diff --git a/C4/Biblio.pm b/C4/Biblio.pm
index f694a19f46..5ddd7c7272 100644
--- a/C4/Biblio.pm
+++ b/C4/Biblio.pm
@@ -99,6 +99,7 @@ use Koha::Logger;
 use Koha::Cache::Memory::Lite;
 use Koha::Caches;
 use Koha::ClassSources;
+use Koha::Authority;
 use Koha::Authority::Types;
 use Koha::Acquisition::Currencies;
 use Koha::BackgroundJob::BatchUpdateBiblioHoldsQueue;
@@ -811,10 +812,35 @@ sub LinkBibHeadingsToAuthorities {
 
                     if ( C4::Context->preference('marcflavour') eq 'MARC21' ) {
                         my $userenv = C4::Context->userenv;
-                        my $library;
-                        if ( $userenv && $userenv->{'branch'} ) {
-                            $library = Koha::Libraries->find( $userenv->{'branch'} );
+
+                        # userenv's library could not exist because of a trick in misc/commit_file.pl
+                        # (see FIXME and set_userenv statement there)
+                        my $library =
+                            $userenv && $userenv->{branch} ? Koha::Libraries->find( $userenv->{branch} ) : undef;
+                        my $marcorgcode =
+                            $library ? $library->get_effective_marcorgcode : C4::Context->preference('MARCOrgCode');
+
+                        # Bug 31925: code the new authority with the source heading's actual
+                        # thesaurus instead of relying on AddAuthority()'s LCSH-only default.
+                        # AddAuthority() only defaults 008/040 when they aren't already present
+                        # on the record, so pre-populating them here is sufficient - no change
+                        # to AddAuthority() itself is needed.
+                        if ( $heading->{thesaurus} ) {
+                            my $date = POSIX::strftime( '%y%m%d', localtime );
+                            $marcrecordauth->insert_fields_ordered(
+                                MARC::Field->new(
+                                    '008', $date . Koha::Authority->default_marc21_008( $heading->{thesaurus} )
+                                )
+                            );
+
+                            my @f040_subfields = ( 'a' => $marcorgcode, 'c' => $marcorgcode );
+                            push @f040_subfields, ( 'f' => $heading->{thesaurus} )
+                                unless exists
+                                $Koha::Authority::MARC21_THESAURUS_TO_CONTROL_FIELD_008_11->{ $heading->{thesaurus} };
+                            $marcrecordauth->insert_fields_ordered(
+                                MARC::Field->new( '040', '', '', @f040_subfields ) );
                         }
+
                         $marcrecordauth->insert_fields_ordered(
                             MARC::Field->new(
                                 '667', '', '',
@@ -825,10 +851,8 @@ sub LinkBibHeadingsToAuthorities {
                         $cite =~ s/^[\s\,]*//;
                         $cite =~ s/[\s\,]*$//;
                         $cite =
-                            C4::Context->preference('GenerateAuthorityField670') . ": ("
-                            . (
-                            $library ? $library->get_effective_marcorgcode : C4::Context->preference('MARCOrgCode') )
-                            . ")"
+                              C4::Context->preference('GenerateAuthorityField670') . ": ("
+                            . "$marcorgcode)"
                             . $bib->subfield( '999', 'c' ) . ": "
                             . $cite;
                         $marcrecordauth->insert_fields_ordered( MARC::Field->new( '670', '', '', 'a' => $cite ) );
diff --git a/t/db_dependent/Biblio.t b/t/db_dependent/Biblio.t
index 1b2e81a7c2..975517886a 100755
--- a/t/db_dependent/Biblio.t
+++ b/t/db_dependent/Biblio.t
@@ -1114,6 +1114,65 @@ subtest "LinkBibHeadingsToAuthorities record generation tests" => sub {
     );
 };
 
+subtest "LinkBibHeadingsToAuthorities codes new authorities with the source thesaurus (bug 31925)" => sub {
+    plan tests => 7;
+
+    my $biblio_mod = Test::MockModule->new('C4::Linker::Default');
+    $biblio_mod->mock(
+        'get_link',
+        sub {
+            return ( undef, undef, 0 );
+        }
+    );
+
+    t::lib::Mocks::mock_preference( 'marcflavour',           'MARC21' );
+    t::lib::Mocks::mock_preference( 'AutoCreateAuthorities', '1' );
+
+    my $linker = C4::Linker::Default->new();
+
+    # Case 1: plain LCSH (ind2=0, no $2) - regression guard, must stay coded 'a'
+    my $biblio = $builder->build_sample_biblio();
+    my $record = $biblio->metadata->record;
+    my $field  = MARC::Field->new( '650', '', '0', 'a' => 'Feminism' );
+    $record->append_fields($field);
+    LinkBibHeadingsToAuthorities( $linker, $record, "", undef, 650 );
+    my $authority = GetAuthority( $record->subfield( '650', '9' ) );
+    is( substr( $authority->field('008')->data(), 11, 1 ), 'a',   'Plain LCSH heading (ind2=0) is coded 008/11=a' );
+    is( $authority->subfield( '040', 'f' ),                undef, 'Plain LCSH heading (ind2=0) gets no 040$f' );
+
+    # Case 2: FAST heading (ind2=7, $2=fast) - the actual bug: must NOT be coded as LCSH
+    $biblio = $builder->build_sample_biblio();
+    $record = $biblio->metadata->record;
+    $field  = MARC::Field->new( '650', '', '7', 'a' => 'Miracles', '2' => 'fast' );
+    $record->append_fields($field);
+    LinkBibHeadingsToAuthorities( $linker, $record, "", undef, 650 );
+    $authority = GetAuthority( $record->subfield( '650', '9' ) );
+    is( substr( $authority->field('008')->data(), 11, 1 ), 'z',    'FAST heading (ind2=7, $2=fast) is coded 008/11=z' );
+    is( $authority->subfield( '040', 'f' ),                'fast', 'FAST heading (ind2=7, $2=fast) gets 040$f=fast' );
+
+    # Case 3: MeSH heading (ind2=2, no $2)
+    $biblio = $builder->build_sample_biblio();
+    $record = $biblio->metadata->record;
+    $field  = MARC::Field->new( '650', '', '2', 'a' => 'Something' );
+    $record->append_fields($field);
+    LinkBibHeadingsToAuthorities( $linker, $record, "", undef, 650 );
+    $authority = GetAuthority( $record->subfield( '650', '9' ) );
+    is( substr( $authority->field('008')->data(), 11, 1 ), 'c',   'MeSH heading (ind2=2) is coded 008/11=c' );
+    is( $authority->subfield( '040', 'f' ),                undef, 'MeSH heading (ind2=2) gets no 040$f' );
+
+    # Case 4: non-subject heading (personal name added entry) is unaffected - no thesaurus semantics
+    $biblio = $builder->build_sample_biblio();
+    $record = $biblio->metadata->record;
+    $field  = MARC::Field->new( '700', '', '', 'a' => 'Doe, Jane' );
+    $record->append_fields($field);
+    LinkBibHeadingsToAuthorities( $linker, $record, "", undef, 700 );
+    $authority = GetAuthority( $record->subfield( '700', '9' ) );
+    is(
+        substr( $authority->field('008')->data(), 11, 1 ), 'a',
+        'Non-subject heading (700) keeps the default LCSH 008/11=a coding, unaffected by this fix'
+    );
+};
+
 subtest 'autoControlNumber tests' => sub {
 
     plan tests => 3;