diff --git a/contrib/pg_freespacemap/Makefile b/contrib/pg_freespacemap/Makefile
index a986ae73d69cc..4afd35ade5a8c 100644
--- a/contrib/pg_freespacemap/Makefile
+++ b/contrib/pg_freespacemap/Makefile
@@ -12,8 +12,11 @@ DATA = pg_freespacemap--1.1.sql \
 	pg_freespacemap--1.0--1.1.sql
 PGFILEDESC = "pg_freespacemap - monitoring of free space map"
 
+EXTRA_INSTALL = contrib/pageinspect
+
 REGRESS_OPTS = --temp-config $(top_srcdir)/contrib/pg_freespacemap/pg_freespacemap.conf
 REGRESS = pg_freespacemap
+TAP_TESTS = 1
 
 # Disabled because these tests require "autovacuum=off", which
 # typical installcheck users do not have (e.g. buildfarm clients).
diff --git a/contrib/pg_freespacemap/meson.build b/contrib/pg_freespacemap/meson.build
index 67b46c77ff3af..4e51c267cd8c8 100644
--- a/contrib/pg_freespacemap/meson.build
+++ b/contrib/pg_freespacemap/meson.build
@@ -40,4 +40,9 @@ tests += {
     # typical runningcheck users do not have (e.g. buildfarm clients).
     'runningcheck': false,
   },
+  'tap': {
+    'tests': [
+      't/001_bulk_extend_fsm.pl',
+    ],
+  },
 }
diff --git a/contrib/pg_freespacemap/t/001_bulk_extend_fsm.pl b/contrib/pg_freespacemap/t/001_bulk_extend_fsm.pl
new file mode 100644
index 0000000000000..d075b5d310aef
--- /dev/null
+++ b/contrib/pg_freespacemap/t/001_bulk_extend_fsm.pl
@@ -0,0 +1,151 @@
+# Copyright (c) 2026, PostgreSQL Global Development Group
+
+# Regression coverage for commit a2fd8d6 ("Include last block in FSM vacuum of
+# bulk extended relation").
+#
+# When a relation is bulk-extended, hio.c records the newly added, not
+# immediately used blocks in the free space map and then calls
+# FreeSpaceMapVacuumRange() to propagate that free space up the FSM tree so
+# other backends can find it.  The end block argument of
+# FreeSpaceMapVacuumRange() is *exclusive*, but the code used to pass the number
+# of the last added block.  When that last block was the first block covered by
+# a brand new FSM leaf page (heap block SlotsPerFSMPage, 2*SlotsPerFSMPage, ...),
+# its free space was recorded in the leaf FSM page but never propagated to the
+# upper FSM levels, staying invisible to FSM searches until the next full FSM
+# vacuum.
+#
+# The test bulk-extends a relation with COPY (which uses a BulkInsertState, and
+# so takes the bulk-extension code path) so that the final extension's last
+# block lands exactly on an FSM leaf-page boundary.  It then checks the
+# invariant that a2fd8d6 restores: because the bulk extension has already
+# propagated the new free space, an immediately following VACUUM must not raise
+# any FSM node value.  With the bug present the boundary block's free space is
+# missing from the upper FSM levels, so the redundant VACUUM raises the parent
+# node from 0, and the test fails.  (pg_freespace() reads the leaf value
+# directly and is unaffected by the bug, so it cannot be used to detect it.)
+
+use strict;
+use warnings FATAL => 'all';
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More;
+
+my $node = PostgreSQL::Test::Cluster->new('main');
+$node->init;
+# Autovacuum (or any other FSM vacuum) would repair the upper FSM levels and
+# mask the bug, so keep it firmly disabled.
+$node->append_conf('postgresql.conf', 'autovacuum = off');
+$node->start;
+
+$node->safe_psql('postgres',
+	'CREATE EXTENSION pg_freespacemap; CREATE EXTENSION pageinspect;');
+
+my $bs = $node->safe_psql('postgres', 'SHOW block_size') + 0;
+
+# SlotsPerFSMPage -- the number of heap blocks covered by one FSM leaf page,
+# and thus the first heap block of the second FSM leaf page.  This is only used
+# to size the test relation so that a bulk extension lands on the boundary; the
+# bug detection below needs no knowledge of the FSM page layout.  See
+# src/include/storage/fsm_internals.h: with SizeOfPageHeaderData = 24 and
+# offsetof(FSMPageData, fp_nodes) = 4, SlotsPerFSMPage = BLCKSZ/2 - 27.
+my $boundary = int($bs / 2) - 27;
+
+# Measure how many of our narrow (single int) tuples fit on a heap page by
+# filling more than one page and counting the tuples that landed on block 0,
+# which is necessarily full.
+$node->safe_psql('postgres',
+	'CREATE TABLE probe (c int) WITH (autovacuum_enabled = off);');
+$node->safe_psql('postgres',
+	'INSERT INTO probe SELECT generate_series(1, 5000);');
+my $rpb = $node->safe_psql('postgres',
+	"SELECT count(*) FROM probe WHERE (ctid::text::point)[0]::int = 0;") + 0;
+ok($rpb > 0, "measured $rpb tuples per heap block");
+
+# Helper: COPY the values 1..$n into an existing table.  safe_psql() feeds the
+# script to psql's stdin, so we can inline the COPY data after the command and
+# terminate it with a backslash-dot, like a hand-written psql script.  COPY uses
+# a BulkInsertState, which is what exercises the bulk-extension code path.
+sub bulk_copy
+{
+	my ($table, $n) = @_;
+	my $script = "COPY $table FROM STDIN;\n" . join("\n", 1 .. $n) . "\n\\.\n";
+	$node->safe_psql('postgres', $script);
+	return;
+}
+
+# The relation is extended in chunks; once the extension size saturates at
+# MAX_BUFFERS_TO_EXTEND_BY (64) the relation size after a COPY is congruent to a
+# fixed value modulo 64, independent of how many rows were copied.  Learn that
+# offset with a small COPY into a separately committed table (a table created in
+# the same transaction as the COPY would skip the FSM entirely).
+$node->safe_psql('postgres',
+	'CREATE TABLE grid_probe (c int) WITH (autovacuum_enabled = off);');
+bulk_copy('grid_probe', 300 * $rpb);
+my $offset =
+  ($node->safe_psql('postgres', "SELECT pg_relation_size('grid_probe') / $bs;")
+	  + 0) % 64;
+
+# To make the final extension end exactly on the boundary block, shift the
+# extension grid by pre-filling some fully packed blocks (committed densely, so
+# nothing is left in the FSM), then bulk-extend across the boundary with one
+# COPY that leaves the data about 32 blocks short of the boundary.
+my $shift = (($boundary + 1) - $offset) % 64;
+
+$node->safe_psql('postgres',
+	'CREATE TABLE fsm_boundary (c int) WITH (autovacuum_enabled = off);');
+$node->safe_psql('postgres',
+	"INSERT INTO fsm_boundary SELECT generate_series(1, $shift * $rpb);")
+  if $shift > 0;
+bulk_copy('fsm_boundary', ($boundary - 32 - $shift) * $rpb);
+
+# Confirm we built the intended geometry: the relation has exactly boundary + 1
+# blocks (the last extended block is the boundary block), and that block is a
+# trailing block whose free space was recorded in the leaf FSM page.
+my $nblocks = $node->safe_psql('postgres',
+	"SELECT pg_relation_size('fsm_boundary') / $bs;") + 0;
+is($nblocks, $boundary + 1,
+	"relation bulk-extended to the FSM leaf-page boundary ($nblocks blocks)");
+
+my $leaf_avail = $node->safe_psql('postgres',
+	"SELECT pg_freespace('fsm_boundary', $boundary);") + 0;
+ok($leaf_avail > 0,
+	"boundary block $boundary has free space in the leaf FSM page");
+
+# The core regression check.  Snapshot every non-zero FSM node, run a redundant
+# VACUUM, and confirm it raised no node value.  A VACUUM legitimately *lowers*
+# stale entries for partially filled pages, but it may only *raise* a node if
+# the bulk extension failed to propagate free space up the tree -- exactly the
+# a2fd8d6 bug.  All of this runs in one psql session so the pre-VACUUM snapshot
+# (a temp table) survives, and VACUUM runs outside any transaction block.
+my $raised = $node->safe_psql('postgres', qq{
+	CREATE TEMP TABLE fsm_before AS
+	    SELECT blk,
+	           split_part(line, ':', 1)::int AS node,
+	           split_part(line, ':', 2)::int AS val
+	      FROM generate_series(0, pg_relation_size('fsm_boundary', 'fsm') / $bs - 1) blk,
+	           LATERAL regexp_split_to_table(
+	               fsm_page_contents(get_raw_page('fsm_boundary', 'fsm', blk)),
+	               E'\\n') AS line
+	     WHERE line LIKE '%:%' AND line NOT LIKE 'fp_next_slot%';
+
+	VACUUM fsm_boundary;
+
+	SELECT count(*)
+	  FROM (SELECT blk,
+	               split_part(line, ':', 1)::int AS node,
+	               split_part(line, ':', 2)::int AS val
+	          FROM generate_series(0, pg_relation_size('fsm_boundary', 'fsm') / $bs - 1) blk,
+	               LATERAL regexp_split_to_table(
+	                   fsm_page_contents(get_raw_page('fsm_boundary', 'fsm', blk)),
+	                   E'\\n') AS line
+	         WHERE line LIKE '%:%' AND line NOT LIKE 'fp_next_slot%') AS fsm_after
+	  LEFT JOIN fsm_before USING (blk, node)
+	 WHERE fsm_after.val > COALESCE(fsm_before.val, 0);
+});
+is($raised, '0',
+	'bulk extension propagated boundary free space: redundant VACUUM raises no FSM node'
+);
+
+$node->stop;
+
+done_testing();