Copilot chat running away

Type: Bug

## Bug Description

When I clicked the chat mode selector to change the mode from "Agent" to "Plan", the text:

Start implementation

unexpectedly appeared in the chat conversation as if I had typed it and pressed Enter.

I did not type or submit that phrase. The only action I took was clicking the "Plan" button in the mode selector.

## Steps to Reproduce

1. Open a VS Code Copilot Chat conversation in Agent mode.
2. Do not type a new message.
3. Click the mode selector and choose "Plan".
4. Observe the conversation.

## Expected Behavior

Changing the mode should only change the active mode. It should not insert or submit any message on behalf of the user.

## Actual Behavior

The phrase "Start implementation" appeared as a user message and was submitted into the conversation.

## Impact

This creates a false user message and can cause the agent to begin making changes or taking actions that the user did not request. It also makes the conversation history inaccurate.

## Environment

- OS: Windows
- Editor: Visual Studio Code
- Extension: GitHub Copilot Chat
- Date observed: 2026-08-16
- Workspace: Existing dbt project
- Previous mode: Agent
- Selected mode: Plan
- Models: Auto
- Permissions: Autopilot

## Additional Notes

The inserted text appeared immediately after clicking the "Plan" mode button. I had not entered that text into the chat input.

VS Code version: Code 1.133.0 (a5b500951314efd502d07465bd138dfbd714a960, 2026-08-11T22:14:08Z)
OS version: Windows_NT x64 10.0.26200
Modes:

System Info

|Item|Value|
|---|---|
|CPUs|Intel(R) Core(TM) 7 150U (12 x 2611)|
|GPU Status|2d_canvas: enabled
GPU0: VENDOR= 0x8086, DEVICE=0xa7ac [Intel(R) Graphics], DRIVER_VENDOR=Intel, DRIVER_VERSION=32.0.101.7085 *ACTIVE*
GPU1: VENDOR= 0x1414, DEVICE=0x008c [Microsoft Basic Render Driver], DRIVER_VERSION=10.0.26100.8875
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)|31.69GB (12.26GB free)|
|Process Argv|--crash-reporter-id 0c1ba873-a720-4add-bb9e-896c488a9f4c|
|Screen Reader|no|
|VM|0%|
Extensions (17)

Name|Identifier|Author|Version
---|---|---|---
BigQuery SQL Language Support|dalbitresb12.bqsql|dalbitresb12|2.0.2
DBCode - SQL & Database Client for Postgres, MySQL, MongoDB & more|dbcode.dbcode|dbcode|1.36.10
dbt|dbtlabsinc.dbt|dbtLabsInc|0.98.0
BigQuery Driver for SQLTools|evidence.sqltools-bigquery-driver|Evidence|0.0.9
.NET Install Tool|ms-dotnettools.vscode-dotnet-runtime|ms-dotnettools|3.1.0
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
Jupyter|ms-toolsai.jupyter|ms-toolsai|2025.9.1
Jupyter Keymap|ms-toolsai.jupyter-keymap|ms-toolsai|1.1.2
Jupyter Notebook Renderers|ms-toolsai.jupyter-renderers|ms-toolsai|1.3.0
Jupyter Cell Tags|ms-toolsai.vscode-jupyter-cell-tags|ms-toolsai|0.1.9
Jupyter Slide Show|ms-toolsai.vscode-jupyter-slideshow|ms-toolsai|0.1.6
SQLTools|mtxr.sqltools|mtxr|0.28.6
Better Jinja|samuelcolvin.jinjahtml|samuelcolvin|0.20.0
Jinja|wholroyd.jinja|wholroyd|0.0.8

A/B Experiments

```
vsliv368:30146709
binariesv615:30325510
nativeloc1:31344060
dwcopilot:31170013
bajee813:31263137
dwoutputs:31242946
copilot_t_ci:31333650
e5gg6876:31282496
pythonrdcb7:31342333
6518g693:31463988
aj953862:31281341
envsactivate1:31551504
cloudbuttont:31379625
3efgi100_wstrepl:31403338
bj468946:31457056
ddidt:31399633
ec5jj548:31422691
cp_cls_t_966_ss:31526232
4je02754:31466945
c3h7c220:31478652
ge8j1254_inline_auto_hint_haiku:31490510
cp_cls_t_1081:31454832
conptydll_true:31498968
e9c30283:31461165
test_treatment2:31471001
c9b86496:31447327
capico_a_098a3278:31555650
ei9d7968:31496641
nes-extended-on:31455476
chat:31457767
8hig5102:31480529
89g7j272:31518289
i2gc6536:31499202
52612955:31516516
ghj88844:31499326
ec1eg703:31520046
ddid_c:31478207
hmra_i5g22:31518061
ja75b849:31566471
7df3h592:31512476
cp_cls_t_1082:31535311
logging_enabled_new:31498466
j0d79568:31499440
jb_cp_cls_t_632:31543129
56dj4588:31512888
32d76977:31512328
ha629193:31508444
cdk-lw-on:31524445
a1ije391_t:31540920
jbcp_cls_pctr_t:31531130
61138546:31518536
cp_intellij_t_nes:31548657
jf4hg949:31526829
ahp-both-windows:31556933
session_8:31532895
chatquotatrajectorynudge-enabled:31548767
ihg5j128:31534457
f412h606:31551416
7g2b5551:31542111
enable_editor_pane_layout:31566738
allow-none:31555437
fd5dc207:31551400
36h42362:31564511
c7c27ce7:31554789
1h923230:31564177
1532g621_copy:31554320
treatment-23-1:31555779
unuse_dynamic_mcp:31555281
0d8dfbc3:31566037
switchtoauto:31559321
0c1h4866:31566224
vrbsty_fls:31561059
session-mark-done:31558131
j66cb542_t:31563077
5b8j3302:31564601

```