Fix branch selector for single message and --no-session mode

- Allow branch selector to open with single user message (changed <= 1 to === 0 check)
- Support in-memory branching for --no-session mode (no files created)
- Add isEnabled() getter to SessionManager
- Update sessionFile getter to return null when sessions disabled
- Update SessionSwitchEvent types to allow null session files
- Add branching tests for single message and --no-session scenarios

fixes #163

packages/coding-agent/src/core/agent-session.ts

@@ -76,7 +76,7 @@ export interface CompactionResult {
 
 /** Session statistics for /session command */
 export interface SessionStats {
-	sessionFile: string;
+	sessionFile: string | null;
 	sessionId: string;
 	userMessages: number;
 	assistantMessages: number;
@@ -320,9 +320,9 @@ export class AgentSession {
 		return this.agent.getQueueMode();
 	}
 
-	/** Current session file path */
-	get sessionFile(): string {
-		return this.sessionManager.getSessionFile();
+	/** Current session file path, or null if sessions are disabled */
+	get sessionFile(): string | null {
+		return this.sessionManager.isEnabled() ? this.sessionManager.getSessionFile() : null;
 	}
 
 	/** Current session ID */
@@ -966,11 +966,15 @@ export class AgentSession {
 			return { selectedText, skipped: true };
 		}
 
-		// Create branched session
+		// Create branched session (returns null in --no-session mode)
 		const newSessionFile = this.sessionManager.createBranchedSessionFromEntries(entries, entryIndex);
-		this.sessionManager.setSessionFile(newSessionFile);
 
-		// Emit session_switch event
+		// Update session file if we have one (file-based mode)
+		if (newSessionFile !== null) {
+			this.sessionManager.setSessionFile(newSessionFile);
+		}
+
+		// Emit session_switch event (in --no-session mode, both files are null)
 		if (this._hookRunner) {
 			this._hookRunner.setSessionFile(newSessionFile);
 			await this._hookRunner.emit({
@@ -981,7 +985,7 @@ export class AgentSession {
 			});
 		}
 
-		// Reload
+		// Reload messages from entries (works for both file and in-memory mode)
 		const loaded = loadSessionFromEntries(this.sessionManager.loadEntries());
 		this.agent.replaceMessages(loaded.messages);
 

packages/coding-agent/src/core/hooks/types.ts

@@ -86,10 +86,10 @@ export interface SessionStartEvent {
  */
 export interface SessionSwitchEvent {
 	type: "session_switch";
-	/** New session file path */
-	newSessionFile: string;
-	/** Previous session file path */
-	previousSessionFile: string;
+	/** New session file path, or null in --no-session mode */
+	newSessionFile: string | null;
+	/** Previous session file path, or null in --no-session mode */
+	previousSessionFile: string | null;
 	/** Reason for the switch */
 	reason: "branch" | "switch";
 }

packages/coding-agent/src/core/session-manager.ts

@@ -231,6 +231,11 @@ export class SessionManager {
 		this.enabled = false;
 	}
 
+	/** Check if session persistence is enabled */
+	isEnabled(): boolean {
+		return this.enabled;
+	}
+
 	private getSessionDirectory(): string {
 		const cwd = process.cwd();
 		// Replace all path separators and colons (for Windows drive letters) with dashes
@@ -637,32 +642,43 @@ export class SessionManager {
 	/**
 	 * Create a branched session from session entries up to (but not including) a specific entry index.
 	 * This preserves compaction events and all entry types.
-	 * Returns the new session file path.
+	 * Returns the new session file path, or null if in --no-session mode (in-memory only).
 	 */
-	createBranchedSessionFromEntries(entries: SessionEntry[], branchBeforeIndex: number): string {
+	createBranchedSessionFromEntries(entries: SessionEntry[], branchBeforeIndex: number): string | null {
 		const newSessionId = uuidv4();
 		const timestamp = new Date().toISOString().replace(/[:.]/g, "-");
 		const newSessionFile = join(this.sessionDir, `${timestamp}_${newSessionId}.jsonl`);
 
-		// Copy all entries up to (but not including) the branch point
+		// Build new entries list (up to but not including branch point)
+		const newEntries: SessionEntry[] = [];
 		for (let i = 0; i < branchBeforeIndex; i++) {
 			const entry = entries[i];
 
 			if (entry.type === "session") {
 				// Rewrite session header with new ID and branchedFrom
-				const newHeader: SessionHeader = {
+				newEntries.push({
 					...entry,
 					id: newSessionId,
 					timestamp: new Date().toISOString(),
-					branchedFrom: this.sessionFile,
-				};
-				appendFileSync(newSessionFile, JSON.stringify(newHeader) + "\n");
+					branchedFrom: this.enabled ? this.sessionFile : undefined,
+				});
 			} else {
 				// Copy other entries as-is
-				appendFileSync(newSessionFile, JSON.stringify(entry) + "\n");
+				newEntries.push(entry);
 			}
 		}
 
-		return newSessionFile;
+		if (this.enabled) {
+			// Write to file
+			for (const entry of newEntries) {
+				appendFileSync(newSessionFile, JSON.stringify(entry) + "\n");
+			}
+			return newSessionFile;
+		} else {
+			// In-memory mode: replace inMemoryEntries, no file created
+			this.inMemoryEntries = newEntries;
+			this.sessionId = newSessionId;
+			return null;
+		}
 	}
 }