Checkpoint: auto-retry after a failed pass; release rate-limit slot on failure

A paste fires exactly one grammar checkpoint, and a failed one never retried
until the next keystroke — stranding the writer on "Petal's helper is resting"
after a paste. Long docs make it worse: their 15-25s checks have a wide window
to catch a transient 502 from the shared Ollama (co-tenant apps load other
models and evict the 9B). A failed pass also burned the per-document rate-limit
slot, so a retry within 30s hit the throttle path and got an empty set back.

- llm.RateLimiter.Release rolls back a slot when its pass fails; Allow now
  returns the recorded timestamp so Release only frees its own slot.
- suggestions.runPass releases the slot on LLM failure before returning 502.
- useCheckpoint auto-retries a failed checkpoint with backoff (3/12/35s),
  keeping the breathing dot up and only flagging "resting" once retries exhaust.

Claude-Session: https://claude.ai/code/session_016Yr6jELuRc7hyzYLccQKZd

internal/llm/checkpoint.go

@@ -127,17 +127,33 @@ func NewRateLimiter(interval time.Duration) *RateLimiter {
 }
 
 // Allow reports whether a checkpoint may run for docID now. When allowed it
-// records the time and returns (true, 0); when throttled it returns
-// (false, retryAfter) where retryAfter is the wait until the next allowed run.
-func (rl *RateLimiter) Allow(docID string) (bool, time.Duration) {
+// records the time and returns (true, 0, recordedAt); when throttled it returns
+// (false, retryAfter, zero) where retryAfter is the wait until the next allowed
+// run. recordedAt lets a caller whose pass fails hand the exact timestamp to
+// Release so it rolls back only its own slot.
+func (rl *RateLimiter) Allow(docID string) (bool, time.Duration, time.Time) {
 	rl.mu.Lock()
 	defer rl.mu.Unlock()
 	now := time.Now()
 	if last, ok := rl.last[docID]; ok {
 		if elapsed := now.Sub(last); elapsed < rl.interval {
-			return false, rl.interval - elapsed
+			return false, rl.interval - elapsed, time.Time{}
 		}
 	}
 	rl.last[docID] = now
-	return true, 0
+	return true, 0, now
+}
+
+// Release rolls back the slot a prior Allow recorded for docID, but only if no
+// newer Allow has since claimed it. Called when an LLM pass fails: Allow runs
+// before the model call, so without this a failed checkpoint would hold the
+// per-document slot for the full interval and a retry (or the frontend's
+// auto-retry) would hit the throttle path and get the stale/empty set back
+// instead of re-running. `at` is the time the failed Allow returned.
+func (rl *RateLimiter) Release(docID string, at time.Time) {
+	rl.mu.Lock()
+	defer rl.mu.Unlock()
+	if last, ok := rl.last[docID]; ok && last.Equal(at) {
+		delete(rl.last, docID)
+	}
 }

internal/llm/checkpoint_test.go

@@ -65,16 +65,31 @@ func TestParseCheckpoint(t *testing.T) {
 func TestRateLimiter(t *testing.T) {
 	rl := NewRateLimiter(30 * time.Second)
 
-	if ok, _ := rl.Allow("doc1"); !ok {
+	ok, _, at := rl.Allow("doc1")
+	if !ok {
 		t.Fatal("first call should be allowed")
 	}
-	if ok, retry := rl.Allow("doc1"); ok || retry <= 0 {
+	if ok, retry, _ := rl.Allow("doc1"); ok || retry <= 0 {
 		t.Fatalf("immediate second call should be throttled, got ok=%v retry=%v", ok, retry)
 	}
 	// A different document is independent.
-	if ok, _ := rl.Allow("doc2"); !ok {
+	if ok, _, _ := rl.Allow("doc2"); !ok {
 		t.Fatal("different doc should be allowed")
 	}
+
+	// Releasing doc1's slot (as a failed pass does) lets the next check re-run
+	// immediately instead of waiting out the interval.
+	rl.Release("doc1", at)
+	if ok, _, _ := rl.Allow("doc1"); !ok {
+		t.Fatal("after Release the next call should be allowed again")
+	}
+	// Release only rolls back its own slot: a stale timestamp must not evict the
+	// slot a newer Allow holds (so a late failure can't cancel a fresh in-flight
+	// check). doc2 still holds its slot from above; a zero-time Release is a no-op.
+	rl.Release("doc2", time.Time{})
+	if ok, _, _ := rl.Allow("doc2"); ok {
+		t.Fatal("stale Release must not free doc2's current slot")
+	}
 }
 
 func TestTruncateDoc(t *testing.T) {

internal/suggestions/handlers.go

@@ -102,7 +102,8 @@ func (h *Handler) runPass(w http.ResponseWriter, r *http.Request, limiter *llm.R
 		return
 	}
 
-	if ok, _ := limiter.Allow(docID); !ok {
+	ok, _, slotAt := limiter.Allow(docID)
+	if !ok {
 		// Throttled: return the existing pending set unchanged rather than an
 		// error, so the frontend keeps showing current suggestions.
 		existing, err := h.fetchPending(docID)
@@ -116,6 +117,10 @@ func (h *Handler) runPass(w http.ResponseWriter, r *http.Request, limiter *llm.R
 
 	raw, err := run(r.Context(), h.Client, contentText, tone)
 	if err != nil {
+		// Allow ran before the model call, so a failed pass would otherwise hold
+		// the per-document slot for the full interval — stranding the frontend's
+		// auto-retry on the throttle path. Release it so a retry can re-run.
+		limiter.Release(docID, slotAt)
 		errorJSON(w, http.StatusBadGateway, "llm pass failed: "+err.Error())
 		return
 	}

web/src/hooks/useCheckpoint.ts

@@ -20,17 +20,29 @@ export function useCheckpoint(docId: string | null) {
   const [llmDown, setLlmDown] = useState(false)
 
   const debounceRef = useRef<ReturnType<typeof setTimeout>>(undefined)
+  // Pending auto-retry timer for a failed checkpoint (see runCheck).
+  const retryRef = useRef<ReturnType<typeof setTimeout>>(undefined)
   const docIdRef = useRef(docId)
   docIdRef.current = docId
 
   // Token to discard responses from a doc we've since navigated away from.
   const runRef = useRef(0)
 
-  const runCheck = useCallback(async () => {
+  // Backoff schedule for re-running a checkpoint that failed. A paste fires
+  // exactly ONE checkpoint, and nothing re-fires until the next keystroke — so a
+  // single transient failure (a momentary stall on the shared inference box)
+  // would otherwise strand the user on "resting" indefinitely after a paste.
+  // These delays clear the server's 30s per-doc floor by the later attempts, and
+  // a failed pass now releases its slot server-side so a retry can truly re-run.
+  const RETRY_DELAYS_MS = [3000, 12000, 35000]
+
+  const runCheck = useCallback(async (attempt = 0) => {
     const id = docIdRef.current
     if (!id) return
+    clearTimeout(retryRef.current)
     const run = ++runRef.current
     setChecking(true)
+    let retrying = false
     try {
       const fresh = await api.checkDoc(id)
       if (run === runRef.current && id === docIdRef.current) {
@@ -39,9 +51,18 @@ export function useCheckpoint(docId: string | null) {
       }
     } catch (err) {
       console.error('checkpoint failed', err)
-      if (run === runRef.current) setLlmDown(true)
+      if (run !== runRef.current) return
+      if (attempt < RETRY_DELAYS_MS.length) {
+        // Keep trying quietly — don't flag "resting" until retries are exhausted.
+        retrying = true
+        retryRef.current = setTimeout(() => void runCheck(attempt + 1), RETRY_DELAYS_MS[attempt])
+      } else {
+        setLlmDown(true)
+      }
     } finally {
-      if (run === runRef.current) setChecking(false)
+      // Stay in the "checking" state while a retry is queued so the breathing dot
+      // keeps reassuring rather than flickering off between attempts.
+      if (run === runRef.current && !retrying) setChecking(false)
     }
   }, [])
 
@@ -51,6 +72,7 @@ export function useCheckpoint(docId: string | null) {
   const runVoice = useCallback(async () => {
     const id = docIdRef.current
     if (!id) return
+    clearTimeout(retryRef.current) // a voice pass supersedes a queued grammar retry
     const run = ++runRef.current
     setVoicing(true)
     try {
@@ -70,13 +92,15 @@ export function useCheckpoint(docId: string | null) {
   // Call on every edit; schedules a check 4s after typing settles.
   const schedule = useCallback(() => {
     clearTimeout(debounceRef.current)
-    debounceRef.current = setTimeout(runCheck, DEBOUNCE_MS)
+    clearTimeout(retryRef.current) // a fresh edit supersedes any queued retry
+    debounceRef.current = setTimeout(() => void runCheck(), DEBOUNCE_MS)
   }, [runCheck])
 
   // Load existing pending suggestions whenever the document changes, and cancel
   // any in-flight debounce from the previous doc.
   useEffect(() => {
     clearTimeout(debounceRef.current)
+    clearTimeout(retryRef.current)
     runRef.current++
     setSuggestions([])
     setChecking(false)
@@ -97,7 +121,13 @@ export function useCheckpoint(docId: string | null) {
     }
   }, [docId])
 
-  useEffect(() => () => clearTimeout(debounceRef.current), [])
+  useEffect(
+    () => () => {
+      clearTimeout(debounceRef.current)
+      clearTimeout(retryRef.current)
+    },
+    [],
+  )
 
   // Drop one suggestion locally (after accept/dismiss) without a refetch.
   const removeSuggestion = useCallback((id: string) => {