diff --git a/packages/host/app/lib/matrix-classes/room.ts b/packages/host/app/lib/matrix-classes/room.ts
index 24e7c3bfd18..3c932957a72 100644
--- a/packages/host/app/lib/matrix-classes/room.ts
+++ b/packages/host/app/lib/matrix-classes/room.ts
@@ -139,7 +139,22 @@ export default class Room {
         event.content.data = JSON.parse(event.content.data);
       }
     }
-    eventId = eventId ?? stateKey; // room state may not necessary have an event ID
+    // Backfilled events carry their latest edit as a server-side aggregation
+    // bundle, and getAggregatedReplacement substitutes that bundle's content
+    // for the event's own — so its data needs the same decoding. Without
+    // this, every rebuilt message whose usage/context arrived on an edit
+    // reads `data` as a wire string and silently loses those fields (the
+    // session token total visibly shrank after a reload because pre-reload
+    // turns' usage vanished this way).
+    let bundledReplace = (event.unsigned as any)?.['m.relations']?.[
+      'm.replace'
+    ];
+    if (
+      bundledReplace?.content?.data &&
+      typeof bundledReplace.content.data === 'string'
+    ) {
+      bundledReplace.content.data = JSON.parse(bundledReplace.content.data);
+    }
     if (!eventId) {
       throw new Error(
         `bug: event ID is undefined for event ${JSON.stringify(
diff --git a/packages/host/tests/unit/room-test.ts b/packages/host/tests/unit/room-test.ts
new file mode 100644
index 00000000000..64a20b289ea
--- /dev/null
+++ b/packages/host/tests/unit/room-test.ts
@@ -0,0 +1,70 @@
+import { module, test } from 'qunit';
+
+import Room from '@cardstack/host/lib/matrix-classes/room';
+
+import type { EventStatus } from 'matrix-js-sdk';
+
+module('Unit | matrix | room', function () {
+  test('addEvent decodes the wire-encoded data of the event and of its aggregation bundle', function (assert) {
+    // `content.data` is a JSON string on the wire. Backfilled events also
+    // carry their latest edit as a server-side aggregation bundle under
+    // unsigned['m.relations']['m.replace'], whose content the room resource
+    // substitutes for the event's own (getAggregatedReplacement) — so a
+    // string left undecoded there silently drops every field read off
+    // `data` on rebuild: token usage, context.agentId, and with it the
+    // agent gate for tool auto-execution.
+    let room = new Room('!room1:localhost');
+    let usage = { promptTokens: 10, completionTokens: 2, costUsd: 0.01 };
+    let event = {
+      event_id: '$original',
+      type: 'm.room.message',
+      room_id: '!room1:localhost',
+      origin_server_ts: 1,
+      status: null as EventStatus | null,
+      content: {
+        msgtype: 'app.boxel.message',
+        body: 'streaming…',
+        data: JSON.stringify({ context: { agentId: 'agent-1' } }),
+      },
+      unsigned: {
+        age: 0,
+        'm.relations': {
+          'm.replace': {
+            event_id: '$edit',
+            type: 'm.room.message',
+            origin_server_ts: 2,
+            content: {
+              msgtype: 'app.boxel.message',
+              body: 'the whole answer',
+              data: JSON.stringify({ context: { agentId: 'agent-1' }, usage }),
+              'm.relates_to': {
+                rel_type: 'm.replace',
+                event_id: '$original',
+              },
+            },
+          },
+        },
+      },
+    };
+
+    room.addEvent(event as any);
+
+    let added = room.events[0] as any;
+    assert.deepEqual(
+      added.content.data.context,
+      { agentId: 'agent-1' },
+      'the event’s own data is decoded',
+    );
+    let bundled = added.unsigned['m.relations']['m.replace'];
+    assert.deepEqual(
+      bundled.content.data.usage,
+      usage,
+      'the aggregation bundle’s data is decoded too, so a rebuilt message keeps its usage',
+    );
+    assert.strictEqual(
+      bundled.content.data.context.agentId,
+      'agent-1',
+      'and keeps its agent id',
+    );
+  });
+});