diff --git a/backend/app/downloads/clip.py b/backend/app/downloads/clip.py
index af2cb46b..4b4b1021 100644
--- a/backend/app/downloads/clip.py
+++ b/backend/app/downloads/clip.py
@@ -78,7 +78,7 @@ class ClipError(Exception):
     message names (the cap itself, so the client never says a stale one)."""
 
     def __init__(self, reason: ClipReason, **params: float):
-        self.reason = reason
+        self.reason: ClipReason = reason
         self.params = params
         super().__init__(reason)
 
diff --git a/backend/app/downloads/credentials.py b/backend/app/downloads/credentials.py
index 39e1cd93..c80a892d 100644
--- a/backend/app/downloads/credentials.py
+++ b/backend/app/downloads/credentials.py
@@ -174,7 +174,7 @@ class InvalidCookies(ValueError):
 
     def __init__(self, reason: CookieReason):
         super().__init__(reason)
-        self.reason = reason
+        self.reason: CookieReason = reason
 
 
 def validate_cookies(raw: str) -> str:
diff --git a/backend/app/downloads/folders.py b/backend/app/downloads/folders.py
index fffb75c0..c440b9de 100644
--- a/backend/app/downloads/folders.py
+++ b/backend/app/downloads/folders.py
@@ -197,7 +197,7 @@ class FolderError(Exception):
 
     def __init__(self, reason: FolderReason) -> None:
         super().__init__(reason)
-        self.reason = reason
+        self.reason: FolderReason = reason
 
 
 def own_folder(db: Session, user_id: int, folder_id: int, *, hold: bool = True) -> FileFolder:
diff --git a/backend/app/downloads/quota.py b/backend/app/downloads/quota.py
index e8d56f85..179b3b92 100644
--- a/backend/app/downloads/quota.py
+++ b/backend/app/downloads/quota.py
@@ -60,7 +60,7 @@ class QuotaExceeded(Exception):
     """Raised by check_enqueue; `reason` names the cap it hit (an i18n key on the client)."""
 
     def __init__(self, reason: QuotaReason, limit: int, current: int):
-        self.reason = reason
+        self.reason: QuotaReason = reason
         self.limit = limit
         self.current = current
         super().__init__(f"download quota exceeded: {reason} ({current}/{limit})")
diff --git a/backend/app/downloads/service.py b/backend/app/downloads/service.py
index ffc40cbf..e2e80910 100644
--- a/backend/app/downloads/service.py
+++ b/backend/app/downloads/service.py
@@ -49,7 +49,7 @@ class EditError(Exception):
     """Bad edit request (empty spec / source not ready / not media); `reason` names the sentence."""
 
     def __init__(self, reason: EditReason):
-        self.reason = reason
+        self.reason: EditReason = reason
         super().__init__(reason)
 
 
diff --git a/backend/app/downloads/urlguard.py b/backend/app/downloads/urlguard.py
index 575c7836..854aa732 100644
--- a/backend/app/downloads/urlguard.py
+++ b/backend/app/downloads/urlguard.py
@@ -59,7 +59,7 @@ class BlockedUrl(Exception):
 
     def __init__(self, reason: UrlBlockedReason):
         super().__init__(reason)
-        self.reason = reason
+        self.reason: UrlBlockedReason = reason
 
 
 def _is_forbidden(ip: ipaddress.IPv4Address | ipaddress.IPv6Address) -> bool:
diff --git a/backend/app/reader/service.py b/backend/app/reader/service.py
index 5eb779c0..fa651005 100644
--- a/backend/app/reader/service.py
+++ b/backend/app/reader/service.py
@@ -311,7 +311,7 @@ class MoveRefused(Exception):
 
     def __init__(self, reason: MoveReason):
         super().__init__(reason)
-        self.reason = reason
+        self.reason: MoveReason = reason
 
 
 def touch(pl: Playlist) -> None:
diff --git a/backend/app/refusal.py b/backend/app/refusal.py
index c092bb55..337a1c8a 100644
--- a/backend/app/refusal.py
+++ b/backend/app/refusal.py
@@ -7,16 +7,19 @@ whatever sentence the route happened to hand-write. `refused` is the generic fam
 (`errors.refused.<reason>`, the params interpolated into the sentence); the older specific codes
 (`not_found`, `youtube`, `playlist`, the admin refusals, …) stay, since each is already translated.
 
-A test (`test_http_details.py`) reads every `HTTPException` in the app and fails on a string detail,
-and the frontend's contract test (`i18n/backendIds.contract.test.ts`) requires every `code` a detail
-carries to be in one record of `lib/api/refusalCodes.ts`, and every reason it can be sent with — a
-literal, or a `Literal` type when it travels in a variable — to have its sentence in both languages.
-A code or reason that scan cannot read fails it until it is listed with what holds it. So the rule is
-enforced, not described.
+A test (`test_http_details.py`) reads every `HTTPException` in the app and fails on a string detail.
+`tests/refusal_sites.py` reads every `{"code": …}` detail from the syntax tree and follows each reason
+to its values: a literal, a `Literal` type on the parameter or attribute it travels in, every call of
+a helper like `refused` below, a map's values. The frontend's contract test
+(`i18n/backendIds.contract.test.ts`) runs it and requires each code in one record of
+`lib/api/refusalCodes.ts`, and each reason's sentence in both languages; a code or reason it cannot
+follow fails it, unless no screen shows that code. So the rule is enforced, not described.
 """
 
 from fastapi import HTTPException
 
+from app.sysconfig import InvalidSetting, SettingReason
+
 
 def refused(status: int, reason: str, **params: object) -> HTTPException:
     """The refusal to raise: `raise refused(409, "view_name_taken", name=name)`. `params` travel in
@@ -24,9 +27,9 @@ def refused(status: int, reason: str, **params: object) -> HTTPException:
     return HTTPException(status_code=status, detail={"code": "refused", "reason": reason, **params})
 
 
-# `sysconfig.InvalidSetting.reason` → the refusal it answers as. A dict of literals, so the frontend's
-# contract test can read every reason this can produce.
-_SETTING_REASONS = {
+# `sysconfig.InvalidSetting.reason` → the refusal it answers as. A dict of literals, so the refusal
+# scan reads every reason this can produce, and holds its keys to `SettingReason`.
+_SETTING_REASONS: dict[SettingReason, str] = {
     "not_a_number": "setting_not_a_number",
     "too_low": "setting_too_low",
     "too_high": "setting_too_high",
@@ -35,6 +38,6 @@ _SETTING_REASONS = {
 }
 
 
-def setting_refused(exc) -> HTTPException:
+def setting_refused(exc: InvalidSetting) -> HTTPException:
     """A value the admin entered that `sysconfig.set_value` refused (`exc`: its `InvalidSetting`)."""
     return refused(400, _SETTING_REASONS[exc.reason], key=exc.key, **exc.params)
diff --git a/backend/app/sysconfig.py b/backend/app/sysconfig.py
index d5c1adb5..74607b6b 100644
--- a/backend/app/sysconfig.py
+++ b/backend/app/sysconfig.py
@@ -443,15 +443,20 @@ def _default(s: ConfigSpec):
     return getattr(settings, s.default_attr)
 
 
+# Every reason `set_value` refuses with. `refusal.setting_refused` looks each one up in its map, which
+# the refusal scan holds to exactly these keys: a reason the map lacks would answer a 500.
+SettingReason = Literal["not_a_number", "too_low", "too_high", "not_a_choice", "no_encryption_key"]
+
+
 class InvalidSetting(ValueError):
     """A value `set_value` refuses, as a reason the UI translates (`errors.refused.setting_<reason>`)
     plus what the sentence needs — the English message is for logs only. A ValueError still, so a
     caller that only needs "refused or not" keeps working."""
 
-    def __init__(self, key: str, reason: str, **params: object):
+    def __init__(self, key: str, reason: SettingReason, **params: object):
         super().__init__(f"{key}: {reason} {params}")
         self.key = key
-        self.reason = reason
+        self.reason: SettingReason = reason
         self.params = params
 
 
diff --git a/backend/tests/refusal_sites.py b/backend/tests/refusal_sites.py
new file mode 100644
index 00000000..edf79344
--- /dev/null
+++ b/backend/tests/refusal_sites.py
@@ -0,0 +1,625 @@
+"""Every refusal the backend can answer, read from its syntax tree: each `{"code": …, "reason": …}` detail,
+with every reason its code can be sent with.
+
+The frontend's contract (`i18n/backendIds.contract.test.ts`) runs this and requires a sentence for each
+code and reason in both languages. A regex over the source read only the shapes it was written for: a
+reason after another key, a status passed by name, a second handler with the same text all went past it.
+Here a reason is followed to where it comes from, and what cannot be followed is reported, never guessed:
+
+- a literal; both arms of `a if c else b`, every operand of `a or b`;
+- a module constant (`urlguard.REASON_MALFORMED`), a local variable's assignments;
+- a parameter or attribute typed with a `Literal` (basedpyright holds every value passed in), the
+  attribute found on the class an `except … as e` catches, or the class a parameter is typed with;
+- a `str` parameter of a helper (`refused`, `not_found`): followed to every call of it, its default
+  included, and a helper passed around as a value is unread;
+- a call: its return annotation; a map lookup (`_SETTING_REASONS[exc.reason]`): the map's values, and
+  its keys must be exactly what the index can be.
+
+Anything else is UNREAD, and the contract fails on it unless no screen shows that code. Stdlib only: it
+runs on the host, beside the frontend's tests (`python -I backend/tests/refusal_sites.py` → JSON).
+"""
+
+import ast
+import json
+import pathlib
+import sys
+from dataclasses import dataclass, field
+
+APP = pathlib.Path(__file__).resolve().parents[1] / "app"
+
+# Files whose `"code"` dicts are data, not refusals: a subtitle track's status payload, TMDB country codes.
+NOT_REFUSALS = {
+    "plex/subtitles.py": "a subtitle track's status payload (its own `code`)",
+    "plex/tmdb.py": "a production country's ISO code",
+}
+
+Node = ast.AST
+
+
+@dataclass
+class Res:
+    values: set[str] = field(default_factory=set)
+    unread: list[str] = field(default_factory=list)
+
+    def __or__(self, other: "Res") -> "Res":
+        return Res(self.values | other.values, self.unread + other.unread)
+
+
+@dataclass
+class Module:
+    path: str  # relative to app/, posix
+    name: str  # dotted, `app.…`
+    tree: ast.Module
+    parents: dict[Node, Node]
+    imports: dict[
+        str, tuple[str, str | None]
+    ]  # local name → (module, symbol or None for the module)
+    top: dict[str, Node]  # module-level bindings: def, class, or the assigned value
+
+
+def _module_name(path: str) -> str:
+    parts = path.removesuffix(".py").split("/")
+    if parts[-1] == "__init__":
+        parts = parts[:-1]
+    return ".".join(["app", *parts])
+
+
+def _load(sources: dict[str, str]) -> dict[str, Module]:
+    mods: dict[str, Module] = {}
+    for path, src in sources.items():
+        tree = ast.parse(src)
+        parents = {child: node for node in ast.walk(tree) for child in ast.iter_child_nodes(node)}
+        imports: dict[str, tuple[str, str | None]] = {}
+        for node in ast.walk(tree):
+            if isinstance(node, ast.ImportFrom) and node.module and node.level == 0:
+                for a in node.names:
+                    imports[a.asname or a.name] = (node.module, a.name)
+            elif isinstance(node, ast.Import):
+                for a in node.names:
+                    if a.asname:
+                        imports[a.asname] = (a.name, None)
+        top: dict[str, Node] = {}
+        for stmt in tree.body:
+            if isinstance(stmt, (ast.FunctionDef, ast.AsyncFunctionDef, ast.ClassDef)):
+                top[stmt.name] = stmt
+            elif isinstance(stmt, ast.Assign):
+                for t in stmt.targets:
+                    if isinstance(t, ast.Name):
+                        top[t.id] = stmt
+            elif isinstance(stmt, ast.AnnAssign) and isinstance(stmt.target, ast.Name):
+                top[stmt.target.id] = stmt
+        name = _module_name(path)
+        mods[name] = Module(path, name, tree, parents, imports, top)
+    return mods
+
+
+class Scan:
+    def __init__(self, sources: dict[str, str]):
+        self.mods = _load(sources)
+        self.errors: list[str] = []
+        self._following: set[tuple[str, str, str]] = set()
+
+    # --- names ---------------------------------------------------------------------------------------
+
+    def lookup(self, m: Module, name: str) -> tuple[Module, Node] | tuple[str, None] | None:
+        """What a module-level name is: (its module, its binding), or (a module's name, None)."""
+        if name in m.top:
+            return m, m.top[name]
+        if name in m.imports:
+            mod, sym = m.imports[name]
+            if sym is None:
+                return (mod, None) if mod in self.mods else None
+            if f"{mod}.{sym}" in self.mods:
+                return f"{mod}.{sym}", None
+            if mod in self.mods:
+                return self.lookup(self.mods[mod], sym)
+        return None
+
+    def lookup_expr(self, m: Module, e: Node) -> tuple[Module, Node] | tuple[str, None] | None:
+        if isinstance(e, ast.Name):
+            return self.lookup(m, e.id)
+        if isinstance(e, ast.Attribute):
+            base = self.lookup_expr(m, e.value)
+            if base and base[1] is None and isinstance(base[0], str):
+                return self.lookup(self.mods[base[0]], e.attr)
+        return None
+
+    def at(self, m: Module, e: Node) -> str:
+        return f"{m.path}:{getattr(e, 'lineno', '?')}: {ast.unparse(e)}"
+
+    def enclosing(self, m: Module, e: Node, kind: type | tuple[type, ...]) -> Node | None:
+        node = m.parents.get(e)
+        while node is not None and not isinstance(node, kind):
+            node = m.parents.get(node)
+        return node
+
+    # --- types ---------------------------------------------------------------------------------------
+
+    def ann_values(self, m: Module, ann: Node | None) -> set[str] | None:
+        """The values a `Literal` annotation allows, or None when it is not one (`str`, a class, …)."""
+        if ann is None:
+            return None
+        if isinstance(ann, ast.Constant):
+            if ann.value is None:
+                return set()
+            if isinstance(ann.value, str):
+                return self.ann_values(m, ast.parse(ann.value, mode="eval").body)
+            return None
+        if isinstance(ann, ast.Subscript) and getattr(ann.value, "id", None) == "Literal":
+            items = ann.slice.elts if isinstance(ann.slice, ast.Tuple) else [ann.slice]
+            if all(isinstance(i, ast.Constant) and isinstance(i.value, str) for i in items):
+                return {i.value for i in items}
+            return None
+        if isinstance(ann, ast.BinOp) and isinstance(ann.op, ast.BitOr):
+            left, right = self.ann_values(m, ann.left), self.ann_values(m, ann.right)
+            return None if left is None or right is None else left | right
+        found = self.lookup_expr(m, ann)
+        if found and isinstance(found[1], ast.Assign) and isinstance(found[0], Module):
+            if isinstance(found[1].value, (ast.Subscript, ast.BinOp, ast.Name, ast.Attribute)):
+                return self.ann_values(found[0], found[1].value)
+        return None
+
+    def classes(self, m: Module, e: Node | None) -> list[tuple[Module, ast.ClassDef]] | None:
+        if e is None:
+            return None
+        if isinstance(e, ast.Tuple):
+            out = [self.classes(m, x) for x in e.elts]
+            return None if any(o is None for o in out) else [c for o in out for c in o]
+        found = self.lookup_expr(m, e)
+        if found and isinstance(found[0], Module) and isinstance(found[1], ast.ClassDef):
+            return [(found[0], found[1])]
+        return None
+
+    def attr_values(self, cm: Module, cls: ast.ClassDef, attr: str) -> set[str] | None:
+        """An attribute's `Literal` values, from its annotation on the class or `self.attr: T` in a method,
+        else from a base class."""
+        for node in ast.walk(cls):
+            if isinstance(node, ast.AnnAssign):
+                t = node.target
+                if (isinstance(t, ast.Name) and t.id == attr and node in cls.body) or (
+                    isinstance(t, ast.Attribute)
+                    and t.attr == attr
+                    and getattr(t.value, "id", None) == "self"
+                ):
+                    return self.ann_values(cm, node.annotation)
+        for base in cls.bases:
+            for bm, bc in self.classes(cm, base) or []:
+                v = self.attr_values(bm, bc, attr)
+                if v is not None:
+                    return v
+        return None
+
+    # --- reasons -------------------------------------------------------------------------------------
+
+    def resolve(self, m: Module, e: Node) -> Res:
+        if isinstance(e, ast.Constant):
+            if e.value is None:
+                return Res()
+            if isinstance(e.value, str):
+                return Res({e.value})
+        if isinstance(e, ast.IfExp):
+            return self.resolve(m, e.body) | self.resolve(m, e.orelse)
+        if isinstance(e, ast.BoolOp) and isinstance(e.op, ast.Or):
+            out = Res()
+            for v in e.values:
+                out = out | self.resolve(m, v)
+            return out
+        if isinstance(e, ast.Name):
+            return self.resolve_name(m, e)
+        if isinstance(e, ast.Attribute):
+            return self.resolve_attr(m, e)
+        if isinstance(e, ast.Call):
+            found = self.lookup_expr(m, e.func)
+            if (
+                found
+                and isinstance(found[0], Module)
+                and isinstance(found[1], (ast.FunctionDef, ast.AsyncFunctionDef))
+            ):
+                v = self.ann_values(found[0], found[1].returns)
+                if v is not None:
+                    return Res(v)
+        if isinstance(e, ast.Subscript):
+            return self.resolve_lookup(m, e)
+        return Res(unread=[self.at(m, e)])
+
+    def resolve_name(self, m: Module, e: ast.Name) -> Res:
+        fn = self.enclosing(m, e, (ast.FunctionDef, ast.AsyncFunctionDef))
+        while fn is not None:
+            assert isinstance(fn, (ast.FunctionDef, ast.AsyncFunctionDef))
+            if self.caught_as(m, e, e.id) is not None:
+                return Res(unread=[self.at(m, e)])  # the exception itself, not a reason
+            params = {a.arg: a for a in [*fn.args.posonlyargs, *fn.args.args, *fn.args.kwonlyargs]}
+            if e.id in params:
+                return self.resolve_param(m, fn, params[e.id], e)
+            if (
+                fn.args.vararg
+                and fn.args.vararg.arg == e.id
+                or fn.args.kwarg
+                and fn.args.kwarg.arg == e.id
+            ):
+                return Res(unread=[self.at(m, e)])
+            assigned = self.assignments(fn, e.id)
+            if assigned:
+                out = Res()
+                for a in assigned:
+                    if (
+                        isinstance(a, ast.AnnAssign)
+                        and (v := self.ann_values(m, a.annotation)) is not None
+                    ):
+                        out = out | Res(v)
+                    elif (
+                        isinstance(a, (ast.Assign, ast.AnnAssign))
+                        and a.value is not None
+                        and not any(
+                            isinstance(t, (ast.Tuple, ast.List)) for t in getattr(a, "targets", [])
+                        )
+                    ):
+                        out = out | self.resolve(m, a.value)
+                    else:
+                        out = out | Res(unread=[self.at(m, e)])
+                return out
+            fn = self.enclosing(m, fn, (ast.FunctionDef, ast.AsyncFunctionDef))
+        found = self.lookup(m, e.id)
+        if (
+            found
+            and isinstance(found[0], Module)
+            and isinstance(found[1], (ast.Assign, ast.AnnAssign))
+        ):
+            stmt = found[1]
+            if stmt.value is not None and isinstance(stmt.value, ast.Constant):
+                return self.resolve(found[0], stmt.value)
+        return Res(unread=[self.at(m, e)])
+
+    def assignments(self, fn: Node, name: str) -> list[Node]:
+        """The statements that bind `name` in this function's own body (not in a nested one)."""
+        out: list[Node] = []
+        stack = list(ast.iter_child_nodes(fn))
+        while stack:
+            node = stack.pop()
+            if isinstance(node, (ast.FunctionDef, ast.AsyncFunctionDef, ast.Lambda, ast.ClassDef)):
+                continue
+            if isinstance(node, ast.Assign):
+                if any(
+                    name in {n.id for n in ast.walk(t) if isinstance(n, ast.Name)}
+                    for t in node.targets
+                ):
+                    out.append(node)
+            elif isinstance(node, ast.AnnAssign) and getattr(node.target, "id", None) == name:
+                out.append(node)
+            elif isinstance(
+                node, (ast.For, ast.With, ast.AugAssign, ast.NamedExpr, ast.comprehension)
+            ):
+                targets = [getattr(node, "target", None)] + [
+                    i.optional_vars
+                    for i in getattr(node, "items", [])
+                    if i.optional_vars is not None
+                ]
+                if any(
+                    t is not None and name in {n.id for n in ast.walk(t) if isinstance(n, ast.Name)}
+                    for t in targets
+                ):
+                    out.append(
+                        node
+                    )  # bound by a loop, a `with`, an augmented or a walrus: not followed
+            stack.extend(ast.iter_child_nodes(node))
+        return out
+
+    def caught_as(self, m: Module, e: Node, name: str) -> ast.ExceptHandler | None:
+        node = m.parents.get(e)
+        while node is not None and not isinstance(node, (ast.FunctionDef, ast.AsyncFunctionDef)):
+            if isinstance(node, ast.ExceptHandler) and node.name == name:
+                return node
+            node = m.parents.get(node)
+        return None
+
+    def resolve_param(self, m: Module, fn: Node, param: ast.arg, e: Node) -> Res:
+        v = self.ann_values(m, param.annotation)
+        if v is not None:
+            return Res(v)
+        if param.annotation is not None and getattr(param.annotation, "id", None) != "str":
+            return Res(unread=[self.at(m, e)])
+        return self.follow(m, fn, param)
+
+    def follow(self, m: Module, fn: Node, param: ast.arg) -> Res:
+        """A `str` parameter of a helper: what every call of the helper passes it."""
+        assert isinstance(fn, (ast.FunctionDef, ast.AsyncFunctionDef))
+        key = (m.name, fn.name, param.arg)
+        if key in self._following:
+            return Res()
+        if m.top.get(fn.name) is not fn:
+            return Res(
+                unread=[
+                    f"{m.path}:{fn.lineno}: {fn.name}({param.arg}) is not a module-level helper"
+                ]
+            )
+        self._following.add(key)
+        positional = [a.arg for a in [*fn.args.posonlyargs, *fn.args.args]]
+        defaults = dict(
+            zip(
+                positional[len(positional) - len(fn.args.defaults) :], fn.args.defaults, strict=True
+            )
+        )
+        defaults |= {
+            a.arg: d
+            for a, d in zip(fn.args.kwonlyargs, fn.args.kw_defaults, strict=True)
+            if d is not None
+        }
+        out = Res()
+        for cm in self.mods.values():
+            for node in ast.walk(cm.tree):
+                if not isinstance(node, (ast.Name, ast.Attribute)) or isinstance(
+                    cm.parents.get(node), ast.Attribute
+                ):
+                    continue
+                found = self.lookup_expr(cm, node)
+                if not (found and found[0] is m and found[1] is fn):
+                    continue
+                call = cm.parents.get(node)
+                if not (isinstance(call, ast.Call) and call.func is node):
+                    out = out | Res(unread=[f"{self.at(cm, node)} (the helper used as a value)"])
+                    continue
+                kw = {k.arg: k.value for k in call.keywords}
+                given = param.arg in positional and positional.index(param.arg) < len(call.args)
+                if param.arg in kw:
+                    out = out | self.resolve(cm, kw[param.arg])
+                elif any(isinstance(a, ast.Starred) for a in call.args):
+                    out = out | Res(unread=[self.at(cm, call)])
+                elif given:  # given by position, a `**` spread cannot pass it again (a TypeError)
+                    out = out | self.resolve(cm, call.args[positional.index(param.arg)])
+                elif None in kw:
+                    out = out | Res(unread=[self.at(cm, call)])
+                elif param.arg in defaults:
+                    out = out | self.resolve(m, defaults[param.arg])
+        return out
+
+    def resolve_attr(self, m: Module, e: ast.Attribute) -> Res:
+        base = e.value
+        if isinstance(base, ast.Name):
+            handler = self.caught_as(m, e, base.id)
+            fn = self.enclosing(m, e, (ast.FunctionDef, ast.AsyncFunctionDef))
+            typed = None
+            if handler is not None:
+                typed = self.classes(m, handler.type)
+            elif fn is not None:
+                params = {
+                    a.arg: a for a in [*fn.args.posonlyargs, *fn.args.args, *fn.args.kwonlyargs]
+                }
+                if base.id in params:
+                    typed = self.classes(m, params[base.id].annotation)
+            if typed is not None:
+                out = Res()
+                for cm, cls in typed:
+                    v = self.attr_values(cm, cls, e.attr)
+                    out = out | (
+                        Res(v)
+                        if v is not None
+                        else Res(
+                            unread=[f"{self.at(m, e)} ({cls.name}.{e.attr} has no Literal type)"]
+                        )
+                    )
+                return out
+        found = self.lookup_expr(m, e)
+        if (
+            found
+            and isinstance(found[0], Module)
+            and isinstance(found[1], (ast.Assign, ast.AnnAssign))
+        ):
+            stmt = found[1]
+            if isinstance(stmt.value, ast.Constant):
+                return self.resolve(found[0], stmt.value)
+        return Res(unread=[self.at(m, e)])
+
+    def resolve_lookup(self, m: Module, e: ast.Subscript) -> Res:
+        found = self.lookup_expr(m, e.value)
+        if not (
+            found
+            and isinstance(found[0], Module)
+            and isinstance(found[1], (ast.Assign, ast.AnnAssign))
+        ):
+            return Res(unread=[self.at(m, e)])
+        table = found[1].value
+        if not isinstance(table, ast.Dict) or not all(
+            isinstance(k, ast.Constant) and isinstance(k.value, str) for k in table.keys
+        ):
+            return Res(unread=[self.at(m, e)])
+        out = Res()
+        for v in table.values:
+            out = out | self.resolve(found[0], v)
+        index = self.resolve(m, e.slice)
+        keys = {k.value for k in table.keys}
+        if index.unread or index.values != keys:
+            self.errors.append(
+                f"{self.at(m, e)}: the map's keys {sorted(keys)} are not what its index can be "
+                f"{sorted(index.values)}{' + unread ' + str(index.unread) if index.unread else ''}"
+            )
+        return out
+
+    # --- sites ---------------------------------------------------------------------------------------
+
+    def spread_is_safe(self, m: Module, e: Node, src: Node) -> bool:
+        """A `**x` in a detail, or `detail |= {…}` from it, cannot carry a reason when `x` is the `**kwargs`
+        of a function that takes `reason` by name: Python binds that one to the parameter."""
+        if isinstance(src, ast.Attribute):
+            return self.spread_is_safe_attr(m, e, src)
+        fn = self.enclosing(m, e, (ast.FunctionDef, ast.AsyncFunctionDef))
+        if fn is None or fn.args.kwarg is None:
+            return False
+        names = {a.arg for a in [*fn.args.posonlyargs, *fn.args.args, *fn.args.kwonlyargs]}
+        kwarg = fn.args.kwarg.arg
+        if isinstance(src, ast.Name):
+            return src.id == kwarg and "reason" in names
+
+        if isinstance(src, ast.DictComp):
+            it = src.generators[0].iter
+            return (
+                isinstance(it, ast.Call)
+                and isinstance(it.func, ast.Attribute)
+                and it.func.attr == "items"
+                and getattr(it.func.value, "id", None) == kwarg
+                and "reason" in names
+            )
+        return False
+
+    def spread_is_safe_attr(self, m: Module, e: Node, src: ast.Attribute) -> bool:
+        """`e.params` of a caught exception whose `__init__(self, reason, **params)` keeps `self.params = params`."""
+        if not isinstance(src.value, ast.Name):
+            return False
+        handler = self.caught_as(m, e, src.value.id)
+        typed = self.classes(m, handler.type) if handler else None
+        return bool(typed) and all(self.keeps_kwargs(cls, src.attr) for _, cls in typed or [])
+
+    def keeps_kwargs(self, cls: ast.ClassDef, attr: str) -> bool:
+        init = next(
+            (s for s in cls.body if isinstance(s, ast.FunctionDef) and s.name == "__init__"), None
+        )
+        if init is None or init.args.kwarg is None:
+            return False
+        names = {a.arg for a in [*init.args.posonlyargs, *init.args.args, *init.args.kwonlyargs]}
+        kwarg = init.args.kwarg.arg
+        return "reason" in names and any(
+            isinstance(n, ast.Assign)
+            and any(
+                isinstance(t, ast.Attribute)
+                and t.attr == attr
+                and getattr(t.value, "id", None) == "self"
+                for t in n.targets
+            )
+            and getattr(n.value, "id", None) == kwarg
+            for n in ast.walk(init)
+        )
+
+    def entries(self, d: Node) -> list[tuple[str | None, Node]]:
+        if isinstance(d, ast.Dict):
+            return [
+                (k.value if isinstance(k, ast.Constant) else None if k is None else "?", v)
+                for k, v in zip(d.keys, d.values, strict=True)
+            ]
+        assert isinstance(d, ast.Call)
+        return [(k.arg, k.value) for k in d.keywords]
+
+    def site(self, m: Module, d: Node, out: dict) -> None:
+        items = self.entries(d)
+        code_expr = next(v for k, v in items if k == "code")
+        code = self.resolve(m, code_expr)
+        if code.unread or len(code.values) != 1:
+            out["unreadCodes"].append(self.at(m, code_expr))
+            return
+        entry = out["codes"].setdefault(
+            next(iter(code.values)), {"reasons": set(), "unread": [], "sites": 0, "reasonless": []}
+        )
+        entry["sites"] += 1
+        res, has_reason = Res(), False
+        for k, v in items:
+            if k == "reason":
+                res, has_reason = res | self.resolve(m, v), True
+            elif k is None and not self.spread_is_safe(m, d, v):
+                res = res | Res(unread=[f"{self.at(m, v)} (a spread that may carry a reason)"])
+        res, more = self.mutations(m, d, res)
+        has_reason = has_reason or more
+        if not has_reason:
+            entry["reasonless"].append(f"{m.path}:{d.lineno}")
+        entry["reasons"] |= res.values
+        entry["unread"] += res.unread
+
+    def mutations(self, m: Module, d: Node, res: Res) -> tuple[Res, bool]:
+        """`detail |= {…}`, `detail["reason"] = …`, `detail.update(…)` on the variable a detail is put in."""
+        stmt = m.parents.get(d)
+        if not (isinstance(stmt, (ast.Assign, ast.AnnAssign)) and stmt.value is d):
+            return res, False
+        target = stmt.targets[0] if isinstance(stmt, ast.Assign) else stmt.target
+        fn = self.enclosing(m, d, (ast.FunctionDef, ast.AsyncFunctionDef))
+        if not isinstance(target, ast.Name) or fn is None:
+            return res, False
+        found = False
+        for node in ast.walk(fn):
+            if isinstance(node, ast.AugAssign) and getattr(node.target, "id", None) == target.id:
+                v = node.value
+                if isinstance(v, ast.Dict):
+                    for k, val in self.entries(v):
+                        if k == "reason":
+                            res, found = res | self.resolve(m, val), True
+                        elif k is None and not self.spread_is_safe(m, node, val):
+                            res = res | Res(
+                                unread=[f"{self.at(m, val)} (a spread that may carry a reason)"]
+                            )
+                elif not self.spread_is_safe(m, node, v):
+                    res = res | Res(unread=[f"{self.at(m, node)} (may carry a reason)"])
+            elif (
+                isinstance(node, ast.Subscript)
+                and getattr(node.value, "id", None) == target.id
+                and isinstance(node.ctx, ast.Store)
+            ):
+                if isinstance(node.slice, ast.Constant) and node.slice.value != "reason":
+                    continue
+                assign = m.parents.get(node)
+                if isinstance(node.slice, ast.Constant) and isinstance(assign, ast.Assign):
+                    res, found = res | self.resolve(m, assign.value), True
+                else:
+                    res = res | Res(unread=[f"{self.at(m, node)} (may carry a reason)"])
+            elif (
+                isinstance(node, ast.Call)
+                and isinstance(node.func, ast.Attribute)
+                and getattr(node.func.value, "id", None) == target.id
+                and node.func.attr in {"update", "setdefault"}
+            ):
+                res = res | Res(unread=[f"{self.at(m, node)} (may carry a reason)"])
+        return res, found
+
+    def run(self) -> dict:
+        out: dict = {
+            "codes": {},
+            "unreadCodes": [],
+            "errors": self.errors,
+            "excluded": {},
+            "aliases": {},
+            "notRefusals": sorted(NOT_REFUSALS),
+        }
+        for m in self.mods.values():
+            for name, stmt in m.top.items():
+                if (
+                    isinstance(stmt, ast.Assign)
+                    and isinstance(stmt.value, ast.Subscript)
+                    and (v := self.ann_values(m, stmt.value))
+                ):
+                    out["aliases"].setdefault(m.path, {})[name] = sorted(v)
+            for node in ast.walk(m.tree):
+                is_dict = isinstance(node, ast.Dict) and any(
+                    isinstance(k, ast.Constant) and k.value == "code" for k in node.keys
+                )
+                is_call = (
+                    isinstance(node, ast.Call)
+                    and getattr(node.func, "id", None) == "dict"
+                    and any(k.arg == "code" for k in node.keywords)
+                )
+                if not (is_dict or is_call):
+                    continue
+                if m.path in NOT_REFUSALS:
+                    out["excluded"][m.path] = out["excluded"].get(m.path, 0) + 1
+                    continue
+                self.site(m, node, out)
+        for entry in out["codes"].values():
+            entry["reasons"] = sorted(entry["reasons"])
+        return out
+
+
+def scan(sources: dict[str, str]) -> dict:
+    return Scan(sources).run()
+
+
+def scan_app(app: pathlib.Path = APP) -> dict:
+    return scan(
+        {
+            p.relative_to(app).as_posix(): p.read_text(encoding="utf-8")
+            for p in sorted(app.rglob("*.py"))
+            if "__pycache__" not in p.parts
+        }
+    )
+
+
+if __name__ == "__main__":
+    json.dump(
+        scan_app(pathlib.Path(sys.argv[1]) if len(sys.argv) > 1 else APP),
+        sys.stdout,
+        indent=1,
+        sort_keys=True,
+    )
diff --git a/backend/tests/test_refusal_sites.py b/backend/tests/test_refusal_sites.py
new file mode 100644
index 00000000..9c6effbb
--- /dev/null
+++ b/backend/tests/test_refusal_sites.py
@@ -0,0 +1,278 @@
+"""The refusal scan (`refusal_sites.py`) on small sources: every shape a reason travels in is followed to
+its values or reported unread. A scan that silently drops a shape is a guard green over nothing, so each
+shape that once went past the regex it replaced is here. Pure: no app import, no database."""
+
+import textwrap
+
+from tests.refusal_sites import scan, scan_app
+
+
+def _scan(**files: str) -> dict:
+    return scan(
+        {f"{name.replace('__', '/')}.py": textwrap.dedent(src) for name, src in files.items()}
+    )
+
+
+def _code(result: dict, code: str) -> dict:
+    return result["codes"][code]
+
+
+REFUSAL = """
+    from fastapi import HTTPException
+
+    def refused(status: int, reason: str, **params: object) -> HTTPException:
+        return HTTPException(status_code=status, detail={"code": "refused", "reason": reason, **params})
+"""
+
+
+def test_a_literal_wherever_it_sits_in_the_dict():
+    r = _scan(
+        a="""
+        def f(x):
+            raise HTTPException(409, {"code": "clip", "limit_s": 5, "reason": "too_long"})
+        def g(x):
+            raise HTTPException(409, {"reason": "before", "code": "clip"})
+        def h(x):
+            raise HTTPException(409, {
+                "code": "clip",  # a comment between
+                "reason": "after_comment",
+            })
+        def i(x):
+            raise HTTPException(409, detail=dict(code="clip", reason="by_call"))
+        """
+    )
+    assert _code(r, "clip")["reasons"] == ["after_comment", "before", "by_call", "too_long"]
+    assert _code(r, "clip")["unread"] == []
+
+
+def test_every_arm_of_a_conditional_and_every_operand_of_or():
+    r = _scan(
+        a="""
+        def f(x, e):
+            raise HTTPException(409, {"code": "edit", "reason": "a" if x else "b"})
+        def g(x):
+            raise HTTPException(409, {"code": "edit", "reason": x.why or "c"})
+        """
+    )
+    assert _code(r, "edit")["reasons"] == ["a", "b", "c"]
+    assert [u.split(": ", 1)[1] for u in _code(r, "edit")["unread"]] == ["x.why"]
+
+
+def test_a_helper_is_followed_to_every_call_however_it_is_written():
+    r = _scan(
+        refusal=REFUSAL,
+        routes__a="""
+        from http import HTTPStatus
+        from app import refusal
+        from app.refusal import refused
+
+        def f(status):
+            raise refused(status, "by_name_status")
+        def g():
+            raise refused(HTTPStatus.CONFLICT, reason="by_keyword")
+        def h():
+            raise refusal.refused(409, "qualified")
+        def i(x):
+            raise refused(409, "a" if x else "second_arm")
+        """,
+    )
+    assert _code(r, "refused")["reasons"] == [
+        "a",
+        "by_keyword",
+        "by_name_status",
+        "qualified",
+        "second_arm",
+    ]
+    assert _code(r, "refused")["unread"] == []
+
+
+def test_a_helper_passed_as_a_value_or_spread_into_is_unread():
+    r = _scan(
+        refusal=REFUSAL,
+        routes__a="""
+        from app.refusal import refused
+
+        def f(args):
+            pick = refused
+            raise refused(*args)
+        """,
+    )
+    unread = _code(r, "refused")["unread"]
+    assert any("the helper used as a value" in u for u in unread)
+    assert any("refused(*args)" in u for u in unread)
+
+
+def test_a_default_is_what_a_call_that_omits_it_sends():
+    r = _scan(
+        routes___common="""
+        def not_found(what: str = "generic"):
+            return HTTPException(404, {"code": "not_found", "reason": what})
+
+        def get_or_404(db, id_, *, what: str = "generic"):
+            raise not_found(what)
+        """,
+        routes__a="""
+        from app.routes._common import get_or_404, not_found
+
+        def f(db):
+            get_or_404(db, 1, what="user")
+            get_or_404(db, 2)
+            raise not_found("debug_session")
+        """,
+    )
+    assert _code(r, "not_found")["reasons"] == ["debug_session", "generic", "user"]
+
+
+def test_a_caught_exception_answers_with_its_attribute_type_every_class_it_catches():
+    r = _scan(
+        errors="""
+        from typing import Literal
+        ClipReason = Literal["bad_range", "too_short"]
+        EditReason = Literal["not_media"]
+
+        class ClipError(Exception):
+            def __init__(self, reason: ClipReason, **params: float):
+                self.reason: ClipReason = reason
+                self.params = params
+
+        class EditError(Exception):
+            def __init__(self, reason: EditReason):
+                self.reason: EditReason = reason
+
+        class Untyped(Exception):
+            def __init__(self, reason: str):
+                self.reason = reason
+
+        class Narrower(ClipError):
+            pass
+        """,
+        routes__a="""
+        from app import errors
+
+        def f():
+            try:
+                pass
+            except (errors.ClipError, errors.EditError) as e:
+                raise HTTPException(422, {"code": "clip", "reason": e.reason, **e.params})
+            except errors.Untyped as e:
+                raise HTTPException(422, {"code": "clip", "reason": e.reason})
+            except errors.Narrower as e:
+                raise HTTPException(422, {"code": "edit", "reason": e.reason})
+        """,
+    )
+    assert _code(r, "clip")["reasons"] == ["bad_range", "not_media", "too_short"]
+    unread = _code(r, "clip")["unread"]
+    assert any("Untyped.reason has no Literal type" in u for u in unread)
+    # EditError keeps no `**params`, so its `e.params` could be anything
+    assert any("e.params" in u for u in unread)
+    assert _code(r, "edit")["reasons"] == ["bad_range", "too_short"]
+
+
+def test_a_typed_parameter_is_its_literal_and_an_untyped_one_is_unread_when_never_called():
+    r = _scan(
+        a="""
+        from typing import Literal
+        Why = Literal["one", "two"]
+
+        def typed(reason: Why):
+            return HTTPException(409, {"code": "playlist", "reason": reason})
+
+        class C:
+            def method(self, reason: str):
+                return HTTPException(409, {"code": "playlist", "reason": reason})
+        """
+    )
+    assert _code(r, "playlist")["reasons"] == ["one", "two"]
+    assert any("is not a module-level helper" in u for u in _code(r, "playlist")["unread"])
+
+
+def test_a_value_the_scan_cannot_follow_is_unread():
+    r = _scan(
+        a="""
+        def f(svc):
+            hit, code = svc.fetch()
+            raise HTTPException(422, {"code": "subtitle", "reason": code})
+        def g(svc):
+            raise HTTPException(422, {"code": "subtitle", "reason": svc.reason()})
+        def h(e):
+            raise HTTPException(422, {"code": "subtitle", "reason": e.reason})
+        """
+    )
+    assert len(_code(r, "subtitle")["unread"]) == 3
+    assert _code(r, "subtitle")["reasons"] == []
+
+
+def test_a_map_lookup_is_its_values_and_its_keys_must_be_what_the_index_can_be():
+    src = """
+        from typing import Literal
+        SettingReason = Literal["too_low", "too_high"]
+
+        class InvalidSetting(ValueError):
+            def __init__(self, key: str, reason: SettingReason, **params: object):
+                self.reason: SettingReason = reason
+                self.params = params
+
+        _MAP: dict[SettingReason, str] = {{{keys}}}
+
+        def refused(status: int, reason: str, **params: object):
+            return HTTPException(status, {{"code": "refused", "reason": reason, **params}})
+
+        def setting_refused(exc: InvalidSetting):
+            return refused(400, _MAP[exc.reason], **exc.params)
+    """
+    whole = _scan(a=src.format(keys='"too_low": "setting_too_low", "too_high": "setting_too_high"'))
+    assert _code(whole, "refused")["reasons"] == ["setting_too_high", "setting_too_low"]
+    assert whole["errors"] == []
+    short = _scan(a=src.format(keys='"too_low": "setting_too_low"'))
+    assert len(short["errors"]) == 1 and "not what its index can be" in short["errors"][0]
+
+
+def test_a_detail_built_up_in_steps_is_read_through_its_variable():
+    r = _scan(
+        a="""
+        def f(status: int, reason: str, user=None, **extra):
+            detail: dict = {"code": "login", "reason": "invalid"}
+            if user is not None:
+                detail |= {"email": user.email, "reason": "suspended"}
+            detail |= {k: v for k, v in extra.items() if v is not None}
+            return HTTPException(status, detail)
+
+        def g(more):
+            detail = {"code": "login"}
+            detail["reason"] = "late"
+            detail.update(more)
+            detail |= more
+            return HTTPException(401, detail)
+        """
+    )
+    entry = _code(r, "login")
+    assert entry["reasons"] == ["invalid", "late", "suspended"]
+    assert sum("may carry a reason" in u for u in entry["unread"]) == 2
+
+
+def test_a_code_must_be_one_the_scan_can_read():
+    r = _scan(
+        a="""
+        BUSY = "busy"
+        def f(code):
+            raise HTTPException(409, {"code": BUSY})
+        def g(code):
+            raise HTTPException(409, {"code": code})
+        """
+    )
+    assert "busy" in r["codes"] and r["codes"]["busy"]["reasonless"]
+    assert len(r["unreadCodes"]) == 1 and r["unreadCodes"][0].endswith(": code")
+
+
+def test_the_data_files_are_excluded_by_name_only():
+    r = _scan(plex__tmdb='x = {"code": "HU", "name": "Hungary"}', plex__other='y = {"code": "HU"}')
+    assert r["excluded"] == {"plex/tmdb.py": 1}
+    assert "HU" in r["codes"]
+
+
+def test_the_app_scans():
+    """The real tree, on the interpreter the backend runs: every refusal it holds is read (the
+    frontend's contract judges what is read)."""
+    result = scan_app()
+    assert {"refused", "not_found", "login", "clip"} <= set(result["codes"])
+    assert result["errors"] == []
diff --git a/docs/development.md b/docs/development.md
index 465b327c..dcbe382d 100644
--- a/docs/development.md
+++ b/docs/development.md
@@ -144,7 +144,8 @@ npm run typecheck     # tsc x3 (app / node / e2e)
 npm run lint          # eslint (typed rules included; warnings fail)
 npm run format:check  # prettier
 npm run knip          # unused files/exports/deps
-npm run test          # vitest (incl. the compiler and gate-register contracts)
+npm run test          # vitest (incl. the compiler and gate-register contracts; the refusal contract runs
+                      # the host's `python` on backend/tests/refusal_sites.py, stdlib only)
 npx vite build        # the production bundle (devOnlyGuard fails it on dev-only code, chunkGuard on a lazy page loaded at boot)
 npm audit --omit=dev --audit-level=high   # what ships to the browser, at high or above
 ```
diff --git a/frontend/src/i18n/backendIds.contract.test.ts b/frontend/src/i18n/backendIds.contract.test.ts
index 524ed7db..436d5ce6 100644
--- a/frontend/src/i18n/backendIds.contract.test.ts
+++ b/frontend/src/i18n/backendIds.contract.test.ts
@@ -1,4 +1,6 @@
+import { execFileSync } from "node:child_process";
 import { readdirSync, readFileSync } from "node:fs";
+import { fileURLToPath } from "node:url";
 import { describe, expect, it } from "vitest";
 
 import { RUN_STATUSES } from "../lib/activityUrlState";
@@ -27,6 +29,25 @@ import { vocabKey } from "../lib/vocab";
 const backend = (file: string) =>
   readFileSync(new URL(`../../../backend/app/${file}`, import.meta.url), "utf-8");
 
+// Every refusal the backend can answer, read from its syntax tree by `backend/tests/refusal_sites.py`
+// (stdlib `ast`, on the host's Python, as the ruff lane is): each code with every reason it can be sent
+// with — a reason in a variable followed to its `Literal` type, its helper's every call, its map — and
+// what it could not follow, which fails below. It also hands back every `Literal` alias it read.
+type RefusalScan = {
+  codes: Record<string, { reasons: string[]; unread: string[]; reasonless: string[] }>;
+  unreadCodes: string[];
+  errors: string[];
+  notRefusals: string[];
+  excluded: Record<string, number>;
+  aliases: Record<string, Record<string, string[]>>;
+};
+const scan = JSON.parse(
+  execFileSync("python", ["-I", "backend/tests/refusal_sites.py"], {
+    cwd: fileURLToPath(new URL("../../../", import.meta.url)),
+    encoding: "utf-8",
+  }),
+) as RefusalScan;
+
 const mods = import.meta.glob("./locales/*/*.json", { eager: true }) as Record<
   string,
   { default: Record<string, unknown> }
@@ -54,12 +75,11 @@ const keysOf = (lang: string): Set<string> => {
 const specs = [
   ...backend("sysconfig.py").matchAll(/ConfigSpec\(\s*"(\w+)",\s*"(\w+)",\s*"(\w+)",\s*"(\w+)"/g),
 ].map((m) => ({ key: m[1]!, type: m[2]!, group: m[3]!, attr: m[4]! }));
-// A choice key's options, as config.py spells them: `<attr>: <Alias> = …` → `<Alias> = Literal[…]`.
+// A choice key's options, as config.py spells them: `<attr>: <Alias> = …`, the alias's values from the scan.
 const configPy = backend("config.py");
 const choicesOf = (attr: string): string[] => {
   const alias = configPy.match(new RegExp(`^\\s+${attr}: (\\w+) =`, "m"))?.[1];
-  const body = alias && configPy.match(new RegExp(`^${alias} = Literal\\[([^\\]]*)\\]`, "m"))?.[1];
-  return [...(body ?? "").matchAll(/"(\w+)"/g)].map((m) => m[1]!);
+  return (alias && scan.aliases["config.py"]?.[alias]) || [];
 };
 const choiceIds = specs
   .filter((s) => s.type === "choice")
@@ -90,16 +110,10 @@ const runEnum = (name: string) =>
 const runStatuses = runEnum("RunStatus");
 const runTriggers = runEnum("RunTrigger");
 const quotaActions = enumValues("quota.py");
-// The admin actions' refusal reasons → `errors.admin.<reason>`. The codes are the refusal records
-// under that prefix (lib/api/refusalCodes), so a code recorded there is scanned here too; a detail
-// dict ruff wraps over several lines still matches.
+// The admin actions' refusals → `errors.admin.<reason>`: the codes recorded under that prefix.
 const adminCodes = Object.entries(REFUSAL_FAMILIES)
   .filter(([, f]) => f.prefix === "errors.admin")
   .map(([code]) => code);
-const adminRefusals = [
-  ...backend("routes/admin.py").matchAll(/\{\s*"code":\s*"(\w+)",\s*"reason":\s*"(\w+)"/g),
-].filter((m) => adminCodes.includes(m[1]!));
-const adminReasons = [...new Set(adminRefusals.map((m) => m[2]!))];
 // The download failure catalog's host-specific entries → `downloads.errors.fail.catalog.<detail>`.
 // A code holds a dot (`facebook.login_wall`), so the reverse check strips the prefix and the part
 // rather than splitting on dots.
@@ -120,66 +134,6 @@ const failCategories = [
   ]),
 ];
 
-// What a structured 404 says is gone → `errors.notFound.<what>`: every `not_found("x")` and every
-// `owned_or_404(..., what="x")` / `get_or_404(..., what="x")` under routes/, read from the sources.
-const routeFiles = (dir = "routes"): string[] =>
-  readdirSync(new URL(`../../../backend/app/${dir}/`, import.meta.url), {
-    withFileTypes: true,
-  }).flatMap((e) =>
-    e.isDirectory()
-      ? routeFiles(`${dir}/${e.name}`)
-      : e.name.endsWith(".py")
-        ? [`${dir}/${e.name}`]
-        : [],
-  );
-const routeSources = (): string[] => routeFiles().map(backend);
-// The words a job's result gives for not doing its work → `scheduler.skip.<vocabKey(word)>` (the Scheduler
-// page, the run notice and the audit row name them; lib/jobResult). Read as the `"skipped": "…"` /
-// `"reason": "…"` literals of the non-HTTP backend: under routes/ (and main.py) a `reason` is a refusal
-// code, which `errors.*` names. A word from an exception's text is not a literal and is not covered.
-const appFiles = (dir: string): string[] =>
-  readdirSync(new URL(`../../../backend/app/${dir}`, import.meta.url), {
-    withFileTypes: true,
-  }).flatMap((e) =>
-    e.isDirectory()
-      ? e.name === "routes" || e.name === "__pycache__"
-        ? []
-        : appFiles(`${dir}${e.name}/`)
-      : e.name.endsWith(".py") && `${dir}${e.name}` !== "main.py"
-        ? [`${dir}${e.name}`]
-        : [],
-  );
-const skipWords = [
-  ...new Set(
-    appFiles("").flatMap((f) =>
-      [...backend(f).matchAll(/"(?:skipped|reason)": "([^"]+)"/g)].map((m) => m[1]!),
-    ),
-  ),
-];
-// A target the scan cannot read — `what=SOME_CONSTANT` — is a 404 no locale is checked for (one hid
-// `debug_session`). Only the helpers' own pass-through in `_common.py` may name it by variable.
-const opaqueWhats = routeFiles()
-  .filter((f) => !f.endsWith("/_common.py"))
-  .flatMap((f) =>
-    [...backend(f).matchAll(/not_found\((?!"|\))[^)]*\)|what=(?!")\S+/g)].map(
-      (m) => `${f}: ${m[0]}`,
-    ),
-  );
-const goneWhats = [
-  ...new Set(
-    routeSources().flatMap((src) =>
-      [...src.matchAll(/not_found\("(\w+)"\)|what="(\w+)"/g)].map((m) => (m[1] ?? m[2])!),
-    ),
-  ),
-];
-// A backend `Name = Literal["a", "b"]` alias, by name: the reasons a typed parameter accepts, which
-// basedpyright then holds at every call site — a reason passed by variable included.
-const literalOf = (file: string, name: string) =>
-  [
-    ...(
-      backend(file).match(new RegExp(`^${name} = Literal\\[([^\\]]*)\\]`, "m"))?.[1] ?? ""
-    ).matchAll(/"(\w+)"/g),
-  ].map((m) => m[1]!);
 // Every backend source, by its path under app/.
 const appPaths = (dir = ""): string[] =>
   readdirSync(new URL(`../../../backend/app/${dir}`, import.meta.url), {
@@ -193,114 +147,32 @@ const appPaths = (dir = ""): string[] =>
         ? [`${dir}${e.name}`]
         : [],
   );
-const appSources = (): string[] => appPaths().map(backend);
-// Every generic refusal → `errors.refused.<reason>`: each `refused(<status>, "<reason>")` anywhere in
-// the backend (not `_refused`, a router's own family), the values of the settings map, which
-// `setting_refused` picks from at runtime, and the folder rules' `FolderReason`, answered with
-// `refused(409, e.reason)`.
-const refusedReasons = [
-  ...new Set([
-    ...appSources().flatMap((src) =>
-      [...src.matchAll(/(?<![\w.])refused\(\s*\d+,\s*"(\w+)"/g)].map((m) => m[1]!),
-    ),
-    ...literalOf("downloads/folders.py", "FolderReason"),
-    ...[
-      ...(backend("refusal.py").split("_SETTING_REASONS = {")[1]?.split("}")[0] ?? "").matchAll(
-        /:\s*"(\w+)"/g,
-      ),
-    ].map((m) => m[1]!),
-  ]),
-];
-// A `refused(…)` whose reason the scan above cannot read. Only two may: a folder rule's `e.reason`
-// (the nearest `except` above it catches `FolderError`, typed `FolderReason`) and the settings map.
-const opaqueRefused = appPaths().flatMap((f) => {
-  const src = backend(f);
-  return [...src.matchAll(/(?<![\w.])refused\(\s*\d+,\s*(?![\s"])([^,)]+)/g)]
-    .filter((m) => {
-      const arg = m[1]!.trim();
-      if (f === "refusal.py" && arg.startsWith("_SETTING_REASONS[")) return false;
-      const caught = [...src.slice(0, m.index).matchAll(/except ([^\n:]*):/g)].pop()?.[1] ?? "";
-      return !(arg === "e.reason" && /FolderError as e$/.test(caught));
-    })
-    .map((m) => `${f}: ${m[0]}`);
-});
-const youtubeReasons = literalOf("youtube/client.py", "YouTubeRefusal");
-const clipReasons = literalOf("downloads/clip.py", "ClipReason");
-
-// Every `"code": …` a backend dict carries, with the `"reason": …` that follows it. Two modules use a
-// `code` key for something that is not a refusal; everything else is one.
-const NOT_REFUSALS: Record<string, string> = {
-  "plex/subtitles.py":
-    "the player's subtitle status vocabulary (`/status`), read by the player itself",
-  "plex/tmdb.py": "a production country's ISO code",
-};
-const detailSites = appPaths()
-  .filter((f) => !(f in NOT_REFUSALS))
-  .flatMap((f) =>
-    [
-      ...backend(f).matchAll(
-        /"code":\s*(?:"(\w+)"|([^\s,}]+))(?:,\s*"reason":\s*(?:"(\w+)"|([^\s,}]+)))?/g,
-      ),
-    ].map((m) => ({ f, code: m[1], codeExpr: m[2], reason: m[3], reasonExpr: m[4] })),
-  );
-const backendCodes = [...new Set(detailSites.flatMap((s) => (s.code ? [s.code] : [])))];
-// A code the scan cannot read (`"code": SOME_CONSTANT`) is a refusal no record is checked for.
-const opaqueCodes = detailSites.filter((s) => !s.code).map((s) => `${s.f}: "code": ${s.codeExpr}`);
-// A reason passed by variable, each pinned to what holds it, so a new one has to say how it is held.
-const OPAQUE_REASONS: Record<string, string> = {
-  "refusal.py: refused ← reason": "a literal at each refused() call, else opaqueRefused",
-  "routes/auth.py: login ← reason": "_login_refused's LoginRefusal",
-  "routes/downloads.py: url_blocked ← urlguard.REASON_MALFORMED": "UrlBlockedReason",
-  "routes/downloads.py: url_blocked ← e.reason": "BlockedUrl's UrlBlockedReason",
-  "routes/downloads.py: cookies ← e.reason": "InvalidCookies' CookieReason",
-  "routes/downloads.py: edit ← e.reason": "EditError's EditReason",
-  "routes/downloads.py: clip ← e.reason": "ClipError's ClipReason",
-  "routes/files.py: quota ← e.reason": "QuotaExceeded's QuotaReason",
-  "routes/playlists.py: playlist ← reason": "_refused's PlaylistRefusal",
-  "routes/playlists.py: playlist_move ← e.reason": "MoveRefused's MoveReason",
-  "routes/plex/playback.py: subtitle ← code": "REFUSAL_UNSHOWN: no screen reads it",
-  "routes/_common.py: not_found ← what": "opaqueWhats",
-  "routes/_common.py: youtube ← reason": "YouTubeRefusal",
-};
-const opaqueReasons = [
+// The words a job's result gives for not doing its work → `scheduler.skip.<vocabKey(word)>` (the Scheduler
+// page, the run notice and the audit row name them; lib/jobResult). Read as the `"skipped": "…"` /
+// `"reason": "…"` literals of the non-HTTP backend: under routes/ (and main.py) a `reason` is a refusal
+// code, which `errors.*` names. A word from an exception's text is not a literal and is not covered.
+const skipWords = [
   ...new Set(
-    detailSites
-      .filter((s) => s.code && s.reasonExpr)
-      .map((s) => `${s.f}: ${s.code} ← ${s.reasonExpr}`),
+    appPaths()
+      .filter((f) => !f.startsWith("routes/") && f !== "main.py")
+      .flatMap((f) =>
+        [...backend(f).matchAll(/"(?:skipped|reason)": "([^"]+)"/g)].map((m) => m[1]!),
+      ),
   ),
 ];
-// A reason written as a literal in a detail dict, wherever that dict is built.
-const literalReasons = (code: string) =>
-  detailSites.filter((s) => s.code === code && s.reason).map((s) => s.reason!);
-// What each refusal code can carry as its reason: the Literal its typed parameter accepts, or the
-// call's own literal — and every literal any detail dict anywhere gives it.
-const reasonsOf: Record<string, string[]> = Object.fromEntries(
-  Object.entries({
-    refused: refusedReasons,
-    youtube: youtubeReasons,
-    not_found: goneWhats,
-    clip: clipReasons,
-    ...Object.fromEntries(adminCodes.map((c) => [c, []])),
-    playlist: literalOf("routes/playlists.py", "PlaylistRefusal"),
-    playlist_move: literalOf("reader/service.py", "MoveReason"),
-    edit: literalOf("downloads/service.py", "EditReason"),
-    cookies: literalOf("downloads/credentials.py", "CookieReason"),
-    url_blocked: literalOf("downloads/urlguard.py", "UrlBlockedReason"),
-    login: literalOf("routes/auth.py", "LoginRefusal"),
-    quota: literalOf("downloads/quota.py", "QuotaReason"),
-    reset: [],
-    collection: [],
-  } as Record<string, string[]>).map(([code, rs]) => [
-    code,
-    [...new Set([...rs, ...literalReasons(code)])],
-  ]),
-);
+
+const backendCodes = Object.keys(scan.codes);
 const recordedCodes = [
   ...Object.keys(REFUSAL_FAMILIES),
   ...Object.keys(REFUSAL_BY_REASON),
   ...Object.keys(REFUSAL_FIXED),
   ...Object.keys(REFUSAL_UNSHOWN),
 ];
+// The codes whose reason picks the sentence, and what each can carry as its reason.
+const reasoned = [...Object.keys(REFUSAL_FAMILIES), ...Object.keys(REFUSAL_BY_REASON)];
+const reasonsOf: Record<string, string[]> = Object.fromEntries(
+  reasoned.map((code) => [code, scan.codes[code]?.reasons ?? []]),
+);
 
 // The closed English vocabularies the backend WRITES into data, named through `vocab.<group>`
 // (lib/vocab) by their letters-and-digits key. A word the backend adds shows in English until named —
@@ -363,10 +235,11 @@ describe("backend ids have their names in both languages", () => {
     expect(runTriggers).toEqual(expect.arrayContaining(["scheduled", "ad_hoc"]));
     expect(adminCodes).toEqual(expect.arrayContaining(["activate", "resend"]));
     expect(failDetails).toEqual(expect.arrayContaining(["facebook.login_wall"]));
-    expect(goneWhats).toEqual(expect.arrayContaining(["playlist", "download", "job"]));
-    expect(opaqueWhats).toEqual([]);
-    expect(youtubeReasons).toEqual(["quota", "reconnect", "unavailable", "gone"]);
-    expect(clipReasons).toEqual(expect.arrayContaining(["too_long", "past_end", "bad_crop"]));
+    expect(reasonsOf.not_found).toEqual(
+      expect.arrayContaining(["playlist", "download", "job", "user", "debug_session"]),
+    );
+    expect(reasonsOf.youtube).toEqual(["gone", "quota", "reconnect", "unavailable"]);
+    expect(reasonsOf.clip).toEqual(expect.arrayContaining(["too_long", "past_end", "bad_crop"]));
     expect(crewJobs).toEqual(
       expect.arrayContaining(["Director", "Creator", "Director of Photography"]),
     );
@@ -382,11 +255,14 @@ describe("backend ids have their names in both languages", () => {
         "no service user",
       ]),
     );
-    expect(refusedReasons).toEqual(
-      expect.arrayContaining(["weak_password", "setting_too_low", "plex_write_failed"]),
+    expect(reasonsOf.refused).toEqual(
+      expect.arrayContaining([
+        "weak_password",
+        "setting_too_low",
+        "plex_write_failed",
+        "file_folder_gone",
+      ]),
     );
-    // Every code the frontend localizes is raised somewhere the scan can see.
-    expect(adminCodes.filter((c) => !adminRefusals.some((m) => m[1] === c))).toEqual([]);
   });
 
   it("every refusal code the backend answers is said by exactly one record, and no record names another", () => {
@@ -397,12 +273,19 @@ describe("backend ids have their names in both languages", () => {
     expect(recordedCodes.filter((c) => !backendCodes.includes(c))).toEqual([]);
   });
 
-  it("every refusal code and reason is one the scans can read, or says what holds it", () => {
-    // The modules set aside as not refusals still exist and still use the key.
-    for (const f of Object.keys(NOT_REFUSALS)) expect(backend(f), f).toContain('"code":');
-    expect(opaqueCodes).toEqual([]);
-    expect(opaqueRefused).toEqual([]);
-    expect(opaqueReasons.sort()).toEqual(Object.keys(OPAQUE_REASONS).sort());
+  it("every refusal code and reason is one the scan can follow", () => {
+    // The modules set aside as not refusals still use the key.
+    expect(Object.keys(scan.excluded).sort()).toEqual(scan.notRefusals);
+    expect(scan.unreadCodes).toEqual([]);
+    expect(scan.errors).toEqual([]);
+    // A reason it could not follow is a sentence nobody is asked for, unless no screen says that code.
+    expect(
+      Object.entries(scan.codes)
+        .filter(([code]) => !(code in REFUSAL_UNSHOWN))
+        .flatMap(([, c]) => c.unread),
+    ).toEqual([]);
+    // A code whose reason picks the sentence, sent without one, says only its fallback.
+    expect(reasoned.flatMap((code) => scan.codes[code]?.reasonless ?? [])).toEqual([]);
   });
 
   it("read every refusal's reasons (a scan that matches nothing is green over nothing)", () => {
@@ -533,33 +416,6 @@ describe("backend ids have their names in both languages", () => {
       expect([...named].filter((d) => !failDetails.includes(d))).toEqual([]);
     });
 
-    it(`${lang}: every "that is gone" a 404 names, and every YouTube refusal`, () => {
-      expect(missing(goneWhats.map((w) => `errors.notFound.${w}`))).toEqual([]);
-      expect(
-        [...idsUnder(keys, "errors.notFound")].filter(
-          (w) => w !== "generic" && !goneWhats.includes(w),
-        ),
-      ).toEqual([]);
-      expect(missing(youtubeReasons.map((r) => `errors.youtube.${r}`))).toEqual([]);
-      expect(
-        [...idsUnder(keys, "errors.youtube")].filter((r) => !youtubeReasons.includes(r)),
-      ).toEqual([]);
-    });
-
-    it(`${lang}: every generic refusal the backend can answer, and none it cannot`, () => {
-      expect(missing(refusedReasons.map((r) => `errors.refused.${r}`))).toEqual([]);
-      expect(
-        [...idsUnder(keys, "errors.refused")].filter((r) => !refusedReasons.includes(r)),
-      ).toEqual([]);
-    });
-
-    it(`${lang}: every reason a clip is refused, and none it cannot be`, () => {
-      expect(missing(clipReasons.map((r) => `clip.errors.${r}`))).toEqual([]);
-      expect([...idsUnder(keys, "clip.errors")].filter((r) => !clipReasons.includes(r))).toEqual(
-        [],
-      );
-    });
-
     it(`${lang}: every crew job, auto-tag and built-in profile the backend writes, and no other`, () => {
       for (const [group, words] of Object.entries(VOCAB)) {
         const wanted = words.map((w) => `vocab.${group}.${vocabKey(w)}`);
@@ -588,32 +444,32 @@ describe("backend ids have their names in both languages", () => {
     });
 
     it(`${lang}: no sentence for a refusal reason the backend cannot send`, () => {
-      // A family whose prefix is its own (the shared `errors.admin` is checked below). A suffixed
-      // family shares its prefix with more than its refusals — the sign-in screens' `gate`,
-      // `addressLabel` — so only an id with the refusal's own key counts as a reason there.
+      // By prefix: the admin actions share `errors.admin`. A suffixed family shares its prefix with more
+      // than its refusals — the sign-in screens' `gate`, `addressLabel` — so only an id with the
+      // refusal's own key counts as a reason there.
+      const byPrefix = new Map<
+        string,
+        { suffix: string; reasons: Set<string>; spare: Set<string> }
+      >();
       for (const [code, f] of Object.entries(REFUSAL_FAMILIES)) {
-        if (f.prefix === "errors.admin") continue;
-        const suffix = "suffix" in f ? f.suffix : "";
-        const fallbackId = f.fallback.startsWith(`${f.prefix}.`)
-          ? f.fallback.slice(f.prefix.length + 1)
-          : null;
-        expect(
-          [...idsUnder(keys, f.prefix)].filter(
-            (r) =>
-              keys.has(`${f.prefix}.${r}${suffix}`) &&
-              r !== fallbackId &&
-              !reasonsOf[code]!.includes(r),
-          ),
-          code,
-        ).toEqual([]);
+        const p = byPrefix.get(f.prefix) ?? {
+          suffix: "suffix" in f ? f.suffix : "",
+          reasons: new Set<string>(),
+          spare: new Set<string>(),
+        };
+        for (const r of reasonsOf[code]!) p.reasons.add(r);
+        if (f.fallback.startsWith(`${f.prefix}.`))
+          p.spare.add(f.fallback.slice(f.prefix.length + 1));
+        byPrefix.set(f.prefix, p);
       }
-    });
-
-    it(`${lang}: every admin refusal reason`, () => {
-      expect(missing(adminReasons.map((r) => `errors.admin.${r}`))).toEqual([]);
-      expect([...idsUnder(keys, "errors.admin")].filter((r) => !adminReasons.includes(r))).toEqual(
-        [],
-      );
+      for (const [prefix, { suffix, reasons, spare }] of byPrefix)
+        expect(
+          [...idsUnder(keys, prefix)].filter(
+            (r) =>
+              (!suffix || keys.has(`${prefix}.${r}${suffix}`)) && !spare.has(r) && !reasons.has(r),
+          ),
+          prefix,
+        ).toEqual([]);
     });
   }
 });