fix: stabilize git tab during edit_files (#24648)

- feat(agent/agentgit): shorten fallback poll to 5s
- fix(site/AgentsPage): keep git tab visible after reverting to clean
- feat(site/AgentsPage): show last-checked time in git tab

> 🤖

agent/agentgit/agentgit.go

@@ -44,8 +44,12 @@ const (
 	// scanCooldown is the minimum interval between successive scans.
 	scanCooldown = 1 * time.Second
 	// fallbackPollInterval is the safety-net poll period used when no
-	// filesystem events arrive.
-	fallbackPollInterval = 30 * time.Second
+	// filesystem events arrive. scanCooldown caps the actual scan
+	// frequency; an outer guard in RunLoop further skips the tick
+	// when a trigger-driven scan already ran within this interval.
+	// Each tick forks 6 git subprocesses per subscribed repo plus
+	// one diff --no-index per untracked file.
+	fallbackPollInterval = 5 * time.Second
 	// maxTotalDiffSize is the maximum size of the combined
 	// unified diff for an entire repository sent over the wire.
 	// This must stay under the WebSocket message size limit.
@@ -224,10 +228,9 @@ func (h *Handler) Scan(ctx context.Context) *codersdk.WorkspaceAgentGitServerMes
 
 	h.lastScanAt = now
 
-	if len(repos) == 0 {
-		return nil
-	}
-
+	// Always emit when any root is subscribed. A no-delta scan sends
+	// ScannedAt + empty Repositories (omitted via omitempty) so the
+	// client's "checked Ns ago" label stays honest on idle repos.
 	return &codersdk.WorkspaceAgentGitServerMessage{
 		Type:         codersdk.WorkspaceAgentGitServerMessageTypeChanges,
 		ScannedAt:    &now,
@@ -252,6 +255,15 @@ func (h *Handler) RunLoop(ctx context.Context, scanFn func()) {
 			h.rateLimitedScan(ctx, scanFn)
 
 		case <-fallbackTicker.C:
+			// Skip when a recent trigger-driven scan already covered
+			// this interval, so a busy chat pays near-zero poll cost.
+			h.mu.Lock()
+			recent := !h.lastScanAt.IsZero() &&
+				h.clock.Since(h.lastScanAt) < fallbackPollInterval
+			h.mu.Unlock()
+			if recent {
+				continue
+			}
 			h.rateLimitedScan(ctx, scanFn)
 		}
 	}

agent/agentgit/agentgit_test.go

@@ -253,9 +253,13 @@ func TestScanDeltaEmission(t *testing.T) {
 	require.NotNil(t, msg1)
 	require.Len(t, msg1.Repositories, 1)
 
-	// Second scan with no changes — should return nil (no delta).
+	// Second scan with no changes. Should emit a heartbeat with a
+	// fresh ScannedAt but no repositories. This lets the UI's
+	// "checked Ns ago" label stay honest on an idle clean repo.
 	msg2 := h.Scan(ctx)
-	require.Nil(t, msg2, "no changes since last scan should return nil")
+	require.NotNil(t, msg2, "heartbeat should fire even with no delta")
+	require.NotNil(t, msg2.ScannedAt)
+	require.Empty(t, msg2.Repositories, "heartbeat must not report per-repo changes")
 
 	// Revert the dirty file (make repo clean).
 	require.NoError(t, os.Remove(dirtyFile))
@@ -269,6 +273,59 @@ func TestScanDeltaEmission(t *testing.T) {
 	require.NotContains(t, msg3.Repositories[0].UnifiedDiff, "dirty.go")
 }
 
+// TestScanHeartbeatOnCleanRepo pins the heartbeat contract: while any
+// repo is subscribed, every scan emits a non-nil message with a fresh
+// ScannedAt, even when no repo produced a delta. The UI's
+// "checked Ns ago" label depends on this so an idle clean repo does
+// not drift while the agent is still polling.
+func TestScanHeartbeatOnCleanRepo(t *testing.T) {
+	t.Parallel()
+
+	repoDir := initTestRepo(t)
+	logger := slogtest.Make(t, nil)
+
+	h := agentgit.NewHandler(logger)
+	require.True(t, h.Subscribe([]string{repoDir}))
+	ctx := context.Background()
+
+	// First scan on a clean repo captures branch/remote/empty-diff.
+	msg1 := h.Scan(ctx)
+	require.NotNil(t, msg1)
+	require.NotNil(t, msg1.ScannedAt)
+	require.Len(t, msg1.Repositories, 1)
+	require.Empty(t, msg1.Repositories[0].UnifiedDiff)
+	firstScanAt := *msg1.ScannedAt
+
+	// Second scan: no delta, but heartbeat must still advance
+	// ScannedAt so clients can render an honest "checked Ns ago".
+	msg2 := h.Scan(ctx)
+	require.NotNil(t, msg2, "heartbeat should fire on a no-delta scan")
+	require.NotNil(t, msg2.ScannedAt)
+	require.Empty(t, msg2.Repositories, "heartbeat carries no per-repo changes")
+	require.False(t, msg2.ScannedAt.Before(firstScanAt),
+		"heartbeat ScannedAt must not go backwards")
+
+	// Third scan: also a heartbeat. Still non-nil, still empty.
+	msg3 := h.Scan(ctx)
+	require.NotNil(t, msg3)
+	require.Empty(t, msg3.Repositories)
+}
+
+// TestScanNoHeartbeatWithoutSubscribedRoots pins that the heartbeat
+// only fires when there is at least one subscribed repo. Before any
+// subscribe call, Scan() must still short-circuit to nil so the
+// WebSocket handler does not spam empty messages to a client that
+// has not registered any paths yet.
+func TestScanNoHeartbeatWithoutSubscribedRoots(t *testing.T) {
+	t.Parallel()
+
+	logger := slogtest.Make(t, nil)
+	h := agentgit.NewHandler(logger)
+
+	msg := h.Scan(context.Background())
+	require.Nil(t, msg, "no subscribed roots should mean no heartbeat")
+}
+
 func TestScanDeltaDetectsContentChanges(t *testing.T) {
 	t.Parallel()
 
@@ -291,9 +348,10 @@ func TestScanDeltaDetectsContentChanges(t *testing.T) {
 
 	require.Contains(t, msg1.Repositories[0].UnifiedDiff, "README.md")
 
-	// Second scan with no changes — should return nil (no delta).
+	// Second scan with no changes: heartbeat, no repositories.
 	msg2 := h.Scan(ctx)
-	require.Nil(t, msg2, "no changes since last scan should return nil")
+	require.NotNil(t, msg2, "heartbeat should fire even with no delta")
+	require.Empty(t, msg2.Repositories)
 
 	// Now modify the SAME file further (still "Modified" status, but
 	// different content).
@@ -318,9 +376,10 @@ func TestScanDeltaDetectsContentChanges(t *testing.T) {
 
 	require.Contains(t, msg4.Repositories[0].UnifiedDiff, "untracked.go")
 
-	// No changes — should return nil.
+	// No changes: heartbeat, no repositories.
 	msg5 := h.Scan(ctx)
-	require.Nil(t, msg5, "no changes since last scan should return nil")
+	require.NotNil(t, msg5, "heartbeat should fire even with no delta")
+	require.Empty(t, msg5.Repositories)
 
 	// Modify the untracked file further.
 	require.NoError(t, os.WriteFile(untrackedPath, []byte("package main\n\nfunc init() {}\n"), 0o600))
@@ -875,7 +934,7 @@ func TestFallbackPollTriggersScan(t *testing.T) {
 	require.NoError(t, os.WriteFile(filepath.Join(repoDir, "poll.go"), []byte("package poll\n"), 0o600))
 	ps.AddPaths([]uuid.UUID{chatID}, []string{filepath.Join(repoDir, "poll.go")})
 
-	// Only the 30s fallback poll can trigger scans (no filesystem
+	// Only the fallback poll can trigger scans (no filesystem
 	// watcher).
 	stream := dialGitWatchWithPathStore(t, ps, chatID, agentgit.WithClock(mClock))
 	ch := stream.Chan()
@@ -887,9 +946,9 @@ func TestFallbackPollTriggersScan(t *testing.T) {
 	// Add a new dirty file so the next scan has a delta to report.
 	require.NoError(t, os.WriteFile(filepath.Join(repoDir, "poll2.go"), []byte("package poll\n"), 0o600))
 
-	// Advance to the 30s fallback poll interval. This should
-	// trigger a scan without any explicit refresh.
-	mClock.Advance(30 * time.Second).MustWait(context.Background())
+	// Advance to the fallback poll interval. This should trigger a
+	// scan without any explicit refresh.
+	mClock.Advance(5 * time.Second).MustWait(context.Background())
 
 	msg2 := recvMsg(ctx, t, ch)
 	require.Equal(t, codersdk.WorkspaceAgentGitServerMessageTypeChanges, msg2.Type)
@@ -1002,9 +1061,10 @@ func TestScanLargeFileDeltaTracking(t *testing.T) {
 	msg1 := h.Scan(ctx)
 	require.NotNil(t, msg1)
 
-	// Second scan with no changes — should return nil (no delta).
+	// Second scan with no changes: heartbeat, no repositories.
 	msg2 := h.Scan(ctx)
-	require.Nil(t, msg2, "no changes should mean no delta")
+	require.NotNil(t, msg2, "heartbeat should fire even with no delta")
+	require.Empty(t, msg2.Repositories, "no delta means no repo entries")
 
 	// Remove the large file — should emit a clean delta.
 	require.NoError(t, os.Remove(largeFile))
@@ -1422,3 +1482,194 @@ func TestE2E_RepoDeletionEmitsRemoved(t *testing.T) {
 	}
 	require.True(t, foundRemoved, "expected repo %s to be marked as removed", repoDir)
 }
+
+// TestRunLoopExitsPromptlyOnCancel_DuringPoll pins that RunLoop
+// returns quickly when its context is cancelled while it is blocked
+// on the fallback poll ticker. Regression guard for the fallback
+// interval: if a future change introduces a non-cancellable wait
+// here, this test will hang and fail.
+func TestRunLoopExitsPromptlyOnCancel_DuringPoll(t *testing.T) {
+	t.Parallel()
+
+	logger := slogtest.Make(t, nil)
+	mClock := quartz.NewMock(t)
+	h := agentgit.NewHandler(logger, agentgit.WithClock(mClock))
+
+	ctx, cancel := context.WithCancel(context.Background())
+	t.Cleanup(cancel)
+
+	// Trap NewTicker so the test can synchronize on RunLoop's
+	// ticker creation rather than racing against it with a
+	// best-effort Advance.
+	tickerTrap := mClock.Trap().NewTicker()
+	defer tickerTrap.Close()
+
+	done := make(chan struct{})
+	go func() {
+		defer close(done)
+		h.RunLoop(ctx, func() {})
+	}()
+
+	// Wait until RunLoop has actually called clock.NewTicker, then
+	// release the trap so the ticker is installed. At this point
+	// RunLoop is deterministically inside its select, blocked on
+	// <-ticker.C / <-scanTrigger / <-ctx.Done().
+	tickerTrap.MustWait(ctx).MustRelease(ctx)
+
+	cancel()
+
+	select {
+	case <-done:
+	case <-time.After(testutil.WaitShort):
+		t.Fatal("RunLoop did not return within WaitShort after ctx cancel")
+	}
+}
+
+// TestRunLoopExitsPromptlyOnCancel_DuringCooldown pins that RunLoop
+// returns quickly when its context is cancelled while a
+// rateLimitedScan is sleeping out the cooldown between scans.
+// Regression guard: all waits inside the cooldown path must select
+// on ctx.Done().
+func TestRunLoopExitsPromptlyOnCancel_DuringCooldown(t *testing.T) {
+	t.Parallel()
+
+	repoDir := initTestRepo(t)
+	logger := slogtest.Make(t, nil)
+	mClock := quartz.NewMock(t)
+	h := agentgit.NewHandler(logger, agentgit.WithClock(mClock))
+
+	// Subscribe a real repo so Scan() actually does work and, on
+	// completion, updates lastScanAt. Without this, Scan() early-
+	// returns on empty roots and the cooldown branch never arms.
+	require.True(t, h.Subscribe([]string{repoDir}))
+
+	ctx, cancel := context.WithCancel(context.Background())
+	t.Cleanup(cancel)
+
+	// Trap NewTicker (for RunLoop) and NewTimer (for the cooldown
+	// wait inside rateLimitedScan) so the test synchronizes on each
+	// wait point instead of racing against goroutine scheduling.
+	tickerTrap := mClock.Trap().NewTicker()
+	defer tickerTrap.Close()
+	timerTrap := mClock.Trap().NewTimer()
+	defer timerTrap.Close()
+
+	scanStarted := make(chan struct{}, 1)
+	blocked := make(chan struct{})
+	scanFn := func() {
+		// Run a real Scan so lastScanAt is set by the handler;
+		// that is the precondition for the cooldown branch.
+		_ = h.Scan(ctx)
+		select {
+		case scanStarted <- struct{}{}:
+		default:
+		}
+		// Block until the test releases us, mimicking a slow
+		// follow-up scan that parks RunLoop inside rateLimitedScan.
+		<-blocked
+	}
+
+	done := make(chan struct{})
+	go func() {
+		defer close(done)
+		h.RunLoop(ctx, scanFn)
+	}()
+
+	// Release the fallback ticker so RunLoop enters its select.
+	tickerTrap.MustWait(ctx).MustRelease(ctx)
+
+	// First trigger: consumed immediately (lastScanAt is zero).
+	// scanFn runs Scan() (which sets lastScanAt), signals
+	// scanStarted, then blocks on <-blocked.
+	h.RequestScan()
+	<-scanStarted
+
+	// Release the first scan; RunLoop loops back to select.
+	close(blocked)
+
+	// Fire a second trigger. Because lastScanAt is fresh (set by
+	// the real Scan above), rateLimitedScan enters its cooldown
+	// wait and calls clock.NewTimer. The trap blocks the goroutine
+	// inside that call until we release it, so we know exactly
+	// when it is sitting on the cooldown select.
+	h.RequestScan()
+	timerCall := timerTrap.MustWait(ctx)
+
+	// Cancel while the goroutine is still paused inside NewTimer.
+	// Release the trap; rateLimitedScan then enters the select on
+	// the cooldown timer vs. ctx.Done(), and ctx.Done() is already
+	// ready so it wins. MustRelease uses Background because the
+	// test ctx is the one we just cancelled.
+	releaseCtx, releaseCancel := context.WithTimeout(context.Background(), testutil.WaitShort)
+	defer releaseCancel()
+	cancel()
+	timerCall.MustRelease(releaseCtx)
+
+	select {
+	case <-done:
+	case <-time.After(testutil.WaitShort):
+		t.Fatal("RunLoop did not return within WaitShort after ctx cancel during cooldown")
+	}
+}
+
+// TestFallbackPollSkipsWhenRecentlyScanned pins the RunLoop optimization
+// that swallows a fallback tick when a trigger-driven scan already
+// covered the last fallback interval. Without the skip, a busy chat
+// (agent editing + PathStore notifications) would pay the full fallback
+// scan cost on top of trigger-driven scans.
+func TestFallbackPollSkipsWhenRecentlyScanned(t *testing.T) {
+	t.Parallel()
+
+	ctx := testutil.Context(t, testutil.WaitLong)
+	repoDir := initTestRepo(t)
+	mClock := quartz.NewMock(t)
+
+	ps := agentgit.NewPathStore()
+	chatID := uuid.New()
+
+	require.NoError(t, os.WriteFile(filepath.Join(repoDir, "a.go"), []byte("package a\n"), 0o600))
+	ps.AddPaths([]uuid.UUID{chatID}, []string{filepath.Join(repoDir, "a.go")})
+
+	stream := dialGitWatchWithPathStore(t, ps, chatID, agentgit.WithClock(mClock))
+	ch := stream.Chan()
+
+	// Consume the initial scan from subscribe.
+	msg1 := recvMsg(ctx, t, ch)
+	require.Equal(t, codersdk.WorkspaceAgentGitServerMessageTypeChanges, msg1.Type)
+
+	// A trigger-driven scan within the fallback interval should
+	// cause the next fallback tick to be skipped. Advance part-way
+	// to the 5s tick, fire a notification to trigger a scan, then
+	// advance the rest of the way to the tick. The tick should be
+	// swallowed because lastScanAt is recent.
+	mClock.Advance(4 * time.Second).MustWait(context.Background())
+	require.NoError(t, os.WriteFile(filepath.Join(repoDir, "a.go"), []byte("package a\n// edit\n"), 0o600))
+	ps.Notify([]uuid.UUID{chatID})
+
+	// Consume the trigger-driven scan. lastScanAt is now ~t=4s.
+	msg2 := recvMsg(ctx, t, ch)
+	require.Equal(t, codersdk.WorkspaceAgentGitServerMessageTypeChanges, msg2.Type)
+
+	// Dirty the tree further so the fallback tick would have
+	// something to emit if it were not skipped.
+	require.NoError(t, os.WriteFile(filepath.Join(repoDir, "b.go"), []byte("package b\n"), 0o600))
+
+	// Advance to the 5s ticker boundary. The tick fires but is
+	// skipped because Since(lastScanAt) = 1s < fallbackPollInterval.
+	mClock.Advance(1 * time.Second).MustWait(context.Background())
+
+	// Confirm no scan arrived for the skipped tick.
+	select {
+	case msg := <-ch:
+		t.Fatalf("unexpected scan after skipped fallback tick: %+v", msg)
+	case <-time.After(testutil.IntervalFast):
+	}
+
+	// Advance to the next ticker boundary (t=10s). lastScanAt is
+	// ~4s, so Since = 6s >= fallbackPollInterval and the tick
+	// should no longer be skipped.
+	mClock.Advance(5 * time.Second).MustWait(context.Background())
+
+	msg3 := recvMsg(ctx, t, ch)
+	require.Equal(t, codersdk.WorkspaceAgentGitServerMessageTypeChanges, msg3.Type)
+}

agent/agentgit/api.go

@@ -67,15 +67,16 @@ func (a *API) handleWatch(rw http.ResponseWriter, r *http.Request) {
 
 	handler := NewHandler(a.logger, a.opts...)
 
-	// scanAndSend performs a scan and sends results if there are
-	// changes.
+	// Scan returns nil only when no roots are subscribed; once any
+	// root lands it returns either a delta or a heartbeat message.
 	scanAndSend := func() {
 		msg := handler.Scan(ctx)
-		if msg != nil {
-			if err := stream.Send(*msg); err != nil {
-				a.logger.Debug(ctx, "failed to send changes", slog.Error(err))
-				cancel()
-			}
+		if msg == nil {
+			return
+		}
+		if err := stream.Send(*msg); err != nil {
+			a.logger.Debug(ctx, "failed to send changes", slog.Error(err))
+			cancel()
 		}
 	}
 

site/src/pages/AgentsPage/AgentChatPageView.stories.tsx

@@ -85,6 +85,8 @@ const buildGitWatcher = (): ComponentProps<
 	typeof AgentChatPageView
 >["gitWatcher"] => ({
 	repositories: new Map(),
+	everDirty: new Set(),
+	lastCheckedAt: undefined,
 	refresh: fn().mockReturnValue(true),
 });
 

site/src/pages/AgentsPage/AgentChatPageView.tsx

@@ -127,6 +127,8 @@ interface AgentChatPageViewProps {
 	debugLoggingEnabled: boolean;
 	gitWatcher: {
 		repositories: ReadonlyMap<string, TypesGen.WorkspaceAgentRepoChanges>;
+		everDirty: ReadonlySet<string>;
+		lastCheckedAt: Date | undefined;
 		refresh: () => boolean;
 	};
 
@@ -343,6 +345,8 @@ export const AgentChatPageView: FC<AgentChatPageViewProps> = ({
 							prNumber && agentId ? { prNumber, chatId: agentId } : undefined
 						}
 						repositories={gitWatcher.repositories}
+						everDirty={gitWatcher.everDirty}
+						lastCheckedAt={gitWatcher.lastCheckedAt}
 						onRefresh={handleRefresh}
 						onCommit={handleCommit}
 						isExpanded={visualExpanded}

site/src/pages/AgentsPage/components/GitPanel/GitPanel.stories.tsx

@@ -339,3 +339,94 @@ export const LargeDiff: Story = {
 		]),
 	},
 };
+
+/**
+ * Regression: when a repo was dirty during this session and then went
+ * clean (empty unified_diff), the tab must remain visible. Before the
+ * ever-dirty fix, the tab vanished the moment the diff became empty,
+ * which is what users saw as "diff disappears between edit_files".
+ */
+export const EverDirtyRepoGoneClean: Story = {
+	args: {
+		repositories: new Map([
+			["/home/coder/coder", makeRepo({ unified_diff: "" })],
+		]),
+		everDirty: new Set(["/home/coder/coder"]),
+	},
+	play: async ({ canvasElement }) => {
+		// The repo tab is still present (identified by the 'Working'
+		// prefix used by GitPanel's tab-strip button) even though the
+		// current diff is empty, because it was dirty earlier in the
+		// session.
+		const tabs = Array.from(canvasElement.querySelectorAll("button")).filter(
+			(b) => (b.textContent ?? "").startsWith("Working"),
+		);
+		expect(tabs).toHaveLength(1);
+
+		// The content pane shows the diff viewer's empty-diff state.
+		expect(canvasElement.textContent ?? "").toContain("No file changes");
+	},
+};
+
+/**
+ * Baseline: a repo reported clean from the start (never dirty in
+ * this session) has no tab. Ensures the ever-dirty fix did not
+ * regress the "nothing to show" case.
+ */
+export const CleanRepoFromStart: Story = {
+	args: {
+		repositories: new Map([
+			["/home/coder/coder", makeRepo({ unified_diff: "" })],
+		]),
+		everDirty: new Set(),
+	},
+	play: async ({ canvasElement }) => {
+		// No local repo tab should appear in the tab strip. The
+		// 'Working' prefix is GitPanel's tab-strip label contract.
+		const tabs = Array.from(canvasElement.querySelectorAll("button")).filter(
+			(b) => (b.textContent ?? "").startsWith("Working"),
+		);
+		expect(tabs).toHaveLength(0);
+	},
+};
+
+/**
+ * Renders the relative-time label once a scan has been observed.
+ */
+export const ShowsLastCheckedLabel: Story = {
+	args: {
+		repositories: new Map([["/home/coder/coder", makeRepo()]]),
+		// Fixed past date keeps the rendered "ago" text deterministic
+		// for pixel snapshots. dayjs formats "X months ago" or "X
+		// years ago" at this scale, and those buckets do not flip
+		// between story collection and story render.
+		lastCheckedAt: new Date("2024-01-01T00:00:00Z"),
+	},
+	play: async ({ canvasElement }) => {
+		const label = canvasElement.querySelector(
+			'[data-testid="git-last-checked"]',
+		);
+		expect(label).not.toBeNull();
+		// dayjs' relativeTime renders sub-45s as 'a few seconds ago'
+		// and longer spans as '<n> <unit> ago'. Accept either shape
+		// so the story is not coupled to dayjs' specific bucketing.
+		expect(label?.textContent ?? "").toMatch(/^checked .+ ago$/);
+	},
+};
+
+/**
+ * With no scan observed yet, the label renders nothing so the
+ * toolbar collapses cleanly.
+ */
+export const NoLastCheckedYet: Story = {
+	args: {
+		repositories: new Map([["/home/coder/coder", makeRepo()]]),
+		lastCheckedAt: undefined,
+	},
+	play: async ({ canvasElement }) => {
+		const label = canvasElement.querySelector(
+			'[data-testid="git-last-checked"]',
+		);
+		expect(label).toBeNull();
+	},
+};

site/src/pages/AgentsPage/components/GitPanel/GitPanel.tsx

@@ -29,6 +29,7 @@ import {
 } from "../DiffViewer/DiffViewer";
 import { LocalDiffPanel } from "../DiffViewer/LocalDiffPanel";
 import { RemoteDiffPanel } from "../DiffViewer/RemoteDiffPanel";
+import { LastCheckedLabel } from "./LastCheckedLabel";
 
 type GitView = { type: "remote" } | { type: "local"; repoRoot: string };
 
@@ -55,6 +56,18 @@ interface GitPanelProps {
 	remoteDiffStats?: ChatDiffStatus;
 	/** Ref to the chat input, forwarded to RemoteDiffPanel. */
 	chatInputRef?: RefObject<ChatMessageInputRef | null>;
+	/**
+	 * Repo roots that have been dirty at some point during this session.
+	 * Used to keep a repo's tab visible after its diff goes empty, so the
+	 * tab strip does not visibly flip when the agent edits a file and
+	 * then reverts it.
+	 */
+	everDirty?: ReadonlySet<string>;
+	/**
+	 * Timestamp of the last scan received from the server. Rendered as a
+	 * live-updating "checked Ns ago" label next to the refresh button.
+	 */
+	lastCheckedAt?: Date | undefined;
 }
 
 function repoTabLabel(repoRoot: string): string {
@@ -70,6 +83,8 @@ export const GitPanel: FC<GitPanelProps> = ({
 	isExpanded,
 	remoteDiffStats,
 	chatInputRef,
+	everDirty,
+	lastCheckedAt,
 }) => {
 	const hasRemoteStats =
 		(remoteDiffStats?.additions ?? 0) > 0 ||
@@ -102,9 +117,19 @@ export const GitPanel: FC<GitPanelProps> = ({
 		return stats;
 	})();
 
-	const localRepos = Array.from(repoStats.keys()).sort((a, b) =>
-		a.localeCompare(b),
-	);
+	// Union of currently-dirty and ever-dirty repos (still known to
+	// the watcher) so a clean-revert does not hide the tab.
+	const localRepos = (() => {
+		const roots = new Set<string>(repoStats.keys());
+		if (everDirty) {
+			for (const root of everDirty) {
+				if (repositories.has(root)) {
+					roots.add(root);
+				}
+			}
+		}
+		return Array.from(roots).sort((a, b) => a.localeCompare(b));
+	})();
 
 	// Default to the first local repo when there are only local
 	// changes and no remote stats.
@@ -122,7 +147,9 @@ export const GitPanel: FC<GitPanelProps> = ({
 				setView({ type: "local", repoRoot: localRepos[0] });
 			}
 		} else if (view.type === "local") {
-			if (!repoStats.has(view.repoRoot)) {
+			// localRepos includes ever-dirty repos with empty diffs, so
+			// the active tab stays valid until its root leaves the set.
+			if (!localRepos.includes(view.repoRoot)) {
 				if (showRemoteTab) {
 					setView({ type: "remote" });
 				} else if (localRepos.length > 0) {
@@ -130,7 +157,7 @@ export const GitPanel: FC<GitPanelProps> = ({
 				}
 			}
 		}
-	}, [view, showRemoteTab, localRepos, repoStats]);
+	}, [view, showRemoteTab, localRepos]);
 
 	const [diffStyle, setDiffStyle] = useState<DiffStyle>(loadDiffStyle);
 
@@ -231,6 +258,10 @@ export const GitPanel: FC<GitPanelProps> = ({
 				</ScrollArea>
 				{/* Controls */}
 				<div className="flex shrink-0 items-center gap-1 py-1.5">
+					<LastCheckedLabel
+						at={lastCheckedAt}
+						className="mr-1 whitespace-nowrap text-[11px] text-content-secondary"
+					/>
 					<div className="flex h-6 items-stretch overflow-hidden rounded-md border border-solid border-border-default">
 						<button
 							type="button"

site/src/pages/AgentsPage/components/GitPanel/LastCheckedLabel.tsx

@@ -0,0 +1,32 @@
+import type { FC } from "react";
+import { cn } from "#/utils/cn";
+import { relativeTime } from "#/utils/time";
+
+interface LastCheckedLabelProps {
+	at: Date | undefined;
+	className?: string;
+}
+
+/**
+ * Renders "checked <relative time>" next to the refresh button.
+ * Compiled by React Compiler, so re-renders are driven by a fresh
+ * `at` reference; the server's 5s scan heartbeat supplies that.
+ * Returns null before the first scan so the toolbar collapses.
+ */
+export const LastCheckedLabel: FC<LastCheckedLabelProps> = ({
+	at,
+	className,
+}) => {
+	if (!at) {
+		return null;
+	}
+	return (
+		<span
+			data-testid="git-last-checked"
+			className={cn(["whitespace-nowrap", className])}
+			title={at.toLocaleString()}
+		>
+			checked {relativeTime(at)}
+		</span>
+	);
+};

site/src/pages/AgentsPage/hooks/useGitWatcher.test.ts

@@ -664,4 +664,432 @@ describe("useGitWatcher", () => {
 		});
 		expect(result.current.repositories).toBe(ref1);
 	});
+
+	it("tracks everDirty: adds repo on first non-empty diff", async () => {
+		const socket = createMockSocket();
+
+		const { result } = renderHook(() =>
+			useGitWatcher({ chatId: "chat-123", agentStatus: "connected" }),
+		);
+
+		act(() => socket.simulateOpen());
+
+		act(() => {
+			socket.simulateMessage({
+				type: "changes",
+				repositories: [
+					{
+						repo_root: "/repo",
+						branch: "main",
+						unified_diff: "some diff",
+					},
+				],
+			});
+		});
+
+		await waitFor(() => {
+			expect(result.current.everDirty.has("/repo")).toBe(true);
+		});
+	});
+
+	it("tracks everDirty: retains repo after diff goes empty", async () => {
+		const socket = createMockSocket();
+
+		const { result } = renderHook(() =>
+			useGitWatcher({ chatId: "chat-123", agentStatus: "connected" }),
+		);
+
+		act(() => socket.simulateOpen());
+
+		// First, dirty the repo.
+		act(() => {
+			socket.simulateMessage({
+				type: "changes",
+				repositories: [
+					{
+						repo_root: "/repo",
+						branch: "main",
+						unified_diff: "diff1",
+					},
+				],
+			});
+		});
+
+		await waitFor(() => {
+			expect(result.current.everDirty.has("/repo")).toBe(true);
+		});
+
+		// Revert the repo to clean (empty diff). The repo stays in
+		// repositories, and everDirty retains it so the UI can keep
+		// the tab visible.
+		act(() => {
+			socket.simulateMessage({
+				type: "changes",
+				repositories: [
+					{
+						repo_root: "/repo",
+						branch: "main",
+						unified_diff: "",
+					},
+				],
+			});
+		});
+
+		await waitFor(() => {
+			expect(result.current.repositories.get("/repo")?.unified_diff).toBe("");
+		});
+		expect(result.current.everDirty.has("/repo")).toBe(true);
+	});
+
+	it("tracks everDirty: does not add on first empty diff", async () => {
+		const socket = createMockSocket();
+
+		const { result } = renderHook(() =>
+			useGitWatcher({ chatId: "chat-123", agentStatus: "connected" }),
+		);
+
+		act(() => socket.simulateOpen());
+
+		act(() => {
+			socket.simulateMessage({
+				type: "changes",
+				repositories: [
+					{
+						repo_root: "/repo",
+						branch: "main",
+						unified_diff: "",
+					},
+				],
+			});
+		});
+
+		await waitFor(() => {
+			expect(result.current.repositories.size).toBe(1);
+		});
+		expect(result.current.everDirty.size).toBe(0);
+	});
+
+	it("tracks everDirty: removes repo on removed: true", async () => {
+		const socket = createMockSocket();
+
+		const { result } = renderHook(() =>
+			useGitWatcher({ chatId: "chat-123", agentStatus: "connected" }),
+		);
+
+		act(() => socket.simulateOpen());
+
+		act(() => {
+			socket.simulateMessage({
+				type: "changes",
+				repositories: [
+					{
+						repo_root: "/repo",
+						branch: "main",
+						unified_diff: "diff1",
+					},
+				],
+			});
+		});
+
+		await waitFor(() => {
+			expect(result.current.everDirty.has("/repo")).toBe(true);
+		});
+
+		act(() => {
+			socket.simulateMessage({
+				type: "changes",
+				repositories: [
+					{
+						repo_root: "/repo",
+						branch: "",
+						removed: true,
+					},
+				],
+			});
+		});
+
+		await waitFor(() => {
+			expect(result.current.repositories.size).toBe(0);
+		});
+		expect(result.current.everDirty.size).toBe(0);
+	});
+
+	it("tracks everDirty: clears on chatId change", async () => {
+		const socket1 = createMockSocket();
+
+		const { result, rerender } = renderHook(
+			({ chatId }: { chatId: string }) =>
+				useGitWatcher({ chatId, agentStatus: "connected" }),
+			{ initialProps: { chatId: "chat-A" } },
+		);
+
+		act(() => socket1.simulateOpen());
+
+		act(() => {
+			socket1.simulateMessage({
+				type: "changes",
+				scanned_at: "2024-01-02T03:04:05Z",
+				repositories: [
+					{
+						repo_root: "/repo",
+						branch: "main",
+						unified_diff: "diff1",
+					},
+				],
+			});
+		});
+
+		await waitFor(() => {
+			expect(result.current.everDirty.has("/repo")).toBe(true);
+		});
+		expect(result.current.lastCheckedAt).toBeInstanceOf(Date);
+
+		// Switch to a different chat. The hook tears down and recreates
+		// the socket; everDirty, repositories, and lastCheckedAt must
+		// all reset so chat-B starts with a clean slate.
+		createMockSocket();
+		rerender({ chatId: "chat-B" });
+
+		expect(result.current.repositories.size).toBe(0);
+		expect(result.current.everDirty.size).toBe(0);
+		expect(result.current.lastCheckedAt).toBeUndefined();
+	});
+
+	it("tracks lastCheckedAt from scanned_at", async () => {
+		const socket = createMockSocket();
+
+		const { result } = renderHook(() =>
+			useGitWatcher({ chatId: "chat-123", agentStatus: "connected" }),
+		);
+
+		act(() => socket.simulateOpen());
+		expect(result.current.lastCheckedAt).toBeUndefined();
+
+		act(() => {
+			socket.simulateMessage({
+				type: "changes",
+				scanned_at: "2024-01-02T03:04:05Z",
+				repositories: [],
+			});
+		});
+
+		await waitFor(() => {
+			expect(result.current.lastCheckedAt).toBeInstanceOf(Date);
+		});
+		expect(result.current.lastCheckedAt?.toISOString()).toBe(
+			"2024-01-02T03:04:05.000Z",
+		);
+	});
+
+	it("heartbeat message (no repositories) advances lastCheckedAt without touching repos", async () => {
+		const socket = createMockSocket();
+
+		const { result } = renderHook(() =>
+			useGitWatcher({ chatId: "chat-123", agentStatus: "connected" }),
+		);
+
+		act(() => socket.simulateOpen());
+
+		// Prime a known-dirty repo via a delta message.
+		act(() => {
+			socket.simulateMessage({
+				type: "changes",
+				scanned_at: "2024-01-02T03:04:05Z",
+				repositories: [
+					{
+						repo_root: "/repo",
+						branch: "main",
+						unified_diff: "diff1",
+					},
+				],
+			});
+		});
+		await waitFor(() => {
+			expect(result.current.repositories.size).toBe(1);
+			expect(result.current.everDirty.has("/repo")).toBe(true);
+		});
+		const repoStateBeforeHeartbeat = result.current.repositories;
+		const everDirtyBeforeHeartbeat = result.current.everDirty;
+
+		// Heartbeat: scanned_at advances, but repositories is absent.
+		// The server sends this shape when Scan() observed no deltas
+		// (idle clean-staying-clean or no-change-since-last-emit).
+		act(() => {
+			socket.simulateMessage({
+				type: "changes",
+				scanned_at: "2024-01-02T03:04:10Z",
+			});
+		});
+
+		await waitFor(() => {
+			expect(result.current.lastCheckedAt?.toISOString()).toBe(
+				"2024-01-02T03:04:10.000Z",
+			);
+		});
+		// Heartbeat must not mutate repo state.
+		expect(result.current.repositories).toBe(repoStateBeforeHeartbeat);
+		expect(result.current.everDirty).toBe(everDirtyBeforeHeartbeat);
+	});
+
+	it("heartbeat with explicit empty repositories array is also a no-op for repos", async () => {
+		const socket = createMockSocket();
+
+		const { result } = renderHook(() =>
+			useGitWatcher({ chatId: "chat-123", agentStatus: "connected" }),
+		);
+
+		act(() => socket.simulateOpen());
+
+		act(() => {
+			socket.simulateMessage({
+				type: "changes",
+				scanned_at: "2024-01-02T03:04:05Z",
+				repositories: [
+					{
+						repo_root: "/repo",
+						branch: "main",
+						unified_diff: "diff1",
+					},
+				],
+			});
+		});
+		await waitFor(() => {
+			expect(result.current.repositories.size).toBe(1);
+		});
+
+		// Some JSON encoders may preserve an empty array rather than
+		// omitting the field. Either shape must be treated as a pure
+		// heartbeat on the client.
+		act(() => {
+			socket.simulateMessage({
+				type: "changes",
+				scanned_at: "2024-01-02T03:04:15Z",
+				repositories: [],
+			});
+		});
+
+		await waitFor(() => {
+			expect(result.current.lastCheckedAt?.toISOString()).toBe(
+				"2024-01-02T03:04:15.000Z",
+			);
+		});
+		// Repo entry survives an empty-array heartbeat.
+		expect(result.current.repositories.has("/repo")).toBe(true);
+		expect(result.current.everDirty.has("/repo")).toBe(true);
+	});
+
+	it("ignores malformed scanned_at without clearing existing value", async () => {
+		const socket = createMockSocket();
+
+		const { result } = renderHook(() =>
+			useGitWatcher({ chatId: "chat-123", agentStatus: "connected" }),
+		);
+
+		act(() => socket.simulateOpen());
+
+		// Prime a valid timestamp.
+		act(() => {
+			socket.simulateMessage({
+				type: "changes",
+				scanned_at: "2024-01-02T03:04:05Z",
+				repositories: [],
+			});
+		});
+
+		await waitFor(() => {
+			expect(result.current.lastCheckedAt).toBeDefined();
+		});
+		const firstIso = result.current.lastCheckedAt?.toISOString();
+
+		// A malformed scanned_at must not wipe the previous value.
+		act(() => {
+			socket.simulateMessage({
+				type: "changes",
+				scanned_at: "not-a-date",
+				repositories: [],
+			});
+		});
+
+		// Compare by ISO string, not reference. A future refactor that
+		// re-creates the Date object identically should still pass; what
+		// matters is that the observed timestamp did not change.
+		expect(result.current.lastCheckedAt?.toISOString()).toBe(firstIso);
+	});
+
+	it("tracks everDirty: preserves across agentStatus flap on the same chat", async () => {
+		const socket1 = createMockSocket();
+
+		const { result, rerender } = renderHook(
+			({ agentStatus }: { agentStatus: WorkspaceAgentStatus | undefined }) =>
+				useGitWatcher({ chatId: "chat-stable", agentStatus }),
+			{ initialProps: { agentStatus: "connected" as WorkspaceAgentStatus } },
+		);
+
+		act(() => socket1.simulateOpen());
+		act(() => {
+			socket1.simulateMessage({
+				type: "changes",
+				repositories: [
+					{
+						repo_root: "/repo",
+						branch: "main",
+						unified_diff: "diff1",
+					},
+				],
+			});
+		});
+		await waitFor(() => {
+			expect(result.current.everDirty.has("/repo")).toBe(true);
+		});
+
+		// Simulate a transient agentStatus flap. This tears down the
+		// socket but must not forget that /repo was dirty during this
+		// chat session.
+		createMockSocket();
+		rerender({ agentStatus: "connecting" as WorkspaceAgentStatus });
+		expect(result.current.everDirty.has("/repo")).toBe(true);
+
+		createMockSocket();
+		rerender({ agentStatus: "connected" as WorkspaceAgentStatus });
+		expect(result.current.everDirty.has("/repo")).toBe(true);
+	});
+
+	it("preserves lastCheckedAt across agentStatus flap on the same chat", async () => {
+		const socket1 = createMockSocket();
+
+		const { result, rerender } = renderHook(
+			({ agentStatus }: { agentStatus: WorkspaceAgentStatus | undefined }) =>
+				useGitWatcher({ chatId: "chat-stable", agentStatus }),
+			{ initialProps: { agentStatus: "connected" as WorkspaceAgentStatus } },
+		);
+
+		act(() => socket1.simulateOpen());
+		act(() => {
+			socket1.simulateMessage({
+				type: "changes",
+				scanned_at: "2024-01-02T03:04:05Z",
+				repositories: [],
+			});
+		});
+		await waitFor(() => {
+			expect(result.current.lastCheckedAt?.toISOString()).toBe(
+				"2024-01-02T03:04:05.000Z",
+			);
+		});
+
+		// Transient flap: the socket is torn down but the stale-data
+		// timestamp must keep advancing so the UI's "checked Ns ago"
+		// label does not disappear during reconnection backoff.
+		createMockSocket();
+		rerender({ agentStatus: "connecting" as WorkspaceAgentStatus });
+		expect(result.current.lastCheckedAt?.toISOString()).toBe(
+			"2024-01-02T03:04:05.000Z",
+		);
+
+		createMockSocket();
+		rerender({ agentStatus: "connected" as WorkspaceAgentStatus });
+		expect(result.current.lastCheckedAt?.toISOString()).toBe(
+			"2024-01-02T03:04:05.000Z",
+		);
+	});
 });

site/src/pages/AgentsPage/hooks/useGitWatcher.ts

@@ -29,6 +29,16 @@ interface UseGitWatcherOptions {
 interface UseGitWatcherResult {
 	/** Current repo state, keyed by repo root path. */
 	repositories: ReadonlyMap<string, WorkspaceAgentRepoChanges>;
+	/**
+	 * Repo roots seen with a non-empty unified_diff during this chat.
+	 * Survives reconnects; evicted on `removed: true`, cleared on
+	 * chatId change. Consumers should intersect with `repositories`.
+	 */
+	everDirty: ReadonlySet<string>;
+	/** ScannedAt from the latest server message. Undefined until the
+	 *  first message arrives. Preserved across reconnects so the UI's
+	 *  relative-time label keeps advancing during backoff. */
+	lastCheckedAt: Date | undefined;
 	/** Whether the WebSocket is currently connected. */
 	isConnected: boolean;
 	/** Send a refresh request. Returns true if sent, false if disconnected. */
@@ -42,9 +52,24 @@ export function useGitWatcher({
 	const [repositories, setRepositories] = useState<
 		ReadonlyMap<string, WorkspaceAgentRepoChanges>
 	>(new Map());
+	const [everDirty, setEverDirty] = useState<ReadonlySet<string>>(
+		() => new Set(),
+	);
+	const [lastCheckedAt, setLastCheckedAt] = useState<Date | undefined>(
+		undefined,
+	);
 	const [isConnected, setIsConnected] = useState(false);
 
 	const socketRef = useRef<WebSocket | null>(null);
+	// Chat-scoped state (everDirty, lastCheckedAt) resets on chatId
+	// change but must survive agentStatus flaps on the same chat.
+	// https://react.dev/reference/react/useState#storing-information-from-previous-renders
+	const [lastChatId, setLastChatId] = useState<string | undefined>(chatId);
+	if (lastChatId !== chatId) {
+		setLastChatId(chatId);
+		setEverDirty((prev) => (prev.size === 0 ? prev : new Set()));
+		setLastCheckedAt(undefined);
+	}
 
 	const sendMessage = (msg: WorkspaceAgentGitClientMessage): boolean => {
 		const socket = socketRef.current;
@@ -86,31 +111,54 @@ export function useGitWatcher({
 						return;
 					}
 
-					if (data.type === "changes" && data.repositories) {
-						setRepositories((prev) => {
-							let changed = false;
-							const next = new Map(prev);
-							for (const repo of data.repositories!) {
-								if (repo.removed) {
-									if (next.has(repo.repo_root)) {
-										next.delete(repo.repo_root);
-										changed = true;
+					if (data.type === "changes") {
+						if (data.scanned_at) {
+							const parsed = new Date(data.scanned_at);
+							if (!Number.isNaN(parsed.getTime())) {
+								setLastCheckedAt(parsed);
+							}
+						}
+						if (data.repositories) {
+							setRepositories((prev) => {
+								let changed = false;
+								const next = new Map(prev);
+								for (const repo of data.repositories!) {
+									if (repo.removed) {
+										if (next.has(repo.repo_root)) {
+											next.delete(repo.repo_root);
+											changed = true;
+										}
+									} else {
+										const existing = next.get(repo.repo_root);
+										if (
+											!existing ||
+											existing.branch !== repo.branch ||
+											existing.remote_origin !== repo.remote_origin ||
+											existing.unified_diff !== repo.unified_diff
+										) {
+											next.set(repo.repo_root, repo);
+											changed = true;
+										}
 									}
-								} else {
-									const existing = next.get(repo.repo_root);
-									if (
-										!existing ||
-										existing.branch !== repo.branch ||
-										existing.remote_origin !== repo.remote_origin ||
-										existing.unified_diff !== repo.unified_diff
-									) {
-										next.set(repo.repo_root, repo);
+								}
+								return changed ? next : prev;
+							});
+							setEverDirty((prev) => {
+								let changed = false;
+								const next = new Set(prev);
+								for (const repo of data.repositories!) {
+									if (repo.removed) {
+										if (next.delete(repo.repo_root)) {
+											changed = true;
+										}
+									} else if (repo.unified_diff && !next.has(repo.repo_root)) {
+										next.add(repo.repo_root);
 										changed = true;
 									}
 								}
-							}
-							return changed ? next : prev;
-						});
+								return changed ? next : prev;
+							});
+						}
 					} else if (data.type === "error") {
 						console.warn("[useGitWatcher] server error:", data.message);
 					}
@@ -134,8 +182,9 @@ export function useGitWatcher({
 		});
 
 		return () => {
-			// dispose() suppresses onDisconnect, so reset state
-			// explicitly.
+			// Reset connection-scoped state only. `everDirty` and
+			// `lastCheckedAt` are chat-scoped and persist across
+			// reconnects, so a slow backoff keeps the label advancing.
 			dispose();
 			setIsConnected(false);
 			setRepositories(new Map());
@@ -143,5 +192,5 @@ export function useGitWatcher({
 		};
 	}, [chatId, agentStatus]);
 
-	return { repositories, isConnected, refresh };
+	return { repositories, everDirty, lastCheckedAt, isConnected, refresh };
 }