diff --git a/src/pipeline/pipeline.c b/src/pipeline/pipeline.c
index 0c3041959..5cc163941 100644
--- a/src/pipeline/pipeline.c
+++ b/src/pipeline/pipeline.c
@@ -401,7 +401,19 @@ static int effective_worker_count(bool initial) {
     if (st && st[0] == '1') {
         return 1;
     }
-    return cbm_default_worker_count(initial);
+    /* Keep the explicit override available for parallel regression tests and
+     * deliberate diagnostics. It is opt-in; the production default below stays
+     * on the crash-free sequential path. */
+    if (getenv("CBM_WORKERS")) {
+        return cbm_default_worker_count(initial);
+    }
+    (void)initial;
+    /* Parallel extraction currently corrupts heap state on real mixed-language
+     * repositories (reproduced with two or more workers on Serena). Keep the
+     * production indexing path deterministic and crash-free until the shared
+     * parser/allocator race is fixed. Correctness is more important than the
+     * parallel speedup. */
+    return 1;
 }
 
 /* Resolve the DB path for this pipeline. Caller must free(). */
diff --git a/tests/test_pipeline.c b/tests/test_pipeline.c
index 6cb2a6150..50646838c 100644
--- a/tests/test_pipeline.c
+++ b/tests/test_pipeline.c
@@ -7192,6 +7192,9 @@ TEST(pipeline_backpressure_futile_nap_disengages) {
     ASSERT_TRUE(cbm_mem_over_budget());
 
     cbm_pp_bp_nap_cycles_reset();
+    char *old_workers = getenv("CBM_WORKERS");
+    char *saved_workers = old_workers ? strdup(old_workers) : NULL;
+    cbm_setenv("CBM_WORKERS", "4", 1);
     char db_path[512];
     snprintf(db_path, sizeof(db_path), "%s/backpressure.db", g_tmpdir);
     cbm_pipeline_t *p = cbm_pipeline_new(g_tmpdir, db_path, CBM_MODE_FULL);
@@ -7201,6 +7204,12 @@ TEST(pipeline_backpressure_futile_nap_disengages) {
 
     /* Restore the caller-visible budget BEFORE asserting. */
     cbm_mem_set_budget_for_tests(saved_budget);
+    if (saved_workers) {
+        cbm_setenv("CBM_WORKERS", saved_workers, 1);
+        free(saved_workers);
+    } else {
+        cbm_unsetenv("CBM_WORKERS");
+    }
     cbm_pipeline_free(p);
     teardown_test_repo();