diff --git a/.kiro/specs/ha-commit-barrier-rollout/tasks.md b/.kiro/specs/ha-commit-barrier-rollout/tasks.md
index c4eae15..be4a7a8 100644
--- a/.kiro/specs/ha-commit-barrier-rollout/tasks.md
+++ b/.kiro/specs/ha-commit-barrier-rollout/tasks.md
@@ -84,7 +84,7 @@
   - Observable completion: new cases FAIL (red) against the unguarded `mark_job_start/finish/fail/update` (job.c:583/638/696/726 anchors)
   - _Requirements: 5.1, 5.2, 5.3_
 
-- [ ] 6.2 Implement the guards to green
+- [x] 6.2 Implement the guards to green
   - Add transition guards at the top of each `mark_job_*` (under the caller-held job WRITE lock, before BEFORE_EVENT fires): duplicate → log + return no-op; terminal → non-terminal → WARN + reject preserving state; forward transitions unchanged
   - Audit all callers (`on_job_*`, `job_ctrl`, `update_job_rusages`) for the new return semantics
   - Observable completion: all task 6.1 tests PASS; existing job lifecycle tests green
diff --git a/.wolf/memory.md b/.wolf/memory.md
index 605a44e..51c354b 100644
--- a/.wolf/memory.md
+++ b/.wolf/memory.md
@@ -1018,3 +1018,9 @@
 | 04:18 | task 6.1 review: RED tests for mark_job_* guards verified | test_commit_barrier_reply.c | APPROVED — 22/3 group, 508/3 census, boundary clean | ~38k |
 | 21:19 | Edited .kiro/specs/ha-commit-barrier-rollout/tasks.md | inline fix | ~13 |
 | 21:19 | Edited .kiro/specs/ha-commit-barrier-rollout/tasks.md | modified 2() | ~243 |
+| 21:23 | Edited src/controller/job.c | modified job_state_is_terminal() | ~330 |
+| 21:23 | Edited src/controller/job.c | added 1 condition(s) | ~224 |
+| 21:24 | Edited src/controller/job.c | added 1 condition(s) | ~212 |
+| 21:24 | Edited src/controller/job.c | added 2 condition(s) | ~294 |
+| 04:37 | task 6.2: mark_job_* transition guards (dup no-op / backward reject / forward merge preserved) | src/controller/job.c | 22 ctrl tests 0 fail, 5 guard tests GREEN | ~14k |
+| 21:54 | Edited .kiro/specs/ha-commit-barrier-rollout/tasks.md | inline fix | ~11 |
diff --git a/src/controller/job.c b/src/controller/job.c
index 5fe05cd..9a960c4 100644
--- a/src/controller/job.c
+++ b/src/controller/job.c
@@ -19,6 +19,28 @@ static int mark_job_finish(cluster_t*, job_t*, xce_jobfinish_t*);
 static int mark_job_fail(cluster_t*, job_t*, job_failinfo_t*);
 static int mark_job_update(cluster_t*, job_t*, int);
 
+/* A job is terminal once it carries a DONE or EXIT bit. Terminal jobs reject
+ * backward (terminal -> non-terminal) transitions and treat re-applied
+ * terminal transitions as duplicates; forward bit-OR merges (PDONE, PERR,
+ * suspend bits) on a terminal job remain legal. */
+#define JOB_STATE_TERMINAL_BITS (JOB_STAT_DONE | JOB_STAT_EXIT)
+
+static inline bool job_state_is_terminal(int state)
+{
+    return (state & JOB_STATE_TERMINAL_BITS) != 0;
+}
+
+/* Shared backward predicate for START and UPDATE (Req 5.2): a transition is
+ * backward when the job is already terminal but the requested next state drops
+ * every terminal bit it currently holds (e.g. START overwriting DONE with a
+ * bare JOB_STAT_RUN, or an UPDATE clearing the terminal bits). A forward merge
+ * that preserves at least one terminal bit (postdone OR'ing PDONE) is NOT
+ * backward. */
+static inline bool job_state_is_backward(int cur, int next)
+{
+    return job_state_is_terminal(cur) && !job_state_is_terminal(next);
+}
+
 static job_t* create_job(cluster_t* c, xce_job_t* j)
 {
     job_t* job = NULL;
@@ -591,6 +613,20 @@ int handle_jobsubmit(cluster_t* c, endpoint_t* ep, xce_request_t* request)
 
 static int mark_job_start(cluster_t* c, job_t* job, job_pidinfo_t* info)
 {
+    int cur = job->job.state;
+
+    /* Transition guard (Req 5.2): START sets the non-terminal JOB_STAT_RUN. A
+     * START on an already-terminal job (stale/duplicate worker report after
+     * the job finished) is a backward transition — reject it, preserve the
+     * terminal state, and fire NO hook (no WAL append). Return success-no-op
+     * so the caller (on_job_start) still reports success. */
+    if (job_state_is_backward(cur, JOB_STAT_RUN)) {
+        WARN("job %ld: rejecting backward START on terminal state 0x%x — "
+             "preserving terminal state",
+             job->job.jobid, cur);
+        return 0;
+    }
+
     xce_run_hooks(&(c->hooksys), XCE_CONTROLLER_HOOK_JOB, HOOK_BEFORE_EVENT,
                   HOOK_DETAIL_JOB_START, c, info, job);
     /* update job information */
@@ -646,6 +682,19 @@ int on_job_start(cluster_t* c, job_pidinfo_t* info)
 
 static int mark_job_finish(cluster_t* c, job_t* job, xce_jobfinish_t* finish)
 {
+    int cur = job->job.state;
+
+    /* Transition guard (Req 5.1): FINISH drives the job to a terminal state
+     * (DONE on exit_code 0, else EXIT). If the job is already terminal the
+     * FINISH has already been applied (retransmit / WAL replay) — log it and
+     * return a success-no-op WITHOUT firing the AFTER_EVENT hook, so no second
+     * WAL entry is appended and current_lsn does not advance. */
+    if (job_state_is_terminal(cur)) {
+        INFO("job %ld: duplicate FINISH on terminal state 0x%x — no-op",
+             job->job.jobid, cur);
+        return 0;
+    }
+
     xce_run_hooks(&(c->hooksys), XCE_CONTROLLER_HOOK_JOB, HOOK_BEFORE_EVENT,
                   HOOK_DETAIL_JOB_EXIT, c, finish, job);
     /* update job information */
@@ -734,6 +783,26 @@ static int mark_job_fail(cluster_t* c, job_t* job, job_failinfo_t* failinfo)
 
 static int mark_job_update(cluster_t* c, job_t* job, int state)
 {
+    int cur = job->job.state;
+
+    /* Transition guard (Req 5.1/5.2), sharing the backward predicate with
+     * START: an UPDATE that clears every terminal bit a terminal job holds is
+     * backward — reject it, preserve the terminal state, fire no hook. A
+     * forward merge that keeps a terminal bit (postdone OR'ing PDONE) is not
+     * backward and proceeds. A no-op update (requested state already current)
+     * is logged and skipped so it appends no duplicate WAL entry. */
+    if (job_state_is_backward(cur, state)) {
+        WARN("job %ld: rejecting backward UPDATE 0x%x -> 0x%x — preserving "
+             "terminal state",
+             job->job.jobid, cur, state);
+        return 0;
+    }
+    if (state == cur) {
+        INFO("job %ld: duplicate UPDATE to state 0x%x — no-op", job->job.jobid,
+             cur);
+        return 0;
+    }
+
     xce_run_hooks(&(c->hooksys), XCE_CONTROLLER_HOOK_JOB, HOOK_BEFORE_EVENT,
                   HOOK_DETAIL_JOB_UPDATE, c, job);
     /* update job state */