diff --git a/open-sse/handlers/chatCore.ts b/open-sse/handlers/chatCore.ts
index 8331cd5780..201b8957f2 100644
--- a/open-sse/handlers/chatCore.ts
+++ b/open-sse/handlers/chatCore.ts
@@ -5,6 +5,7 @@ import { extractSystemRoleMessages } from "./chatCore/claudeSystemRole.ts";
 export { extractSystemRoleMessages } from "./chatCore/claudeSystemRole.ts";
 import { checkIdempotencyCache } from "./chatCore/idempotency.ts";
 import { checkSemanticCache } from "./chatCore/semanticCache.ts";
+import { applyClientUsageBuffer } from "./chatCore/clientUsageBuffer.ts";
 import { sanitizeChatRequestBody } from "./chatCore/sanitization.ts";
 import {
   getHeaderValueCaseInsensitive,
@@ -68,7 +69,6 @@ import { resolveStreamReadinessTimeout } from "../utils/streamReadinessPolicy.ts
 import { createStreamController, pipeWithDisconnect } from "../utils/streamHandler.ts";
 import * as streamFailure from "../utils/streamFailureFinalization.ts";
 import { createSseHeartbeatTransform, shapeForClientFormat } from "../utils/sseHeartbeat.ts";
-import { addBufferToUsage, filterUsageForFormat, estimateUsage } from "../utils/usageTracking.ts";
 import {
   refreshWithRetry,
   isUnrecoverableRefreshError,
@@ -3470,20 +3470,7 @@ export async function handleChatCore({
       translatedResponse = sanitizeOpenAIResponse(translatedResponse);
     }
 
-    // Add buffer and filter usage for client (to prevent CLI context errors)
-    if (translatedResponse?.usage) {
-      const buffered = addBufferToUsage(translatedResponse.usage);
-      translatedResponse.usage = filterUsageForFormat(buffered, clientResponseFormat);
-    } else {
-      // Fallback: estimate usage when provider returned no usage block
-      const contentLength = JSON.stringify(
-        translatedResponse?.choices?.[0]?.message?.content || ""
-      ).length;
-      if (contentLength > 0) {
-        const estimated = estimateUsage(body, contentLength, clientResponseFormat);
-        translatedResponse.usage = filterUsageForFormat(estimated, clientResponseFormat);
-      }
-    }
+    applyClientUsageBuffer(translatedResponse, body, clientResponseFormat);
 
     if (memoryOwnerId && memorySettings?.enabled && memorySettings.maxTokens > 0) {
       const requestMemoryText = extractMemoryTextFromRequestBody(body as Record<string, unknown>);
diff --git a/open-sse/handlers/chatCore/clientUsageBuffer.ts b/open-sse/handlers/chatCore/clientUsageBuffer.ts
new file mode 100644
index 0000000000..12101b73d0
--- /dev/null
+++ b/open-sse/handlers/chatCore/clientUsageBuffer.ts
@@ -0,0 +1,54 @@
+/**
+ * chatCore client usage buffer/estimate (Quality Gate v2 / Fase 9 — chatCore god-file
+ * decomposition, #3501).
+ *
+ * Extracted from handleChatCore's non-streaming success path: add a buffer to the response usage
+ * and filter it for the client format (to prevent CLI context errors); if the provider returned no
+ * usage block, fall back to estimating from the serialized content length. Mutates
+ * `translatedResponse.usage` in place — byte-identical to the previous inline block, including the
+ * `?.usage` guard, the `JSON.stringify(... || "")` content-length, and the `> 0` estimate gate.
+ */
+import {
+  addBufferToUsage as defaultAddBuffer,
+  filterUsageForFormat as defaultFilterUsage,
+  estimateUsage as defaultEstimateUsage,
+} from "../../utils/usageTracking.ts";
+
+type ResponseLike = {
+  usage?: unknown;
+  choices?: Array<{ message?: { content?: unknown } }>;
+} | null | undefined;
+
+export interface ClientUsageBufferDeps {
+  addBufferToUsage: typeof defaultAddBuffer;
+  filterUsageForFormat: typeof defaultFilterUsage;
+  estimateUsage: typeof defaultEstimateUsage;
+}
+
+const DEFAULT_DEPS: ClientUsageBufferDeps = {
+  addBufferToUsage: defaultAddBuffer,
+  filterUsageForFormat: defaultFilterUsage,
+  estimateUsage: defaultEstimateUsage,
+};
+
+export function applyClientUsageBuffer(
+  translatedResponse: ResponseLike,
+  body: unknown,
+  clientResponseFormat: unknown,
+  deps: ClientUsageBufferDeps = DEFAULT_DEPS
+): void {
+  // Add buffer and filter usage for client (to prevent CLI context errors)
+  if (translatedResponse?.usage) {
+    const buffered = deps.addBufferToUsage(translatedResponse.usage);
+    translatedResponse.usage = deps.filterUsageForFormat(buffered, clientResponseFormat);
+  } else {
+    // Fallback: estimate usage when provider returned no usage block
+    const contentLength = JSON.stringify(
+      translatedResponse?.choices?.[0]?.message?.content || ""
+    ).length;
+    if (contentLength > 0) {
+      const estimated = deps.estimateUsage(body, contentLength, clientResponseFormat);
+      translatedResponse.usage = deps.filterUsageForFormat(estimated, clientResponseFormat);
+    }
+  }
+}
diff --git a/tests/unit/chatcore-client-usage-buffer.test.ts b/tests/unit/chatcore-client-usage-buffer.test.ts
new file mode 100644
index 0000000000..ab58ab4fd2
--- /dev/null
+++ b/tests/unit/chatcore-client-usage-buffer.test.ts
@@ -0,0 +1,78 @@
+// Characterization of applyClientUsageBuffer — the non-streaming usage buffer/estimate block
+// extracted from handleChatCore (chatCore god-file decomposition, #3501). Deps are injected so the
+// buffer-vs-estimate branch and the in-place mutation of translatedResponse.usage are observable.
+// Locks: usage present → buffer+filter; usage absent → estimate from content length; empty content
+// (length 2 from JSON.stringify("")) still estimates; the mutation target is translatedResponse.
+import { test } from "node:test";
+import assert from "node:assert/strict";
+
+const { applyClientUsageBuffer } = await import(
+  "../../open-sse/handlers/chatCore/clientUsageBuffer.ts"
+);
+
+function makeDeps(overrides: Record<string, unknown> = {}) {
+  const calls = { buffer: [] as unknown[], estimate: [] as unknown[], filter: [] as unknown[] };
+  const deps = {
+    addBufferToUsage: (u: unknown) => {
+      calls.buffer.push(u);
+      return { ...(u as object), _buffered: true };
+    },
+    estimateUsage: (...a: unknown[]) => {
+      calls.estimate.push(a);
+      return { _estimated: true };
+    },
+    filterUsageForFormat: (u: unknown, _fmt: unknown) => {
+      calls.filter.push(u);
+      return { ...(u as object), _filtered: true };
+    },
+    ...overrides,
+  } as Parameters<typeof applyClientUsageBuffer>[3];
+  return { deps, calls };
+}
+
+test("usage present → buffer then filter, mutates in place", () => {
+  const { deps, calls } = makeDeps();
+  const resp: Record<string, unknown> = { usage: { prompt_tokens: 5 } };
+  applyClientUsageBuffer(resp, { messages: [] }, "openai", deps);
+  assert.equal(calls.buffer.length, 1);
+  assert.equal(calls.estimate.length, 0);
+  assert.equal((resp.usage as Record<string, unknown>)._buffered, true);
+  assert.equal((resp.usage as Record<string, unknown>)._filtered, true);
+});
+
+test("no usage but content present → estimate then filter", () => {
+  const { deps, calls } = makeDeps();
+  const resp: Record<string, unknown> = {
+    choices: [{ message: { content: "hello world" } }],
+  };
+  applyClientUsageBuffer(resp, { messages: [] }, "openai", deps);
+  assert.equal(calls.buffer.length, 0);
+  assert.equal(calls.estimate.length, 1);
+  assert.equal((resp.usage as Record<string, unknown>)._estimated, true);
+  assert.equal((resp.usage as Record<string, unknown>)._filtered, true);
+  // estimateUsage receives (body, contentLength, format)
+  const args = calls.estimate[0] as unknown[];
+  assert.equal(args[2], "openai");
+  assert.equal(typeof args[1], "number");
+});
+
+test("empty content → JSON.stringify('') length 2 > 0 still estimates", () => {
+  const { deps, calls } = makeDeps();
+  const resp: Record<string, unknown> = {};
+  applyClientUsageBuffer(resp, {}, "claude", deps);
+  // content "" → JSON.stringify("") = '""' length 2 → contentLength 2 > 0
+  assert.equal(calls.estimate.length, 1);
+  const args = calls.estimate[0] as unknown[];
+  assert.equal(args[1], 2);
+});
+
+test("content length is computed from choices[0].message.content", () => {
+  const { deps, calls } = makeDeps();
+  const resp: Record<string, unknown> = {
+    choices: [{ message: { content: "abc" } }],
+  };
+  applyClientUsageBuffer(resp, {}, "openai", deps);
+  // JSON.stringify("abc") = '"abc"' → length 5
+  const args = calls.estimate[0] as unknown[];
+  assert.equal(args[1], 5);
+});