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Harden ACP session restoration

Fail safely until pending local history can be applied, preserve desired models across transient restore failures, discard cancelled trailing prompts, and avoid redundant fork parking. Co-Authored-By: siGit Code <noreply@sigit.si>

Seto Elkahfi committed Sep 17, 2026 at 08:40 UTC be210818e08d4d7459a1e57a1d22043cbecb075a

2 files changed +86 -17

src/backend.rs

+50 -8

@@ -221,12 +221,18 @@ impl LocalBackend {

221 }

222 }

223

224 - async fn apply_pending_history(&self) {

225 - if self.engine.info().await.status != EngineStatus::Ready {

226 - return;

224 + async fn apply_pending_history(&self) -> Result<(), BackendError> {

225 + if self.pending_history.lock().await.is_none() {

226 + return Ok(());

227 + }

228 + let status = self.engine.info().await.status;

229 + if status != EngineStatus::Ready {

230 + return Err(format!(

231 + "on-device model is not ready to restore session history (status: {status})"

232 + ));

233 }

234 let Some(history) = self.pending_history.lock().await.take() else {

229 - return;

235 + return Ok(());

236 };

237

238 self.engine.clear_history().await;

@@ -248,6 +254,7 @@ impl LocalBackend {

254 };

255 self.engine.push_history(message).await;

256 }

257 + Ok(())

258 }

259 }

260

@@ -270,7 +277,7 @@ impl InferenceBackend for LocalBackend {

277 tools: &[ToolSpec],

278 sink: Option<&TokenSink>,

279 ) -> Result<TurnResult, BackendError> {

273 - self.apply_pending_history().await;

280 + self.apply_pending_history().await?;

281 // onde's tool-aware path is non-streaming: it has to buffer the whole

282 // reply to detect tool calls. We can only stream when no tools are on

283 // offer (a plain answer), which is exactly the tools-disabled case.

@@ -301,7 +308,7 @@ impl InferenceBackend for LocalBackend {

308 allow_tool_calls: bool,

309 sink: Option<&TokenSink>,

310 ) -> Result<TurnResult, BackendError> {

304 - self.apply_pending_history().await;

311 + self.apply_pending_history().await?;

312 let onde_results: Vec<onde::inference::ToolResult> = results

313 .into_iter()

314 .map(|result| onde::inference::ToolResult {

@@ -365,11 +372,15 @@ impl InferenceBackend for LocalBackend {

372

373 async fn restore_history(&self, history: Vec<serde_json::Value>) {

374 *self.pending_history.lock().await = Some(history);

368 - self.apply_pending_history().await;

375 + if self.engine.info().await.status == EngineStatus::Ready {

376 + // Ready was just observed, so failure here can only mean a status

377 + // transition; keep the pending copy for the next inference call.

378 + let _ = self.apply_pending_history().await;

379 + }

380 }

381

382 async fn compact_history(&self, keep_last: usize) -> Result<(), BackendError> {

372 - self.apply_pending_history().await;

383 + self.apply_pending_history().await?;

384 let snapshot = self.engine.history().await;

385 // One plain (tool-free) inference round produces the summary. On error

386 // history is untouched — send_message only mutates it on success, and

@@ -1240,6 +1251,17 @@ pub fn carryover_history(snapshot: Vec<serde_json::Value>) -> Vec<serde_json::Va

1251 }

1252 }

1253

1254 + // Session switches happen only after the serialized turn has ended. A

1255 + // trailing user message therefore belongs to a cancelled/failed inference

1256 + // request with no answer; carrying it would make the next activation feed

1257 + // the model an orphaned prompt that the client considers cancelled.

1258 + while carried

1259 + .last()

1260 + .is_some_and(|message| message["role"] == "user")

1261 + {

1262 + carried.pop();

1263 + }

1264 +

1265 carried

1266 }

1267

@@ -1433,6 +1455,19 @@ mod tests {

1455 assert_eq!(carried[1]["content"], "hi");

1456 }

1457

1458 + #[test]

1459 + fn carryover_drops_a_cancelled_trailing_user_message() {

1460 + let carried = carryover_history(vec![

1461 + serde_json::json!({ "role": "system", "content": "prompt" }),

1462 + serde_json::json!({ "role": "user", "content": "completed question" }),

1463 + serde_json::json!({ "role": "assistant", "content": "completed answer" }),

1464 + serde_json::json!({ "role": "user", "content": "cancelled question" }),

1465 + ]);

1466 +

1467 + assert_eq!(carried.len(), 2, "{carried:#?}");

1468 + assert_eq!(carried.last().unwrap()["content"], "completed answer");

1469 + }

1470 +

1471 #[test]

1472 fn carryover_strips_a_tool_call_that_never_got_a_result() {

1473 // Switching mid-turn: the assistant asked for a tool, nothing answered.

@@ -1613,6 +1648,13 @@ mod tests {

1648 .contains("thread A")),

1649 "restore_history must replace, not append to, the local engine: {restored:#?}"

1650 );

1651 +

1652 + let error = backend

1653 + .send_message_with_tools("must not run", &[], None)

1654 + .await

1655 + .unwrap_err();

1656 + assert!(error.contains("not ready"), "{error}");

1657 + assert_eq!(backend.history_snapshot().await, restored);

1658 }

1659

1660 /// Minimal scripted OpenAI-compatible endpoint: accepts one HTTP request on

src/main.rs

+36 -9

@@ -1526,7 +1526,6 @@ impl SiGitAgent {

1526 let snapshot = self.backend.lock().await.history_snapshot().await;

1527 if let Some(state) = self.sessions.lock().unwrap().get_mut(&active) {

1528 state.conversation = backend::carryover_history(snapshot);

1529 - state.model_id = self.current_model.lock().unwrap().model_id.clone();

1529 }

1530 }

1531

@@ -1539,8 +1538,15 @@ impl SiGitAgent {

1538 /// replaces its parked state (notably `session/load`); every caller holds

1539 /// `turn_lock`, so parking and replacement cannot race another turn.

1540 async fn open_session(&self, session_id: &SessionId, state: SessionState) {

1542 - let key = session_id.to_string();

1541 self.park_active_session().await;

1542 + self.open_parked_session(session_id, state).await;

1543 + }

1544 +

1545 + /// Install a new session after the caller has already parked the live one.

1546 + /// Fork needs this split so it can read an up-to-date source state without

1547 + /// making `open_session` snapshot the same live backend a second time.

1548 + async fn open_parked_session(&self, session_id: &SessionId, state: SessionState) {

1549 + let key = session_id.to_string();

1550 self.sessions

1551 .lock()

1552 .unwrap()

@@ -1643,7 +1649,6 @@ impl SiGitAgent {

1649 log::warn!(

1650 "session({key}): cannot restore cloud model without a signed-in account"

1651 );

1646 - self.record_active_model();

1652 }

1653 return;

1654 }

@@ -1655,7 +1660,6 @@ impl SiGitAgent {

1660 }

1661 Err(error) => {

1662 log::warn!("session({key}): could not restore model {target}: {error}");

1658 - self.record_active_model();

1663 }

1664 }

1665 }

@@ -1702,9 +1706,6 @@ impl SiGitAgent {

1706 /// wrong project is worse than one that isn't listed at all.

1707 async fn persist_session(&self, session_id: &SessionId, snapshot: &[serde_json::Value]) {

1708 let key = session_id.to_string();

1705 - if self.active_session.lock().unwrap().as_deref() == Some(key.as_str()) {

1706 - self.record_active_model();

1707 - }

1709 if let Err(error) = session_store::save(&key, snapshot) {

1710 log::warn!("session({session_id}) save failed: {error}");

1711 return;

@@ -1881,7 +1882,7 @@ impl SiGitAgent {

1882 cx.send_notification(SessionNotification::new(new_id.clone(), update))

1883 .ok();

1884 }

1884 - self.open_session(&new_id, state).await;

1885 + self.open_parked_session(&new_id, state).await;

1886 log::info!(

1887 "fork_session: copied {copied} message(s), replayed {replayed} update(s) for {new_id}"

1888 );

@@ -2414,6 +2415,9 @@ impl SiGitAgent {

2415 tool_call_id: tc.id.clone(),

2416 content: output,

2417 });

2418 + // Cancellation cannot preempt a synchronous foreground tool:

2419 + // it is observed here as soon as that tool exits. Background

2420 + // commands remain cancellable through `kill_command`.

2421 if cancellation.cancelled.load(Ordering::Acquire) {

2422 for pending in &result.tool_calls[call_index + 1..] {

2423 tool_results.push(BackendToolResult {

@@ -2892,6 +2896,12 @@ impl SiGitAgent {

2896 "set_session_config_option: {} is already the active selection, skipping",

2897 current.display_name

2898 );

2899 + // A restored preferred model may have fallen back temporarily.

2900 + // Explicitly picking the live model adopts that fallback as the

2901 + // session's new preference even though no backend switch runs.

2902 + drop(current);

2903 + self.record_active_model();

2904 + let current = self.current_model.lock().unwrap();

2905 let config_options = build_config_options(&current, &args.session_id.to_string());

2906 return Ok(SetSessionConfigOptionResponse::new(config_options));

2907 }

@@ -4655,11 +4665,12 @@ mod tests {

4665 .join("\n")

4666 }

4667

4658 - /// Append a user message to the live conversation.

4668 + /// Append one completed exchange to the live conversation.

4669 async fn say(agent: &SiGitAgent, text: &str) {

4670 let backend = agent.backend.lock().await.clone();

4671 let mut history = backend.history_snapshot().await;

4672 history.push(serde_json::json!({ "role": "user", "content": text }));

4673 + history.push(serde_json::json!({ "role": "assistant", "content": "acknowledged" }));

4674 backend.restore_history(history).await;

4675 }

4676

@@ -4851,6 +4862,22 @@ mod tests {

4862 assert_eq!(sessions["thread-b"].model_id, "sigit-cloud:test-b");

4863 drop(sessions);

4864

4865 + // A transient restore failure may leave another model live, but parking

4866 + // must not erase the session's desired selection; its next activation

4867 + // should retry that preference.

4868 + agent

4869 + .sessions

4870 + .lock()

4871 + .unwrap()

4872 + .get_mut("thread-b")

4873 + .unwrap()

4874 + .model_id = "sigit-cloud:preferred".into();

4875 + agent.park_active_session().await;

4876 + assert_eq!(

4877 + agent.sessions.lock().unwrap()["thread-b"].model_id,

4878 + "sigit-cloud:preferred"

4879 + );

4880 +

4881 std::env::set_current_dir(prev_cwd).unwrap();

4882 tools::set_active_session(None);

4883 workspace::replace_additional_roots(Vec::new());