fix: compaction keeps original ID, forks old history to archive, chains via compactedFrom

Reworked compaction to match the confirmed design:
- The compacted conversation KEEPS its original ID (messaging between
  agents is unaffected — the ID never changes)
- The old full history is forked to a new archive conversation (new UUID)
- The archive inherits the source's compactedFrom, creating a chain:
  A → Y → X (walk compactedFrom backward)
- A's history is replaced with [summary + recent N]
- A.compactedFrom = archive ID

forkHistory: inherit compactedFrom from source (not set to sourceId),
so archives chain backward to previous archives.

FE: no tab switching needed — the ID doesn't change. Just reload history.

6 files changed, 24 insertions, 17 deletions

diff --git a/packages/conversation-store/src/store.ts b/packages/conversation-store/src/store.ts
index d713de3..700be1e 100644
--- a/packages/conversation-store/src/store.ts
+++ b/packages/conversation-store/src/store.ts
@@ -113,7 +113,7 @@ export interface ConversationStore {
 	 * Set the `compactedFrom` field on a conversation's metadata, pointing to
 	 * the archive conversation that holds the pre-compaction history.
 	 */
-	readonly setCompactedFrom: (conversationId: string, archiveId: string) => Promise<void>;
+	readonly setCompactedFrom: (conversationId: string, newConversationId: string) => Promise<void>;
 }
 
 export const conversationStoreHandle = defineService<ConversationStore>("conversation-store/store");
@@ -592,7 +592,9 @@ export function createConversationStore(
 			}
 			await storage.set(seqKey(targetId), String(Math.max(seq - 1, 0)));
 
-			// Copy metadata with archive title + closed status + compactedFrom.
+			// Copy metadata with archive title + closed status.
+			// Inherit compactedFrom from the source so archives chain:
+			// A → Y → X (each archive points to the previous one).
 			const metaRaw = await storage.get(metaKey(sourceId));
 			if (metaRaw !== null) {
 				const existing = parseMetaRow(metaRaw);
@@ -602,7 +604,9 @@ export function createConversationStore(
 						lastActivityAt: existing.lastActivityAt,
 						title: `Archive: ${existing.title}`,
 						status: "closed",
-						compactedFrom: sourceId,
+						...(existing.compactedFrom !== undefined
+							? { compactedFrom: existing.compactedFrom }
+							: {}),
 					};
 					await storage.set(metaKey(targetId), JSON.stringify(row));
 				}
@@ -630,7 +634,7 @@ export function createConversationStore(
 			}
 		},
 
-		async setCompactedFrom(conversationId, archiveId) {
+		async setCompactedFrom(conversationId, newConversationId) {
 			const raw = await storage.get(metaKey(conversationId));
 			const existing = raw !== null ? parseMetaRow(raw) : null;
 			const ts = now();
@@ -642,7 +646,7 @@ export function createConversationStore(
 			};
 			await storage.set(
 				metaKey(conversationId),
-				JSON.stringify({ ...row, compactedFrom: archiveId }),
+				JSON.stringify({ ...row, compactedFrom: newConversationId }),
 			);
 		},
 	};
diff --git a/packages/session-orchestrator/src/orchestrator.ts b/packages/session-orchestrator/src/orchestrator.ts
index 21c068c..b46ecc1 100644
--- a/packages/session-orchestrator/src/orchestrator.ts
+++ b/packages/session-orchestrator/src/orchestrator.ts
@@ -132,7 +132,7 @@ export const conversationStatusChanged: EventHookDescriptor<ConversationStatusCh
 /** Payload for the conversationCompacted bus event. */
 export interface ConversationCompactedPayload {
 	readonly conversationId: string;
-	readonly archiveId: string;
+	readonly newConversationId: string;
 	readonly messagesSummarized: number;
 	readonly messagesKept: number;
 }
@@ -802,8 +802,11 @@ export function createCompactionService(
 				return { error: "model produced empty summary" };
 			}
 
-			// Non-destructive: fork the full pre-compaction history to an
-			// archive conversation before replacing it.
+			// Non-destructive: fork the full pre-compaction history to a new
+			// archive conversation. The original conversation keeps its ID
+			// (so messaging between agents still works) and gets the compacted
+			// content. The archive inherits the original's compactedFrom,
+			// creating a chain: A → Y → X → ...
 			const archiveId = crypto.randomUUID();
 			await deps.conversationStore.forkHistory(conversationId, archiveId);
 
@@ -823,14 +826,14 @@ export function createCompactionService(
 
 			const result: CompactionResult = {
 				summary,
-				archiveId,
+				newConversationId: archiveId,
 				messagesSummarized: toSummarize.length,
 				messagesKept: toKeep.length,
 			};
 
 			deps.emit(conversationCompacted, {
 				conversationId,
-				archiveId,
+				newConversationId: archiveId,
 				messagesSummarized: toSummarize.length,
 				messagesKept: toKeep.length,
 			});
diff --git a/packages/transport-contract/src/index.ts b/packages/transport-contract/src/index.ts
index 2316c96..5745672 100644
--- a/packages/transport-contract/src/index.ts
+++ b/packages/transport-contract/src/index.ts
@@ -521,7 +521,7 @@ export interface ConversationStatusChangedMessage {
 export interface ConversationCompactedMessage {
 	readonly type: "conversation.compacted";
 	readonly conversationId: string;
-	readonly archiveId: string;
+	readonly newConversationId: string;
 	readonly messagesSummarized: number;
 	readonly messagesKept: number;
 }
@@ -577,7 +577,7 @@ export interface TitleResponse {
  */
 export interface CompactResponse {
 	readonly conversationId: string;
-	readonly archiveId: string;
+	readonly newConversationId: string;
 	readonly messagesSummarized: number;
 	readonly messagesKept: number;
 }
diff --git a/packages/transport-http/src/app.ts b/packages/transport-http/src/app.ts
index 233e30e..64e46fd 100644
--- a/packages/transport-http/src/app.ts
+++ b/packages/transport-http/src/app.ts
@@ -725,7 +725,7 @@ export function createApp(opts: CreateServerOptions): Hono {
 
 		const response: CompactResponse = {
 			conversationId,
-			archiveId: result.archiveId,
+			newConversationId: result.newConversationId,
 			messagesSummarized: result.messagesSummarized,
 			messagesKept: result.messagesKept,
 		};
diff --git a/packages/transport-ws/src/extension.ts b/packages/transport-ws/src/extension.ts
index abc69e7..b42f434 100644
--- a/packages/transport-ws/src/extension.ts
+++ b/packages/transport-ws/src/extension.ts
@@ -153,11 +153,11 @@ export function createTransportWsExtension(): Extension {
 			disposers.push(
 				host.on(
 					conversationCompacted,
-					({ conversationId, archiveId, messagesSummarized, messagesKept }) => {
+					({ conversationId, newConversationId, messagesSummarized, messagesKept }) => {
 						broadcast({
 							type: "conversation.compacted",
 							conversationId,
-							archiveId,
+							newConversationId,
 							messagesSummarized,
 							messagesKept,
 						});
diff --git a/packages/wire/src/index.ts b/packages/wire/src/index.ts
index 8c85f89..4ab8825 100644
--- a/packages/wire/src/index.ts
+++ b/packages/wire/src/index.ts
@@ -535,12 +535,12 @@ export interface ConversationMeta {
  * Result of a compaction operation. `summary` is the text the model produced;
  * `messagesKept` is how many recent messages were retained after the summary;
  * `messagesSummarized` is how many old messages were replaced by the summary.
- * `archiveId` is the ID of the new conversation that holds the full
+ * `newConversationId` is the ID of the new conversation that holds the full
  * pre-compaction history (non-destructive — the original history is preserved).
  */
 export interface CompactionResult {
 	readonly summary: string;
-	readonly archiveId: string;
+	readonly newConversationId: string;
 	readonly messagesSummarized: number;
 	readonly messagesKept: number;
 }