From 0e6909f050eeb15e8f6c05185511f3788357ddb3 Mon Sep 17 00:00:00 2001
From: David Brailovsky <davidbrai@gmail.com>
Date: Mon, 13 Jul 2026 18:07:59 +0200
Subject: [PATCH] anthropic-messages: skip usage fields if empty (#6611)

fixes: #6567
---
 packages/ai/src/api/anthropic-messages.ts     | 40 ++++++++++---------
 .../ai/test/anthropic-sse-parsing.test.ts     | 28 +++++++++++++
 2 files changed, 49 insertions(+), 19 deletions(-)

diff --git a/packages/ai/src/api/anthropic-messages.ts b/packages/ai/src/api/anthropic-messages.ts
index e5dc8bc6..6c419bce 100644
--- a/packages/ai/src/api/anthropic-messages.ts
+++ b/packages/ai/src/api/anthropic-messages.ts
@@ -703,25 +703,27 @@ export const stream: StreamFunction<"anthropic-messages", AnthropicOptions> = (
 					}
 					// Only update usage fields if present (not null).
 					// Preserves input_tokens from message_start when proxies omit it in message_delta.
-					if (event.usage.input_tokens != null) {
-						output.usage.input = event.usage.input_tokens;
-					}
-					if (event.usage.output_tokens != null) {
-						output.usage.output = event.usage.output_tokens;
-					}
-					if (event.usage.cache_read_input_tokens != null) {
-						output.usage.cacheRead = event.usage.cache_read_input_tokens;
-					}
-					if (event.usage.cache_creation_input_tokens != null) {
-						output.usage.cacheWrite = event.usage.cache_creation_input_tokens;
-					}
-					// Anthropic reports reasoning tokens in `output_tokens_details.thinking_tokens` on the
-					// final message_delta usage (a subset of output_tokens). SDK 0.91.1 omits the field from
-					// its Usage type, so read it through a narrow cast. Verified against the live API.
-					const thinkingTokens = (event.usage as { output_tokens_details?: { thinking_tokens?: number } })
-						.output_tokens_details?.thinking_tokens;
-					if (thinkingTokens != null) {
-						output.usage.reasoning = thinkingTokens;
+					if (event.usage) {
+						if (event.usage.input_tokens != null) {
+							output.usage.input = event.usage.input_tokens;
+						}
+						if (event.usage.output_tokens != null) {
+							output.usage.output = event.usage.output_tokens;
+						}
+						if (event.usage.cache_read_input_tokens != null) {
+							output.usage.cacheRead = event.usage.cache_read_input_tokens;
+						}
+						if (event.usage.cache_creation_input_tokens != null) {
+							output.usage.cacheWrite = event.usage.cache_creation_input_tokens;
+						}
+						// Anthropic reports reasoning tokens in `output_tokens_details.thinking_tokens` on the
+						// final message_delta usage (a subset of output_tokens). SDK 0.91.1 omits the field from
+						// its Usage type, so read it through a narrow cast. Verified against the live API.
+						const thinkingTokens = (event.usage as { output_tokens_details?: { thinking_tokens?: number } })
+							.output_tokens_details?.thinking_tokens;
+						if (thinkingTokens != null) {
+							output.usage.reasoning = thinkingTokens;
+						}
 					}
 					// Anthropic doesn't provide total_tokens, compute from components
 					output.usage.totalTokens =
diff --git a/packages/ai/test/anthropic-sse-parsing.test.ts b/packages/ai/test/anthropic-sse-parsing.test.ts
index e510ec55..0cdd0577 100644
--- a/packages/ai/test/anthropic-sse-parsing.test.ts
+++ b/packages/ai/test/anthropic-sse-parsing.test.ts
@@ -224,6 +224,34 @@ describe("Anthropic raw SSE parsing", () => {
 		expect(result.errorMessage).toBe(explanation);
 	});
 
+	it("treats message_delta without usage as a no-op for usage accumulation", async () => {
+		const model = getModel("anthropic", "claude-haiku-4-5");
+		const context: Context = {
+			messages: [{ role: "user", content: "Say hello.", timestamp: Date.now() }],
+		};
+		const response = createSseResponse(
+			minimalAnthropicEvents.map((event) =>
+				event.event === "message_delta"
+					? {
+							event: "message_delta",
+							data: JSON.stringify({ type: "message_delta", delta: { stop_reason: "end_turn" } }),
+						}
+					: event,
+			),
+		);
+
+		const stream = streamAnthropic(model, context, {
+			client: createFakeAnthropicClient(response),
+		});
+		const result = await stream.result();
+
+		expect(result.stopReason).toBe("stop");
+		expect(result.errorMessage).toBeUndefined();
+		expect(result.content).toEqual([{ type: "text", text: "Hello" }]);
+		expect(result.usage.input).toBe(12);
+		expect(result.usage.totalTokens).toBe(12);
+	});
+
 	it("ignores unknown SSE events after message_stop", async () => {
 		const model = getModel("anthropic", "claude-haiku-4-5");
 		const context: Context = {