Implicit fork + resume for chat and agent sessions

[big-guy]

Summary

Opening a new agent surface — a Chat (json-claude) tab or an xterm Claude/Codex tab — now branches off or resumes a prior session implicitly, driven by what's already open in the worktree:

• A same-kind agent tab already open → fork from the active one (tip).
• No agent tabs of any kind open → resume the worktree's last known session.
• Otherwise → blank.

Grouping is by agent kind, not tab surface: Chat and xterm Claude share ~/.claude/projects, so they fork from / resume each other; Codex tabs form their own group. Fork uses each agent's native mechanism (Claude --fork-session, Codex fork <id>); the agent mints the new session id and we bind it to the tab via the existing hook/init discovery path.

Design

The crux is decoupling the tab id from the agent session id (they were forced equal). The tab id stays the stable slice/instance/React key; the on-disk session id lives in TerminalTab.sessionId (persisted, late-bindable for forks) and JsonClaudeInstance.claudeSessionId. This lets a forked session own its own transcript without breaking tab identity, and survives reload.

• session-birth.ts — pure, dependency-injected decision logic (resolveTabBirth / deriveSpawnSpec / agentKindOfTab) + SessionSpawnSpec.
• agents — AgentSpawnOpts.forkFromSessionId + AgentModule.listSessions; Claude/Codex buildSpawnArgs emit fork flags.
• json-claude-manager — spawn-spec drives --session-id/--resume/--fork-session; the forked session_id is captured from the init event onto the tab.
• index.ts — per-kind birthDeps via getAgent(kind); createChatTab/createAgentTab + jsonClaude:addChatTab / agent:addTab IPCs.
• panes-fsm — first-tab resume-last for chat and xterm (bypassed for prompted/teleport launches).
• renderer — addChatTab/addAgentTab backend methods; transient forkFromSessionId threaded through XTerminal.

Behavior note

Because grouping is cross-surface, a worktree with only an xterm Claude tab open → a new Chat now forks it (previously: blank). Intentional, and matches "fork when another Claude tab is open."

Testing

tsc -b, electron-vite build, and the full vitest suite (1464 tests) pass. New coverage: session-birth (chat+codex birth, cross-surface fork, resume excludes open ids), json-claude-manager (spawn-spec flags + fork id discovery), Claude/Codex buildSpawnArgs fork/resume/blank.

Caveat: Claude --fork-session was verified end-to-end (new session id minted, fresh self-consistent transcript, source untouched). The Codex fork path is covered by unit tests against the documented codex fork <id> syntax but not run live — worth a manual smoke before relying on Codex forking.

🤖 Generated with Claude Code