diff --git a/.buildkite/scripts/build_pr.sh b/.buildkite/scripts/build_pr.sh
index 506a5a6d0ee11..50781309a672d 100755
--- a/.buildkite/scripts/build_pr.sh
+++ b/.buildkite/scripts/build_pr.sh
@@ -41,6 +41,25 @@ buildkite-agent \
 
 
 if [[ "${GITHUB_PR_BASE_REPO}" != 'docs' ]]; then
+  # main/master are hardcoded to always skip the build, without needing conf.yaml/YAML at all:
+  # we no longer build legacy AsciiDoc docs against these branches.
+  if [[ "${GITHUB_PR_TARGET_BRANCH}" == "main" || "${GITHUB_PR_TARGET_BRANCH}" == "master" ]]; then
+    echo "Target branch '${GITHUB_PR_TARGET_BRANCH}' is main/master — skipping build (reporting success)."
+    exit 0
+  fi
+
+  # Build only if conf.yaml lists the target branch as a legacy AsciiDoc branch
+  # for this repo — otherwise skip but report success, since docs-build-pr is a
+  # required check and we want it green rather than failing or left pending.
+  # The helper exits non-zero only when conf.yaml itself couldn't be read (e.g.
+  # the perl YAML module is missing), which short-circuits the condition below
+  # so we fail open and build as today, rather than silently going green.
+  if legacy_branches=$(perl "$(dirname "$0")/legacy_branches.pl" "$GITHUB_PR_BASE_REPO") \
+       && ! grep -qxF "${GITHUB_PR_TARGET_BRANCH}" <<< "${legacy_branches}"; then
+    echo "Target branch '${GITHUB_PR_TARGET_BRANCH}' is not a legacy AsciiDoc branch in conf.yaml for ${GITHUB_PR_BASE_REPO} — skipping build (reporting success)."
+    exit 0
+  fi
+
   # Buildkite PR bot for repositories other than the `elastic/docs` repo are configured to
   # always checkout the master branch of the `elastic/docs` repo (where the build logic resides).
   # We first need to checkout the product repo / branch in a sub directory, that we'll reference
diff --git a/.buildkite/scripts/legacy_branches.pl b/.buildkite/scripts/legacy_branches.pl
new file mode 100755
index 0000000000000..e89723462f223
--- /dev/null
+++ b/.buildkite/scripts/legacy_branches.pl
@@ -0,0 +1,88 @@
+#!/usr/bin/env perl
+use strict;
+use warnings;
+use YAML qw(LoadFile);
+
+# Usage: legacy_branches.pl <github-repo-name>
+# Prints one legacy (AsciiDoc) branch per line that the given repo still carries in conf.yaml.
+#
+# Exit 0: conf.yaml was read successfully. The list is empty if the repo is
+#         fully migrated to docs-builder or isn't in conf.yaml at all — either
+#         way it has no legacy branches, so the caller should skip the build.
+# Exit 1: conf.yaml couldn't be loaded (e.g. missing YAML module, parse error)
+#         — caller should build as today (fail open), since we can't tell.
+
+my ($github_repo) = @ARGV
+    or die "Usage: $0 <github-repo-name>\n";
+
+# conf.yaml lives in the repo root, which is also the working directory when
+# build_pr.sh runs — same assumption used by build_docs.pl (see build_docs.pl:964).
+my $conf = eval { LoadFile('conf.yaml') };
+if ($@) {
+    warn "Failed to load conf.yaml: $@\n";
+    exit 1;
+}
+
+# Sources in conf.yaml reference the conf key (e.g. "esf"), which for most repos
+# is already the GitHub repo name but for some (e.g. "elastic-serverless-forwarder")
+# differs from it. Try the GitHub name as a conf key directly first, then fall back
+# to matching it against each repo's URL basename.
+my $conf_key = exists $conf->{repos}{$github_repo} ? $github_repo : undef;
+unless ( defined $conf_key ) {
+    while ( my ( $key, $url ) = each %{ $conf->{repos} } ) {
+        ( my $name = $url ) =~ s{.*/|\.git$}{}g;    # URL -> repo name (strip path and .git)
+        if ( $name eq $github_repo ) {
+            $conf_key = $key;
+            last;
+        }
+    }
+}
+
+unless ( defined $conf_key ) {
+    exit 0;    # repo not in conf — no legacy branches, caller should skip
+}
+
+# Walk conf.yaml contents and collect every git branch for which this repo still
+# appears as a source, using the same branch-resolution rules as the build itself
+# (ES::Book:104-109 + ES::BranchTracker:63):
+#   - git branch = LHS of each branches entry (scalar or single-key hash)
+#   - map_branches{book_branch} applied to get the actual git branch in the source repo
+#   - branches in exclude_branches skipped
+my %branches;
+walk_entries( $conf->{contents}, $conf_key, \%branches );
+
+print "$_\n" for sort keys %branches;
+exit 0;
+
+
+sub walk_entries {
+    my ( $entries, $conf_key, $branches ) = @_;
+    for my $entry (@$entries) {
+        if ( $entry->{sections} ) {
+            walk_entries( $entry->{sections}, $conf_key, $branches );
+        } else {
+            collect_book_branches( $entry, $conf_key, $branches );
+        }
+    }
+}
+
+sub collect_book_branches {
+    my ( $book, $conf_key, $branches ) = @_;
+    my $branch_list = $book->{branches} or return;
+    my $sources     = $book->{sources}  or return;
+
+    my @matching = grep { ( $_->{repo} // '' ) eq $conf_key } @$sources;
+    return unless @matching;
+
+    for my $source (@matching) {
+        my $map  = $source->{map_branches}    // {};
+        my %excl = map { $_ => 1 } @{ $source->{exclude_branches} // [] };
+
+        for my $entry (@$branch_list) {
+            # Each entry is a scalar or a single-key hash { book_branch => display_title }
+            my ($branch) = ref $entry eq 'HASH' ? keys %$entry : ($entry);
+            next if $excl{$branch};
+            $branches->{ $map->{$branch} // $branch } = 1;
+        }
+    }
+}
diff --git a/.buildkite/scripts/legacy_branches.t b/.buildkite/scripts/legacy_branches.t
new file mode 100755
index 0000000000000..8905f5faa7742
--- /dev/null
+++ b/.buildkite/scripts/legacy_branches.t
@@ -0,0 +1,31 @@
+#!/usr/bin/env perl
+use strict;
+use warnings;
+use Test::More;
+use FindBin qw($RealBin);
+
+# Regression check for legacy_branches.pl's repo-name resolution: it must
+# handle both a GitHub repo name that already matches a conf.yaml key
+# directly (e.g. "esf" itself) and one that only matches via a source repo's
+# URL basename (e.g. "elastic-serverless-forwarder" -> "esf"). Run from the
+# repo root since legacy_branches.pl reads ./conf.yaml relative to cwd.
+
+chdir "$RealBin/../.." or die "Can't chdir to repo root: $!";
+
+sub run {
+    my ($repo) = @_;
+    my $out = `perl .buildkite/scripts/legacy_branches.pl @{[quotemeta $repo]} 2>/dev/null`;
+    return ( $? >> 8, $out );
+}
+
+for my $repo (qw(kibana-cn swiftype esf elastic-serverless-forwarder)) {
+    my ( $exit, $out ) = run($repo);
+    is( $exit, 0, "$repo: exits 0" );
+    isnt( $out, '', "$repo: has at least one legacy branch (conf key resolved)" );
+}
+
+my ( $exit, $out ) = run('some-repo-not-in-conf-yaml');
+is( $exit, 0, 'unknown repo: exits 0' );
+is( $out, '', 'unknown repo: no legacy branches' );
+
+done_testing();