diff --git a/apps/effectcraft-web/js/host.js b/apps/effectcraft-web/js/host.js
index cdc71f9d..486865ae 100644
--- a/apps/effectcraft-web/js/host.js
+++ b/apps/effectcraft-web/js/host.js
@@ -278,8 +278,8 @@ export function audioStop() {
 
 // ------------------------------------------------------------------ workers
 
-// Idle workers are reused; each keeps the files it was sent (path → size) so a file goes to a
-// worker once.
+// Idle workers are reused; each keeps the files it was sent (path → version) so a file goes to a
+// worker once, and again when it was replaced (even by one of the same size, #218).
 const idle = [];
 const running = new Map(); // job id → {worker, done}
 // Job workers open a WebGPU device of their own (unless `?nogpuworkers`): their jobs' frames
@@ -310,7 +310,8 @@ function spawn(base) {
   return w;
 }
 
-/// Run a job in a worker. `files`: [[path, Uint8Array]] the job reads; `onMessage(type, json,
+/// Run a job in a worker. `files`: [[path, Uint8Array, version]] the job reads (`version`
+/// changes with every write of the file); `onMessage(type, json,
 /// path, bytes)` receives "reply" (json), "file" (path, bytes: a rendered file to download),
 /// "store" (path, bytes: a file to keep in browser storage) and "error" (json = message).
 export function workerRun(id, json, files, base, onMessage) {
@@ -339,9 +340,10 @@ export function workerRun(id, json, files, base, onMessage) {
       onMessage("error", String(e.message || "worker error"), null, null);
       w.worker.terminate();
     };
-    for (const [path, bytes] of files) {
-      if (w.files.get(path) === bytes.length) continue;
-      w.files.set(path, bytes.length);
+    for (const [path, bytes, version] of files) {
+      const tag = version ?? bytes.length;
+      if (w.files.get(path) === tag) continue;
+      w.files.set(path, tag);
       w.worker.postMessage({ type: "file", path, bytes });
     }
     w.worker.postMessage({ type: "job", json });
diff --git a/apps/effectcraft-web/src/files.rs b/apps/effectcraft-web/src/files.rs
index ef0889ba..ca6699ea 100644
--- a/apps/effectcraft-web/src/files.rs
+++ b/apps/effectcraft-web/src/files.rs
@@ -35,6 +35,12 @@ pub fn get(path: &str) -> Option<Arc<[u8]>> {
     STORE.file(path)
 }
 
+/// A file's bytes and version (new with every write: workers resend a file whose version they
+/// haven't seen).
+pub fn get_versioned(path: &str) -> Option<(Arc<[u8]>, u64)> {
+    STORE.file_versioned(path)
+}
+
 /// `(path, size)` of every file in the table.
 pub fn list() -> Vec<(String, usize)> {
     STORE.files().into_iter().map(|(p, n, _, _)| (p, n)).collect()
diff --git a/apps/effectcraft-web/src/frames.rs b/apps/effectcraft-web/src/frames.rs
index fc703eba..523fd03c 100644
--- a/apps/effectcraft-web/src/frames.rs
+++ b/apps/effectcraft-web/src/frames.rs
@@ -19,7 +19,7 @@
 //! frame times. Without WebGPU, frames render on the worker's CPU.
 
 use std::cell::RefCell;
-use std::collections::HashSet;
+use std::collections::HashMap;
 use std::sync::Arc;
 
 use effectcraft_engine::offload::footage_files;
@@ -45,7 +45,8 @@ extern "C" {
 struct Fw {
     index: u32,
     mirror: Mirror,
-    files: HashSet<String>,
+    /// The footage files it was sent, with their versions (a replaced file goes again).
+    files: HashMap<String, u64>,
     busy: Option<(u64, RemoteDone)>,
     alive: bool,
     /// Its GPU adapter (after it reported), or why it has none.
@@ -99,7 +100,7 @@ impl WebFrames {
             let mut s = s.borrow_mut();
             for _ in 0..n {
                 let index = frame_worker_start(&crate::page_url(""), on.as_ref().unchecked_ref(), gpu);
-                s.workers.push(Fw { index, mirror: Mirror::default(), files: HashSet::new(), busy: None, alive: true, gpu: None });
+                s.workers.push(Fw { index, mirror: Mirror::default(), files: HashMap::new(), busy: None, alive: true, gpu: None });
             }
         });
         HANDLER.with(|h| *h.borrow_mut() = Some(on));
@@ -116,14 +117,13 @@ impl WebFrames {
             // Footage the worker hasn't got yet.
             let w = &mut s.workers[k];
             for p in footage_files(&job.project) {
-                if w.files.contains(&p) {
+                let Some((d, version)) = crate::files::get_versioned(&p) else { continue };
+                if w.files.get(&p) == Some(&version) {
                     continue;
                 }
-                if let Some(d) = crate::files::get(&p) {
-                    let bytes = js_sys::Uint8Array::from(&d[..]);
-                    frame_worker_post(w.index, &obj(&[("type", "file".into()), ("path", p.as_str().into()), ("bytes", bytes.into())]), &js_sys::Array::new());
-                    w.files.insert(p);
-                }
+                let bytes = js_sys::Uint8Array::from(&d[..]);
+                frame_worker_post(w.index, &obj(&[("type", "file".into()), ("path", p.as_str().into()), ("bytes", bytes.into())]), &js_sys::Array::new());
+                w.files.insert(p, version);
             }
             let sync = w.mirror.sync(job.revision, &job.project);
             let index = w.index;
diff --git a/apps/effectcraft-web/src/store.rs b/apps/effectcraft-web/src/store.rs
index ae02649b..7017b620 100644
--- a/apps/effectcraft-web/src/store.rs
+++ b/apps/effectcraft-web/src/store.rs
@@ -42,6 +42,9 @@ pub struct Entry {
     pub modified: f64,
     /// Written to the browser's storage (render outputs are not).
     pub persist: bool,
+    /// Which write this is: new with every write of the key, so a file replaced by another of
+    /// the same size still reads as changed (workers are sent it again, #218).
+    pub version: u64,
 }
 
 /// A queued write for the browser storage: `data` = `None` deletes the key.
@@ -58,17 +61,21 @@ pub struct Mirror {
     entries: BTreeMap<String, Entry>,
     /// Keys changed since the last [`Mirror::take_pending`], in first-change order.
     dirty: Vec<String>,
+    /// The last [`Entry::version`] given.
+    version: u64,
 }
 
 impl Mirror {
     /// An entry read from the browser's storage at startup (nothing to flush).
     pub fn load(&mut self, key: &str, data: Arc<[u8]>, modified: f64) {
-        self.entries.insert(key.to_string(), Entry { data, modified, persist: true });
+        let version = self.next_version();
+        self.entries.insert(key.to_string(), Entry { data, modified, persist: true, version });
     }
 
     pub fn put(&mut self, key: &str, data: Arc<[u8]>, modified: f64, persist: bool) {
         let was_persisted = self.entries.get(key).is_some_and(|e| e.persist);
-        self.entries.insert(key.to_string(), Entry { data, modified, persist });
+        let version = self.next_version();
+        self.entries.insert(key.to_string(), Entry { data, modified, persist, version });
         if persist || was_persisted {
             self.touch(key);
         }
@@ -86,6 +93,11 @@ impl Mirror {
         }
     }
 
+    fn next_version(&mut self) -> u64 {
+        self.version = self.version.wrapping_add(1);
+        self.version
+    }
+
     fn touch(&mut self, key: &str) {
         if !self.dirty.iter().any(|k| k == key) {
             self.dirty.push(key.to_string());
@@ -172,6 +184,11 @@ impl Store {
         self.lock().get(&file_key(path)).map(|e| e.data.clone())
     }
 
+    /// The file under `path` with its [`Entry::version`].
+    pub fn file_versioned(&self, path: &str) -> Option<(Arc<[u8]>, u64)> {
+        self.lock().get(&file_key(path)).map(|e| (e.data.clone(), e.version))
+    }
+
     pub fn remove_file(&self, path: &str) -> bool {
         let r = self.lock().remove(&file_key(path));
         self.changed();
@@ -412,6 +429,20 @@ mod tests {
         assert_eq!(m.list("files").len(), 1);
     }
 
+    /// A file replaced by different bytes of the same size gets a new version, so workers are
+    /// sent it again (#218).
+    #[test]
+    fn versions_change_with_every_write() {
+        let s = store();
+        s.put_file("/collision.png", Arc::from(&b"red!"[..]), true);
+        let (_, a) = s.file_versioned("/collision.png").unwrap();
+        s.put_file("/collision.png", Arc::from(&b"blue"[..]), true);
+        let (data, b) = s.file_versioned("/collision.png").unwrap();
+        assert_ne!(a, b);
+        assert_eq!(&data[..], b"blue");
+        assert_eq!(s.file_versioned("/collision.png").map(|f| f.1), Some(b), "reading doesn't change it");
+    }
+
     #[test]
     fn keys_and_paths_round_trip() {
         assert_eq!(file_key("/a b/c.mov"), "files/a b/c.mov");
diff --git a/apps/effectcraft-web/src/worker.rs b/apps/effectcraft-web/src/worker.rs
index 81c5fb31..bd86fd5d 100644
--- a/apps/effectcraft-web/src/worker.rs
+++ b/apps/effectcraft-web/src/worker.rs
@@ -60,8 +60,8 @@ impl Offload for WorkerOffload {
         let id = req.id;
         let files = js_sys::Array::new();
         for p in &req.files {
-            if let Some(d) = crate::files::get(p) {
-                files.push(&js_sys::Array::of2(&JsValue::from_str(p), &js_sys::Uint8Array::from(&d[..])));
+            if let Some((d, version)) = crate::files::get_versioned(p) {
+                files.push(&js_sys::Array::of3(&JsValue::from_str(p), &js_sys::Uint8Array::from(&d[..]), &JsValue::from_f64(version as f64)));
             }
         }
         let json = serde_json::to_string(&req).map_err(|e| e.to_string())?;
diff --git a/apps/effectcraft-web/tests/workers.mjs b/apps/effectcraft-web/tests/workers.mjs
new file mode 100644
index 00000000..96e3f1f0
--- /dev/null
+++ b/apps/effectcraft-web/tests/workers.mjs
@@ -0,0 +1,56 @@
+// Job-worker plumbing of the web app (js/host.js) under Node with a fake Worker,
+// no browser or Wasm build needed:
+//
+//   node apps/effectcraft-web/tests/workers.mjs
+//
+// - A reused worker is sent a file again when it was replaced, even by one of the same size
+//   (#218), and not when it is unchanged.
+// Exits non-zero on failure.
+import assert from "node:assert/strict";
+import { dirname, join } from "node:path";
+import { fileURLToPath, pathToFileURL } from "node:url";
+
+const here = dirname(fileURLToPath(import.meta.url));
+globalThis.self = globalThis;
+
+// A Worker that starts on "init" and finishes each job at once.
+let failStart = false;
+const workers = [];
+class FakeWorker {
+  constructor() {
+    this.posted = [];
+    this.terminated = false;
+    workers.push(this);
+  }
+  postMessage(m) {
+    this.posted.push(m);
+    setTimeout(() => {
+      if (m.type === "init") {
+        if (failStart) this.onerror?.({ message: "worker failed to start" });
+        else this.onmessage?.({ data: { type: "ready" } });
+      } else if (m.type === "job") {
+        this.onmessage?.({ data: { type: "reply", json: "{}", done: true } });
+      }
+    });
+  }
+  terminate() {
+    this.terminated = true;
+  }
+}
+globalThis.Worker = FakeWorker;
+
+const host = await import(pathToFileURL(join(here, "../js/host.js")));
+const run = (id, files) =>
+  new Promise((resolve) => host.workerRun(id, "{}", files, "http://localhost/", (kind, json) => kind !== "file" && resolve({ kind, json })));
+const sent = (w) => w.posted.filter((m) => m.type === "file").map((m) => m.bytes[0]);
+
+// #218: equal-length replacement.
+const red = new Uint8Array(512).fill(1);
+const blue = new Uint8Array(512).fill(2);
+assert.equal((await run(1, [["/collision.png", red, 1]])).kind, "reply");
+assert.equal((await run(2, [["/collision.png", blue, 2]])).kind, "reply");
+assert.equal((await run(3, [["/collision.png", blue, 2]])).kind, "reply");
+assert.equal(workers.length, 1, "the worker is reused");
+assert.deepEqual(sent(workers[0]), [1, 2], "the replacement is sent once, the unchanged file not again");
+
+console.log("workers: ok");