GPT-5.6 Sol local tools fail in Codex Desktop on Windows: code-mode host exited during handshake

Summary

On Windows x64, local tool execution in Codex Desktop fails reproducibly with GPT-5.6 Sol during the code-mode host handshake. The same simple local command succeeds in the same app/environment after switching the task to GPT-5.5.

Observed error:

code-mode host exited during handshake

This looks like a GPT-5.6-specific Codex Desktop / code-mode host tool-negotiation issue, not a general shell, filesystem, sandbox, model-availability, or MFA/authentication issue.

Environment

• OS: Windows x64 / AMD64
• Codex Desktop package observed: OpenAI.Codex_26.820.7780.0_x64__2p2nqsd0c76g0
• Bundled Codex runtime observed: codex-cli 0.150.0-alpha.8
• Also tested runtime: codex-cli 0.149.0
• Failing models: gpt-5.6-sol, gpt-5.6-luna
• Working model: gpt-5.5

Expected behavior

GPT-5.6 Sol should be able to use local Codex tools in Codex Desktop on Windows x64, or fall back to the same working local command execution path used by GPT-5.5. A simple directory listing should not fail before the command actually runs.

Actual behavior

When GPT-5.6 Sol attempts to run a simple local command, the tool call fails before command execution with:

code-mode host exited during handshake

Switching the same task/environment to GPT-5.5 allows the local command to run successfully and list the expected directories.

Reproduction outline

1. Open Codex Desktop on Windows x64.
2. Start a local task using gpt-5.6-sol.
3. Ask Codex to run a simple local command, such as listing the current directory.
4. The tool call fails with code-mode host exited during handshake.
5. Switch the task to gpt-5.5.
6. Run the same type of local directory-listing command.
7. The command succeeds.

A/B evidence from local session logs

I have multiple local session logs showing the same pattern:

• With gpt-5.6-sol: comp_hash=3000, multi_agent_version=v2, local tool call fails with code-mode host exited during handshake.
• With gpt-5.6-luna: same handshake failure.
• With gpt-5.5: comp_hash=2911, local command execution succeeds.

I can provide the session JSONL files privately if useful. I am not attaching them publicly because they contain local transcript details.

Local model catalog observations

The local model cache shows a relevant routing difference:

gpt-5.6-sol:

{
  "use_responses_lite": true,
  "tool_mode": "code_mode_only",
  "multi_agent_version": "v2",
  "comp_hash": "3000",
  "max_context_window": 872000
}

gpt-5.5:

{
  "use_responses_lite": false,
  "tool_mode": "<absent>",
  "multi_agent_version": "<absent>",
  "comp_hash": "2911",
  "max_context_window": 272000
}

Temporarily changing the local cache fields for GPT-5.6 entries did not produce a durable fix; fresh GPT-5.6 tasks still showed comp_hash=3000 and failed. This suggests the effective GPT-5.6 tool route is not controlled solely by models_cache.json, or it is refreshed/overridden by the app/backend.

Mitigation attempts already tested

• Switching from GPT-5.6 Sol to GPT-5.5: local command execution works.
• Testing GPT-5.6 Luna: same handshake failure.
• Temporarily patching local model-cache routing fields for GPT-5.6: no durable fix.
• Testing Codex runtime 0.149.0 instead of 0.150.0-alpha.8: GPT-5.6 Sol still fails with the same handshake error.
• Checking sandbox setup logs: setup completed with errors=[].
• Checking for MFA / 403-style failures in the relevant logs: no 403 Forbidden, Multi-factor authentication required, or mfa_required evidence was found.
• Manually starting codex-code-mode-host.exe --listen ws://127.0.0.1:0: the host can start/listen manually, so the binary does not appear to be simply missing or unable to launch.

Request

Could the Codex team check whether the GPT-5.6 Sol/Terra/Luna tool route currently has a compatibility issue with Codex Desktop's Windows code-mode host handshake, especially around:

• tool_mode=code_mode_only
• use_responses_lite=true
• multi_agent_version=v2
• comp_hash=3000
• Desktop runtime 0.150.0-alpha.8 and 0.149.0 on Windows x64
• the transition from GPT-5.5-style local shell execution to the GPT-5.6 code-mode host execution path