diff --git a/open-sse/utils/usageTracking.js b/open-sse/utils/usageTracking.js
index 24518ef3..0d66f6f3 100644
--- a/open-sse/utils/usageTracking.js
+++ b/open-sse/utils/usageTracking.js
@@ -190,7 +190,10 @@ export function canonicalizeUsage(usage) {
     prompt = prompt + cached + cacheCreation;
   } else {
     // OpenAI/Gemini path (or already-canonical input): prompt already includes cached_tokens.
-    cached = num(usage.cached_tokens);
+    // Mirror the cacheCreation fallback above: buildUsage() only ever emits the
+    // nested prompt_tokens_details.cached_tokens shape, so without this the
+    // cache-read count is silently dropped on every buildUsage()-derived usage.
+    cached = num(usage.cached_tokens ?? usage.prompt_tokens_details?.cached_tokens);
   }
 
   const result = {
diff --git a/tests/unit/cached-token-usage.test.js b/tests/unit/cached-token-usage.test.js
index 878110d0..9b3ebd97 100644
--- a/tests/unit/cached-token-usage.test.js
+++ b/tests/unit/cached-token-usage.test.js
@@ -1,7 +1,7 @@
 import { describe, it, expect } from "vitest";
 import { canonicalizeUsage, extractUsage, mergeUsage } from "../../open-sse/utils/usageTracking.js";
 import { calculateCostFromTokens } from "../../open-sse/providers/pricing.js";
-import { toOpenAIUsage } from "../../open-sse/translator/concerns/usage.js";
+import { buildUsage, toOpenAIUsage } from "../../open-sse/translator/concerns/usage.js";
 
 // Canonical convention (single source of truth for storage + cost):
 //   prompt_tokens             = total input INCLUDING cache read + cache creation
@@ -49,6 +49,18 @@ describe("canonicalizeUsage", () => {
     expect(out.reasoning_tokens).toBe(40);
   });
 
+  it("reads cached_tokens from the nested buildUsage() shape", () => {
+    // buildUsage() only emits cache reads under prompt_tokens_details. The
+    // Responses translator overwrites state.usage with that shape on
+    // response.completed, so a top-level-only read silently drops the cache
+    // count for every Responses provider (codex, grok-cli, ...).
+    const out = canonicalizeUsage(
+      buildUsage({ promptTokens: 330, completionTokens: 50, totalTokens: 380, cachedTokens: 200 })
+    );
+    expect(out.prompt_tokens).toBe(330);
+    expect(out.cached_tokens).toBe(200);
+  });
+
   it("handles no-cache usage", () => {
     const out = canonicalizeUsage({ prompt_tokens: 100, completion_tokens: 50 });
     expect(out.prompt_tokens).toBe(100);