diff --git a/Cargo.lock b/Cargo.lock
index b632368ea..cb00b1a44 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1146,6 +1146,15 @@ dependencies = [
  "zip 9.1.0",
 ]
 
+[[package]]
+name = "craft-web-host"
+version = "0.1.0"
+source = "git+https://github.com/storytold/craft-libs?rev=2e2312ddaf21bc3d85cd3395d0955c7d239306f6#2e2312ddaf21bc3d85cd3395d0955c7d239306f6"
+dependencies = [
+ "wasm-bindgen",
+ "web-sys",
+]
+
 [[package]]
 name = "crc32fast"
 version = "1.5.2"
@@ -2584,6 +2593,7 @@ dependencies = [
 name = "filmcraft-web"
 version = "0.17.0"
 dependencies = [
+ "craft-web-host",
  "eframe",
  "egui",
  "filmcraft-codecs",
diff --git a/apps/filmcraft-web/Cargo.toml b/apps/filmcraft-web/Cargo.toml
index 10affd2c0..54751628c 100644
--- a/apps/filmcraft-web/Cargo.toml
+++ b/apps/filmcraft-web/Cargo.toml
@@ -16,6 +16,8 @@ crate-type = ["cdylib", "rlib"]
 # and clippy stay cheap). Build with `cargo xtask web` (see docs/web.md).
 [target.'cfg(target_arch = "wasm32")'.dependencies]
 filmcraft-engine = { workspace = true }
+# The host messages (storytold/craft-libs): one contract and one bridge for the web builds.
+craft-web-host = { git = "https://github.com/storytold/craft-libs", rev = "2e2312ddaf21bc3d85cd3395d0955c7d239306f6", version = "0.1.0" }
 filmcraft-ui-egui = { workspace = true }
 filmcraft-media = { workspace = true }
 filmcraft-codecs = { workspace = true }
diff --git a/apps/filmcraft-web/src/host.rs b/apps/filmcraft-web/src/host.rs
index ca3d464fa..7f0b5b260 100644
--- a/apps/filmcraft-web/src/host.rs
+++ b/apps/filmcraft-web/src/host.rs
@@ -1,39 +1,22 @@
 //! Host modes: a page that runs FilmCraft (as an `<iframe>`, or in its own document) opens
-//! projects in it and stores what it saves instead of the browser downloading. The same
-//! messages as the other Craft apps' bridges (WordCraft, GridCraft, PdfCraft), with the
-//! `filmcraft:` prefix.
+//! projects in it and stores what it saves instead of the browser downloading. The messages and
+//! the two transports are `craft-web-host`'s (storytold/craft-libs), the same contract as the
+//! other Craft apps' bridges, with the `filmcraft:` prefix. This module is FilmCraft's part of it.
 //!
-//! ## `?host=parent` — an `<iframe>` (such as a Nextcloud app)
-//!
-//! The framing page and FilmCraft talk over `postMessage` between same-origin windows. Messages
-//! are plain objects with a `type`:
-//!
-//! - `filmcraft:ready` (to the parent): `{ version }`, sent once the app is running and
-//!   listening; send a file now.
-//! - `filmcraft:open` (from the parent): `{ name, bytes }`, bytes an `ArrayBuffer` or
-//!   `Uint8Array`. A `.fcproj` opens like a project picked with File › Open or dropped on the
-//!   page; any other file imports like a dropped one. Over [`MAX_OPEN_BYTES`] (512 MiB, refused
-//!   before any byte is copied) or malformed: a message in the app's status bar, never a crash.
-//! - `filmcraft:save` (to the parent): `{ name, saveAs, bytes }` (a transferred `ArrayBuffer`)
-//!   for every file FilmCraft would otherwise download — see the `saveAs` rule below. The
-//!   parent stores it and reports failures itself.
-//! - `filmcraft:dirty` (to the parent): `{ dirty }` whenever unsaved changes appear or go away
-//!   (the session's own unsaved-changes state, reported only when it changes).
-//! - `filmcraft:failed` (to the parent): `{ error }` when the app couldn't start.
-//!
-//! Only the parent window on the page's own origin is heard or answered. With no parent, or a
-//! parent on another origin (messages to it are dropped silently, so saves would vanish while
-//! the document looked saved), there is no bridge: the normal app with downloads.
-//!
-//! ## `data-host="page"` — the page's own document
-//!
-//! A page that loads the module into its own document (its own `<canvas id="filmcraft_canvas"
-//! data-host="page">`, no iframe) gets the same behaviour over DOM `CustomEvent`s on the canvas
-//! instead of `postMessage`: the same four events outward (`ready`, `save`, `dirty`, `failed`,
-//! with the message fields as the event's `detail`), and the page dispatches `filmcraft:open`
-//! on the canvas with `detail: { name, bytes }` to open a file. Events don't bubble, so two
-//! editors on one page can't hear each other, and the canvas is the only target FilmCraft
-//! listens on.
+//! - `?host=parent` (an `<iframe>`, such as a Nextcloud app) talks to the same-origin parent over
+//!   `postMessage`; `data-host="page"` on the canvas (the page's own document) uses DOM
+//!   `CustomEvent`s on the canvas instead. `?host=parent` wins when both are present. With no
+//!   parent, or a parent on another origin, there is no bridge: the normal app with downloads.
+//! - `filmcraft:ready` once the app is running and listening; send a file now. FilmCraft reads
+//!   bytes, so it says `file: false` and a host sends `bytes`.
+//! - `filmcraft:open` (from the host): `{ name, bytes }`. A `.fcproj` opens like a project picked
+//!   with File › Open or dropped on the page; any other file imports like a dropped one. Over
+//!   [`MAX_OPEN_BYTES`] (512 MiB, refused before any byte is copied) or malformed: a message in
+//!   the app's status bar, never a crash.
+//! - `filmcraft:save` (to the host): `{ name, saveAs, bytes }` for every file FilmCraft would
+//!   otherwise download — see the `saveAs` rule below.
+//! - `filmcraft:dirty` whenever unsaved changes appear or go away (the session's own
+//!   unsaved-changes state), `filmcraft:failed` when the app couldn't start.
 //!
 //! ## The `saveAs` rule
 //!
@@ -64,103 +47,36 @@
 //! unsaved. FilmCraft has no user names that work could be signed with, so there is no
 //! `?author=`.
 
-use std::cell::{Cell, RefCell};
+use std::cell::RefCell;
 use std::rc::Rc;
 
+use craft_web_host::web::{FileData, Host, Message};
 use filmcraft_engine::Session;
-use wasm_bindgen::JsCast as _;
-use wasm_bindgen::JsValue;
-use wasm_bindgen::closure::Closure;
 
 use crate::hostmode::{MAX_OPEN_BYTES, is_project, is_save_as, open_name, open_size_ok};
 
-/// How a host page is reached. The modes behave the same; they differ only in how an event goes
-/// out ([`Transport::send`]) and how an open request comes in ([`Bridge::install`]).
-#[derive(Clone)]
-enum Transport {
-    /// `?host=parent`: `postMessage` between same-origin windows.
-    Parent { window: web_sys::Window, parent: web_sys::Window, origin: String },
-    /// `data-host="page"`: DOM `CustomEvent`s on the editor's own canvas.
-    Page { canvas: web_sys::HtmlCanvasElement },
-}
-
-impl Transport {
-    /// Hand an event to the host. In parent mode this is a `postMessage` whose object carries a
-    /// `type` field; in page mode a `CustomEvent` of that name on the canvas, whose `detail`
-    /// holds the fields and which never bubbles (two editors on one page must not hear each
-    /// other). `transfer` hands an `ArrayBuffer` to the parent instead of copying it (parent
-    /// mode only; the page's document already shares the buffer's bytes).
-    fn send(&self, event: &str, fields: &[(&str, JsValue)], transfer: Option<&js_sys::ArrayBuffer>) -> Result<(), String> {
-        match self {
-            Transport::Parent { parent, origin, .. } => {
-                let mut message_fields = Vec::with_capacity(fields.len() + 1);
-                message_fields.push(("type", JsValue::from_str(event)));
-                message_fields.extend_from_slice(fields);
-                let message = message(&message_fields).ok_or_else(|| "couldn't build the message".to_string())?;
-                let sent = match transfer {
-                    Some(buffer) => parent.post_message_with_transfer(&message, origin, &js_sys::Array::of1(buffer)),
-                    None => parent.post_message(&message, origin),
-                };
-                sent.map_err(|e| js_error(&e, "couldn't post the message"))
-            }
-            Transport::Page { canvas } => {
-                let detail = message(fields).ok_or_else(|| "couldn't build the event detail".to_string())?;
-                let init = web_sys::CustomEventInit::new();
-                // The builder methods (`bubbles(..)`, `detail(..)`) are deprecated in web-sys
-                // 0.3.106 and fail clippy -D warnings; use the setters.
-                init.set_bubbles(false);
-                init.set_detail(&detail);
-                let custom = web_sys::CustomEvent::new_with_event_init_dict(event, &init).map_err(|e| js_error(&e, "couldn't create the event"))?;
-                // The bool says whether a listener called `preventDefault`; not a send failure.
-                canvas.dispatch_event(&custom).map_err(|e| js_error(&e, "couldn't dispatch the event"))?;
-                Ok(())
-            }
-        }
-    }
-}
+/// The app id in the host messages (`filmcraft:ready`).
+const APP: &str = "filmcraft";
 
 /// The host bridge in host mode. Cloned into the write hook and the open listener; the state
 /// they share is behind `Rc`.
 #[derive(Clone)]
 pub struct Bridge {
-    transport: Transport,
+    host: Rc<Host>,
     /// Files written since the last frame: their paths (the bytes stay in the file table,
     /// [`crate::fs::mem_bytes`], so a large export is never copied a second time).
     written: Rc<RefCell<Vec<String>>>,
     /// The document file the host knows: the project it opened here, and every file it was
     /// sent with `saveAs: true`.
     host_document: Rc<RefCell<Option<String>>>,
-    /// The unsaved-changes state last reported to the host.
-    reported: Rc<Cell<Option<bool>>>,
 }
 
 impl Bridge {
-    /// The parent bridge for `?host=parent`. `None` when the page isn't framed (there is no
-    /// parent to talk to), so opening that URL directly stays the normal app with downloads, or
-    /// when the parent is on another origin: messages only go to this origin, so the browser
-    /// would drop every save without an error while the document showed as saved.
-    pub fn new() -> Option<Self> {
-        let window = web_sys::window()?;
-        let parent = window.parent().ok().flatten()?;
-        if js_sys::Object::is(&parent, &window) {
-            return None;
-        }
-        let origin = window.location().origin().ok()?;
-        // An opaque origin (a `file://` page in some browsers) can't be a `postMessage` target.
-        if origin == "null" {
-            return None;
-        }
-        // Reading another origin's location throws, which is `None` too.
-        if parent.location().origin().ok()? != origin {
-            return None;
-        }
-        Some(Self { transport: Transport::Parent { window, parent, origin }, written: Rc::default(), host_document: Rc::default(), reported: Rc::default() })
-    }
-
-    /// The page bridge for `data-host="page"`: the host page runs FilmCraft in its own document
-    /// and hears the editor's DOM `CustomEvent`s on this canvas.
-    pub fn for_page(canvas: &web_sys::HtmlCanvasElement) -> Self {
-        Self { transport: Transport::Page { canvas: canvas.clone() }, written: Rc::default(), host_document: Rc::default(), reported: Rc::default() }
+    /// The bridge for this page, or `None` when FilmCraft isn't hosted: no `?host=parent` and no
+    /// `data-host="page"` on the canvas, an unframed `?host=parent` page (it stays the normal app
+    /// with downloads), or a parent on another origin.
+    pub fn connect(canvas: &web_sys::HtmlCanvasElement) -> Option<Self> {
+        Some(Self { host: Rc::new(Host::connect(APP, canvas)?), written: Rc::default(), host_document: Rc::default() })
     }
 
     /// Install the bridge: hear the host's files, and hand the app's writes to it instead of
@@ -173,22 +89,20 @@ impl Bridge {
 
     /// After each frame's commands (the web app's per-frame logic pass): files written this
     /// frame go to the host, with the `saveAs` decision from the session's state (the module
-    /// doc has the rule), and the host hears when unsaved changes appear or go away.
+    /// doc has the rule), and the host hears when unsaved changes appear or go away (the crate
+    /// does not repeat a state it already reported).
     pub fn sync(&self, session: &Session) {
         self.flush_saves(session);
-        let dirty = session.is_dirty();
-        if self.reported.get() != Some(dirty) && self.transport.send("filmcraft:dirty", &[("dirty", dirty.into())], None).is_ok() {
-            self.reported.set(Some(dirty));
-        }
+        self.host.post_dirty(session.is_dirty());
     }
 
     pub fn post_ready(&self) {
-        let _ = self.transport.send("filmcraft:ready", &[("version", env!("CARGO_PKG_VERSION").into())], None);
+        let _ = self.host.post_ready(env!("CARGO_PKG_VERSION"), false);
     }
 
     /// The app couldn't start (no WebGPU or WebGL2, for example): let the host say so.
     pub fn post_failed(&self, error: &str) {
-        let _ = self.transport.send("filmcraft:failed", &[("error", error.into())], None);
+        let _ = self.host.post_failed(error, None);
     }
 
     /// The host's project opened: the host has this file as the document, so a Save of it goes
@@ -226,7 +140,7 @@ impl Bridge {
             // a file right back.
             let before = self.host_document.borrow().clone();
             let save_as = is_save_as(&path, before.as_deref(), current.as_deref());
-            match self.post_save(&name, &bytes, save_as) {
+            match self.host.post_save(&name, &bytes, None, save_as) {
                 // The host has this file as the document now, unless it opened another one while
                 // handling the save; a failed hand-over changes nothing, so a retry is still a Save As.
                 Ok(()) if save_as => {
@@ -241,66 +155,31 @@ impl Bridge {
         }
     }
 
-    fn post_save(&self, name: &str, bytes: &[u8], save_as: bool) -> Result<(), String> {
-        // `filmcraft:save` carries the bytes as an `ArrayBuffer`; the parent bridge transfers it.
-        let buffer = js_sys::Uint8Array::from(bytes).buffer();
-        let fields: [(&str, JsValue); 3] = [("name", name.into()), ("saveAs", save_as.into()), ("bytes", buffer.clone().into())];
-        self.transport.send("filmcraft:save", &fields, Some(&buffer))
-    }
-
-    /// Hear `filmcraft:open` — from the same-origin parent (a `message` on the window) or from
-    /// the host document (a `filmcraft:open` event on the canvas) — and open or import the file
-    /// like one the user picked or dropped.
+    /// Hear `filmcraft:open` and open or import the file like one the user picked or dropped.
     fn listen(&self) {
         let bridge = self.clone();
-        match &self.transport {
-            Transport::Parent { window, parent, origin } => {
-                let parent = parent.clone();
-                let origin = origin.clone();
-                let on_message = Closure::<dyn FnMut(web_sys::MessageEvent)>::new(move |event: web_sys::MessageEvent| {
-                    if event.origin() != origin || !event.source().is_some_and(|s| js_sys::Object::is(&s, &parent)) {
-                        return;
-                    }
-                    let data = event.data();
-                    if string(&data, "type").as_deref() != Some("filmcraft:open") {
-                        return;
-                    }
-                    deliver_open(&bridge, &data);
-                });
-                if window.add_event_listener_with_callback("message", on_message.as_ref().unchecked_ref()).is_ok() {
-                    // The listener lives as long as the page.
-                    on_message.forget();
-                }
+        self.host.listen(move |message| {
+            if let Message::Open { name, data } = message {
+                deliver_open(&bridge, &name, &data);
             }
-            Transport::Page { canvas } => {
-                let on_open = Closure::<dyn FnMut(web_sys::Event)>::new(move |event: web_sys::Event| {
-                    // A plain `Event` of the same name carries no detail; only a `CustomEvent`.
-                    let Some(open) = event.dyn_ref::<web_sys::CustomEvent>() else { return };
-                    deliver_open(&bridge, &open.detail());
-                });
-                if canvas.add_event_listener_with_callback("filmcraft:open", on_open.as_ref().unchecked_ref()).is_ok() {
-                    // The listener lives as long as the page.
-                    on_open.forget();
-                }
-            }
-        }
+        });
     }
 }
 
-/// Handle a `filmcraft:open` payload: `{ name, bytes }`, bytes an `ArrayBuffer` or
-/// `Uint8Array` — checked, then opened or imported like a file the user picked or dropped.
-fn deliver_open(bridge: &Bridge, data: &JsValue) {
-    let Ok(raw) = js_sys::Reflect::get(data, &JsValue::from_str("bytes")) else { return };
-    if !raw.is_instance_of::<js_sys::ArrayBuffer>() && raw.dyn_ref::<js_sys::Uint8Array>().is_none() {
-        return rejected_open("the bytes are neither an ArrayBuffer nor a Uint8Array".to_string());
-    }
-    let Some(len) = byte_length(&raw) else { return rejected_open("the bytes have no readable length".to_string()) };
+/// Handle a `filmcraft:open` payload: checked, then opened or imported like a file the user
+/// picked or dropped.
+fn deliver_open(bridge: &Bridge, name: &str, data: &FileData) {
+    let FileData::Bytes(array) = data else {
+        return rejected_open("FilmCraft reads bytes, not a File".to_string());
+    };
+    // The data's own length, as a number: a 4 GiB view must not wrap to 0 and pass the check.
+    let len = data.size() as u64;
     if !open_size_ok(len) {
         return rejected_open(format!("{len} bytes is over the {} MiB host open limit", MAX_OPEN_BYTES >> 20));
     }
-    let name = open_name(string(data, "name").as_deref());
+    let name = open_name(name);
     if is_project(&name) {
-        let Some(bytes) = bytes_from(&raw) else { return rejected_open("couldn't read the bytes".to_string()) };
+        let bytes = array.to_vec();
         let path = format!("/projects/{name}");
         let opened = bridge.clone();
         wasm_bindgen_futures::spawn_local(async move {
@@ -313,7 +192,7 @@ fn deliver_open(bridge: &Bridge, data: &JsValue) {
         });
     } else {
         // Media registers like a dropped file: a `Blob` over the bytes, never copied whole.
-        let Some(blob) = blob_from_bytes(&raw) else { return rejected_open("couldn't wrap the bytes in a Blob".to_string()) };
+        let Some(blob) = blob_from_bytes(array) else { return rejected_open("couldn't wrap the bytes in a Blob".to_string()) };
         wasm_bindgen_futures::spawn_local(async move { crate::import::import_blob(name, blob).await });
     }
 }
@@ -324,39 +203,11 @@ fn rejected_open(why: String) {
     crate::post(move |w, _| w.app.status(format!("the host's file was not opened: {why}")));
 }
 
-/// The payload's `byteLength`, read as a number: the bindings' `u32` would wrap a 4 GiB view
-/// to 0 and let it past the size check.
-fn byte_length(v: &JsValue) -> Option<u64> {
-    let n = js_sys::Reflect::get(v, &JsValue::from_str("byteLength")).ok()?.as_f64()?;
-    if !n.is_finite() || n < 0.0 {
-        return None;
-    }
-    Some(n as u64)
-}
-
-/// The payload as Rust bytes (a project opens from them): one copy, from an `ArrayBuffer` or a
-/// `Uint8Array`.
-fn bytes_from(v: &JsValue) -> Option<Vec<u8>> {
-    if let Some(array) = v.dyn_ref::<js_sys::Uint8Array>() {
-        Some(array.to_vec())
-    } else if v.is_instance_of::<js_sys::ArrayBuffer>() {
-        Some(js_sys::Uint8Array::new(v).to_vec())
-    } else {
-        None
-    }
-}
-
 /// The payload as a `Blob` (media registers like a dropped file): a view over the bytes, no
 /// copy on the wasm side — the `Blob` constructor copies into its own store.
-fn blob_from_bytes(v: &JsValue) -> Option<web_sys::Blob> {
+fn blob_from_bytes(view: &js_sys::Uint8Array) -> Option<web_sys::Blob> {
     let parts = js_sys::Array::new();
-    if let Some(view) = v.dyn_ref::<js_sys::Uint8Array>() {
-        parts.push(view);
-    } else if v.is_instance_of::<js_sys::ArrayBuffer>() {
-        parts.push(&js_sys::Uint8Array::new(v));
-    } else {
-        return None;
-    }
+    parts.push(view);
     let opts = web_sys::BlobPropertyBag::new();
     opts.set_type("application/octet-stream");
     web_sys::Blob::new_with_u8_array_sequence_and_options(&parts, &opts).ok()
@@ -365,22 +216,3 @@ fn blob_from_bytes(v: &JsValue) -> Option<web_sys::Blob> {
 fn file_name(path: &str) -> String {
     std::path::Path::new(path).file_name().map(|n| n.to_string_lossy().to_string()).unwrap_or_else(|| path.to_string())
 }
-
-/// A message object or `CustomEvent` detail with the given fields, or `None` if one couldn't be
-/// built (a field that isn't a valid property name, say).
-fn message(fields: &[(&str, JsValue)]) -> Option<js_sys::Object> {
-    let m = js_sys::Object::new();
-    for (key, value) in fields {
-        js_sys::Reflect::set(&m, &JsValue::from_str(key), value).ok()?;
-    }
-    Some(m)
-}
-
-fn string(data: &JsValue, key: &str) -> Option<String> {
-    js_sys::Reflect::get(data, &JsValue::from_str(key)).ok()?.as_string()
-}
-
-/// A `JsValue` error as a string, after `what` went wrong.
-fn js_error(e: &JsValue, what: &str) -> String {
-    format!("{what}: {}", e.as_string().unwrap_or_else(|| format!("{e:?}")))
-}
diff --git a/apps/filmcraft-web/src/hostmode.rs b/apps/filmcraft-web/src/hostmode.rs
index 441d42b78..582b1f000 100644
--- a/apps/filmcraft-web/src/hostmode.rs
+++ b/apps/filmcraft-web/src/hostmode.rs
@@ -1,9 +1,6 @@
 //! Pure host-mode rules, shared by the bridge ([`crate::host`]) and testable without a browser:
-//! what opts a page into hosting FilmCraft in its own document, and the name and kind the
-//! bridge reads out of an open request. Pure, so it is tested natively in `tests/it/hostmode.rs`.
-
-/// The one `data-host` value that opts a page in: the exact string `"page"`.
-pub const PAGE_HOST: &str = "page";
+//! the name and kind the bridge reads out of an open request, and the `saveAs` rule. Which pages
+//! host the app is `craft-web-host`'s. Pure, so it is tested natively in `tests/it/hostmode.rs`.
 
 /// The file name used when an open request carries no usable one: an untitled project — what
 /// a host that never names a file still opens (FilmCraft's document is its project file).
@@ -13,18 +10,16 @@ pub const DEFAULT_OPEN_NAME: &str = "Untitled.fcproj";
 /// file dropped on the page.
 pub const PROJECT_EXT: &str = ".fcproj";
 
-/// Does this `data-host` value ask for page host mode? Only the exact value [`PAGE_HOST`]
-/// counts; anything else is ignored, so the page host mode stays opt-in. `?host=parent`, the
-/// parent bridge's URL opt-in, is answered before this is asked.
-pub fn page_host_requested(data_host: Option<&str>) -> bool {
-    data_host == Some(PAGE_HOST)
-}
+/// The name `craft-web-host` gives an open request that carries no usable one (it cleans a host's
+/// name and falls back to this).
+const NAMELESS: &str = "document";
 
-/// A usable file name for an open request: the host's name when it isn't blank, else the
-/// [`DEFAULT_OPEN_NAME`]. `/` and `\` become `_`, as [`crate::fs::register_blob`] does for
-/// dropped names — the name becomes a path in the virtual file table.
-pub fn open_name(name: Option<&str>) -> String {
-    let name = name.filter(|n| !n.trim().is_empty()).unwrap_or(DEFAULT_OPEN_NAME);
+/// A usable file name for an open request: the cleaned name the crate hands over, or
+/// [`DEFAULT_OPEN_NAME`] when the host sent none. `/` and `\` become `_`, as
+/// [`crate::fs::register_blob`] does for dropped names — the name becomes a path in the virtual
+/// file table.
+pub fn open_name(name: &str) -> String {
+    let name = if name == NAMELESS || name.trim().is_empty() { DEFAULT_OPEN_NAME } else { name };
     name.chars().map(|c| if c == '/' || c == '\\' { '_' } else { c }).collect()
 }
 
diff --git a/apps/filmcraft-web/src/lib.rs b/apps/filmcraft-web/src/lib.rs
index 5c4f17ef4..384e06756 100644
--- a/apps/filmcraft-web/src/lib.rs
+++ b/apps/filmcraft-web/src/lib.rs
@@ -133,10 +133,6 @@ fn query_flag(name: &str) -> bool {
     web_sys::window().and_then(|w| w.location().search().ok()).and_then(|s| web_sys::UrlSearchParams::new_with_str(&s).ok()).is_some_and(|p| p.has(name))
 }
 
-fn query_param(name: &str) -> Option<String> {
-    web_sys::window().and_then(|w| w.location().search().ok()).and_then(|s| web_sys::UrlSearchParams::new_with_str(&s).ok()).and_then(|p| p.get(name))
-}
-
 /// Entry point (called by the page's bootstrap script once the wasm module is instantiated).
 #[wasm_bindgen]
 pub async fn start(canvas_id: String) -> Result<(), JsValue> {
@@ -145,19 +141,13 @@ pub async fn start(canvas_id: String) -> Result<(), JsValue> {
     let doc = web_sys::window().and_then(|w| w.document()).ok_or("no document")?;
     let canvas: web_sys::HtmlCanvasElement = doc.get_element_by_id(&canvas_id).ok_or("no canvas")?.dyn_into()?;
 
-    // Host modes: `?host=parent`, a same-origin page framing FilmCraft in an `<iframe>`, opens
-    // files in it and stores what it saves (`host`); unframed there is no parent, so that URL
-    // on its own stays the normal app with downloads. `data-host="page"` on the canvas opts a
-    // page in instead: it runs FilmCraft in its own document and talks over DOM `CustomEvent`s
-    // on the canvas. In host mode the host sends the document to edit, so no demo project
-    // opens and nothing from an earlier visit is recovered.
-    let host = if query_param("host").as_deref() == Some("parent") {
-        host::Bridge::new()
-    } else if hostmode::page_host_requested(canvas.get_attribute("data-host").as_deref()) {
-        Some(host::Bridge::for_page(&canvas))
-    } else {
-        None
-    };
+    // Host modes (`craft-web-host`): `?host=parent`, a same-origin page framing FilmCraft in an
+    // `<iframe>`, opens files in it and stores what it saves (`host`); unframed there is no parent,
+    // so that URL on its own stays the normal app with downloads. `data-host="page"` on the canvas
+    // opts a page in instead: it runs FilmCraft in its own document and talks over DOM
+    // `CustomEvent`s on the canvas. In host mode the host sends the document to edit, so no demo
+    // project opens and nothing from an earlier visit is recovered.
+    let host = host::Bridge::connect(&canvas);
     let in_host_mode = host.is_some();
 
     let isolated = js_sys::Reflect::get(&js_sys::global(), &"crossOriginIsolated".into()).ok().and_then(|v| v.as_bool()).unwrap_or(false);
diff --git a/apps/filmcraft-web/tests/it/hostmode.rs b/apps/filmcraft-web/tests/it/hostmode.rs
index 1f3f66f12..5d46ab8a6 100644
--- a/apps/filmcraft-web/tests/it/hostmode.rs
+++ b/apps/filmcraft-web/tests/it/hostmode.rs
@@ -1,26 +1,18 @@
-//! The page host mode's pure rules (the `data-host` opt-in, open names, project or media). They run
+//! The page host mode's pure rules (open names, project or media). They run
 //! natively even though the rest of the crate is wasm32-only.
 
 #[path = "../../src/hostmode.rs"]
 mod rules;
 
-use rules::{DEFAULT_OPEN_NAME, MAX_OPEN_BYTES, is_project, is_save_as, open_name, open_size_ok, page_host_requested};
-
-#[test]
-fn only_the_exact_page_value_opts_in() {
-    assert!(page_host_requested(Some("page")));
-    for off in [None, Some(""), Some("Page"), Some("PAGE"), Some(" page"), Some("page "), Some("parent")] {
-        assert!(!page_host_requested(off), "{off:?} must keep the page host mode off");
-    }
-}
+use rules::{DEFAULT_OPEN_NAME, MAX_OPEN_BYTES, is_project, is_save_as, open_name, open_size_ok};
 
 #[test]
 fn open_names_fall_back_to_the_default_and_lose_path_separators() {
-    assert_eq!(open_name(Some("Trailer v2.fcproj")), "Trailer v2.fcproj");
-    assert_eq!(open_name(Some(" Sheet2/../../etc.fcproj ")), " Sheet2_.._.._etc.fcproj ");
-    assert_eq!(open_name(Some("")), DEFAULT_OPEN_NAME);
-    assert_eq!(open_name(Some("   ")), DEFAULT_OPEN_NAME);
-    assert_eq!(open_name(None), DEFAULT_OPEN_NAME);
+    assert_eq!(open_name("Trailer v2.fcproj"), "Trailer v2.fcproj");
+    assert_eq!(open_name("Sheet2/../etc.fcproj"), "Sheet2_.._etc.fcproj");
+    assert_eq!(open_name("document"), DEFAULT_OPEN_NAME, "what the crate says for a missing or blank name");
+    assert_eq!(open_name(""), DEFAULT_OPEN_NAME);
+    assert_eq!(open_name("   "), DEFAULT_OPEN_NAME);
 }
 
 #[test]
diff --git a/packaging/web/README.md b/packaging/web/README.md
index 11c29b1ee..bc4852a04 100644
--- a/packaging/web/README.md
+++ b/packaging/web/README.md
@@ -83,7 +83,7 @@ downloading them. The two sides talk `postMessage` with plain window messages th
 
 | Message | Direction | Fields |
 |---|---|---|
-| `filmcraft:ready` | app → page | `version`: the app is running and listening; send a file now |
+| `filmcraft:ready` | app → page | `version`, `appVersion`: the app is running and listening; send a file now |
 | `filmcraft:open` | page → app | `name`, `bytes` (an `ArrayBuffer` or `Uint8Array`): a `.fcproj` opens like File › Open, anything else imports like a file dropped on the page. Over 512 MiB or malformed: refused with a message in the app, nothing crashes |
 | `filmcraft:save` | app → page | `name`, `saveAs`, `bytes` (a transferred `ArrayBuffer`): every file the app would otherwise download — Save, Save As, Save a Copy, video / frame / audio exports, captions, interchange (AAF, OMF, EDL, markers), extracted audio, the schema backup. The page stores it and reports failures itself. `saveAs` says whether the file becomes the document's file — the project the next Save writes: `true` when the app adopts the file and the page didn't already have it as the document (Save As, and the first Save of a project the page never sent); `false` for a Save of the document under its own name and for every copy (Save a Copy, exports, the schema backup beside an upgraded project) |
 | `filmcraft:dirty` | app → page | `dirty`: unsaved changes appeared or went away |
@@ -159,6 +159,8 @@ await start("filmcraft_canvas"); // the canvas id the app renders on
   present it takes precedence, so a same-origin framed page uses the parent bridge, while an
   unframed or cross-origin page gets no bridge and keeps normal downloads.
 
+The messages and both transports are the shared contract of [`craft-web-host`](https://github.com/storytold/craft-libs/tree/main/crates/web-host) (storytold/craft-libs#420), the same for every Craft app: its README lists every message, including `saved`, with the compatibility notes. In `filmcraft:ready`, `version` is the contract's (`"1"`) and `appVersion` is this app's release. FilmCraft reads bytes, so a host sends `bytes`, not a `File`.
+
 ### Trust and limits
 
 - **Who is trusted.** In page host mode the page and FilmCraft share one document, so any script on the page can drive the app: open files in it and receive what it saves. Use this mode only for a page you control. The iframe mode (`?host=parent`) answers only the parent window, on the page's own origin.