diff --git a/client_v2/src/lib/api/map.ts b/client_v2/src/lib/api/map.ts
index 2cbdb603..efd65edc 100644
--- a/client_v2/src/lib/api/map.ts
+++ b/client_v2/src/lib/api/map.ts
@@ -64,6 +64,8 @@ export function mapSpool(s: Json): Spool {
 		unused: (s.used_weight ?? 0) === 0,
 		remaining: s.remaining_weight ?? 0,
 		initial: s.initial_weight ?? f.weight ?? 0,
+		initialOverride: s.initial_weight ?? undefined,
+		usedWeight: s.used_weight ?? 0,
 		location: s.location ?? '',
 		lot: s.lot_nr ?? '',
 		price: s.price ?? undefined,
diff --git a/client_v2/src/lib/stores/inventory.svelte.ts b/client_v2/src/lib/stores/inventory.svelte.ts
index 1b67b81d..7768610c 100644
--- a/client_v2/src/lib/stores/inventory.svelte.ts
+++ b/client_v2/src/lib/stores/inventory.svelte.ts
@@ -83,6 +83,28 @@ class Inventory {
 		if (v) this.upsertVendor({ ...v, ...patch });
 	}
 
+	/**
+	 * Recompute the weights of cached spools that inherit them from `f`.
+	 *
+	 * The server does not re-broadcast a filament's spools when the filament changes
+	 * (that fan-out is unbounded — see spoolman/database/filament.py `update`), so a
+	 * filament event is our only signal. Everything else a spool shows about its
+	 * filament resolves through `filamentById` and is already live; the exception is
+	 * a spool with no `initial_weight` of its own, whose initial/remaining weights the
+	 * server derived from `filament.weight`. Mirror `Spool.from_db`'s formula —
+	 * including its clamp at 0 — so an over-used spool doesn't go negative.
+	 */
+	private reweighSpoolsOfFilament(f: Filament) {
+		let changed = false;
+		const next = this.spools.map((s) => {
+			if (s.filamentId !== f.id || s.initialOverride !== undefined) return s;
+			if (s.initial === f.weight) return s;
+			changed = true;
+			return { ...s, initial: f.weight, remaining: Math.max(f.weight - s.usedWeight, 0) };
+		});
+		if (changed) this.spools = next;
+	}
+
 	// --- live events (raw API payload → cache) ------------------------------
 
 	ingest(event: LiveEvent) {
@@ -104,7 +126,9 @@ class Inventory {
 			const id = String(event.id);
 			if (event.type === 'deleted') this.filaments = this.filaments.filter((f) => f.id !== id);
 			else if (payload) {
-				this.upsertFilament(mapFilament(payload));
+				const f = mapFilament(payload);
+				this.upsertFilament(f);
+				this.reweighSpoolsOfFilament(f);
 				const v = payload.vendor as Record<string, unknown> | undefined;
 				if (v) this.upsertVendor(mapVendor(v));
 			}
diff --git a/client_v2/src/lib/types.ts b/client_v2/src/lib/types.ts
index 16f85b4a..e2d35172 100644
--- a/client_v2/src/lib/types.ts
+++ b/client_v2/src/lib/types.ts
@@ -56,8 +56,13 @@ export interface Spool {
 	unused: boolean;
 	/** Remaining filament weight in grams. */
 	remaining: number;
-	/** Initial (net) weight in grams. */
+	/** Initial (net) weight in grams; falls back to the filament's weight. */
 	initial: number;
+	/** Per-spool initial-weight override; undefined means `initial` came from the
+	 *  filament and must be recomputed when the filament's weight changes. */
+	initialOverride?: number;
+	/** Filament drawn from this spool so far, in grams. */
+	usedWeight: number;
 	location: string;
 	lot: string;
 	/** Per-spool price override; undefined falls back to the filament's price. */
diff --git a/spoolman/database/filament.py b/spoolman/database/filament.py
index f059b08d..a3006445 100644
--- a/spoolman/database/filament.py
+++ b/spoolman/database/filament.py
@@ -190,27 +190,19 @@ async def update(
         else:
             setattr(filament, k, v)
     await db.commit()
+    # A spool's response embeds its filament and carries values derived from it
+    # (remaining_length, and the initial_weight/price fall-backs), so this edit changed
+    # every one of this filament's spool payloads without touching a spool row.
+    # We deliberately do NOT re-broadcast those spools: a filament shared by hundreds of
+    # spools would turn one PATCH into hundreds of ORM loads and websocket frames inside
+    # the request. Subscribers are expected to treat a filament event as invalidating the
+    # spools that reference it and refetch what they still have on screen — see
+    # client_v2/src/lib/stores/inventory.svelte.ts. The same reasoning applies to vendor
+    # updates, which is why there is no vendor-side fan-out either.
     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/tests/filament/test_notify_spools.py b/tests_integration/tests/filament/test_notify_spools.py
index e4c7ef8f..4d559b79 100644
--- a/tests_integration/tests/filament/test_notify_spools.py
+++ b/tests_integration/tests/filament/test_notify_spools.py
@@ -1,13 +1,18 @@
-"""Integration tests: a filament update re-broadcasts its spools over websockets.
+"""Integration tests: a filament update emits only a filament event, not a spool fan-out.
 
 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.
+initial_weight/price fall-backs), so editing a filament does change those derived values
+without touching any spool row. Re-broadcasting every affected spool was tried and
+deliberately dropped: a filament shared by hundreds of spools turned one PATCH into
+hundreds of ORM loads and websocket frames inside the request. Subscribers are instead
+expected to treat a filament event as invalidating the spools that reference it.
+
+These tests pin that contract from both sides so the fan-out isn't reintroduced by
+accident.
 """
 
 import asyncio
+import contextlib
 import json
 from typing import Any
 
@@ -19,18 +24,21 @@ from ..conftest import URL
 
 WS_URL = URL.replace("http://", "ws://", 1)
 
+# How long to wait for an event we expect NOT to arrive. Long enough that a
+# re-introduced fan-out would reliably land inside it, short enough to keep the
+# suite quick across all four database backends.
+QUIET_PERIOD_S = 2.0
+
 
 @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.
+async def test_filament_update_does_not_fan_out_to_spools(random_filament: dict[str, Any]) -> None:
+    """Patching a filament emits no spool events, however many spools reference it."""
     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:
@@ -38,19 +46,34 @@ async def test_filament_update_notifies_spools(random_filament: dict[str, Any])
             # 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},
+                json={"density": random_filament["density"] * 2},
             ).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
+            # Nothing should reach a /spool subscriber.
+            with contextlib.suppress(TimeoutError):
+                event = json.loads(await asyncio.wait_for(ws.recv(), timeout=QUIET_PERIOD_S))
+                pytest.fail(f"filament update fanned out to a spool event: {event}")
     finally:
         httpx.delete(f"{URL}/api/v1/spool/{spool['id']}").raise_for_status()
+
+
+@pytest.mark.asyncio
+async def test_filament_update_emits_filament_event(random_filament: dict[str, Any]) -> None:
+    """The filament event subscribers rely on for invalidation is still emitted."""
+    async with connect(f"{WS_URL}/api/v1/filament") as ws:
+        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"] == "filament"
+    assert event["payload"]["id"] == random_filament["id"]
+    assert event["payload"]["density"] == new_density