VS Code extension cannot reopen some local sessions that CLI can resume

Summary

Some local Codex sessions become permanently unopenable in the VS Code Codex extension, while the same sessions can still be resumed successfully with the Codex CLI (/resume).

This is not a temporary WebView glitch: reloading VS Code, restarting VS Code Remote, reinstalling the extension, and updating extension versions do not make the affected sessions open again. Once a session hits this state, clicking it in the Codex sidebar leaves the page blank with no loading indicator.

Observed behavior

• Affected sessions are visible/clickable in the VS Code Codex sidebar.
• Clicking an affected session opens a blank page.
• There is no visible loading state or error in the sidebar.
• The model indicator in the lower-right UI falls back to custom or 5.4.
• The same session was actually created and used with gpt-5.5 and medium reasoning.
• Smaller sessions often still open normally in the extension and keep showing gpt-5.5 medium.
• The affected sessions remain readable/resumable in the Codex CLI via /resume.

Why this looks like an extension/session hydration issue

The local JSONL session file is present and parseable. One concrete affected session:

thread_id: 019dd6e6-44ba-7772-abab-cc47e88f36de
thread_name: 添加查询模式附加组件
session file: ~/.codex/sessions/2026/04/29/rollout-2026-04-29T01-41-48-019dd6e6-44ba-7772-abab-cc47e88f36de.jsonl
size: 719858 bytes
jsonl lines: 401
originator: codex_vscode
source: vscode
session cli_version: 0.126.0-alpha.8
model in turn_context: gpt-5.5
effort in turn_context: medium

Validation:

All 401 JSONL lines parse successfully with JSON.parse.
Codex CLI can resume the session.
VS Code extension cannot open/render the session.

This has happened with multiple sessions over time. Large session size may be a trigger, but may not be the only trigger.

Local environment

VS Code Codex extension id: openai.chatgpt
Installed extension versions observed:
  openai.chatgpt-26.422.62136-linux-x64
  openai.chatgpt-26.422.71525-linux-x64

Bundled extension CLI:
  codex-cli 0.126.0-alpha.8

User/system CLI:
  codex-cli 0.125.0

Host:
  VS Code Remote on Linux

Relevant logs

~/.vscode-server/data/logs/20260429T003025/exthost3/remoteexthost.log:

2026-04-29 02:12:19.807 [error] PendingMigrationError: navigator is now a global in nodejs, please see https://aka.ms/vscode-extensions/navigator for additional info on this error.
    at /home/.../.vscode-server/extensions/openai.chatgpt-26.422.71525-linux-x64/out/extension.js:...

~/.vscode-server/data/logs/20260429T003025/exthost2/openai.chatgpt/Codex.log around the affected session creation/open period:

[warning] [IpcClient] Received broadcast but no handler is configured method=thread-stream-state-changed
[warning] [IpcClient] Received broadcast but no handler is configured method=thread-read-state-changed
[error] [desktop-notifications][global-error] ResizeObserver loop completed with undelivered notifications.

Steps to reproduce

1. Use the VS Code Codex extension for a longer/local session until the session grows large or otherwise reaches the problematic state.
2. Close/reopen the Codex sidebar, reload VS Code, or restart VS Code Remote.
3. Click the affected session in the Codex sidebar.
4. Observe a blank page with no loading indicator.
5. Resume the same session with Codex CLI /resume.

Expected behavior

The VS Code extension should either:

• open/render any local session that the CLI can resume, or
• show a clear recoverable error if a session is too large or unsupported, or
• safely summarize/page/trim the UI rendering without making the thread permanently inaccessible from the extension.

It should not silently show a blank page or show a misleading fallback model (custom / 5.4) when the persisted turn context says gpt-5.5 medium.

Related issues

This appears related to earlier VS Code extension/session restore issues:

• #5902: VS Code extension crashes after a single conversation exceeds the session size limit.
• #14812: VS Code extension loses thread history after restart even though session files still exist.
• #15368: session list cap / local thread hydration issues under heavy usage.
• #3834: session files exist under .codex/sessions but the extension does not read them after a format change.