[Windows][Codex Desktop 26.820.7780.0] GPT-5.6 Sol Ultra tool calls fail with "code-mode host exited during handshake", while GPT-5.5 works

[hxlacceptedmsn]

What version of the Codex App are you using (From “About Codex” dialog)?

26.820.7780.0

What subscription do you have?

ChatGPT Pro 20x

What platform is your computer?

Microsoft Windows NT 10.0.26200.0 x64

What issue are you seeing?

After recovering the separate Windows Desktop 26.820 bundled-resource
relocation/startup regression described in #40700, Codex Desktop and
its app server start normally.

However, GPT-5.6 Sol Ultra cannot execute any local shell or tool call.
Every attempt fails before the first command is executed with:

code-mode host exited during handshake

The same command works normally in a new GPT-5.5 Extra High session on
the same machine, using the same Codex Desktop installation, account,
working directory, and PowerShell environment.

This is not another report of the original:

Unable to locate the Codex CLI binary

startup failure. The desktop UI and app-server initialization are now
working. This report concerns a remaining model-specific Code Mode
handshake failure.

Model A/B result:

GPT-5.6 Sol Ultra:

code-mode host exited during handshake

GPT-5.5 Extra High:

Command executed successfully.

CODE_MODE_SMOKE_OK

A full two-turn health check under GPT-5.5 also passes shell execution,
temporary file creation, file read/write, recursive search, SHA-256
calculation, cross-turn state persistence, and cleanup.

Possibly related to #32759, but this reproduction is specifically on
Windows Codex Desktop 26.820.7780.0 with bundled CLI
0.150.0-alpha.8.

### What steps can reproduce the bug?

1. Launch Codex Desktop on Windows.

2. Confirm that the app and app server initialize normally.

3. Open a new session and select GPT-5.6 Sol Ultra.

4. Send this prompt:

```text
Call the terminal tool exactly once and run the following PowerShell
command. Do not create or modify any files, do not retry, and do not
infer the result without running the command:

Write-Output 'CODE_MODE_SMOKE_OK'

Clearly distinguish between a command that actually executed and a
command that did not execute. On failure, report only the original tool
error.
Observe that no PowerShell command runs and the result is:
code-mode host exited during handshake
Open another new session on the same machine and select GPT-5.5
Extra High.
Send the exact same prompt.
Observe that the command executes successfully and returns:
CODE_MODE_SMOKE_OK

I reproduced the GPT-5.6 failure more than once. It occurs before the
first shell command runs.

### What is the expected behavior?

GPT-5.6 Sol Ultra should initialize the local Code Mode host and execute
the same PowerShell command that GPT-5.5 executes successfully.

Expected output:

```text
CODE_MODE_SMOKE_OK

A model change should not cause the local code-mode host process to exit
during the initial handshake.

### Additional information

Environment:

```text
Codex Desktop package: OpenAI.Codex 26.820.7780.0 x64
Bundled CLI: codex-cli 0.150.0-alpha.8
PowerShell: 7.6.5
Authentication: ChatGPT account
CODEX_CLI_PATH: unset at Process, User, and Machine scope
Package status: Ok
Package signature kind: Store

The desktop is using this recovered per-user CLI:

%LOCALAPPDATA%\OpenAI\Codex\bin\d0097be4feba73d0\codex.exe

The matching Code Mode host exists beside it:

%LOCALAPPDATA%\OpenAI\Codex\bin\d0097be4feba73d0\
codex-code-mode-host.exe

Local host validation:

Cached host exists: True
Official MSIX source host exists: True
Cached host length: 65880368
MSIX source host length: 65880368
Cached host encrypted: False
MSIX source host encrypted: True
Lengths match: True
SHA-256 hashes match: True
Authenticode signature: Valid
Signer: OpenAI OpCo, LLC
codex-code-mode-host.exe --help exit code: 0

SHA-256 of both the cached and official package host:

3B93FC5DF0B38358D7FFFB290A3F11A56CECBEBBF70EB62BF8C58D6D994B28E3

Windows event checks found no matching events in:

Application
Microsoft-Windows-CodeIntegrity/Operational
Microsoft-Windows-Windows Defender/Operational

The Codex Desktop app-server log shows successful initialization:

[AppServerConnection]
initialize_handshake_result
outcome=success
transportKind=stdio

Therefore the desktop-to-app-server handshake is healthy. The failure
occurs later in the GPT-5.6 Code Mode host path.

Background:

The installation originally encountered the separate 26.820 Windows
bundled-resource relocation regression in #40700. Startup was recovered
by materializing version-specific per-user caches from the currently
installed official Store package.

No WindowsApps files, ACLs, ownership, or encryption attributes were
modified. CODEX_CLI_PATH was not set.

GPT-5.5 executes shell commands and passes a full two-turn local-tool
health check on the same installation. This makes a missing, corrupted,
encrypted, or Windows-security-blocked local host binary unlikely.

Related:

#32759: same model-specific handshake error on another platform/CLI
#40700: separate Windows Desktop startup/resource-relocation regression

[kinglamchan]

I can reproduce this on the same environment:

• Codex Desktop 26.820.7780.0
• Bundled CLI 0.150.0-alpha.8
• Microsoft Windows NT 10.0.26200.0 x64
• ChatGPT Pro 5x

The same code-mode host exited during handshake error occurs before any local command runs, including in existing tasks, new tasks, and a new worktree. Restarting Codex Desktop and rebooting Windows did not help.

A manually started WebSocket code-mode host successfully completed a real exec tool call, while the normal Desktop stdio path still fails.

Additional reproduction:

Image generation also fails under GPT-5.6 with the same error:

code-mode host exited during handshake

The request fails before any image is generated. Text chat continues to work normally.

This suggests the issue affects non-shell tool calls as well, not only terminal commands.

[Gzy1996]

I can reproduce the same regression on Windows with Codex Desktop 26.820.7780.0.

Observed behavior:

• GPT-5.6 Sol: every local tool/shell request fails before the command runs with code-mode host exited during handshake.
• The failure reproduces in existing tasks, forked tasks, and brand-new tasks, so it is not project-specific.
• Switching only the model to GPT-5.5 on the same machine, same desktop installation, same account, same PowerShell environment, and same working directory makes local execution work immediately.

Smoke test used after switching to GPT-5.5:

Write-Output 'CODE_MODE_SMOKE_OK'

Result: command actually executed, exit code 0, output CODE_MODE_SMOKE_OK.

Additional troubleshooting already performed before this A/B test:

• App repair/reinstall and multiple Windows restarts: no change.
• Package status reports OK and AppX registration was rebuilt.
• User runtime caches were rebuilt.
• CODEX_CLI_PATH was cleared.
• Defender/Application logs did not show a corresponding block/crash.
• A separately installed official npm Codex CLI works (codex-cli 0.149.1), but pointing Desktop at that CLI did not restore GPT-5.6 tool execution.

This strongly suggests a GPT-5.6 / Code Mode handshake regression in Desktop 26.820.7780.0 rather than a generic local execution, project path, or PowerShell problem.

[xjin8747-beep]

I can reproduce this on another Windows machine with the same Desktop build, and I have one additional A/B result that may help narrow the failure path.

Environment:

• Codex Desktop: 26.820.7780.0
• Desktop bundled/cached CLI: codex-cli 0.150.0-alpha.8
• Desktop runtime path: %LOCALAPPDATA%\OpenAI\Codex\bin\d0097be4feba73d0\codex.exe
• Cached codex-code-mode-host.exe exists beside it, size 65880368 bytes
• Separately installed official CLI: codex-cli 0.149.1

Desktop behavior:

• GPT-5.6 Sol: local tool use fails with code-mode host exited during handshake
• GPT-5.5: local tool use works normally
• Restarting the Desktop app multiple times did not help
• The regression started immediately after updating the Desktop app today

Additional isolation:

1. Running the separately installed 0.149.1 CLI directly with GPT-5.6 successfully executes PowerShell tools.
2. More importantly, running the exact Desktop cached 0.150.0-alpha.8 binary directly also succeeds with GPT-5.6:

& "$env:LOCALAPPDATA\OpenAI\Codex\bin\d0097be4feba73d0\codex.exe" exec --skip-git-repo-check -m gpt-5.6-sol "Run pwd using the shell tool."

The command ultimately executed successfully and returned the working directory. The same cached CLI also successfully read local Skill files before executing pwd.

Other checks:

• CODEX_CODE_MODE_HOST_PATH is unset
• no duplicate codex.exe entries were found on PATH
• no matching Application Error / WER crash event was found for codex-code-mode-host.exe

This suggests the cached codex.exe + codex-code-mode-host.exe binaries themselves are functional with GPT-5.6 when invoked directly, while the failure is specific to the Desktop-integrated Code Mode startup/handshake path.

[hxlacceptedmsn]

Confirmed: rolling back the complete Windows Desktop package fixes the
GPT-5.6 Code Mode handshake failure on the same machine.

Controlled version A/B:

Broken

Codex Desktop: 26.820.7780.0 x64
Bundled CLI: 0.150.0-alpha.8
Model: GPT-5.6 Sol Ultra
Result: code-mode host exited during handshake

The failure reproduced repeatedly before the first PowerShell command
was executed.

GPT-5.5 Extra High continued to execute local tools normally on the same
installation.

Working after rollback

I downloaded the official OpenAI CDN MSIX for:

OpenAI.Codex 26.818.8289.0 x64

I performed an in-place AppX downgrade with
-ForceUpdateFromAnyVersion. The resulting package state is:

Version: 26.818.8289.0
Architecture: X64
SignatureKind: Store
Status: Ok

CODEX_CLI_PATH remains unset at Process, User, and Machine scope.

After launching 26.818.8289.0, GPT-5.6 Sol successfully executed the
same PowerShell smoke test that failed on 26.820:

Write-Output 'CODE_MODE_SMOKE_OK'

Result:

Command actually executed
Exit code: 0
Output: CODE_MODE_SMOKE_OK

PowerShell 7 remains installed, and no AppX-volume, WindowsApps ACL,
ownership, Defender, PATH, or environment-variable changes were made
between the failing and working GPT-5.6 tests.

This strongly isolates the regression to the Desktop/runtime combination
introduced in 26.820.7780.0, rather than to PowerShell, the project,
the account, or a generally broken local tool environment.

[Sprith-J]

I’m seeing the same issue on Windows Codex Desktop 26.820.7780.0. I added a detailed Windows repro with GPT-5.6 Sol + Luna failures, GPT-5.5 success, and GPT-5.6 CLI/app-server controls here: #32759 (comment)

[CloudyRiver9]

Related Windows Desktop report with additional diagnostics: #40913

Key extra finding there: from the ChatGPT Desktop process context, spawning the bundled runtime fails at the lowest level:

C:\Users\c\AppData\Local\OpenAI\Codex\runtimes\cua_node\950613ca46815e82\bin\node.exe --version
nodeVersionError.code = 3221225506  (0xC0000022)

while the exact same node.exe launched manually from PowerShell succeeds (v24.19.0, exit 0). A full clean-room reset of all Codex/Desktop state, reinstall, disabling code_mode_host, Defender exclusions, and stopping Nahimic did not change the result. The main Desktop -> codex app-server stdio handshake succeeds; only the local code-mode execution child-chain fails. This points to the Desktop/MSIX packaged child-process context rather than the binaries or user state.