diff --git a/tl/src/tl/events.cljs b/tl/src/tl/events.cljs
index 67a47b6..969047a 100644
--- a/tl/src/tl/events.cljs
+++ b/tl/src/tl/events.cljs
@@ -75,7 +75,7 @@
   (let [stack (vec (valid-stack scene stack))
         ctx   (peek stack)]
     (cond-> (assoc view :stack stack)
-      playhead (assoc-in [:playheads ctx] playhead))))
+      playhead (assoc-in [:playheads ctx] (scene/assert-frame "route playhead" playhead)))))
 
 (defn- route-state [db]
   (let [ctx (peek (get-in db [:view :stack]))]
@@ -601,7 +601,8 @@
 ;; moved the stack, so snap back to the editing context before seeking the frame.
 (rf/reg-event-fx ::preview-frame
                  (fn [{:keys [db]} [_ local]]
-                   (let [stack (get-in db [:view :linking :stack])
+                   (let [local (scene/assert-frame "preview frame" local)
+                         stack (get-in db [:view :linking :stack])
                          ctx   (peek stack)
                          db    (-> db (assoc-in [:view :stack] stack)
                                       (assoc-in [:view :playheads ctx] local))
@@ -611,7 +612,8 @@
 
 (rf/reg-event-fx ::set-playhead
                  (fn [{:keys [db]} [_ ctx lf]]
-                   (let [next-db (-> db
+                   (let [lf      (scene/assert-frame "playhead" lf)
+                         next-db (-> db
                                      (assoc-in [:view :playheads ctx] lf)
                                      (sync-draft-mark-for-playhead ctx lf))]
                      (sync-route {:db next-db} next-db))))
@@ -841,8 +843,8 @@
                          (scene/seg-local segs seg-id (or frame 0))
                          (scene/seg-local segs seg-id (or frame (scene/seg-length segs seg-id))))
              keep      (:keep pt)
-             lo        (js/Math.round (min new-local keep))
-             hi        (js/Math.round (max new-local keep))]
+             lo        (scene/assert-frame "proxy range start" (min new-local keep))
+             hi        (scene/assert-frame "proxy range end" (max new-local keep))]
          {:db (-> db (assoc-in [:scene :groups (:proxy pt)]
                                (scene/roll-proxy scene (:parent g) proxy lo (max (inc lo) hi)))
                      (assoc-in [:view :pt] :new))})
@@ -903,9 +905,8 @@
          ;; For a normal (non-transcluded) mark the two are the same.
          ctx   (peek (get-in db [:view :stack]))
          len   (scene/length (scene/content-segments scene ctx))
-         ;; the dragged endpoint is a whole frame; the FIXED endpoint arrives
-         ;; EXACT (fractional) so its mark re-derives identically. Only the fixed
-         ;; end must avoid rounding here — the clamp keeps [la lb) valid half-open.
+         la    (scene/assert-frame "proxy roll start" la)
+         lb    (scene/assert-frame "proxy roll end" lb)
          la*   (max 0 (min la (dec len)))
          lb*   (max (inc la*) (min lb len))]
      (if (= :proxy (:type proxy))
@@ -918,7 +919,8 @@
 (rf/reg-event-db
  ::set-proxy-frame
  (fn [db [_ pid which frame]]
-   (let [marks (get-in db [:scene :groups pid :marks])
+   (let [frame (scene/assert-frame "proxy endpoint frame" frame)
+         marks (get-in db [:scene :groups pid :marks])
          idx   (if (= which :start) 0 (dec (count marks)))]
      (assoc-in db [:scene :groups pid :marks idx which :at] frame))))
 
diff --git a/tl/src/tl/otio.cljs b/tl/src/tl/otio.cljs
index e1d7883..7600a33 100644
--- a/tl/src/tl/otio.cljs
+++ b/tl/src/tl/otio.cljs
@@ -13,14 +13,20 @@
 (defn- clip? [item]
   (str/starts-with? (:OTIO_SCHEMA item "") "Clip"))
 
-(defn- frames
-  "Frame number of a RationalTime, as a WHOLE frame. This is the ONE place a
-   fraction can enter: a rate conform (e.g. 23.976 NTSC) gives fractional source
-   positions (start_time). A frame is absolute and integer — you can't seek to half
-   a frame — so we snap here, at the boundary. Everything downstream is integer and
-   nothing else rounds (durations are already whole, so timeline tiling is exact)."
+(defn- timeline-frames
+  "Frame count/position for timeline math. These must already be integer frames."
   [rational-time]
-  (js/Math.round (:value rational-time)))
+  (let [v (:value rational-time)]
+    (when-not (integer? v)
+      (throw (js/Error. (str "OTIO RationalTime value must be an integer frame, got " v))))
+    v))
+
+(defn- media-frame
+  "Source media position as an integer frame index. Some OTIO exports carry
+   rate-conformed source starts as sub-frame RationalTime values; the app model is
+   frame-index based, so source starts are normalized once at this import boundary."
+  [rational-time]
+  (int (:value rational-time)))
 
 (defn- clip-starts
   "Source start_time (frames) of every clip across all tracks."
@@ -28,15 +34,15 @@
   (for [t tracks
         c (:children t)
         :when (clip? c)]
-    (frames (get-in c [:source_range :start_time]))))
+    (media-frame (get-in c [:source_range :start_time]))))
 
 (defn- parse-track [media-offset idx track]
-  (loop [pos 0.0
+  (loop [pos 0
          items (:children track)
          ci 0
          clips (transient [])]
     (if-let [item (first items)]
-      (let [dur (frames (get-in item [:source_range :duration]))
+      (let [dur (timeline-frames (get-in item [:source_range :duration]))
             is-clip (clip? item)]
         (recur (+ pos dur)
                (rest items)
@@ -46,7 +52,7 @@
                                :name     (:name item)
                                :start    pos                              ; timeline frame, 0-based
                                :duration dur
-                               :media-in (- (frames (get-in item [:source_range :start_time]))
+                               :media-in (- (media-frame (get-in item [:source_range :start_time]))
                                             media-offset)})               ; frame into local .mov
                  clips)))
       {:id       idx
@@ -62,11 +68,11 @@
   (let [tracks       (get-in otio [:tracks :children])
         fps          (get-in otio [:global_start_time :rate] 24)
         starts       (clip-starts tracks)
-        media-offset (if (seq starts) (apply min starts) 0.0)
+        media-offset (if (seq starts) (apply min starts) 0)
         parsed       (vec (map-indexed (partial parse-track media-offset) tracks))]
     {:fps          fps
      :media-offset media-offset
-     :duration     (reduce max 0.0 (for [t parsed
-                                         c (:clips t)]
-                                     (+ (:media-in c) (:duration c))))
+     :duration     (reduce max 0 (for [t parsed
+                                       c (:clips t)]
+                                   (+ (:media-in c) (:duration c))))
      :tracks       parsed}))
diff --git a/tl/src/tl/routes.cljs b/tl/src/tl/routes.cljs
index b2bf1fe..e277b5d 100644
--- a/tl/src/tl/routes.cljs
+++ b/tl/src/tl/routes.cljs
@@ -2,7 +2,8 @@
   (:require
    [clojure.string :as str]
    [reitit.frontend :as reitit]
-   [reitit.frontend.easy :as rfe]))
+   [reitit.frontend.easy :as rfe]
+   [tl.scene :as scene]))
 
 (def routes
   [["/" {:name :projects}]
@@ -54,15 +55,18 @@
                      (into {} (map (fn [[k v]] [(keyword k) v])
                                    (:query-params match))))
         stack (some-> (:stack qp) (str/split #","))
-        f     (some-> (:f qp) js/parseFloat)]
+        fstr  (some-> (:f qp) str)
+        f     (when (and fstr (re-matches #"\d+" fstr))
+                (scene/assert-frame "route playhead" (js/parseInt fstr 10)))]
     (cond-> {}
       (seq stack) (assoc :stack (mapv keyword stack))
-      (and f (not (js/isNaN f))) (assoc :playhead f))))
+      f (assoc :playhead f))))
 
 (defn project-url [id stack playhead]
   (let [stack-param (->> (rest stack) (map name) (str/join ","))
         query       (query-string {:stack stack-param
-                                   :f (some-> playhead js/Math.round)})]
+                                   :f (when (some? playhead)
+                                        (scene/assert-frame "route playhead" playhead))})]
     (str (href :project/show {:id id}) query)))
 
 (defonce ^:private last-replaced (atom nil))
diff --git a/tl/src/tl/scene.cljs b/tl/src/tl/scene.cljs
index 8aea34c..79a3551 100644
--- a/tl/src/tl/scene.cljs
+++ b/tl/src/tl/scene.cljs
@@ -16,6 +16,28 @@
 
 (defn- grp [scene gid] (get-in scene [:groups gid]))
 
+(defn frame?
+  "True when `n` is a concrete integer frame coordinate."
+  [n]
+  (and (number? n) (integer? n) (not (js/isNaN n))))
+
+(defn assert-frame
+  "Return `n` after asserting it is an integer frame. This is intentionally a
+   runtime check, not cljs.core/assert, so production builds keep the invariant."
+  [label n]
+  (when-not (frame? n)
+    (throw (js/Error. (str label " must be an integer frame, got " (pr-str n)))))
+  n)
+
+(defn assert-range
+  "Return `[lo hi]` after asserting a valid integer half-open frame range."
+  [label [lo hi]]
+  (assert-frame (str label " start") lo)
+  (assert-frame (str label " end") hi)
+  (when (> lo hi)
+    (throw (js/Error. (str label " must be ordered, got " (pr-str [lo hi])))))
+  [lo hi])
+
 (defn- find-mark
   "[owning-gid mark] for a mark id anywhere in the scene, or nil."
   [scene mid]
@@ -31,7 +53,10 @@
 (defn local->source
   "The source frame shown at local frame `lf` (clamped to the end)."
   [segs lf]
+  (assert-frame "local frame" lf)
   (or (some (fn [{:keys [src local]}]
+              (assert-range "segment source" src)
+              (assert-range "segment local" local)
               (let [[c d] local [a _] src]
                 (when (and (<= c lf) (< lf d)) (+ a (- lf c)))))
             segs)
@@ -40,7 +65,10 @@
 (defn source->local
   "Local frame for source frame `sf` (first segment containing it), or nil."
   [segs sf]
+  (assert-frame "source frame" sf)
   (some (fn [{:keys [src local]}]
+          (assert-range "segment source" src)
+          (assert-range "segment local" local)
           (let [[a b] src [c _] local]
             (when (and (<= a sf) (< sf b)) (+ c (- sf a)))))
         segs))
@@ -48,7 +76,10 @@
 (defn pieces
   "Where source range [sa sb) lands in local coords: a list of [lo hi)."
   [segs sa sb]
+  (assert-range "source range" [sa sb])
   (vec (keep (fn [{:keys [src local]}]
+               (assert-range "segment source" src)
+               (assert-range "segment local" local)
                (let [[a b] src [c _] local
                      lo (max sa a) hi (min sb b)]
                  (when (< lo hi) [(+ c (- lo a)) (+ c (- hi a))])))
@@ -58,13 +89,11 @@
   "Coalesce [lo hi) ranges that meet at a boundary into single bars, so a
    continuous run spanning several clips reads as one piece. Ranges are half-open,
    so adjacent clips share a boundary (A.hi == B.lo) and merge exactly — no gap to
-   fudge. Endpoints snap to whole frames first (a bar covers the frames it
-   touches); only a real (≥1 frame) gap splits. Called per-mark, so it never fuses
-   two distinct marks."
+   fudge. Called per-mark, so it never fuses two distinct marks."
   [bars]
   (reduce (fn [acc [lo hi]]
-            (let [lo (js/Math.floor lo) hi (js/Math.ceil hi)
-                  [plo phi] (peek acc)]
+            (assert-range "bar" [lo hi])
+            (let [[plo phi] (peek acc)]
               (if (and plo (<= lo phi))
                 (conj (pop acc) [plo (max phi hi)])
                 (conj acc [lo hi]))))
@@ -74,8 +103,11 @@
 (defn slice
   "Sub-segments of `segs` covering local range [la lb), src + local re-cut."
   [segs la lb]
+  (assert-range "slice" [la lb])
   (vec
    (keep (fn [{:keys [src local] :as seg}]
+           (assert-range "segment source" src)
+           (assert-range "segment local" local)
            (let [[a _] src [c d] local
                  lo (max la c) hi (min lb d)]
              (when (< lo hi)
@@ -323,9 +355,9 @@
 
 (defn mark-extent
   "EXACT context-local [lo hi] of mark `mark-id` of annotation `gid` — the true
-   min piece-start / max piece-end, WITHOUT merge-bars' floor/ceil. Endpoint
-   editing must use this (not a rounded lane bar) or the fixed end drifts a frame
-   per re-roll. `ctx-segs` = content-segments of ctx."
+   min piece-start / max piece-end, without merge-bars. Endpoint editing must use
+   this exact lane extent or the fixed end drifts a frame per edit. `ctx-segs` =
+   content-segments of ctx."
   [scene gid mark-id ctx-segs]
   (let [pcs (->> (resolve scene gid)
                  (filter #(= mark-id (:mark %)))
@@ -402,10 +434,10 @@
 (defn selection->marks
   "Split local range [la lb) of context `ctx` into a run of single-clip ref
    marks, one per content segment it crosses (the 'no cross-clip marks' rule).
-   Each references the segment's source id with the right offsets. Offsets are
-   ROUNDED to whole frames here — this is where user frame-accuracy is applied
-   (clips stay exact for tiling; the mark snaps to a whole frame of its clip)."
+   Each references the segment's source id with the right offsets. Inputs and
+   derived offsets must already be integer frames."
   [scene ctx la lb]
+  (assert-range "selection" [la lb])
   (mapv (fn [{:keys [mark src]}]
           ;; :at is the target's OWN local frame (src->at), matching resolve-mark's
           ;; slice — correct even when the target is a scattered multi-clip proxy.
@@ -413,8 +445,8 @@
           (let [[a b] src
                 a'    (src->at scene mark a)]
             {:id (str (random-uuid))                ; string so it survives JSON
-             :start {:ref mark :at (js/Math.round a')}
-             :end   {:ref mark :at (js/Math.round (+ a' (- b a)))}}))
+             :start {:ref mark :at (assert-frame "selection start offset" a')}
+             :end   {:ref mark :at (assert-frame "selection end offset" (+ a' (- b a)))}}))
         (slice (content-segments scene ctx) la lb)))
 
 (defn reconcile-run
@@ -542,7 +574,7 @@
    shared basis for both the annotation editor rows and the jump popover, so they
    can never disagree on how a point reads."
   [scene {:keys [ref at]}]
-  {:seg ref :f (js/Math.round (at->local scene ref at))})
+  {:seg ref :f (assert-frame "display point" (at->local scene ref at))})
 
 (defn- clip-row
   "Editor row {:s … :e …} for a single-clip/subclip ref mark, in mark time."
@@ -580,22 +612,23 @@
    clip mark-group per clip (source range + track), and the root timeline. The
    otio is only a seed — nothing here reads it again.
 
-   Clip ranges are kept EXACT (OTIO's fractional RationalTime), NOT rounded:
-   adjacent clips must share their half-open boundary exactly to tile without a
-   gap, and independently rounding :start vs length breaks that. Frame-accuracy is
-   applied where it belongs — at the mark the user creates (selection->marks
-   rounds :at) — and at display, never by rounding clips or bars."
+   Clip ranges are integer frame ranges. Fractional OTIO input is rejected before
+   this point; from here on, frame math asserts instead of snapping."
   [{:keys [duration tracks]}]
+  (assert-frame "OTIO duration" duration)
   (let [vtracks   (filter #(= :video (:kind %)) tracks)
         track-map (into {} (map (fn [t] [(keyword (str "t" (:index t))) {:name (:name t)}])) vtracks)
         clips     (into {} (for [t vtracks c (:clips t)]
-                             [(keyword (:id c))
-                              {:type :clip :parent nil :name (:name c)
-                               :start (:start c)              ; timeline position (frames)
-                               :marks [{:id    (keyword (str (:id c) "-m"))
-                                        :start (:media-in c)
-                                        :end   (+ (:media-in c) (:duration c))
-                                        :track (keyword (str "t" (:index t)))}]}]))]
+                             (let [start    (assert-frame "clip timeline start" (:start c))
+                                   media-in (assert-frame "clip media-in" (:media-in c))
+                                   duration (assert-frame "clip duration" (:duration c))]
+                               [(keyword (:id c))
+                                {:type :clip :parent nil :name (:name c)
+                                 :start start              ; timeline position (frames)
+                                 :marks [{:id    (keyword (str (:id c) "-m"))
+                                          :start media-in
+                                          :end   (+ media-in duration)
+                                          :track (keyword (str "t" (:index t)))}]}])))]
     {:tracks track-map
      :groups (assoc clips :root {:type :timeline :parent nil
                                  :marks [{:id :root-m :start 0 :end duration}]})}))
@@ -684,7 +717,7 @@
                           (last csegs))]
               {:local lo
                :seg   (:mark seg)
-               :f     (js/Math.round (- lo (first (:local seg [0 0]))))}))
+               :f     (assert-frame "jump target frame" (- lo (first (:local seg [0 0]))))}))
           (runs scene ctx gid))))
 
 (defn linkables
diff --git a/tl/src/tl/subs.cljs b/tl/src/tl/subs.cljs
index 56bb6f1..342dfdb 100644
--- a/tl/src/tl/subs.cljs
+++ b/tl/src/tl/subs.cljs
@@ -183,7 +183,7 @@
                        :hidden (boolean hidden)
                        :tags (vec (get-in g [:meta :tags]))
                        ;; jump targets labelled from the marks' clip refs (same as
-                       ;; the editor) — not re-derived from a floored bar frame
+                       ;; the editor) — not re-derived from a display bar frame
                        :jumps jumps
                        :start (or (ffirst bars) 0) :bars bars}))))))
           (sort-by (juxt :broken :start))      ; broken annotations sink to the bottom
diff --git a/tl/src/tl/views.cljs b/tl/src/tl/views.cljs
index 5e28053..8daf1f8 100644
--- a/tl/src/tl/views.cljs
+++ b/tl/src/tl/views.cljs
@@ -38,6 +38,14 @@
 
 (defn- px [local fps zoom] (* (/ local fps) zoom))
 
+(defn- frame-index
+  "Convert a continuous UI/video coordinate to the integer frame index it is over.
+   After this edge conversion, frame values are asserted, not snapped."
+  [label n]
+  (when-not (and (number? n) (not (js/isNaN n)))
+    (throw (js/Error. (str label " must be numeric, got " (pr-str n)))))
+  (scene/assert-frame label (int n)))
+
 (defn- clip-thumbnail-layer [thumbs fps zoom row-h {:keys [mark thumb thumb-start local]}]
   (when-let [tiles (seq (get-in thumbs [:clips (or thumb mark)]))]
     (let [[c d] local
@@ -125,7 +133,7 @@
 (defn- play-tick []
   (when-let [{:keys [ctx segs fps idx]} @play]
     (let [v  @video-el
-          sf (* (.-currentTime v) fps)
+          sf (frame-index "video source frame" (* (.-currentTime v) fps))
           {:keys [pending-ns]} @play
           {[ss se] :src [ls _] :local} (nth segs idx)]
       (cond
@@ -206,7 +214,8 @@
    clip under the landing frame into view vertically — jumps always do both."
   ([local] (goto! local false))
   ([local follow?]
-   (let [ctx  @(rf/subscribe [::subs/context])
+   (let [local (scene/assert-frame "goto frame" local)
+         ctx  @(rf/subscribe [::subs/context])
          segs @(rf/subscribe [::subs/segments])
          fps  @(rf/subscribe [::subs/fps])
          zoom @(rf/subscribe [::subs/zoom])]
@@ -399,10 +408,10 @@
         linking @(rf/subscribe [::subs/linking])
         auth? (some? @(rf/subscribe [::subs/draft-group]))
         seg   (some (fn [{[c d] :local :as s}] (when (and (<= c ph) (< ph d)) s)) segs)
-        cf    (when seg (js/Math.round (- ph (first (:local seg)))))
+        cf    (when seg (scene/assert-frame "clip frame" (- ph (first (:local seg)))))
         click? (and auth? seg)]
     [:div.frame-readout
-     [:span.fr-abs (str (js/Math.round ph) "f")]
+     [:span.fr-abs (str (scene/assert-frame "playhead" ph) "f")]
      (when seg
        [:span.fr-clip {:class (when click? "clickable")
                        :title (when click? (if linking "Click to link this frame"
@@ -449,7 +458,7 @@
   (.preventDefault ev)
   (let [to (fn [clientX]
              (let [x (- clientX (.-left (.getBoundingClientRect content)))]
-               (goto! (max 0 (* (/ x zoom) fps)))))
+               (goto! (frame-index "scrub frame" (max 0 (* (/ x zoom) fps))))))
         move (fn [e] (to (.-clientX e)))
         up   (fn up [_] (.removeEventListener js/document "mousemove" move)
                         (.removeEventListener js/document "mouseup" up))]
@@ -471,12 +480,11 @@
                   (let [d (d-of e)]
                     (when (or @moved? (> (js/Math.abs d) 3))
                       (reset! moved? true)
-                      ;; round ONLY the dragged endpoint; the fixed endpoint stays
-                      ;; exact so selection->marks re-derives its mark identically.
                       (let [[la lb] (case mode
-                                      :move  [(js/Math.round (+ lo d)) (js/Math.round (+ hi d))]
-                                      :start [(js/Math.round (+ lo d)) hi]
-                                      :end   [lo (js/Math.round (+ hi d))])]
+                                      :move  [(frame-index "drag start frame" (+ lo d))
+                                              (frame-index "drag end frame" (+ hi d))]
+                                      :start [(frame-index "drag start frame" (+ lo d)) hi]
+                                      :end   [lo (frame-index "drag end frame" (+ hi d))])]
                         (rf/dispatch [::events/reroll-proxy ann mark-id la lb])))))
         up      (fn up [_]
                   (.removeEventListener js/document "mousemove" move)
@@ -500,7 +508,7 @@
   (.preventDefault ev)
   (let [rect   (.getBoundingClientRect content)
         sx     (.-clientX ev)
-        to     (fn [cx] (max 0 (* (/ (- cx (.-left rect)) zoom) fps)))
+        to     (fn [cx] (frame-index "selection frame" (max 0 (* (/ (- cx (.-left rect)) zoom) fps))))
         a      (to sx)
         moved? (atom false)
         mv     (fn [e]
@@ -513,7 +521,7 @@
                  (let [was @moved? b (to (.-clientX e)) lo (min a b) hi (max a b)]
                    (reset! region-sel nil)
                    (if (and was (> (- hi lo) 0.5))
-                     (rf/dispatch [::events/draft-select-range (js/Math.round lo) (js/Math.round hi)])
+                     (rf/dispatch [::events/draft-select-range lo hi])
                      (on-click a))))]                   ; click (no drag)
     (.addEventListener js/document "mousemove" mv)
     (.addEventListener js/document "mouseup" up)))
@@ -693,9 +701,8 @@
             ;; visual pieces the mark has, it gets one handle pair, at its ends.
             (for [a visible :when (:draft a)
                   mid (distinct (map #(nth % 2) (:bars a)))
-                  ;; EXACT context-local extent (never the rounded bar). The drag
-                  ;; keeps the FIXED endpoint at this exact value so its mark
-                  ;; re-derives identically — that's what stops the other end drifting.
+                  ;; Exact context-local extent. The drag keeps the fixed endpoint
+                  ;; at this value so its mark re-derives identically.
                   :let [ext (scene/mark-extent scene (:id a) mid segs)]
                   :when ext
                   :let [[lo hi] ext
@@ -833,7 +840,7 @@
                                      :title (when-not lf "linked clip no longer in this timeline")
                                      :on-click #(goto-link! lf)}
                     (when-not lf "△ ") label
-                    (when lf [:span.link-f (str " " (js/Math.round lf) "f")])])))))
+                    (when lf [:span.link-f (str " " (scene/assert-frame "link frame" lf) "f")])])))))
 
 (defn- build-chip [{:keys [label ref at kind]}]
   (let [span      (js/document.createElement "span")
@@ -865,7 +872,7 @@
     (when lf
       (let [f (js/document.createElement "span")]
         (set! (.-className f) "link-f")
-        (set! (.-textContent f) (str " " (js/Math.round lf) "f"))
+        (set! (.-textContent f) (str " " (scene/assert-frame "link frame" lf) "f"))
         (.appendChild span f)))
     span))
 
@@ -953,7 +960,7 @@
                         :point {:kind :script-note :ref id}})))))))))
 
 (defn- point-label [cand off local]
-  (str (:label cand) " @" (js/Math.round local) "f"
+  (str (:label cand) " @" (scene/assert-frame "point label frame" local) "f"
        (when (and (not (:abs cand)) (pos? off))
          (str " +" off))))
 
@@ -1066,18 +1073,18 @@
                   [:span.cand-label (:label c)]
                   (when (:group c) [:span.cand-group (str " " (:group c))])
                   (when-not (contains? #{:timeline :script-note} (:kind c))
-                    [:span.link-f (str " " (js/Math.round (:local c)) "f")])])))])))
+                    [:span.link-f (str " " (scene/assert-frame "candidate frame" (:local c)) "f")])])))])))
 
 (defn- local->draft-point [segs local]
   (some (fn [{m :mark [c d] :local}]
           (when (and (<= c local) (< local d))
-            {:seg m :f (js/Math.round (- local c))}))
+            {:seg m :f (scene/assert-frame "draft point frame" (- local c))}))
         segs))
 
 (defn- local->draft-end-point [segs local]
   (some (fn [{m :mark [c d] :local}]
           (when (and (<= c local) (<= local d))
-            {:seg m :f (js/Math.round (- local c))}))
+            {:seg m :f (scene/assert-frame "draft endpoint frame" (- local c))}))
         segs))
 
 ;; --- shared string autocomplete ------------------------------------------
@@ -1369,7 +1376,7 @@
   (let [len (pt-len scene segs seg)]
     [:div.pt-chip.pending
      [:span.pt-chip-name (pt-name scene segs seg)]
-     [:input.pt-frame {:type "number" :value (js/Math.round f) :disabled true}]
+     [:input.pt-frame {:type "number" :value (scene/assert-frame "pending frame" f) :disabled true}]
      [:span.pt-dur (str "/" len)]]))
 
 (defn- empty-frame-chip [label]
@@ -1741,7 +1748,7 @@
         len  @(rf/subscribe [::subs/length])
         authed? @(rf/subscribe [::subs/authed?])
         authoring? (some? @(rf/subscribe [::subs/draft-group]))
-        ph-now #(js/Math.round @(rf/subscribe [::subs/playhead]))]
+        ph-now #(scene/assert-frame "playhead" @(rf/subscribe [::subs/playhead]))]
     [:div.toolbar
      [:div.transport
       [:button.step-btn {:title "Previous frame" :disabled start?
@@ -2083,7 +2090,7 @@
          (when (and url (seq @pages))
            [:div.script-controls
             [:button {:title "Zoom out" :on-click #(bump (fn [z] (* z 0.9)))} "−"]
-            [:span.zoom-read (str (js/Math.round (* scale 100)) "%")]
+            [:span.zoom-read (str (int (* scale 100)) "%")]
             [:button {:title "Zoom in" :on-click #(bump (fn [z] (* z 1.1)))} "＋"]
             [:button {:title "Fit width" :on-click fit!} "Fit"]])]))))
 
@@ -2271,7 +2278,7 @@
           (when uploading?
             [:div.upload-progress
              [:div.upload-track [:div.upload-fill {:style {:width (str (* 100 prog) "%")}}]]
-             [:span.upload-pct (str (js/Math.round (* 100 prog)) "%")]])
+             [:span.upload-pct (str (int (* 100 prog)) "%")]])
           [:div.form-actions
            [:button.save {:disabled (or uploading? (str/blank? @pname) (not @otio) (not @clip))
                           :on-click #(rf/dispatch [::events/create-project @pname @otio @clip])}
diff --git a/tl/test/tl/flow_test.cljs b/tl/test/tl/flow_test.cljs
index f0da29a..079f461 100644
--- a/tl/test/tl/flow_test.cljs
+++ b/tl/test/tl/flow_test.cljs
@@ -127,3 +127,35 @@
         (let [bars (bars-in :annB :annC)]
           (is (some #(= bid (nth % 2)) bars) "after the drag the B-mark still resolves in annB (no vanish)")
           (is (= [50 140] (some (fn [[lo hi m]] (when (= m bid) [lo hi])) bars)) "handle moved end to 140"))))))
+
+(deftest draft-range-events-keep-integer-frames
+  (testing "basic drag selection through the real event path stores only integer frames"
+    (setup! clips-scene [:root])
+    (rf/dispatch-sync [::ev/open-draft])
+    (rf/dispatch-sync [::ev/draft-select-range 25 175])
+    (let [[gid g] (some (fn [[gid g]] (when (:draft g) [gid g])) (:groups (scene*)))
+          mark    (first (:marks g))
+          pid     (get-in mark [:start :ref])
+          proxy   (get-in (scene*) [:groups pid])
+          ats     (mapcat (fn [m] [(get-in m [:start :at]) (get-in m [:end :at])]) (:marks proxy))]
+      (is gid "draft exists")
+      (is (= :proxy (:type proxy)))
+      (is (every? integer? ats))
+      (is (= [[25 175 (:id mark)]] (bars-in :root gid)))))
+  (testing "fractional drag selection fails instead of being changed into nearby frames"
+    (setup! clips-scene [:root])
+    (rf/dispatch-sync [::ev/open-draft])
+    (is (thrown-with-msg? js/Error #"integer frame"
+          (rf/dispatch-sync [::ev/draft-select-range 25.5 175])))))
+
+(deftest reroll-proxy-rejects-fractional-frames
+  (testing "live proxy reroll accepts integer handle positions and rejects fractional ones"
+    (setup! clips-scene [:root])
+    (rf/dispatch-sync [::ev/open-draft])
+    (rf/dispatch-sync [::ev/draft-select-range 25 175])
+    (let [[gid g] (some (fn [[gid g]] (when (:draft g) [gid g])) (:groups (scene*)))
+          mark-id (:id (first (:marks g)))]
+      (rf/dispatch-sync [::ev/reroll-proxy gid mark-id 30 170])
+      (is (= [[30 170 mark-id]] (bars-in :root gid)))
+      (is (thrown-with-msg? js/Error #"integer frame"
+            (rf/dispatch-sync [::ev/reroll-proxy gid mark-id 30.25 170]))))))
diff --git a/tl/test/tl/frame_policy_test.cljs b/tl/test/tl/frame_policy_test.cljs
new file mode 100644
index 0000000..3c6a7f0
--- /dev/null
+++ b/tl/test/tl/frame_policy_test.cljs
@@ -0,0 +1,27 @@
+(ns tl.frame-policy-test
+  (:require [cljs.test :refer-macros [deftest is testing]]
+            [clojure.string :as str]))
+
+(def fs (js/require "fs"))
+(def path (js/require "path"))
+
+(defn- cljs-files [dir]
+  (mapcat (fn [name]
+            (let [p (.join path dir name)
+                  st (.statSync fs p)]
+              (cond
+                (.isDirectory st) (cljs-files p)
+                (str/ends-with? name ".cljs") [p]
+                :else [])))
+          (array-seq (.readdirSync fs dir))))
+
+(deftest no-math-round-in-frame-code
+  (testing "frame code must not use the JS rounding API"
+    (let [needle (str "Math" "." "round")
+          hits (->> (concat (cljs-files "src") (cljs-files "test"))
+                    (remove #(str/ends-with? % "frame_policy_test.cljs"))
+                    (keep (fn [p]
+                            (when (str/includes? (.readFileSync fs p "utf8") needle)
+                              p)))
+                    vec)]
+      (is (= [] hits)))))
diff --git a/tl/test/tl/routes_test.cljs b/tl/test/tl/routes_test.cljs
new file mode 100644
index 0000000..fd0c111
--- /dev/null
+++ b/tl/test/tl/routes_test.cljs
@@ -0,0 +1,15 @@
+(ns tl.routes-test
+  (:require [cljs.test :refer-macros [deftest is testing]]
+            [tl.routes :as routes]))
+
+(deftest view-state-parses-integer-frame-query
+  (testing "route playhead accepts string and numeric integer query params"
+    (set! (.. js/window -location -hash) "")
+    (is (= {:playhead 42}
+           (routes/view-state {:query-params {"f" "42"}})))
+    (is (= {:playhead 42}
+           (routes/view-state {:query-params {"f" 42}}))))
+  (testing "fractional route playhead is ignored instead of changed"
+    (set! (.. js/window -location -hash) "")
+    (is (= {}
+           (routes/view-state {:query-params {"f" "42.5"}})))))
diff --git a/tl/test/tl/scene_test.cljs b/tl/test/tl/scene_test.cljs
index aa8616c..aca12f9 100644
--- a/tl/test/tl/scene_test.cljs
+++ b/tl/test/tl/scene_test.cljs
@@ -1,6 +1,7 @@
 (ns tl.scene-test
   (:require [cljs.test :refer-macros [deftest is testing]]
             [tl.md :as md]
+            [tl.otio :as otio]
             [tl.scene :as s]))
 
 ;; --- shared fixture -------------------------------------------------------
@@ -16,6 +17,8 @@
 
 (defn with-group [scene gid g] (assoc-in scene [:groups gid] g))
 (defn refm [id clip a b] {:id id :start {:ref clip :at a} :end {:ref clip :at b}})
+(defn err-msg [f]
+  (try (f) nil (catch js/Error e (.-message e))))
 
 ;; X = inter-x: B[50,100), A[0,50), B[0,50), A[50,100) — four 50-frame subclips.
 (def inter-x
@@ -408,26 +411,55 @@
         (is (= #{:t0 :t1} (s/tracks segs)))
         (is (= 200 (s/length segs)))))))
 
-(deftest from-otio-keeps-clip-ranges-exact
-  (testing "clips keep OTIO's exact fractional ranges (so adjacent clips tile);
-            frame-accuracy is applied at mark creation, not here"
-    (let [parsed {:fps 24 :duration 199.6
-                  :tracks [{:index 0 :kind :video :name "W"
-                            :clips [{:id "t0-c0" :name "a" :start 0.2 :media-in 188.87 :duration 100.4}]}]}
-          mark   (first (get-in (s/from-otio parsed) [:groups :t0-c0 :marks]))]
-      (is (= 188.87 (:start mark)))                            ; exact, not rounded
-      (is (= 289.27 (:end mark))))))
+(deftest otio-normalizes-source-starts-and-rejects-fractional-durations
+  (testing "fractional OTIO source starts are normalized at import"
+    (let [clip {:OTIO_SCHEMA "Clip.2"
+                :name "a"
+                :source_range {:start_time {:value 10.5 :rate 24}
+                               :duration {:value 5 :rate 24}}}
+          otio {:global_start_time {:rate 24}
+                :tracks {:children [{:name "V" :kind "Video" :children [clip]}]}}
+          parsed (otio/parse otio)]
+      (is (= 10 (:media-offset parsed)))
+      (is (= 0 (get-in parsed [:tracks 0 :clips 0 :media-in])))
+      (is (integer? (get-in parsed [:tracks 0 :clips 0 :media-in])))))
+  (testing "fractional OTIO durations still fail because timeline math is integer"
+    (let [clip {:OTIO_SCHEMA "Clip.2"
+                :name "a"
+                :source_range {:start_time {:value 10 :rate 24}
+                               :duration {:value 5.5 :rate 24}}}
+          otio {:global_start_time {:rate 24}
+                :tracks {:children [{:name "V" :kind "Video" :children [clip]}]}}
+          msg  (err-msg #(otio/parse otio))]
+      (is (re-find #"integer frame" msg))))
+  (testing "from-otio also asserts parsed frame fields are integers"
+    (let [clip {:id "t0-c0" :name "a" :start 0.2 :media-in 188.87 :duration 100.4}
+          parsed {:fps 24 :duration 199.6
+                  :tracks [{:index 0 :kind :video :name "W" :clips [clip]}]}
+          msg (err-msg #(s/from-otio parsed))]
+      (is (re-find #"integer frame" msg)))))
 
-(deftest selection-rounds-at-to-whole-frames
-  (testing "a selection over a clip with fractional layout still yields whole-frame
-            :at offsets (clip stays exact, the mark snaps)"
+(deftest real-otio-source-starts-import-as-integer-media-frames
+  (testing "the bundled Challengers OTIO has fractional source starts but imports into integer model frames"
+    (let [raw    (js->clj (js/JSON.parse (.readFileSync (js/require "fs") "resources/public/one_two_three.otio" "utf8"))
+                          :keywordize-keys true)
+          parsed (otio/parse raw)
+          scene  (s/from-otio parsed)
+          media-ins (for [t (:tracks parsed) c (:clips t)] (:media-in c))]
+      (is (seq media-ins))
+      (is (every? integer? media-ins))
+      (is (every? integer? (mapcat (fn [[_ g]]
+                                     (mapcat (juxt :start :end) (:marks g)))
+                                   (:groups scene)))))))
+
+(deftest selection-rejects-fractional-frames
+  (testing "a selection over fractional clip layout fails instead of changing frames"
     (let [scene {:tracks {:t0 {:name "W"}}
                  :groups {:root {:type :timeline :parent nil :marks [{:id :m/r :start 0 :end 100}]}
                           :fr   {:type :clip :parent nil :start 0.3   ; fractional timeline pos
                                  :marks [{:id :m/fr :start 10.4 :end 110.4 :track :t0}]}}}
-          run   (s/selection->marks scene :root 20 60)]
-      (is (seq run))
-      (is (every? integer? (mapcat (juxt #(get-in % [:start :at]) #(get-in % [:end :at])) run))))))
+          msg (err-msg #(s/selection->marks scene :root 20 60))]
+      (is (re-find #"integer frame" msg)))))
 
 ;; =========================================================================
 ;; Suite 4 — draft rows <-> marks (the two-input editor)
@@ -460,13 +492,13 @@
         (is (= 0  (get-in (first marks) [:start :at])))
         (is (= 30 (get-in (first marks) [:end :at])))))))
 
-(deftest merge-bars-coalesces-continuous-run
-  (testing "sub-frame OTIO gaps collapse to one bar; a real gap stays split"
-    ;; foobar's real bars (continuous 5-clip selection, ~0.9-frame source gaps)
+(deftest merge-bars-coalesces-continuous-integer-runs
+  (testing "integer-adjacent bars merge; fractional bars fail"
     (is (= [[128 542]]
-           (s/merge-bars [[128.87 190.87] [191.80 265.80] [266.73 384.73]
-                          [385.61 413.61] [414.58 541.58]])))
+           (s/merge-bars [[128 191] [191 266] [266 386]
+                          [386 414] [414 542]])))
     (is (= [[0 50] [200 260]] (s/merge-bars [[0 50] [200 260]])))   ; real gap → two
+    (is (re-find #"integer frame" (err-msg #(s/merge-bars [[0.2 50]]))))
     (is (= [] (s/merge-bars [])))))
 
 (deftest restore-annotations-rekeywordizes-json