Remove three kinds of duplication in the backend

Stage 2, items 1, 4, and 5 of `plan/17-refactor.md`.

**One path resolver in `worldstate`.** `apply_delta` and `apply_override` routed
`flags.<name>`, `milestones.<id>`, `world.<stat>`, `player.<stat>`, and
`npc.<id>.<stat>` with parallel code, about 100 lines each. `_resolve` now says
what a path points at and returns either a target or the rejection to report.
Each function keeps its own write rule, because the rules genuinely differ: an
override sets a number rather than adding to it, ignores `cooldown`,
`max_delta_per_turn`, and the rule that a counter only counts up, and can un-set
a milestone.

A differential check ran both implementations over 3960 payloads: twenty paths,
fourteen values, three starting states, plus every three-path combination. The
results are identical except that 674 rejections from `apply_override` now carry
a `fix` string. `apply_delta` already worded those, and the world-state editor
renders them, so an override that names an unknown flag now explains itself the
way a delta does.

**`sse`, `SSE_HEADERS`, and `turn_error` move to `app/sse.py`.** Two routers
stream, and `chat.py` had to import from `routers.adventures` to reach them.

**`get_adventure_or_404` becomes the `current_adventure` dependency.** All 32
handlers repeated the call as their first statement. The ownership check now
reads in the signature and runs before the body. FastAPI caches a dependency for
one request, so the handler's `db` is the session the adventure came from.

The generated OpenAPI document is byte-identical except on `rename_branch`,
where `branch_id` is now listed before `adventure_id`, because that handler no
longer names `adventure_id` itself. Parameter order in the document is
cosmetic.

Six tests in `test_state_revert.py` call `undo_turn` and `retry_action`
directly rather than over HTTP. They pass the adventure they already hold
instead of an id.

549 tests pass.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014Dix4oGV3njgWRdu7P9t6r

backend/app/routers/adventures/__init__.py

@@ -42,17 +42,15 @@ from ... import limits  # noqa: F401  `adventures.limits` is patched by tests.
 from .crud import SNIPPET_MAX, _snippet
 from .paging import ACTION_PAGE
 from .takes import retry_action, undo_turn
-from .turns import SSE_HEADERS, sse, world_delta_of
+from .turns import world_delta_of
 
 __all__ = [
     "ACTION_PAGE",
     "SNIPPET_MAX",
-    "SSE_HEADERS",
     "_snippet",
     "limits",
     "retry_action",
     "router",
-    "sse",
     "undo_turn",
     "world_delta_of",
 ]

backend/app/routers/adventures/actions.py

@@ -11,18 +11,17 @@ from sqlalchemy.orm import Session
 from ... import models, schemas, tree
 from ...database import get_db
 
-from .deps import CurrentUser, get_adventure_or_404, router
+from .deps import CurrentUser, current_adventure, router
 from .nodes import delete_turn
 from .paging import ACTION_PAGE, action_window, annotate_takes
 
 
 @router.get("/{adventure_id}/actions", response_model=schemas.ActionPage)
 def list_actions(
-    adventure_id: int,
     before_id: int | None = None,
     limit: int = ACTION_PAGE,
     db: Session = Depends(get_db),
-    user: models.User = CurrentUser,
+    adventure: models.Adventure = Depends(current_adventure),
 ):
     """Returns a page of the story, working backwards from the newest action.
 
@@ -30,7 +29,6 @@ def list_actions(
     asks for what comes before it. Omit `before_id` for the newest window. See
     `action_window` for why this anchors on a row rather than an offset.
     """
-    adventure = get_adventure_or_404(adventure_id, db, user)
     limit = max(1, min(limit, ACTION_PAGE * 4))
     actions, total, has_more = action_window(
         db, adventure, before_id=before_id, limit=limit
@@ -51,9 +49,8 @@ def update_action(
     action_id: int,
     payload: schemas.ActionUpdate,
     db: Session = Depends(get_db),
-    user: models.User = CurrentUser,
+    adventure: models.Adventure = Depends(current_adventure),
 ):
-    get_adventure_or_404(adventure_id, db, user)
     action = db.get(models.Action, action_id)
     if action is None or action.adventure_id != adventure_id:
         raise HTTPException(404, "Action not found")
@@ -70,9 +67,8 @@ def delete_action(
     adventure_id: int,
     action_id: int,
     db: Session = Depends(get_db),
-    user: models.User = CurrentUser,
+    adventure: models.Adventure = Depends(current_adventure),
 ):
-    adventure = get_adventure_or_404(adventure_id, db, user)
     action = db.get(models.Action, action_id)
     if action is None or action.adventure_id != adventure_id:
         raise HTTPException(404, "Action not found")

backend/app/routers/adventures/branches.py

@@ -18,14 +18,15 @@ from ...context import lineage
 from ...database import get_db
 
 from . import turns
-from .deps import CurrentUser, get_adventure_or_404, router
+from .deps import CurrentUser, current_adventure, router
 from .nodes import db_tip
 from .paging import current_window
 
 
 @router.get("/{adventure_id}/branches", response_model=list[schemas.BranchOut])
 def list_branches(
-    adventure_id: int, db: Session = Depends(get_db), user: models.User = CurrentUser
+    db: Session = Depends(get_db),
+    adventure: models.Adventure = Depends(current_adventure),
 ):
     """Returns every branch of the adventure and where each one leaves its parent.
 
@@ -35,7 +36,6 @@ def list_branches(
     `actions`, never one query per branch, so a view of a hundred forks does not
     cost a hundred round trips.
     """
-    adventure = get_adventure_or_404(adventure_id, db, user)
     branches = (
         db.query(models.Branch)
         .filter(models.Branch.adventure_id == adventure.id)
@@ -95,11 +95,10 @@ def get_branch_or_404(
     "/{adventure_id}/branches/{branch_id}", response_model=schemas.BranchOut
 )
 def rename_branch(
-    adventure_id: int,
     branch_id: int,
     payload: schemas.BranchRename,
     db: Session = Depends(get_db),
-    user: models.User = CurrentUser,
+    adventure: models.Adventure = Depends(current_adventure),
 ):
     """Names a branch, or clears the name to leave it unnamed.
 
@@ -107,7 +106,6 @@ def rename_branch(
     name anyone chose, and storing one gives the client an empty label to draw
     instead of the fork depth.
     """
-    adventure = get_adventure_or_404(adventure_id, db, user)
     branch = get_branch_or_404(adventure, branch_id, db)
     name = (payload.name or "").strip()
     branch.name = name or None
@@ -145,7 +143,7 @@ def delete_branch(
     adventure_id: int,
     branch_id: int,
     db: Session = Depends(get_db),
-    user: models.User = CurrentUser,
+    adventure: models.Adventure = Depends(current_adventure),
 ):
     """Deletes a branch and everything forked from it.
 
@@ -163,7 +161,6 @@ def delete_branch(
     cascade on `branches.parent_branch_id`, so the delete is a single statement
     however deep the subtree is.
     """
-    adventure = get_adventure_or_404(adventure_id, db, user)
     branch = get_branch_or_404(adventure, branch_id, db)
     if branch.parent_branch_id is None:
         raise HTTPException(
@@ -238,7 +235,7 @@ def switch_branch(
     adventure_id: int,
     branch_id: int,
     db: Session = Depends(get_db),
-    user: models.User = CurrentUser,
+    adventure: models.Adventure = Depends(current_adventure),
 ):
     """Reads and plays a different branch of the story.
 
@@ -249,7 +246,6 @@ def switch_branch(
     another branch's numbers, including the world-state cooldown clock inside the
     snapshot.
     """
-    adventure = get_adventure_or_404(adventure_id, db, user)
     branch = db.get(models.Branch, branch_id)
     if branch is None or branch.adventure_id != adventure.id:
         raise HTTPException(404, "Branch not found")

backend/app/routers/adventures/bundle_io.py

@@ -10,19 +10,19 @@ from sqlalchemy.orm import Session
 from ... import analytics, bundle, limits, models, schemas
 from ...database import get_db
 
-from .deps import CurrentUser, get_adventure_or_404, router
+from .deps import CurrentUser, current_adventure, router
 
 
 @router.get("/{adventure_id}/export")
 def export_adventure(
-    adventure_id: int, db: Session = Depends(get_db), user: models.User = CurrentUser
+    db: Session = Depends(get_db),
+    adv: models.Adventure = Depends(current_adventure),
 ):
     """Returns a full backup: plot components, story cards, scripts, state, and tree.
 
     `app/bundle.py` owns the format, in both of its versions. A backup outlives
     the schema, so no call site decides anything about its shape.
     """
-    adv = get_adventure_or_404(adventure_id, db, user)
     return bundle.export(db, adv)
 
 

backend/app/routers/adventures/crud.py

@@ -12,7 +12,7 @@ from sqlalchemy.orm.attributes import set_committed_value
 from ... import analytics, attempts, images, limits, memorybank, models, schemas, tree, worldstate
 from ...database import get_db
 
-from .deps import CurrentUser, get_adventure_or_404, router
+from .deps import CurrentUser, current_adventure, router
 from .paging import action_window, annotate_takes
 from .scenario_text import fill_placeholders, scenario_card_specs
 
@@ -229,7 +229,8 @@ def create_adventure(
 
 @router.get("/{adventure_id}", response_model=schemas.AdventureOut)
 def get_adventure(
-    adventure_id: int, db: Session = Depends(get_db), user: models.User = CurrentUser
+    db: Session = Depends(get_db),
+    adventure: models.Adventure = Depends(current_adventure),
 ):
     """Returns the adventure and the newest window of its story.
 
@@ -238,7 +239,6 @@ def get_adventure(
     exist above. `GET /{id}/actions` serves the older pages as the reader
     scrolls up.
     """
-    adventure = get_adventure_or_404(adventure_id, db, user)
     actions, total, _ = action_window(db, adventure)
     # Annotate before handing over the window. This path serializes through the
     # relationship rather than building `ActionOut` itself, so the pager numbers
@@ -259,7 +259,7 @@ def get_adventure(
 
 @router.get("/{adventure_id}/script-state")
 def get_script_state(
-    adventure_id: int, db: Session = Depends(get_db), user: models.User = CurrentUser
+    adventure: models.Adventure = Depends(current_adventure),
 ):
     """Returns the scripting `state` object.
 
@@ -267,21 +267,19 @@ def get_script_state(
     `state.x`, persisted after each hook. It stays `{}` until a script sets a
     variable.
     """
-    adventure = get_adventure_or_404(adventure_id, db, user)
     state = adventure.script_state if isinstance(adventure.script_state, dict) else {}
     return {"state": state}
 
 
 @router.get("/{adventure_id}/world-state")
 def get_world_state(
-    adventure_id: int, db: Session = Depends(get_db), user: models.User = CurrentUser
+    adventure: models.Adventure = Depends(current_adventure),
 ):
     """Returns the live RPG world state and the scenario's `stat_schema`.
 
     The play view uses both to render the character sheet and the milestones.
     `schema` is null when the adventure has no RPG layer.
     """
-    adventure = get_adventure_or_404(adventure_id, db, user)
     schema = adventure.scenario.stat_schema if adventure.scenario else None
     state = adventure.world_state if isinstance(adventure.world_state, dict) else {}
     return {
@@ -292,10 +290,9 @@ def get_world_state(
 
 @router.put("/{adventure_id}/world-state")
 def override_world_state(
-    adventure_id: int,
     overrides: dict = Body(...),
     db: Session = Depends(get_db),
-    user: models.User = CurrentUser,
+    adventure: models.Adventure = Depends(current_adventure),
 ):
     """Edits the live RPG values directly, as a manual correction rather than a turn.
 
@@ -303,7 +300,6 @@ def override_world_state(
     `milestones.y` to their new absolute values. The endpoint rejects unknown
     paths and wrong types one at a time, and applies the rest.
     """
-    adventure = get_adventure_or_404(adventure_id, db, user)
     schema = adventure.scenario.stat_schema if adventure.scenario else None
     if not worldstate.has_schema(schema):
         raise HTTPException(400, "This adventure has no RPG world-state layer")
@@ -316,12 +312,10 @@ def override_world_state(
 
 @router.patch("/{adventure_id}", response_model=schemas.AdventureOut)
 def update_adventure(
-    adventure_id: int,
     payload: schemas.AdventureUpdate,
     db: Session = Depends(get_db),
-    user: models.User = CurrentUser,
+    adventure: models.Adventure = Depends(current_adventure),
 ):
-    adventure = get_adventure_or_404(adventure_id, db, user)
     for field, value in payload.model_dump(exclude_unset=True).items():
         setattr(adventure, field, value)
     db.commit()
@@ -330,9 +324,10 @@ def update_adventure(
 
 @router.delete("/{adventure_id}", status_code=204)
 def delete_adventure(
-    adventure_id: int, db: Session = Depends(get_db), user: models.User = CurrentUser
+    adventure_id: int,
+    db: Session = Depends(get_db),
+    adventure: models.Adventure = Depends(current_adventure),
 ):
-    adventure = get_adventure_or_404(adventure_id, db, user)
     db.delete(adventure)
     db.commit()
     # No later request reads this adventure's vectors, so drop them now. The

backend/app/routers/adventures/deps.py

@@ -9,6 +9,7 @@ from fastapi import APIRouter, Depends, HTTPException
 from sqlalchemy.orm import Session
 
 from ... import auth, models
+from ...database import get_db
 
 
 router = APIRouter(prefix="/api/adventures", tags=["adventures"])
@@ -23,3 +24,19 @@ def get_adventure_or_404(
     if adventure is None or adventure.user_id != user.id:
         raise HTTPException(404, "Adventure not found")
     return adventure
+
+
+def current_adventure(
+    adventure_id: int,
+    db: Session = Depends(get_db),
+    user: models.User = Depends(auth.get_current_user),
+) -> models.Adventure:
+    """Resolves the `{adventure_id}` in the path to the caller's own adventure.
+
+    Declare this in a handler's signature. The ownership check then reads in the
+    signature, where you look for it, and it runs before the handler body.
+
+    FastAPI caches a dependency for the length of one request, so the handler's
+    own `db` is the same session this one loaded the adventure from.
+    """
+    return get_adventure_or_404(adventure_id, db, user)

backend/app/routers/adventures/insights.py

@@ -12,15 +12,16 @@ from ...context import build_context
 from ...database import get_db
 from ..settings import get_settings
 
-from .deps import CurrentUser, get_adventure_or_404, router
+from .deps import CurrentUser, current_adventure, router
 
 
 @router.get("/{adventure_id}/context")
 async def dry_run_context(
-    adventure_id: int, db: Session = Depends(get_db), user: models.User = CurrentUser
+    db: Session = Depends(get_db),
+    user: models.User = CurrentUser,
+    adventure: models.Adventure = Depends(current_adventure),
 ):
     """Returns what the app would send to the AI if the player continued now."""
-    adventure = get_adventure_or_404(adventure_id, db, user)
     settings = get_settings(db, user)
     if auth.resolve_provider_config(settings).using_demo:
         memories = (
@@ -39,9 +40,8 @@ def action_context(
     adventure_id: int,
     action_id: int,
     db: Session = Depends(get_db),
-    user: models.User = CurrentUser,
+    adventure: models.Adventure = Depends(current_adventure),
 ):
-    get_adventure_or_404(adventure_id, db, user)
     action = db.get(models.Action, action_id)
     if action is None or action.adventure_id != adventure_id:
         raise HTTPException(404, "Action not found")

backend/app/routers/adventures/memories.py

@@ -11,7 +11,7 @@ from ... import limits, memorybank, models, schemas, tree
 from ...context import lineage
 from ...database import get_db
 
-from .deps import CurrentUser, get_adventure_or_404, router
+from .deps import CurrentUser, current_adventure, router
 
 
 # The columns `schemas.MemoryOut` renders. `embedded` is a real column and
@@ -32,9 +32,10 @@ MEMORY_LIST_COLUMNS = (
 
 @router.get("/{adventure_id}/memories", response_model=list[schemas.MemoryOut])
 def list_memories(
-    adventure_id: int, db: Session = Depends(get_db), user: models.User = CurrentUser
+    adventure_id: int,
+    db: Session = Depends(get_db),
+    adventure: models.Adventure = Depends(current_adventure),
 ):
-    adventure = get_adventure_or_404(adventure_id, db, user)
     # Name the columns in a query rather than walk `adventure.memories`.
     # Retrieval used to walk the relationship, which is why a turn cost
     # megabytes: a relationship load returns whole entities, so it reads
@@ -63,13 +64,12 @@ def list_memories(
 
 @router.post("/{adventure_id}/memories", response_model=schemas.MemoryOut, status_code=201)
 def create_memory(
-    adventure_id: int,
     payload: schemas.MemoryCreate,
     db: Session = Depends(get_db),
     user: models.User = CurrentUser,
+    adventure: models.Adventure = Depends(current_adventure),
 ):
     """Adds a memory manually. The next post-turn pass embeds it."""
-    adventure = get_adventure_or_404(adventure_id, db, user)
     limits.check_row_cap("memories", db, user, adventure=adventure)
     if not payload.text.strip():
         raise HTTPException(400, "Memory text cannot be empty")
@@ -88,9 +88,8 @@ def update_memory(
     memory_id: int,
     payload: schemas.MemoryUpdate,
     db: Session = Depends(get_db),
-    user: models.User = CurrentUser,
+    adventure: models.Adventure = Depends(current_adventure),
 ):
-    get_adventure_or_404(adventure_id, db, user)
     memory = db.get(models.Memory, memory_id)
     if memory is None or memory.adventure_id != adventure_id:
         raise HTTPException(404, "Memory not found")
@@ -108,9 +107,8 @@ def delete_memory(
     adventure_id: int,
     memory_id: int,
     db: Session = Depends(get_db),
-    user: models.User = CurrentUser,
+    adventure: models.Adventure = Depends(current_adventure),
 ):
-    get_adventure_or_404(adventure_id, db, user)
     memory = db.get(models.Memory, memory_id)
     if memory is None or memory.adventure_id != adventure_id:
         raise HTTPException(404, "Memory not found")

backend/app/routers/adventures/refresh.py

@@ -14,7 +14,7 @@ from ... import models, schemas, worldstate
 from ...database import get_db
 
 from . import turns
-from .deps import CurrentUser, get_adventure_or_404, router
+from .deps import CurrentUser, current_adventure, router
 from .scenario_text import (
     CARD_FIELDS, SCENARIO_TEXT_FIELDS, fill_placeholders, scenario_card_specs,
     scenario_placeholder_names,
@@ -113,10 +113,11 @@ def plan_refresh(
 
 @router.get("/{adventure_id}/refresh", response_model=schemas.RefreshPlan)
 def preview_refresh(
-    adventure_id: int, db: Session = Depends(get_db), user: models.User = CurrentUser
+    db: Session = Depends(get_db),
+    user: models.User = CurrentUser,
+    adventure: models.Adventure = Depends(current_adventure),
 ):
     """Returns what "Update from scenario" would change, for the confirm dialog."""
-    adventure = get_adventure_or_404(adventure_id, db, user)
     scenario = resolve_source_scenario(adventure, db, user)
     if scenario is None:
         raise HTTPException(404, "No scenario to update from")
@@ -137,6 +138,7 @@ def refresh_from_scenario(
     payload: schemas.AdventureRefresh = Body(default=schemas.AdventureRefresh()),
     db: Session = Depends(get_db),
     user: models.User = CurrentUser,
+    adventure: models.Adventure = Depends(current_adventure),
 ):
     """Copies the scenario's current plot text, story cards, and stat schema over
     this adventure's copy.
@@ -150,7 +152,6 @@ def refresh_from_scenario(
     cards; and, through `worldstate.reconcile`, the live value of every stat the
     schema still defines.
     """
-    adventure = get_adventure_or_404(adventure_id, db, user)
     scenario = resolve_source_scenario(adventure, db, user)
     if scenario is None:
         raise HTTPException(404, "No scenario to update from")

backend/app/routers/adventures/scripts.py

@@ -11,7 +11,7 @@ from sqlalchemy.orm import Session
 from ... import models, schemas
 from ...database import get_db
 
-from .deps import CurrentUser, get_adventure_or_404, router
+from .deps import CurrentUser, current_adventure, router
 
 
 # Fields that are copied from a library Script into its adventure-script
@@ -59,9 +59,10 @@ def _mark_out_of_date(
 
 @router.get("/{adventure_id}/scripts", response_model=list[schemas.AdventureScriptOut])
 def list_adventure_scripts(
-    adventure_id: int, db: Session = Depends(get_db), user: models.User = CurrentUser
+    db: Session = Depends(get_db),
+    user: models.User = CurrentUser,
+    adventure: models.Adventure = Depends(current_adventure),
 ):
-    adventure = get_adventure_or_404(adventure_id, db, user)
     return [_mark_out_of_date(s, db, user) for s in adventure.scripts]
 
 
@@ -74,12 +75,12 @@ def sync_adventure_script(
     adv_script_id: int,
     db: Session = Depends(get_db),
     user: models.User = CurrentUser,
+    adventure: models.Adventure = Depends(current_adventure),
 ):
     """Overwrites this copy's code with the latest from its library script.
 
     `enabled`, `position`, and the adventure's shared `script_state` are kept.
     """
-    get_adventure_or_404(adventure_id, db, user)
     script = db.get(models.AdventureScript, adv_script_id)
     if script is None or script.adventure_id != adventure_id:
         raise HTTPException(404, "Script not found")
@@ -104,9 +105,8 @@ def update_adventure_script(
     adv_script_id: int,
     payload: schemas.AdventureScriptUpdate,
     db: Session = Depends(get_db),
-    user: models.User = CurrentUser,
+    adventure: models.Adventure = Depends(current_adventure),
 ):
-    get_adventure_or_404(adventure_id, db, user)
     script = db.get(models.AdventureScript, adv_script_id)
     if script is None or script.adventure_id != adventure_id:
         raise HTTPException(404, "Script not found")

backend/app/routers/adventures/takes.py

@@ -16,12 +16,12 @@ from ...context import cursors
 from ...context import lineage
 from ...database import get_db
 from ...scripting import ScriptPipeline
+from ...sse import SSE_HEADERS
 
 from . import turns
-from .deps import CurrentUser, get_adventure_or_404, router
+from .deps import CurrentUser, current_adventure, router
 from .nodes import delete_turn, last_action, stand_on
 from .paging import action_window, annotate_takes, current_window
-from .turns import SSE_HEADERS
 
 
 @router.post("/{adventure_id}/retry")
@@ -30,6 +30,7 @@ def retry_action(
     request: Request,
     db: Session = Depends(get_db),
     user: models.User = CurrentUser,
+    adventure: models.Adventure = Depends(current_adventure),
 ):
     """Regenerates the last AI action and keeps the discarded attempt.
 
@@ -38,7 +39,6 @@ def retry_action(
     attempt is stored as a sibling at the same coordinate. No text the AI wrote
     is rewritten or deleted.
     """
-    adventure = get_adventure_or_404(adventure_id, db, user)
     limits.rate_limit("turn", request, user)
     turns.check_demo_cap(db, user)
     turns.acquire_turn_lock(adventure_id)
@@ -79,7 +79,7 @@ def list_variants(
     adventure_id: int,
     action_id: int,
     db: Session = Depends(get_db),
-    user: models.User = CurrentUser,
+    adventure: models.Adventure = Depends(current_adventure),
 ):
     """Returns every attempt made for one AI turn.
 
@@ -90,7 +90,6 @@ def list_variants(
     Switching changes which row the story tells, and a client that holds an id it
     received a moment ago still has to be able to ask about the same turn.
     """
-    get_adventure_or_404(adventure_id, db, user)
     action = db.get(models.Action, action_id)
     if action is None or action.adventure_id != adventure_id:
         raise HTTPException(404, "Action not found")
@@ -119,7 +118,7 @@ def select_variant(
     action_id: int,
     payload: schemas.VariantSelect,
     db: Session = Depends(get_db),
-    user: models.User = CurrentUser,
+    adventure: models.Adventure = Depends(current_adventure),
 ):
     """Makes an earlier attempt live again and restores the state it produced.
 
@@ -130,7 +129,6 @@ def select_variant(
     would leave the story contradicting itself. The attempts of earlier turns
     stay readable through `list_variants`.
     """
-    adventure = get_adventure_or_404(adventure_id, db, user)
     action = db.get(models.Action, action_id)
     if action is None or action.adventure_id != adventure_id:
         raise HTTPException(404, "Action not found")
@@ -172,7 +170,7 @@ def fork_from_attempt(
     adventure_id: int,
     action_id: int,
     db: Session = Depends(get_db),
-    user: models.User = CurrentUser,
+    adventure: models.Adventure = Depends(current_adventure),
 ):
     """Continues the story from this attempt, forking a branch if one is needed.
 
@@ -184,7 +182,6 @@ def fork_from_attempt(
     * The story has moved past its turn, so the endpoint forks. The attempt gets
       a branch of its own, and the line it leaves keeps every turn it has.
     """
-    adventure = get_adventure_or_404(adventure_id, db, user)
     action = db.get(models.Action, action_id)
     if action is None or action.adventure_id != adventure_id:
         raise HTTPException(404, "Action not found")
@@ -236,6 +233,7 @@ def add_take(
     request: Request,
     db: Session = Depends(get_db),
     user: models.User = CurrentUser,
+    adventure: models.Adventure = Depends(current_adventure),
 ):
     """Plays a turn again, whoever wrote it.
 
@@ -256,7 +254,6 @@ def add_take(
     turn, so the new attempt is written at the same depth under the same parent,
     and the line it leaves is unchanged. No node below is copied.
     """
-    adventure = get_adventure_or_404(adventure_id, db, user)
     limits.rate_limit("turn", request, user)
     limits.check_row_cap("actions", db, user, adventure=adventure)
     turns.check_demo_cap(db, user)
@@ -317,7 +314,9 @@ def add_take(
 
 @router.post("/{adventure_id}/undo", response_model=schemas.ActionPage)
 def undo_turn(
-    adventure_id: int, db: Session = Depends(get_db), user: models.User = CurrentUser
+    adventure_id: int,
+    db: Session = Depends(get_db),
+    adventure: models.Adventure = Depends(current_adventure),
 ):
     """Deletes the last turn: the trailing AI action and its player action, if any.
 
@@ -325,7 +324,6 @@ def undo_turn(
     ran, and it prunes any memory that summarized the removed actions. The turn
     lock prevents an undo while a turn is still generating.
     """
-    adventure = get_adventure_or_404(adventure_id, db, user)
     turns.acquire_turn_lock(adventure_id)
     try:
         # Only the last turn is removed, so fetch the two actions it can

backend/app/routers/adventures/turns.py

@@ -6,7 +6,6 @@ lock guards one set only while one module owns it. And a test that replaces
 `OpenAICompatibleProvider`, `generate_turn`, or `check_demo_cap` patches this
 module, which every caller reads through.
 """
-import json
 import threading
 
 from fastapi import Depends, HTTPException, Request
@@ -20,9 +19,10 @@ from ...context import build_context, cursors
 from ...database import get_db
 from ...providers import OpenAICompatibleProvider, PromptParts, ProviderError
 from ...scripting import ScriptPipeline
+from ...sse import SSE_HEADERS, sse, turn_error
 from ..settings import get_settings
 
-from .deps import CurrentUser, get_adventure_or_404, router
+from .deps import CurrentUser, current_adventure, router
 from .nodes import _move_to_after, next_depth, next_index
 from .paging import annotate_takes
 
@@ -98,27 +98,6 @@ def format_player_input(action_type: str, text: str) -> str:
     return text  # The "story" type is appended as raw text.
 
 
-def sse(obj: dict) -> str:
-    return f"data: {json.dumps(obj)}\n\n"
-
-
-def turn_error(detail: str, **extra) -> str:
-    """Returns an SSE error for a turn that could not be produced, and counts it.
-
-    A failed turn is still an HTTP 200 response, so the middleware's status-code
-    tally cannot see it. This metric exists so that a demo whose model refuses
-    every request does not report as healthy.
-    """
-    analytics.record(analytics.M_EVENT, analytics.EV_TURN_ERROR)
-    return sse({"type": "error", "detail": detail, **extra})
-
-
-# `no-cache` stops an intermediary from caching the stream. `X-Accel-Buffering`
-# makes nginx-style reverse proxies, which hosted deploys use, flush each event
-# immediately rather than buffer it.
-SSE_HEADERS = {"Cache-Control": "no-cache", "X-Accel-Buffering": "no"}
-
-
 def action_json(action: models.Action, db: Session | None = None) -> dict:
     """Serializes one action for the wire.
 
@@ -425,8 +404,8 @@ def create_action(
     request: Request,
     db: Session = Depends(get_db),
     user: models.User = CurrentUser,
+    adventure: models.Adventure = Depends(current_adventure),
 ):
-    adventure = get_adventure_or_404(adventure_id, db, user)
     limits.rate_limit("turn", request, user)
     limits.check_row_cap("actions", db, user, adventure=adventure)
     check_demo_cap(db, user)

backend/app/routers/chat.py

@@ -19,7 +19,7 @@ from sqlalchemy.orm import Session
 from .. import auth, limits, models, schemas
 from ..database import get_db
 from ..providers import OpenAICompatibleProvider, ProviderError
-from .adventures import SSE_HEADERS, sse
+from ..sse import SSE_HEADERS, sse
 from .settings import get_settings, list_endpoint_models
 
 router = APIRouter(prefix="/api/chat", tags=["chat"])

backend/app/sse.py

@@ -0,0 +1,30 @@
+"""Server-sent events: the wire format, the headers, and the error frame.
+
+Two routers stream: the turn engine in `routers/adventures/turns.py` and the
+chat scratchpad in `routers/chat.py`. Both send JSON objects as SSE `data:`
+frames, so the format lives here rather than in either one.
+"""
+import json
+
+from . import analytics
+
+# `no-cache` stops an intermediary from caching the stream. `X-Accel-Buffering`
+# makes nginx-style reverse proxies, which hosted deploys use, flush each event
+# immediately rather than buffer it.
+SSE_HEADERS = {"Cache-Control": "no-cache", "X-Accel-Buffering": "no"}
+
+
+def sse(obj: dict) -> str:
+    """Returns one SSE frame carrying `obj` as JSON."""
+    return f"data: {json.dumps(obj)}\n\n"
+
+
+def turn_error(detail: str, **extra) -> str:
+    """Returns an SSE error for a turn that could not be produced, and counts it.
+
+    A failed turn is still an HTTP 200 response, so the middleware's status-code
+    tally cannot see it. This metric exists so that a demo whose model refuses
+    every request does not report as healthy.
+    """
+    analytics.record(analytics.M_EVENT, analytics.EV_TURN_ERROR)
+    return sse({"type": "error", "detail": detail, **extra})

backend/app/worldstate/apply.py

@@ -5,10 +5,105 @@ place. A proposed change that breaks a limit is recorded as refused rather than
 dropped, because the player and the model both need to see that it did not land.
 """
 import copy
+from typing import NamedTuple
 
 from .schema import STAT_SECTIONS, _initials, instantiate, npc_name
 
 
+class _Target(NamedTuple):
+    """Where one path writes, and what the schema says about the value there."""
+
+    kind: str                # "flag", "milestone", or "stat"
+    section: str             # "flags", "milestones", "player", "world", or "npc"
+    key: str                 # the key inside the container
+    stat_def: dict | None    # the stat's definition, or None for a flag or milestone
+    npc_id: str | None       # the character, for `npc.<id>.<stat>`, else None
+    npc_stats: dict | None   # that character's stat defs, used to build a new block
+
+    def container(self, ws: dict) -> dict:
+        """Returns the dict this path writes into, and creates it if it is missing.
+
+        Call this only when you are about to write. Creating the container is a
+        side effect, and a rejected path must not leave an empty section behind.
+        """
+        if self.npc_id is None:
+            return ws.setdefault(self.section, {})
+        return ws.setdefault("npc", {}).setdefault(self.npc_id, _initials(self.npc_stats))
+
+
+def _resolve(path: str, stat_schema: dict) -> tuple[_Target | None, dict | None]:
+    """Routes one path to the value it names, without deciding what happens to it.
+
+    Returns `(target, None)` when the schema defines the path, and
+    `(None, rejection)` when it does not. The rejection is the entry that goes
+    into a report's `rejected` list, with its reason and its fix already worded.
+
+    `apply_delta` and `apply_override` route paths identically and differ only in
+    what they write, so the routing lives here and each function keeps its own
+    write rule.
+    """
+    parts = path.split(".")
+
+    # `flags.<name>` is a boolean the scenario declares.
+    if parts[0] == "flags" and len(parts) == 2:
+        flag_defs = stat_schema.get("flags") or {}
+        if parts[1] not in flag_defs:
+            return None, {
+                "path": path, "reason": "unknown flag",
+                "fix": f"There is no flag `{parts[1]}`. {_names_phrase('flag', flag_defs)}",
+            }
+        return _Target("flag", "flags", parts[1], None, None, None), None
+
+    # `milestones.<id>` records that a milestone is reached.
+    if parts[0] == "milestones" and len(parts) == 2:
+        milestones = stat_schema.get("milestones") or {}
+        if parts[1] not in milestones:
+            return None, {
+                "path": path, "reason": "unknown milestone",
+                "fix": f"There is no milestone `{parts[1]}`. "
+                       f"{_names_phrase('milestone', milestones)}",
+            }
+        return _Target("milestone", "milestones", parts[1], None, None, None), None
+
+    # `world.<stat>` and `player.<stat>`.
+    if parts[0] in STAT_SECTIONS and len(parts) == 2:
+        stat_defs = stat_schema.get(parts[0]) or {}
+        stat_def = stat_defs.get(parts[1])
+        if not isinstance(stat_def, dict):
+            return None, {
+                "path": path, "reason": "unknown stat",
+                "fix": f"`{parts[0]}` has no stat `{parts[1]}`. "
+                       f"{_names_phrase('stat', stat_defs)}",
+            }
+        return _Target("stat", parts[0], parts[1], stat_def, None, None), None
+
+    # `npc.<id>.<stat>`. Each character carries its own stat definitions.
+    if parts[0] == "npc" and len(parts) == 3:
+        npcs = stat_schema.get("npcs") or {}
+        ndef = npcs.get(parts[1])
+        if not isinstance(ndef, dict):
+            return None, {
+                "path": path, "reason": "unknown npc",
+                "fix": f"There is no character `{parts[1]}`. "
+                       f"{_names_phrase('character', npcs)}",
+            }
+        stat_defs = ndef.get("stats") or {}
+        stat_def = stat_defs.get(parts[2])
+        if not isinstance(stat_def, dict):
+            return None, {
+                "path": path, "reason": "unknown npc stat",
+                "fix": f"`{npc_name(ndef, parts[1])}` has no stat `{parts[2]}`. "
+                       f"{_names_phrase('stat', stat_defs)}",
+            }
+        return _Target("stat", "npc", parts[2], stat_def, parts[1], stat_defs), None
+
+    return None, {
+        "path": path, "reason": "unknown path",
+        "fix": f"`{path}` is not a tracked value. Use player.<stat>, "
+               f"world.<stat>, npc.<id>.<stat>, flags.<name> or milestones.<id>.",
+    }
+
+
 def _coerce_number(value):
     if isinstance(value, bool):  # `bool` is an `int` subclass, so reject it here.
         return None
@@ -177,10 +272,6 @@ def apply_override(world_state: dict, stat_schema: dict, overrides: dict) -> tup
     if not isinstance(overrides, dict):
         return ws, report
 
-    milestones = stat_schema.get("milestones") or {}
-    flag_defs = stat_schema.get("flags") or {}
-    npcs = stat_schema.get("npcs") or {}
-
     def set_stat(container: dict, key: str, stat_def: dict, value, path: str) -> None:
         if stat_def.get("type") == "text":
             if not isinstance(value, str):
@@ -212,80 +303,42 @@ def apply_override(world_state: dict, stat_schema: dict, overrides: dict) -> tup
 
     for raw_path, value in overrides.items():
         path = str(raw_path)
-        parts = path.split(".")
+        target, rejection = _resolve(path, stat_schema)
+        if rejection is not None:
+            report["rejected"].append(rejection)
+            continue
 
-        if parts[0] == "flags" and len(parts) == 2:
-            fid = parts[1]
-            if fid not in flag_defs:
-                report["rejected"].append({"path": path, "reason": "unknown flag"})
-                continue
+        if target.kind == "flag":
             if not isinstance(value, bool):
-                report["rejected"].append({"path": path, "reason": "not a boolean"})
+                report["rejected"].append({
+                    "path": path, "reason": "not a boolean",
+                    "fix": f"`{path}` takes true or false.",
+                })
                 continue
-            flags = ws.setdefault("flags", {})
-            old = bool(flags.get(fid, False))
-            flags[fid] = value
+            flags = target.container(ws)
+            old = bool(flags.get(target.key, False))
+            flags[target.key] = value
             report["applied"].append({"path": path, "old": old, "new": value})
-            continue
 
-        if parts[0] == "milestones" and len(parts) == 2:
-            mid = parts[1]
-            if mid not in milestones:
-                report["rejected"].append({"path": path, "reason": "unknown milestone"})
-                continue
+        elif target.kind == "milestone":
+            # An override toggles a milestone in both directions, so unlike a
+            # delta it takes false as well as true.
             if not isinstance(value, bool):
-                report["rejected"].append({"path": path, "reason": "not a boolean"})
+                report["rejected"].append({
+                    "path": path, "reason": "not a boolean",
+                    "fix": f"`{path}` takes true or false.",
+                })
                 continue
-            reached = ws.setdefault("milestones", {})
-            old = bool(reached.get(mid, {}).get("reached"))
+            reached = target.container(ws)
+            old = bool(reached.get(target.key, {}).get("reached"))
             if value:
-                reached[mid] = {"reached": True}
+                reached[target.key] = {"reached": True}
             else:
-                reached.pop(mid, None)
+                reached.pop(target.key, None)
             report["applied"].append({"path": path, "old": old, "new": value})
-            continue
 
-        if parts[0] in STAT_SECTIONS and len(parts) == 2:
-            stat_def = (stat_schema.get(parts[0]) or {}).get(parts[1])
-            if not isinstance(stat_def, dict):
-                report["rejected"].append({
-                    "path": path, "reason": "unknown stat",
-                    "fix": f"`{parts[0]}` has no stat `{parts[1]}`. "
-                           f"{_names_phrase('stat', stat_schema.get(parts[0]) or {})}",
-                })
-                continue
-            container = ws.setdefault(parts[0], {})
-            set_stat(container, parts[1], stat_def, value, path)
-            continue
-
-        if parts[0] == "npc" and len(parts) == 3:
-            ndef = npcs.get(parts[1])
-            if not isinstance(ndef, dict):
-                report["rejected"].append({
-                    "path": path, "reason": "unknown npc",
-                    "fix": f"There is no character `{parts[1]}`. "
-                           f"{_names_phrase('character', npcs)}",
-                })
-                continue
-            stat_defs = ndef.get("stats") or {}
-            stat_def = stat_defs.get(parts[2])
-            if not isinstance(stat_def, dict):
-                report["rejected"].append({
-                    "path": path, "reason": "unknown npc stat",
-                    "fix": f"`{npc_name(ndef, parts[1])}` has no stat `{parts[2]}`. "
-                           f"{_names_phrase('stat', stat_defs)}",
-                })
-                continue
-            npc_state = ws.setdefault("npc", {})
-            container = npc_state.setdefault(parts[1], _initials(stat_defs))
-            set_stat(container, parts[2], stat_def, value, path)
-            continue
-
-        report["rejected"].append({
-            "path": path, "reason": "unknown path",
-            "fix": f"`{path}` is not a tracked value. Use player.<stat>, "
-                   f"world.<stat>, npc.<id>.<stat>, flags.<name> or milestones.<id>.",
-        })
+        else:
+            set_stat(target.container(ws), target.key, target.stat_def, value, path)
 
     return ws, report
 
@@ -307,46 +360,29 @@ def apply_delta(world_state: dict, stat_schema: dict, delta: dict,
     if not isinstance(delta, dict):
         return ws, report
 
-    milestones = stat_schema.get("milestones") or {}
-    flag_defs = stat_schema.get("flags") or {}
-    npcs = stat_schema.get("npcs") or {}
-
     for raw_path, change in delta.items():
         path = str(raw_path)
-        parts = path.split(".")
-
-        # `flags.<name>` is a two-way boolean, and either value is accepted.
-        if parts[0] == "flags" and len(parts) == 2:
-            fid = parts[1]
-            if fid not in flag_defs:
-                report["rejected"].append({
-                    "path": path, "reason": "unknown flag",
-                    "fix": f"There is no flag `{fid}`. {_names_phrase('flag', flag_defs)}",
-                })
+        target, rejection = _resolve(path, stat_schema)
+        if rejection is not None:
+            report["rejected"].append(rejection)
             continue
+
+        if target.kind == "flag":
+            # A flag goes both ways, so either value is accepted.
             if not isinstance(change, bool):
                 report["rejected"].append({
                     "path": path, "reason": "not a boolean",
                     "fix": f"`{path}` takes true or false.",
                 })
                 continue
-            flags = ws.setdefault("flags", {})
-            old = bool(flags.get(fid, False))
+            flags = target.container(ws)
+            old = bool(flags.get(target.key, False))
             if change != old:
-                flags[fid] = change
+                flags[target.key] = change
                 report["applied"].append({"path": path, "old": old, "new": change})
-            continue
 
-        # `milestones.<id>` is a sticky boolean, and only `true` is accepted.
-        if parts[0] == "milestones" and len(parts) == 2:
-            mid = parts[1]
-            if mid not in milestones:
-                report["rejected"].append({
-                    "path": path, "reason": "unknown milestone",
-                    "fix": f"There is no milestone `{mid}`. "
-                           f"{_names_phrase('milestone', milestones)}",
-                })
-                continue
+        elif target.kind == "milestone":
+            # A milestone is sticky, so only true is accepted.
             if change is not True:
                 report["rejected"].append({
                     "path": path, "reason": "not true",
@@ -354,65 +390,18 @@ def apply_delta(world_state: dict, stat_schema: dict, delta: dict,
                            f"reached once and never taken back.",
                 })
                 continue
-            reached = ws.setdefault("milestones", {})
-            if reached.get(mid, {}).get("reached"):
+            reached = target.container(ws)
+            if reached.get(target.key, {}).get("reached"):
                 continue  # Already reached, so do nothing.
-            reached[mid] = {"reached": True, "at": action_index}
+            reached[target.key] = {"reached": True, "at": action_index}
             report["applied"].append({"path": path, "old": False, "new": True})
-            continue
 
-        # world.<stat> / player.<stat>
-        if parts[0] in STAT_SECTIONS and len(parts) == 2:
-            stat_def = (stat_schema.get(parts[0]) or {}).get(parts[1])
-            if not isinstance(stat_def, dict):
-                report["rejected"].append({
-                    "path": path, "reason": "unknown stat",
-                    "fix": f"`{parts[0]}` has no stat `{parts[1]}`. "
-                           f"{_names_phrase('stat', stat_schema.get(parts[0]) or {})}",
-                })
-                continue
-            container = ws.setdefault(parts[0], {})
-            if stat_def.get("type") == "text":
-                _apply_text_stat(container, parts[1], stat_def, change, path,
-                                 action_index, meta, report)
+        elif target.stat_def.get("type") == "text":
+            _apply_text_stat(target.container(ws), target.key, target.stat_def, change,
+                             path, action_index, meta, report)
+
         else:
-                _apply_stat(container, parts[1], stat_def, change, path,
-                            action_index, meta, report)
-            continue
-
-        # `npc.<npcId>.<stat>`. Each NPC has its own stat definitions.
-        if parts[0] == "npc" and len(parts) == 3:
-            ndef = npcs.get(parts[1])
-            if not isinstance(ndef, dict):
-                report["rejected"].append({
-                    "path": path, "reason": "unknown npc",
-                    "fix": f"There is no character `{parts[1]}`. "
-                           f"{_names_phrase('character', npcs)}",
-                })
-                continue
-            stat_defs = ndef.get("stats") or {}
-            stat_def = stat_defs.get(parts[2])
-            if not isinstance(stat_def, dict):
-                report["rejected"].append({
-                    "path": path, "reason": "unknown npc stat",
-                    "fix": f"`{npc_name(ndef, parts[1])}` has no stat `{parts[2]}`. "
-                           f"{_names_phrase('stat', stat_defs)}",
-                })
-                continue
-            npc_state = ws.setdefault("npc", {})
-            container = npc_state.setdefault(parts[1], _initials(stat_defs))
-            if stat_def.get("type") == "text":
-                _apply_text_stat(container, parts[2], stat_def, change, path,
-                                 action_index, meta, report)
-            else:
-                _apply_stat(container, parts[2], stat_def, change, path,
-                            action_index, meta, report)
-            continue
-
-        report["rejected"].append({
-            "path": path, "reason": "unknown path",
-            "fix": f"`{path}` is not a tracked value. Use player.<stat>, "
-                   f"world.<stat>, npc.<id>.<stat>, flags.<name> or milestones.<id>.",
-        })
+            _apply_stat(target.container(ws), target.key, target.stat_def, change,
+                        path, action_index, meta, report)
 
     return ws, report

backend/tests/test_state_revert.py

@@ -81,7 +81,7 @@ def test_undo_reverts_state_to_before_the_turn(db):
     _add(db, adv, 2, "ai", state_after={"gold": 10})
     db.commit()
 
-    adventures.undo_turn(adv.id, db=db, user=user)
+    adventures.undo_turn(adv.id, db=db, adventure=adv)
 
     assert adv.script_state == {"gold": 0}
     assert [a.type for a in adv.actions] == ["start"]
@@ -95,7 +95,7 @@ def test_undo_of_bare_continue_uses_the_node_in_front(db):
     _add(db, adv, 1, "ai", state_after={"gold": 5})
     db.commit()
 
-    adventures.undo_turn(adv.id, db=db, user=user)
+    adventures.undo_turn(adv.id, db=db, adventure=adv)
 
     assert adv.script_state == {"gold": 0}
     assert [a.type for a in adv.actions] == ["start"]
@@ -110,7 +110,7 @@ def test_undo_leaves_state_untouched_when_snapshot_missing(db):
     _add(db, adv, 2, "ai")
     _forget_snapshots(db, adv)
 
-    adventures.undo_turn(adv.id, db=db, user=user)
+    adventures.undo_turn(adv.id, db=db, adventure=adv)
 
     assert adv.script_state == {"gold": 10}
 
@@ -120,7 +120,7 @@ def test_undo_raises_when_nothing_to_undo(db):
     _add(db, adv, 0, "start")
     db.commit()
     with pytest.raises(HTTPException) as exc:
-        adventures.undo_turn(adv.id, db=db, user=user)
+        adventures.undo_turn(adv.id, db=db, adventure=adv)
     assert exc.value.status_code == 400
 
 
@@ -133,7 +133,7 @@ def test_undo_blocked_by_active_turn_lock(db):
     adventures.turns.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)
+            adventures.undo_turn(adv.id, db=db, adventure=adv)
         assert exc.value.status_code == 409
         # The failed undo must not have released someone else's lock.
         assert adv.id in adventures.turns._active_turns
@@ -151,7 +151,7 @@ def test_undo_prunes_memory_covering_removed_actions(db):
     db.add_all([covering, keep])
     db.commit()
 
-    adventures.undo_turn(adv.id, db=db, user=user)  # removes indexes 2 & 3
+    adventures.undo_turn(adv.id, db=db, adventure=adv)  # removes indexes 2 & 3
 
     texts = {m.text for m in adv.memories}
     assert texts == {"k"}
@@ -228,7 +228,7 @@ def test_retry_restores_the_state_the_turn_started_from(db, monkeypatch):
             yield  # make it an async generator
     monkeypatch.setattr(adventures.turns, "generate_turn", _noop)
 
-    adventures.retry_action(adv.id, request=None, db=db, user=user)
+    adventures.retry_action(adv.id, request=None, db=db, user=user, adventure=adv)
 
     assert adv.script_state == {"gold": 10}
     # Nothing is written until a replacement actually arrives: the attempt on