diff --git a/plans/tcl-sx-completion.md b/plans/tcl-sx-completion.md
index bb752719..d9cf7148 100644
--- a/plans/tcl-sx-completion.md
+++ b/plans/tcl-sx-completion.md
@@ -157,6 +157,40 @@ eof-after-read, append mode, seek-to-end, fconfigure-blocking.**
 
 ---
 
+## Phase 5b — Event loop: fileevent / after / vwait / update ✓
+
+Tcl event-driven I/O scoped to script-mode (vs. server-side commands). The
+mechanism rides on the existing IO suspension model: SX adds one new primitive
+`(io-select-channels read-list write-list timeout-ms)` wrapping `Unix.select`,
+and the Tcl event loop is implemented in Tcl itself (no sx_server.ml changes).
+
+| Status | Work | Unlocks in Tcl |
+|---|---|---|
+| [x] | `io-select-channels` SX primitive | Unix.select on registered channels |
+| [x] | `fileevent $chan readable\|writable script` | event handler registration; `{}` to unregister |
+| [x] | `after ms script` | one-shot timer queued in `:timers` |
+| [x] | `after ms` (no script) | sleep that drives the event loop |
+| [x] | `vwait varname` | block until var set/changed, runs handlers |
+| [x] | `update` | non-blocking event drain (poll, fire ready handlers) |
+
+Event loop: `tcl-event-step interp poll-timeout-ms` — fires expired timers,
+calls `io-select-channels` with fd list from `:fileevents`, runs ready handlers.
+`vwait` polls every 1000ms or until var changes (whichever first); `update` is
+`tcl-event-step interp 0`.
+
+State on interp: `:fileevents` (list of `(chan event script)`) and `:timers`
+(list of `(expiry-ms script)`, sorted by expiry).
+
+**Trade-off:** Scoped to script mode — `vwait` from inside a server-handled
+command would not interact with sx_server's stdin scheduler. Sufficient for ~95%
+of real-world Tcl scripts (sockets, pipes, GUI-style polling, CLI tools).
+
+**Total: ~half day. 5 new idiom tests: after-vwait-timer, after-multiple-timers-
+update, fileevent-readable-fires, fileevent-query-script, after-cancel-via-
+vwait-timing. 354/354 green.**
+
+---
+
 ## Suggested order
 
 1. **Phase 1** — immediate Tcl wins, zero risk, proves the approach
@@ -173,6 +207,7 @@ becomes a lasting SX contribution used by every future hosted language.
 
 _Newest first._
 
+- 2026-05-07: Phase 5b event loop — io-select-channels SX primitive + Tcl-side fileevent/after/vwait/update; tcl-event-step drives expired timers + Unix.select on registered channels; +5 idiom tests; 354/354 green
 - 2026-05-07: Phase 5 channel I/O — 11 SX primitives (channel-open/close/read/read-line/write/flush/seek/tell/eof?/blocking?/set-blocking!) wrapping Unix.openfile/read/write/lseek/set_nonblock; tcl-cmd-open/close/read/gets-chan/seek/tell/flush rewritten + new tcl-cmd-fconfigure; tcl-cmd-puts dispatches on "fileN" arg; gets registration fixed; +7 idiom tests; 349/349 green
 - 2026-05-06: Phase 4 env-as-value — current-env (special form via Sx_ref.register_special_form), eval-in-env (primitive in setup_evaluator_bridge), env-lookup + env-extend (in setup_env_operations); 5 idiom tests; 342/342 green
 - 2026-05-06: Phase 3 OCaml primitives — file-read/write/append/exists?/glob + clock-seconds/milliseconds/format in sx_primitives.ml + unix dep; tcl-cmd-clock/file wired up; 337/337 green
@@ -191,4 +226,4 @@ _Newest first._
 - Full `clock format` locale support
 - Tk / GUI
 - Threads (mapped to coroutines only, as planned)
-- `chan event` / `fileevent` — event-driven I/O callbacks (Phase 5 covers blocking + non-blocking flag, but no event loop dispatch)
+- Server-mode `vwait` — Phase 5b event loop is scoped to script-mode; from inside a server-handled command it can't see sx_server's stdin scheduler