[Insiders] "Steer with Message" appears to terminate an active agent run instead of continuing it

Type: Bug

## Environment

- OS: Windows 10 Enterprise 22H2
- VS Code: `1.133.0-insider`
- GitHub Copilot extension: `0.61.2026081103`
- Occurred: `2026-08-12`
- Chat session ID: `d773f79e-bb44-4416-9511-cf53bb213b13`

## Summary

When an agent is actively performing tool calls, using **Steer with Message** appears to start a normal replacement turn that produces a final response without continuing the active workflow.

The steering message is accepted and generates a response, but the agent does not continue the work that was already in progress. The transcript records the message as an ordinary `user.message` and does not contain any steering, interruption, cancellation, replacement, or continuation metadata.

## Steps to Reproduce

1. Start a long-running agent task that performs one or more tool calls. (Local harness)
2. While the agent is still active, use **Steer with Message**.
3. Send a short redirecting instruction.
4. Observe the assistant response and whether the original workflow continues.
5. Inspect the Copilot transcript/debug artifacts for the session lifecycle.

## Expected Behavior

The steering message should redirect the active agent run while preserving the current task state.

The previous run should be interrupted or superseded cleanly, and the assistant should continue from the latest available state. Any interruption, replacement, or steering relationship should also be represented in the diagnostic metadata.

## Actual Behavior

The message is recorded as a normal `user.message` event with only `content` and `attachments` fields.

A new `assistant.turn_start` follows immediately. The resulting assistant message contains no tool requests, and the turn ends normally without continuing the active tool workflow.

The preceding assistant turn had started tool execution but had not emitted an `assistant.turn_end` before the steering message arrived. No explicit interruption or cancellation event was recorded.

## Sanitized Diagnostic Evidence

The relevant event sequence was:

```text
2026-08-12T13:29:31.660Z assistant.turn_start
2026-08-12T13:29:31.944Z tool.execution_start
2026-08-12T13:29:55.316Z tool.execution_complete
2026-08-12T13:29:58.043Z user.message
2026-08-12T13:29:58.046Z assistant.turn_start
2026-08-12T13:30:06.175Z assistant.message, toolRequests=0
2026-08-12T13:30:06.176Z assistant.turn_end
2026-08-12T13:30:24.702Z user.message
2026-08-12T13:30:24.704Z assistant.turn_start
```
VS Code version: Code - Insiders 1.133.0-insider (2a084000e4f05e682580153cd96a94847fec52ae, 2026-08-11T10:13:30Z)
OS version: Windows_NT x64 10.0.19045
Modes:

System Info

|Item|Value|
|---|---|
|CPUs|Intel(R) Core(TM) i7-6700 CPU @ 3.40GHz (8 x 3408)|
|GPU Status|2d_canvas: enabled
GPU0: VENDOR= 0x8086, DEVICE=0x1912 [Intel(R) HD Graphics 530], DRIVER_VENDOR=Intel, DRIVER_VERSION=30.0.101.1339 *ACTIVE*
GPU1: VENDOR= 0x1414, DEVICE=0x008c [Microsoft Basic Render Driver], DRIVER_VERSION=10.0.19041.7548
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)|31.88GB (3.67GB free)|
|Process Argv|--crash-reporter-id 58a304ea-fc3e-4cda-bd6d-91d16fd366cd|
|Screen Reader|no|
|VM|0%|
Extensions (10)

Name|Identifier|Author|Version
---|---|---|---
Container Tools|ms-azuretools.vscode-containers|ms-azuretools|2.4.5
C# Dev Kit|ms-dotnettools.csdevkit|ms-dotnettools|3.20.199
C#|ms-dotnettools.csharp|ms-dotnettools|2.140.9
.NET Install Tool|ms-dotnettools.vscode-dotnet-runtime|ms-dotnettools|3.1.0
Dev Containers|ms-vscode-remote.remote-containers|ms-vscode-remote|0.467.0
PowerShell|ms-vscode.powershell|ms-vscode|2025.4.0
Chat Customizations Evaluations|ms-vscode.vscode-chat-customizations-evaluations|ms-vscode|1.0.8
AI Engineering Fluency|robbos.ai-engineering-fluency|RobBos|0.17.1
vscode-pdf|tomoki1207.pdf|tomoki1207|1.2.2
Mermaid Preview|vstirbu.vscode-mermaid-preview|vstirbu|2.1.2

A/B Experiments

```
vsliv368:30146709
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
55ia6109:31457054
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_control1:31471004
c9b86496:31447327
ei9d7968:31462942
nes-extended-on:31455475
89g7j272:31506658
7e884298:31462391
i2gc6536:31472020
52612955:31508042
h08i8180_copy:31475366
23c7c724:31520045
ddid_c:31478205
hmra_i5g22:31518061
bjc72774_agent_sandbox:31538428
7df3h592:31491241
cp_cls_t_1082:31535311
logging_enabled_new:31490725
jb_cp_cls_t_632:31543129
b1g4i769:31503653
ha629193:31508444
cdk-lw-on:31524445
a1ije391_t:31533808
g4a0e106:31510899
jbcp_cls_pctr_t:31531130
cp_intellij_t_nes:31548657
gf14b233:31526830
hide-eh-duplicates:31543293
replace-eh-with-ah:31545853
ihg5j128:31534457
f412h606:31543414
enable_editor_pane_layout:31551200
52258b4d:31547953
36h42362:31560305
c7c27ce7:31551212
6577i413_copy:31558653
1h923230:31564177
1532g621_copy:31554310
treatment-23-1:31555779
unuse_dynamic_mcp:31555281
0c1h4866:31562347
vrbsty_tr:31561056
cch_brk_fls:31561679
ai4g2856_c:31561844
j66cb542_t:31561847
5b8j3302:31564600

```