diff --git a/packages/core/src/__tests__/agent-token-usage.test.ts b/packages/core/src/__tests__/agent-token-usage.test.ts
new file mode 100644
index 000000000..6e651f8ff
--- /dev/null
+++ b/packages/core/src/__tests__/agent-token-usage.test.ts
@@ -0,0 +1,72 @@
+import { afterEach, beforeEach, describe, expect, it } from "vitest";
+import { AgentStore } from "../agent-store.js";
+import { aggregateAgentTokenUsage } from "../agent-token-usage.js";
+import { createTaskStoreTestHarness } from "./store-test-helpers.js";
+
+describe("aggregateAgentTokenUsage", () => {
+  const harness = createTaskStoreTestHarness();
+  let agentStore: AgentStore;
+
+  beforeEach(async () => {
+    await harness.beforeEach();
+    agentStore = new AgentStore({ rootDir: harness.rootDir() });
+    await agentStore.init();
+  });
+
+  afterEach(async () => {
+    await harness.afterEach();
+  });
+
+  it("returns null when agent does not exist", async () => {
+    const result = await aggregateAgentTokenUsage({ taskStore: harness.store(), agentStore, agentId: "missing" });
+    expect(result).toBeNull();
+  });
+
+  it("returns null for ephemeral agents", async () => {
+    const ephemeral = await agentStore.createAgent({ name: "temp", role: "executor", reportsTo: "FN-1", metadata: { type: "spawned" } });
+    const result = await aggregateAgentTokenUsage({ taskStore: harness.store(), agentStore, agentId: ephemeral.id });
+    expect(result).toBeNull();
+  });
+
+  it("aggregates usage across windows", async () => {
+    const agent = await agentStore.createAgent({ name: "exec", role: "executor" });
+    await harness.store().createTask({
+      description: "recent",
+      assignedAgentId: agent.id,
+      tokenUsage: {
+        inputTokens: 100,
+        outputTokens: 10,
+        cachedTokens: 50,
+        cacheWriteTokens: 5,
+        totalTokens: 165,
+        firstUsedAt: "2026-05-13T09:00:00.000Z",
+        lastUsedAt: "2026-05-13T11:00:00.000Z",
+      },
+    });
+    await harness.store().createTask({
+      description: "older",
+      assignedAgentId: agent.id,
+      tokenUsage: {
+        inputTokens: 40,
+        outputTokens: 4,
+        cachedTokens: 10,
+        cacheWriteTokens: 1,
+        totalTokens: 55,
+        firstUsedAt: "2026-05-05T09:00:00.000Z",
+        lastUsedAt: "2026-05-05T11:00:00.000Z",
+      },
+    });
+
+    const result = await aggregateAgentTokenUsage({
+      taskStore: harness.store(),
+      agentStore,
+      agentId: agent.id,
+      now: new Date("2026-05-13T12:00:00.000Z"),
+    });
+
+    expect(result).not.toBeNull();
+    expect(result?.allTime).toMatchObject({ totalInputTokens: 140, totalCachedTokens: 60, totalCacheWriteTokens: 6, totalOutputTokens: 14, nTasks: 2 });
+    expect(result?.last24h).toMatchObject({ totalInputTokens: 100, totalCachedTokens: 50, totalCacheWriteTokens: 5, totalOutputTokens: 10, nTasks: 1 });
+    expect(result?.last7d).toMatchObject({ totalInputTokens: 100, totalCachedTokens: 50, totalCacheWriteTokens: 5, totalOutputTokens: 10, nTasks: 1 });
+  });
+});
diff --git a/packages/core/src/agent-token-usage.ts b/packages/core/src/agent-token-usage.ts
new file mode 100644
index 000000000..4496f7681
--- /dev/null
+++ b/packages/core/src/agent-token-usage.ts
@@ -0,0 +1,106 @@
+import type { AgentStore } from "./agent-store.js";
+import type { TaskStore } from "./store.js";
+import { isEphemeralAgent, type AgentRole } from "./types.js";
+
+export interface AgentTokenUsageWindowSummary {
+  totalInputTokens: number;
+  totalCachedTokens: number;
+  totalCacheWriteTokens: number;
+  totalOutputTokens: number;
+  nTasks: number;
+  hitRatio: number;
+}
+
+export interface AgentTokenUsageSummary {
+  agentId: string;
+  role: AgentRole;
+  last24h: AgentTokenUsageWindowSummary;
+  last7d: AgentTokenUsageWindowSummary;
+  allTime: AgentTokenUsageWindowSummary;
+}
+
+export async function aggregateAgentTokenUsage({
+  taskStore,
+  agentStore,
+  agentId,
+  now = new Date(),
+}: {
+  taskStore: TaskStore;
+  agentStore: AgentStore;
+  agentId: string;
+  now?: Date;
+}): Promise<AgentTokenUsageSummary | null> {
+  const agent = await agentStore.getAgent(agentId);
+  if (!agent || isEphemeralAgent(agent)) {
+    return null;
+  }
+
+  const tasks = await taskStore.listTasks({ slim: true, includeArchived: true });
+  const nowMs = now.getTime();
+  const last24hMs = nowMs - (24 * 60 * 60 * 1000);
+  const last7dMs = nowMs - (7 * 24 * 60 * 60 * 1000);
+
+  const allTime = createWindowSummary();
+  const last24h = createWindowSummary();
+  const last7d = createWindowSummary();
+
+  for (const task of tasks) {
+    if (!task.tokenUsage) continue;
+    const matchesAgent = task.assignedAgentId === agentId || task.sourceAgentId === agentId || task.checkedOutBy === agentId;
+    if (!matchesAgent) continue;
+
+    const usage = task.tokenUsage;
+    applyTaskUsage(allTime, usage.inputTokens ?? 0, usage.cachedTokens ?? 0, usage.outputTokens ?? 0, usage.cacheWriteTokens ?? 0);
+
+    const lastUsedAtMs = Date.parse(usage.lastUsedAt ?? "");
+    if (!Number.isFinite(lastUsedAtMs)) continue;
+
+    if (lastUsedAtMs >= last24hMs) {
+      applyTaskUsage(last24h, usage.inputTokens ?? 0, usage.cachedTokens ?? 0, usage.outputTokens ?? 0, usage.cacheWriteTokens ?? 0);
+    }
+    if (lastUsedAtMs >= last7dMs) {
+      applyTaskUsage(last7d, usage.inputTokens ?? 0, usage.cachedTokens ?? 0, usage.outputTokens ?? 0, usage.cacheWriteTokens ?? 0);
+    }
+  }
+
+  return {
+    agentId,
+    role: agent.role as AgentRole,
+    last24h: finalizeWindowSummary(last24h),
+    last7d: finalizeWindowSummary(last7d),
+    allTime: finalizeWindowSummary(allTime),
+  };
+}
+
+function createWindowSummary(): AgentTokenUsageWindowSummary {
+  return {
+    totalInputTokens: 0,
+    totalCachedTokens: 0,
+    totalCacheWriteTokens: 0,
+    totalOutputTokens: 0,
+    nTasks: 0,
+    hitRatio: 0,
+  };
+}
+
+function applyTaskUsage(
+  summary: AgentTokenUsageWindowSummary,
+  inputTokens: number,
+  cachedTokens: number,
+  outputTokens: number,
+  cacheWriteTokens: number,
+): void {
+  summary.totalInputTokens += inputTokens;
+  summary.totalCachedTokens += cachedTokens;
+  summary.totalCacheWriteTokens += cacheWriteTokens;
+  summary.totalOutputTokens += outputTokens;
+  summary.nTasks += 1;
+}
+
+function finalizeWindowSummary(summary: AgentTokenUsageWindowSummary): AgentTokenUsageWindowSummary {
+  const denominator = summary.totalInputTokens + summary.totalCachedTokens;
+  return {
+    ...summary,
+    hitRatio: denominator > 0 ? summary.totalCachedTokens / denominator : 0,
+  };
+}
diff --git a/packages/core/src/index.ts b/packages/core/src/index.ts
index 4c4c98e59..046476818 100644
--- a/packages/core/src/index.ts
+++ b/packages/core/src/index.ts
@@ -87,6 +87,8 @@ export type {
   AgentProvisioningPolicyDecision,
 } from "./agent-provisioning-policy.js";
 export { TaskStore } from "./store.js";
+export { aggregateAgentTokenUsage } from "./agent-token-usage.js";
+export type { AgentTokenUsageSummary, AgentTokenUsageWindowSummary } from "./agent-token-usage.js";
 export {
   STALLED_REVIEW_REENQUEUE_THRESHOLD,
   STALLED_REVIEW_INVALID_TRANSITION_THRESHOLD,