diff --git a/lib/events/api.sx b/lib/events/api.sx
index 5258fc6d..aa5b2d03 100644
--- a/lib/events/api.sx
+++ b/lib/events/api.sx
@@ -249,3 +249,29 @@
   (fn
     (b store actor ws we)
     (> (len (ev/conflicts-p b store actor ws we)) 0)))
+
+;; ---- conflict-checked booking ----
+;; Capacity is per-event, but an attendee should not be double-booked against
+;; THEMSELVES across different events. Would booking `actor` into `occ` overlap
+;; an existing booking of theirs elsewhere? (Derived from persist availability;
+;; an existing booking into `occ` itself is excluded — that's idempotent.)
+(define
+  ev/would-time-conflict?
+  (fn
+    (b store actor occ)
+    (and
+      (not (ev-actor-booked? b (ev-occ-key occ) actor))
+      (not (ev/free-p? b store actor (get occ :start) (get occ :end))))))
+
+;; Book `actor` into `occ` only if it doesn't clash with their other bookings.
+;; Re-booking the same occurrence is idempotent (:already); a clash returns
+;; :time-conflict; otherwise the normal ev/book-occ! result (:booked / :full).
+(define
+  ev/book-checked!
+  (fn
+    (b store actor occ)
+    (cond
+      ((ev-actor-booked? b (ev-occ-key occ) actor) (ev/book-occ! b store actor occ))
+      ((ev/would-time-conflict? b store actor occ)
+        {:status :time-conflict :actor actor :occ-key (ev-occ-key occ)})
+      (else (ev/book-occ! b store actor occ)))))
diff --git a/lib/events/scoreboard.json b/lib/events/scoreboard.json
index 18e86561..7df5fc32 100644
--- a/lib/events/scoreboard.json
+++ b/lib/events/scoreboard.json
@@ -1,13 +1,13 @@
 {
   "lang": "events",
-  "total_passed": 303,
+  "total_passed": 311,
   "total_failed": 0,
-  "total": 303,
+  "total": 311,
   "suites": [
     {"name":"calendar","passed":51,"failed":0,"total":51},
     {"name":"timezone","passed":17,"failed":0,"total":17},
     {"name":"availability","passed":22,"failed":0,"total":22},
-    {"name":"api","passed":24,"failed":0,"total":24},
+    {"name":"api","passed":32,"failed":0,"total":32},
     {"name":"booking","passed":82,"failed":0,"total":82},
     {"name":"booking-notify","passed":11,"failed":0,"total":11},
     {"name":"ticket","passed":31,"failed":0,"total":31},
@@ -16,5 +16,5 @@
     {"name":"federation","passed":29,"failed":0,"total":29},
     {"name":"integration","passed":8,"failed":0,"total":8}
   ],
-  "generated": "2026-06-07T11:51:43+00:00"
+  "generated": "2026-06-07T13:59:09+00:00"
 }
diff --git a/lib/events/scoreboard.md b/lib/events/scoreboard.md
index 15f6602f..f7eb1a50 100644
--- a/lib/events/scoreboard.md
+++ b/lib/events/scoreboard.md
@@ -1,13 +1,13 @@
 # events scoreboard
 
-**303 / 303 passing** (0 failure(s)).
+**311 / 311 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
 | calendar | 51 | 51 | ok |
 | timezone | 17 | 17 | ok |
 | availability | 22 | 22 | ok |
-| api | 24 | 24 | ok |
+| api | 32 | 32 | ok |
 | booking | 82 | 82 | ok |
 | booking-notify | 11 | 11 | ok |
 | ticket | 31 | 31 | ok |
diff --git a/lib/events/tests/api.sx b/lib/events/tests/api.sx
index d459a2fb..fb681f8f 100644
--- a/lib/events/tests/api.sx
+++ b/lib/events/tests/api.sx
@@ -259,6 +259,66 @@
                             20))
                         true))))))))))))
 
+;; ---- conflict-checked booking ----
+(define
+  ev-api-cf-run-all!
+  (fn
+    ()
+    (let
+      ((b (persist/open))
+        (store
+          (ev/schedule
+            (ev/schedule
+              (ev/schedule (ev/empty) (quote a) (ev-dt 2026 6 1 9 0) 60 nil 10)
+              (quote bb)
+              (ev-dt 2026 6 1 9 30)
+              60
+              nil
+              10)
+            (quote c)
+            (ev-dt 2026 6 1 11 0)
+            60
+            nil
+            10)))
+      (let
+        ((oa (ev-occ (quote a) (ev-dt 2026 6 1 9 0) 60))
+          (ob (ev-occ (quote bb) (ev-dt 2026 6 1 9 30) 60))
+          (oc (ev-occ (quote c) (ev-dt 2026 6 1 11 0) 60)))
+        (do
+          (ev-api-check!
+            "first checked booking succeeds"
+            (get (ev/book-checked! b store (quote nia) oa) :status)
+            :booked)
+          (ev-api-check!
+            "overlapping different-event booking is a time conflict"
+            (get (ev/book-checked! b store (quote nia) ob) :status)
+            :time-conflict)
+          (ev-api-check!
+            "the clashing booking did not land on the roster"
+            (ev/roster-occ b ob)
+            (list))
+          (ev-api-check!
+            "a non-overlapping booking is allowed"
+            (get (ev/book-checked! b store (quote nia) oc) :status)
+            :booked)
+          (ev-api-check!
+            "re-booking the same occurrence is idempotent, not a conflict"
+            (get (ev/book-checked! b store (quote nia) oa) :status)
+            :already)
+          ;; a different actor is unaffected by nia's bookings
+          (ev-api-check!
+            "another actor may take the overlapping slot"
+            (get (ev/book-checked! b store (quote ola) ob) :status)
+            :booked)
+          (ev-api-check!
+            "would-time-conflict? predicate agrees"
+            (ev/would-time-conflict? b store (quote nia) ob)
+            true)
+          (ev-api-check!
+            "would-time-conflict? false for a free slot"
+            (ev/would-time-conflict? b store (quote zed) ob)
+            false))))))
+
 (define
   ev-api-tests-run!
   (fn
@@ -268,4 +328,5 @@
       (set! ev-api-fail 0)
       (set! ev-api-failures (list))
       (ev-api-run-all!)
+      (ev-api-cf-run-all!)
       {:failures ev-api-failures :total (+ ev-api-pass ev-api-fail) :passed ev-api-pass :failed ev-api-fail})))
diff --git a/plans/events-on-sx.md b/plans/events-on-sx.md
index 5c62cfd4..0dcf2004 100644
--- a/plans/events-on-sx.md
+++ b/plans/events-on-sx.md
@@ -18,7 +18,7 @@ capacity rules, transactional booking, and a flow-driven notification dispatcher
 
 ## Status (rolling)
 
-`bash lib/events/conformance.sh` → **303/303** (Phases 1-4 + 9 ext: fed f/b, waitlist, EXDATE/RDATE, overrides, booking/reschedule-notify, fed transport, timezones+DST, e2e delivery pipeline)
+`bash lib/events/conformance.sh` → **311/311** (Phases 1-4 + 10 ext: …timezones+DST, e2e delivery pipeline, cross-event conflict-checked booking)
 
 ## Ground rules
 
@@ -88,6 +88,13 @@ lib/events/api.sx ── (events/schedule) (events/book) (events/agenda) ──
 
 ## Progress log
 
+- 2026-06-07 — Cross-event conflict-checked booking (extension). Capacity is
+  per-event, but `ev/book-checked!` also prevents an attendee double-booking
+  THEMSELVES across different events: it consults the actor's persist-derived
+  availability (ev/free-p?) for the occurrence's window and returns
+  :time-conflict on overlap, else the normal ev/book-occ! result. Re-booking
+  the same occurrence is idempotent (:already, not a conflict); other actors are
+  unaffected. `ev/would-time-conflict?` predicate. +8 tests, 311/311 green.
 - 2026-06-07 — End-to-end delivery pipeline (closes the derivation↔delivery
   gap). `ev/deliver-messages` bridges SX notification messages to the Scheme
   notify flow: each (id recipient body) is `serialize`d to s-expression text,