diff --git a/plugins/tracing/dist/index.mjs b/plugins/tracing/dist/index.mjs
index cdc80c5..f96eb74 100644
--- a/plugins/tracing/dist/index.mjs
+++ b/plugins/tracing/dist/index.mjs
@@ -46981,15 +46981,28 @@ async function seededTraceParent(config$1, sessionMeta, turnNumber) {
 		return;
 	}
 }
+function isTokenCount(value) {
+	return typeof value === "number" && Number.isSafeInteger(value) && value >= 0;
+}
+/** Send Codex's inclusive counts using Langfuse's strict OpenAI usage schema. */
 function toUsageDetails(usage) {
 	if (!usage) return void 0;
-	const details = {};
-	if (typeof usage.input_tokens === "number") details.input = usage.input_tokens;
-	if (typeof usage.output_tokens === "number") details.output = usage.output_tokens;
-	if (typeof usage.total_tokens === "number") details.total = usage.total_tokens;
-	if (typeof usage.cached_input_tokens === "number") details.cache_read_input_tokens = usage.cached_input_tokens;
-	if (typeof usage.reasoning_output_tokens === "number") details.reasoning_tokens = usage.reasoning_output_tokens;
-	return Object.keys(details).length > 0 ? details : void 0;
+	const { input_tokens: input, output_tokens: output, total_tokens: total, cached_input_tokens: cached$1, reasoning_output_tokens: reasoning } = usage;
+	if (!isTokenCount(input) || !isTokenCount(output) || !isTokenCount(total) || total !== input + output) {
+		debugLog("dropping usage: missing or inconsistent token counts", usage);
+		return;
+	}
+	if (cached$1 !== void 0 && (!isTokenCount(cached$1) || cached$1 > input) || reasoning !== void 0 && (!isTokenCount(reasoning) || reasoning > output)) {
+		debugLog("dropping usage: implausible cached/reasoning details", usage);
+		return;
+	}
+	return {
+		prompt_tokens: input,
+		completion_tokens: output,
+		total_tokens: total,
+		...cached$1 !== void 0 ? { prompt_tokens_details: { cached_tokens: cached$1 } } : {},
+		...reasoning !== void 0 ? { completion_tokens_details: { reasoning_tokens: reasoning } } : {}
+	};
 }
 /** Build a clip() that truncates long strings to `maxChars`. */
 function makeClip(maxChars) {
diff --git a/plugins/tracing/src/trace.ts b/plugins/tracing/src/trace.ts
index 3f5e2d0..24b4f0e 100644
--- a/plugins/tracing/src/trace.ts
+++ b/plugins/tracing/src/trace.ts
@@ -6,6 +6,7 @@ import {
   createTraceId,
   propagateAttributes,
   startObservation,
+  type LangfuseGenerationAttributes,
   type LangfuseObservation,
 } from "@langfuse/tracing";
 import { TraceFlags, type SpanContext } from "@opentelemetry/api";
@@ -109,19 +110,51 @@ async function seededTraceParent(
   }
 }
 
-function toUsageDetails(usage: TokenUsage | undefined): Record<string, number> | undefined {
+function isTokenCount(value: number | undefined): value is number {
+  return typeof value === "number" && Number.isSafeInteger(value) && value >= 0;
+}
+
+/** Send Codex's inclusive counts using Langfuse's strict OpenAI usage schema. */
+function toUsageDetails(
+  usage: TokenUsage | undefined,
+): LangfuseGenerationAttributes["usageDetails"] {
   if (!usage) return undefined;
-  const details: Record<string, number> = {};
-  if (typeof usage.input_tokens === "number") details.input = usage.input_tokens;
-  if (typeof usage.output_tokens === "number") details.output = usage.output_tokens;
-  if (typeof usage.total_tokens === "number") details.total = usage.total_tokens;
-  if (typeof usage.cached_input_tokens === "number") {
-    details.cache_read_input_tokens = usage.cached_input_tokens;
+  const {
+    input_tokens: input,
+    output_tokens: output,
+    total_tokens: total,
+    cached_input_tokens: cached,
+    reasoning_output_tokens: reasoning,
+  } = usage;
+
+  if (
+    !isTokenCount(input) ||
+    !isTokenCount(output) ||
+    !isTokenCount(total) ||
+    total !== input + output
+  ) {
+    debugLog("dropping usage: missing or inconsistent token counts", usage);
+    return undefined;
   }
-  if (typeof usage.reasoning_output_tokens === "number") {
-    details.reasoning_tokens = usage.reasoning_output_tokens;
+  if (
+    (cached !== undefined && (!isTokenCount(cached) || cached > input)) ||
+    (reasoning !== undefined && (!isTokenCount(reasoning) || reasoning > output))
+  ) {
+    debugLog("dropping usage: implausible cached/reasoning details", usage);
+    return undefined;
   }
-  return Object.keys(details).length > 0 ? details : undefined;
+
+  // The runtime supports this documented shape, but the SDK type still
+  // exposes only its legacy camelCase usage interface.
+  return {
+    prompt_tokens: input,
+    completion_tokens: output,
+    total_tokens: total,
+    ...(cached !== undefined ? { prompt_tokens_details: { cached_tokens: cached } } : {}),
+    ...(reasoning !== undefined
+      ? { completion_tokens_details: { reasoning_tokens: reasoning } }
+      : {}),
+  } as unknown as LangfuseGenerationAttributes["usageDetails"];
 }
 
 type Clip = {
diff --git a/plugins/tracing/src/types.ts b/plugins/tracing/src/types.ts
index 5f1de5a..36abc2c 100644
--- a/plugins/tracing/src/types.ts
+++ b/plugins/tracing/src/types.ts
@@ -110,6 +110,7 @@ export type TurnContextPayload = {
   [key: string]: unknown;
 };
 
+/** Codex reports top-level input/output counts inclusive of their details. */
 export type TokenUsage = {
   input_tokens?: number;
   output_tokens?: number;
diff --git a/plugins/tracing/test/trace.test.ts b/plugins/tracing/test/trace.test.ts
index e3c0c95..ca9890a 100644
--- a/plugins/tracing/test/trace.test.ts
+++ b/plugins/tracing/test/trace.test.ts
@@ -85,19 +85,38 @@ describe("convertRollout", () => {
 
     // Two generations, both children of the root, named "LLM" (the model name
     // lives in the model attribute, not the observation name).
-    const generations = spans.filter((s) => obsType(s) === "generation");
+    const generations = spans
+      .filter((s) => obsType(s) === "generation")
+      .sort((a, b) => startMs(a) - startMs(b));
     expect(generations).toHaveLength(2);
     for (const gen of generations) {
       expect(gen.name).toBe("LLM");
       expect(parentId(gen)).toBe(root!.spanContext().spanId);
       expect(attr(gen, "langfuse.observation.model.name")).toBe("gpt-5.4");
     }
-    // First generation carries token usage.
-    const usage = generations
-      .map((g) => attr(g, "langfuse.observation.usage_details"))
-      .find((u) => u.includes("120"));
-    expect(usage, "expected usage details with 120 total tokens").toBeTruthy();
-
+    // Usage is sent in Langfuse's OpenAI-compatible shape. Langfuse then
+    // normalizes the inclusive parent counts and nested detail counts.
+    const usages = generations.map((generation) => {
+      const usage = attr(generation, "langfuse.observation.usage_details");
+      expect(usage, "expected generation usage details").not.toBe("");
+      return JSON.parse(usage);
+    });
+    expect(usages).toEqual([
+      {
+        prompt_tokens: 100,
+        completion_tokens: 20,
+        total_tokens: 120,
+        prompt_tokens_details: { cached_tokens: 0 },
+        completion_tokens_details: { reasoning_tokens: 5 },
+      },
+      {
+        prompt_tokens: 150,
+        completion_tokens: 30,
+        total_tokens: 180,
+        prompt_tokens_details: { cached_tokens: 50 },
+        completion_tokens_details: { reasoning_tokens: 0 },
+      },
+    ]);
     // One tool span, nested under a generation, with the captured command output.
     const tools = spans.filter((s) => obsType(s) === "tool");
     expect(tools).toHaveLength(1);