Chat History not syncing to other devices

### Description

Cross-device Copilot chat session is present in the cloud session index with full transcript but does not hydrate into Chat History or the Agents window.
VS Code Insiders: 1.134.0-insider
Bundled Copilot Chat: 0.62.2026081303
Session ID: 85fab69d-83ce-4b9d-9888-871514075371
Multi-root workspace; both chat.sessionSync.enabled and local indexing are enabled.

### Issue Details

Item|Value
---|---
Issue Category|Bug
Target|Visual Studio Code - Insiders
VS Code Version|Code - Insiders 1.134.0-insider (72f433c37ae15b2f1fb8e292a908964ad1673fbc, 2026-08-13T04:55:17Z)
OS Version|Windows_NT x64 10.0.26220

System Info

Item|Value
---|---
VS Code Version|Code - Insiders 1.134.0-insider (72f433c37ae15b2f1fb8e292a908964ad1673fbc, 2026-08-13T04:55:17Z)
OS Version|Windows_NT x64 10.0.26220
CPUs|13th Gen Intel(R) Core(TM) i7-13800H (20 x 2918)
GPU Status|2d_canvas: enabled
GPU0: VENDOR= 0x8086, DEVICE=0xa7a0 [Intel(R) Iris(R) Xe Graphics], DRIVER_VENDOR=Intel, DRIVER_VERSION=32.0.101.6737 *ACTIVE*
GPU1: VENDOR= 0x10de, DEVICE=0x28a1 [NVIDIA GeForce RTX 4050 Laptop GPU], DRIVER_VERSION=32.0.15.9155
GPU2: VENDOR= 0x1414, DEVICE=0x008c [Microsoft Basic Render Driver], DRIVER_VERSION=10.0.26100.9061
Machine model name:
Machine model version:
direct_rendering_display_compositor: disabled_off_ok
gpu_compositing: enabled
multiple_raster_threads: enabled_on
opengl: enabled_on
rasterization: enabled
raw_draw: disabled_off_ok
skia_graphite: disabled_off
trees_in_viz: disabled_off
video_decode: enabled
video_encode: enabled
webgl: enabled
webgpu: enabled
webnn: disabled_off
Load (avg)|
Memory (System)|31.83GB (10.53GB free)
Process Argv|--crash-reporter-id 860ade74-a273-464d-bcb1-bee7779aaee2
Screen Reader|no
VM|0%
User Agent|Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Code-Insiders/1.134.0-insider Chrome/148.0.7778.280 Electron/42.8.1 Safari/537.36
Installation pure|true

Extensions (4)

Name|Identifier|Author|Version
---|---|---|---
Remote - SSH|ms-vscode-remote.remote-ssh|ms-vscode-remote|0.125.2026080721
Remote - SSH: Editing Configuration Files|ms-vscode-remote.remote-ssh-edit|ms-vscode-remote|0.87.0
PowerShell|ms-vscode.powershell|ms-vscode|2026.1.2
Remote Explorer|ms-vscode.remote-explorer|ms-vscode|0.6.2026031809

Theme extensions: 9

A/B Experiments

```
vsliv368cf:30146710
pythonvspyt551cf:31249598
nativeloc1:31118317
dwcopilot:31158714
dwoutputs:31242946
copilot_t_ci:31333650
g012b348:31231168
pythonrdcb7:31268811
pythonpcpt1cf:31399617
6518g693:31302842
b6b4d950:31327385
envsactivate1:31349248
editstats-enabled:31346256
cloudbuttont:31366566
3efgi100_wstrepl:31403338
bj468946:31457056
gpt-5-copilot-3x-gkv:31561747
metis_rerank_treatment1:31411021
cp_cls_t_966_ss:31526232
4je02754:31455664
8hhj4413:31478653
ge8j1254_inline_auto_hint_haiku:31490507
cp_cls_c_1081:31454833
conptydll_true:31485575
e9c30283:31453065
test_treatment2:31471001
c9b86496:31447327
control_6dc23131:31555649
ei9d7968:31462942
nes-extended-on:31455475
de16b994:31458079
89g7j272:31506658
7e884298:31462391
i2gc6536:31472020
52612955:31508042
h08i8180:31475367
23c7c724:31520045
ddid_c:31478205
hmra_i5g22:-9
bjc72774_agent_sandbox:31538428
7df3h592:31491241
cp_cls_t_1082:31535311
logging_enabled_new:31490725
jb_cp_cls_t_632:31543129
32d76977:31503652
ha629193:31508444
cdk-lw-on:31524445
a1ije391_t:31533808
g4a0e106:31510899
jbcp_cls_pctr_t:31531130
3f904173:31522231
cp_intellij_t_nes:31548657
jf4hg949:31526829
hide-eh-duplicates:31543293
replace-eh-with-ah:31545853
ihg5j128:31534457
f412h606:31543414
enable_editor_pane_layout:31551200
52258b4d:31547953
36h42362:31560305
c7c27ce7:31551212
voicemodeon:31556058
a81hg326:31551415
6577i413_copy:31558653
1h923230:31564177
0dg73464:31554309
gpt56sol_default:31557178
treatment-23-1:31555779
unuse_dynamic_mcp:31555281
maidictation:31561119
0c1h4866:31562347
vrbsty_fls:31561057
cch_brk_tru:31561680
ai4g2856_c:31561844
be7id924_c:31561848
a3a5c856:31562480
5b8j3302:31564600
mangle-name-control:31565184
```