diff --git a/docs/testing.md b/docs/testing.md
index 01ae305..4adcfd5 100644
--- a/docs/testing.md
+++ b/docs/testing.md
@@ -78,6 +78,26 @@ value-stream codecs (round-trip over randomized and boundary inputs):
 test/pbt/run.sh [seed] [iterations]
 ```
 
+## Before merging: build every major
+
+```sh
+test/build_all_versions.sh
+```
+
+No clusters and no suites, just a compile against each installed major, about a
+minute for all five. Run it before merging anything that touches a version guard,
+a table access method callback signature, or `columnar_compat.h`.
+
+The per-PR gate runs the suites on two majors, which is the right trade for test
+time and structurally cannot see a defect on a major it never builds.
+`scan_analyze_next_block` changed signature at PG17; a change guarded the
+callback at PG18 instead; PG15, PG16, PG18 and PG19 all built, and `main` did not
+compile on PG17 at all while a two-major gate reported it green. This check
+catches that in a minute. The full matrix remains the thorough answer.
+
+Do not leave a branch broken on a supported major while a fix is pending. Land
+the fix.
+
 ## The version matrix
 
 To build and run every suite across a set of PostgreSQL majors in one pass, each
diff --git a/test/build_all_versions.sh b/test/build_all_versions.sh
new file mode 100755
index 0000000..7262f53
--- /dev/null
+++ b/test/build_all_versions.sh
@@ -0,0 +1,76 @@
+#!/usr/bin/env bash
+#
+# Compile the extension against every installed major.
+#
+# The per-PR gate runs the suites on two majors, which is the right trade for
+# test time and cannot see a defect on a major it never builds. That is not
+# hypothetical: scan_analyze_next_block changed signature at PG17, a change
+# guarded the callback at PG18 instead, and PG15, PG16, PG18 and PG19 all built
+# fine while main did not compile on PG17 at all. A two-major gate reported it
+# green.
+#
+# This is the cheap half of the answer: no clusters, no suites, just a compile
+# against each major, which takes about a minute for all five. Run it before
+# merging anything that touches a version guard, a table AM callback signature,
+# or columnar_compat.h. The full matrix remains the thorough half.
+#
+# Usage:
+#   test/build_all_versions.sh [pg_config ...]
+#
+# With no arguments it builds against the same default set the version matrix
+# uses. Exits non-zero on the first major that fails, and prints its errors.
+
+set -uo pipefail
+
+SRCDIR="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+
+DEFAULT_CONFIGS=(
+	/usr/local/pg15/bin/pg_config
+	/usr/local/pg16/bin/pg_config
+	/usr/local/pg17/bin/pg_config
+	/usr/local/pgsql/bin/pg_config
+	/usr/local/pg19/bin/pg_config
+)
+
+if [ "$#" -gt 0 ]; then
+	CONFIGS=("$@")
+else
+	CONFIGS=("${DEFAULT_CONFIGS[@]}")
+fi
+
+echo "== pgColumnar build check across majors =="
+
+failed=0
+for pgc in "${CONFIGS[@]}"; do
+	if [ ! -x "$pgc" ]; then
+		printf '  SKIP  %-34s (not executable)\n' "$pgc"
+		continue
+	fi
+
+	ver="$("$pgc" --version)"
+	log="$(mktemp /tmp/pgc-build-XXXXXX.log)"
+
+	make -C "$SRCDIR" clean PG_CONFIG="$pgc" >/dev/null 2>&1
+	if make -C "$SRCDIR" PG_CONFIG="$pgc" > "$log" 2>&1; then
+		# -Werror is not set, so a warning still builds; report it rather than
+		# let a new one accumulate unnoticed across majors.
+		warns="$(grep -cE '^[^ ].*\bwarning:' "$log")"
+		printf '  OK    %-34s %s warning(s)\n' "$ver" "$warns"
+		[ "$warns" != "0" ] && grep -E '^[^ ].*\bwarning:' "$log" | head -5 | sed 's/^/          /'
+	else
+		printf '  FAIL  %s\n' "$ver"
+		grep -E '\berror:' "$log" | head -8 | sed 's/^/          /'
+		failed=1
+	fi
+	rm -f "$log"
+done
+
+# Leave no object tree behind from whichever major happened to be last: the next
+# build against a different major would link objects compiled for this one.
+make -C "$SRCDIR" clean >/dev/null 2>&1 || true
+
+if [ "$failed" != "0" ]; then
+	echo "build_all_versions.sh: FAILED"
+	exit 1
+fi
+echo "build_all_versions.sh: PASSED"
diff --git a/test/harness_selftest.sh b/test/harness_selftest.sh
index 539488f..cfbd203 100755
--- a/test/harness_selftest.sh
+++ b/test/harness_selftest.sh
@@ -158,13 +158,15 @@ check "guard rejects a foreign cluster" "$_verdict" "foreign"
 TESTDIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 RUNNER="$TESTDIR/run_all_versions.sh"
 
-# Not suites: the shared library, the runner itself, and the two developer
-# helpers that build rather than test. native_scale is a suite but is opt-in by
+# Not suites: the shared library, the two runners, and the two developer helpers
+# that build rather than test. build_all_versions compiles against every major
+# and is run before merging a change that touches a version guard; it takes no
+# cluster and reports per major, so the matrix cannot run it as a suite. native_scale is a suite but is opt-in by
 # design and says so in its own header: it runs at a row count the matrix should
 # not carry.
 not_a_suite() {
 	case "$1" in
-		lib|run_all_versions|devloop|rebuild|native_scale) return 0 ;;
+		lib|run_all_versions|build_all_versions|devloop|rebuild|native_scale) return 0 ;;
 		*) return 1 ;;
 	esac
 }