diff --git a/.buildkite/scripts/build_pr.sh b/.buildkite/scripts/build_pr.sh
index 506a5a6d0ee11..9fc51e6b5e2ac 100755
--- a/.buildkite/scripts/build_pr.sh
+++ b/.buildkite/scripts/build_pr.sh
@@ -41,6 +41,18 @@ buildkite-agent \
 
 
 if [[ "${GITHUB_PR_BASE_REPO}" != 'docs' ]]; then
+  # Skip the build but report success when the PR targets a branch that no longer carries
+  # legacy AsciiDoc docs (per conf.yaml). docs-build-pr is a required check, so we exit 0
+  # to keep it green rather than failing or leaving it pending.
+  # The helper exits non-zero on a parse error or unknown repo, which short-circuits the
+  # condition below so we build as today — i.e. fail open, never block a real build.
+  if legacy_branches=$(perl "$(dirname "$0")/legacy_branches.pl" "$GITHUB_PR_BASE_REPO") \
+       && [[ -n "${legacy_branches}" ]] \
+       && ! grep -qxF "${GITHUB_PR_TARGET_BRANCH}" <<< "${legacy_branches}"; then
+    echo "Target branch '${GITHUB_PR_TARGET_BRANCH}' has no legacy AsciiDoc docs in conf.yaml for ${GITHUB_PR_BASE_REPO} — skipping build (reporting success)."
+    exit 0
+  fi
+
   # Buildkite PR bot for repositories other than the `elastic/docs` repo are configured to
   # always checkout the master branch of the `elastic/docs` repo (where the build logic resides).
   # We first need to checkout the product repo / branch in a sub directory, that we'll reference
diff --git a/.buildkite/scripts/legacy_branches.pl b/.buildkite/scripts/legacy_branches.pl
new file mode 100755
index 0000000000000..6b5a000759e3b
--- /dev/null
+++ b/.buildkite/scripts/legacy_branches.pl
@@ -0,0 +1,81 @@
+#!/usr/bin/env perl
+use strict;
+use warnings;
+use YAML qw(LoadFile);
+
+# Usage: legacy_branches.pl <github-repo-name>
+# Prints one legacy (AsciiDoc) branch per line that the given repo still carries in conf.yaml.
+#
+# Exit 0: repo found; list may be empty if fully migrated to docs-builder.
+# Exit 2: repo not found in conf.yaml — caller should build as today (fail open).
+# Exit 1: YAML parse / load error — caller should build as today (fail open).
+
+my ($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;
+}
+
+# Build a map: GitHub repo name (URL basename minus .git) -> conf repo key.
+# Sources in conf.yaml reference the conf key (e.g. "esf"), not the GitHub name
+# (e.g. "elastic-serverless-forwarder"), so we need this translation.
+my %github_to_key;
+while ( my ( $key, $url ) = each %{ $conf->{repos} } ) {
+    ( my $name = $url ) =~ s{.*/|\.git$}{}g;    # URL -> repo name (strip path and .git)
+    $github_to_key{$name} = $key;
+}
+
+my $conf_key = $github_to_key{$github_repo};
+unless ( defined $conf_key ) {
+    exit 2;    # repo not in conf — caller builds as today
+}
+
+# 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;
+        }
+    }
+}