diff --git a/internal/api/agent.go b/internal/api/agent.go
index 2b19313..622d8c1 100644
--- a/internal/api/agent.go
+++ b/internal/api/agent.go
@@ -8,6 +8,8 @@ import (
 	"io"
 	"log/slog"
 	"net/http"
+	"sync"
+	"time"
 
 	mdagent "gitea.stevedudenhoeffer.com/steve/majordomo/agent"
 	"gitea.stevedudenhoeffer.com/steve/majordomo/llm"
@@ -62,13 +64,19 @@ func (h *handlers) agentChat(c *gin.Context) {
 		return
 	}
 
-	send := openEventStream(c)
+	stream := openEventStream(c)
+	send := stream.send
 
+	// A model thinking hard between tool calls sends nothing for a while, and an
+	// idle proxy will cut a quiet connection. A comment frame every 20s keeps it
+	// open; SSE ignores comments, so this costs the client nothing.
+	stopBeat := stream.keepAlive(20 * time.Second)
 	turn, err := h.agent.Run(c.Request.Context(), actor.ID, req.GardenID, req.Message,
 		replayHistory(history),
 		func(s mdagent.Step) {
 			send(chatEvent{Step: &stepEvent{Index: s.Index, Tools: toolNames(s)}})
 		})
+	stopBeat()
 	if err != nil {
 		// The stream is already open, so an error is an event rather than a
 		// status code — the client has committed to reading a stream by now.
@@ -93,25 +101,71 @@ func (h *handlers) agentChat(c *gin.Context) {
 	send(chatEvent{Done: turn})
 }
 
-// openEventStream puts the response into SSE mode and returns a sender.
+// eventStream serializes writes to one SSE response.
+//
+// The mutex is load-bearing, not decoration: step events are sent from the
+// agent's run goroutine while the keep-alive ticker writes from its own, and two
+// goroutines writing a ResponseWriter concurrently is a data race that corrupts
+// frames long before it crashes anything.
+type eventStream struct {
+	c  *gin.Context
+	mu sync.Mutex
+}
+
+// openEventStream puts the response into SSE mode.
 //
 // Headers go out before the first write and the stream is flushed immediately,
 // so a proxy holding the response until it looks complete can't reintroduce
 // exactly the silence streaming exists to remove.
-func openEventStream(c *gin.Context) func(chatEvent) {
+func openEventStream(c *gin.Context) *eventStream {
 	c.Header("Content-Type", "text/event-stream")
 	c.Header("Cache-Control", "no-cache")
 	c.Header("X-Accel-Buffering", "no")
 	c.Writer.Flush()
+	return &eventStream{c: c}
+}
 
-	return func(ev chatEvent) {
-		b, err := json.Marshal(ev)
-		if err != nil {
-			slog.Error("api: encode chat event", "error", err)
-			return
+func (s *eventStream) send(ev chatEvent) {
+	b, err := json.Marshal(ev)
+	if err != nil {
+		slog.Error("api: encode chat event", "error", err)
+		return
+	}
+	s.write(fmt.Sprintf("data: %s\n\n", b))
+}
+
+func (s *eventStream) write(frame string) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	_, _ = io.WriteString(s.c.Writer, frame)
+	s.c.Writer.Flush()
+}
+
+// keepAlive writes an SSE comment frame on an interval until the returned
+// function is called, so a long silence while the model thinks doesn't look like
+// a dead connection to whatever sits in between. SSE ignores comment frames, so
+// this costs the client nothing.
+func (s *eventStream) keepAlive(every time.Duration) func() {
+	done := make(chan struct{})
+	stopped := make(chan struct{})
+	go func() {
+		defer close(stopped)
+		t := time.NewTicker(every)
+		defer t.Stop()
+		for {
+			select {
+			case <-done:
+				return
+			case <-s.c.Request.Context().Done():
+				return
+			case <-t.C:
+				s.write(": keep-alive\n\n")
+			}
 		}
-		_, _ = fmt.Fprintf(c.Writer, "data: %s\n\n", b)
-		c.Writer.Flush()
+	}()
+	return func() {
+		close(done)
+		<-stopped
 	}
 }
 
diff --git a/internal/service/revisions.go b/internal/service/revisions.go
index a0ae39b..273f469 100644
--- a/internal/service/revisions.go
+++ b/internal/service/revisions.go
@@ -130,11 +130,7 @@ func (s *Service) WithChangeSet(ctx context.Context, actorID, gardenID int64, op
 		// which is exactly the situation undo exists for. Record what happened,
 		// mark the summary, and still report the failure.
 		sc.summary = partialSummary(sc.summary)
-		// Detached from cancellation on purpose. The commonest reason fn failed is
-		// that ctx was cancelled or timed out — and using that same dead context
-		// to record what committed would fail too, losing the history for changes
-		// that really happened. That is precisely the case this path exists for.
-		if _, cerr := s.commitScope(context.WithoutCancel(ctx), sc, nil); cerr != nil {
+		if _, cerr := s.commitScope(ctx, sc, nil); cerr != nil {
 			slog.Error("service: partial turn could not be recorded", "error", cerr, "garden", gardenID)
 		}
 		return nil, err
@@ -154,11 +150,23 @@ func partialSummary(summary string) string {
 // commitScope writes a scope's buffered revisions as one change set. revertsID is
 // set only by RevertChangeSet. A scope with no revisions writes nothing — an
 // operation that changed nothing doesn't belong in history.
+//
+// The write is DETACHED FROM CANCELLATION, always. By the time it runs, the data
+// changes it describes have already committed — so cancelling it cannot undo
+// anything, it can only lose the record of what happened and leave real changes
+// with no way to undo them.
+//
+// This was originally only done on the failure path, on the reasoning that a
+// cancelled context is why fn failed. That missed the commoner case: an agent
+// turn whose client disconnects can still COMPLETE, and then the success path
+// commits with a dead context and loses the history anyway. Found in production
+// with 18 plantings and no change set behind them.
 func (s *Service) commitScope(ctx context.Context, sc *changeScope, revertsID *int64) (*domain.ChangeSet, error) {
 	revs := sc.taken()
 	if len(revs) == 0 {
 		return nil, nil
 	}
+	ctx = context.WithoutCancel(ctx)
 	return s.store.WriteChangeSet(ctx, &domain.ChangeSet{
 		GardenID:   sc.gardenID,
 		ActorID:    sc.actorID,
@@ -312,10 +320,9 @@ func (s *Service) RevertChangeSet(ctx context.Context, actorID, changeSetID int6
 		changes, conflict, err := s.applyInverse(ctx, r, applied)
 		if err != nil {
 			// Record what actually landed before surfacing the failure, so the
-			// partial revert is visible and undoable rather than orphaned. Detached
-			// from cancellation for the same reason as WithChangeSet's path: a
-			// timed-out context can't be used to write the record of what it did.
-			if _, cerr := s.commitScope(context.WithoutCancel(ctx), sc, &target.ID); cerr != nil {
+			// partial revert is visible and undoable rather than orphaned.
+			// (commitScope detaches from cancellation itself.)
+			if _, cerr := s.commitScope(ctx, sc, &target.ID); cerr != nil {
 				slog.Error("service: partial revert could not be recorded", "error", cerr, "changeSet", changeSetID)
 			}
 			return nil, nil, err
diff --git a/internal/service/revisions_test.go b/internal/service/revisions_test.go
index fecd22f..15234f9 100644
--- a/internal/service/revisions_test.go
+++ b/internal/service/revisions_test.go
@@ -805,3 +805,57 @@ func countsTotal(counts []domain.ChangeCount) int {
 	}
 	return n
 }
+
+// TestSucceededTurnRecordsEvenIfTheCallerWentAway is the production bug.
+//
+// The failure path was detached from cancellation; the SUCCESS path was not. An
+// agent turn whose client disconnects can still COMPLETE — and then the success
+// path committed with a dead context, the write failed, and real changes were
+// left with no change set and no way to undo them. Found live with 18 plantings
+// behind no history at all.
+//
+// Nothing about a commit needs the caller to still be there: by the time it
+// runs, the data it describes has already been written.
+func TestSucceededTurnRecordsEvenIfTheCallerWentAway(t *testing.T) {
+	s := newTestService(t, openConfig())
+	owner := seedUser(t, s, "a@example.com")
+	g := seedGarden(t, s, owner)
+	bed := seedBed(t, s, owner, g.ID)
+	plant := seedOwnPlant(t, s, owner, 15)
+
+	ctx, cancel := context.WithCancel(context.Background())
+	before := len(history(t, s, owner, g.ID))
+
+	// fn does its work and SUCCEEDS, but the caller goes away before it returns.
+	cs, err := s.WithChangeSet(ctx, owner, g.ID, ChangeSetOptions{
+		Source: domain.SourceAgent, Summary: "plant beans in the second bed",
+	}, func(ctx context.Context) error {
+		if _, err := s.FillNamedRegion(ctx, owner, bed.ID, "all", plant.ID, nil); err != nil {
+			return err
+		}
+		cancel() // the client disconnects, mid-turn, after the work landed
+		return nil
+	})
+	if err != nil {
+		t.Fatalf("WithChangeSet: %v", err)
+	}
+	if cs == nil {
+		t.Fatal("the turn changed things but produced no change set")
+	}
+
+	after := history(t, s, owner, g.ID)
+	if len(after) != before+1 {
+		t.Fatalf("recorded %d change sets, want 1", len(after)-before)
+	}
+	if after[0].Summary != "plant beans in the second bed" {
+		t.Errorf("summary = %q; a completed turn shouldn't be marked partial", after[0].Summary)
+	}
+	// And it's undoable, which is the entire point.
+	if _, conflicts, err := s.RevertChangeSet(context.Background(), owner, cs.ID, domain.SourceUI); err != nil || len(conflicts) != 0 {
+		t.Fatalf("the recorded turn should be undoable: err=%v conflicts=%+v", err, conflicts)
+	}
+	active, _ := s.store.ListActivePlantingsForObject(context.Background(), bed.ID)
+	if len(active) != 0 {
+		t.Errorf("%d plantings survived the undo", len(active))
+	}
+}