diff --git a/src/mcp/mcp.c b/src/mcp/mcp.c
index e2fc55049..b75cc4c27 100644
--- a/src/mcp/mcp.c
+++ b/src/mcp/mcp.c
@@ -8353,6 +8353,25 @@ static char *handle_index_repository(cbm_mcp_server_t *srv, const char *args) {
             srv, doc, root, project_name, repo_path, persistence, p, excluded_dirs, excluded_count,
             file_errors, file_error_count, has_logfile ? logfile_path : NULL);
         yyjson_mut_obj_add_str(doc, root, "status", degraded ? "degraded" : "indexed");
+    } else if (rc == CBM_PIPELINE_ABORT_PRESERVE_DB) {
+        /* The truthful abort message: the old generic "check repo_path" hint
+         * sent people debugging a path that was fine, when the run aborted
+         * pre-publication (semantic inputs changed mid-run, or a discovery/
+         * manifest phase failed transiently) and the previous index is
+         * intact. A 99-test cascade on the Windows leg traced back to
+         * exactly this — the response said error, but not which kind. */
+        yyjson_mut_obj_add_str(doc, root, "status", "aborted_previous_preserved");
+        yyjson_mut_obj_add_str(doc, root, "hint",
+                               "Indexing aborted before publication; the previous index is "
+                               "intact and still serving. Causes: files changed while the run "
+                               "was in flight, or a discovery/manifest phase failed "
+                               "transiently. Retry; if it repeats, check the run log.");
+    } else if (rc == CBM_PIPELINE_PERSIST_FAILED) {
+        yyjson_mut_obj_add_str(doc, root, "status", "persist_failed");
+        yyjson_mut_obj_add_str(doc, root, "hint",
+                               "The validated staging database could not be published. Check "
+                               "free disk space and permissions on the cache directory; the "
+                               "previous index may have been rolled back.");
     } else {
         yyjson_mut_obj_add_str(doc, root, "status", "error");
         yyjson_mut_obj_add_str(doc, root, "hint",
diff --git a/src/pipeline/pipeline.h b/src/pipeline/pipeline.h
index df684ede5..ff8e132d7 100644
--- a/src/pipeline/pipeline.h
+++ b/src/pipeline/pipeline.h
@@ -63,9 +63,9 @@ void cbm_pipeline_free(cbm_pipeline_t *p);
  * need to know whether the PREVIOUS generation survived can distinguish the
  * failures by value: the run publishes by renaming a fully validated staging
  * database over the destination, so every abort before that rename leaves the
- * existing database in place. Those codes (CBM_PIPELINE_ABORT_PRESERVE_DB and
- * CBM_PIPELINE_PERSIST_FAILED) are defined in pipeline_internal.h alongside the
- * stages that raise them. */
+ * existing database in place. */
+#define CBM_PIPELINE_ABORT_PRESERVE_DB (-2) /* aborted pre-publication; previous DB intact */
+#define CBM_PIPELINE_PERSIST_FAILED (-4)    /* staging/rollback failure during publication */
 int cbm_pipeline_run(cbm_pipeline_t *p);
 
 /* Request cancellation of a running pipeline (thread-safe). */
diff --git a/src/pipeline/pipeline_internal.h b/src/pipeline/pipeline_internal.h
index 754fa4ffa..fc99c4296 100644
--- a/src/pipeline/pipeline_internal.h
+++ b/src/pipeline/pipeline_internal.h
@@ -29,12 +29,12 @@
 /* Route node QN buffer size (must fit __route__METHOD__/full/url/path) */
 #define CBM_ROUTE_QN_SIZE 768
 
-/* Incremental integrity failure: abort the run and preserve the existing DB.
- * Distinct from CBM_NOT_FOUND, which the orchestrator uses as the normal
+/* CBM_PIPELINE_ABORT_PRESERVE_DB / CBM_PIPELINE_PERSIST_FAILED moved to
+ * pipeline.h — callers legitimately distinguish them (the header's contract
+ * always said so). FORCE_FULL_REINDEX never escapes the orchestrator and
+ * stays internal. All three are distinct from CBM_NOT_FOUND, the normal
  * "no incremental route; continue with a full index" sentinel. */
-#define CBM_PIPELINE_ABORT_PRESERVE_DB (-2)
 #define CBM_PIPELINE_FORCE_FULL_REINDEX (-3)
-#define CBM_PIPELINE_PERSIST_FAILED (-4)
 
 /* Canonicalize route-path parameter placeholders (":id", "{id}", "<id>",
  * "${...}") to a single "{}" token so that client call sites and server
diff --git a/tests/test_incremental.c b/tests/test_incremental.c
index 7ae91f971..af261ccf3 100644
--- a/tests/test_incremental.c
+++ b/tests/test_incremental.c
@@ -107,7 +107,20 @@ static int reformat_files(const char *subdir, int max_files) {
 static char *index_repo(void) {
     char args[512];
     snprintf(args, sizeof(args), "{\"repo_path\":\"%s\"}", g_repodir);
-    return cbm_mcp_handle_tool(g_srv, "index_repository", args);
+    char *resp = cbm_mcp_handle_tool(g_srv, "index_repository", args);
+    /* Fail-closed at the source: every caller asserts resp != NULL, but an
+     * error response (e.g. a transient pre-publication abort that preserves
+     * the previous index) used to slip through and cascade into dozens of
+     * mysterious count/query failures downstream — a 99-test pile-up on the
+     * Windows leg traced back to exactly this. Surface the real cause once,
+     * here, and let the existing non-NULL asserts fail loudly. */
+    if (resp && strstr(resp, "\"isError\":true")) {
+        fprintf(stderr, "  index_repo: index_repository returned an error response:\n  %.400s\n",
+                resp);
+        free(resp);
+        return NULL;
+    }
+    return resp;
 }
 
 /* Timed index: returns response, sets *elapsed_ms and *peak_rss_mb */