Module engine 

Core engine (issue #5261).

Move, don’t rewrite: the turn loop, session, thread manager, the TUI’s run_event_loop, and the chat client’s request-building are all destined for this crate. Only request-building and fragments have moved so far. The turn loop still lives in crates/tui/src/core/engine/turn_loop.rs and is what every interactive and headless turn runs today; this module is the boundary that move lands against, not the current owner of turn execution. The TUI crate depends on core, not the reverse.

Approved crates that the engine needs are already in crates/core’s Cargo.toml: config, execpolicy, protocol, state, tools, mcp, hooks, agent. Things that stay in the TUI (ratatui, crossterm, prompt_zones rendering) are not imported here; the engine is terminal-free so it can start a session with no TUI attached.

This module is intentionally small on this first cut: it formalizes the ThreadId/SessionId boundary, the Op-in / EventMsg-out channels in crates/protocol, the Journal leaf, and the Thread-owned headless spawn that TUI and codewhale exec both go through. The full turn loop, guards (StuckGuard, ReadRepeatGuard, ToolCallBudget), stream retry budget, and the four-way RuntimeThreadManager split live in the thread/ submodules so follow-ons (#5262, #5263, #5264) have a place to land without another boundary move.

Back-compat: persisted state.json / threads shape is unchanged.

Modules

thread

RuntimeThreadManager split per #3313 (issue #5261).

Structs

Engine

EngineConfig

EngineHandle

Handle to communicate with the core engine via the Op-in / EventMsg-out channels. The TUI’s EngineHandle and the headless exec both hold this type; handle.steer, cancel, approve_tool_call etc are the same code path in both modes so crates/execpolicy stays the authority identically.

Enums

CancelReason

Reason the active turn was cancelled.

Functions

spawn_engine

Spawn the engine in a background task (mirrors spawn_engine in the old crates/tui/src/core/engine.rs). Returns the handle that TUI, CLI exec, app-server, and tests all share — one Op-in / EventMsg-out API.

spawn_headless_thread

Start a headless session and expose its shared operation channel.

spawn_supervised

Spawn with supervision (mirrors spawn_supervised).