diff --git a/backend/app/memorybank.py b/backend/app/memorybank.py
index c1f9979..4b9928c 100644
--- a/backend/app/memorybank.py
+++ b/backend/app/memorybank.py
@@ -84,6 +84,21 @@ def story_actions(adventure: models.Adventure) -> list[models.Action]:
     return [a for a in adventure.actions if a.text.strip()]
 
 
+def prune_dangling_memories(adventure: models.Adventure, db: Session) -> int:
+    """Delete memories that summarized actions which no longer exist (e.g. after
+    undo). source_start/source_end are Action.index values; a memory is dangling
+    if any covered action is past the current end of the story. Returns the count
+    removed. Cursors are self-healing in run_post_turn, so this is cleanup only."""
+    max_index = max((a.index for a in adventure.actions), default=-1)
+    dangling = [
+        m for m in adventure.memories
+        if m.source_end is not None and m.source_end > max_index
+    ]
+    for m in dangling:
+        db.delete(m)
+    return len(dangling)
+
+
 # ---------- Retrieval (runs inside the turn, before build_context) ----------
 
 async def retrieve_memories(
diff --git a/backend/app/migrations.py b/backend/app/migrations.py
index 332e2b2..5cbc461 100644
--- a/backend/app/migrations.py
+++ b/backend/app/migrations.py
@@ -75,6 +75,10 @@ MIGRATIONS: list[tuple[int, str]] = [
     # re-synced on demand. NULL for copies made before this column existed.
     (24, "ALTER TABLE adventure_scripts ADD COLUMN source_script_id INTEGER "
          "REFERENCES scripts(id) ON DELETE SET NULL"),
+    # Per-action snapshot of the shared script_state as it was before that
+    # action's hooks ran, enabling undo/retry to roll state back. JSON is valid
+    # on both SQLite and Postgres.
+    (25, "ALTER TABLE actions ADD COLUMN state_before JSON"),
 ]
 
 LATEST_VERSION = max((v for v, _ in MIGRATIONS), default=1)
diff --git a/backend/app/models.py b/backend/app/models.py
index df6c624..ae0fbe1 100644
--- a/backend/app/models.py
+++ b/backend/app/models.py
@@ -181,6 +181,10 @@ class Action(Base):
     # Reasoning-model "thinking" that preceded the text (AI actions only).
     reasoning: Mapped[str | None] = mapped_column(Text, nullable=True)
     context_snapshot: Mapped[dict | None] = mapped_column(JSON, nullable=True)
+    # Copy of Adventure.script_state as it was immediately BEFORE this action's
+    # script hooks ran, so undo/retry can roll the shared scoreboard back.
+    # NULL for actions created before this column existed.
+    state_before: Mapped[dict | None] = mapped_column(JSON, nullable=True)
     created_at: Mapped[datetime] = mapped_column(DateTime, default=utcnow)
 
     adventure: Mapped[Adventure] = relationship(back_populates="actions")
diff --git a/backend/app/routers/adventures.py b/backend/app/routers/adventures.py
index 93ca82b..3d815bd 100644
--- a/backend/app/routers/adventures.py
+++ b/backend/app/routers/adventures.py
@@ -1,3 +1,4 @@
+import copy
 import json
 import re
 import threading
@@ -153,6 +154,13 @@ def get_script_state(
     return {"state": state}
 
 
+def snapshot_state(adventure: models.Adventure) -> dict:
+    """Deep copy of the shared script_state, to staple onto an action so undo/
+    retry can restore it. Independent of later hook mutations."""
+    state = adventure.script_state if isinstance(adventure.script_state, dict) else {}
+    return copy.deepcopy(state)
+
+
 @router.patch("/{adventure_id}", response_model=schemas.AdventureOut)
 def update_adventure(
     adventure_id: int,
@@ -258,6 +266,9 @@ async def generate_turn(
         )
     else:
         memories = await memorybank.retrieve_memories(adventure, settings, update_stats=True)
+    # Scoreboard as it stands before this AI turn's context/output hooks mutate
+    # it — stapled onto the AI action so retry can start over from here.
+    state_before = snapshot_state(adventure)
     system_text, story_text, snapshot = build_context(adventure, settings, memories)
 
     # onModelContext: scripts see (and may rewrite) the whole assembled context.
@@ -327,6 +338,7 @@ async def generate_turn(
         text=text,
         reasoning="".join(reasoning_chunks).strip() or None,
         context_snapshot=snapshot,
+        state_before=state_before,
     )
     db.add(ai_action)
     adventure.updated_at = models.utcnow()
@@ -362,6 +374,9 @@ async def run_player_turn(
 
     # An empty do/say/story is just a continue.
     if payload.type != "continue" and payload.text.strip():
+        # Scoreboard before the input hook mutates it — the pre-turn state that
+        # undo restores to (the AI action keeps its own post-input snapshot).
+        state_before = snapshot_state(adventure)
         # onInput sees the formatted text (as in AI Dungeon: "> You ...").
         formatted = format_player_input(payload.type, payload.text)
         modified, stop = pipeline.run("input", formatted)
@@ -374,6 +389,7 @@ async def run_player_turn(
             index=next_index(adventure),
             type=payload.type,
             text=modified,
+            state_before=state_before,
         )
         db.add(player_action)
         db.commit()
@@ -426,7 +442,13 @@ def retry_action(
     acquire_turn_lock(adventure_id)
     try:
         if adventure.actions and adventure.actions[-1].type == "ai":
-            db.delete(adventure.actions[-1])
+            last_ai = adventure.actions[-1]
+            # Roll the scoreboard back to before this AI turn's hooks ran, so
+            # regenerating starts fresh instead of stacking output mutations on
+            # top of the discarded attempt. NULL for pre-migration actions.
+            if last_ai.state_before is not None:
+                adventure.script_state = copy.deepcopy(last_ai.state_before)
+            db.delete(last_ai)
             db.commit()
             db.refresh(adventure)
     except BaseException:
@@ -446,18 +468,34 @@ def retry_action(
 def undo_turn(
     adventure_id: int, db: Session = Depends(get_db), user: models.User = CurrentUser
 ):
-    """Delete the last turn: the trailing AI action plus its player action, if any."""
+    """Delete the last turn: the trailing AI action plus its player action, if any.
+
+    Also rolls the shared script_state back to before that turn ran and prunes
+    any memory that summarized the removed actions. The turn lock guards against
+    undoing while a turn is still generating."""
     adventure = get_adventure_or_404(adventure_id, db, user)
-    actions = list(adventure.actions)
-    if not actions or actions[-1].type == "start":
-        raise HTTPException(400, "Nothing to undo")
-    last = actions.pop()
-    db.delete(last)
-    if last.type == "ai" and actions and actions[-1].type in ("do", "say", "story"):
-        db.delete(actions.pop())
-    db.commit()
-    db.refresh(adventure)
-    return adventure.actions
+    acquire_turn_lock(adventure_id)
+    try:
+        actions = list(adventure.actions)
+        if not actions or actions[-1].type == "start":
+            raise HTTPException(400, "Nothing to undo")
+        last = actions.pop()
+        # The earliest action removed in this turn holds the pre-turn scoreboard.
+        first_removed = last
+        db.delete(last)
+        if last.type == "ai" and actions and actions[-1].type in ("do", "say", "story"):
+            first_removed = actions.pop()
+            db.delete(first_removed)
+        if first_removed.state_before is not None:
+            adventure.script_state = copy.deepcopy(first_removed.state_before)
+        db.flush()  # apply deletes so pruning sees the shrunken action list
+        db.expire(adventure, ["actions"])
+        memorybank.prune_dangling_memories(adventure, db)
+        db.commit()
+        db.refresh(adventure)
+        return adventure.actions
+    finally:
+        _active_turns.discard(adventure_id)
 
 
 # ---------- Import / Export ----------
diff --git a/backend/tests/test_state_revert.py b/backend/tests/test_state_revert.py
new file mode 100644
index 0000000..4e75552
--- /dev/null
+++ b/backend/tests/test_state_revert.py
@@ -0,0 +1,201 @@
+"""Tests for undo/retry rolling back the shared script_state scoreboard
+(plan/11-state-revert-and-retry-fix.md).
+
+Run from the backend dir:  python -m pytest tests/test_state_revert.py -v
+"""
+import os
+import tempfile
+
+# Point the app at a throwaway SQLite file BEFORE importing anything that binds
+# the engine at import time (app.database reads AIDND_DB_PATH on import).
+_tmp = tempfile.NamedTemporaryFile(suffix=".db", delete=False)
+_tmp.close()
+os.environ["AIDND_DB_PATH"] = _tmp.name
+os.environ.pop("AIDND_DATABASE_URL", None)
+os.environ.pop("DATABASE_URL", None)
+
+import pytest
+from fastapi import HTTPException
+
+from app import memorybank, models
+from app.database import Base, SessionLocal, engine
+from app.routers import adventures
+
+
+@pytest.fixture()
+def db():
+    Base.metadata.create_all(bind=engine)
+    session = SessionLocal()
+    try:
+        yield session
+    finally:
+        session.close()
+        Base.metadata.drop_all(bind=engine)
+        adventures._active_turns.clear()
+
+
+def _make_adventure(db, script_state):
+    user = models.User(is_guest=False)
+    db.add(user)
+    db.flush()
+    adv = models.Adventure(user_id=user.id, title="T", script_state=script_state)
+    db.add(adv)
+    db.flush()
+    return user, adv
+
+
+def _add(db, adv, index, type_, text="x", state_before=None):
+    a = models.Action(
+        adventure_id=adv.id, index=index, type=type_, text=text,
+        state_before=state_before,
+    )
+    db.add(a)
+    db.flush()
+    return a
+
+
+# ---------------------------------------------------------------- undo
+
+def test_undo_reverts_state_to_before_the_turn(db):
+    # A turn took the scoreboard from {gold:0} -> {gold:10}. The player action
+    # carries the pre-turn snapshot; current state is the mutated one.
+    user, adv = _make_adventure(db, {"gold": 10})
+    _add(db, adv, 0, "start", state_before=None)
+    _add(db, adv, 1, "do", state_before={"gold": 0})
+    _add(db, adv, 2, "ai", state_before={"gold": 0})
+    db.commit()
+
+    adventures.undo_turn(adv.id, db=db, user=user)
+
+    assert adv.script_state == {"gold": 0}
+    assert [a.type for a in adv.actions] == ["start"]
+
+
+def test_undo_of_bare_continue_uses_ai_snapshot(db):
+    # A "continue" turn has no player action; the AI action's own snapshot is
+    # the pre-turn state.
+    user, adv = _make_adventure(db, {"gold": 5})
+    _add(db, adv, 0, "start")
+    _add(db, adv, 1, "ai", state_before={"gold": 0})
+    db.commit()
+
+    adventures.undo_turn(adv.id, db=db, user=user)
+
+    assert adv.script_state == {"gold": 0}
+    assert [a.type for a in adv.actions] == ["start"]
+
+
+def test_undo_leaves_state_untouched_when_snapshot_missing(db):
+    # Pre-migration actions have state_before = NULL: don't clobber the state.
+    user, adv = _make_adventure(db, {"gold": 10})
+    _add(db, adv, 0, "start")
+    _add(db, adv, 1, "do", state_before=None)
+    _add(db, adv, 2, "ai", state_before=None)
+    db.commit()
+
+    adventures.undo_turn(adv.id, db=db, user=user)
+
+    assert adv.script_state == {"gold": 10}
+
+
+def test_undo_raises_when_nothing_to_undo(db):
+    user, adv = _make_adventure(db, {})
+    _add(db, adv, 0, "start")
+    db.commit()
+    with pytest.raises(HTTPException) as exc:
+        adventures.undo_turn(adv.id, db=db, user=user)
+    assert exc.value.status_code == 400
+
+
+def test_undo_blocked_by_active_turn_lock(db):
+    user, adv = _make_adventure(db, {})
+    _add(db, adv, 0, "start")
+    _add(db, adv, 1, "ai", state_before={})
+    db.commit()
+
+    adventures.acquire_turn_lock(adv.id)  # a turn is "generating"
+    try:
+        with pytest.raises(HTTPException) as exc:
+            adventures.undo_turn(adv.id, db=db, user=user)
+        assert exc.value.status_code == 409
+        # The failed undo must not have released someone else's lock.
+        assert adv.id in adventures._active_turns
+    finally:
+        adventures._active_turns.discard(adv.id)
+
+
+def test_undo_prunes_memory_covering_removed_actions(db):
+    user, adv = _make_adventure(db, {})
+    for i in range(4):
+        _add(db, adv, i, "ai" if i % 2 else "do", state_before={})
+    # A memory summarizing actions up to index 3, which undo will delete.
+    covering = models.Memory(adventure_id=adv.id, text="m", source_start=0, source_end=3)
+    keep = models.Memory(adventure_id=adv.id, text="k", source_start=0, source_end=1)
+    db.add_all([covering, keep])
+    db.commit()
+
+    adventures.undo_turn(adv.id, db=db, user=user)  # removes indexes 2 & 3
+
+    texts = {m.text for m in adv.memories}
+    assert texts == {"k"}
+
+
+# ---------------------------------------------------------------- prune helper
+
+def test_prune_dangling_memories_counts_and_removes(db):
+    user, adv = _make_adventure(db, {})
+    _add(db, adv, 0, "do")
+    _add(db, adv, 1, "ai")
+    db.add_all([
+        models.Memory(adventure_id=adv.id, text="live", source_start=0, source_end=1),
+        models.Memory(adventure_id=adv.id, text="dead", source_start=2, source_end=5),
+    ])
+    db.commit()
+
+    removed = memorybank.prune_dangling_memories(adv, db)
+    db.commit()
+    db.refresh(adv)  # expire_on_commit=False: reload the memories collection
+
+    assert removed == 1
+    assert {m.text for m in adv.memories} == {"live"}
+
+
+# ---------------------------------------------------------------- snapshot
+
+def test_snapshot_state_is_an_independent_deep_copy(db):
+    _, adv = _make_adventure(db, {"nested": {"n": 1}})
+    snap = adventures.snapshot_state(adv)
+    adv.script_state["nested"]["n"] = 99
+    assert snap == {"nested": {"n": 1}}  # unaffected by later mutation
+
+
+def test_snapshot_state_handles_non_dict(db):
+    _, adv = _make_adventure(db, {})
+    adv.script_state = None
+    assert adventures.snapshot_state(adv) == {}
+
+
+# ---------------------------------------------------------------- retry
+
+def test_retry_restores_state_before_regenerating(db, monkeypatch):
+    # Retry deletes the last AI action and must roll the scoreboard back to that
+    # action's snapshot so regeneration doesn't stack output mutations.
+    user, adv = _make_adventure(db, {"gold": 20})  # 20 = double-applied bug value
+    _add(db, adv, 0, "start")
+    _add(db, adv, 1, "do", state_before={"gold": 0})
+    _add(db, adv, 2, "ai", state_before={"gold": 10})
+    db.commit()
+
+    monkeypatch.setattr(adventures.limits, "rate_limit", lambda *a, **k: None)
+    monkeypatch.setattr(adventures, "check_demo_cap", lambda *a, **k: None)
+
+    async def _noop(*a, **k):
+        if False:
+            yield  # make it an async generator
+    monkeypatch.setattr(adventures, "generate_turn", _noop)
+
+    adventures.retry_action(adv.id, request=None, db=db, user=user)
+
+    assert adv.script_state == {"gold": 10}
+    assert [a.type for a in adv.actions] == ["start", "do"]
+    adventures._active_turns.discard(adv.id)
diff --git a/backend/tests/test_turn_flow_integration.py b/backend/tests/test_turn_flow_integration.py
new file mode 100644
index 0000000..c199db2
--- /dev/null
+++ b/backend/tests/test_turn_flow_integration.py
@@ -0,0 +1,136 @@
+"""End-to-end HTTP tests for undo/retry state revert, driving real turns through
+the actual routes + scripting engine with only the LLM provider mocked.
+
+A script's output hook adds 10 gold each turn; we assert the shared scoreboard
+behaves correctly across play / undo / retry.
+
+    python -m pytest tests/test_turn_flow_integration.py -v
+"""
+import os
+import tempfile
+
+_tmp = tempfile.NamedTemporaryFile(suffix=".db", delete=False)
+_tmp.close()
+os.environ["AIDND_DB_PATH"] = _tmp.name
+os.environ.pop("AIDND_DATABASE_URL", None)
+os.environ.pop("DATABASE_URL", None)
+
+import pytest
+from fastapi import Depends
+from fastapi.testclient import TestClient
+
+from app import auth, limits, models
+from app.database import Base, SessionLocal, engine, get_db
+from app.main import app
+from app.providers import PromptParts
+from app.routers import adventures
+
+GOLD_SCRIPT = """
+const modifier = (text) => {
+  state.gold = (state.gold || 0) + 10;
+  return { text };
+};
+modifier(text);
+"""
+
+
+class FakeProvider:
+    """Stand-in for OpenAICompatibleProvider: streams one fixed line, no network."""
+    def __init__(self, *a, **k):
+        pass
+
+    async def generate(self, parts: PromptParts, *, temperature, max_tokens):
+        yield ("text", "The torch flickers as you press onward.")
+
+
+@pytest.fixture()
+def client(monkeypatch):
+    Base.metadata.create_all(bind=engine)
+    setup = SessionLocal()
+    user = models.User(is_guest=False, email="tester@example.com")
+    setup.add(user)
+    setup.flush()
+    setup.add(models.Settings(user_id=user.id, api_key="enc:dummy", model="test-model"))
+    adv = models.Adventure(user_id=user.id, title="Cave", script_state={})
+    setup.add(adv)
+    setup.flush()
+    setup.add(models.Action(adventure_id=adv.id, index=0, type="start", text="You enter a cave."))
+    setup.add(models.AdventureScript(
+        adventure_id=adv.id, position=0, enabled=True, name="Gold",
+        output_js=GOLD_SCRIPT,
+    ))
+    setup.commit()
+    adv_id, user_id = adv.id, user.id
+    setup.close()
+
+    # Force a real (non-demo) turn that uses our fake provider.
+    monkeypatch.setattr(adventures, "OpenAICompatibleProvider", FakeProvider)
+    monkeypatch.setattr(auth, "resolve_provider_config", lambda s: auth.ProviderConfig(
+        "http://fake", "k", "test-model", False))
+    monkeypatch.setattr(limits, "rate_limit", lambda *a, **k: None)
+    monkeypatch.setattr(limits, "check_row_cap", lambda *a, **k: None)
+
+    def _current_user(db=Depends(get_db)):
+        return db.get(models.User, user_id)
+
+    app.dependency_overrides[auth.get_current_user] = _current_user
+
+    c = TestClient(app)
+    c.adv_id = adv_id
+    try:
+        yield c
+    finally:
+        app.dependency_overrides.clear()
+        adventures._active_turns.clear()
+        Base.metadata.drop_all(bind=engine)
+
+
+def _state(adv_id):
+    db = SessionLocal()
+    try:
+        return db.get(models.Adventure, adv_id).script_state
+    finally:
+        db.close()
+
+
+def _play(client, type_="do", text="look around"):
+    r = client.post(f"/api/adventures/{client.adv_id}/actions", json={"type": type_, "text": text})
+    assert r.status_code == 200, r.text
+    return r
+
+
+def test_play_then_undo_reverts_gold(client):
+    assert _state(client.adv_id) == {}
+    _play(client)
+    assert _state(client.adv_id) == {"gold": 10}
+
+    r = client.post(f"/api/adventures/{client.adv_id}/undo")
+    assert r.status_code == 200, r.text
+    assert _state(client.adv_id) == {}  # scoreboard rolled back
+
+
+def test_two_turns_then_undo_reverts_only_last(client):
+    _play(client)
+    _play(client)
+    assert _state(client.adv_id) == {"gold": 20}
+
+    client.post(f"/api/adventures/{client.adv_id}/undo")
+    assert _state(client.adv_id) == {"gold": 10}  # back to after turn 1, not 0
+
+
+def test_retry_does_not_double_apply_gold(client):
+    _play(client)
+    assert _state(client.adv_id) == {"gold": 10}
+
+    # Before the fix this produced 20 (output hook ran twice); now it stays 10.
+    r = client.post(f"/api/adventures/{client.adv_id}/retry")
+    assert r.status_code == 200, r.text
+    assert _state(client.adv_id) == {"gold": 10}
+
+
+def test_retry_then_undo_still_clean(client):
+    _play(client)
+    client.post(f"/api/adventures/{client.adv_id}/retry")
+    assert _state(client.adv_id) == {"gold": 10}
+    client.post(f"/api/adventures/{client.adv_id}/undo")
+    assert _state(client.adv_id) == {}
diff --git a/plan/11-state-revert-and-retry-fix.md b/plan/11-state-revert-and-retry-fix.md
new file mode 100644
index 0000000..18eda1c
--- /dev/null
+++ b/plan/11-state-revert-and-retry-fix.md
@@ -0,0 +1,81 @@
+# Plan: undo/retry state revert (+ concurrency lock)
+
+Fixes three linked issues around `script_state` (the shared per-adventure
+"scoreboard" scripts write to) and undo/retry.
+
+## Background
+
+- `script_state` is one shared dict on `Adventure` (`models.py:90`), mutated in
+  exactly ONE place: `pipeline.py:89` (`self.adventure.script_state = state`).
+- Today, undo (`adventures.py:445`) and retry (`:422`) delete *actions* but never
+  touch `script_state`, so state never rolls back.
+
+## Issue 1 — retry double-applies state (pre-existing bug)
+
+Retry deletes the last AI action and regenerates. The output hook already mutated
+`script_state` on the first attempt; regenerating runs it again, stacking the change
+(e.g. "add 10 gold" → 20 gold after one retry). Same root cause as undo not
+reverting.
+
+## Issue 4 — undo has no concurrency guard
+
+Turns take `acquire_turn_lock` (`:189`); undo does not, so undo can race a turn
+that is still streaming.
+
+## Issue 2 — Memory Bank leftovers (smaller than expected)
+
+`run_post_turn` already clamps `memory_cursor`/`summary_cursor` down to the current
+action count (`memorybank.py:178-182`), so there is NO cursor stall. The only
+remainder: a `Memory` created from a turn that was later undone stays behind, its
+`source_start/source_end` now pointing past the end of the story.
+
+---
+
+## The fix
+
+### 1. Snapshot state per turn  (Issue 1 + enables undo revert)
+
+- Add column `state_before: JSON nullable` to `Action` (`models.py`).
+- Migration: append `(25, "ALTER TABLE actions ADD COLUMN state_before JSON")`
+  to `migrations.py`. `JSON` is valid on both SQLite and Postgres.
+- In `run_player_turn` / `generate_turn`, when the FIRST action of a turn is
+  created, stash `copy.deepcopy(adventure.script_state)` onto it — captured
+  *before* any hook runs. (Player action for do/say/story; the AI action for a
+  bare `continue`.)
+- Fix retry directly: before regenerating, restore
+  `adventure.script_state` from the deleted AI action's `state_before` so the
+  output hook starts from the pre-turn scoreboard instead of the mutated one.
+
+### 2. Revert on undo  (Issue depends on #1)
+
+- In `undo_turn`, after deleting the popped actions, set
+  `adventure.script_state = <deleted player/first action>.state_before` (fall back
+  to `{}` if null, i.e. pre-migration turns), then commit.
+- Only wire this into undo + retry — NOT the arbitrary
+  `delete_action` endpoint (`:830`); mid-history state revert is undefined.
+
+### 3. Lock undo  (Issue 4)
+
+- Wrap `undo_turn` body in `acquire_turn_lock(adventure_id)` /
+  `_active_turns.discard(...)` in a `finally`. It's synchronous (not SSE), so no
+  `with_turn_lock` wrapper needed — just acquire and discard.
+
+### 4. Clean up dangling memories  (Issue 2, optional)
+
+- In `undo_turn` after deleting actions, delete any `Memory` whose `source_start`
+  is >= the new story-action count (i.e. summarized a turn that no longer exists).
+  Cursors already self-heal, so this is polish, not correctness.
+
+## Known limits (document, don't fix)
+
+- Story cards a script created (`_apply_cards`, `pipeline.py:87`) are NOT reverted —
+  only text + script_state roll back.
+- Pre-migration turns have `state_before = NULL` → undo falls back to `{}`.
+- Demo turn cap is not refunded on undo (intentional).
+
+## Test checklist
+
+- Script that increments a counter: play → undo → counter back to prior value.
+- Same script: play → retry → counter changes once, not twice.
+- Undo during an active stream returns 409, doesn't corrupt state.
+- Undo a turn old enough to have been summarized: no orphaned memory left.