diff --git a/include/starkernel/timer.h b/include/starkernel/timer.h
index 8675b66..79f0d61 100644
--- a/include/starkernel/timer.h
+++ b/include/starkernel/timer.h
@@ -87,7 +87,14 @@ typedef struct time_window {
  */
 typedef struct time_trust_state {
     /* Core counters */
-    uint64_t     ticks;            /* TIME-TICKS: monotonic heartbeat count */
+    volatile uint64_t ticks;       /* TIME-TICKS: monotonic heartbeat count --
+                                     * written directly in ISR context
+                                     * (heartbeat_tick(), all three archs) and
+                                     * read directly by mainline
+                                     * (heartbeat_ticks()); genuinely
+                                     * concurrent, unlike every other field in
+                                     * this struct (FABRIC.md item 4.5a/4.5b,
+                                     * 2026-08-11). */
     uint64_t     last_tsc;         /* TSC at last heartbeat */
     uint64_t     expected_delta;   /* Expected TSC ticks per heartbeat */