feat: harden workflow gates and expose token usage

frontend/src/store/sessionStore.test.ts

@@ -38,6 +38,26 @@ test("text_delta accumulates into transcript", () => {
   expect(useSessionStore.getState().transcript.at(-1)?.text).toBe("Analisi");
 });
 
+test("usage events accumulate billed token categories but keep the latest context occupancy", () => {
+  const store = useSessionStore.getState();
+  store.applyEvent({
+    type: "usage",
+    usage: { input: 60_000, cacheRead: 400_000, output: 20_000, totalTokens: 80_000, contextWindow: 200_000 },
+  });
+  store.applyEvent({
+    type: "usage",
+    usage: { input: 50_348, cacheRead: 233_344, output: 13_652, totalTokens: 55_935, contextWindow: 200_000 },
+  });
+
+  expect(useSessionStore.getState().tokenUsage).toEqual({
+    input: 110_348,
+    cacheRead: 633_344,
+    output: 33_652,
+    totalTokens: 55_935,
+    contextWindow: 200_000,
+  });
+});
+
 test("folds prompt, lifecycle, streams, tool updates, and gates in chronological order", () => {
   const store = useSessionStore.getState();
   store.setPhase("F1");

frontend/src/store/sessionStore.ts

@@ -1,5 +1,5 @@
 import { create } from "zustand";
-import type { ActivityEntry, StreamEvent, WidgetDescriptor } from "../api/types";
+import type { ActivityEntry, StreamEvent, TokenUsage, WidgetDescriptor } from "../api/types";
 
 interface Entry {
   role: "assistant";
@@ -21,6 +21,7 @@ interface SessionState {
   // system event — the only end-of-turn signal on the final workflow step, where no
   // follow-up gate arrives to release the spinner.
   agentActive: boolean;
+  tokenUsage: TokenUsage | null;
   applyEvent: (e: StreamEvent) => void;
   clearPending: () => void;
   resetSession: () => void;
@@ -65,6 +66,7 @@ const empty = {
   phaseError: null as string | null,
   seenGateIds: new Set<string>(),
   agentActive: false,
+  tokenUsage: null as TokenUsage | null,
 };
 
 export const useSessionStore = create<SessionState>((set) => ({
@@ -145,6 +147,17 @@ export const useSessionStore = create<SessionState>((set) => ({
         }
         return { activityLog, agentActive: true };
       }
+      if (e.type === "usage") {
+        return {
+          tokenUsage: {
+            input: (st.tokenUsage?.input ?? 0) + e.usage.input,
+            cacheRead: (st.tokenUsage?.cacheRead ?? 0) + e.usage.cacheRead,
+            output: (st.tokenUsage?.output ?? 0) + e.usage.output,
+            totalTokens: e.usage.totalTokens,
+            contextWindow: e.usage.contextWindow,
+          },
+        };
+      }
       if (e.type === "info") {
         const level = e.level ?? "info";
         const stepMessages = [...st.stepMessages, { level, text: e.text }];