diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,14 @@
 # Changelog
 
+## 0.5.0.0 — 2026-08-22
+
+### Added
+
+- `underMemoryContext` and `inLegacyMemorySpaceOnly`, generic `Applicative` helpers that apply an
+  existing context's permission, space, and actor decision in a fixed order while letting Memory
+  and Session retain their distinct public error types. These helpers consume a context; they do
+  not mint or widen one.
+
 ## 0.4.1.0 — 2026-08-18
 
 ### Changed
diff --git a/kioku-api.cabal b/kioku-api.cabal
--- a/kioku-api.cabal
+++ b/kioku-api.cabal
@@ -1,6 +1,6 @@
 cabal-version:   3.0
 name:            kioku-api
-version:         0.4.1.0
+version:         0.5.0.0
 synopsis:        Reusable agent memory wire types
 description:
   Wire contract for kioku: custom prelude, TypeID identifiers, memory
@@ -78,7 +78,7 @@
     , base         >=4.21     && <5
     , bytestring   >=0.11     && <0.13
     , containers   >=0.6      && <0.8
-    , kioku-api    ^>=0.4.1.0
+    , kioku-api    ^>=0.5.0.0
     , tasty        >=1.5      && <1.6
     , tasty-hunit  >=0.10     && <0.11
     , text         >=2.1      && <2.2
diff --git a/src/Kioku/Api/Access.hs b/src/Kioku/Api/Access.hs
--- a/src/Kioku/Api/Access.hs
+++ b/src/Kioku/Api/Access.hs
@@ -110,6 +110,10 @@
     memoryContextRecordedActor,
     assumeAuthorizedMemoryContext,
     authorizeMemoryAccess,
+
+    -- * Applying an authorized decision
+    underMemoryContext,
+    inLegacyMemorySpaceOnly,
   )
 where
 
@@ -117,6 +121,42 @@
 import Data.Set qualified as Set
 import Kioku.Api.Access.Internal
 import Kioku.Prelude
+
+-- | Gate an operation on the permission, space, and actor already captured by a context.
+--
+-- The three error constructors keep this policy independent of any caller's error vocabulary.
+-- Permission is deliberately checked first so an unauthorized caller cannot use later mismatch
+-- errors to inspect the context's space or actor.
+underMemoryContext ::
+  (Applicative f) =>
+  (MemoryPermission -> err) ->
+  (MemorySpaceId -> MemorySpaceId -> err) ->
+  (RecordedPrincipal -> RecordedPrincipal -> err) ->
+  MemoryAccessContext ->
+  MemoryPermission ->
+  MemorySpaceId ->
+  RecordedPrincipal ->
+  f (Either err a) ->
+  f (Either err a)
+underMemoryContext permissionError spaceError actorError context permission space actor run
+  | not (memoryContextAllows permission context) = pure (Left (permissionError permission))
+  | space /= authorizedSpace = pure (Left (spaceError space authorizedSpace))
+  | actor /= authorizedActor = pure (Left (actorError actor authorizedActor))
+  | otherwise = run
+  where
+    authorizedSpace = memoryContextSpace context
+    authorizedActor = memoryContextRecordedActor context
+
+-- | Confine a deprecated compatibility operation to the explicit legacy memory space.
+inLegacyMemorySpaceOnly ::
+  (Applicative f) =>
+  (MemorySpaceId -> MemorySpaceId -> err) ->
+  MemorySpaceId ->
+  f (Either err a) ->
+  f (Either err a)
+inLegacyMemorySpaceOnly spaceError space run
+  | space /= legacyMemorySpaceId = pure (Left (spaceError space legacyMemorySpaceId))
+  | otherwise = run
 
 -- | Run the three gates for one memory space and a set of requested actions, and mint a
 -- 'MemoryAccessContext' only if every one of them passes.
diff --git a/test/Kioku/Api/AccessSpec.hs b/test/Kioku/Api/AccessSpec.hs
--- a/test/Kioku/Api/AccessSpec.hs
+++ b/test/Kioku/Api/AccessSpec.hs
@@ -26,7 +26,8 @@
       jsonRoundTripTests,
       objectRefTests,
       bindingTests,
-      contextTests
+      contextTests,
+      contextGateTests
     ]
 
 -- | A memory space id is Kioku's own object identifier, so Kioku owns its rules. It has to
@@ -297,6 +298,89 @@
         memoryContextAllows MemoryForget context @?= False
         memoryContextPermissions context @?= Set.singleton MemoryRead
     ]
+
+-- | The gate owns branch ordering but not an application's error type. Distinct sentinel
+-- constructors make both properties visible without importing either core write-error type.
+contextGateTests :: TestTree
+contextGateTests =
+  testGroup
+    "context operation gates"
+    [ testCase "permission denial precedes space and actor mismatches" do
+        let context = readOnlyContext
+        underMemoryContext
+          GatePermission
+          GateSpace
+          GateActor
+          context
+          MemoryRecord
+          spaceTwo
+          otherRecordedActor
+          successfulAction
+          @?= Just (Left (GatePermission MemoryRecord)),
+      testCase "space mismatch precedes actor mismatch" do
+        let context = assumeAuthorizedMemoryContext spaceOne testActor
+        underMemoryContext
+          GatePermission
+          GateSpace
+          GateActor
+          context
+          MemoryRecord
+          spaceTwo
+          otherRecordedActor
+          successfulAction
+          @?= Just (Left (GateSpace spaceTwo spaceOne)),
+      testCase "actor mismatch uses the injected constructor" do
+        let context = assumeAuthorizedMemoryContext spaceOne testActor
+        underMemoryContext
+          GatePermission
+          GateSpace
+          GateActor
+          context
+          MemoryRecord
+          spaceOne
+          otherRecordedActor
+          successfulAction
+          @?= Just (Left (GateActor otherRecordedActor (KnownPrincipal (actorPrincipal testActor)))),
+      testCase "a matching context runs the supplied action" do
+        let context = assumeAuthorizedMemoryContext spaceOne testActor
+        underMemoryContext
+          GatePermission
+          GateSpace
+          GateActor
+          context
+          MemoryRecord
+          spaceOne
+          (memoryContextRecordedActor context)
+          successfulAction
+          @?= successfulAction,
+      testCase "the legacy gate refuses another space through the injected constructor" do
+        inLegacyMemorySpaceOnly GateSpace spaceTwo successfulAction
+          @?= Just (Left (GateSpace spaceTwo legacyMemorySpaceId)),
+      testCase "the legacy gate runs an operation in the explicit legacy space" do
+        inLegacyMemorySpaceOnly GateSpace legacyMemorySpaceId successfulAction
+          @?= successfulAction
+    ]
+
+data GateError
+  = GatePermission !MemoryPermission
+  | GateSpace !MemorySpaceId !MemorySpaceId
+  | GateActor !RecordedPrincipal !RecordedPrincipal
+  deriving stock (Eq, Show)
+
+successfulAction :: Maybe (Either GateError Text)
+successfulAction = Just (Right "ran")
+
+readOnlyContext :: MemoryAccessContext
+readOnlyContext =
+  Internal.MemoryAccessContext
+    { Internal.memorySpaceId = spaceOne,
+      Internal.actor = testActor,
+      Internal.grantedPermissions = Set.singleton MemoryRead,
+      Internal.decisionToken = Nothing
+    }
+
+otherRecordedActor :: RecordedPrincipal
+otherRecordedActor = KnownPrincipal (expectRight (mkPrincipalRef "person_01h9xk3v7hf8b9c0d1e2f3g4h6"))
 
 testActor :: MemoryActor
 testActor = MemoryActor (expectRight (mkPrincipalRef "person_01h9xk3v7hf8b9c0d1e2f3g4h5"))