fix(anthropic): Record thinking tokens as metric

.changeset/anthropic-thinking-tokens-metric.md

@@ -0,0 +1,5 @@
+---
+"braintrust": patch
+---
+
+fix(anthropic): Record thinking tokens as metric

e2e/scenarios/anthropic-instrumentation/__snapshots__/anthropic-v0-latest-wrapped.span-tree.txt

@@ -573,6 +573,7 @@ span_tree:
     │         }
     │       }
     │       metrics: {
+    │         "completion_reasoning_tokens": 42,
     │         "completion_tokens": 48,
     │         "prompt_cache_creation_1h_tokens": 0,
     │         "prompt_cache_creation_5m_tokens": 0,

e2e/scenarios/anthropic-instrumentation/__snapshots__/anthropic-v0-latest.span-tree.txt

@@ -573,6 +573,7 @@ span_tree:
     │         }
     │       }
     │       metrics: {
+    │         "completion_reasoning_tokens": 42,
     │         "completion_tokens": 48,
     │         "prompt_cache_creation_1h_tokens": 0,
     │         "prompt_cache_creation_5m_tokens": 0,

e2e/scenarios/anthropic-instrumentation/assertions.ts

@@ -746,7 +746,12 @@ export function defineAnthropicInstrumentationAssertions(options: {
           time_to_first_token: expect.any(Number),
           prompt_tokens: expect.any(Number),
           completion_tokens: expect.any(Number),
+          completion_reasoning_tokens: expect.any(Number),
         });
+        const metrics = span?.metrics as Record<string, number>;
+        expect(metrics.completion_reasoning_tokens).toBeLessThanOrEqual(
+          metrics.completion_tokens,
+        );
         expect(
           output?.content?.some((block) => block.type === "thinking"),
         ).toBe(true);

js/src/instrumentation/plugins/anthropic-plugin.test.ts

@@ -159,6 +159,42 @@ describe("parseMetricsFromUsage", () => {
     });
   });
 
+  it("should map output_tokens_details.thinking_tokens to completion_reasoning_tokens", () => {
+    const usage = {
+      input_tokens: 100,
+      output_tokens: 80,
+      output_tokens_details: {
+        thinking_tokens: 20,
+      },
+    };
+
+    const result = parseMetricsFromUsageForTest(usage);
+
+    expect(result).toEqual({
+      prompt_tokens: 100,
+      completion_tokens: 80,
+      completion_reasoning_tokens: 20,
+    });
+  });
+
+  it("should ignore a null or non-number output token breakdown", () => {
+    expect(
+      parseMetricsFromUsageForTest({
+        input_tokens: 100,
+        output_tokens: 80,
+        output_tokens_details: null,
+      }),
+    ).toEqual({ prompt_tokens: 100, completion_tokens: 80 });
+
+    expect(
+      parseMetricsFromUsageForTest({
+        input_tokens: 100,
+        output_tokens: 80,
+        output_tokens_details: { thinking_tokens: "not a number" },
+      }),
+    ).toEqual({ prompt_tokens: 100, completion_tokens: 80 });
+  });
+
   it("should ignore non-number token values", () => {
     const usage = {
       input_tokens: "not a number",
@@ -283,6 +319,40 @@ describe("aggregateAnthropicStreamChunks", () => {
     });
   });
 
+  it("should carry thinking tokens through from message_delta without double counting", () => {
+    const chunks = [
+      {
+        type: "message_start",
+        message: {
+          usage: {
+            input_tokens: 10,
+            output_tokens: 0,
+          },
+        },
+      },
+      {
+        type: "content_block_delta",
+        delta: { type: "thinking_delta", thinking: "hmm" },
+      },
+      {
+        type: "message_delta",
+        usage: {
+          output_tokens: 80,
+          output_tokens_details: { thinking_tokens: 20 },
+        },
+      },
+    ];
+
+    const result = aggregateAnthropicStreamChunksForTest(chunks);
+
+    expect(result.metrics).toMatchObject({
+      prompt_tokens: 10,
+      completion_tokens: 80,
+      completion_reasoning_tokens: 20,
+      tokens: 90,
+    });
+  });
+
   it("should concatenate multiple text deltas", () => {
     const chunks = [
       {

js/src/instrumentation/plugins/anthropic-plugin.ts

@@ -843,6 +843,14 @@ export function parseMetricsFromUsage(
     }
   }
 
+  // Thinking tokens are already included in output_tokens.
+  if (isObject(usage.output_tokens_details)) {
+    const thinkingTokens = usage.output_tokens_details.thinking_tokens;
+    if (typeof thinkingTokens === "number") {
+      metrics.completion_reasoning_tokens = thinkingTokens;
+    }
+  }
+
   if (isObject(usage.server_tool_use)) {
     for (const [name, value] of Object.entries(usage.server_tool_use)) {
       if (typeof value === "number") {

js/src/vendor-sdk-types/anthropic.ts

@@ -195,6 +195,10 @@ export interface AnthropicUsage {
   cache_read_input_tokens?: number;
   cache_creation_input_tokens?: number;
   cache_creation?: AnthropicCacheCreationUsage | null;
+  output_tokens_details?: {
+    thinking_tokens?: number;
+    [key: string]: unknown;
+  } | null;
   server_tool_use?: AnthropicServerToolUseUsage;
   [key: string]: unknown;
 }