diff --git a/changelog.d/features/9571-plugin-streaming-usage-timing.md b/changelog.d/features/9571-plugin-streaming-usage-timing.md
new file mode 100644
index 0000000000..84720d2df5
--- /dev/null
+++ b/changelog.d/features/9571-plugin-streaming-usage-timing.md
@@ -0,0 +1,11 @@
+- **feat(plugins):** add onStreamComplete built-in event exposing streaming usage and timing (#9571)
+
+  Adds a new `onStreamComplete` plugin event that fires after an SSE stream is fully
+  consumed, carrying usage token counts and timing metrics (latency, TTFT). Built-in
+  events now include `onStreamComplete` as a fire-and-forget lifecycle hook.
+
+  Payload: `status`, `usage` (prompt_tokens, completion_tokens, reasoning_tokens,
+  cache_read_input_tokens, cache_creation_input_tokens), `timing` (latencyMs, ttft),
+  `model`, `provider`, `errorCode`.
+
+  Non-breaking — existing `onResponse` hooks with `{ streamed: true }` remain unchanged.
diff --git a/open-sse/handlers/chatCore.ts b/open-sse/handlers/chatCore.ts
index e71de50bd8..4130bed738 100644
--- a/open-sse/handlers/chatCore.ts
+++ b/open-sse/handlers/chatCore.ts
@@ -251,7 +251,10 @@ import { recordCompressionCacheStats } from "./chatCore/compressionCacheStats.ts
 import { writeCavemanOutputAnalytics } from "./chatCore/cavemanOutputAnalytics.ts";
 import { scheduleQuotaShareConsumption } from "./chatCore/quotaShareConsumption.ts";
 import { emitRequestGamificationEvent } from "./chatCore/gamificationEvent.ts";
-import { runPluginOnResponseHook } from "./chatCore/pluginOnResponse.ts";
+import {
+  runPluginOnResponseHook,
+  runPluginOnStreamCompleteHook,
+} from "./chatCore/pluginOnResponse.ts";
 import { scheduleStreamingQuotaShareConsumption } from "./chatCore/streamingQuotaShare.ts";
 import { recordStreamingUsageStats } from "./chatCore/streamingUsageStats.ts";
 import { recordStreamingCost } from "./chatCore/streamingCost.ts";
@@ -4934,6 +4937,17 @@ export async function handleChatCore({
       streamUsage,
       log,
     });
+
+    // Plugin onStreamComplete hook — fire-and-forget, fail-open (#9571)
+    runPluginOnStreamCompleteHook({
+      status: normalizedStreamStatus,
+      usage: streamUsage as Record<string, unknown> | undefined,
+      ttft,
+      model,
+      provider,
+      errorCode: streamErrorCode,
+      startTime,
+    });
   };
 
   const streamFailureFinalizers = streamFailure.createStreamFailureFinalizers({
diff --git a/open-sse/handlers/chatCore/pluginOnResponse.ts b/open-sse/handlers/chatCore/pluginOnResponse.ts
index 63055e2e74..eb100c5b5e 100644
--- a/open-sse/handlers/chatCore/pluginOnResponse.ts
+++ b/open-sse/handlers/chatCore/pluginOnResponse.ts
@@ -45,3 +45,57 @@ export async function runPluginOnResponseHook(args: {
     /* plugin onResponse optional */
   }
 }
+
+/**
+ * Payload passed to plugin onStreamComplete hooks after a streaming response is consumed.
+ * Carries usage token counts, timing metrics (latency, TTFT), model, provider, and error code.
+ */
+export type PluginOnStreamCompletePayload = {
+  status: number;
+  usage?: {
+    prompt_tokens?: number;
+    completion_tokens?: number;
+    reasoning_tokens?: number;
+    cache_read_input_tokens?: number;
+    cache_creation_input_tokens?: number;
+  };
+  timing?: {
+    latencyMs: number;
+    ttft?: number;
+  };
+  model?: string;
+  provider?: string;
+  errorCode?: string;
+};
+
+/**
+ * Run plugin onStreamComplete hooks — fire-and-forget and fail-open.
+ * Called inside the onStreamComplete callback (chatCore.ts) where usage and timing data
+ * converge after an SSE stream is fully consumed.
+ */
+export async function runPluginOnStreamCompleteHook(args: {
+  status: number;
+  usage?: Record<string, unknown>;
+  ttft?: number;
+  model: string | null | undefined;
+  provider: string | null | undefined;
+  errorCode?: string | null | undefined;
+  startTime: number;
+}): Promise<void> {
+  try {
+    const { runOnStreamComplete } = await import("@/lib/plugins/hooks");
+    runOnStreamComplete({
+      status: args.status,
+      usage: args.usage as PluginOnStreamCompletePayload["usage"],
+      timing: {
+        latencyMs: Date.now() - args.startTime,
+        ttft: args.ttft,
+      },
+      model: args.model ?? undefined,
+      provider: args.provider ?? undefined,
+      errorCode: args.errorCode ?? undefined,
+    }).catch(() => {});
+  } catch (_) {
+    /* plugin onStreamComplete optional */
+  }
+}
diff --git a/tests/unit/chatcore-plugin-onresponse.test.ts b/tests/unit/chatcore-plugin-onresponse.test.ts
index ffd8161fd9..5125a07647 100644
--- a/tests/unit/chatcore-plugin-onresponse.test.ts
+++ b/tests/unit/chatcore-plugin-onresponse.test.ts
@@ -6,9 +6,7 @@
 import { test, after } from "node:test";
 import assert from "node:assert/strict";
 
-const { registerHook, unregisterHook } = await import("../../src/lib/plugins/hooks.ts");
-const { runPluginOnResponseHook } =
-  await import("../../open-sse/handlers/chatCore/pluginOnResponse.ts");
+const { registerHook, unregisterHook } = await import("../../src/lib/plugins/hooks.ts");const { runPluginOnResponseHook, runPluginOnStreamCompleteHook } = await import("../../open-sse/handlers/chatCore/pluginOnResponse.ts");
 
 async function waitFor(pred: () => boolean, timeoutMs = 2000): Promise<void> {
   const deadline = Date.now() + timeoutMs;
@@ -19,6 +17,7 @@ async function waitFor(pred: () => boolean, timeoutMs = 2000): Promise<void> {
 
 after(() => {
   unregisterHook("onResponse", "test-onresponse-plugin");
+  unregisterHook("onStreamComplete", "test-onstreamcomplete-plugin");
 });
 
 test("no registered hooks → resolves without throwing (no-op)", async () => {
@@ -143,3 +142,140 @@ test("a throwing hook never rejects the caller (fail-open)", async () => {
   );
   await new Promise((r) => setTimeout(r, 30));
 });
+
+// ── onStreamComplete hook tests (#9571) ──
+
+test("onStreamComplete: no registered hooks resolves without throwing (no-op)", async () => {
+  const start = Date.now();
+  await assert.doesNotReject(
+    runPluginOnStreamCompleteHook({
+      status: 200,
+      usage: { prompt_tokens: 10, completion_tokens: 20 },
+      ttft: 150,
+      model: "gpt-4",
+      provider: "openai",
+      errorCode: undefined,
+      startTime: start - 500,
+    })
+  );
+});
+
+test("onStreamComplete: registered hook receives usage + timing payload", async () => {
+  let captured: Record<string, unknown> | undefined;
+  registerHook(
+    "onStreamComplete",
+    "test-onstreamcomplete-plugin",
+    async (payload: Record<string, unknown>) => {
+      captured = payload;
+    }
+  );
+
+  const startTime = Date.now() - 500;
+  await runPluginOnStreamCompleteHook({
+    status: 200,
+    usage: { prompt_tokens: 42, completion_tokens: 100, reasoning_tokens: 5 },
+    ttft: 200,
+    model: "claude-3-opus",
+    provider: "anthropic",
+    errorCode: undefined,
+    startTime,
+  });
+
+  await waitFor(() => captured !== undefined);
+  assert.ok(captured, "expected onStreamComplete hook to be invoked");
+
+  // payload shape: status, usage, timing, model, provider
+  assert.equal(captured!.status, 200);
+  assert.ok(captured!.usage, "usage should be present");
+  assert.equal((captured!.usage as Record<string, number>).prompt_tokens, 42);
+  assert.equal((captured!.usage as Record<string, number>).completion_tokens, 100);
+  assert.equal((captured!.usage as Record<string, number>).reasoning_tokens, 5);
+
+  assert.ok(captured!.timing, "timing should be present");
+  const timing = captured!.timing as Record<string, number>;
+  assert.equal(timing.ttft, 200);
+  assert.ok(timing.latencyMs > 450, "latencyMs should be near 500");
+
+  assert.equal(captured!.model, "claude-3-opus");
+  assert.equal(captured!.provider, "anthropic");
+  assert.equal(captured!.errorCode, undefined);
+});
+
+test("onStreamComplete: payload includes cache token fields when present", async () => {
+  let captured: Record<string, unknown> | undefined;
+  registerHook(
+    "onStreamComplete",
+    "test-onstreamcomplete-plugin",
+    async (payload: Record<string, unknown>) => {
+      captured = payload;
+    }
+  );
+
+  await runPluginOnStreamCompleteHook({
+    status: 200,
+    usage: {
+      prompt_tokens: 50,
+      completion_tokens: 30,
+      cache_read_input_tokens: 20,
+      cache_creation_input_tokens: 10,
+    },
+    ttft: 100,
+    model: "gpt-4",
+    provider: "openai",
+    errorCode: undefined,
+    startTime: Date.now(),
+  });
+
+  await waitFor(() => captured !== undefined);
+  assert.ok(captured);
+  const usage = captured!.usage as Record<string, number>;
+  assert.equal(usage.cache_read_input_tokens, 20);
+  assert.equal(usage.cache_creation_input_tokens, 10);
+});
+
+test("onStreamComplete: throwing hook never rejects the caller (fail-open)", async () => {
+  registerHook("onStreamComplete", "test-onstreamcomplete-plugin", async () => {
+    throw new Error("stream-complete-boom");
+  });
+
+  await assert.doesNotReject(
+    runPluginOnStreamCompleteHook({
+      status: 500,
+      usage: undefined,
+      ttft: undefined,
+      model: "gpt-4",
+      provider: "openai",
+      errorCode: "upstream_error",
+      startTime: Date.now(),
+    })
+  );
+  await new Promise((r) => setTimeout(r, 30));
+});
+
+test("onStreamComplete: errorCode is passed through when provided", async () => {
+  let captured: Record<string, unknown> | undefined;
+  registerHook(
+    "onStreamComplete",
+    "test-onstreamcomplete-plugin",
+    async (payload: Record<string, unknown>) => {
+      captured = payload;
+    }
+  );
+
+  await runPluginOnStreamCompleteHook({
+    status: 502,
+    usage: undefined,
+    ttft: undefined,
+    model: "grok-3",
+    provider: "xai",
+    errorCode: "upstream_timeout",
+    startTime: Date.now(),
+  });
+
+  await waitFor(() => captured !== undefined);
+  assert.ok(captured);
+  assert.equal(captured!.status, 502);
+  assert.equal(captured!.errorCode, "upstream_timeout");
+  assert.equal(captured!.model, "grok-3");
+  assert.equal(captured!.provider, "xai");
+});