Code added after a previous chat gets deleted when you go back to it

Type: Bug

There are two copilot chats: chats A and B. Chat A came first. Chat B added some new code. You go back to Chat A and enter a prompt which would involve changing the files as Chat B and the the code added by Chat B just disappears.

VS Code version: Code 1.137.0 (645f29cc3176500b4b5762ba887cf2a7f0ffdf2c, 2026-09-08T13:43:59-07:00)
OS version: Windows_NT x64 10.0.26100
Modes:

System Info

|Item|Value|
|---|---|
|CPUs|Intel(R) Core(TM) Ultra 5 235U (14 x 3187)|
|GPU Status|2d_canvas: enabled
GPU0: VENDOR= 0x8086, DEVICE=0x7d41 [Intel(R) Graphics], DRIVER_VENDOR=Intel, DRIVER_VERSION=32.0.101.8826 *ACTIVE*
GPU1: VENDOR= 0x1414, DEVICE=0x008c [Microsoft Basic Render Driver], DRIVER_VERSION=10.0.26100.8972
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)|undefined|
|Memory (System)|31.43GB (7.41GB free)|
|Process Argv|--crash-reporter-id e546e131-6b24-43ef-a052-fb00957fab69|
|Screen Reader|no|
|VM|0%|
Extensions (11)

Name|Identifier|Author|Version
---|---|---|---
Cucumber|cucumberopen.cucumber-official|CucumberOpen|1.11.0
GitHub Actions|github.vscode-github-actions|github|0.32.3
Python Debugger|ms-python.debugpy|ms-python|2026.6.0
Python|ms-python.python|ms-python|2026.4.0
Pylance|ms-python.vscode-pylance|ms-python|2026.3.1
Python Environments|ms-python.vscode-python-envs|ms-python|1.36.0
Language Support for Java(TM) by Red Hat|redhat.java|redhat|1.56.0
vscode-pdf|tomoki1207.pdf|tomoki1207|1.2.2
Gradle for Java|vscjava.vscode-gradle|vscjava|3.18.0
Debugger for Java|vscjava.vscode-java-debug|vscjava|0.59.0
Extension Pack for Java|vscjava.vscode-java-pack|vscjava|0.31.1

A/B Experiments

```
vsliv368cf:30146710
binariesv615:30325510
nativeloc1:31344060
dwcopilot:31170013
dwoutputs:31242946
copilot_t_ci:31333650
e5gg6876:31282496
pythonrdcb7:31342333
6518g693:31463988
aj953862:31281341
4f60g487:31327383
envsactivate1:31551504
gh487529:31544284
cloudbuttont:31379625
42190218_ostrepl:31403339
ddidt:31399633
ec5jj548:31422691
cp_cls_t_966_ss:31526232
4je02754:31466945
8hhj4413:31478653
ge8j1254_inline_auto_hint_haiku:31490510
cp_cls_c_1081:31454833
conptydll_true:31498968
core-renderer-profiling-on:31440367
e9c30283:31461165
c9b86496:31447327
ei9d7968:31496641
chat:31457767
8hig5102:31480529
89g7j272:31518289
i2gc6536:31499202
52612955:31516516
ddid_c:31478207
hmra_i5g22:31518061
ja75b849:31580633
7df3h592:31512476
cp_cls_t_1082:31535311
logging_enabled_new:31498466
db5d2638:31499441
jb_cp_cls_t_632:31543129
748c7209:31512887
32d76977:31512328
ha629193:31508444
jh5f2457_c:31540921
jbcp_cls_pctr_t:31531130
cp_intellij_t_nes:31548657
jf4hg949:31526829
ahp-both-windows:31556933
ihg5j128:31534457
9gg7f176:31542112
enable_editor_pane_layout:31569726
allow-none:31555437
ac562147:31564510
c7c27ce7:31554789
1h923230:31564177
treatment-23-1:31555779
unuse_dynamic_mcp:31555281
0d8dfbc3:31570291
switchtoauto:31559321
0c1h4866:31566224
vrbsty_fls:31561059
session-mark-done:31558131
5b8j3302:31564601
mangle-name-treatment:31572634
allow-signed-out:31581176
intellij_nes_preview_t:31580086
multiv2:31574985
2bfb8b39:31576696
unread:31581112
aw-autom-on:31583758
revealsession_treatment_7ed7e840:31582194
permission_prompt_control:1332567
ccr_pr_nudge_control:1319470
vsc_wsm_c:1340633
eh853263:1370294
1g5a5742:1379227
3aced641:1389836

```