Add provider handoff compaction on model switches

[juliusmarminge]

Summary

• Add provider switch handoff flow so mid-thread model changes compact the active thread, restart the target provider, and prepend a handoff summary to the next user turn.
• Implement compaction support in both Codex and Claude provider adapters, plus server-side session lifecycle handling for thread/compact/start.
• Tighten runtime ingestion and session bookkeeping so events from a replaced provider session do not leak into the active thread state.
• Update web chat/session logic to preserve draft behavior and render handoff context cleanly.
• Add coverage for provider switching, compaction, runtime ingestion, and UI-facing state transitions.

Testing

• bun fmt
• bun lint
• bun typecheck
• bun run test
• Added/updated tests for provider adapter compaction, provider command handoff, runtime ingestion guards, and chat/session draft behavior.

---

Note

Medium Risk
Changes provider/session lifecycle by allowing mid-thread provider switching with compaction and handoff, which can impact routing, state correctness, and event ingestion across concurrent sessions.

Overview
Enables mid-thread provider switching by compacting the active provider thread, starting a new session on the target provider, and prepending a <provider_handoff> summary to the next user turn.

Adds a compactThread capability across adapters and ProviderService, plus stopSessionForProvider to cleanly terminate the replaced provider session without breaking the new active binding; updates runtime ingestion guards to ignore lifecycle/error events from non-active providers.

Updates web chat logic to keep provider/model selection consistent during handoff and render provider.handoff.* activities as dedicated transition banners, with expanded test coverage for the new handoff/compaction flow.

^{Reviewed by Cursor Bugbot for commit 84b54f7. Bugbot is set up for automated code reviews on this repo. Configure here.}

Note

Add provider handoff compaction when switching AI providers mid-thread

• When a thread switches providers mid-conversation, the orchestration layer compacts the current thread context via the source provider, starts a new session for the target provider, and sends the first turn with a <provider_handoff> preamble containing the compaction summary.
• Both Claude and Codex adapters gain a compactThread operation: Claude enqueues /compact and awaits a PostCompact hook callback (60s timeout); Codex triggers thread/compact/start and waits for a thread/compacted notification.
• ProviderService gains compactThread and stopSessionForProvider methods to support provider-specific session teardown without disturbing other active bindings.
• Provider handoff progress is emitted as provider.handoff.compacting and provider.handoff.completed activities, surfaced in the work log and rendered in the chat UI as a provider-to-provider transition pill with animated state indicators.
• deriveEffectiveComposerModelState is fixed to ignore model selections from a different provider than the currently selected one, preventing cross-provider model bleedthrough.
• Behavioral Change: lifecycle events from providers that don't match the thread's active session provider are now silently ignored under STRICT_PROVIDER_LIFECYCLE_GUARD.

^{Macroscope summarized 84b54f7.}

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[cursor]

Cursor Bugbot has reviewed your changes and found 2 potential issues.

Bugbot Autofix prepared fixes for both issues found in the latest run.

• ✅ Fixed: Unguarded cache clear on mismatched provider session exit
  • Added shouldApplyThreadLifecycle guard to the clearTurnStateForSession call so it only runs when the exiting provider matches the active session, preventing a replaced provider's exit from wiping the new provider's turn caches.
• ✅ Fixed: Broad looksLikeCompactionItem matches any item with summary
  • Removed the overly broad typeof record.summary === "string" fallback from looksLikeCompactionItem, keeping only the specific compact_summary and compactSummary checks that reliably identify compaction items.

Or push these changes by commenting:

@cursor push d4c9511f80

Preview (d4c9511f80)

diff --git a/apps/server/src/orchestration/Layers/ProviderRuntimeIngestion.ts b/apps/server/src/orchestration/Layers/ProviderRuntimeIngestion.ts
--- a/apps/server/src/orchestration/Layers/ProviderRuntimeIngestion.ts
+++ b/apps/server/src/orchestration/Layers/ProviderRuntimeIngestion.ts
@@ -1147,7 +1147,7 @@
       }
     }
 
-    if (event.type === "session.exited") {
+    if (event.type === "session.exited" && shouldApplyThreadLifecycle) {
       yield* clearTurnStateForSession(thread.id);
     }
 

diff --git a/apps/server/src/provider/handoffSummary.ts b/apps/server/src/provider/handoffSummary.ts
--- a/apps/server/src/provider/handoffSummary.ts
+++ b/apps/server/src/provider/handoffSummary.ts
@@ -72,11 +72,7 @@
   if (COMPACTION_TYPE_SNIPPETS.some((snippet) => type.includes(snippet))) {
     return true;
   }
-  return (
-    typeof record.compact_summary === "string" ||
-    typeof record.compactSummary === "string" ||
-    typeof record.summary === "string"
-  );
+  return typeof record.compact_summary === "string" || typeof record.compactSummary === "string";
 }
 
 export function extractCompactionSummaryFromSnapshot(snapshot: {

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^{Reviewed by Cursor Bugbot for commit 84b54f7. Configure here.}

[cursor]

Unguarded cache clear on mismatched provider session exit

High Severity

After a provider switch, the old provider's session.exited event is correctly blocked from updating thread session state by the new guard at lines 895–904 (via shouldApplyThreadLifecycle). However, clearTurnStateForSession at line 1150 runs unconditionally for any session.exited event, regardless of whether the event's provider matches the active session. When the replaced provider exits, this wipes the turn message/proposed-plan caches that may already belong to the new provider's in-progress turn.

Additional Locations (1)

• apps/server/src/orchestration/Layers/ProviderRuntimeIngestion.ts#L894-L904

 

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[cursor]

Broad looksLikeCompactionItem matches any item with summary

Medium Severity

looksLikeCompactionItem returns true for any object that has a summary property of type string (line 78). Since many regular thread items (e.g. tool results, assistant messages) commonly carry a summary field, this heuristic is overly broad and can misidentify non-compaction items as compaction items, extracting incorrect text as the handoff summary.

 

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[macroscopeapp]

Approvability

Verdict: Needs human review

This PR introduces a new provider handoff feature with significant runtime behavior changes across orchestration, provider adapters, and UI. Two unresolved review comments identify potential bugs: a high-severity cache clearing issue during provider switches and a medium-severity heuristic issue in compaction summary extraction.

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