diff --git a/packages/llm/src/protocols/anthropic-messages.ts b/packages/llm/src/protocols/anthropic-messages.ts
index 1c0dcd32a433..80642101a87a 100644
--- a/packages/llm/src/protocols/anthropic-messages.ts
+++ b/packages/llm/src/protocols/anthropic-messages.ts
@@ -171,7 +171,7 @@ const AnthropicMessagesBody = Schema.Struct(AnthropicBodyFields)
 export type AnthropicMessagesBody = Schema.Schema.Type<typeof AnthropicMessagesBody>
 
 const AnthropicUsage = Schema.Struct({
-  input_tokens: Schema.optional(Schema.Number),
+  input_tokens: optionalNull(Schema.Number),
   output_tokens: Schema.optional(Schema.Number),
   cache_creation_input_tokens: optionalNull(Schema.Number),
   cache_read_input_tokens: optionalNull(Schema.Number),
@@ -572,7 +572,7 @@ const mapFinishReason = (reason: string | null | undefined): FinishReason => {
 // `outputTokens` carries the combined total.
 const mapUsage = (usage: AnthropicUsage | undefined): Usage | undefined => {
   if (!usage) return undefined
-  const nonCached = usage.input_tokens
+  const nonCached = usage.input_tokens ?? undefined
   const cacheRead = usage.cache_read_input_tokens ?? undefined
   const cacheWrite = usage.cache_creation_input_tokens ?? undefined
   const inputTokens = ProviderShared.sumTokens(nonCached, cacheRead, cacheWrite)
diff --git a/packages/llm/test/provider/anthropic-messages.test.ts b/packages/llm/test/provider/anthropic-messages.test.ts
index 898931295849..90a0a90344a2 100644
--- a/packages/llm/test/provider/anthropic-messages.test.ts
+++ b/packages/llm/test/provider/anthropic-messages.test.ts
@@ -407,6 +407,34 @@ describe("Anthropic Messages route", () => {
     }),
   )
 
+  it.effect("retains earlier input usage when the terminal count is null", () =>
+    Effect.gen(function* () {
+      const body = sseEvents(
+        { type: "message_start", message: { usage: { input_tokens: 5 } } },
+        { type: "content_block_start", index: 0, content_block: { type: "text", text: "" } },
+        { type: "content_block_delta", index: 0, delta: { type: "text_delta", text: "Hello" } },
+        { type: "content_block_stop", index: 0 },
+        {
+          type: "message_delta",
+          delta: { stop_reason: "end_turn" },
+          usage: { input_tokens: null, output_tokens: 2 },
+        },
+        { type: "message_stop" },
+      )
+      const response = yield* LLMClient.generate(request).pipe(Effect.provide(fixedResponse(body)))
+
+      expect(response.usage).toMatchObject({
+        inputTokens: 5,
+        outputTokens: 2,
+        nonCachedInputTokens: 5,
+        totalTokens: 7,
+        providerMetadata: {
+          anthropic: { input_tokens: null, output_tokens: 2 },
+        },
+      })
+    }),
+  )
+
   it.effect("assembles streamed tool call input", () =>
     Effect.gen(function* () {
       const body = sseEvents(