diff --git a/hosts/ocaml/lib/sx_primitives.ml b/hosts/ocaml/lib/sx_primitives.ml
index bd30b828..58f7d5c4 100644
--- a/hosts/ocaml/lib/sx_primitives.ml
+++ b/hosts/ocaml/lib/sx_primitives.ml
@@ -3264,6 +3264,50 @@ let () =
       Nil
     | _ -> raise (Eval_error "channel-set-blocking!: (channel bool)"));
 
+  (* io-select-channels: (read-list write-list timeout-ms) → {:readable [...] :writable [...]}
+     timeout-ms < 0 → block indefinitely; 0 → poll. Returns ready channel names. *)
+  register "io-select-channels" (fun args ->
+    let to_ms v = match v with
+      | Integer n -> n
+      | Number n -> int_of_float n
+      | _ -> raise (Eval_error "io-select-channels: timeout must be a number")
+    in
+    let to_list v = match v with
+      | List xs | ListRef { contents = xs } -> xs
+      | Nil -> []
+      | _ -> raise (Eval_error "io-select-channels: expected list")
+    in
+    let chan_name_of v = match v with
+      | String s -> s
+      | _ -> raise (Eval_error "io-select-channels: channel must be a string")
+    in
+    let (read_list, write_list, timeout_ms) = match args with
+      | [r; w; t] -> (to_list r, to_list w, to_ms t)
+      | _ -> raise (Eval_error "io-select-channels: (read-list write-list timeout-ms)")
+    in
+    let read_pairs = List.map (fun v ->
+      let name = chan_name_of v in
+      let (fd, _, _, _) = chan_get name in (name, fd)) read_list in
+    let write_pairs = List.map (fun v ->
+      let name = chan_name_of v in
+      let (fd, _, _, _) = chan_get name in (name, fd)) write_list in
+    let read_fds = List.map snd read_pairs in
+    let write_fds = List.map snd write_pairs in
+    let timeout = if timeout_ms < 0 then -1.0 else float_of_int timeout_ms /. 1000.0 in
+    let (ready_r, ready_w, _) =
+      try Unix.select read_fds write_fds [] timeout
+      with Unix.Unix_error (Unix.EINTR, _, _) -> ([], [], [])
+    in
+    let names_of pairs ready =
+      List.filter_map (fun (n, fd) ->
+        if List.exists (fun rfd -> rfd = fd) ready then Some (String n) else None
+      ) pairs
+    in
+    let d = Hashtbl.create 2 in
+    Hashtbl.replace d "readable" (List (names_of read_pairs ready_r));
+    Hashtbl.replace d "writable" (List (names_of write_pairs ready_w));
+    Dict d);
+
   (* === Clock === *)
   register "clock-seconds" (fun args ->
     match args with
diff --git a/lib/tcl/runtime.sx b/lib/tcl/runtime.sx
index 81bf37c4..4457dc55 100644
--- a/lib/tcl/runtime.sx
+++ b/lib/tcl/runtime.sx
@@ -2990,6 +2990,216 @@
             (assoc interp :result "")))))))
 
 
+; ============================================================
+; Event loop: fileevent / after / vwait / update (Phase 5b)
+; ============================================================
+
+; :fileevents is list of (chan event script) tuples
+; :timers is list of (expiry-ms script) tuples, sorted ascending by expiry
+
+(define
+  tcl-fileevent-set
+  (fn
+    (interp chan event script)
+    (let
+      ((existing (or (get interp :fileevents) (list))))
+      (let
+        ((filtered
+            (filter
+              (fn (e) (not (and (equal? (first e) chan) (equal? (nth e 1) event))))
+              existing)))
+        (let
+          ((new-list
+              (if (equal? script "")
+                filtered
+                (append filtered (list (list chan event script))))))
+          (assoc interp :fileevents new-list))))))
+
+(define
+  tcl-fileevent-get
+  (fn
+    (interp chan event)
+    (let
+      ((events (or (get interp :fileevents) (list))))
+      (let
+        ((matches
+            (filter
+              (fn (e) (and (equal? (first e) chan) (equal? (nth e 1) event)))
+              events)))
+        (if (= 0 (len matches)) "" (nth (first matches) 2))))))
+
+(define
+  tcl-timer-insert
+  (fn
+    (timers new-timer)
+    (cond
+      ((= 0 (len timers)) (list new-timer))
+      ((<= (first new-timer) (first (first timers))) (cons new-timer timers))
+      (else (cons (first timers) (tcl-timer-insert (rest timers) new-timer))))))
+
+(define
+  tcl-timer-add
+  (fn
+    (interp ms script)
+    (let
+      ((expiry (+ (clock-milliseconds) ms)))
+      (let
+        ((existing (or (get interp :timers) (list))))
+        (assoc interp :timers (tcl-timer-insert existing (list expiry script)))))))
+
+; Run one iteration of the event loop.
+; poll-timeout-ms: -1 = block indefinitely, 0 = poll, N>0 = wait up to N ms.
+; Returns updated interp.
+(define
+  tcl-event-step
+  (fn
+    (interp poll-timeout-ms)
+    (let
+      ((timers (or (get interp :timers) (list))) (now-ms (clock-milliseconds)))
+      (let
+        ((expired (filter (fn (t) (<= (first t) now-ms)) timers))
+          (remaining (filter (fn (t) (> (first t) now-ms)) timers)))
+        (let
+          ((interp1
+              (reduce
+                (fn (acc t) (tcl-eval-string acc (nth t 1)))
+                (assoc interp :timers remaining)
+                expired)))
+          (let
+            ((events (or (get interp1 :fileevents) (list))))
+            (let
+              ((read-chans
+                  (map
+                    (fn (e) (first e))
+                    (filter (fn (e) (equal? (nth e 1) "readable")) events)))
+                (write-chans
+                  (map
+                    (fn (e) (first e))
+                    (filter (fn (e) (equal? (nth e 1) "writable")) events)))
+                (next-timer-delta
+                  (if
+                    (= 0 (len remaining))
+                    -1
+                    (- (first (first remaining)) (clock-milliseconds)))))
+              (let
+                ((effective-timeout
+                    (cond
+                      ((and (>= poll-timeout-ms 0) (>= next-timer-delta 0))
+                        (min poll-timeout-ms next-timer-delta))
+                      ((>= poll-timeout-ms 0) poll-timeout-ms)
+                      ((>= next-timer-delta 0) next-timer-delta)
+                      (else -1))))
+                (if
+                  (and
+                    (= 0 (len read-chans))
+                    (= 0 (len write-chans)))
+                  ; nothing to select on; if timeout > 0, do a no-op wait via select
+                  (if
+                    (> effective-timeout 0)
+                    (let
+                      ((_ (io-select-channels (list) (list) effective-timeout)))
+                      interp1)
+                    interp1)
+                  (let
+                    ((select-result
+                        (io-select-channels read-chans write-chans effective-timeout)))
+                    (let
+                      ((ready-r (or (get select-result :readable) (list)))
+                        (ready-w (or (get select-result :writable) (list))))
+                      (let
+                        ((interp2
+                            (reduce
+                              (fn (acc chan)
+                                (let
+                                  ((script (tcl-fileevent-get acc chan "readable")))
+                                  (if (equal? script "") acc (tcl-eval-string acc script))))
+                              interp1
+                              ready-r)))
+                        (reduce
+                          (fn (acc chan)
+                            (let
+                              ((script (tcl-fileevent-get acc chan "writable")))
+                              (if (equal? script "") acc (tcl-eval-string acc script))))
+                          interp2
+                          ready-w)))))))))))))
+
+(define
+  tcl-cmd-fileevent
+  (fn
+    (interp args)
+    (let
+      ((chan (first args)) (event (nth args 1)))
+      (if
+        (= 2 (len args))
+        (assoc interp :result (tcl-fileevent-get interp chan event))
+        (let
+          ((script (nth args 2)))
+          (assoc (tcl-fileevent-set interp chan event script) :result ""))))))
+
+(define
+  tcl-cmd-after
+  (fn
+    (interp args)
+    (if
+      (= 0 (len args))
+      (error "after: wrong # args")
+      (let
+        ((ms (parse-int (first args))))
+        (if
+          (= 1 (len args))
+          ; pure sleep — drive event loop until ms elapsed
+          (let
+            ((target-ms (+ (clock-milliseconds) ms)))
+            (assoc (tcl-after-sleep-loop interp target-ms) :result ""))
+          ; schedule timer
+          (let
+            ((script (join " " (rest args))))
+            (assoc (tcl-timer-add interp ms script) :result "")))))))
+
+(define
+  tcl-after-sleep-loop
+  (fn
+    (interp target-ms)
+    (let
+      ((now (clock-milliseconds)))
+      (if
+        (>= now target-ms)
+        interp
+        (tcl-after-sleep-loop
+          (tcl-event-step interp (- target-ms now))
+          target-ms)))))
+
+(define
+  tcl-cmd-vwait
+  (fn
+    (interp args)
+    (if
+      (= 0 (len args))
+      (error "vwait: wrong # args")
+      (let
+        ((name (first args)))
+        (let
+          ((initial (frame-lookup (get interp :frame) name)))
+          (assoc (tcl-vwait-loop interp name initial) :result ""))))))
+
+(define
+  tcl-vwait-loop
+  (fn
+    (interp name initial)
+    (let
+      ((cur (frame-lookup (get interp :frame) name)))
+      (if
+        (and (not (nil? cur)) (not (equal? cur initial)))
+        interp
+        (tcl-vwait-loop (tcl-event-step interp 1000) name initial)))))
+
+(define
+  tcl-cmd-update
+  (fn
+    (interp args)
+    (assoc (tcl-event-step interp 0) :result "")))
+
+
 (define
   tcl-cmd-array
   (fn
@@ -3471,6 +3681,14 @@
                                                                                                                       ((i (tcl-register i "flush" tcl-cmd-flush)))
                                                                                                                       (let
                                                                                                                         ((i (tcl-register i "fconfigure" tcl-cmd-fconfigure)))
+                                                                                                                        (let
+                                                                                                                          ((i (tcl-register i "fileevent" tcl-cmd-fileevent)))
+                                                                                                                        (let
+                                                                                                                          ((i (tcl-register i "after" tcl-cmd-after)))
+                                                                                                                        (let
+                                                                                                                          ((i (tcl-register i "vwait" tcl-cmd-vwait)))
+                                                                                                                        (let
+                                                                                                                          ((i (tcl-register i "update" tcl-cmd-update)))
                                                                                                                         (let
                                                                                                                           ((i (tcl-register i "file" tcl-cmd-file)))
                                                                                                                           (let
@@ -3482,4 +3700,4 @@
                                                                                                                                 (tcl-register
                                                                                                                                   i
                                                                                                                                   "array"
-                                                                                                                                  tcl-cmd-array)))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))
+                                                                                                                                  tcl-cmd-array)))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))
diff --git a/lib/tcl/tests/idioms.sx b/lib/tcl/tests/idioms.sx
index b84aaafd..17738dc9 100644
--- a/lib/tcl/tests/idioms.sx
+++ b/lib/tcl/tests/idioms.sx
@@ -238,6 +238,42 @@
         :result)
       "0")
 
+    ; 33-37. Phase 5b event loop: after / vwait / fileevent / update
+    (ok "after-vwait-timer"
+      (get
+        (run
+          "after 30 {set ::done fired}\nvwait ::done\nset ::done")
+        :result)
+      "fired")
+
+    (ok "after-multiple-timers-update"
+      (get
+        (run
+          "set ::n 0\nafter 0 {incr ::n}\nafter 0 {incr ::n}\nafter 0 {incr ::n}\nupdate\nset ::n")
+        :result)
+      "3")
+
+    (ok "fileevent-readable-fires"
+      (get
+        (run
+          "set f /tmp/tcl-phase5b-1.txt\nset c [open $f w]\nputs -nonewline $c hi\nclose $c\nset c [open $f r]\nfileevent $c readable {set ::ready 1; fileevent $::ch readable {}}\nset ::ch $c\nvwait ::ready\nclose $c\nfile delete $f\nset ::ready")
+        :result)
+      "1")
+
+    (ok "fileevent-query-script"
+      (get
+        (run
+          "set f /tmp/tcl-phase5b-2.txt\nset c [open $f w]\nputs -nonewline $c x\nclose $c\nset c [open $f r]\nfileevent $c readable {puts hello}\nset s [fileevent $c readable]\nclose $c\nfile delete $f\nreturn $s")
+        :result)
+      "puts hello")
+
+    (ok "after-cancel-via-vwait-timing"
+      (get
+        (run
+          "set ::counter 0\nafter 10 {incr ::counter}\nafter 50 {set ::done 1}\nvwait ::done\nset ::counter")
+        :result)
+      "1")
+
     (dict
       "passed"
       tcl-idiom-pass