diff --git a/src/app/control/mod.rs b/src/app/control/mod.rs
index 3d44a7fb2..ecb1d3271 100644
--- a/src/app/control/mod.rs
+++ b/src/app/control/mod.rs
@@ -131,6 +131,24 @@ struct Operation {
 fn failure(code: &str, error: impl ToString) -> Value {
     json!({"ok":false,"status":"failed","code":code,"error":error.to_string()})
 }
+/// `{"op":"open","name":"plan.dwg","data_base64":"…"}`: the web build has no
+/// filesystem path to open, so the caller sends the file itself. `name` is
+/// reduced to its file name; it labels the tab and the recent-files entry.
+fn open_bytes_request(req: &Value) -> Result<(String, Vec<u8>), Value> {
+    use base64::Engine as _;
+    let name = string(req, "name")?;
+    let name = name.rsplit(['/', '\\']).next().unwrap_or(name);
+    if name.is_empty() {
+        return Err(failure("invalid_request", "Missing name"));
+    }
+    let bytes = base64::engine::general_purpose::STANDARD
+        .decode(req["data_base64"].as_str().unwrap_or_default())
+        .map_err(|e| failure("invalid_request", format!("data_base64: {e}")))?;
+    if bytes.is_empty() {
+        return Err(failure("invalid_request", "data_base64 is empty"));
+    }
+    Ok((name.to_string(), bytes))
+}
 fn string<'a>(req: &'a Value, key: &str) -> Result<&'a str, Value> {
     req[key]
         .as_str()
@@ -703,6 +721,24 @@ impl OpenCADStudio {
         let i = self.active_tab;
         Ok(match req["op"].as_str().unwrap_or("") {
             "new" => self.update(Message::TabNew),
+            "open" if req.get("data_base64").is_some() => {
+                let (name, bytes) = open_bytes_request(req)?;
+                #[cfg(target_arch = "wasm32")]
+                {
+                    if self.opening.is_some() {
+                        return Err(failure("busy", "Another drawing is still opening"));
+                    }
+                    self.open_web_bytes(name, bytes)
+                }
+                #[cfg(not(target_arch = "wasm32"))]
+                {
+                    let _ = (name, bytes);
+                    return Err(failure(
+                        "invalid_request",
+                        "data_base64 is for the web build; supply path",
+                    ));
+                }
+            }
             "open" => self.update(Message::OpenExternal(std::path::PathBuf::from(string(
                 req, "path",
             )?))),
@@ -1409,6 +1445,41 @@ mod tests {
         let _ = std::fs::remove_file(path);
     }
 
+    #[test]
+    fn open_from_bytes_validates_the_request() {
+        let ok = open_bytes_request(&json!({"name":"C:\\plans\\plan.dwg","data_base64":"AAEC"}));
+        assert_eq!(ok, Ok(("plan.dwg".to_string(), vec![0, 1, 2])));
+        for (req, error) in [
+            (json!({"data_base64":"AAEC"}), "Missing name"),
+            (json!({"name":"dir/","data_base64":"AAEC"}), "Missing name"),
+            (
+                json!({"name":"a.dwg","data_base64":""}),
+                "data_base64 is empty",
+            ),
+        ] {
+            assert_eq!(
+                open_bytes_request(&req).unwrap_err()["error"],
+                error,
+                "{req}"
+            );
+        }
+        let bad = open_bytes_request(&json!({"name":"a.dwg","data_base64":"not base64!"}));
+        assert_eq!(bad.unwrap_err()["code"], "invalid_request");
+    }
+    #[test]
+    fn open_from_bytes_is_refused_natively_without_touching_documents() {
+        let mut app = OpenCADStudio::new_for_test();
+        request(&mut app, json!({"op":"new"}));
+        let tabs = app.tabs.len();
+        let reply = request(
+            &mut app,
+            json!({"op":"open","name":"a.dxf","data_base64":"AAEC"}),
+        );
+        assert_eq!(reply["code"], "invalid_request", "{reply}");
+        assert_eq!(app.tabs.len(), tabs);
+        assert!(app.opening.is_none());
+    }
+
     #[test]
     fn embed_image_op_places_an_ole2frame_and_undoes_it() {
         let mut app = OpenCADStudio::new_for_test();
diff --git a/src/app/update/file.rs b/src/app/update/file.rs
index d4404fad9..f9715c9bb 100644
--- a/src/app/update/file.rs
+++ b/src/app/update/file.rs
@@ -1182,31 +1182,53 @@ impl OpenCADStudio {
         }
         #[cfg(target_arch = "wasm32")]
         {
-            // `FileOpened` only installs the result when an open is in
-            // progress, so mark one. The browser picker + parse happen
-            // inside `pick_and_load_web`; the real name is unknown until
-            // then, so show a generic label meanwhile.
-            let state = std::sync::Arc::new(crate::io::OpenProgressState::new(
-                crate::app::OPEN_PHASE_READING,
-            ));
-            let open_id = self.next_open_id();
-            self.opening = Some(crate::app::OpenProgress {
-                id: open_id,
-                name: "Opening…".into(),
-                source_path: None,
-                size_bytes: 0,
-                state: state.clone(),
-                started: Instant::now(),
-                recovery_error: None,
-                recovery_read_stats: None,
-                recovery_bytes: None,
-            });
+            // The browser picker + parse happen inside `pick_and_load_web`;
+            // the real name is unknown until then, so show a generic label
+            // meanwhile.
+            let (open_id, state) = self.begin_web_open("Opening…".into(), 0);
             Task::perform(crate::io::pick_and_load_web(state), move |outcome| {
                 Message::WebFileOpened(open_id, outcome)
             })
         }
     }
 
+    /// Web: open a drawing from bytes the caller already holds, through the
+    /// same path as a file chosen in the browser picker.
+    #[cfg(target_arch = "wasm32")]
+    pub(in crate::app) fn open_web_bytes(&mut self, name: String, bytes: Vec<u8>) -> Task<Message> {
+        let (open_id, state) = self.begin_web_open(name.clone(), bytes.len() as u64);
+        Task::perform(
+            crate::io::open_bytes_web(name, std::sync::Arc::from(bytes), state),
+            move |outcome| Message::WebFileOpened(open_id, outcome),
+        )
+    }
+
+    /// `FileOpened` only installs the result when an open is in progress, so
+    /// mark one.
+    #[cfg(target_arch = "wasm32")]
+    fn begin_web_open(
+        &mut self,
+        name: String,
+        size_bytes: u64,
+    ) -> (u64, std::sync::Arc<crate::io::OpenProgressState>) {
+        let state = std::sync::Arc::new(crate::io::OpenProgressState::new(
+            crate::app::OPEN_PHASE_READING,
+        ));
+        let open_id = self.next_open_id();
+        self.opening = Some(crate::app::OpenProgress {
+            id: open_id,
+            name,
+            source_path: None,
+            size_bytes,
+            state: state.clone(),
+            started: Instant::now(),
+            recovery_error: None,
+            recovery_read_stats: None,
+            recovery_bytes: None,
+        });
+        (open_id, state)
+    }
+
     pub(in crate::app) fn next_open_id(&mut self) -> u64 {
         self.open_job_serial = self.open_job_serial.wrapping_add(1).max(1);
         self.open_job_serial
diff --git a/src/io/mod.rs b/src/io/mod.rs
index 38a115cbe..67053222f 100644
--- a/src/io/mod.rs
+++ b/src/io/mod.rs
@@ -526,6 +526,18 @@ pub async fn pick_and_load_web(
     let name = handle.file_name();
     progress.set(crate::app::OPEN_PHASE_READING, 500, 1, 2);
     let bytes: Arc<[u8]> = Arc::from(handle.read().await);
+    open_bytes_web(name, bytes, progress).await
+}
+
+/// Web file open from bytes the caller already holds (the control channel's
+/// `open` with `data_base64`). Same parse, caching and recovery handling as a
+/// file chosen in the browser picker.
+#[cfg(target_arch = "wasm32")]
+pub async fn open_bytes_web(
+    name: String,
+    bytes: Arc<[u8]>,
+    progress: Arc<OpenProgressState>,
+) -> WebOpenOutcome {
     let size_bytes = bytes.len() as u64;
     let result = load_web_bytes(&name, &bytes, progress.clone(), false, "", None).await;
     let keep_for_recovery = result