diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5f55ce7f..19cc366b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -51,6 +51,9 @@ changes accumulate. Track in-flight protocol changes via PRs touching
 
 ### Fixed
 
+- `chat/usage` reducer now updates the matching turn in `turns` when the
+  `turnId` refers to a completed (non-active) turn, rather than ignoring the
+  action.
 - Corrected the `ACTION_INTRODUCED_IN` entries for `annotations/set`,
   `annotations/removed`, `annotations/entrySet`, and `annotations/entryRemoved`
   from `0.3.0` to `0.4.0`. The annotations channel first shipped in the
diff --git a/clients/go/CHANGELOG.md b/clients/go/CHANGELOG.md
index e8ff02d6..e1d93a2f 100644
--- a/clients/go/CHANGELOG.md
+++ b/clients/go/CHANGELOG.md
@@ -43,6 +43,12 @@ tag whose matching `## [X.Y.Z]` heading is missing from this file.
   published tools by re-dispatching `SessionActiveClientSetAction` with its
   full, updated entry.
 
+### Fixed
+
+- `ApplyActionToChat` now updates the matching turn in `Turns` when a
+  `ChatUsageAction` targets a completed (non-active) turn, rather than
+  ignoring the action.
+
 ## [0.4.0] — 2026-06-19
 
 Implements AHP 0.4.0.
diff --git a/clients/go/ahp/reducers.go b/clients/go/ahp/reducers.go
index 1348b7a3..59543cdf 100644
--- a/clients/go/ahp/reducers.go
+++ b/clients/go/ahp/reducers.go
@@ -472,12 +472,19 @@ func ApplyActionToChat(state *ahptypes.ChatState, action ahptypes.StateAction) R
 			return tc
 		})
 	case *ahptypes.ChatUsageAction:
-		if state.ActiveTurn == nil || state.ActiveTurn.Id != a.TurnId {
-			return ReduceOutcomeNoOp
+		if state.ActiveTurn != nil && state.ActiveTurn.Id == a.TurnId {
+			usage := a.Usage
+			state.ActiveTurn.Usage = &usage
+			return ReduceOutcomeApplied
 		}
-		usage := a.Usage
-		state.ActiveTurn.Usage = &usage
-		return ReduceOutcomeApplied
+		for i := range state.Turns {
+			if state.Turns[i].Id == a.TurnId {
+				usage := a.Usage
+				state.Turns[i].Usage = &usage
+				return ReduceOutcomeApplied
+			}
+		}
+		return ReduceOutcomeNoOp
 	case *ahptypes.ChatReasoningAction:
 		return updateResponsePart(state, a.TurnId, a.PartId, func(p *ahptypes.ResponsePart) {
 			if r, ok := p.Value.(*ahptypes.ReasoningResponsePart); ok {
diff --git a/clients/kotlin/CHANGELOG.md b/clients/kotlin/CHANGELOG.md
index 9a564c32..7aac71c3 100644
--- a/clients/kotlin/CHANGELOG.md
+++ b/clients/kotlin/CHANGELOG.md
@@ -45,6 +45,12 @@ versions (`*-SNAPSHOT`) are explicitly rejected by the publish pipeline; bump
   published tools by re-dispatching `StateActionSessionActiveClientSet` with its
   full, updated entry.
 
+### Fixed
+
+- `chatReducer` now updates the matching turn in `turns` when a
+  `StateActionChatUsage` targets a completed (non-active) turn, rather than
+  ignoring the action.
+
 ## [0.4.0] — 2026-06-19
 
 Implements AHP 0.4.0.
diff --git a/clients/kotlin/src/main/kotlin/com/microsoft/agenthostprotocol/Reducers.kt b/clients/kotlin/src/main/kotlin/com/microsoft/agenthostprotocol/Reducers.kt
index 7d78ad5d..1409aec9 100644
--- a/clients/kotlin/src/main/kotlin/com/microsoft/agenthostprotocol/Reducers.kt
+++ b/clients/kotlin/src/main/kotlin/com/microsoft/agenthostprotocol/Reducers.kt
@@ -975,10 +975,17 @@ public fun chatReducer(state: ChatState, action: StateAction): ChatState = when
     is StateActionChatUsage -> {
         val a = action.value
         val activeTurn = state.activeTurn
-        if (activeTurn == null || activeTurn.id != a.turnId) {
-            state
-        } else {
+        if (activeTurn != null && activeTurn.id == a.turnId) {
             state.copy(activeTurn = activeTurn.copy(usage = a.usage))
+        } else {
+            val idx = state.turns.indexOfFirst { it.id == a.turnId }
+            if (idx < 0) {
+                state
+            } else {
+                val turns = state.turns.toMutableList()
+                turns[idx] = turns[idx].copy(usage = a.usage)
+                state.copy(turns = turns)
+            }
         }
     }
 
diff --git a/clients/rust/CHANGELOG.md b/clients/rust/CHANGELOG.md
index ed90eda0..d426b4e5 100644
--- a/clients/rust/CHANGELOG.md
+++ b/clients/rust/CHANGELOG.md
@@ -52,6 +52,12 @@ matching `## [X.Y.Z]` heading is missing from this file.
   published tools by re-dispatching `SessionActiveClientSet` with its full,
   updated entry.
 
+### Fixed
+
+- `apply_action_to_chat` now updates the matching turn in `turns` when a
+  `StateAction::ChatUsage` targets a completed (non-active) turn, rather than
+  ignoring the action.
+
 ## [0.4.0] — 2026-06-19
 
 Implements AHP 0.4.0.
diff --git a/clients/rust/crates/ahp/src/reducers.rs b/clients/rust/crates/ahp/src/reducers.rs
index 74e83755..c0f220d2 100644
--- a/clients/rust/crates/ahp/src/reducers.rs
+++ b/clients/rust/crates/ahp/src/reducers.rs
@@ -850,13 +850,16 @@ pub fn apply_action_to_chat(state: &mut ChatState, action: &StateAction) -> Redu
         }
         StateAction::ChatToolCallContentChanged(a) => apply_tool_call_content_changed(state, a),
         StateAction::ChatUsage(a) => {
-            let Some(active) = state.active_turn.as_mut() else {
+            if let Some(active) = state.active_turn.as_mut() {
+                if active.id == a.turn_id {
+                    active.usage = Some(a.usage.clone());
+                    return ReduceOutcome::Applied;
+                }
+            }
+            let Some(turn) = state.turns.iter_mut().find(|t| t.id == a.turn_id) else {
                 return ReduceOutcome::NoOp;
             };
-            if active.id != a.turn_id {
-                return ReduceOutcome::NoOp;
-            }
-            active.usage = Some(a.usage.clone());
+            turn.usage = Some(a.usage.clone());
             ReduceOutcome::Applied
         }
         StateAction::ChatReasoning(a) => update_response_part(state, &a.turn_id, &a.part_id, |p| {
diff --git a/clients/swift/AgentHostProtocol/Sources/AgentHostProtocol/Reducers.swift b/clients/swift/AgentHostProtocol/Sources/AgentHostProtocol/Reducers.swift
index 9ce944a4..a90e3e19 100644
--- a/clients/swift/AgentHostProtocol/Sources/AgentHostProtocol/Reducers.swift
+++ b/clients/swift/AgentHostProtocol/Sources/AgentHostProtocol/Reducers.swift
@@ -338,12 +338,17 @@ public func chatReducer(state: ChatState, action: StateAction) -> ChatState {
         }
 
     case .chatUsage(let a):
-        guard var activeTurn = state.activeTurn, activeTurn.id == a.turnId else {
+        if var activeTurn = state.activeTurn, activeTurn.id == a.turnId {
+            activeTurn.usage = a.usage
+            var next = state
+            next.activeTurn = activeTurn
+            return next
+        }
+        guard let idx = state.turns.firstIndex(where: { $0.id == a.turnId }) else {
             return state
         }
-        activeTurn.usage = a.usage
         var next = state
-        next.activeTurn = activeTurn
+        next.turns[idx].usage = a.usage
         return next
 
     case .chatReasoning(let a):
diff --git a/clients/swift/CHANGELOG.md b/clients/swift/CHANGELOG.md
index c6d857ac..7950135c 100644
--- a/clients/swift/CHANGELOG.md
+++ b/clients/swift/CHANGELOG.md
@@ -47,6 +47,12 @@ the tag matches the version pinned in [`VERSION`](VERSION).
   published tools by re-dispatching `StateAction.sessionActiveClientSet` with its
   full, updated entry.
 
+### Fixed
+
+- `chatReducer` now updates the matching turn in `turns` when a
+  `StateAction.chatUsage` targets a completed (non-active) turn, rather than
+  ignoring the action.
+
 ## [0.4.0] — 2026-06-19
 
 Implements AHP 0.4.0.
diff --git a/clients/typescript/CHANGELOG.md b/clients/typescript/CHANGELOG.md
index ab7e9ef7..eaec32e0 100644
--- a/clients/typescript/CHANGELOG.md
+++ b/clients/typescript/CHANGELOG.md
@@ -51,6 +51,8 @@ hotfix escape hatch.
 
 ### Fixed
 
+- `chatReducer` now updates the matching turn in `turns` when a `chat/usage`
+  action targets a completed (non-active) turn, rather than ignoring the action.
 - Hosted session summary caches now apply `_meta` updates from
   `root/sessionSummaryChanged` notifications.
 - Corrected the `ACTION_INTRODUCED_IN` entries for `annotations/set`,
diff --git a/types/channels-chat/reducer.ts b/types/channels-chat/reducer.ts
index 24d93841..161b9589 100644
--- a/types/channels-chat/reducer.ts
+++ b/types/channels-chat/reducer.ts
@@ -502,14 +502,21 @@ export function chatReducer(state: ChatState, action: ChatAction, log?: (msg: st
       });
 
 
-    case ActionType.ChatUsage:
-      if (!state.activeTurn || state.activeTurn.id !== action.turnId) {
+    case ActionType.ChatUsage: {
+      if (state.activeTurn && state.activeTurn.id === action.turnId) {
+        return {
+          ...state,
+          activeTurn: { ...state.activeTurn, usage: action.usage },
+        };
+      }
+      const idx = state.turns.findIndex(t => t.id === action.turnId);
+      if (idx === -1) {
         return state;
       }
-      return {
-        ...state,
-        activeTurn: { ...state.activeTurn, usage: action.usage },
-      };
+      const turns = state.turns.slice();
+      turns[idx] = { ...turns[idx], usage: action.usage };
+      return { ...state, turns };
+    }
 
     case ActionType.ChatReasoning:
       return updateResponsePart(state, action.turnId, action.partId, part => {