fix(ai): emit per-flush deltas to keep ai.toolCalls linear step count

packages/evlog/bench/core/ai-flush.bench.ts

@@ -0,0 +1,60 @@
+import { bench, describe } from 'vitest'
+import type { LanguageModelV3 } from '@ai-sdk/provider'
+import { createAILogger } from '../../src/ai/index'
+import { createLogger } from '../../src/logger'
+import type { RequestLogger } from '../../src/types'
+
+function makeMockResult(toolName: string) {
+  return {
+    content: [{ type: 'tool-call', toolCallId: `tc-${toolName}`, toolName, args: '{}' }],
+    finishReason: { unified: 'tool-calls', raw: undefined },
+    usage: {
+      inputTokens: { total: 100, cacheRead: undefined, cacheWrite: undefined },
+      outputTokens: { total: 20, reasoning: undefined },
+    },
+    response: { modelId: 'claude-sonnet-4.6' },
+  }
+}
+
+function makeWrappedModel(steps: number) {
+  const calls: ReturnType<typeof makeMockResult>[] = []
+  for (let i = 0; i < steps; i++) calls.push(makeMockResult(`tool-${i}`))
+  let i = 0
+  return {
+    specificationVersion: 'v3',
+    provider: 'anthropic',
+    modelId: 'claude-sonnet-4.6',
+    defaultObjectGenerationMode: 'json',
+    doGenerate: () => Promise.resolve(calls[i++]),
+    doStream: () => Promise.resolve({ stream: new ReadableStream() }),
+  } as unknown as LanguageModelV3
+}
+
+async function runScenario(steps: number, withSubscriber: boolean): Promise<void> {
+  const log = createLogger()
+  const ai = createAILogger(log as unknown as RequestLogger)
+  if (withSubscriber) ai.onUpdate(() => { /* noop */ })
+  const wrapped = ai.wrap(makeWrappedModel(steps))
+  for (let i = 0; i < steps; i++) {
+    await wrapped.doGenerate({} as any)
+  }
+  log.emit({ _forceKeep: true } as any)
+}
+
+describe('multi-step agent flush throughput', () => {
+  bench('6 steps, no subscriber', async () => {
+    await runScenario(6, false)
+  })
+
+  bench('6 steps, with subscriber', async () => {
+    await runScenario(6, true)
+  })
+
+  bench('50 steps, no subscriber', async () => {
+    await runScenario(50, false)
+  })
+
+  bench('50 steps, with subscriber', async () => {
+    await runScenario(50, true)
+  })
+})