fix(openai-shim): preserve final streaming usage chunks

Handle OpenAI-compatible SSE responses that send usage in a trailing empty-choices chunk so token accounting and budget enforcement stay correct.

src/services/api/openaiShim.test.ts

@@ -0,0 +1,135 @@
+import { afterEach, beforeEach, expect, test } from 'bun:test'
+import { createOpenAIShimClient } from './openaiShim.ts'
+
+type FetchType = typeof globalThis.fetch
+
+const originalEnv = {
+  OPENAI_BASE_URL: process.env.OPENAI_BASE_URL,
+  OPENAI_API_KEY: process.env.OPENAI_API_KEY,
+}
+
+const originalFetch = globalThis.fetch
+
+function makeSseResponse(lines: string[]): Response {
+  const encoder = new TextEncoder()
+  return new Response(
+    new ReadableStream({
+      start(controller) {
+        for (const line of lines) {
+          controller.enqueue(encoder.encode(line))
+        }
+        controller.close()
+      },
+    }),
+    {
+      headers: {
+        'Content-Type': 'text/event-stream',
+      },
+    },
+  )
+}
+
+function makeStreamChunks(chunks: unknown[]): string[] {
+  return [
+    ...chunks.map(chunk => `data: ${JSON.stringify(chunk)}\n\n`),
+    'data: [DONE]\n\n',
+  ]
+}
+
+beforeEach(() => {
+  process.env.OPENAI_BASE_URL = 'http://example.test/v1'
+  process.env.OPENAI_API_KEY = 'test-key'
+})
+
+afterEach(() => {
+  process.env.OPENAI_BASE_URL = originalEnv.OPENAI_BASE_URL
+  process.env.OPENAI_API_KEY = originalEnv.OPENAI_API_KEY
+  globalThis.fetch = originalFetch
+})
+
+test('preserves usage from final OpenAI stream chunk with empty choices', async () => {
+  globalThis.fetch = (async (_input, init) => {
+    const url = typeof _input === 'string' ? _input : _input.url
+    expect(url).toBe('http://example.test/v1/chat/completions')
+
+    const body = JSON.parse(String(init?.body))
+    expect(body.stream).toBe(true)
+    expect(body.stream_options).toEqual({ include_usage: true })
+
+    const chunks = makeStreamChunks([
+      {
+        id: 'chatcmpl-1',
+        object: 'chat.completion.chunk',
+        model: 'fake-model',
+        choices: [
+          {
+            index: 0,
+            delta: { role: 'assistant', content: 'hello world' },
+            finish_reason: null,
+          },
+        ],
+      },
+      {
+        id: 'chatcmpl-1',
+        object: 'chat.completion.chunk',
+        model: 'fake-model',
+        choices: [
+          {
+            index: 0,
+            delta: {},
+            finish_reason: 'stop',
+          },
+        ],
+      },
+      {
+        id: 'chatcmpl-1',
+        object: 'chat.completion.chunk',
+        model: 'fake-model',
+        choices: [],
+        usage: {
+          prompt_tokens: 123,
+          completion_tokens: 45,
+          total_tokens: 168,
+        },
+      },
+    ])
+
+    return makeSseResponse(chunks)
+  }) as FetchType
+
+  const client = createOpenAIShimClient({}) as {
+    beta: {
+      messages: {
+        create: (
+          params: Record<string, unknown>,
+          options?: Record<string, unknown>,
+        ) => Promise<unknown> & {
+          withResponse: () => Promise<{ data: AsyncIterable<Record<string, unknown>> }>
+        }
+      }
+    }
+  }
+
+  const result = await client.beta.messages
+    .create({
+      model: 'fake-model',
+      system: 'test system',
+      messages: [{ role: 'user', content: 'hello' }],
+      max_tokens: 64,
+      stream: true,
+    })
+    .withResponse()
+
+  const events: Array<Record<string, unknown>> = []
+  for await (const event of result.data) {
+    events.push(event)
+  }
+
+  const usageEvent = events.find(
+    event => event.type === 'message_delta' && typeof event.usage === 'object' && event.usage !== null,
+  ) as { usage?: { input_tokens?: number; output_tokens?: number } } | undefined
+
+  expect(usageEvent).toBeDefined()
+  expect(usageEvent?.usage?.input_tokens).toBe(123)
+  expect(usageEvent?.usage?.output_tokens).toBe(45)
+})

src/services/api/openaiShim.ts

@@ -267,6 +267,19 @@ function makeMessageId(): string {
   return `msg_${Math.random().toString(36).slice(2)}${Date.now().toString(36)}`
 }
 
+function convertChunkUsage(
+  usage: OpenAIStreamChunk['usage'] | undefined,
+): Partial<AnthropicUsage> | undefined {
+  if (!usage) return undefined
+
+  return {
+    input_tokens: usage.prompt_tokens ?? 0,
+    output_tokens: usage.completion_tokens ?? 0,
+    cache_creation_input_tokens: 0,
+    cache_read_input_tokens: 0,
+  }
+}
+
 /**
  * Async generator that transforms an OpenAI SSE stream into
  * Anthropic-format BetaRawMessageStreamEvent objects.
@@ -279,6 +292,8 @@ async function* openaiStreamToAnthropic(
   let contentBlockIndex = 0
   const activeToolCalls = new Map<number, { id: string; name: string; index: number }>()
   let hasEmittedContentStart = false
+  let lastStopReason: 'tool_use' | 'max_tokens' | 'end_turn' | null = null
+  let hasEmittedFinalUsage = false
 
   // Emit message_start
   yield {
@@ -326,6 +341,8 @@ async function* openaiStreamToAnthropic(
         continue
       }
 
+      const chunkUsage = convertChunkUsage(chunk.usage)
+
       for (const choice of chunk.choices ?? []) {
         const delta = choice.delta
 
@@ -427,16 +444,31 @@ async function* openaiStreamToAnthropic(
               : choice.finish_reason === 'length'
                 ? 'max_tokens'
                 : 'end_turn'
+          lastStopReason = stopReason
 
           yield {
             type: 'message_delta',
             delta: { stop_reason: stopReason, stop_sequence: null },
-            usage: {
-              output_tokens: chunk.usage?.completion_tokens ?? 0,
-            },
+            ...(chunkUsage ? { usage: chunkUsage } : {}),
+          }
+          if (chunkUsage) {
+            hasEmittedFinalUsage = true
           }
         }
       }
+
+      if (
+        !hasEmittedFinalUsage &&
+        chunkUsage &&
+        (chunk.choices?.length ?? 0) === 0
+      ) {
+        yield {
+          type: 'message_delta',
+          delta: { stop_reason: lastStopReason, stop_sequence: null },
+          usage: chunkUsage,
+        }
+        hasEmittedFinalUsage = true
+      }
     }
   }