Stop regenerating the world on every check

A card vanishing and coming back seconds later, with different words, was
never about latency: every pass deleted its whole family and re-inserted
it, so each round minted new row ids. The rail keys on suggestion.id, so a
full remount was guaranteed — new id, new created_at (hence the re-fired
chime), and a fresh explanation from a model that re-reasons every time it
is asked. One unchanged mistake carried three different explanations in a
single sitting.

Passes now reconcile instead of replace. A re-proposed edit keeps its row:
its id, its created_at, and the wording she has already read. And the
grammar checkpoint stops asking about sentences nobody touched — the
document is split into hashed sentences, checked_chunks records which ones
a family has read, and only the difference is sent. When nothing changed
it doesn't call the model at all, and doesn't spend its rate-limit slot on
having done nothing.

The tone is part of a sentence's identity: cached advice was written for
the old register, so switching doc type re-reads every line.

replaceMechanics reconciles too, which mattered more than expected — the
rule pack fires 250 ms after a keystroke, so it was re-minting every local
card's id several times a sentence.

Only the grammar checkpoint is chunked. Voice is a property of the whole
document, and the collocation coach is a button she pressed asking for a
fresh read.

No client change was needed; stable ids were the whole of it.

Claude-Session: https://claude.ai/code/session_016y6gyuHkQXPiEuW8RGQyua

UX_REVIEW_2026-07-27.md

@@ -143,6 +143,81 @@ instability, doubles the pause after each accept, and burns qwen3.5 tokens.
 or position of any other card; re-check traffic after a one-sentence edit
 contains only that sentence's chunk; explanations are stable across rounds.
 
+### 2 — DONE (fourth session). Server-side; the client needed nothing.
+
+Both halves shipped, and they turned out to be one idea. The root cause of
+the vanish/reappear was structural: **every pass deleted its whole family
+and re-inserted it**, so each round minted new row ids. The rail keys its
+cards on `suggestion.id`, so a full remount was guaranteed — new id, new
+`created_at` (hence the re-fired arrival chime), and a freshly-worded
+explanation from a model that re-reasons every time it's asked.
+
+**Implemented:**
+
+- `chunk.go` — splits the document into sentences and hashes each. Newlines
+  always break; ASCII terminators need trailing whitespace (so `3.50` and
+  `Ms.` stay whole); `。！？` break outright, since Chinese runs sentences
+  together with no space and she writes both languages in one document. The
+  hash normalizes quotes and whitespace runs through the existing
+  `normalizeForDedup`, so the editor's constant quote rewriting and a
+  reflowed paragraph cost nothing. Identity is the hash, not the position —
+  insert a paragraph at the top and every sentence below keeps its cards.
+- `reconcile.go` — passes now *reconcile* rather than replace. A row on a
+  sentence this pass didn't ask about is kept untouched; a row whose
+  sentence is gone is dropped; a row on a sentence that was re-read survives
+  only if the model proposed the same edit again, keeping its id,
+  `created_at` and its **original explanation**. Re-proposals are matched on
+  `(original, replacement)` normalized — not on type, so a re-labelled edit
+  keeps the label she's already reading.
+- `checked_chunks` (migration `0014`) records which sentences a family has
+  read. The grammar checkpoint asks only about the difference. **When
+  nothing changed it doesn't call the model at all** — and doesn't consume
+  its rate-limit slot, so an idle check can't throttle the next real edit.
+- The tone is folded into a sentence's hash, so switching doc type still
+  re-reads every line: the same sentence gets different advice as an
+  academic essay than as a journal entry, and cached advice was written for
+  the old register.
+- `replaceMechanics` reconciles too. This mattered more than expected: the
+  rule pack fires 250 ms after a keystroke (item 3b), so it was re-minting
+  every local card's id several times a sentence.
+
+**Deliberately not done:**
+
+- Only the grammar checkpoint is chunked. Voice is a property of the
+  document as a whole — a sentence isn't inconsistent with itself — and the
+  collocation coach is a button she presses asking for a fresh read. Both
+  still read everything, but both now reconcile, so they keep their ids.
+- No client change. With stable ids the existing code already does what the
+  item asked for: the rail keeps its card DOM, an expanded card survives a
+  re-check, and the chime (which keys on id) stops re-firing for advice she
+  is already reading. The one-card optimistic removal on accept was already
+  there.
+
+**Sentences the model can't be trusted to have read.** Two guards the plan
+didn't anticipate, both found while writing the tests: a finding is
+attributed to a sentence the model was *actually shown* before falling back
+to the whole document (a short span like "the the" can occur twice, and
+crediting the cached copy would drop it); and a cached row whose quoted span
+no longer matches byte-for-byte is dropped *and* its sentence re-opened,
+rather than caching advice the frontend can't anchor.
+
+**Verified on the running binary**, not just in tests — per the handoff's
+own advice. Against a stand-in model server: three checks over a two-
+sentence document, editing only the second. The model received exactly
+`She goes to market yesterday.` and never saw the first sentence; the
+untouched card kept its id and its first explanation across all three
+rounds; the fixed sentence's card was dropped; the idle re-check made zero
+model calls. Mechanics identity confirmed the same way (a finding kept its
+row id while its span moved).
+
+Coverage: `chunk_test.go` (splitting, CJK, decimals, cosmetic churn) and
+`stability_test.go` (untouched cards keep id + explanation, unchanged
+document skips the model, deleted sentence drops its card, tone change
+re-opens everything, mechanics rows keep identity). Two existing tests
+changed contract deliberately — `TestFickleEditsSuppressed` and
+`TestCollocationPassCoexists` both re-checked a document nobody had edited,
+which is now a no-op; they edit the text between passes, as she always does.
+
 ## 3. Perceived latency: mask the LLM round-trip
 
 Measured ~8–15 s from typing-stop to cards, with only a small "Checking…"
@@ -338,6 +413,14 @@ reproducible) and item 5's original premise (re-scoped, much cheaper now).
 *(Third session: item 3b done — see the subsection under item 3. Item 3's
 incremental-surfacing half remains. Untouched: 2, 4, 6, 7, 8.)*
 
+*(Fourth session: item 2 done — see the subsection under it. **Neither 3b
+nor 2 is deployed yet**: both sit on unmerged topic branches
+(`feat/instant-local-rules`, then `feat/stable-suggestions` stacked on it)
+and `main` is still at `ba06d90`. Untouched: 4, 6, 7, 8. Item 3's
+incremental-surfacing half is now cheap — the chunking it was waiting on
+exists — but it needs streaming, which the current `/check` shape doesn't
+do.)*
+
 **Suggested next:** item 3b, the instant local rules layer — but it is
 **largely already built, in `main`**, and the item as written doesn't know
 that. Before writing any rules engine, read:

internal/db/db.go

@@ -497,6 +497,32 @@ CREATE INDEX idx_suggestions_resolved ON suggestions(status, resolved_at);
 			stmt: `
 ALTER TABLE suggestions ADD COLUMN source TEXT NOT NULL DEFAULT 'llm';
 UPDATE suggestions SET source = 'local' WHERE type = 'mechanics';
+`,
+		},
+		{
+			// Sentence-level identity, so a re-check stops regenerating the world.
+			// Every pass used to delete its whole family and re-insert it, which
+			// meant accepting one edit gave every other card a new id and a newly
+			// worded explanation — the rail visibly emptied and refilled, and the
+			// model was asked again about sentences nobody had touched.
+			//
+			// `chunk_hash` records which sentence a suggestion belongs to, and
+			// checked_chunks records which sentences a family has already read. A
+			// re-check then asks only about the difference and keeps the rest of
+			// the rows exactly as they are, id and wording included.
+			//
+			// Existing rows get '' — "sentence unknown", which reads as in-play, so
+			// they are simply reconciled on the next pass like any fresh finding.
+			name: "0014_suggestion_chunk_hash",
+			stmt: `
+ALTER TABLE suggestions ADD COLUMN chunk_hash TEXT NOT NULL DEFAULT '';
+
+CREATE TABLE checked_chunks (
+    doc_id TEXT NOT NULL REFERENCES documents(id) ON DELETE CASCADE,
+    family TEXT NOT NULL,
+    hash   TEXT NOT NULL,
+    PRIMARY KEY (doc_id, family, hash)
+);
 `,
 		},
 	}

internal/suggestions/chunk.go

@@ -0,0 +1,145 @@
+package suggestions
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"strings"
+	"unicode"
+)
+
+// Chunking splits a document into sentence-sized units so a re-check can ask the
+// model only about the sentences that actually changed. Accepting one edit used
+// to re-run the whole document: every card vanished, came back with a new id and
+// a freshly-worded explanation, and spans re-merged into different shapes. The
+// sentences she didn't touch have nothing new to say about themselves, so their
+// suggestions are simply kept (see reconcilePending).
+//
+// A chunk's identity is its hash, not its position — she inserts a paragraph at
+// the top and every sentence below keeps its suggestions.
+
+// chunk is one sentence of the document, with the hash that identifies it.
+type chunk struct {
+	text string
+	hash string
+}
+
+// asciiTerminators end a sentence only when whitespace (or the end of the text)
+// follows, so "3.5" and "Ms." don't split mid-word — a wrong split costs only a
+// slightly smaller chunk, but a split inside a number would churn its hash on
+// every keystroke around it.
+const asciiTerminators = ".!?"
+
+// cjkTerminators end a sentence outright: Chinese runs sentences together with
+// no space after 。, and she writes in both languages in one document.
+const cjkTerminators = "。！？"
+
+// closers are swallowed into the sentence they close, so the quote mark travels
+// with the sentence rather than opening the next one.
+const closers = `)]}"'’”」』`
+
+// splitChunks divides text into sentences, dropping whitespace-only runs.
+// Newlines always break a chunk, so a list or a line of dialogue is its own unit.
+//
+// `salt` distinguishes two *readings* of the same sentence. The grammar
+// checkpoint's advice depends on the document's tone — the same line gets
+// different notes as an academic essay than as a journal entry — so switching
+// tone must re-open every sentence rather than serve back advice written for the
+// old register.
+func splitChunks(text, salt string) []chunk {
+	var out []chunk
+	runes := []rune(text)
+	start := 0
+	add := func(end int) {
+		if s := string(runes[start:end]); strings.TrimSpace(s) != "" {
+			out = append(out, chunk{text: s, hash: hashChunk(s, salt)})
+		}
+		start = end
+	}
+
+	for i := 0; i < len(runes); i++ {
+		r := runes[i]
+		if r == '\n' {
+			add(i + 1)
+			continue
+		}
+		cjk := strings.ContainsRune(cjkTerminators, r)
+		if !cjk && !strings.ContainsRune(asciiTerminators, r) {
+			continue
+		}
+		// Swallow a run of terminators ("?!", "…") and any closing punctuation.
+		j := i + 1
+		for j < len(runes) && (strings.ContainsRune(asciiTerminators+cjkTerminators+closers, runes[j])) {
+			j++
+		}
+		if cjk || j >= len(runes) || unicode.IsSpace(runes[j]) {
+			add(j)
+			i = j - 1
+		}
+	}
+	if start < len(runes) {
+		add(len(runes))
+	}
+	return out
+}
+
+// hashChunk identifies a sentence by its content under the same normalization
+// the suppression logic uses: quote style and whitespace runs churn constantly
+// (the editor rewrites quotes as she types, a paragraph reflows) and none of
+// that changes what the sentence says, so none of it should cost a re-check.
+func hashChunk(s, salt string) string {
+	sum := sha256.Sum256([]byte(salt + "\x00" + normalizeForDedup(s)))
+	return hex.EncodeToString(sum[:])[:16]
+}
+
+// hashSet indexes chunks by hash — "is this sentence in the document?"
+func hashSet(chunks []chunk) map[string]bool {
+	out := make(map[string]bool, len(chunks))
+	for _, c := range chunks {
+		out[c.hash] = true
+	}
+	return out
+}
+
+// changedChunks returns the chunks whose hash wasn't in the last checked set,
+// in document order and deduplicated — a sentence repeated verbatim is one
+// question, not two.
+func changedChunks(chunks []chunk, checked map[string]bool) []chunk {
+	seen := make(map[string]bool, len(chunks))
+	var out []chunk
+	for _, c := range chunks {
+		if checked[c.hash] || seen[c.hash] {
+			continue
+		}
+		seen[c.hash] = true
+		out = append(out, c)
+	}
+	return out
+}
+
+// joinChunks renders a chunk set as the text to hand the model: one sentence per
+// line, so two sentences pulled from opposite ends of the document don't read as
+// one run-on.
+func joinChunks(chunks []chunk) string {
+	parts := make([]string, 0, len(chunks))
+	for _, c := range chunks {
+		parts = append(parts, strings.TrimSpace(c.text))
+	}
+	return strings.Join(parts, "\n")
+}
+
+// chunkFor names the sentence a suggestion belongs to: the first chunk whose
+// text contains the flagged span. Returns "" when the span straddles a sentence
+// boundary or the model paraphrased what it quoted — such a row is re-examined
+// on every pass rather than cached, which is the safe direction.
+func chunkFor(original string, chunks []chunk) string {
+	o := normalizeForDedup(original)
+	if o == "" {
+		return ""
+	}
+	for _, c := range chunks {
+		if strings.Contains(normalizeForDedup(c.text), o) {
+			return c.hash
+		}
+	}
+	return ""
+}

internal/suggestions/chunk_test.go

@@ -0,0 +1,118 @@
+package suggestions
+
+import (
+	"strings"
+	"testing"
+)
+
+func texts(chunks []chunk) []string {
+	out := make([]string, 0, len(chunks))
+	for _, c := range chunks {
+		out = append(out, strings.TrimSpace(c.text))
+	}
+	return out
+}
+
+func TestSplitChunks(t *testing.T) {
+	cases := []struct {
+		name string
+		in   string
+		want []string
+	}{
+		{
+			name: "plain sentences",
+			in:   "I has two apple. She go to market yesterday! Why?",
+			want: []string{"I has two apple.", "She go to market yesterday!", "Why?"},
+		},
+		{
+			// A decimal must not split, or the sentence's identity would churn
+			// while she types the number.
+			name: "decimals stay whole",
+			in:   "It costs 3.50 today. Tomorrow, more.",
+			want: []string{"It costs 3.50 today.", "Tomorrow, more."},
+		},
+		{
+			name: "closing quote travels with its sentence",
+			in:   `He said "early," and left. She stayed.`,
+			want: []string{`He said "early," and left.`, "She stayed."},
+		},
+		{
+			// Chinese runs sentences together with no space after 。 — she writes
+			// in both languages in one document.
+			name: "cjk terminators split without a space",
+			in:   "我想说这句话。但是不知道用英语怎么说。",
+			want: []string{"我想说这句话。", "但是不知道用英语怎么说。"},
+		},
+		{
+			name: "newlines break chunks",
+			in:   "A list item\nAnother item\n",
+			want: []string{"A list item", "Another item"},
+		},
+		{
+			name: "blank runs are dropped",
+			in:   "\n\n  \nOnly this.\n\n",
+			want: []string{"Only this."},
+		},
+		{
+			name: "trailing fragment is its own chunk",
+			in:   "Done. Still writing",
+			want: []string{"Done.", "Still writing"},
+		},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			got := texts(splitChunks(tc.in, ""))
+			if len(got) != len(tc.want) {
+				t.Fatalf("want %q, got %q", tc.want, got)
+			}
+			for i := range got {
+				if got[i] != tc.want[i] {
+					t.Fatalf("chunk %d: want %q, got %q", i, tc.want[i], got[i])
+				}
+			}
+		})
+	}
+}
+
+// A sentence's identity survives the churn that doesn't change what it says:
+// the editor rewrites quotes as she types, and a paragraph reflows.
+func TestChunkIdentityIgnoresCosmeticChurn(t *testing.T) {
+	a := splitChunks(`She said "hello" softly.`, "")
+	b := splitChunks("She said “hello”   softly.", "")
+	if len(a) != 1 || len(b) != 1 {
+		t.Fatalf("want one chunk each, got %d and %d", len(a), len(b))
+	}
+	if a[0].hash != b[0].hash {
+		t.Fatalf("quote/whitespace churn changed the sentence's identity")
+	}
+	if same := splitChunks(`She said "hello" softly.`, "academic"); same[0].hash == a[0].hash {
+		t.Fatalf("a different tone must be a different reading of the sentence")
+	}
+}
+
+func TestChangedChunksAndLookup(t *testing.T) {
+	chunks := splitChunks("One thing. Another thing. One thing.", "")
+	if len(chunks) != 3 {
+		t.Fatalf("want 3 chunks, got %d", len(chunks))
+	}
+
+	// A repeated sentence is one question, not two.
+	if got := changedChunks(chunks, nil); len(got) != 2 {
+		t.Fatalf("want 2 distinct changed chunks, got %d", len(got))
+	}
+
+	checked := hashSet(chunks[:1])
+	changed := changedChunks(chunks, checked)
+	if len(changed) != 1 || strings.TrimSpace(changed[0].text) != "Another thing." {
+		t.Fatalf("want only the unread sentence, got %q", texts(changed))
+	}
+
+	if chunkFor("Another", chunks) != chunks[1].hash {
+		t.Fatalf("span was attributed to the wrong sentence")
+	}
+	// A span the document doesn't contain has no sentence, so it is never cached.
+	if chunkFor("nowhere in here", chunks) != "" {
+		t.Fatalf("unanchorable span should have no chunk")
+	}
+}

internal/suggestions/handlers.go

@@ -160,16 +160,21 @@ func (h *Handler) mechanics(w http.ResponseWriter, r *http.Request) {
 	httputil.WriteJSON(w, http.StatusOK, out)
 }
 
-// replaceMechanics swaps the document's pending offline rows for the supplied
+// replaceMechanics brings the document's pending offline rows in line with the
-// findings in one transaction, leaving the LLM families and actioned rows
+// supplied findings in one transaction, leaving the LLM families and actioned
-// untouched. Findings the user already accepted or dismissed are suppressed (the
+// rows untouched. Findings the user already accepted or dismissed are suppressed
-// detector has no memory between runs), and malformed spans are skipped.
+// (the detector has no memory between runs), and malformed spans are skipped.
 //
-// The DELETE is scoped by *source*, not by type: the rule pack owns both the
+// A finding the detector still reports keeps its existing row — same id, same
-// mechanics family and its share of the collocation family, and every run is a
+// created_at — and only its offsets move. This pass fires 250 ms after a
-// full recompute of the document, so everything it wrote last time goes. Scoping
+// keystroke, so deleting and re-inserting the family would hand every card a new
-// by type instead would strand offline collocations the current text no longer
+// identity several times a sentence: the rail would remount, a card expanded for
-// warrants — the one row nobody would ever replace.
+// Ask Petal would collapse under her, and the arrival chime would re-fire.
+//
+// The scope is *source*, not type: the rule pack owns both the mechanics family
+// and its share of the collocation family, and every run is a full recompute of
+// the document. Scoping by type instead would strand offline collocations the
+// current text no longer warrants — the one row nobody would ever replace.
 func (h *Handler) replaceMechanics(docID string, findings []mechanicsFinding) error {
 	tx, err := h.DB.Begin()
 	if err != nil {
@@ -177,18 +182,18 @@ func (h *Handler) replaceMechanics(docID string, findings []mechanicsFinding) er
 	}
 	defer tx.Rollback()
 
-	if _, err := tx.Exec(
+	existing, err := loadPending(tx, docID, "source = '"+db.SuggestionSourceLocal+"'")
-		`DELETE FROM suggestions WHERE doc_id = ? AND status = ? AND source = ?`,
+	if err != nil {
-		docID, db.SuggestionStatusPending, db.SuggestionSourceLocal,
-	); err != nil {
 		return err
 	}
+	index := indexByEdit(existing)
 
 	sup, err := buildSuppressor(tx, docID)
 	if err != nil {
 		return err
 	}
 
+	kept := make(map[string]bool, len(existing))
 	for _, f := range findings {
 		if f.From < 0 || f.To <= f.From || strings.TrimSpace(f.Original) == "" {
 			continue // malformed span — the client re-anchors by string anyway
@@ -196,16 +201,34 @@ func (h *Handler) replaceMechanics(docID string, findings []mechanicsFinding) er
 		if sup.suppressed(f.Original, f.Replacement) {
 			continue
 		}
+		typ := localType(f.Type)
+		if row, ok := index.take(f.Original, f.Replacement, f.From); ok {
+			kept[row.id] = true
+			if err := reposition(tx, row, f.From, f.To, ""); err != nil {
+				return err
+			}
+			continue
+		}
 		if _, err := tx.Exec(
 			`INSERT INTO suggestions (doc_id, from_pos, to_pos, original, replacement, explanation, type, source)
 			 VALUES (?, ?, ?, ?, ?, ?, ?, ?)`,
 			docID, f.From, f.To, f.Original, f.Replacement, f.Explanation,
-			localType(f.Type), db.SuggestionSourceLocal,
+			typ, db.SuggestionSourceLocal,
 		); err != nil {
 			return err
 		}
 	}
 
+	// Whatever the detector no longer reports, she has fixed.
+	for _, row := range existing {
+		if kept[row.id] {
+			continue
+		}
+		if _, err := tx.Exec(`DELETE FROM suggestions WHERE id = ?`, row.id); err != nil {
+			return err
+		}
+	}
+
 	return tx.Commit()
 }
 
@@ -258,11 +281,64 @@ func (h *Handler) runPass(w http.ResponseWriter, r *http.Request, limiter *llm.R
 		return
 	}
 
-	// Nothing to analyze on an empty document — skip the LLM round-trip.
+	// Nothing to analyze on an empty document — skip the LLM round-trip. The
+	// family's rows go with the text they were about.
 	if strings.TrimSpace(contentText) == "" {
-		httputil.WriteJSON(w, http.StatusOK, []db.Suggestion{})
+		if err := h.reconcilePending(docID, contentText, nil, scope, nil, nil, false); err != nil {
+			httputil.ServerError(w, err)
 			return
 		}
+		out, err := h.fetchPending(userID, docID)
+		if err != nil {
+			httputil.ServerError(w, err)
+			return
+		}
+		httputil.WriteJSON(w, http.StatusOK, out)
+		return
+	}
+
+	// Decide what to ask about before spending anything: a chunked pass asks only
+	// about the sentences that changed since it last read the document, and when
+	// none did it doesn't call the model at all — nor consume its rate-limit slot,
+	// so the next real edit isn't throttled by a check that had nothing to do.
+	//
+	// Only a chunked pass consults that record, so only it needs the tone folded
+	// into a sentence's identity.
+	salt := ""
+	if scope.chunked {
+		salt = tone
+	}
+	chunks := splitChunks(contentText, salt)
+	askText, fresh := contentText, chunks
+	if scope.chunked {
+		checked, err := h.checkedChunks(docID, scope.family)
+		if err != nil {
+			httputil.ServerError(w, err)
+			return
+		}
+		changed := changedChunks(chunks, checked)
+		if len(changed) == 0 {
+			// Every sentence has already been read. Drop the rows whose sentence is
+			// gone, keep the rest exactly as they are, and answer immediately.
+			if err := h.reconcilePending(docID, contentText, nil, scope, chunks, nil, false); err != nil {
+				httputil.ServerError(w, err)
+				return
+			}
+			out, err := h.fetchPending(userID, docID)
+			if err != nil {
+				httputil.ServerError(w, err)
+				return
+			}
+			httputil.WriteJSON(w, http.StatusOK, out)
+			return
+		}
+		// When every sentence is new — a first pass, a paste, a tone switch — hand
+		// over the document verbatim so the model reads it with its paragraphing
+		// intact. Otherwise send just the delta, one sentence per line.
+		if len(changed) < len(hashSet(chunks)) {
+			askText, fresh = joinChunks(changed), changed
+		}
+	}
 
 	ok, _, slotAt := limiter.Allow(docID)
 	if !ok {
@@ -277,7 +353,7 @@ func (h *Handler) runPass(w http.ResponseWriter, r *http.Request, limiter *llm.R
 		return
 	}
 
-	raw, err := run(r.Context(), h.Client, contentText, tone, llm.LangFor(pairLang))
+	raw, err := run(r.Context(), h.Client, askText, tone, llm.LangFor(pairLang))
 	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
@@ -287,7 +363,9 @@ func (h *Handler) runPass(w http.ResponseWriter, r *http.Request, limiter *llm.R
 		return
 	}
 
-	if err := h.replacePending(docID, contentText, raw, scope); err != nil {
+	// A whole-document pass re-read everything, so every one of its rows is up for
+	// re-proposal; a chunked pass only puts the sentences it asked about in play.
+	if err := h.reconcilePending(docID, contentText, raw, scope, chunks, fresh, !scope.chunked); err != nil {
 		httputil.ServerError(w, err)
 		return
 	}
@@ -308,78 +386,49 @@ func (h *Handler) runPass(w http.ResponseWriter, r *http.Request, limiter *llm.R
 // inserts. The grammar checkpoint and voice pass each own a disjoint family, so
 // running one never disturbs the other's pending flags.
 type pendingScope struct {
-	deleteWhere string // extra WHERE clause scoping the DELETE to this family
+	deleteWhere string // extra WHERE clause scoping this pass to its own family
 	forceType   string // if set, every inserted row gets this type; else normalizeType
+	// family keys the sentences this pass has already read (see checked_chunks).
+	family string
+	// chunked passes re-read only the sentences that changed. True for the typing-
+	// cadence grammar checkpoint, which fires constantly and must feel still;
+	// false for the explicit whole-document passes, where she pressed a button
+	// asking for a fresh read of everything.
+	chunked bool
 }
 
-// Every scope below is confined to source='llm'. The offline rule pack replaces
+// Every scope below is confined to source='llm'. The offline rule pack owns its
-// its own rows wholesale on each edit (see replaceMechanics) and its findings
+// own rows and recomputes them on each edit (see replaceMechanics); its findings
 // must survive all three model passes — including the collocation coach, which
 // now shares the collocation family with it.
 var (
 	// grammarScope owns the grammar/phrasing/idiom/clarity flags — everything but
 	// the other self-owned families (voice, collocation), which run on their own
 	// cadence/pass and must survive a grammar checkpoint. Notably the offline pass
-	// writes its rows in the same /check request just before this DELETE runs, so
+	// writes its rows in the same /check request just before this pass reconciles,
-	// the source clause is also what keeps them alive.
+	// so the source clause is also what keeps them alive.
-	grammarScope = pendingScope{deleteWhere: "source = 'llm' AND type NOT IN ('voice','collocation')", forceType: ""}
+	grammarScope = pendingScope{
-	// voiceScope owns the model's voice flags only.
+		deleteWhere: "source = 'llm' AND type NOT IN ('voice','collocation')",
-	voiceScope = pendingScope{deleteWhere: "source = 'llm' AND type = 'voice'", forceType: db.SuggestionTypeVoice}
+		family:      "grammar",
+		chunked:     true,
+	}
+	// voiceScope owns the model's voice flags only. Voice is a property of the
+	// document as a whole — a sentence isn't inconsistent with itself — so this
+	// pass always reads everything.
+	voiceScope = pendingScope{
+		deleteWhere: "source = 'llm' AND type = 'voice'",
+		forceType:   db.SuggestionTypeVoice,
+		family:      "voice",
+	}
 	// collocationScope owns the model's collocation flags only — the rule pack's
 	// share of the same family is left standing.
-	collocationScope = pendingScope{deleteWhere: "source = 'llm' AND type = 'collocation'", forceType: db.SuggestionTypeCollocation}
+	collocationScope = pendingScope{
+		deleteWhere: "source = 'llm' AND type = 'collocation'",
+		forceType:   db.SuggestionTypeCollocation,
+		family:      "collocation",
+	}
 )
 
-// replacePending swaps a document's pending suggestions within one family for a
-// fresh batch in a single transaction. Accepted/rejected suggestions and the
-// other family's pending rows are left untouched.
-//
-// Suggestions touching a sentence the user already settled are suppressed from
-// the fresh batch (see suppressor): not just the identical edit re-proposed, but
-// reversals and re-polishing of the model's own just-accepted output — the
-// "fickle, keeps going back and forth on a few sentences" behavior. The model has
-// no memory between passes, so without this it re-opens resolved sentences every
-// checkpoint.
-func (h *Handler) replacePending(docID, contentText string, raw []llm.RawSuggestion, scope pendingScope) error {
-	tx, err := h.DB.Begin()
-	if err != nil {
-		return err
-	}
-	defer tx.Rollback()
-
-	if _, err := tx.Exec(
-		`DELETE FROM suggestions WHERE doc_id = ? AND status = ? AND `+scope.deleteWhere,
-		docID, db.SuggestionStatusPending,
-	); err != nil {
-		return err
-	}
-
-	sup, err := buildSuppressor(tx, docID)
-	if err != nil {
-		return err
-	}
-
-	for _, s := range raw {
-		if sup.suppressed(s.Original, s.Replacement) {
-			continue
-		}
-		typ := scope.forceType
-		if typ == "" {
-			typ = normalizeType(s.Type)
-		}
-		from, to := locate(contentText, s.Original)
-		if _, err := tx.Exec(
-			`INSERT INTO suggestions (doc_id, from_pos, to_pos, original, replacement, explanation, type, source)
-			 VALUES (?, ?, ?, ?, ?, ?, ?, ?)`,
-			docID, from, to, s.Original, s.Replacement, s.Explanation, typ, db.SuggestionSourceLLM,
-		); err != nil {
-			return err
-		}
-	}
-
-	return tx.Commit()
-}
-
 // dedupQuoteReplacer folds every straight/curly single- and double-quote variant
 // (and backtick/acute accent) onto one canonical character. The editor and the
 // model both rewrite quotes between passes — a sentence accepted with "…" comes

internal/suggestions/handlers_test.go

@@ -20,10 +20,17 @@ import (
 type stubClient struct {
 	response string
 	calls    int
+	// The full prompt of the most recent call, so a test can assert which
+	// sentences a chunked pass actually asked about.
+	lastPrompt string
 }
 
-func (s *stubClient) Complete(_ context.Context, _ llm.CompletionRequest) (string, error) {
+func (s *stubClient) Complete(_ context.Context, req llm.CompletionRequest) (string, error) {
 	s.calls++
+	s.lastPrompt = ""
+	for _, m := range req.Messages {
+		s.lastPrompt += m.Content + "\n"
+	}
 	return s.response, nil
 }
 
@@ -64,6 +71,17 @@ func newTestServer(t *testing.T, client llm.LLMClient) (http.Handler, string, *H
 	return authed, docID, h
 }
 
+// setDocText rewrites the seeded document, standing in for the writer editing.
+// The grammar checkpoint only asks the model about sentences that changed since
+// it last read the document, so a test that wants a second real pass has to
+// change something first — as she always has.
+func setDocText(t *testing.T, h *Handler, docID, text string) {
+	t.Helper()
+	if _, err := h.DB.Exec(`UPDATE documents SET content_text = ? WHERE id = ?`, text, docID); err != nil {
+		t.Fatalf("update doc text: %v", err)
+	}
+}
+
 func do(t *testing.T, srv http.Handler, method, path, body string) *httptest.ResponseRecorder {
 	t.Helper()
 	var r *http.Request
@@ -183,6 +201,7 @@ func TestFickleEditsSuppressed(t *testing.T) {
 	]}`}
 	srv, docID, h := newTestServer(t, client)
 	h.Limit = llm.NewRateLimiter(0)
+	setDocText(t, h, docID, `He left "early," because of the rain. The cat always have a calm face.`)
 
 	rec := do(t, srv, http.MethodPost, "/docs/"+docID+"/check", "")
 	var got []db.Suggestion
@@ -194,6 +213,10 @@ func TestFickleEditsSuppressed(t *testing.T) {
 		do(t, srv, http.MethodPost, "/suggestions/"+s.ID+"/accept", "")
 	}
 
+	// Both edits are now in the document, which is what re-opens those sentences
+	// for a second reading.
+	setDocText(t, h, docID, `He left "early," due to the rain. The cat always has a calm face.`)
+
 	// Reversal of the first accept (note the " → ' quote churn) and a re-polish of
 	// the second accept must both be dropped; only the unrelated edit survives.
 	client.response = `{"suggestions":[
@@ -314,7 +337,9 @@ func TestCollocationPassCoexists(t *testing.T) {
 		t.Fatalf("collocation response should carry all three families, got %+v", got)
 	}
 
-	// A grammar checkpoint must NOT wipe the voice or collocation flags.
+	// A grammar checkpoint must NOT wipe the voice or collocation flags. She fixes
+	// the flagged sentence, so its own grammar row goes and nothing replaces it.
+	setDocText(t, h, docID, "I have two apples.")
 	client.response = `{"suggestions":[]}`
 	rec = do(t, srv, http.MethodPost, "/docs/"+docID+"/check", "")
 	if rec.Code != http.StatusOK {

internal/suggestions/reconcile.go

@@ -0,0 +1,293 @@
+package suggestions
+
+import (
+	"database/sql"
+
+	"gitea.parodia.dev/drwily/petal/internal/db"
+	"gitea.parodia.dev/drwily/petal/internal/llm"
+)
+
+// Reconciliation replaces the old "delete the family, insert the new batch"
+// shape of every pass. A suggestion the pass proposes again is the *same*
+// suggestion: it keeps its row, and therefore its id, its created_at and — most
+// visibly — the explanation it was first given. The model re-words its reasoning
+// every time it is asked, so re-inserting meant one unchanged mistake carried
+// three different explanations in a single sitting.
+//
+// The id is what the frontend keys its cards on, so a stable id is also what
+// keeps the rail from emptying and refilling, a card from collapsing mid-read,
+// and the arrival chime from re-firing for advice she has already seen.
+
+// pendingRow is the part of an existing pending suggestion reconciliation cares
+// about.
+type pendingRow struct {
+	id          string
+	original    string
+	replacement string
+	chunkHash   string
+	from        int
+}
+
+// loadPending reads the pending rows a pass owns. `where` is the pass's own
+// scoping clause (by source, and for the model passes by family) — the same
+// fragment that used to scope its DELETE.
+func loadPending(tx *sql.Tx, docID, where string) ([]pendingRow, error) {
+	rows, err := tx.Query(
+		`SELECT id, original, replacement, chunk_hash, from_pos FROM suggestions
+		  WHERE doc_id = ? AND status = ? AND `+where,
+		docID, db.SuggestionStatusPending,
+	)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+
+	var out []pendingRow
+	for rows.Next() {
+		var r pendingRow
+		if err := rows.Scan(&r.id, &r.original, &r.replacement, &r.chunkHash, &r.from); err != nil {
+			return nil, err
+		}
+		out = append(out, r)
+	}
+	return out, rows.Err()
+}
+
+// editKey identifies an edit by what it proposes, not where: "this exact change
+// to this exact text". Normalized like the suppression comparisons, so the
+// editor's quote rewriting and a reflowed paragraph don't read as a new edit.
+func editKey(original, replacement string) string {
+	return normalizeForDedup(original) + "\x00" + normalizeForDedup(replacement)
+}
+
+// editIndex matches freshly proposed edits against the rows already standing.
+type editIndex struct {
+	rows  []pendingRow
+	used  []bool
+	byKey map[string][]int
+}
+
+func indexByEdit(rows []pendingRow) *editIndex {
+	idx := &editIndex{rows: rows, used: make([]bool, len(rows)), byKey: map[string][]int{}}
+	for i, r := range rows {
+		k := editKey(r.original, r.replacement)
+		idx.byKey[k] = append(idx.byKey[k], i)
+	}
+	return idx
+}
+
+// take claims the standing row for this edit, if there is one. When a document
+// repeats the same mistake, `near` (the fresh span's start) picks the closest
+// standing row, so two identical cards keep their own identities instead of
+// trading them whenever the text between them grows.
+func (i *editIndex) take(original, replacement string, near int) (pendingRow, bool) {
+	best, bestDist := -1, 0
+	for _, n := range i.byKey[editKey(original, replacement)] {
+		if i.used[n] {
+			continue
+		}
+		d := i.rows[n].from - near
+		if d < 0 {
+			d = -d
+		}
+		if best < 0 || d < bestDist {
+			best, bestDist = n, d
+		}
+	}
+	if best < 0 {
+		return pendingRow{}, false
+	}
+	i.used[best] = true
+	return i.rows[best], true
+}
+
+// reposition updates the advisory offsets (and the sentence a row belongs to)
+// without touching anything the writer can see. The frontend re-anchors by
+// string at render time, so these only matter for the local-vs-model span
+// arbitration in dedupeSpans.
+func reposition(tx *sql.Tx, row pendingRow, from, to int, chunkHash string) error {
+	if row.from == from && row.chunkHash == chunkHash {
+		return nil
+	}
+	_, err := tx.Exec(
+		`UPDATE suggestions SET from_pos = ?, to_pos = ?, chunk_hash = ? WHERE id = ?`,
+		from, to, chunkHash, row.id,
+	)
+	return err
+}
+
+// reconcilePending brings a model pass's family in line with what it just
+// proposed, sentence by sentence:
+//
+//   - A row on a sentence this pass didn't ask about is kept untouched — that
+//     is the whole point of chunking. Only its offsets are refreshed.
+//   - A row on a sentence that no longer exists in the document is dropped: she
+//     rewrote or deleted it.
+//   - A row on a sentence the pass *did* ask about survives only if the model
+//     proposed the same edit again, in which case it keeps its identity.
+//
+// `fresh` names the sentences the model was asked about (nil when it wasn't
+// called at all). inPlayAll marks the whole-document passes — voice and the
+// collocation coach — where every row is up for re-proposal because the model
+// just re-read everything.
+func (h *Handler) reconcilePending(
+	docID, contentText string,
+	raw []llm.RawSuggestion,
+	scope pendingScope,
+	chunks, fresh []chunk,
+	inPlayAll bool,
+) error {
+	tx, err := h.DB.Begin()
+	if err != nil {
+		return err
+	}
+	defer tx.Rollback()
+
+	existing, err := loadPending(tx, docID, scope.deleteWhere)
+	if err != nil {
+		return err
+	}
+
+	present := hashSet(chunks)
+	asked := hashSet(fresh)
+	modelRan := inPlayAll || fresh != nil
+
+	// Sentences to hand back to the model next time, because a row we were
+	// caching on them turned out to be unanchorable (see below).
+	reopen := map[string]bool{}
+
+	var inPlay []pendingRow
+	for _, r := range existing {
+		switch {
+		// A row whose sentence we can't name is never cached — it is re-examined
+		// whenever the model speaks, and left alone when it doesn't.
+		case inPlayAll, r.chunkHash == "" && modelRan, asked[r.chunkHash]:
+			inPlay = append(inPlay, r)
+		case r.chunkHash != "" && !present[r.chunkHash]:
+			if _, err := tx.Exec(`DELETE FROM suggestions WHERE id = ?`, r.id); err != nil {
+				return err
+			}
+		default:
+			// Untouched sentence: keep the card exactly as she last saw it.
+			from, to := locate(contentText, r.original)
+			if from < 0 {
+				// The sentence is unchanged in substance but the quoted span no
+				// longer matches byte for byte — a quote mark the editor rewrote
+				// inside it, say. The frontend anchors by that string, so this card
+				// can't be shown; drop it and let the sentence be read again rather
+				// than cache advice nobody can see.
+				reopen[r.chunkHash] = true
+				if _, err := tx.Exec(`DELETE FROM suggestions WHERE id = ?`, r.id); err != nil {
+					return err
+				}
+				continue
+			}
+			if err := reposition(tx, r, from, to, r.chunkHash); err != nil {
+				return err
+			}
+		}
+	}
+	for h := range reopen {
+		delete(present, h)
+	}
+
+	sup, err := buildSuppressor(tx, docID)
+	if err != nil {
+		return err
+	}
+
+	index := indexByEdit(inPlay)
+	kept := make(map[string]bool, len(inPlay))
+	for _, s := range raw {
+		if sup.suppressed(s.Original, s.Replacement) {
+			continue
+		}
+		from, to := locate(contentText, s.Original)
+		// Attribute the finding to a sentence the model was actually shown before
+		// falling back to the whole document: a short span ("the the") can occur in
+		// two sentences, and crediting it to the cached one would drop it as advice
+		// we already have.
+		hash := chunkFor(s.Original, fresh)
+		if hash == "" {
+			hash = chunkFor(s.Original, chunks)
+		}
+		// A sentence we didn't ask about already has whatever advice it deserves.
+		// The model can't normally quote one — it was only shown the delta — but if
+		// it wanders there anyway, the cached card stands rather than gaining a
+		// twin.
+		if !inPlayAll && hash != "" && present[hash] && !asked[hash] {
+			continue
+		}
+		if row, ok := index.take(s.Original, s.Replacement, from); ok {
+			kept[row.id] = true
+			if err := reposition(tx, row, from, to, hash); err != nil {
+				return err
+			}
+			continue
+		}
+		typ := scope.forceType
+		if typ == "" {
+			typ = normalizeType(s.Type)
+		}
+		if _, err := tx.Exec(
+			`INSERT INTO suggestions (doc_id, from_pos, to_pos, original, replacement, explanation, type, source, chunk_hash)
+			 VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?)`,
+			docID, from, to, s.Original, s.Replacement, s.Explanation, typ, db.SuggestionSourceLLM, hash,
+		); err != nil {
+			return err
+		}
+	}
+
+	// Asked about and not proposed again: the model has changed its mind, or she
+	// has fixed it.
+	for _, r := range inPlay {
+		if kept[r.id] {
+			continue
+		}
+		if _, err := tx.Exec(`DELETE FROM suggestions WHERE id = ?`, r.id); err != nil {
+			return err
+		}
+	}
+
+	// Record the sentences this family has now read. Every sentence still in the
+	// document has been read by *some* pass: the ones just asked about now, the
+	// rest in an earlier round.
+	if scope.chunked {
+		if _, err := tx.Exec(
+			`DELETE FROM checked_chunks WHERE doc_id = ? AND family = ?`, docID, scope.family,
+		); err != nil {
+			return err
+		}
+		for h := range present {
+			if _, err := tx.Exec(
+				`INSERT INTO checked_chunks (doc_id, family, hash) VALUES (?, ?, ?)`,
+				docID, scope.family, h,
+			); err != nil {
+				return err
+			}
+		}
+	}
+
+	return tx.Commit()
+}
+
+// checkedChunks loads the sentences a family read on its last pass.
+func (h *Handler) checkedChunks(docID, family string) (map[string]bool, error) {
+	rows, err := h.DB.Query(
+		`SELECT hash FROM checked_chunks WHERE doc_id = ? AND family = ?`, docID, family,
+	)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+
+	out := map[string]bool{}
+	for rows.Next() {
+		var hash string
+		if err := rows.Scan(&hash); err != nil {
+			return nil, err
+		}
+		out[hash] = true
+	}
+	return out, rows.Err()
+}

internal/suggestions/stability_test.go

@@ -0,0 +1,191 @@
+package suggestions
+
+import (
+	"encoding/json"
+	"net/http"
+	"strings"
+	"testing"
+
+	"gitea.parodia.dev/drwily/petal/internal/db"
+	"gitea.parodia.dev/drwily/petal/internal/llm"
+)
+
+// byOriginal indexes a pending set by the text each card flags.
+func byOriginal(in []db.Suggestion) map[string]db.Suggestion {
+	out := map[string]db.Suggestion{}
+	for _, s := range in {
+		out[s.Original] = s
+	}
+	return out
+}
+
+// TestUntouchedSentencesKeepTheirCards is the heart of the stability work: she
+// edits one sentence, and the cards on every other sentence stay exactly as they
+// were — same id (so the rail keeps the card instead of remounting it), same
+// explanation (the model re-words its reasoning every time it is asked, and one
+// unchanged mistake used to carry three different explanations in a sitting).
+// The model is only asked about the sentence that changed.
+func TestUntouchedSentencesKeepTheirCards(t *testing.T) {
+	client := &stubClient{response: `{"suggestions":[
+		{"original":"I has two apple","replacement":"I have two apples","explanation":"first wording","type":"grammar"},
+		{"original":"She go to market","replacement":"She goes to market","explanation":"agreement","type":"grammar"}
+	]}`}
+	srv, docID, h := newTestServer(t, client)
+	h.Limit = llm.NewRateLimiter(0)
+	setDocText(t, h, docID, "I has two apple. She go to market yesterday.")
+
+	rec := do(t, srv, http.MethodPost, "/docs/"+docID+"/check", "")
+	var first []db.Suggestion
+	if err := json.Unmarshal(rec.Body.Bytes(), &first); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	if len(first) != 2 {
+		t.Fatalf("first pass: want 2, got %d: %+v", len(first), first)
+	}
+	kept := byOriginal(first)["I has two apple"]
+
+	// She fixes only the second sentence. The model, asked again, re-words its
+	// reasoning about the first — which it must never get the chance to do.
+	setDocText(t, h, docID, "I has two apple. She goes to market yesterday.")
+	client.response = `{"suggestions":[
+		{"original":"I has two apple","replacement":"I have two apples","explanation":"REWORDED","type":"grammar"}
+	]}`
+	rec = do(t, srv, http.MethodPost, "/docs/"+docID+"/check", "")
+	var second []db.Suggestion
+	if err := json.Unmarshal(rec.Body.Bytes(), &second); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+
+	if strings.Contains(client.lastPrompt, "I has two apple") {
+		t.Fatalf("untouched sentence was sent to the model:\n%s", client.lastPrompt)
+	}
+	if !strings.Contains(client.lastPrompt, "She goes to market") {
+		t.Fatalf("edited sentence was not sent to the model:\n%s", client.lastPrompt)
+	}
+
+	now := byOriginal(second)["I has two apple"]
+	if now.ID != kept.ID {
+		t.Fatalf("card was remounted: id %q became %q", kept.ID, now.ID)
+	}
+	if now.Explanation != "first wording" {
+		t.Fatalf("explanation drifted: %q", now.Explanation)
+	}
+	// The fixed sentence's card is gone, and the model's stray re-proposal for the
+	// cached sentence did not become a second card.
+	if len(second) != 1 {
+		t.Fatalf("want exactly one card left, got %d: %+v", len(second), second)
+	}
+}
+
+// TestUnchangedDocumentSkipsTheModel proves a check with nothing new to read
+// costs nothing: no model call, and every card left standing untouched. This is
+// the doc-open and tone-less re-check path.
+func TestUnchangedDocumentSkipsTheModel(t *testing.T) {
+	client := &stubClient{response: `{"suggestions":[
+		{"original":"I has","replacement":"I have","explanation":"agreement","type":"grammar"}
+	]}`}
+	srv, docID, h := newTestServer(t, client)
+	h.Limit = llm.NewRateLimiter(0)
+
+	rec := do(t, srv, http.MethodPost, "/docs/"+docID+"/check", "")
+	var first []db.Suggestion
+	_ = json.Unmarshal(rec.Body.Bytes(), &first)
+	if len(first) != 1 || client.calls != 1 {
+		t.Fatalf("first pass: %d cards, %d calls", len(first), client.calls)
+	}
+
+	rec = do(t, srv, http.MethodPost, "/docs/"+docID+"/check", "")
+	var second []db.Suggestion
+	if err := json.Unmarshal(rec.Body.Bytes(), &second); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	if client.calls != 1 {
+		t.Fatalf("re-checking an unedited document called the model %d times", client.calls)
+	}
+	if len(second) != 1 || second[0].ID != first[0].ID {
+		t.Fatalf("card did not survive an idle re-check: %+v", second)
+	}
+}
+
+// TestDeletedSentenceDropsItsCard covers the other half of the skip path: she
+// removes a flagged sentence outright, so nothing changed that the model could
+// be asked about — but its card must still go.
+func TestDeletedSentenceDropsItsCard(t *testing.T) {
+	client := &stubClient{response: `{"suggestions":[
+		{"original":"I has","replacement":"I have","explanation":"agreement","type":"grammar"}
+	]}`}
+	srv, docID, h := newTestServer(t, client)
+	h.Limit = llm.NewRateLimiter(0)
+
+	do(t, srv, http.MethodPost, "/docs/"+docID+"/check", "")
+	setDocText(t, h, docID, "")
+
+	rec := do(t, srv, http.MethodPost, "/docs/"+docID+"/check", "")
+	var got []db.Suggestion
+	if err := json.Unmarshal(rec.Body.Bytes(), &got); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	if len(got) != 0 {
+		t.Fatalf("card outlived its sentence: %+v", got)
+	}
+}
+
+// TestToneChangeReopensEverySentence: the checkpoint's advice is written for the
+// document's tone, so switching from a journal to an academic essay has to
+// re-read sentences that haven't changed a character.
+func TestToneChangeReopensEverySentence(t *testing.T) {
+	client := &stubClient{response: `{"suggestions":[
+		{"original":"I has","replacement":"I have","explanation":"agreement","type":"grammar"}
+	]}`}
+	srv, docID, h := newTestServer(t, client)
+	h.Limit = llm.NewRateLimiter(0)
+
+	do(t, srv, http.MethodPost, "/docs/"+docID+"/check", "")
+	if _, err := h.DB.Exec(`UPDATE documents SET tone = 'academic' WHERE id = ?`, docID); err != nil {
+		t.Fatalf("set tone: %v", err)
+	}
+	do(t, srv, http.MethodPost, "/docs/"+docID+"/check", "")
+
+	if client.calls != 2 {
+		t.Fatalf("tone change did not re-read the document: %d model calls", client.calls)
+	}
+}
+
+// TestMechanicsFindingsKeepTheirRows: the rule pack re-runs 250 ms after every
+// keystroke. A finding it still reports must keep its row, or the rail would
+// remount several times a sentence — collapsing a card she has open, and
+// re-firing the arrival chime for advice she is already reading.
+func TestMechanicsFindingsKeepTheirRows(t *testing.T) {
+	srv, docID, _ := newTestServer(t, &stubClient{response: `{"suggestions":[]}`})
+	body := `{"findings":[
+		{"from":0,"to":5,"original":"I has","replacement":"I have","explanation":"agreement"},
+		{"from":6,"to":15,"original":"two apple","replacement":"two apples","explanation":"plural"}
+	]}`
+
+	rec := do(t, srv, http.MethodPost, "/docs/"+docID+"/mechanics", body)
+	var first []db.Suggestion
+	if err := json.Unmarshal(rec.Body.Bytes(), &first); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	if len(first) != 2 {
+		t.Fatalf("want 2 rows, got %d", len(first))
+	}
+
+	// She types elsewhere: same findings, shifted spans, one of them now fixed.
+	rec = do(t, srv, http.MethodPost, "/docs/"+docID+"/mechanics", `{"findings":[
+		{"from":20,"to":25,"original":"I has","replacement":"I have","explanation":"agreement"}
+	]}`)
+	var second []db.Suggestion
+	if err := json.Unmarshal(rec.Body.Bytes(), &second); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	if len(second) != 1 {
+		t.Fatalf("want 1 row, got %d: %+v", len(second), second)
+	}
+	if second[0].ID != byOriginal(first)["I has"].ID {
+		t.Fatalf("surviving finding was given a new identity: %+v", second[0])
+	}
+	if second[0].FromPos != 20 {
+		t.Fatalf("span did not follow the text: %+v", second[0])
+	}
+}