diff --git a/spoolman/database/filament.py b/spoolman/database/filament.py
index caa42e7e..5bffef80 100644
--- a/spoolman/database/filament.py
+++ b/spoolman/database/filament.py
@@ -191,9 +191,26 @@ async def update(
             setattr(filament, k, v)
     await db.commit()
     await filament_changed(filament, EventType.UPDATED)
+    # A spool's response carries fields derived from its filament (remaining_length,
+    # and the initial_weight/price fall-backs to the filament's values). Those change
+    # when the filament changes, yet no spool row was touched — so subscribers to the
+    # affected spools would keep stale derived values until each spool is next edited.
+    # Re-broadcast the filament's spools so websocket clients refresh them.
+    await _notify_filament_spools(db, filament_id)
     return filament
 
 
+async def _notify_filament_spools(db: AsyncSession, filament_id: int) -> None:
+    """Emit a spool 'updated' event for every spool belonging to a filament."""
+    # Local import: spool.py imports this module at load time, so importing it at the
+    # top here would be a circular import.
+    from spoolman.database import spool  # noqa: PLC0415
+
+    spools, _ = await spool.find(db=db, filament_id=filament_id, allow_archived=True)
+    for s in spools:
+        await spool.spool_changed(s, EventType.UPDATED)
+
+
 async def delete(db: AsyncSession, filament_id: int) -> None:
     """Delete a filament object."""
     filament = await get_by_id(db, filament_id)
diff --git a/tests_integration/requirements.txt b/tests_integration/requirements.txt
index 0d2da6db..fec45362 100644
--- a/tests_integration/requirements.txt
+++ b/tests_integration/requirements.txt
@@ -1,3 +1,4 @@
 pytest==9.1.1
 pytest-asyncio==1.4.0
 httpx==0.27.0
+websockets==13.1
diff --git a/tests_integration/tests/filament/test_notify_spools.py b/tests_integration/tests/filament/test_notify_spools.py
new file mode 100644
index 00000000..e4c7ef8f
--- /dev/null
+++ b/tests_integration/tests/filament/test_notify_spools.py
@@ -0,0 +1,56 @@
+"""Integration tests: a filament update re-broadcasts its spools over websockets.
+
+Spool responses carry fields derived from the filament (e.g. remaining_length, and the
+initial_weight/price fall-backs). Editing the filament changes those derived values
+without touching any spool row, so the server must emit a spool 'updated' event for
+every spool of the filament — otherwise websocket subscribers keep stale values until
+the spool itself is next edited.
+"""
+
+import asyncio
+import json
+from typing import Any
+
+import httpx
+import pytest
+from websockets.asyncio.client import connect
+
+from ..conftest import URL
+
+WS_URL = URL.replace("http://", "ws://", 1)
+
+
+@pytest.mark.asyncio
+async def test_filament_update_notifies_spools(random_filament: dict[str, Any]) -> None:
+    """Patching a filament's density emits a spool 'updated' event with fresh derived fields."""
+    # Setup: one spool for this filament.
+    result = httpx.post(
+        f"{URL}/api/v1/spool",
+        json={"filament_id": random_filament["id"], "remaining_weight": 500},
+    )
+    result.raise_for_status()
+    spool = result.json()
+    original_length = spool["remaining_length"]
+
+    try:
+        async with connect(f"{WS_URL}/api/v1/spool") as ws:
+            # The subscription is registered synchronously right after the handshake,
+            # but give the server a beat before triggering the event.
+            await asyncio.sleep(0.2)
+
+            new_density = random_filament["density"] * 2
+            httpx.patch(
+                f"{URL}/api/v1/filament/{random_filament['id']}",
+                json={"density": new_density},
+            ).raise_for_status()
+
+            event = json.loads(await asyncio.wait_for(ws.recv(), timeout=10))
+
+        assert event["type"] == "updated"
+        assert event["resource"] == "spool"
+        assert event["payload"]["id"] == spool["id"]
+        # The embedded filament and the derived length must reflect the new density.
+        assert event["payload"]["filament"]["density"] == new_density
+        assert event["payload"]["remaining_length"] != original_length
+    finally:
+        httpx.delete(f"{URL}/api/v1/spool/{spool['id']}").raise_for_status()