diff --git a/agent/commands.go b/agent/commands.go
index 3f59448..e7322bb 100644
--- a/agent/commands.go
+++ b/agent/commands.go
@@ -12,7 +12,7 @@ import (
 
 )
 
-func (a *sessionHost) handleCommand(ctx context.Context, input string) bool {
+func (a *harness) handleCommand(ctx context.Context, input string) bool {
 	if !strings.HasPrefix(input, "/") {
 		return false
 	}
diff --git a/agent/core_test.go b/agent/core_test.go
index 26fb052..b4a9d28 100644
--- a/agent/core_test.go
+++ b/agent/core_test.go
@@ -83,7 +83,7 @@ func defaultBE() *mockBackend {
 
 // newCore builds a minimal *agent for tests, bypassing loadSystemPrompt
 // by directly setting Preamble on Runtime.
-func newCore(t *testing.T, be backend.Backend, hooks Hooks) *sessionHost {
+func newCore(t *testing.T, be backend.Backend, hooks Hooks) *harness {
 	t.Helper()
 	t.Setenv("OLLIE", "")
 	if be == nil {
@@ -107,7 +107,7 @@ func newCore(t *testing.T, be backend.Backend, hooks Hooks) *sessionHost {
 		NewDispatcher: tools.NewDispatcher,
 	})
 	t.Cleanup(c.Close)
-	return c.(*sessionHost)
+	return c.(*harness)
 }
 
 // collectEvents runs Submit synchronously and returns all emitted events.
@@ -1788,7 +1788,7 @@ func TestSetSessionID_UpdatesPreamble(t *testing.T) {
 		CWD:           t.TempDir(),
 		Runtime:       env,
 		NewDispatcher: tools.NewDispatcher,
-	}).(*sessionHost)
+	}).(*harness)
 	t.Cleanup(c.Close)
 
 	newID := NewSessionID()
@@ -2270,7 +2270,7 @@ func (m *mockEnvServer) get(k string) string {
 	return m.env[k]
 }
 
-func newCoreWithExecServer(t *testing.T, srv *mockEnvServer) *sessionHost {
+func newCoreWithExecServer(t *testing.T, srv *mockEnvServer) *harness {
 	t.Helper()
 	t.Setenv("OLLIE", "")
 	d := tools.NewDispatcher()
@@ -2291,7 +2291,7 @@ func newCoreWithExecServer(t *testing.T, srv *mockEnvServer) *sessionHost {
 		NewDispatcher: tools.NewDispatcher,
 	})
 	t.Cleanup(c.Close)
-	return c.(*sessionHost)
+	return c.(*harness)
 }
 
 func TestSetEnv_PropagatestoExecuteServer(t *testing.T) {
diff --git a/agent/host.go b/agent/harness.go
similarity index 93%
rename from agent/host.go
rename to agent/harness.go
index 20c76c4..8a64658 100644
--- a/agent/host.go
+++ b/agent/harness.go
@@ -307,9 +307,9 @@ type AgentCoreConfig struct {
 	BaseLayers []string
 }
 
-// sessionHost is the Core implementation. It owns all sessionHost and session state
+// harness is the Core implementation. It owns all harness and session state
 // but has no knowledge of how output is rendered.
-type sessionHost struct {
+type harness struct {
 	sess            *session.Session
 	r            *Agent
 	log             *olog.Logger
@@ -337,12 +337,12 @@ type sessionHost struct {
 // ToolCallCount returns the total number of tool calls executed in this
 // session. The counter is monotonically increasing and never resets.
 // Blocked calls (pre-tool hook exit 2) are not counted.
-func (a *sessionHost) ToolCallCount() int64 {
+func (a *harness) ToolCallCount() int64 {
 	return a.toolCallCount.Load()
 }
 
 // SetEnv stores a session-scoped variable and propagates it to the execute server.
-func (a *sessionHost) SetEnv(key, value string) {
+func (a *harness) SetEnv(key, value string) {
 	a.sess.SetEnv(key, value)
 	if a.r.runtime == nil || a.r.runtime.Dispatcher == nil {
 		return
@@ -355,7 +355,7 @@ func (a *sessionHost) SetEnv(key, value string) {
 }
 
 // pushSessionEnv injects OLLIE_SESSION_ID into the execute server subprocess env.
-func (a *sessionHost) pushSessionEnv() {
+func (a *harness) pushSessionEnv() {
 	if a.r.runtime == nil || a.r.runtime.Dispatcher == nil || a.sess.ID() == "" {
 		return
 	}
@@ -370,14 +370,14 @@ func (a *sessionHost) pushSessionEnv() {
 }
 
 // pushLockDir sets the flock directory on the execute server to the session tmpdir.
-func (a *sessionHost) pushLockDir() {
+func (a *harness) pushLockDir() {
 	if a.r.runtime == nil || a.r.runtime.Dispatcher == nil || a.sess.ID() == "" {
 		return
 	}
 
 }
 
-var _ Core = (*sessionHost)(nil) // compile-time interface check
+var _ Core = (*harness)(nil) // compile-time interface check
 
 var sweepTmpOnce sync.Once
 
@@ -453,7 +453,7 @@ func NewAgentCore(cfg AgentCoreConfig) Core {
 		log = olog.NewWriter("core", olog.LevelError+1, io.Discard, io.Discard)
 	}
 
-	a := &sessionHost{
+	a := &harness{
 		sess: session.New(session.Config{
 			ID:    cfg.SessionID,
 			Uname: cfg.Uname,
@@ -495,7 +495,7 @@ func NewAgentCore(cfg AgentCoreConfig) Core {
 }
 
 // Close releases resources for this session, including its tmpdir.
-func (a *sessionHost) Close() {
+func (a *harness) Close() {
 	a.log.Debug("Close() session=%q", a.sess.ID())
 	a.flushSave()
 	if a.r.runtime != nil && a.r.runtime.Dispatcher != nil {
@@ -512,7 +512,7 @@ func (a *sessionHost) Close() {
 }
 
 // execServer returns the execute server if available, or nil.
-func (a *sessionHost) execServer() interface{} {
+func (a *harness) execServer() interface{} {
 	if a.r.runtime == nil || a.r.runtime.Dispatcher == nil {
 		return nil
 	}
@@ -520,7 +520,7 @@ func (a *sessionHost) execServer() interface{} {
 	return srv
 }
 
-func (a *sessionHost) Detach() bool {
+func (a *harness) Detach() bool {
 	if srv := a.execServer(); srv != nil {
 		if d, ok := srv.(interface{ Detach() bool }); ok {
 			return d.Detach()
@@ -529,7 +529,7 @@ func (a *sessionHost) Detach() bool {
 	return false
 }
 
-func (a *sessionHost) ListDetached() []DetachedInfo {
+func (a *harness) ListDetached() []DetachedInfo {
 	if srv := a.execServer(); srv != nil {
 		type listDetacher interface {
 			ListDetachedRaw() []any
@@ -564,7 +564,7 @@ func (a *sessionHost) ListDetached() []DetachedInfo {
 	return nil
 }
 
-func (a *sessionHost) SignalDetached(pid, signal int) error {
+func (a *harness) SignalDetached(pid, signal int) error {
 	if srv := a.execServer(); srv != nil {
 		type signaler interface {
 			SignalDetached(int, syscall.Signal) error
@@ -576,7 +576,7 @@ func (a *sessionHost) SignalDetached(pid, signal int) error {
 	return fmt.Errorf("no execute server available")
 }
 
-func (a *sessionHost) GetDetachedOutput(pid int) (string, error) {
+func (a *harness) GetDetachedOutput(pid int) (string, error) {
 	if srv := a.execServer(); srv != nil {
 		type outputGetter interface {
 			GetDetachedOutput(int) (string, error)
@@ -588,7 +588,7 @@ func (a *sessionHost) GetDetachedOutput(pid int) (string, error) {
 	return "", fmt.Errorf("no execute server available")
 }
 
-func (a *sessionHost) DismissDetached(pid int) bool {
+func (a *harness) DismissDetached(pid int) bool {
 	if srv := a.execServer(); srv != nil {
 		type dismisser interface {
 			DismissDetached(int) bool
@@ -600,7 +600,7 @@ func (a *sessionHost) DismissDetached(pid int) bool {
 	return false
 }
 
-func (a *sessionHost) InjectSystemEvent(content string) {
+func (a *harness) InjectSystemEvent(content string) {
 	a.Queue("<detached-process-result>\n" + content + "\n</detached-process-result>")
 }
 
@@ -622,7 +622,7 @@ func classifyReaction(emoji string) (category, description string, positive bool
 	}
 }
 
-func (a *sessionHost) Reactions() map[string]string {
+func (a *harness) Reactions() map[string]string {
 	result := make(map[string]string)
 	if a.r.history == nil {
 		return result
@@ -633,11 +633,11 @@ func (a *sessionHost) Reactions() map[string]string {
 	return result
 }
 
-func (a *sessionHost) React(emoji string) {
+func (a *harness) React(emoji string) {
 	_ = a.ReactTo("", emoji)
 }
 
-func (a *sessionHost) ReactTo(responseID, emoji string) error {
+func (a *harness) ReactTo(responseID, emoji string) error {
 	if a.r.history == nil {
 		return fmt.Errorf("no active session")
 	}
@@ -687,33 +687,33 @@ func (a *sessionHost) ReactTo(responseID, emoji string) error {
 	return nil
 }
 
-func (a *sessionHost) AgentName() string {
+func (a *harness) AgentName() string {
 	v := a.r.agentName
 	a.log.Debug("AgentName() = %q", v)
 	return v
 }
-func (a *sessionHost) BackendName() string {
+func (a *harness) BackendName() string {
 	v := a.r.runtime.Backend.Name()
 	a.log.Debug("BackendName() = %q", v)
 	return v
 }
-func (a *sessionHost) ModelName() string {
+func (a *harness) ModelName() string {
 	v := a.r.runtime.Backend.Model()
 	a.log.Debug("ModelName() = %q", v)
 	return v
 }
 
-func (a *sessionHost) State() string {
+func (a *harness) State() string {
 	return a.sess.State()
 }
 
-func (a *sessionHost) notifyChange() {
+func (a *harness) notifyChange() {
 	a.changeMu.Lock()
 	a.changeCond.Broadcast()
 	a.changeMu.Unlock()
 }
 
-func (a *sessionHost) setState(state string) {
+func (a *harness) setState(state string) {
 	a.sess.SetState(state)
 	a.log.Debug("state -> %q", state)
 	a.notifyChange()
@@ -721,7 +721,7 @@ func (a *sessionHost) setState(state string) {
 
 // WaitChange blocks until the named field changes from current, then returns
 // the new value. Returns ("", false) if ctx is cancelled.
-func (a *sessionHost) WaitChange(ctx context.Context, field, current string) (string, bool) {
+func (a *harness) WaitChange(ctx context.Context, field, current string) (string, bool) {
 	read := func() string {
 		switch field {
 		case WatchState:
@@ -759,14 +759,14 @@ func (a *sessionHost) WaitChange(ctx context.Context, field, current string) (st
 }
 
 
-func (a *sessionHost) Reply() string {
+func (a *harness) Reply() string {
 	r := a.sess.Reply()
 	a.log.Debug("Reply() len=%d", len(r))
 	return r
 }
 
 // CWD returns the current working directory for tool execution.
-func (a *sessionHost) CWD() string {
+func (a *harness) CWD() string {
 	if a.sess.CWD() != "" {
 		a.log.Debug("CWD() = %q", a.sess.CWD())
 		return a.sess.CWD()
@@ -778,7 +778,7 @@ func (a *sessionHost) CWD() string {
 
 // SetCWD changes the working directory for tool execution and updates the
 // system prompt. Returns an error if the path does not exist.
-func (a *sessionHost) SetCWD(dir string) error {
+func (a *harness) SetCWD(dir string) error {
 	a.log.Debug("SetCWD(%q)", dir)
 	dir = paths.ExpandHome(dir)
 	if dir != "" {
@@ -806,7 +806,7 @@ func (a *sessionHost) SetCWD(dir string) error {
 
 // SetSessionID renames the session. It updates the in-memory ID, renames
 // persisted files on disk, and propagates to the execute server env.
-func (a *sessionHost) SetSessionID(newID string) error {
+func (a *harness) SetSessionID(newID string) error {
 	a.log.Debug("SetSessionID(%q) old=%q", newID, a.sess.ID())
 	oldID := a.sess.ID()
 	if oldID == newID {
@@ -843,7 +843,7 @@ const defaultContextLength = 128000
 const defaultToolResultMaxBytes = 131072
 
 // autoCompactLimit returns the token threshold for auto-compaction (75%).
-func (a *sessionHost) autoCompactLimit(ctx context.Context) int {
+func (a *harness) autoCompactLimit(ctx context.Context) int {
 	ctxLen := a.r.runtime.Backend.ContextLength(ctx)
 	if ctxLen <= 0 {
 		ctxLen = defaultContextLength
@@ -852,7 +852,7 @@ func (a *sessionHost) autoCompactLimit(ctx context.Context) int {
 }
 
 // autoWarnLimit returns the token threshold for a context-usage warning (60%).
-func (a *sessionHost) autoWarnLimit(ctx context.Context) int {
+func (a *harness) autoWarnLimit(ctx context.Context) int {
 	ctxLen := a.r.runtime.Backend.ContextLength(ctx)
 	if ctxLen <= 0 {
 		ctxLen = defaultContextLength
@@ -863,7 +863,7 @@ func (a *sessionHost) autoWarnLimit(ctx context.Context) int {
 // spawnContext assembles the agent context injected at each session refresh
 // point (session start, post-clear, post-compaction). It combines the
 // agent-specific prompt with any agentSpawn hook output.
-func (a *sessionHost) spawnContext(ctx context.Context) string {
+func (a *harness) spawnContext(ctx context.Context) string {
 	result := a.r.runtime.Hooks.Run(ctx, HookAgentSpawn, map[string]string{
 		"session_id": a.sess.ID(),
 		"agent":      a.r.agentName,
@@ -886,7 +886,7 @@ func (a *sessionHost) spawnContext(ctx context.Context) string {
 // runCompact executes a full compaction cycle: pre-hook, compact, spawn-context
 // re-injection, post-hook. Returns (n compacted, error). Returns (0, nil) if
 // the pre-hook blocked or there was nothing to compact. Caller manages setState.
-func (a *sessionHost) runCompact(ctx context.Context, trigger string) (int, error) {
+func (a *harness) runCompact(ctx context.Context, trigger string) (int, error) {
 	payload := map[string]string{"session_id": a.sess.ID(), "trigger": trigger, "cwd": a.CWD()}
 	pre := a.r.runtime.Hooks.Run(ctx, HookPreCompact, payload, a.log)
 	if pre.Warning != "" {
@@ -933,11 +933,11 @@ func (a *sessionHost) runCompact(ctx context.Context, trigger string) (int, erro
 	return n, nil
 }
 
-func (a *sessionHost) activeSessionPath(id, suffix string) string {
+func (a *harness) activeSessionPath(id, suffix string) string {
 	return filepath.Join(a.sessionsDir, "active", id+suffix)
 }
 
-func (a *sessionHost) saveSession() {
+func (a *harness) saveSession() {
 	a.saveMu.Lock()
 	a.saveDirty = true
 	if a.saveTimer == nil {
@@ -947,7 +947,7 @@ func (a *sessionHost) saveSession() {
 }
 
 // flushSave immediately persists the session if dirty.
-func (a *sessionHost) flushSave() {
+func (a *harness) flushSave() {
 	a.saveMu.Lock()
 	dirty := a.saveDirty
 	a.saveDirty = false
@@ -975,7 +975,7 @@ func (a *sessionHost) flushSave() {
 
 // SaveSession writes the current session state to the given path, including
 // backend and model metadata for external restore.
-func (a *sessionHost) SaveSession(path string) error {
+func (a *harness) SaveSession(path string) error {
 	a.mu.RLock()
 	defer a.mu.RUnlock()
 	if a.r.history == nil {
@@ -985,7 +985,7 @@ func (a *sessionHost) SaveSession(path string) error {
 		a.r.runtime.Backend.Name(), a.r.runtime.Backend.Model(), a.CWD(), a.remote)
 }
 
-func (a *sessionHost) getActionCancel() context.CancelCauseFunc {
+func (a *harness) getActionCancel() context.CancelCauseFunc {
 	if a := a.r.currentAction.Load(); a != nil {
 		return a.cancel
 	}
@@ -994,7 +994,7 @@ func (a *sessionHost) getActionCancel() context.CancelCauseFunc {
 
 // Interrupt cancels the current in-progress agent turn.
 // Returns true if an action was running and was cancelled.
-func (a *sessionHost) Interrupt(cause error) bool {
+func (a *harness) Interrupt(cause error) bool {
 	a.log.Debug("Interrupt() cause=%v", cause)
 	if cancel := a.getActionCancel(); cancel != nil {
 		cancel(cause)
@@ -1003,7 +1003,7 @@ func (a *sessionHost) Interrupt(cause error) bool {
 	return false
 }
 
-func (a *sessionHost) Inject(prompt string) {
+func (a *harness) Inject(prompt string) {
 	// If an inject is already pending, fall back to the normal FIFO so nothing
 	// is lost. Use CompareAndSwap to avoid a race between the nil check and store.
 	if !a.pendingInject.CompareAndSwap(nil, &prompt) {
@@ -1014,42 +1014,42 @@ func (a *sessionHost) Inject(prompt string) {
 	a.emit(Event{Role: "user", Content: prompt})
 }
 
-func (a *sessionHost) injectRewrite(prompt string) {
+func (a *harness) injectRewrite(prompt string) {
 	a.pendingInject.Store(&prompt)
 	a.emit(Event{Role: "info", Content: "\n"})
 	a.emit(Event{Role: "user", Content: prompt})
 }
 
-func (a *sessionHost) Queue(prompt string) {
+func (a *harness) Queue(prompt string) {
 	a.sess.Queue(prompt)
 	a.sess.Bus().Publish("queued", prompt)
 }
 
-func (a *sessionHost) drainQueue() {
+func (a *harness) drainQueue() {
 	if prompt, ok := a.sess.PopQueue(); ok {
 		a.Submit(context.Background(), prompt)
 	}
 }
 
-func (a *sessionHost) Bus() *pubsub.Bus {
+func (a *harness) Bus() *pubsub.Bus {
 	return a.sess.Bus()
 }
 
-func (a *sessionHost) emit(ev Event) {
+func (a *harness) emit(ev Event) {
 	a.sess.Bus().Publish("event", ev)
 }
 
-func (a *sessionHost) PopQueue() (string, bool) {
+func (a *harness) PopQueue() (string, bool) {
 	return a.sess.PopQueue()
 }
 
-func (a *sessionHost) IsRunning() bool {
+func (a *harness) IsRunning() bool {
 	v := a.r.currentAction.Load() != nil
 	a.log.Debug("IsRunning() = %v", v)
 	return v
 }
 
-func (a *sessionHost) CtxSz() string {
+func (a *harness) CtxSz() string {
 	if a.r.history == nil {
 		a.log.Debug("CtxSz() no session")
 		return "no active session"
@@ -1065,7 +1065,7 @@ func (a *sessionHost) CtxSz() string {
 	return v
 }
 
-func (a *sessionHost) Cost() string {
+func (a *harness) Cost() string {
 	if a.r.history == nil {
 		return "no active session"
 	}
@@ -1073,7 +1073,7 @@ func (a *sessionHost) Cost() string {
 		a.r.history.LastTurnCostUSD, a.r.history.SessionCostUSD)
 }
 
-func (a *sessionHost) Usage() string {
+func (a *harness) Usage() string {
 	if a.r.history == nil {
 		a.log.Debug("Usage() no session")
 		return "no active session"
@@ -1091,7 +1091,7 @@ func (a *sessionHost) Usage() string {
 	return str
 }
 
-func (a *sessionHost) Context() []backend.Message {
+func (a *harness) Context() []backend.Message {
 	a.mu.RLock()
 	var msgs []backend.Message
 	if a.r.history != nil {
@@ -1104,30 +1104,30 @@ func (a *sessionHost) Context() []backend.Message {
 	return msgs
 }
 
-func (a *sessionHost) SystemPrompt() string {
+func (a *harness) SystemPrompt() string {
 	a.log.Debug("SystemPrompt() len=%d", len(a.r.runtime.Preamble))
 	return a.r.runtime.Preamble
 }
 
-func (a *sessionHost) GenerationParams() backend.GenerationParams {
+func (a *harness) GenerationParams() backend.GenerationParams {
 	a.mu.RLock()
 	defer a.mu.RUnlock()
 	return a.r.runtime.GenParams
 }
 
-func (a *sessionHost) CompactionModel() string {
+func (a *harness) CompactionModel() string {
 	a.mu.RLock()
 	defer a.mu.RUnlock()
 	return a.r.runtime.CompactionModel
 }
 
-func (a *sessionHost) SetCompactionModel(model string) {
+func (a *harness) SetCompactionModel(model string) {
 	a.mu.Lock()
 	defer a.mu.Unlock()
 	a.r.runtime.CompactionModel = model
 }
 
-func (a *sessionHost) SetGenerationParams(params backend.GenerationParams) error {
+func (a *harness) SetGenerationParams(params backend.GenerationParams) error {
 	if a.IsRunning() {
 		return fmt.Errorf("cannot change params while agent is running")
 	}
@@ -1137,7 +1137,7 @@ func (a *sessionHost) SetGenerationParams(params backend.GenerationParams) error
 	return nil
 }
 
-func (a *sessionHost) ListModels() string {
+func (a *harness) ListModels() string {
 	a.log.Debug("ListModels()")
 	models := a.r.runtime.Backend.Models(context.Background())
 	slices.Sort(models)
@@ -1166,7 +1166,7 @@ func firstSentence(s string) string {
 //
 // Continuations (post-turn hook context, unconsumed inject, FIFO drain) are
 // handled via an explicit loop rather than recursion to avoid stack growth.
-func (a *sessionHost) Submit(ctx context.Context, input string) {
+func (a *harness) Submit(ctx context.Context, input string) {
 	defer func() {
 		if r := recover(); r != nil {
 			a.log.Error("panic: %v\n%s", r, debug.Stack())
@@ -1213,7 +1213,7 @@ func (a *sessionHost) Submit(ctx context.Context, input string) {
 
 // executeTurn runs a single agent turn and returns the next prompt to execute,
 // or "" if there is nothing more to do.
-func (a *sessionHost) executeTurn(ctx context.Context, input string) string {
+func (a *harness) executeTurn(ctx context.Context, input string) string {
 	a.emit(Event{Role: "user", Content: input})
 
 	hookResult := a.r.runtime.Hooks.Run(ctx, HookPreTurn, map[string]string{
diff --git a/agent/waitchange_test.go b/agent/waitchange_test.go
index f0af55e..98c2175 100644
--- a/agent/waitchange_test.go
+++ b/agent/waitchange_test.go
@@ -12,10 +12,10 @@ import (
 )
 
 // newTestCore returns a minimal agent with session wired up.
-func newTestCore(initialState string) *sessionHost {
+func newTestCore(initialState string) *harness {
 	sess := session.New(session.Config{ID: "test"})
 	sess.SetState(initialState)
-	a := &sessionHost{
+	a := &harness{
 		sess: sess,
 		log:  olog.NewWriter("test", olog.LevelError+1, io.Discard, io.Discard),
 	}