Feature: ephemeral / non-persisted sessions (align with Codex app-server ephemeral threads) #448

Summary

Please consider exposing an ephemeral / non-persisted session option over ACP, analogous to Codex app-server鈥檚 thread/start / thread/fork with ephemeral: true (in-memory only; no rollout path / not listed in durable thread history).

Today, ACP clients that use @agentclientprotocol/codex-acp can call session/new and keep a multi-turn context for the life of that session, but there is no first-class way to ask the Agent to skip writing session rollouts under CODEX_HOME (e.g. ~/.codex/sessions/). Client-side 鈥渄on鈥檛 persist UI history鈥?does not prevent Agent-side disk usage.

Motivation / use cases

ACP clients such as desktop editors (we build a Markdown editor + reader, Inkdown) want two modes:

1. Durable coding agent sessions 鈥?current ACP path (session/new + optional session/load / session/resume) is a great fit.
2. Lightweight, disposable chats 鈥?e.g. selection translate, 鈥渉elp me draft this annotation鈥? short Q&A over a snippet. These should:
   • remember context only for the active short chat
   • not accumulate transcripts on disk after the UI closes
   • avoid polluting the user鈥檚 Codex session list / ~/.codex/sessions

Codex app-server already documents this for its own protocol (ephemeral: true 鈫?in-memory thread, thread.path is null). Clients on ACP cannot call app-server methods on the same stdio pipe; bridging would mean a second transport. Aligning ACP/codex-acp with the existing ephemeral semantics would let editors stay on one Agent process model.

What we鈥檙e asking for

Any of the following would work; (A) is preferred if protocol evolution is acceptable:

A. Protocol + adapter (preferred)

• Advertise a capability, e.g. sessionCapabilities.ephemeral (name TBD).
• Allow session/new (and maybe session/resume/fork if applicable) to pass ephemeral: true (or equivalent).
• When set: do not persist rollouts / durable session metadata; closing the session drops memory (optionally via session/close when advertised).

B. Adapter-only mapping (faster if protocol change is hard)

• Document env/CLI flags for codex-acp that force Codex ephemeral / no-session-persist behavior for new sessions (or for the whole process).
• Ideally still expose something queryable in initialize so Clients know whether 鈥渘o disk transcript鈥?is actually guaranteed.

C. Clarification

If the intentional design is 鈥淎CP always uses durable Codex sessions; use app-server for ephemeral鈥? please document that clearly so Client authors don鈥檛 assume session/new is disposable.

Non-goals

• Replacing durable session/load / resume for coding workflows.
• Asking ACP to proxy raw app-server thread/* RPCs on the same connection.

References

• Codex app-server: thread/start / thread/fork with ephemeral: true (in-memory temporary thread; no durable path).
• Codex CLI: codex exec --ephemeral (skip writing session rollout files) 鈥?similar product intent for throwaway runs.
• ACP session setup: https://agentclientprotocol.com/protocol/session-setup

Environment

• Client: Electron ACP Client (Inkdown)
• Agent: @agentclientprotocol/codex-acp (default npm package)

Thanks for considering this 鈥?happy to refine the capability shape if you prefer a different name or placement under initialize / session/new.