What version of the IDE extension are you using?

JetBrains AI Assistant plugin: 253.30387.186

What subscription do you have?

ChatGPT Plus (paid)

Which IDE are you using?

IntelliJ IDEA 2025.3.2 (Build IU-253.30387.90) (Windows)

What platform is your computer?

Microsoft Windows NT 10.0.26200.0 x64

What issue are you seeing?

After the IDE integration started offering gpt-5.3-codex models, using AI Chat (Codex via ACP) becomes extremely laggy / effectively unusable.

After a few prompts with gpt-5.3-codex, the IDE enters a broken state: even switching back to gpt-5.2-codex remains unusable, because the ACP agent repeatedly attempts to session/resume a persisted session and fails with an internal error:

1. JSON-RPC -32603 Internal error
2. details: "no rollout found for thread id "
3. This triggers a loop: start ACP process → fail resume → stop server → kill process tree → restart.
4. The loop correlates with severe UI lag in the AI Chat tool window (almost unusable).

Error excerpt:
`method: "session/resume"
params: {
sessionId: "<SESSION_ID_REDACTED>",
cwd: "<PROJECT_PATH_REDACTED>",
mcpServers: [],
}
code: -32603,
message: "Internal error",
data: {
details: "no rollout found for thread id <SESSION_ID_REDACTED>"
}

Failed to initialize ACP session ... Internal error
... Killing process tree ...
`

What steps can reproduce the bug?

1. Open IntelliJ IDEA project.
2. Open AI Chat (JetBrains AI Assistant) and select Codex / ACP integration.
3. Select gpt-5.3-codex (e.g., medium) and send a few prompts (any normal chat prompts).
4. Observe: AI Chat becomes very laggy; soon it becomes nearly unusable.
5. Switch model back to gpt-5.2-codex and try again.
6. Observe: still laggy/unusable; ACP keeps retrying session/resume for the same sessionId and fails with “no rollout found…”, restarting the process repeatedly.

Notes: once triggered, the issue persists across attempts because the IDE keeps trying to resume the same session/thread.

What is the expected behavior?

1. AI Chat should remain responsive/usable after switching to gpt-5.3-codex, and switching back to gpt-5.2-codex should recover if needed.
2. If a persisted thread/session cannot be resumed (state missing/expired), the integration should gracefully start a new session (or return a recoverable error), instead of entering a restart loop with Internal error.

Additional information

1. Regression: before gpt-5.3-codex became available in the IDE integration, gpt-5.2-codex worked normally.
2. Codex CLI usage is still workable; the problem seems tied to IDE ACP integration + session resume state.
3. Attaching acp.log with timestamps around the failure helps (shows repeated session/resume + no rollout found + process kill/restart loop).

acp.log