_name_hash = git_env_bool("GIT_TEST_FULL_NAME_HASH", 0);
 
 	if (use_delta_islands)
 		strvec_push(&rp, "--topo-order");
diff --git a/ci/run-build-and-tests.sh b/ci/run-build-and-tests.sh
index 2e28d02b20..75b40f07bb 100755
--- a/ci/run-build-and-tests.sh
+++ b/ci/run-build-and-tests.sh
@@ -30,6 +30,7 @@ linux-TEST-vars)
 	export GIT_TEST_NO_WRITE_REV_INDEX=1
 	export GIT_TEST_CHECKOUT_WORKERS=2
 	export GIT_TEST_PACK_USE_BITMAP_BOUNDARY_TRAVERSAL=1
+	export GIT_TEST_FULL_NAME_HASH=1
 	;;
 linux-clang)
 	export GIT_TEST_DEFAULT_HASH=sha1
diff --git a/t/README b/t/README
index 8dcb778e26..e761d0b0b5 100644
--- a/t/README
+++ b/t/README
@@ -491,6 +491,10 @@ a test and then fails then the whole test run will abort. This can help to make
 sure the expected tests are executed and not silently skipped when their
 dependency breaks or is simply not present in a new environment.
 
+GIT_TEST_FULL_NAME_HASH=<boolean>, when true, sets the default name-hash
+function in 'git pack-objects' to be the one used by the --full-name-hash
+option.
+
 Naming Tests
 ------------
 
diff --git a/t/t5510-fetch.sh b/t/t5510-fetch.sh
index 0890b9f61c..be79c72495 100755
--- a/t/t5510-fetch.sh
+++ b/t/t5510-fetch.sh
@@ -1062,7 +1062,12 @@ test_expect_success 'all boundary commits are excluded' '
 	test_tick &&
 	git merge otherside &&
 	ad=$(git log --no-walk --format=%ad HEAD) &&
-	git bundle create twoside-boundary.bdl main --since="$ad" &&
+
+	# If the --full-name-hash function is used here, then no delta
+	# pair is found and the bundle does not expand to three objects
+	# when fixing the thin object.
+	GIT_TEST_FULL_NAME_HASH=0 \
+		git bundle create twoside-boundary.bdl main --since="$ad" &&
 	test_bundle_object_count --thin twoside-boundary.bdl 3
 '
 
diff --git a/t/t5616-partial-clone.sh b/t/t5616-partial-clone.sh
index c53e93be2f..425aa8d878 100755
--- a/t/t5616-partial-clone.sh
+++ b/t/t5616-partial-clone.sh
@@ -516,7 +516,18 @@ test_expect_success 'fetch lazy-fetches only to resolve deltas' '
 	# Exercise to make sure it works. Git will not fetch anything from the
 	# promisor remote other than for the big tree (because it needs to
 	# resolve the delta).
-	GIT_TRACE_PACKET="$(pwd)/trace" git -C client \
+	#
+	# TODO: the --full-name-hash option is disabled here, since this test
+	# is fundamentally broken! When GIT_TEST_FULL_NAME_HASH=1, the server
+	# recognizes delta bases in a different way and then sends a _blob_ to
+	# the client with a delta base that the client does not have! This is
+	# because the client is cloned from "promisor-server" with tree:0 but
+	# is now fetching from "server" withot any filter. This is violating the
+	# promise to the server that all reachable objects exist and could be
+	# used as delta bases!
+	GIT_TRACE_PACKET="$(pwd)/trace" \
+	GIT_TEST_FULL_NAME_HASH=0 \
+		git -C client \
 		fetch "file://$(pwd)/server" main &&
 
 	# Verify the assumption that the client needed to fetch the delta base
@@ -535,7 +546,18 @@ test_expect_success 'fetch lazy-fetches only to resolve deltas, protocol v2' '
 	# Exercise to make sure it works. Git will not fetch anything from the
 	# promisor remote other than for the big blob (because it needs to
 	# resolve the delta).
-	GIT_TRACE_PACKET="$(pwd)/trace" git -C client \
+	#
+	# TODO: the --full-name-hash option is disabled here, since this test
+	# is fundamentally broken! When GIT_TEST_FULL_NAME_HASH=1, the server
+	# recognizes delta bases in a different way and then sends a _blob_ to
+	# the client with a delta base that the client does not have! This is
+	# because the client is cloned from "promisor-server" with tree:0 but
+	# is now fetching from "server" withot any filter. This is violating the
+	# promise to the server that all reachable objects exist and could be
+	# used as delta bases!
+	GIT_TRACE_PACKET="$(pwd)/trace" \
+	GIT_TEST_FULL_NAME_HASH=0 \
+		git -C client \
 		fetch "file://$(pwd)/server" main &&
 
 	# Verify that protocol version 2 was used.
diff --git a/t/t6020-bundle-misc.sh b/t/t6020-bundle-misc.sh
index 34b5cd62c2..553a20d10e 100755
--- a/t/t6020-bundle-misc.sh
+++ b/t/t6020-bundle-misc.sh
@@ -247,7 +247,11 @@ test_expect_success 'create bundle with --since option' '
 	EOF
 	test_cmp expect actual &&
 
-	git bundle create since.bdl \
+	# If the --full-name-hash option is used, then one fewer
+	# delta base is found and this counts a different number
+	# of objects after performing --fix-thin.
+	GIT_TEST_FULL_NAME_HASH=0 \
+		git bundle create since.bdl \
 		--since "Thu Apr 7 15:27:00 2005 -0700" \
 		--all &&