[ACP] session/resume restores the agent but does not replay history; loadSession is missing, so ACP clients show a blank transcript

[apple-ouyang]

Summary. @deepseek-ai/dsh-acp advertises sessionCapabilities.resume and implements session/resume, which restores the persisted Agent (JSONL) for the model. It does not advertise loadSession, does not implement session/load, and session/resume returns only { configOptions } with zero historical session/update notifications. ACP clients that hydrate their transcript from a load-time replay (Paseo 0.8.0 is one) therefore open an existing dsh session as a blank chat, even though the session file still exists under ~/.dsh.

This is adjacent to #6322 (missing available_commands_update) but a separate gap: that one is discovery; this one is transcript restore.

ACP semantics. Current ACP distinguishes:

• session/load — restore and replay history as session/update so the client can render the transcript (loadSession: true).
• session/resume — reattach runtime state without replaying the transcript (sessionCapabilities.resume).

dsh implements the second and omits the first. Clients that only paint history from the load replay therefore show nothing.

Where. packages/acp/acp (@deepseek-ai/dsh-acp@0.1.5-rc.1):

• initialize returns agentCapabilities.sessionCapabilities = { close, list, resume } and does not set loadSession: true.
• RPC wiring registers session/new, session/list, session/resume, session/close, session/prompt — there is no session/load handler.
• resumeSession restores via ctx.agents.resume({ resumeSessionId }) (so the model can continue), then returns { configOptions }. It never walks the JSONL / projection to emit user_message / agent_message_chunk / agent_thought_chunk / tool_call updates.

Measured (dsh 0.1.5-rc.1, macOS, dsh --profile acp over stdio):

INIT agentCapabilities: {
  "mcpCapabilities": {"http": true},
  "promptCapabilities": {"image": true, "audio": false, "embeddedContext": false},
  "sessionCapabilities": {"close": {}, "list": {}, "resume": {}}
}
# loadSession is absent
SESSION/NEW returns { sessionId, configOptions } only
SESSION UPDATE KINDS during resume: []

Paseo 0.8.0 (packages/server/src/server/agent/providers/acp-agent.ts):

• UI timeline is filled from streamHistory().
• streamHistory() is populated only on the agentCapabilities.loadSession === true path, which calls session/load and records replayed session/update events.
• The sessionCapabilities.resume branch calls unstable_resumeSession and does not set historyPending, so the renderer gets an empty generator.

Result in the Paseo UI: reopen a dsh ACP thread → blank transcript. Sibling providers in the same app (Pi / Codex) that implement loadSession + replay still show history. dsh TUI / dsh web can still open the same JSONL, so the data is not lost — only the ACP client cannot see it.

Request.

1. Advertise loadSession: true from initialize.
2. Implement session/load: restore the persisted Agent (same as resume) and replay the stored conversation as session/update notifications (user / assistant / thought / tool_call / tool_call_update) before returning, matching Gemini CLI / Goose ACP.
3. Keep session/resume as a no-replay reattach, per current ACP spec.

That unblocks every ACP client that hydrates from load replay (Paseo, Zed-style hosts, agent-shell) without changing TUI/web.

[wxhxn041009]

Supplementary evidence: the replay projectors already exist; the missing piece is a call site.

Confirming the same gap on Windows 11 + Zed (custom ACP agent entry, dsh --profile acp), dsh 0.1.5-rc.2:

INIT agentCapabilities:
  sessionCapabilities: { close, list, resume }   // loadSession absent
SESSION/RESUME → { configOptions } only; 0 session/update frames

Two details that may help whoever picks this up:

1. The projection layer is already implemented. In @deepseek-ai/dsh-acp@0.1.5-rc.2 the log → ACP update converters exist as module-level functions, not as something that needs writing:

• assistantUpdates(ctx, session, event) → agent_message_chunk / agent_thought_chunk / usage_update
• toolCallUpdate(event) → tool_call
• toolResultUpdate(ctx, event) → tool_call_update
• usageUpdate(ctx, session, event)

2. The gap is the call site, not the mapping. onSessionEvent(session, event) is the only caller of those functions, and it is wired to the live-event path only. AcpSession.resume() restores context and returns { configOptions } without ever invoking a projector over stored events. So there is currently no code path that converts persisted entries into updates — which is consistent with the documented "log is restored without replaying old updates".

Client side is also ready: @agentclientprotocol/sdk@1.4.0 already defines the session_load method and the user_message_chunk update type, so replaying user turns needs no SDK change.

One discrepancy worth noting: this box reports promptCapabilities.image: false while your macOS run shows true. Likely attachment-store or route configuration rather than a version difference — happy to share the full initialize frame if that is useful.

[apple-ouyang]

Local patch, verified against Paseo 0.8.0.

CONTRIBUTING.md says this repo cannot take external PRs, so the source lives on a fork instead of a PR against master:

• Branch: https://github.com/apple-ouyang/deepseek-harness/tree/feat/acp-session-load-replay
• Diff vs master: apple-ouyang/deepseek-harness@master...feat/acp-session-load-replay
• Commit: f83bb9cfb feat(acp): advertise loadSession and replay JSONL history on session/load

What it does (packages/acp/acp):

1. initialize sets loadSession: true.
2. session/load restores the Agent the same way as session/resume, then replays JSONL as ACP session/update (user_message_chunk / agent_message_chunk / agent_thought_chunk / tool_call / tool_call_update). Plugin-injected user context is skipped.
3. session/resume is unchanged (no replay), matching current ACP semantics.

Measured (dsh --profile acp, stdio):

session/new → prompt "OK" → session/close → session/load
replay n=3 kinds { user_message_chunk: 1, agent_message_chunk: 1, usage_update: 1 }

Paseo then paints the previous turn. Reopening a dsh ACP thread in Paseo 0.8.0 shows history instead of a blank chat.

Happy to adjust the replay mapping if the team wants a different event → update table. Cherry-pick of that branch is the smallest landing.

[apple-ouyang]

ACP work on the fork (verified locally with Paseo). Same branch as the slash-command emit:

https://github.com/apple-ouyang/deepseek-harness/tree/feat/acp-session-load-replay

Commit	What
f83bb9cfb	loadSession: true + session/load JSONL replay (user_message_chunk / agent_message_chunk / tool calls). session/resume still no-replay.
5ad7ce301	available_commands_update from ctx.skills after new/resume/load
cce50ddc8	defer that notification until after session/new returns (Paseo binds sessionId then)

Paseo still needed to wait for the notification on generic ACP providers; that is a Paseo bug, not dsh:

• Generic ACP providers show an empty slash palette when commands arrive after session/new getpaseo/paseo#4759
• fix(acp): wait for available_commands_update on generic ACP providers getpaseo/paseo#4760

Cherry-pick of the fork branch is still the smallest landing on this repo (CONTRIBUTING.md blocks external PRs).