"Invalid String Length" in Copilot Chat

[HeatPhoenix]

Type: Bug

I consistently get "Invalid String Length" as a response in a Github Copilot chat. The specific model, mode and so on doesn't seem to matter at all.

Rolling back to a question/answer before that allows us to use the chat again normally.

Extension version: 0.37.2026020404
VS Code version: Code - Insiders 1.109.0-insider (bdd88df, 2026-02-04T02:00:40.203Z)
OS version: Windows_NT x64 10.0.19045
Modes:
Remote OS version: Linux x64 6.6.87.2-microsoft-standard-WSL2

System Info

Item	Value
CPUs	11th Gen Intel(R) Core(TM) i7-11700 @ 2.50GHz (16 x 2496)
GPU Status	2d_canvas: enabled GPU0: VENDOR= 0x8086, DEVICE=0x4c8a [Intel(R) UHD Graphics 750], DRIVER_VENDOR=Intel, DRIVER_VERSION=32.0.101.7082 ACTIVE GPU1: VENDOR= 0x10de, DEVICE=0x2782 [NVIDIA GeForce RTX 4070 Ti], DRIVER_VERSION=32.0.15.9159 GPU2: VENDOR= 0x1414, DEVICE=0x008c [Microsoft Basic Render Driver], DRIVER_VERSION=10.0.19041.5794 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 webgl2: enabled webgpu: enabled webnn: disabled_off
Load (avg)	undefined
Memory (System)	63.81GB (30.21GB free)
Process Argv	--crash-reporter-id 4d044748-42fe-4894-aaec-ab86075f55bb
Screen Reader	no
VM	0%

Item	Value
Remote	WSL: Ubuntu
OS	Linux x64 6.6.87.2-microsoft-standard-WSL2
CPUs	11th Gen Intel(R) Core(TM) i7-11700 @ 2.50GHz (16 x 0)
Memory (System)	31.25GB (28.54GB free)
VM	0%

A/B Experiments

vsliv368cf:30146710
pythonvspyt551:31249597
nativeloc1:31118317
dwcopilot:31158714
dwoutputs:31242946
copilot_t_ci:31333650
g012b348:31231168
pythonrdcb7:31268811
pythonpcpt1:31399616
6518g693:31302842
6abeh943:31336334
envsactivate1:31349248
editstats-enabled:31346256
cloudbuttont:31366566
3efgi100_wstrepl:31403338
use-responses-api:31390341
je187915:31454425
none:31438490
cp_cls_c_966_ss:31454199
chatvisible:31417128
inlinechat_v2_hd992725:31445440
4je02754:31455664
ge8j1254_inline_auto_hint_haiku:31426735
a5gib710:31434435
38bie571_auto:31429954
7ig2g208:31429686
406hc587_ask_agent:31428392
preserve_tokens:31444546
cp_cls_c_1081:31454833
copilot-nes-oct-trt:31432609
ia-use-proxy-models-svc:31446143
nes-slash-models-on:31441832
a43f0576c:31442820
e9c30283:31453065
subagent_3ccgc321:31449956
diag_5_20:31448840
nes-trig-10:31449242
idci7584:31454084
edit_mode_hidden:31453871
nes-ftch-new:31454641
nes_chat_context_enabled:31451401
showstats:31452402
e3e4d672:31454087
864ei723_large_tool_results_to_disk:31455802
nes-fast-add:31454624
i54ji102:31455230
b1f5h706:31455228
hg17d649:31455236
nes-extended-on:31455475

[lweberru]

Is there any workaround?
I can't use the chat at all right now.
Here the issue is the same with another environment:
Visual Studio Code Version: 1.111.0
ElectronBuildId: 13330601
github.copilot-chat Version: 0.39.2
OS Version: Darwin arm64 24.6.0

Exception:

Error serializing chat session for storage. The session will be lost if the window is closed. Please report this issue to the VS Code team:

RangeError: Invalid string length
at JSON.stringify (<anonymous>)
at Zte.write (vscode-file://vscode-app/Applications/Visual%20Studio%20Code.app/Contents/Resources/app/out/vs/workbench/workbench.desktop.main.js:2908:644)
at eie.writeSession (vscode-file://vscode-app/Applications/Visual%20Studio%20Code.app/Contents/Resources/app/out/vs/workbench/workbench.desktop.main.js:2910:11578)
at vscode-file://vscode-app/Applications/Visual%20Studio%20Code.app/Contents/Resources/app/out/vs/workbench/workbench.desktop.main.js:2910:8538
at Array.map (<anonymous>)
at vscode-file://vscode-app/Applications/Visual%20Studio%20Code.app/Contents/Resources/app/out/vs/workbench/workbench.desktop.main.js:2910:8526
at vscode-file://vscode-app/Applications/Visual%20Studio%20Code.app/Contents/Resources/app/out/vs/workbench/workbench.desktop.main.js:406:73746

[marcoangelo]

Lukas Weberruß (@lweberru) if you click the retry it seems to work, at least for me.

[vonsofh]

reload doesnt fix, new chat does

[lgcgroup2014]

more than 4h hours Invalid string length, very disappointing!

[Rowdster]

This is a terrible issue. My only recourse is to use a new chat and ask it to make a best effort to recover that last chat by title. Ridiculous.

[chrmarti]

Thanks for the report and the additional cross-platform details. This is covered by #314556, which tracks the underlying unbounded chat-session growth and resulting Invalid string length failures; this report adds confirmed Windows/WSL and macOS persistence-stack evidence there.

/duplicate of #314556