fix: preserve fork start config before stopping session (#278)

Resolve the fork start configuration before stopping the active session
so forks can be created from either the source session or current
config. Reuse the resolved config when starting the fork and store it
for the new session.

sdk/apps/code/sidecar/chat-session.ts

@@ -57,6 +57,13 @@ function readSessionMetadataTitle(sessionId: string): string | undefined {
 	return typeof title === "string" ? title.trim() || undefined : undefined;
 }
 
+function readSessionMetadata(sessionId: string): JsonRecord | undefined {
+	const manifest = readSessionManifest(sessionId);
+	return manifest?.metadata && typeof manifest.metadata === "object"
+		? (manifest.metadata as JsonRecord)
+		: undefined;
+}
+
 function derivePromptFromMessages(messages: unknown[]): string {
 	for (const msg of messages) {
 		if (!msg || typeof msg !== "object") continue;
@@ -598,6 +605,112 @@ async function handleAbort(
 	return { sessionId, ok: true };
 }
 
+async function handleFork(
+	ctx: SidecarContext,
+	request: ChatSessionCommandRequest,
+): Promise<unknown> {
+	const sourceSessionId = request.sessionId?.trim();
+	if (!sourceSessionId) throw new Error("sessionId is required");
+	const manager = getSessionManager(ctx);
+	const sourceMessages =
+		readPersistedChatMessages(sourceSessionId) ??
+		ctx.liveSessions.get(sourceSessionId)?.messages;
+	if (!sourceMessages?.length) {
+		throw new Error(`No messages found for session ${sourceSessionId}`);
+	}
+
+	const sourceSession = await manager.get(sourceSessionId);
+	const sourceMetadata =
+		(sourceSession?.metadata && typeof sourceSession.metadata === "object"
+			? (sourceSession.metadata as JsonRecord)
+			: undefined) ?? readSessionMetadata(sourceSessionId);
+	const liveConfig = ctx.liveSessions.get(sourceSessionId)?.config;
+	const forkConfig: JsonRecord = {
+		...(liveConfig ?? {}),
+		...(request.config ?? {}),
+		sessionId: undefined,
+		provider:
+			sourceSession?.provider ||
+			liveConfig?.provider ||
+			request.config?.provider ||
+			request.config?.providerId ||
+			"",
+		model:
+			sourceSession?.model ||
+			liveConfig?.model ||
+			request.config?.model ||
+			request.config?.modelId ||
+			"",
+		cwd:
+			sourceSession?.cwd ||
+			sourceSession?.workspaceRoot ||
+			liveConfig?.cwd ||
+			request.config?.cwd ||
+			request.config?.workspaceRoot ||
+			request.config?.workspace_root ||
+			"",
+		workspaceRoot:
+			sourceSession?.workspaceRoot ||
+			sourceSession?.cwd ||
+			liveConfig?.workspaceRoot ||
+			request.config?.workspaceRoot ||
+			request.config?.workspace_root ||
+			request.config?.cwd ||
+			"",
+	};
+	const checkpointMetadata =
+		sourceMetadata?.checkpoint !== undefined
+			? { checkpoints: sourceMetadata.checkpoint }
+			: {};
+	const forkMetadata: JsonRecord = {
+		...(sourceMetadata ?? {}),
+		fork: {
+			forkedFromSessionId: sourceSessionId,
+			forkedAt: new Date().toISOString(),
+			source: sourceSession?.source ?? "desktop",
+			...checkpointMetadata,
+		},
+	};
+	const systemPrompt = await resolveSystemPrompt(forkConfig);
+	const startResult = await manager.start({
+		...splitCoreSessionConfig(
+			buildCoreSessionConfig({
+				...forkConfig,
+				systemPrompt,
+				initialMessages: sourceMessages,
+			}) as any,
+		),
+		source: SessionSource.DESKTOP,
+		interactive: true,
+		initialMessages: sourceMessages as any[],
+		sessionMetadata: forkMetadata,
+		toolPolicies: resolveToolPolicies(forkConfig),
+	});
+	const newSessionId = startResult.sessionId;
+	ctx.liveSessions.delete(sourceSessionId);
+	ctx.liveSessions.set(
+		newSessionId,
+		createLiveSession(forkConfig, {
+			messages: sourceMessages,
+			prompt: derivePromptFromMessages(sourceMessages),
+			title: readSessionMetadataTitle(sourceSessionId),
+			status: "idle",
+		}),
+	);
+	sendPromptsInQueueSnapshot(ctx, sourceSessionId);
+	sendPromptsInQueueSnapshot(ctx, newSessionId);
+	let messages: unknown[] = sourceMessages;
+	try {
+		const read = await manager.readMessages(newSessionId);
+		if (read?.length > 0) messages = read;
+	} catch {}
+	return {
+		sessionId: newSessionId,
+		forkedFromSessionId: sourceSessionId,
+		messages,
+	};
+}
+
 async function handleReset(
 	ctx: SidecarContext,
 	request: ChatSessionCommandRequest,
@@ -787,6 +900,7 @@ const ACTION_HANDLERS: Record<
 	send: handleSend,
 	stop: handleStop,
 	abort: handleAbort,
+	fork: handleFork,
 	reset: handleReset,
 	restore_checkpoint: handleRestoreCheckpoint,
 	pending_prompts: handlePendingPrompts,

sdk/apps/code/sidecar/types.ts

@@ -14,6 +14,7 @@ export type ChatSessionCommandRequest = {
 		| "send"
 		| "stop"
 		| "abort"
+		| "fork"
 		| "reset"
 		| "restore_checkpoint"
 		| "pending_prompts"