Codex-5.3 is not accessible

[lilinjia09]

Type: Bug

Title: Selecting “Codex” in VS Code Copilot Chat consistently fails with 400 unsupported_api_for_model

Date/time: 2026-03-15 (local time)

Product: GitHub Copilot Chat (VS Code)
Account type: Student

Steps to reproduce:

1. Open GitHub Copilot Chat in VS Code
2. Switch the model/mode to “Codex”
3. Send any prompt

Expected result:
Copilot Chat returns a normal response.

Actual result:
The request fails with HTTP 400:
{"error":{"message":"model "gpt-5.3-codex" is not accessible via the /chat/completions endpoint","code":"unsupported_api_for_model"}}

Notes:

• This is reproducible every time.
• It happens both with and without an HTTP proxy.
• Copilot Chat works in the default (non-Codex) mode.

Relevant log lines:
2026-03-15 23:47:29.752 [info] Request ID for failed request: b0d85006-ecc2-4f94-a128-0130a5683c6e
2026-03-15 23:47:29.753 [error] Request Failed: 400 {"error":{"message":"model "gpt-5.3-codex" is not accessible via the /chat/completions endpoint","code":"unsupported_api_for_model"}}

Extension version: 0.24.1
VS Code version: Code 1.97.2 (e54c774e0add60467559eb0d1e229c6452cf8447, 2025-02-12T23:20:35.343Z)
OS version: Windows_NT x64 10.0.26100
Modes:

Logs

Debug: [LanguageModelAccess] ADDED language model gpt-4.1-2025-04-14
Debug: [LanguageModelAccess] ADDED language model oswe-vscode-prime
Debug: [LanguageModelAccess] ADDED language model gpt-5.2
Debug: [LanguageModelAccess] ADDED language model gpt-3.5-turbo-0613
Debug: [LanguageModelAccess] ADDED language model gpt-4
Debug: [LanguageModelAccess] ADDED language model gpt-4-0125-preview
Debug: [LanguageModelAccess] DID UPDATE language models
Debug: [LanguageModelAccess] UPDATING language models
Trace: Resolving chat model
Trace: Resolved chat model
Debug: [LanguageModelAccess] DID UPDATE language models
Trace: Resolving chat model
Trace: Resolved chat model
Trace: Building intent detector
Trace: Resolving chat model
Trace: Resolved chat model
Trace: Built intent detector
Info: message 0 returned. finish reason: [stop]
Info: request done: requestId: [54f1b657-f477-43f2-bf80-87a140c3bdaf] model deployment ID: []
Trace: Resolving chat model
Trace: Resolved chat model
Trace: [panel] chat request received from extension host
Trace: Resolving chat model
Trace: Processing intent
Trace: Resolving chat model
Trace: Resolved chat model
Trace: Resolved chat model
Trace: Processed intent
Trace: Built prompt
Trace: Sending prompt to model
Info: Request ID for failed request: 8e70e7b2-d4e6-439f-b0d8-35cb9ebf65c3
Error: Request Failed: 400 {"error":{"message":"model \"gpt-5.3-codex\" is not accessible via the /chat/completions endpoint","code":"unsupported_api_for_model"}}
Debug: [context keys] Window state change. Needs offline check: false, active: true, focused: false.

Debug: [context keys] Window state change. Needs offline check: false, active: true, focused: true.

Trace: Resolving embedding model

Trace: Resolved embedding model

Trace: Resolving embedding model

Trace: Resolved embedding model

Debug: [context keys] Window state change. Needs offline check: false, active: true, focused: false.

Debug: [context keys] Window state change. Needs offline check: false, active: true, focused: true.

Trace: Resolving embedding model

Trace: Resolved embedding model

Trace: Resolving embedding model

Trace: Resolved embedding model

Trace: Resolving embedding model

Trace: Resolved embedding model

Trace: Resolving embedding model

Trace: Resolved embedding model

Trace: Resolving embedding model

Trace: Resolved embedding model

System Info

Item	Value
CPUs	Intel(R) Core(TM) i9-14900HX (32 x 2419)
GPU Status	2d_canvas: enabled canvas_oop_rasterization: enabled_on 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 video_decode: enabled video_encode: enabled vulkan: disabled_off webgl: enabled webgl2: enabled webgpu: enabled webnn: disabled_off
Load (avg)	undefined
Memory (System)	31.63GB (13.10GB free)
Process Argv	--crash-reporter-id 87d630f4-9ce4-4293-8402-753414273626
Screen Reader	no
VM	0%

A/B Experiments

vsliv368cf:30146710
binariesv615:30325510
nativeloc1:31344060
dwcopilot:31170013
dwoutputs:31242946
copilot_t_ci:31333650
f5992895:31277799
pythonrdcb7:31342333
aj953862:31281341
6abeh943:31336334
envsactivate1:31464700
cloudbuttont:31379625
aihoversummaries_f:31469309
3efgi100_wstrepl:31403338
d472g771:31401736
ec5jj548:31422691
cp_cls_t_966_ss:31454198
cp_cls_c_1081:31454833
e9c30283:31461165
test_treatment2:31471001
idci7584:31464702
chat:31457767
cpptoolsoff-v2:31475362

[lilinjia09]

I was using systerm version, now change to user version. Everything works now!

[KristinThompson6]

I was using systerm version, now change to user version. Everything works now!

I also have this question,how can I change to user version

[lilinjia09]

I was using systerm version, now change to user version. Everything works now!

I also have this question,how can I change to user version

go to official website and redownload. Make sure you download the user version. There is no need to uninstall system version. Then open,the user version, all the extension will be left. Except for copilot. It will remind you to reinstall. I'm a rookie in CS, and I don't know if there will be any residual files. But now I can use codex 5.3 and agent model of copilot.