diff --git a/scripts/msan.sh b/scripts/msan.sh
index d74deb55d..954608890 100755
--- a/scripts/msan.sh
+++ b/scripts/msan.sh
@@ -79,6 +79,27 @@ ulimit -s 262144 2>/dev/null || true
 export CBM_THREAD_STACK_MB="${CBM_THREAD_STACK_MB:-256}"
 export LD_LIBRARY_PATH="$MSAN_PREFIX/lib${LD_LIBRARY_PATH:+:$LD_LIBRARY_PATH}"
 
+expected_build_config=$'sanitized=1 test_seams=1\n'
+build_config_marker="__cbm_build_config_end__"
+captured_build_config=""
+actual_build_config=""
+build_config_probe_ok=0
+if captured_build_config="$(./build/msan/test-runner --build-config && printf '%s' "$build_config_marker")"; then
+    build_config_probe_ok=1
+    actual_build_config="${captured_build_config%"$build_config_marker"}"
+else
+    actual_build_config="$captured_build_config"
+fi
+if [ "$build_config_probe_ok" -ne 1 ] ||
+    [ "$actual_build_config" != "$expected_build_config" ]; then
+    reported_build_config="${actual_build_config//$'\n'/\\n}"
+    printf '%s\n' 'ERROR: build config mismatch for build/msan/test-runner' >&2
+    printf '%s\n' '  expected: sanitized=1 test_seams=1' >&2
+    printf '  reported: %s\n' "${reported_build_config:-<empty>}" >&2
+    printf '%s\n' 'Refusing to run suites; check sanitizer flags and CBM_SANITIZED_BUILD wiring.' >&2
+    exit 1
+fi
+
 echo "=== MSan lane: $(clang --version | head -1) ==="
 
 # KNOWN RED — THREE DISTINCT CAUSES, whitelisted per cause (O10)
diff --git a/scripts/test.sh b/scripts/test.sh
index 0dc575b04..70aa3d7ae 100755
--- a/scripts/test.sh
+++ b/scripts/test.sh
@@ -145,6 +145,7 @@ source "$ROOT/scripts/path-safety.sh"
 MAKE_ARGS=()
 BUILD_DIR="build/c"
 SANITIZE_GIVEN=0
+SANITIZE_VALUE=""
 prev_arg=""
 for arg in "$@"; do
     case "$arg" in
@@ -154,7 +155,11 @@ for arg in "$@"; do
         --tsan) ;; # already handled
         --suites|--suites=*) ;; # already handled (value skipped via prev_arg below)
         BUILD_DIR=*) BUILD_DIR="${arg#BUILD_DIR=}"; MAKE_ARGS+=("$arg") ;;
-        SANITIZE=*) SANITIZE_GIVEN=1; MAKE_ARGS+=("$arg") ;;
+        SANITIZE=*)
+            SANITIZE_GIVEN=1
+            SANITIZE_VALUE="${arg#SANITIZE=}"
+            MAKE_ARGS+=("$arg")
+            ;;
         *=*)
             if [[ "${prev_arg:-}" != "--suites" ]]; then
                 MAKE_ARGS+=("$arg") # forward any VAR=VAL to make
@@ -174,6 +179,42 @@ if [ "$SANITIZE_GIVEN" -eq 0 ] && [ "${MSYSTEM:-}" = "CLANGARM64" ]; then
     MAKE_ARGS+=("SANITIZE=-fsanitize=undefined -fsanitize-trap=undefined -fstack-protector-strong -fno-omit-frame-pointer")
 fi
 
+EXPECTED_SANITIZED=1
+if [ "$SANITIZE_GIVEN" -eq 1 ]; then
+    case "$SANITIZE_VALUE" in
+        *[![:space:]]*) EXPECTED_SANITIZED=1 ;;
+        *) EXPECTED_SANITIZED=0 ;;
+    esac
+fi
+
+# Refuse to run suites when the built runner disagrees with the lane that
+# produced it. Exact output keeps missing, duplicate, or future unhandled
+# fields fail-closed instead of silently weakening the gate.
+assert_test_runner_build_config() {
+    local runner="$1"
+    local expected_sanitized="$2"
+    local expected="sanitized=$expected_sanitized test_seams=1"
+    local marker="__cbm_build_config_end__"
+    local captured=""
+    local actual=""
+    local probe_ok=0
+
+    if captured="$("$runner" --build-config && printf '%s' "$marker")"; then
+        probe_ok=1
+        actual="${captured%"$marker"}"
+    else
+        actual="$captured"
+    fi
+    if [ "$probe_ok" -ne 1 ] || [ "$actual" != "$expected"$'\n' ]; then
+        local reported="${actual//$'\n'/\\n}"
+        printf 'ERROR: build config mismatch for %s\n' "$runner" >&2
+        printf '  expected: %s\n' "$expected" >&2
+        printf '  reported: %s\n' "${reported:-<empty>}" >&2
+        printf '%s\n' 'Refusing to run suites; check sanitizer flags and CBM_SANITIZED_BUILD wiring.' >&2
+        return 1
+    fi
+}
+
 print_env "test.sh"
 
 # ── TSan mode (--tsan): the data-race gate ──
@@ -182,6 +223,7 @@ print_env "test.sh"
 if [ "$TSAN" -eq 1 ]; then
     echo "=== test.sh: TSan leg (make test-tsan) ==="
     make -j"$NPROC" -f Makefile.cbm "$BUILD_DIR/test-runner-tsan" ${MAKE_ARGS[@]+"${MAKE_ARGS[@]}"}
+    assert_test_runner_build_config "$BUILD_DIR/test-runner-tsan" 1
     make -f Makefile.cbm test-tsan ${MAKE_ARGS[@]+"${MAKE_ARGS[@]}"}
     exit "$?"
 fi
@@ -192,6 +234,7 @@ fi
 if [ -n "$SUITES" ]; then
     echo "=== test.sh: ITERATION mode — suites: $SUITES (incremental build) ==="
     make -j"$NPROC" -f Makefile.cbm "$BUILD_DIR/test-runner" ${MAKE_ARGS[@]+"${MAKE_ARGS[@]}"}
+    assert_test_runner_build_config "$BUILD_DIR/test-runner" "$EXPECTED_SANITIZED"
     # shellcheck disable=SC2086  # suite list is deliberately word-split
     "$BUILD_DIR/test-runner" $SUITES
     exit "$?"
@@ -275,6 +318,7 @@ BUILD_DIR="$BUILD_DIR" scripts/clean.sh
 # pass/fail/skip totals aggregate to the same numbers as the sequential run).
 # CBM_TEST_SEQUENTIAL=1 restores the single-process runner.
 make -j"$NPROC" -f Makefile.cbm "$BUILD_DIR/test-runner" ${MAKE_ARGS[@]+"${MAKE_ARGS[@]}"}
+assert_test_runner_build_config "$BUILD_DIR/test-runner" "$EXPECTED_SANITIZED"
 if [ "${CBM_TEST_SEQUENTIAL:-0}" = "1" ]; then
     make -f Makefile.cbm test ${MAKE_ARGS[@]+"${MAKE_ARGS[@]}"}
 else
diff --git a/tests/test_main.c b/tests/test_main.c
index ba6f26d45..19e7b620c 100644
--- a/tests/test_main.c
+++ b/tests/test_main.c
@@ -17,6 +17,7 @@ int tf_skip_count = 0;
 #include "foundation/log.h"        /* crash-durable worker log probe */
 #include "foundation/mem.h"        /* cbm_mem_init — worker budget */
 #include "foundation/platform.h"   /* cbm_file_exists — blocking-git marker */
+#include "foundation/sanitized.h"  /* CBM_SANITIZED — --build-config contract */
 #include "daemon/runtime.h"        /* bounded worker response probe */
 #include "daemon/ipc.h"            /* Windows private-lock re-exec probe */
 #include "daemon/version_cohort.h" /* Windows crash-turnover re-exec probe */
@@ -817,6 +818,31 @@ extern void suite_dump_verify_io(void);
 extern void cbm_kind_in_set_free_cache(void);
 
 int main(int argc, char **argv) {
+    int blocking_git_rc = tf_maybe_run_blocking_git_probe(argc, argv);
+    if (blocking_git_rc >= 0) {
+        return blocking_git_rc;
+    }
+    /* Installation tests use this executable as a structurally real candidate.
+     * Mirror the production binary's minimal verification contract. */
+    if (argc == 2 && strcmp(argv[1], "--version") == 0) {
+        (void)puts("codebase-memory-mcp test-runner");
+        return 0;
+    }
+    if (argc == 2 && strcmp(argv[1], "--build-config") == 0) {
+#ifdef _WIN32
+        if (_setmode(cbm_fileno(stdout), _O_BINARY) == -1) {
+            fprintf(stderr, "failed to set build-config stdout to binary mode\n");
+            return 2;
+        }
+#endif
+#if defined(CBM_ENABLE_TEST_SEAMS) && CBM_ENABLE_TEST_SEAMS
+        const int test_seams = 1;
+#else
+        const int test_seams = 0;
+#endif
+        (void)printf("sanitized=%d test_seams=%d\n", CBM_SANITIZED, test_seams);
+        return 0;
+    }
     /* Skip the multi-hundred-MB executable-image hash that computes the exact
      * build fingerprint: it is tens of seconds per spawned worker/daemon under
      * ASan on constrained CI runners and the sole cause of the daemon-family
@@ -828,16 +854,6 @@ int main(int argc, char **argv) {
         (void)cbm_setenv("CBM_TEST_BUILD_FINGERPRINT",
                          "0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef", 1);
     }
-    int blocking_git_rc = tf_maybe_run_blocking_git_probe(argc, argv);
-    if (blocking_git_rc >= 0) {
-        return blocking_git_rc;
-    }
-    /* Installation tests use this executable as a structurally real candidate.
-     * Mirror the production binary's minimal verification contract. */
-    if (argc == 2 && strcmp(argv[1], "--version") == 0) {
-        (void)puts("codebase-memory-mcp test-runner");
-        return 0;
-    }
     int mcp_idxfailclosed_rc = tf_maybe_run_mcp_idxfailclosed_probe(argc, argv);
     if (mcp_idxfailclosed_rc >= 0) {
         return mcp_idxfailclosed_rc;