diff --git a/docs/automation/web.md b/docs/automation/web.md
index 89b0b430a..0bebaf578 100644
--- a/docs/automation/web.md
+++ b/docs/automation/web.md
@@ -71,6 +71,22 @@ and `data_base64`, and the tab is marked clean. `target_format` and `target_vers
 default to the drawing's own; a conversion that would drop unsupported records needs
 `allow_lossy: true`, as on native.
 
+## Supplying fonts
+
+The web build has no font folder. A text style whose TrueType or OpenType file is not one
+of the bundled Noto subsets falls back to them. The host page can hand the file over:
+
+```json
+{"op":"font_register","file_name":"Brand-Regular.ttf","data_base64":"…"}
+```
+
+`file_name` is the name the drawing's text styles use; it is matched case-insensitively and
+without a directory, as for installed fonts. The reply carries the `family` inside the file
+and the number of `faces` it added. Open drawings lay their text out again. The font stays
+registered for the session and is never written into saved files. Bytes without a readable
+face fail with `font_invalid`; a missing `file_name` or empty or invalid base64 with
+`invalid_request`.
+
 ## Receiving saved files in the host page
 
 A page that embeds the app can take every saved or exported file itself instead of a
diff --git a/src/app/control/mod.rs b/src/app/control/mod.rs
index 895eacb15..b78eebea4 100644
--- a/src/app/control/mod.rs
+++ b/src/app/control/mod.rs
@@ -204,6 +204,26 @@ fn open_bytes_request(req: &Value) -> Result<(String, Vec<u8>), Value> {
     }
     Ok((name.to_string(), bytes))
 }
+/// `{"op":"font_register","file_name":"Brand-Regular.ttf","data_base64":"…"}`:
+/// a host hands over a TrueType/OpenType file that text styles reference by
+/// name. The web build has no font folder, so this is how such styles get
+/// their real face instead of a fallback.
+fn font_register_request(req: &Value) -> Result<(String, Vec<u8>), Value> {
+    use base64::Engine as _;
+    let name = string(req, "file_name")?;
+    let name = name.rsplit(['/', '\\']).next().unwrap_or(name).trim();
+    if name.is_empty() {
+        return Err(failure("invalid_request", "Missing file_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()
@@ -775,7 +795,12 @@ impl OpenCADStudio {
             }
         // Interactive requests operate on the active tab by definition, so
         // a client may omit document_id (the GUI-hosted REST channel does).
-        } else if !query && !matches!(op, "new" | "open" | "stop" | "user_select" | "getpoint") {
+        } else if !query
+            && !matches!(
+                op,
+                "new" | "open" | "font_register" | "stop" | "user_select" | "getpoint"
+            )
+        {
             return (
                 failure("document_required", "Read state and supply document_id"),
                 Task::none(),
@@ -979,6 +1004,24 @@ impl OpenCADStudio {
             "open" => self.update(Message::OpenExternal(std::path::PathBuf::from(string(
                 req, "path",
             )?))),
+            "font_register" => {
+                let (name, bytes) = font_register_request(req)?;
+                let (family, faces) =
+                    crate::scene::text::sysfont::register_font_file(&name, &bytes)
+                        .ok_or_else(|| failure("font_invalid", "data_base64 holds no font face"))?;
+                // Same invalidation as a fetched web font: styles that fell
+                // back while the file was missing lay out again.
+                crate::scene::text::ttf_glyph::clear_fallback_cache();
+                for tab in self.tabs.iter_mut() {
+                    tab.scene.invalidate_text_geometry_dependencies();
+                }
+                self.set_control_result(json!({
+                    "file_name": name,
+                    "family": family,
+                    "faces": faces,
+                }));
+                Task::none()
+            }
             "activate" | "switch_document" => {
                 let i = self
                     .tabs
@@ -2306,6 +2349,59 @@ mod tests {
         let bad = open_bytes_request(&json!({"name":"a.dwg","data_base64":"not base64!"}));
         assert_eq!(bad.unwrap_err()["code"], "invalid_request");
     }
+    #[test]
+    fn font_register_validates_the_request() {
+        let ok = font_register_request(
+            &json!({"file_name":"C:\\Fonts\\Brand.ttf","data_base64":"AAEC"}),
+        );
+        assert_eq!(ok, Ok(("Brand.ttf".to_string(), vec![0, 1, 2])));
+        for req in [
+            json!({"data_base64":"AAEC"}),
+            json!({"file_name":"fonts/","data_base64":"AAEC"}),
+            json!({"file_name":"a.ttf","data_base64":""}),
+            json!({"file_name":"a.ttf","data_base64":"not base64!"}),
+        ] {
+            assert_eq!(
+                font_register_request(&req).unwrap_err()["code"],
+                "invalid_request",
+                "{req}"
+            );
+        }
+    }
+
+    #[test]
+    fn font_register_makes_a_style_file_resolve() {
+        use base64::Engine as _;
+        let bytes = std::fs::read(concat!(env!("CARGO_MANIFEST_DIR"), "/web/fonts/greek.ttf"))
+            .expect("bundled web font");
+        let mut app = OpenCADStudio::new_for_test();
+        request(&mut app, json!({"op":"new"}));
+        let reply = request(
+            &mut app,
+            json!({
+                "op":"font_register",
+                "file_name":"Control-Registered.ttf",
+                "data_base64": base64::engine::general_purpose::STANDARD.encode(&bytes),
+            }),
+        );
+        assert_eq!(reply["ok"], true, "{reply}");
+        let family = reply["result"]["family"]
+            .as_str()
+            .expect("family")
+            .to_string();
+        assert!(reply["result"]["faces"].as_u64().unwrap() >= 1, "{reply}");
+        assert_eq!(
+            crate::scene::text::sysfont::family_for_reference("control-registered.ttf"),
+            Some(family)
+        );
+
+        let refused = request(
+            &mut app,
+            json!({"op":"font_register","file_name":"x.ttf","data_base64":"AAEC"}),
+        );
+        assert_eq!(refused["code"], "font_invalid", "{refused}");
+    }
+
     #[test]
     fn open_from_bytes_is_refused_natively_without_touching_documents() {
         let mut app = OpenCADStudio::new_for_test();
diff --git a/src/scene/text/sysfont.rs b/src/scene/text/sysfont.rs
index 60dfe96d4..e66304e5f 100644
--- a/src/scene/text/sysfont.rs
+++ b/src/scene/text/sysfont.rs
@@ -22,8 +22,9 @@ struct SysFonts {
     families: Mutex<Vec<String>>,
     /// Installed face file name (lower-case) → the family that face holds.
     /// A drawing references a TrueType style by *file* name, so this is what
-    /// makes such a style resolve — see [`family_for_reference`].
-    by_file: HashMap<String, String>,
+    /// makes such a style resolve — see [`family_for_reference`]. Files a host
+    /// registers at run time join it ([`register_font_file`]).
+    by_file: Mutex<HashMap<String, String>>,
 }
 
 static FONTS: OnceLock<SysFonts> = OnceLock::new();
@@ -60,7 +61,7 @@ fn fonts() -> &'static SysFonts {
         SysFonts {
             db: Mutex::new(db),
             families: Mutex::new(families),
-            by_file,
+            by_file: Mutex::new(by_file),
         }
     })
 }
@@ -320,7 +321,12 @@ fn family_for_file(name: &str) -> Option<String> {
     if file.is_empty() {
         return None;
     }
-    fonts().by_file.get(&file.to_ascii_lowercase()).cloned()
+    fonts()
+        .by_file
+        .lock()
+        .unwrap()
+        .get(&file.to_ascii_lowercase())
+        .cloned()
 }
 
 /// The installed family a drawing's font reference names, or `None` when
@@ -371,10 +377,70 @@ pub fn register_web_font(bytes: &[u8]) {
     canonical_cache().lock().unwrap().clear();
 }
 
+/// Add a font file a host supplies at run time (the `font_register` control
+/// op): its faces join the database and `file_name` resolves to the family of
+/// the first face, so a text style naming that file finds it. Returns that
+/// family and the number of faces the file added, or `None` when the bytes
+/// hold no face fontdb can read. Like [`register_web_font`], the family list
+/// is rebuilt and the canonical-name memo cleared; callers invalidate the glyph
+/// caches and re-layout text.
+pub fn register_font_file(file_name: &str, bytes: &[u8]) -> Option<(String, usize)> {
+    let file = file_name.rsplit(['/', '\\']).next()?.trim();
+    if file.is_empty() {
+        return None;
+    }
+    let sys = fonts();
+    let (family, faces, names) = {
+        let mut db = sys.db.lock().unwrap();
+        let before: std::collections::HashSet<fontdb::ID> = db.faces().map(|f| f.id).collect();
+        db.load_font_data(bytes.to_vec());
+        let added: Vec<String> = db
+            .faces()
+            .filter(|face| !before.contains(&face.id))
+            .filter_map(|face| face.families.first().map(|(name, _)| name.clone()))
+            .collect();
+        let family = added.first()?.clone();
+        (family, added.len(), family_names(&db))
+    };
+    sys.by_file
+        .lock()
+        .unwrap()
+        .insert(file.to_ascii_lowercase(), family.clone());
+    *sys.families.lock().unwrap() = names;
+    canonical_cache().lock().unwrap().clear();
+    Some((family, faces))
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
 
+    /// A host-registered file resolves by its file name — any case, bare or
+    /// with a path — to the family inside it; bytes without a face are refused
+    /// and leave nothing behind.
+    #[test]
+    fn registered_font_file_resolves_by_file_name() {
+        let bytes = std::fs::read(concat!(env!("CARGO_MANIFEST_DIR"), "/web/fonts/greek.ttf"))
+            .expect("bundled web font");
+        let (family, faces) =
+            register_font_file("C:\\Fonts\\HostRegistered-Test.ttf", &bytes).expect("a face");
+        assert!(faces >= 1);
+        assert!(!family.is_empty());
+        assert_eq!(
+            family_for_reference("hostregistered-test.TTF").as_deref(),
+            Some(family.as_str())
+        );
+        assert_eq!(
+            family_for_reference("fonts/HostRegistered-Test.ttf").as_deref(),
+            Some(family.as_str())
+        );
+        assert!(has_family(&family));
+
+        assert_eq!(register_font_file("Broken-Test.ttf", b"not a font"), None);
+        assert_eq!(family_for_reference("Broken-Test.ttf"), None);
+        assert_eq!(register_font_file("", &bytes), None);
+    }
+
     /// A DWG stores a style's TrueType font as a file name, so every installed
     /// face has to be reachable by the name of its file — case-insensitively,
     /// and whether it comes as a bare name or as a path.