feat(chat): consume CR-5 history windowing — server-windowed cold loads + show-earlier backfill

Re-pinned [email protected]>0.10.0 + [email protected]>0.6.1 (reply
frontend-history-windowing-handoff.md); re-mirrored both .dispatch references.

- HistorySync port gains optional { limit?, beforeSeq? } (CR-5 params); the
  app's createHistorySync appends them to GET /conversations/:id.
- COLD-cache fresh load now fetches ?sinceSeq=0&limit=<floor(0.75xL)> — a huge
  conversation no longer ships whole to show 192 chunks. A warm-cache tail sync
  stays unwindowed (windowing a tail that outgrew the limit would leave a
  silent seq gap behind the cache).
- hasEarlier now derives from the [email protected] CONTRACT (1-based gap-free seqs):
  loaded window starting above seq 1 => older history exists — covering both
  locally-trimmed AND server-windowed transcripts (the watermark stays as the
  merge floor only).
- showEarlier(): local cache first; when the cache doesn't reach far enough
  back, backfills the missing older run via ?beforeSeq=<oldestKnown>&limit=
  and persists it (next page-in is local). latestSeq windowed-read caveat is
  satisfied structurally (tail cursor derives from the cache's max seq).
- live-probe: +6 CR-5 checks (seq origin, newest-k ascending, short-chat
  exactness, beforeSeq paging, 400 validation x2). NOT yet run live — backend
  was down at commit time; run pending.
- backend-handoff.md: CR-5 RESOLVED, pins/mirrors current. 602 tests green x2.

14 files changed, 384 insertions, 115 deletions

diff --git a/.dispatch/transport-contract.reference.md b/.dispatch/transport-contract.reference.md
index e6ab799..774cfb0 100644
--- a/.dispatch/transport-contract.reference.md
+++ b/.dispatch/transport-contract.reference.md
@@ -5,10 +5,23 @@
 > hangs on a permission prompt). Your CODE still imports `@dispatch/transport-contract` normally —
 > this file is for READING only.
 >
-> **Orchestrator:** SNAPSHOT of `[email protected]` (CR-4 cache-warming lifecycle shipped).
-> Depends on `@dispatch/[email protected]` (see `wire.reference.md`) + `@dispatch/[email protected]` (see
+> **Orchestrator:** SNAPSHOT of `[email protected]` (CR-5 history windowing shipped).
+> Depends on `@dispatch/[email protected]` (see `wire.reference.md`) + `@dispatch/[email protected]` (see
 > `ui-contract.reference.md`).
 >
+> **2026-06-12 delta (CR-5 history windowing — package bumped `0.9.0` → `0.10.0`):** NO type-shape
+> change — `GET /conversations/:id` gains two OPTIONAL query params alongside `sinceSeq`:
+> **`limit=<k>`** (the NEWEST `k` chunks of the selection, still ASCENDING; a selection with ≤ `k`
+> chunks is returned whole; omitted = full selection, byte-identical to the old behavior) and
+> **`beforeSeq=<s>`** (exclusive upper bound `seq < s`; combined: `sinceSeq < seq < beforeSeq`).
+> `limit`/`beforeSeq` must be POSITIVE integers (`sinceSeq` may still be 0); malformed/zero/negative
+> → HTTP 400 `{ error }` naming the param. Seq numbering is now a WRITTEN CONTRACT: 1-based,
+> monotonic, gap-free (see `[email protected]` `StoredChunk`), so `hasOlder = oldestLoaded.seq > 1` — there
+> is deliberately NO `earliestSeq`/`hasOlder` field. CAVEAT: on a windowed read, `latestSeq`
+> describes the returned WINDOW; never regress a tail cursor from a `beforeSeq` backfill page.
+> Intended flows: fresh load `?sinceSeq=0&limit=<k>` · tail sync `?sinceSeq=<cursor>` (no limit) ·
+> page older in `?beforeSeq=<oldestLoadedSeq>&limit=<k>`.
+>
 > **2026-06-12 delta (CR-4 cache-warming lifecycle — package bumped `0.8.0` → `0.9.0`):** adds
 > `POST /conversations/:id/close` (`CloseConversationResponse`) — the EXPLICIT "user closed this
 > conversation's tab" affordance, distinct from a socket disconnect / `chat.unsubscribe` (which
@@ -92,9 +105,11 @@
 - `POST /chat` — body `ChatRequest` (JSON); response NDJSON stream, one `AgentEvent` per line;
   resolved id also in `X-Conversation-Id` header.
 - `GET /models` — `ModelsResponse`.
-- `GET /conversations/:id?sinceSeq=<n>` — `ConversationHistoryResponse`: RAW, append-order,
-  seq-ordered slice with `seq > n` (NOT reconciled — dangling tool-calls returned as-is).
-  `latestSeq` = last chunk's `seq`, or the requested `sinceSeq` when caught up (empty `chunks`).
+- `GET /conversations/:id?sinceSeq=<n>&beforeSeq=<s>&limit=<k>` — `ConversationHistoryResponse`:
+  RAW, append-order, seq-ordered slice with `n < seq < s`, windowed to the NEWEST `k` (all params
+  optional; NOT reconciled — dangling tool-calls returned as-is). `latestSeq` = last chunk's `seq`,
+  or the requested `sinceSeq` when caught up (empty `chunks`) — a TAIL cursor only; do not regress
+  a cursor from a windowed/backfill read. `limit`/`beforeSeq` must be positive ints → else 400.
 - `GET /conversations/:id/metrics` — `ConversationMetricsResponse`: every SEALED turn's `TurnMetrics`
   in turn order (per-turn token + timing; NOT seq-filtered). IMPLEMENTED + LIVE-VERIFIED (probe 17/17).
 - `POST /chat/warm` — body `WarmRequest` (JSON) → `200 WarmResponse` (cache-warm usage incl.
@@ -182,23 +197,49 @@ export interface ModelsResponse {
 }
 
 /**
- * Response body for `GET /conversations/:id?sinceSeq=<n>` — the incremental
- * read-side history endpoint a long-lived client uses to (re)hydrate a
- * conversation cheaply.
+ * Response body for
+ * `GET /conversations/:id?sinceSeq=<n>&beforeSeq=<s>&limit=<k>` — the
+ * incremental read-side history endpoint a long-lived client uses to
+ * (re)hydrate a conversation cheaply. All three query params are OPTIONAL and
+ * combine as one SELECTION + one WINDOW:
+ *
+ * - **Selection** — `sinceSeq` (exclusive lower bound, `seq > n`; omitted/0 =
+ *   from the start) and `beforeSeq` (exclusive upper bound, `seq < s`; omitted
+ *   = to the end). Together: `n < seq < s`.
+ * - **Window** — `limit=<k>` returns only the NEWEST `k` chunks of the
+ *   selection (the response stays ASCENDING by seq). A selection with ≤ `k`
+ *   chunks is returned whole. `limit` omitted = the full selection — exactly
+ *   the pre-windowing behavior, so existing clients are unchanged.
+ * - `limit` and `beforeSeq` must be POSITIVE integers (`sinceSeq` may be 0);
+ *   malformed, zero, or negative values → HTTP 400 `{ error }`.
+ *
+ * Intended client flows: fresh load = `?sinceSeq=0&limit=<k>` (newest window);
+ * tail sync = `?sinceSeq=<cursor>` (no limit); page older history in =
+ * `?beforeSeq=<oldestLoadedSeq>&limit=<k>`.
+ *
+ * Seq numbering is **1-based and gap-free** (a CONTRACTUAL GUARANTEE — see
+ * `StoredChunk` in `@dispatch/wire`): a client can derive "older chunks exist"
+ * purely from `oldestLoaded.seq > 1`; there is deliberately no
+ * `earliestSeq`/`hasOlder` response field.
  *
  * `chunks` is the RAW, append-order, seq-ordered slice of the conversation log
- * with `seq > sinceSeq` (or the whole log when `sinceSeq` is omitted/0). It is
- * NOT reconciled: a dangling tool-call is returned as-is (rendered as an
- * interrupted call). Reconciliation is a turn-path concern — the server repairs
- * history only when it feeds a provider, never on this read path — which is what
- * preserves the per-chunk `seq` cursor invariant (a synthesized repair chunk
- * would have no seq).
+ * selected + windowed as above. It is NOT reconciled: a dangling tool-call is
+ * returned as-is (rendered as an interrupted call). Reconciliation is a
+ * turn-path concern — the server repairs history only when it feeds a provider,
+ * never on this read path — which is what preserves the per-chunk `seq` cursor
+ * invariant (a synthesized repair chunk would have no seq).
  *
  * `latestSeq` is the `seq` of the LAST chunk in this response, or — when the
  * slice is empty (the client is already caught up) — the requested `sinceSeq`
  * (0 for a full read of an empty conversation). So after applying the response a
  * client's new cursor is always `latestSeq`, and an empty `chunks` means
- * "nothing new past your cursor".
+ * "nothing new past your cursor". CAVEAT (windowed reads): `latestSeq` is a
+ * TAIL-sync cursor — on a `beforeSeq` backfill page (or any `limit`ed read that
+ * did not reach the log's true tail) it describes the returned window, NOT the
+ * conversation's high-water mark, so a client must not regress its sync cursor
+ * from a backfill response. (A true server-side high-water mark independent of
+ * the filter is deferred until a consumer needs it — it would require widening
+ * the store contract.)
  */
 export interface ConversationHistoryResponse {
 	readonly chunks: readonly StoredChunk[];
diff --git a/.dispatch/wire.reference.md b/.dispatch/wire.reference.md
index 40f94cf..1d761bf 100644
--- a/.dispatch/wire.reference.md
+++ b/.dispatch/wire.reference.md
@@ -4,8 +4,14 @@
 > types WITHOUT following the `file:` dep symlink out of this repo (which hangs on a permission
 > prompt). Your CODE still imports `@dispatch/wire` normally — this file is for READING only.
 >
-> **Orchestrator:** SNAPSHOT of `[email protected]` (the metrics types below + the `user-message` turn event
-> shipped + version-bumped). Regenerate whenever `@dispatch/wire` changes.
+> **Orchestrator:** SNAPSHOT of `[email protected]` (doc-only bump: the 1-based gap-free seq guarantee
+> codified on `StoredChunk`). Regenerate whenever `@dispatch/wire` changes.
+>
+> **2026-06-12 delta (CR-5 history windowing — package bumped `0.6.0` → `0.6.1`, DOC-ONLY):** the
+> per-conversation `seq` numbering is now a WRITTEN CONTRACTUAL GUARANTEE on `StoredChunk`:
+> **1-based, monotonic, gap-free** — a conversation's first chunk is always `seq === 1` and
+> numbering never skips. A client holding only a windowed suffix of the log derives "older chunks
+> exist server-side" purely from `oldestLoaded.seq > 1` (no `earliestSeq`/`hasOlder` field exists).
 >
 > **2026-06-12 delta (CR-3 user-message handoff — package bumped `0.5.0` → `0.6.0`, ADDITIVE):** adds a
 > new `AgentEvent` union member `TurnInputEvent` (`{ type: "user-message"; conversationId; turnId; text }`)
@@ -168,11 +174,17 @@ export interface ChatMessage {
 
 /**
  * A persisted chunk plus its sync metadata. The append-only conversation log
- * stamps every chunk with a monotonic, gap-free, per-conversation `seq` (the
- * sync cursor, assigned in append order) and records the `role` of the message
- * it belongs to. This makes a flat seq-ordered stream both incrementally
- * syncable ("give me chunks after seq N") and regroupable into messages by the
- * client. `chunk` is the content unit — `Chunk` carries no storage/sync cursor
+ * stamps every chunk with a **1-based**, monotonic, gap-free, per-conversation
+ * `seq` (the sync cursor, assigned in append order) and records the `role` of
+ * the message it belongs to. This makes a flat seq-ordered stream both
+ * incrementally syncable ("give me chunks after seq N") and regroupable into
+ * messages by the client.
+ *
+ * The 1-based start is a CONTRACTUAL GUARANTEE (not an implementation detail):
+ * a conversation's first chunk is always `seq === 1` and numbering never skips,
+ * so a client holding only a windowed suffix of the log can derive "older
+ * chunks exist server-side" purely from `oldestLoaded.seq > 1` — no separate
+ * has-older flag is needed (or provided). `chunk` is the content unit — `Chunk` carries no storage/sync cursor
  * (`seq` lives here on the envelope, not on the chunk, since it is assigned by
  * the store and the provider has no use for it). A chunk MAY still carry
  * generation provenance assigned at production time (e.g. a tool chunk's
diff --git a/AGENTS.md b/AGENTS.md
index a4c2f36..bc16ef5 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -139,7 +139,8 @@ streaming; tabs + model selector + DaisyUI/dracula), plus per-conversation cwd +
 context size, cache-warming (+ retention/timer), markdown, smart auto-scroll, multi-client
 live view (subscribe/reconnect + the user prompt on the event stream), and the chat limit
 (bulk quarter-unload past `dispatch.chatLimit`, 75% fresh-load window, show-earlier page-in;
-`core/chunks/trim.ts`; backend CR-5 open for `?limit=`/`?beforeSeq=`). Plan in
+`core/chunks/trim.ts`; CR-5 `?limit=`/`?beforeSeq=` CONSUMED — server-windowed cold loads +
+show-earlier server backfill; `hasOlder` from the 1-based gap-free seq contract). Plan in
 `../arch-rewrite/notes/frontend-design.md` §10.
 
 ## Reports
diff --git a/GLOSSARY.md b/GLOSSARY.md
index 2f4f199..a9c7017 100644
--- a/GLOSSARY.md
+++ b/GLOSSARY.md
@@ -39,4 +39,4 @@
 | **TPS** (tokens per second) | A FE-DERIVED decode rate: `outputTokens / (decodeMs / 1000)` (per step; per turn over Σ `decodeMs`), falling back to `genTotalMs` when `decodeMs` is absent. The backend-recommended basis (excludes first-token latency). Not carried on the wire; omitted when timing is absent. | throughput |
 | **chat limit** | The max LOADED chunks per conversation (default 256; localStorage `dispatch.chatLimit`, no UI yet) before the oldest quarter is unloaded. Counts **chunks** (committed + provisional + accumulating). Policy in `core/chunks/trim.ts`. | chunk limit, message limit, history limit |
 | **unload** | Drop the oldest COMMITTED chunks from the in-memory transcript (and DOM) past the **chat limit** — in BULK (`ceil(limit/4)` per pass, deferred while the reader is scrolled up), never one-per-delta (old Dispatch's scroll-jump bug). Purely local: the IndexedDB cache and the server keep everything; `TranscriptState.hiddenBeforeSeq` is the watermark. Distinct from the conversation-cache's cross-conversation **eviction**. | evict (reserved for the cross-conversation cache), prune, drop |
-| **show earlier** | The affordance at the top of a transcript with unloaded history ("Show earlier messages"): pages one unload-unit back in from the local cache (later: the server via CR-5 `?beforeSeq=`), preserving the reader's scroll position. | load more, pagination |
+| **show earlier** | The affordance at the top of a transcript with unloaded history ("Show earlier messages"): pages one unload-unit back in — local cache first, then the server (CR-5 `?beforeSeq=&limit=`) when the cache doesn't reach far enough back — preserving the reader's scroll position. Offered whenever the loaded window starts above seq 1 (the [email protected] 1-based gap-free seq contract). | load more, pagination |
diff --git a/backend-handoff.md b/backend-handoff.md
index 4410b44..5193d8f 100644
--- a/backend-handoff.md
+++ b/backend-handoff.md
@@ -5,15 +5,13 @@
 > **From:** dispatch-web orchestrator · **To:** arch-rewrite orchestrator · **Courier:** the user.
 > `lsp` does NOT span the repos (AGENTS.md § Backend seam) — every cross-repo ask flows through here.
 
-_Last updated: 2026-06-12 (CR-5 opened: chat-limit history windowing). **FE is current on
-`[email protected]` / `[email protected]` / `[email protected]`.** All handoffs to date are
-consumed: surfaces + WS, conversation transcript/metrics, tabs + model selector, cache-warming
-(incl. authoritative timer + retention + cache-rate fix + the CR-4 lifecycle below),
-**per-conversation cwd + LSP status**, **context size**, and **turn continuity + multi-client
-live view**.
-**Open asks: ONE — CR-5** (`?limit=`/`?beforeSeq=` on `GET /conversations/:id`; NOT a blocker,
-courier doc `backend-handoff-chat-limit.md`). CR-1/CR-2/CR-4 all RESOLVED ✅ (see §2); §3 lists
-likely next asks.
+_Last updated: 2026-06-12 (CR-5 consumed). **FE is current on `[email protected]` /
+`[email protected]` / `[email protected]`.** All handoffs to date are consumed: surfaces + WS,
+conversation transcript/metrics, tabs + model selector, cache-warming (incl. authoritative timer
++ retention + cache-rate fix + the CR-4 lifecycle below), **per-conversation cwd + LSP status**,
+**context size**, **turn continuity + multi-client live view**, and the **chat limit + CR-5
+history windowing** (below).
+**Open asks: NONE.** CR-1/CR-2/CR-4/CR-5 all RESOLVED ✅ (see §2); §3 lists likely next asks.
 **CR-3 (watcher couldn't see the USER prompt until seal) → RESOLVED ✅** — backend shipped the
 `user-message` turn event; FE re-pinned + consumption live.
 The cwd/LSP draft-path verification (`backend-handoff-cwd-lsp.md`) came back **all ✅ confirmed**._
@@ -63,7 +61,7 @@ backend ask — but the max-limit denominator is now a live FE need; see §3.
 
 ## 1. Pinned backend contracts (consumed by the FE)
 
-Pinned as `file:` deps: **`[email protected]`; `[email protected]`; `[email protected]`**.
+Pinned as `file:` deps: **`[email protected]`; `[email protected]`; `[email protected]`**.
 
 | Package | Used for |
 |---|---|
@@ -72,7 +70,8 @@ Pinned as `file:` deps: **`[email protected]`; `[email protected]`; `transport-contract
 | `@dispatch/transport-contract` | `ChatRequest`/`ModelsResponse`/`ConversationHistoryResponse`/`ConversationMetricsResponse` + `WarmRequest`/`WarmResponse` + `CwdResponse`/`SetCwdRequest` + LSP (`LspStatusResponse`/`LspServerInfo`/`LspServerState`) + WS chat ops + `WsClientMessage`/`WsServerMessage` |
 
 Endpoints in use (HTTP **24203**, WS **24205**, CORS `*` incl. `PUT`):
-`POST /chat` (NDJSON) · `GET /models` · `GET /conversations/:id?sinceSeq=<n>` ·
+`POST /chat` (NDJSON) · `GET /models` ·
+`GET /conversations/:id?sinceSeq=<n>&beforeSeq=<s>&limit=<k>` (CR-5 windowing) ·
 `GET /conversations/:id/metrics` · `GET`/`PUT /conversations/:id/cwd` ·
 `GET /conversations/:id/lsp` · `POST /chat/warm` · `POST /conversations/:id/close` (explicit
 tab-close: abort turn + stop/disable warming) · WS `chat.send`→`chat.delta` ·
@@ -80,28 +79,32 @@ WS `chat.subscribe`/`chat.unsubscribe` (watch a conversation's turns without sen
 
 Mirrored in-repo for headless agents: `.dispatch/{ui-contract,wire,transport-contract}.reference.md`
 (regenerate on any contract bump; all current as of `[email protected]` /
-`[email protected]` / `[email protected]`).
+`[email protected]` / `[email protected]`).
 
 ## 2. Open asks FOR THE BACKEND
 
-**One open: CR-5.** Resolved history below it.
-
-### CR-5 — history windowing for the FE chat limit → **OPEN (not a blocker)** (courier `backend-handoff-chat-limit.md`)
-
-The FE is shipping a **chat limit** (default 256 chunks, localStorage-configurable): past the
-limit it bulk-unloads the oldest `ceil(L/4)` chunks (scroll-jump-free, unlike old Dispatch's
-one-per-delta eviction), and a fresh page load shows only the newest `floor(0.75×L)`. Works
-today by fetching the full history and windowing in memory — the ask makes the FRESH-BROWSER
-load cheap (today `?sinceSeq=0` returns the whole conversation; a 10k-chunk chat downloads
-megabytes to show 192 chunks). Additive `transport-contract` asks:
-- **`?limit=<n>`** on `GET /conversations/:id` — newest `n` of the selection, still ascending;
-  **≤ `n` chunks exist ⇒ return everything** (the FE always sends it; short chats must stay
-  exact). Absent ⇒ today's behavior.
-- **`?beforeSeq=<s>`** — selection `seq < s` (with `limit`: newest `n` below `s`) — the
-  "Show earlier messages" page-in path once the FE's local cache is exhausted.
-- **`earliestSeq?`/`hasOlder?` response field** — OR simply confirm in writing that seqs start
-  at 1 gap-free, and the FE derives `hasOlder` from `chunks[0].seq > 1` (cheapest, preferred).
-Full consumption plan + sequencing in the courier doc.
+**None open.** Resolved history below.
+
+### CR-5 — history windowing for the FE chat limit → **RESOLVED ✅** (courier `backend-handoff-chat-limit.md`; reply `frontend-history-windowing-handoff.md`; consumed)
+
+Backend shipped everything asked (`[email protected]`, `[email protected]` doc-only):
+`?limit=<k>` (newest-k of the selection, ascending; ≤ k ⇒ whole selection, exact) +
+`?beforeSeq=<s>` (exclusive `seq < s`; combined `sinceSeq < seq < s`) on
+`GET /conversations/:id`; ask #3 answered via our preferred cheapest option — a WRITTEN
+contractual guarantee that per-conversation seqs are **1-based, monotonic, gap-free** (codified
+on `StoredChunk`), so the FE derives `hasOlder = oldestLoaded.seq > 1` (no new response field).
+Validation: `limit`/`beforeSeq` must be positive ints, else 400 (FE never sends `beforeSeq=0` —
+`oldestLoaded.seq === 1` already means "nothing older"). **FE consumed:** re-pinned + re-mirrored
+both `.dispatch/*.reference.md`; `HistorySync` port gained an optional `{ limit?, beforeSeq? }`
+window; a COLD-cache fresh load now fetches `?sinceSeq=0&limit=<floor(0.75×L)>` (a warm-cache
+tail sync stays unwindowed — windowing a tail that outgrew the limit would leave a silent seq
+gap behind the cache); `hasEarlier` is seq-derived per the new contract; "Show earlier messages"
+pages from the local cache first and BACKFILLS the missing older run via
+`?beforeSeq=<oldestKnown>&limit=` (persisted to cache, so the next page-in is local). The
+`latestSeq` windowed-read caveat is satisfied structurally: the FE's tail cursor derives from
+the cache's max seq, never from a response's `latestSeq`. Chat-limit recap (FE-side, shipped
+with CR-5 still open): default 256 (`localStorage["dispatch.chatLimit"]`), bulk quarter-unload
+past the limit gated on reader-at-bottom, 75% fresh-load window.
 
 ### CR-1 — Loaded Extensions as a true table → **RESOLVED ✅** (shipped + consumed)
 
diff --git a/scripts/live-probe.ts b/scripts/live-probe.ts
index 7099b44..f44a136 100644
--- a/scripts/live-probe.ts
+++ b/scripts/live-probe.ts
@@ -75,13 +75,27 @@ function fail(msg: string): never {
 	process.exit(1);
 }
 
-async function historySync(id: string, sinceSeq: number): Promise<ConversationHistoryResponse> {
-	const url = `${HTTP_BASE}/conversations/${encodeURIComponent(id)}?sinceSeq=${sinceSeq}`;
+async function historySync(
+	id: string,
+	sinceSeq: number,
+	window?: { limit?: number; beforeSeq?: number },
+): Promise<ConversationHistoryResponse> {
+	let url = `${HTTP_BASE}/conversations/${encodeURIComponent(id)}?sinceSeq=${sinceSeq}`;
+	if (window?.limit !== undefined) url += `&limit=${window.limit}`;
+	if (window?.beforeSeq !== undefined) url += `&beforeSeq=${window.beforeSeq}`;
 	const res = await fetch(url, { headers: { Origin: "http://localhost:24204" } });
 	if (!res.ok) fail(`history fetch ${res.status} for ${url}`);
 	return (await res.json()) as ConversationHistoryResponse;
 }
 
+/** Raw history GET that returns the status (for the CR-5 validation checks). */
+async function historyStatus(id: string, query: string): Promise<number> {
+	const url = `${HTTP_BASE}/conversations/${encodeURIComponent(id)}?${query}`;
+	const res = await fetch(url, { headers: { Origin: "http://localhost:24204" } });
+	await res.arrayBuffer(); // drain
+	return res.status;
+}
+
 /** Durable metrics fetch — returns the response, or the HTTP status when not OK
  * (the endpoint is being implemented backend-side; the FE tolerates a 404). */
 async function metricsSync(id: string): Promise<ConversationMetricsResponse | { status: number }> {
@@ -203,6 +217,45 @@ async function main() {
 		.join("");
 	record("turn 1 committed transcript has assistant text", committedText.length > 0);
 
+	// ─── CR-5: history windowing (?limit= / ?beforeSeq=, [email protected]) ───────
+	const logLen = hist.chunks.length;
+	record(
+		"CR-5 seq origin: first chunk is seq 1 (1-based gap-free contract)",
+		hist.chunks[0]?.seq === 1,
+		`first seq=${hist.chunks[0]?.seq}`,
+	);
+	const win = await historySync(textConv, 0, { limit: 2 });
+	record(
+		"CR-5 ?limit=2 returns the NEWEST 2, ascending, latestSeq = window tail",
+		win.chunks.length === Math.min(2, logLen) &&
+			win.chunks[0]?.seq === Math.max(1, logLen - 1) &&
+			win.chunks[win.chunks.length - 1]?.seq === logLen &&
+			win.latestSeq === logLen,
+		`seqs=[${win.chunks.map((c) => c.seq).join(",")}] latestSeq=${win.latestSeq}`,
+	);
+	const whole = await historySync(textConv, 0, { limit: 200 });
+	record(
+		"CR-5 ?limit= larger than the log returns everything (short-chat flow exact)",
+		whole.chunks.length === logLen,
+		`${whole.chunks.length}/${logLen} chunks`,
+	);
+	const oldestLoaded = win.chunks[0]?.seq ?? 0;
+	if (oldestLoaded > 1) {
+		const back = await historySync(textConv, 0, { beforeSeq: oldestLoaded, limit: 50 });
+		record(
+			"CR-5 ?beforeSeq= pages the older run (seq < bound, ascending from 1)",
+			back.chunks.length === oldestLoaded - 1 &&
+				back.chunks[0]?.seq === 1 &&
+				back.chunks.every((c) => c.seq < oldestLoaded),
+			`seqs=[${back.chunks.map((c) => c.seq).join(",")}]`,
+		);
+	}
+	record("CR-5 limit=0 rejected with 400", (await historyStatus(textConv, "limit=0")) === 400);
+	record(
+		"CR-5 beforeSeq=-1 rejected with 400",
+		(await historyStatus(textConv, "beforeSeq=-1")) === 400,
+	);
+
 	// ─── Metrics: LIVE token + timing ([email protected] usage/step-complete/done) ──────
 	// (TurnMetricsEntry is `{ turnId, steps, total }` — the turn aggregate lives on
 	// `total`, present once the live `done` folded.)
diff --git a/src/app/store.svelte.ts b/src/app/store.svelte.ts
index 379805f..999f2be 100644
--- a/src/app/store.svelte.ts
+++ b/src/app/store.svelte.ts
@@ -25,7 +25,7 @@ import {
 	subscribe as protocolSubscribe,
 	unsubscribe as protocolUnsubscribe,
 } from "../core/protocol";
-import type { ChatStore, MetricsSync } from "../features/chat";
+import type { ChatStore, HistorySync, MetricsSync } from "../features/chat";
 import { createChatStore } from "../features/chat";
 import type { ConversationCache } from "../features/conversation-cache";
 import { createConversationCache } from "../features/conversation-cache";
@@ -111,12 +111,13 @@ export interface CreateAppStoreOptions {
 	localStorage?: Storage;
 }
 
-function createHistorySync(
-	httpBase: string,
-	fetchImpl: typeof fetch,
-): (conversationId: string, sinceSeq: number) => Promise<ConversationHistoryResponse> {
-	return async (conversationId: string, sinceSeq: number) => {
-		const url = `${httpBase}/conversations/${encodeURIComponent(conversationId)}?sinceSeq=${sinceSeq}`;
+function createHistorySync(httpBase: string, fetchImpl: typeof fetch): HistorySync {
+	return async (conversationId, sinceSeq, window) => {
+		let url = `${httpBase}/conversations/${encodeURIComponent(conversationId)}?sinceSeq=${sinceSeq}`;
+		// CR-5 windowing ([email protected]): both must be positive
+		// integers when present (the server 400s otherwise; callers guarantee it).
+		if (window?.limit !== undefined) url += `&limit=${window.limit}`;
+		if (window?.beforeSeq !== undefined) url += `&beforeSeq=${window.beforeSeq}`;
 		const res = await fetchImpl(url);
 		if (!res.ok) {
 			throw new Error(`History sync failed: ${res.status}`);
diff --git a/src/core/chunks/trim.test.ts b/src/core/chunks/trim.test.ts
index 091b646..7914f35 100644
--- a/src/core/chunks/trim.test.ts
+++ b/src/core/chunks/trim.test.ts
@@ -177,28 +177,43 @@ describe("restoreEarlier", () => {
 		expect(selectHasEarlier(restored)).toBe(true);
 	});
 
-	it("clears the watermark when the restore exhausts known earlier history", () => {
+	it("restoring down to seq 1 reaches the contractual origin (hasEarlier clears)", () => {
 		const windowed = windowTranscript(stateWith(chunks(1, 100)), 75); // hidden: 1..25
 		const restored = restoreEarlier(windowed, chunks(1, 100), 64);
 		expect(restored.committed).toHaveLength(100);
 		expect(restored.committed[0]?.seq).toBe(1);
-		expect(restored.hiddenBeforeSeq).toBe(0);
+		expect(restored.hiddenBeforeSeq).toBe(1); // floor at the origin — inert
 		expect(restored.hiddenThinkingCount).toBe(0);
 		expect(selectHasEarlier(restored)).toBe(false);
 	});
 
-	it("clears the watermark when nothing is actually below it", () => {
+	it("is the identity when nothing older is known locally (server may still hold more)", () => {
 		const windowed = windowTranscript(stateWith(chunks(50, 200)), 75);
 		const restored = restoreEarlier(windowed, [], 64);
-		expect(restored.hiddenBeforeSeq).toBe(0);
-		expect(restored.committed).toEqual(windowed.committed);
+		expect(restored).toBe(windowed);
+		// seqs are 1-based gap-free: window starts at 126 ⇒ older chunks DO exist.
+		expect(selectHasEarlier(restored)).toBe(true);
 	});
 
-	it("is the identity when nothing is hidden", () => {
+	it("is the identity when the window already starts at seq 1", () => {
 		const state = stateWith(chunks(1, 10));
 		expect(restoreEarlier(state, chunks(1, 10), 5)).toBe(state);
 	});
 
+	it("works on a server-windowed transcript (no local watermark)", () => {
+		// A cold-cache fresh load with `?limit=` commits a suffix (seq 809..1000)
+		// with hiddenBeforeSeq still 0 — hasEarlier derives from seq > 1, and a
+		// backfilled run merges below it.
+		const state = stateWith(chunks(809, 1000));
+		expect(state.hiddenBeforeSeq).toBe(0);
+		expect(selectHasEarlier(state)).toBe(true);
+		const restored = restoreEarlier(state, chunks(745, 808), 64);
+		expect(restored.committed[0]?.seq).toBe(745);
+		expect(restored.committed).toHaveLength(192 + 64);
+		expect(restored.hiddenBeforeSeq).toBe(745);
+		expect(selectHasEarlier(restored)).toBe(true);
+	});
+
 	it("decrements the hidden thinking count by the restored thinking chunks", () => {
 		const committed = [chunk(1, "thinking"), chunk(2), chunk(3, "thinking"), ...chunks(4, 12)];
 		const trimmed = trimTranscript(stateWith(committed), 10); // drops 3: seqs 1..3 (2 thinking)
@@ -213,6 +228,7 @@ describe("restoreEarlier", () => {
 		const trimmed = trimTranscript(stateWith(original), 100);
 		const restored = restoreEarlier(trimmed, original, 1000);
 		expect(restored.committed).toEqual(original);
-		expect(restored.hiddenBeforeSeq).toBe(0);
+		expect(restored.hiddenBeforeSeq).toBe(1);
+		expect(selectHasEarlier(restored)).toBe(false);
 	});
 });
diff --git a/src/core/chunks/trim.ts b/src/core/chunks/trim.ts
index 1733027..94065b3 100644
--- a/src/core/chunks/trim.ts
+++ b/src/core/chunks/trim.ts
@@ -110,40 +110,52 @@ export function windowTranscript(state: TranscriptState, maxCommitted: number):
 }
 
 /**
- * Page earlier (unloaded) history back in — the "Show earlier messages" action.
+ * The oldest LOADED seq — the start of the transcript's loaded window. Usually
+ * `committed[0].seq`; falls back to the watermark when a trim emptied the
+ * committed list (all-provisional overflow). 0 = window start unknown/origin.
+ */
+function oldestLoadedSeq(state: TranscriptState): number {
+	return state.committed[0]?.seq ?? state.hiddenBeforeSeq;
+}
+
+/**
+ * Page earlier history back in — the "Show earlier messages" action.
  *
- * `earlier` must be ALL locally-known chunks below the watermark (typically the
- * full cached conversation; chunks at/above the watermark are ignored). The
- * newest `count` of them are merged back in front of `committed` and the
- * watermark lowers to the new oldest loaded seq — or clears to 0 when this
- * restore exhausts the known earlier history (nothing left to offer).
+ * `earlier` is every locally-known chunk older than the loaded window
+ * (typically the full cached conversation, possibly extended by a CR-5
+ * `?beforeSeq=` backfill; chunks at/inside the window are ignored). The newest
+ * `count` of them are merged back in front of `committed`, and the watermark
+ * follows the new window start so history merges still can't resurrect what
+ * remains unloaded. Identity when the window already starts at seq 1 (the
+ * contractual origin) or nothing older is known locally.
  */
 export function restoreEarlier(
 	state: TranscriptState,
 	earlier: readonly StoredChunk[],
 	count: number,
 ): TranscriptState {
-	if (state.hiddenBeforeSeq <= 0) return state;
-	const below = earlier.filter((c) => c.seq < state.hiddenBeforeSeq).sort((a, b) => a.seq - b.seq);
-	if (below.length === 0) {
-		// Nothing is actually hidden below the watermark: clear it so the
-		// "Show earlier" affordance disappears.
-		return { ...state, hiddenBeforeSeq: 0, hiddenThinkingCount: 0 };
-	}
+	const oldest = oldestLoadedSeq(state);
+	if (oldest <= 1) return state;
+	const below = earlier.filter((c) => c.seq < oldest).sort((a, b) => a.seq - b.seq);
+	if (below.length === 0) return state;
 	const keep = below.slice(-Math.max(1, count));
-	const exhausted = keep.length === below.length;
 	const firstKept = keep[0];
 	return {
 		...state,
 		committed: [...keep, ...state.committed],
-		hiddenBeforeSeq: exhausted || firstKept === undefined ? 0 : firstKept.seq,
-		hiddenThinkingCount: exhausted
-			? 0
-			: Math.max(0, state.hiddenThinkingCount - countThinking(keep)),
+		hiddenBeforeSeq: firstKept?.seq ?? state.hiddenBeforeSeq,
+		hiddenThinkingCount: Math.max(0, state.hiddenThinkingCount - countThinking(keep)),
 	};
 }
 
-/** Whether unloaded earlier history exists to offer ("Show earlier messages"). */
+/**
+ * Whether earlier history exists below the loaded window — drives the
+ * "Show earlier messages" affordance. Derived from the [email protected] CONTRACT
+ * that per-conversation seqs are 1-based and gap-free: a loaded window that
+ * starts above seq 1 means older chunks exist (locally cached or server-side),
+ * whether the window came from a local trim or a server-windowed (`?limit=`)
+ * fresh load.
+ */
 export function selectHasEarlier(state: TranscriptState): boolean {
-	return state.hiddenBeforeSeq > 0;
+	return oldestLoadedSeq(state) > 1;
 }
diff --git a/src/features/chat/index.ts b/src/features/chat/index.ts
index 18ed693..139a64f 100644
--- a/src/features/chat/index.ts
+++ b/src/features/chat/index.ts
@@ -1,7 +1,7 @@
 export type { RenderedChunk, RenderGroup, ToolBatchEntry } from "../../core/chunks";
 export { groupRenderedChunks } from "../../core/chunks";
 export type { TurnMetricsEntry } from "../../core/metrics";
-export type { ChatTransport, HistorySync, MetricsSync } from "./ports";
+export type { ChatTransport, HistorySync, HistoryWindow, MetricsSync } from "./ports";
 export type { ChatStore, ChatStoreDependencies } from "./store.svelte";
 export { createChatStore } from "./store.svelte";
 export { default as ChatView } from "./ui/ChatView.svelte";
diff --git a/src/features/chat/ports.ts b/src/features/chat/ports.ts
index e28ebf6..f8c665f 100644
--- a/src/features/chat/ports.ts
+++ b/src/features/chat/ports.ts
@@ -9,10 +9,29 @@ export interface ChatTransport {
 	send(msg: ChatSendMessage): void;
 }
 
-/** Injected history-sync port — fetches incremental history from the server. */
+/**
+ * Optional windowing for a history fetch ([email protected], CR-5).
+ * Both must be POSITIVE integers when present (the server 400s otherwise).
+ */
+export interface HistoryWindow {
+	/** Return only the NEWEST `limit` chunks of the selection (still ascending). */
+	readonly limit?: number;
+	/** Exclusive upper bound: only chunks with `seq < beforeSeq` (backfill paging). */
+	readonly beforeSeq?: number;
+}
+
+/**
+ * Injected history-sync port — fetches incremental history from the server
+ * (`GET /conversations/:id?sinceSeq=&beforeSeq=&limit=`). NOTE the contract
+ * caveat: on a windowed/backfill read the response's `latestSeq` describes the
+ * returned window, not the conversation's high-water mark — never regress a
+ * tail cursor from it (the FE's cursor comes from the cache's max seq, which
+ * satisfies this naturally).
+ */
 export type HistorySync = (
 	conversationId: string,
 	sinceSeq: number,
+	window?: HistoryWindow,
 ) => Promise<ConversationHistoryResponse>;
 
 /** Injected metrics-sync port — fetches persisted per-turn metrics from the server. */
diff --git a/src/features/chat/store.svelte.ts b/src/features/chat/store.svelte.ts
index 5ca28af..e74980d 100644
--- a/src/features/chat/store.svelte.ts
+++ b/src/features/chat/store.svelte.ts
@@ -92,10 +92,10 @@ export interface ChatStore {
 	setModel(model: string): void;
 	load(): Promise<void>;
 	/**
-	 * Page one unload-unit (`ceil(limit/4)`) of earlier history back in from the
-	 * local cache — the "Show earlier messages" action. (When the backend ships
-	 * CR-5 `?beforeSeq=`, this can fall through to the server once the cache is
-	 * exhausted.)
+	 * Page one unload-unit (`ceil(limit/4)`) of earlier history back in — the
+	 * "Show earlier messages" action. Local cache first; when the cache doesn't
+	 * reach far enough back (a server-windowed fresh load), the missing older
+	 * run is fetched via CR-5 `?beforeSeq=&limit=` and persisted to the cache.
 	 */
 	showEarlier(): Promise<void>;
 	/**
@@ -129,12 +129,21 @@ export function createChatStore(deps: ChatStoreDependencies): ChatStore {
 		transcript = trimTranscript(transcript, chatLimit);
 	}
 
-	async function syncTail(): Promise<void> {
+	/**
+	 * Pull `seq > cache-cursor` from the server and fold it in. `coldLimit`, when
+	 * given AND the cache is empty (a truly fresh browser), windows the fetch to
+	 * the newest N chunks (CR-5 `?limit=`) so a huge conversation doesn't ship
+	 * whole. It is deliberately NOT applied to a warm-cache tail: windowing a
+	 * tail that grew past N while we were away would leave a silent seq GAP
+	 * between the cache and the fetched window.
+	 */
+	async function syncTail(coldLimit?: number): Promise<void> {
 		if (disposed || _pendingSync) return;
 		_pendingSync = true;
 		try {
 			const since = await deps.cache.sinceSeq(deps.conversationId);
-			const res = await deps.historySync(deps.conversationId, since);
+			const window = since === 0 && coldLimit !== undefined ? { limit: coldLimit } : undefined;
+			const res = await deps.historySync(deps.conversationId, since, window);
 			const merged = await deps.cache.commit(deps.conversationId, res.chunks);
 			transcript = applyHistory(transcript, merged);
 			maybeTrim();
@@ -227,24 +236,48 @@ export function createChatStore(deps: ChatStoreDependencies): ChatStore {
 
 		async load(): Promise<void> {
 			// Fresh load shows only the newest 75% of the limit — headroom before the
-			// first trim. Window the cached slice SYNCHRONOUSLY with its apply (no
-			// render in between), and again after the tail sync (a cold cache means
-			// syncTail pulled the whole history in one response).
+			// first trim. A warm cache is windowed locally (synchronously with its
+			// apply — no render in between); a COLD cache passes the window to the
+			// server instead (CR-5 `?limit=`), so a huge conversation never ships
+			// whole. The post-sync window re-asserts the cap either way.
 			const windowSize = initialWindowSize(chatLimit);
 			const cached = await deps.cache.load(deps.conversationId);
 			if (cached.length > 0) {
 				transcript = windowTranscript(applyHistory(transcript, cached), windowSize);
 			}
-			await syncTail();
+			await syncTail(windowSize);
 			transcript = windowTranscript(transcript, windowSize);
 			await syncMetrics();
 		},
 
 		async showEarlier(): Promise<void> {
 			if (disposed) return;
-			if (!selectHasEarlier(transcript)) return;
-			const cached = await deps.cache.load(deps.conversationId);
-			transcript = restoreEarlier(transcript, cached, unloadCount(chatLimit));
+			const oldest = transcript.committed[0]?.seq ?? transcript.hiddenBeforeSeq;
+			if (oldest <= 1) return;
+			const want = unloadCount(chatLimit);
+			try {
+				let earlier = (await deps.cache.load(deps.conversationId)).filter((c) => c.seq < oldest);
+				// The local cache may not reach far enough back (a server-windowed
+				// fresh load cached only the window): page the missing OLDER run in
+				// from the server (CR-5 `?beforeSeq=&limit=`) and persist it, so the
+				// next page-in is local. Seqs are gap-free, so the fetched run is
+				// contiguous with what we hold. NOTE: the backfill response's
+				// `latestSeq` is a window cursor — never fed to the tail cursor
+				// (ours derives from the cache's max seq).
+				const oldestKnown = earlier[0]?.seq ?? oldest;
+				if (earlier.length < want && oldestKnown > 1) {
+					const res = await deps.historySync(deps.conversationId, 0, {
+						beforeSeq: oldestKnown,
+						limit: want - earlier.length,
+					});
+					const merged = await deps.cache.commit(deps.conversationId, res.chunks);
+					earlier = merged.filter((c) => c.seq < oldest);
+				}
+				transcript = restoreEarlier(transcript, earlier, want);
+				_error = null;
+			} catch (err) {
+				_error = err instanceof Error ? err.message : String(err);
+			}
 		},
 
 		resync(): void {
diff --git a/src/features/chat/store.test.ts b/src/features/chat/store.test.ts
index 5c798d6..3232009 100644
--- a/src/features/chat/store.test.ts
+++ b/src/features/chat/store.test.ts
@@ -1054,12 +1054,12 @@ describe("createChatStore", () => {
 		store.dispose();
 	});
 
-	it("chat limit: a cold cache (fresh browser) windows the full server history to 75%", async () => {
+	it("chat limit: a cold cache (fresh browser) asks the SERVER for the 75% window (CR-5 ?limit=)", async () => {
 		const transport = createFakeTransport();
 		const historySync = createFakeHistorySync();
 		const metricsSync = createFakeMetricsSync();
 		const cache = createFakeCache();
-		// Backend has no limit param yet (CR-5): sinceSeq=0 returns EVERYTHING.
+		// The server holds 500 chunks; the windowed fetch returns the newest 75.
 		historySync.returnChunks = Array.from({ length: 500 }, (_, i) => makeStoredChunk(i + 1));
 
 		const store = createChatStore({
@@ -1073,12 +1073,77 @@ describe("createChatStore", () => {
 
 		await store.load();
 
+		// The cold-cache initial sync carried the window (`?sinceSeq=0&limit=75`).
+		expect(historySync.calls[0]?.sinceSeq).toBe(0);
+		expect(historySync.calls[0]?.window).toEqual({ limit: 75 });
+
 		expect(store.chunks).toHaveLength(75);
 		expect(store.chunks[0]?.seq).toBe(426);
+		// hasEarlier derives from the 1-based gap-free seq contract (426 > 1) —
+		// no local watermark was ever set.
+		expect(store.hasEarlier).toBe(true);
+		// Only the window was shipped + cached (the point of CR-5).
+		const cached = await cache.impl.load(CONV_ID);
+		expect(cached).toHaveLength(75);
+
+		store.dispose();
+	});
+
+	it("chat limit: a warm cache syncs the tail UNWINDOWED (no seq gap behind the cache)", async () => {
+		const transport = createFakeTransport();
+		const historySync = createFakeHistorySync();
+		const metricsSync = createFakeMetricsSync();
+		const cache = createFakeCache();
+		await cache.impl.commit(CONV_ID, [makeStoredChunk(1), makeStoredChunk(2)]);
+		historySync.returnChunks = [makeStoredChunk(3)];
+
+		const store = createChatStore({
+			conversationId: CONV_ID,
+			transport: transport.impl,
+			historySync: historySync.impl,
+			metricsSync: metricsSync.impl,
+			cache: cache.impl,
+			chatLimit: 100,
+		});
+
+		await store.load();
+
+		expect(historySync.calls[0]?.sinceSeq).toBe(2);
+		expect(historySync.calls[0]?.window).toBeUndefined();
+
+		store.dispose();
+	});
+
+	it("chat limit: showEarlier backfills from the server when the cache is too shallow (CR-5 ?beforeSeq=)", async () => {
+		const transport = createFakeTransport();
+		const historySync = createFakeHistorySync();
+		const metricsSync = createFakeMetricsSync();
+		const cache = createFakeCache();
+		historySync.returnChunks = Array.from({ length: 500 }, (_, i) => makeStoredChunk(i + 1));
+
+		const store = createChatStore({
+			conversationId: CONV_ID,
+			transport: transport.impl,
+			historySync: historySync.impl,
+			metricsSync: metricsSync.impl,
+			cache: cache.impl,
+			chatLimit: 100,
+		});
+
+		await store.load(); // server-windowed: loaded + cached = 426..500
+		expect(store.chunks[0]?.seq).toBe(426);
+
+		await store.showEarlier();
+
+		// Nothing below 426 was cached → fetched the missing run from the server.
+		const backfill = historySync.calls[1];
+		expect(backfill?.window).toEqual({ beforeSeq: 426, limit: 25 });
+		expect(store.chunks).toHaveLength(100);
+		expect(store.chunks[0]?.seq).toBe(401);
 		expect(store.hasEarlier).toBe(true);
-		// The full history is still CACHED locally (show-earlier pages from it).
+		// The backfilled run is persisted: the NEXT page-in is cache-local.
 		const cached = await cache.impl.load(CONV_ID);
-		expect(cached).toHaveLength(500);
+		expect(cached).toHaveLength(100);
 
 		store.dispose();
 	});
@@ -1109,6 +1174,8 @@ describe("createChatStore", () => {
 		expect(store.chunks).toHaveLength(100);
 		expect(store.chunks[0]?.seq).toBe(401);
 		expect(store.hasEarlier).toBe(true);
+		// The cache reached deep enough — no server backfill was needed.
+		expect(historySync.calls).toHaveLength(1);
 
 		store.dispose();
 	});
diff --git a/src/features/chat/test-helpers.ts b/src/features/chat/test-helpers.ts
index 07dad26..6bb98a1 100644
--- a/src/features/chat/test-helpers.ts
+++ b/src/features/chat/test-helpers.ts
@@ -1,6 +1,6 @@
 import type { StoredChunk } from "@dispatch/wire";
 import type { ConversationCache } from "../conversation-cache";
-import type { ChatTransport, HistorySync, MetricsSync } from "./ports";
+import type { ChatTransport, HistorySync, HistoryWindow, MetricsSync } from "./ports";
 
 export interface FakeTransport {
 	readonly sent: import("@dispatch/transport-contract").ChatSendMessage[];
@@ -20,14 +20,14 @@ export function createFakeTransport(): FakeTransport {
 }
 
 export interface FakeHistorySync {
-	readonly calls: Array<{ conversationId: string; sinceSeq: number }>;
+	readonly calls: Array<{ conversationId: string; sinceSeq: number; window?: HistoryWindow }>;
 	/** Set the chunks to return on the next call. */
 	returnChunks: readonly StoredChunk[];
 	readonly impl: HistorySync;
 }
 
 export function createFakeHistorySync(): FakeHistorySync {
-	const calls: Array<{ conversationId: string; sinceSeq: number }> = [];
+	const calls: Array<{ conversationId: string; sinceSeq: number; window?: HistoryWindow }> = [];
 	let returnChunks: readonly StoredChunk[] = [];
 	return {
 		calls,
@@ -37,9 +37,20 @@ export function createFakeHistorySync(): FakeHistorySync {
 		set returnChunks(v: readonly StoredChunk[]) {
 			returnChunks = v;
 		},
-		impl: async (conversationId, sinceSeq) => {
-			calls.push({ conversationId, sinceSeq });
-			const chunks = returnChunks;
+		impl: async (conversationId, sinceSeq, window) => {
+			calls.push({ conversationId, sinceSeq, ...(window !== undefined ? { window } : {}) });
+			// Apply the CR-5 WINDOW semantics (`beforeSeq` bound, then newest-`limit`)
+			// so store tests exercise the real windowed flows. `sinceSeq` filtering is
+			// deliberately NOT applied — tests set `returnChunks` to the slice they
+			// mean the server to hold past the cursor.
+			let chunks = returnChunks;
+			const before = window?.beforeSeq;
+			if (before !== undefined) {
+				chunks = chunks.filter((c) => c.seq < before);
+			}
+			if (window?.limit !== undefined && chunks.length > window.limit) {
+				chunks = chunks.slice(-window.limit);
+			}
 			const latestSeq = chunks.length > 0 ? Math.max(...chunks.map((c) => c.seq)) : sinceSeq;
 			return { chunks, latestSeq };
 		},