PLAN.md

@@ -648,7 +648,10 @@ cannot submit; three-way admin filter.
   - **Margin** (interview): pick the candidate whose resulting margin (own+move−opp)
     is closest to **[1,30]** when playing to win / **[−30,−1]** when playing to lose,
     tie-broken toward the conservative edge; no legal play → exchange the full rack
-    when the bag can refill it, else pass.
+    when the bag can refill it, else pass. *(Later refined — separate PR: on ≈20% of
+    opening/midgame moves the robot flips its intent for that one move, the chance
+    tapering to 0 over the last 14 bag tiles and 0 at an empty bag, so the endgame
+    stays strict; deterministic from the seed.)*
   - **Substitution** (interview): a matchmaker **reaper** (`Reap`/`RunReaper`)
     substitutes a pooled robot after a **10 s** wait (`BACKEND_LOBBY_ROBOT_WAIT`),
     `NewMatchmaker` now takes a `RobotProvider`. A waiter learns of a match — human

PRERELEASE.md

@@ -511,3 +511,11 @@ Then Stage 18.
     (`GameRow`/`GameDetailView` gain `VsAI`); (4) `games_started_total` / `games_abandoned_total` gain a
     **`vs_ai`** attribute and the Grafana *Game domain* dashboard splits started/abandoned into **human**
     and **AI** panels.
+  - **Follow-up (separate PR — strategy deviation):** the robot now plays **≈20%** of opening/midgame moves
+    *against* its per-game `playToWin` intent (toward the opposite margin band — a winning robot eases off, a
+    losing one surges ahead), tapering linearly to **0 over the last 14 bag tiles** and **0 once the bag is
+    empty**, so the endgame follows the chosen strategy strictly while earlier outcomes can swing the human's
+    way. Deterministic from the seed (`mix(seed,"deviate",moveCount)`), applied to **both** robot paths via
+    the shared `selectMove`; the per-game intent (and the admin card) is unchanged. Tests: `robot` unit
+    (taper bounds + monotonicity, never-in-endgame, determinism, ~20% distribution). Bake-back:
+    `docs/ARCHITECTURE.md` §7, `docs/FUNCTIONAL.md` (+`_ru`), `backend/README.md`, `PLAN.md` Stage 5.

backend/README.md

@@ -50,7 +50,8 @@ each a `kind='robot'` identity, provisioned at startup with chat and friend
 requests blocked — backs human-like, per-language composed names. A background driver plays the
 robot's moves through the public game API as an ordinary seated player (so only
 `internal/engine` imports the solver): it decides once per game whether to play to
-win (≈ 40%), targets a small score margin, and times its moves with a move-number-aware
+win (≈ 40%), targets a small score margin — with an occasional off-strategy move that tapers to
+none as the bag empties — and times its moves with a move-number-aware
 right-skewed delay (quick openings, long endgames), a night-sleep window anchored to the opponent's timezone, and nudge
 behaviour — all derived deterministically from the game seed, so it keeps no extra
 state. A background **reaper** seats a pooled robot (matching the game's language) in any open

backend/internal/robot/driver.go

@@ -67,7 +67,8 @@ func (s *Service) handle(ctx context.Context, rt game.RobotTurn, now time.Time)
 	// Honest-AI game: the robot moves the instant it is its turn — no sleep window and
 	// no proactive nudge (chat and nudge are disabled in these games, and the player
 	// chose an opponent that "moves at once"). It still plays to the same per-game
-	// strength (playToWin) and margin band as the human-mimicry path.
+	// strength (playToWin), margin band and occasional off-strategy deviation as the
+	// human-mimicry path.
 	if rt.VsAI {
 		if rt.ToMove == rt.RobotSeat {
 			return s.act(ctx, rt, now)
@@ -164,9 +165,11 @@ func (s *Service) maybeNudge(ctx context.Context, rt game.RobotTurn, now time.Ti
 	return nil
 }
 
-// act reads the live turn, chooses a move by margin and submits it. State that
-// has moved on since the scan (a finished game, a turn that is no longer the
-// robot's) surfaces as a benign error and is skipped.
+// act reads the live turn, chooses a move by margin — usually toward the robot's
+// per-game intent, but with an occasional off-strategy deviation that fades to none
+// as the bag empties — and submits it. State that has moved on since the scan (a
+// finished game, a turn that is no longer the robot's) surfaces as a benign error
+// and is skipped.
 func (s *Service) act(ctx context.Context, rt game.RobotTurn, now time.Time) error {
 	st, err := s.games.GameState(ctx, rt.GameID, rt.RobotID)
 	if err != nil {
@@ -179,7 +182,11 @@ func (s *Service) act(ctx context.Context, rt game.RobotTurn, now time.Time) err
 
 	myScore := st.Game.Seats[st.Seat].Score
 	oppScore := bestOpponentScore(st.Game.Seats, st.Seat)
-	d := selectMove(cands, myScore, oppScore, playToWin(rt.Seed), defaultBand, st.Rack, st.BagLen)
+	win := playToWin(rt.Seed)
+	if deviates(rt.Seed, rt.MoveCount, st.BagLen) {
+		win = !win // an occasional off-strategy move; never once the bag is empty
+	}
+	d := selectMove(cands, myScore, oppScore, win, defaultBand, st.Rack, st.BagLen)
 
 	var res game.MoveResult
 	switch d.kind {

backend/internal/robot/strategy.go

@@ -23,6 +23,15 @@ const (
 	// human wins about 60% of games (docs/ARCHITECTURE.md §7).
 	playToWinPercent = 40
 
+	// The robot occasionally plays a single move against its per-game win/lose
+	// intent (an off-strategy "wobble"), so the chosen strategy may not pan out —
+	// which favours the human. deviateMaxProb is the peak probability of that, held
+	// through the opening and midgame; it tapers linearly to 0 over the last
+	// deviateTaperTiles tiles left in the bag, reaching 0 once the bag is empty, so
+	// the endgame follows the chosen strategy strictly (docs/ARCHITECTURE.md §7).
+	deviateMaxProb    = 0.20
+	deviateTaperTiles = 14
+
 	// The robot's think time depends on how far the game has progressed: early moves
 	// are quick and late moves can be long (endgame deliberation). The delay is drawn
 	// from a band that interpolates with the move count from [delayEarlyLoMinutes,
@@ -129,6 +138,34 @@ func PlayToWin(seed int64) bool { return playToWin(seed) }
 // win in any given game (the admin console shows it alongside the per-game decision).
 const PlayToWinTargetPercent = playToWinPercent
 
+// deviateProb is the probability that the robot plays a single move against its
+// per-game win/lose intent, given the number of tiles left in the bag. It is
+// deviateMaxProb through the opening and midgame, then tapers linearly to 0 over
+// the last deviateTaperTiles tiles, reaching 0 once the bag is empty so the endgame
+// follows the chosen strategy strictly.
+func deviateProb(bagLen int) float64 {
+	switch {
+	case bagLen <= 0:
+		return 0
+	case bagLen >= deviateTaperTiles:
+		return deviateMaxProb
+	default:
+		return deviateMaxProb * float64(bagLen) / float64(deviateTaperTiles)
+	}
+}
+
+// deviates reports whether the robot deviates from its per-game win/lose intent on
+// the move at moveCount: a deterministic per-turn draw (mix/unitFloat, like the
+// think-time sampling) against deviateProb(bagLen), so it is reproducible across
+// restarts and never fires once the bag is empty.
+func deviates(seed int64, moveCount, bagLen int) bool {
+	p := deviateProb(bagLen)
+	if p <= 0 {
+		return false
+	}
+	return unitFloat(mix(seed, "deviate", moveCount)) < p
+}
+
 // NextMoveAt is the deterministic instant the robot is scheduled to play the move at
 // moveCount, given when the turn started and the opponent's timezone (which anchors the
 // robot's sleep window). It is the sampled think-time delay, deferred to the end of the

backend/internal/robot/strategy_test.go

@@ -1,6 +1,7 @@
 package robot
 
 import (
+	"math"
 	"sort"
 	"testing"
 	"time"
@@ -238,6 +239,77 @@ func TestPlayToWinExport(t *testing.T) {
 	}
 }
 
+// TestDeviateProbTaper checks the deviation probability is deviateMaxProb while the
+// bag holds at least deviateTaperTiles tiles, halves at the taper midpoint, is 0
+// once the bag is empty, and stays within [0, deviateMaxProb] and non-decreasing.
+func TestDeviateProbTaper(t *testing.T) {
+	if p := deviateProb(0); p != 0 {
+		t.Errorf("deviateProb(0) = %v, want 0 (strict endgame)", p)
+	}
+	if p := deviateProb(deviateTaperTiles); p != deviateMaxProb {
+		t.Errorf("deviateProb(%d) = %v, want %v", deviateTaperTiles, p, deviateMaxProb)
+	}
+	if p := deviateProb(deviateTaperTiles + 50); p != deviateMaxProb {
+		t.Errorf("deviateProb above the taper = %v, want %v (capped)", p, deviateMaxProb)
+	}
+	if p := deviateProb(deviateTaperTiles / 2); math.Abs(p-deviateMaxProb/2) > 1e-9 {
+		t.Errorf("deviateProb at half taper = %v, want ~%v", p, deviateMaxProb/2)
+	}
+	prev := -1.0
+	for bag := 0; bag <= deviateTaperTiles+5; bag++ {
+		p := deviateProb(bag)
+		if p < 0 || p > deviateMaxProb {
+			t.Fatalf("deviateProb(%d) = %v out of [0,%v]", bag, p, deviateMaxProb)
+		}
+		if p < prev {
+			t.Fatalf("deviateProb not non-decreasing: bag %d gives %v after %v", bag, p, prev)
+		}
+		prev = p
+	}
+}
+
+// TestDeviatesNeverInEndgame checks the robot never deviates once the bag is empty,
+// for every seed and move count, so the endgame follows the chosen strategy strictly.
+func TestDeviatesNeverInEndgame(t *testing.T) {
+	for seed := int64(1); seed <= 5000; seed++ {
+		for mc := 0; mc < 40; mc++ {
+			if deviates(seed, mc, 0) {
+				t.Fatalf("deviates with an empty bag for seed=%d mc=%d", seed, mc)
+			}
+		}
+	}
+}
+
+// TestDeviatesDeterministic checks the per-turn deviation draw is reproducible for a
+// (seed, moveCount, bagLen), so the driver recomputes the same decision on every scan.
+func TestDeviatesDeterministic(t *testing.T) {
+	for seed := int64(1); seed <= 500; seed++ {
+		for mc := 0; mc < 30; mc++ {
+			got := deviates(seed, mc, deviateTaperTiles)
+			if deviates(seed, mc, deviateTaperTiles) != got {
+				t.Fatalf("deviates not deterministic for seed=%d mc=%d", seed, mc)
+			}
+		}
+	}
+}
+
+// TestDeviatesDistribution checks the deviation rate over many games lands near
+// deviateMaxProb while the bag is full (above the taper), at a fixed move count.
+func TestDeviatesDistribution(t *testing.T) {
+	const n = 20000
+	hits := 0
+	for seed := int64(1); seed <= n; seed++ {
+		if deviates(seed, 3, deviateTaperTiles+20) {
+			hits++
+		}
+	}
+	pct := float64(hits) / float64(n) * 100
+	want := deviateMaxProb * 100
+	if pct < want-2 || pct > want+2 {
+		t.Errorf("deviation rate = %.1f%%, want ~%.0f%% (±2)", pct, want)
+	}
+}
+
 // TestProactiveNudgeGap checks the proactive-nudge schedule: the first gap (refIdle 0) is
 // ~60-90 min, every gap stays within [60 min, 6 h] and is deterministic, and the gap lengthens
 // as the idle grows (the median at 12 h idle exceeds the median at the start).

docs/ARCHITECTURE.md

@@ -407,7 +407,14 @@ English game the Latin pool.
   (`engine.Candidates`) the move whose resulting lead (playing to win) or deficit
   (playing to lose) is closest to a small band (**1–30 points**), rather than
   always the maximum; with no legal play it exchanges a full rack when the bag can
-  refill it, else passes.
+  refill it, else passes. On **≈20%** of moves through the opening and midgame it
+  **deviates** — playing that single move toward the *opposite* band (a winning
+  robot eases off, a losing one surges ahead), so the chosen strategy may not pan
+  out, which favours the human; the deviation chance tapers linearly to **0 over
+  the last 14 tiles in the bag** and is **0 once the bag is empty**, so the endgame
+  follows the per-game intent strictly. It is **deterministic from the seed**
+  (`mix(seed,"deviate",moveCount)`), a per-move wobble that leaves the per-game
+  play-to-win intent (and the admin card) unchanged.
 - **Timing**: the per-move delay is **move-number-aware** — a right-skewed sample
   (exponent k=4, short delays frequent) from a band that interpolates from
   **[3, 10] min** at the first move to **[10, 90] min** by ~28 moves, so openings are

docs/FUNCTIONAL.md

@@ -141,8 +141,10 @@ When auto-match finds no human within the wait window (1.5–3 minutes), a robot
 takes the empty seat of the game you are already waiting in. It is meant to feel like a person: it
 decides once per game whether to play to win (about 40% of the time, so the human
 wins most games), aims for a close score rather than crushing or throwing the game,
-and plays at a human pace — short thinking times for most moves, the occasional long
-one, and a night-time pause that tracks the player's own day. It answers a nudge
+now and then plays a single move against that plan — a surprise lead or a slack move —
+yet holds to the plan once the bag empties, and plays at a human pace — short thinking
+times for most moves, the occasional long one, and a night-time pause that tracks the
+player's own day. It answers a nudge
 within a few minutes and nudges back when the player has been away a long time. It
 carries a human-like, language-appropriate name (a Russian game draws mostly Russian
 names); it does not chat, and **silently ignores friend requests** — a request to a
@@ -150,8 +152,9 @@ robot stays pending and expires, exactly like a human who never responds.
 
 The same robot also backs the **honest-AI quick game** the player chooses directly (above). There
 it makes no pretence: it is shown as **🤖** everywhere, joins and moves at once (no thinking time
-or night pause), keeps the same strength (it still plays to win only about 40% of the time), and
-chat, nudge and add-friend are off. AI games are **practice** — they never count toward a player's
+or night pause), keeps the same strength (it still plays to win only about 40% of the time, with
+the same occasional move against its plan that fades out by the endgame), and chat, nudge and
+add-friend are off. AI games are **practice** — they never count toward a player's
 statistics.
 
 ### Social: friends, block, chat, nudge

docs/FUNCTIONAL_ru.md

@@ -146,8 +146,10 @@ nudge) приходят от бота **этой партии** — по язы
 в которой вы уже ждёте, занимает робот-соперник. Он задуман неотличимым от человека:
 один раз за партию решает, играть ли на победу (примерно в 40% случаев, так что
 человек выигрывает большинство партий), целится в близкий счёт, а не в разгром или
-поддавки, и ходит с человеческим темпом — чаще короткие раздумья, изредка долгие, и
-ночная пауза, подстроенная под день игрока. На nudge отвечает за несколько минут и
+поддавки, время от времени делает один ход вопреки этому плану — неожиданный отрыв или
+слабый ход, — но к концу партии (когда мешок пуст) держится плана, и ходит с
+человеческим темпом — чаще короткие раздумья, изредка долгие, и ночная пауза,
+подстроенная под день игрока. На nudge отвечает за несколько минут и
 сам шлёт nudge, когда игрок надолго пропал. Носит человекоподобное имя, подходящее
 языку партии (в русской партии — в основном русские имена); не общается в чате и
 **молча игнорирует заявки в друзья** — заявка роботу остаётся в ожидании и истекает,
@@ -155,8 +157,8 @@ nudge) приходят от бота **этой партии** — по язы
 
 Тот же робот стоит и за **честной игрой против ИИ**, которую игрок выбирает напрямую (выше). Там он
 не притворяется: везде показан как **🤖**, присоединяется и ходит сразу (без раздумий и ночной
-паузы), сохраняет ту же силу (по-прежнему играет на победу лишь примерно в 40% партий), а чат, nudge
-и «добавить в друзья» выключены. Партии с ИИ — это **тренировка**: они не идут в статистику игрока.
+паузы), сохраняет ту же силу (по-прежнему играет на победу лишь примерно в 40% партий, с теми же
+редкими ходами вопреки плану, затухающими к эндшпилю), а чат, nudge и «добавить в друзья» выключены. Партии с ИИ — это **тренировка**: они не идут в статистику игрока.
 
 ### Социальное: друзья, блок, чат, nudge
 Подружиться можно двумя способами: погасить **одноразовый код**, который выпускает