frontier_llm refusal for Claude Fable 5 results in an error

With VSCode Copilot, when working on a repository about LLMs (such as https://github.com/iree-org/iree) and asking Claude Fable 5 a prompt, an error message appears and nothing is returned.

When looking at the log, this warning can be seen:
```
2026-07-08 15:33:20.510 [warning] [messagesAPI] Refusal received: category='frontier_llm' for model claude-fable-5
```

This warning is described here : https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback
> The request could assist the development of competing AI models, which is restricted under Anthropic's commercial terms. Benign machine learning work can also trigger this category.

Instead of returning nothing, such request should either fallback to another model, or fail with a more explicit error message.

How to reproduce:

git clone https://github.com/iree-org/iree

Load the directory into VSCode

Ask Claude Fable 5 this :
> I want to dive into IREE microkernel support. How does it work? Can I plug kernels written outside of IREE into it? Ideally, I would like to compile kernels from outside of IREE, pack them in a .so, and then use them from IREE.
>
> Please note that my target is an offload RISC-V CPU running on Linux ; we have a custom offload stack for communicating with that CPU (through PCI) and we also already have a HAL driver (not currently in this repo) for IREE. Through the offload stack we can dynamically send a .so from the host then load it.

VS Code version: Code 1.128.0 (fc3def6774c76082adf699d366f31a557ce5573f, 2026-07-07T15:14:24-07:00)
OS version: Windows_NT x64 10.0.26200
Modes:

System Info

|Item|Value|
|---|---|
|CPUs|13th Gen Intel(R) Core(TM) i5-1335U (12 x 2496)|
|GPU Status|2d_canvas: enabled
GPU0: VENDOR= 0x8086, DEVICE=0xa7a1 [Intel(R) Iris(R) Xe Graphics], DRIVER_VENDOR=Intel, DRIVER_VERSION=32.0.101.7084 *ACTIVE*
GPU1: VENDOR= 0x1414, DEVICE=0x008c [Microsoft Basic Render Driver], DRIVER_VERSION=10.0.26100.8521
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)|15.69GB (1.41GB free)|
|Process Argv|--crash-reporter-id 12ae8c8c-3282-463a-8257-154ff21cb4af|
|Screen Reader|no|
|VM|0%|

Extensions (7)

Name|Identifier|Author|Version
---|---|---|---
GitLens — Git supercharged|eamodio.gitlens|eamodio|18.2.0
Dev Containers|ms-vscode-remote.remote-containers|ms-vscode-remote|0.463.0
Remote - SSH|ms-vscode-remote.remote-ssh|ms-vscode-remote|0.124.0
Remote - SSH: Editing Configuration Files|ms-vscode-remote.remote-ssh-edit|ms-vscode-remote|0.87.0
Remote Explorer|ms-vscode.remote-explorer|ms-vscode|0.5.0
Jenkinsfile Support|secanis.jenkinsfile-support|secanis|0.1.0
vscode-pdf|tomoki1207.pdf|tomoki1207|1.2.2

A/B Experiments

```
vsliv368:30146709
binariesv615:30325510
nativeloc1:31344060
dwcopilot:31170013
dwoutputs:31242946
copilot_t_ci:31333650
e5gg6876:31282496
pythonrdcb7:31342333
6518g693:31463988
aj953862:31281341
6abeh943:31336334
envsactivate1:31538736
cloudbuttont:31379625
3efgi100_wstrepl:31403338
ddidt:31399633
ec5jj548:31422691
cp_cls_t_966_ss:31526232
4je02754:31466945
c3h7c220:31478652
ge8j1254_inline_auto_hint_haiku:31490510
38bie571_auto:31478677
cp_cls_t_1081:31454832
conptydll_true:31498968
ia-use-proxy-models-svc:31452481
e9c30283:31461165
test_control1:31471004
c9b86496:31447327
capi_b_098a3278:31544978
idci7584:31464702
e3e4d672:31494082
ei9d7968:31496641
nes-extended-on:31455476
chat:31457767
8hig5102:31480529
89g7j272:31518289
7e187181:31503455
i2gc6536:31499202
52612955:31516516
ghj88844:31499326
h08i8180_copy:31475368
ddid_t:31478206
hmra_i5g22:31518061
ja75b849:31546597
7df3h592:31512476
cp_cls_t_1082:31535311
logging_enabled_new:31498466
db5d2638:31499441
jb_cp_cls_t_632:31543129
56dj4588:31512888
32d76977:31512328
ha629193:31508444
cdk-lw-on:31524445
f9bf7985:31543584
31fi7170_t:31522396
a1ije391_t:31540920
api_cot_off:31513532
g4a0e106:31529594
8hfba999:31537718
prpt_lrg:31518466
jbcp_cls_pctr_t:31531130
61138546:31518536
cp_intellij_t_nes:31533504
jf4hg949:31526829
nes-curr-leftover-1500:31538366
h561g132:31533256
sessions_3:31532893
ihg5j128:31534457
jh7i3328:31546038
9gg7f176:31542112
nes-nc-nf-2k:31546582

```