fix(openai): preserve first valid usage in stream when cost metadata frames follow

[xiaoyuyu6420]

⚠️ 提交说明 / PR Notice

Important

• 请提供人工撰写的简洁摘要，避免直接粘贴未经整理的 AI 输出。

📝 变更描述 / Description

部分 openai 兼容上游（如 opencode zen / opencode go）的 SSE 流中，含 usage 的帧之后会追加一个或多个无 usage 的 cost 元数据帧。原 OaiStreamHandler 只从最后一帧（lastStreamData）提取 usage：

• handleLastResponse 在 Usage == null 时不会把 containStreamUsage 置为 true
• 因此 usage 最终落到 ResponseText2Usage 估算路径
• 结果：prompt_tokens / completion_tokens 通常变成 0，缓存命中统计和计费异常

修复：在 StreamScannerHandler 的循环里，对 Chat Completions 模式且尚未拿到 usage 的情况，扫描每一帧的 usage 字段（用 strings.Contains 预过滤避免每帧都做完整 JSON 解析），一旦命中 service.ValidUsage 的帧就保存。后续无 usage 的 cost 帧不会覆盖；最后一帧若仍带 usage，handleLastResponse 会照常更新（已有逻辑不变）。已有的音频模型 secondLastStreamData 机制也保持不变。

🚀 变更类型 / Type of change

• 🐛 Bug 修复 (Bug fix)
• ✨ 新功能 (New feature)
• ⚡ 性能优化 / 重构 (Refactor)
• 📝 文档更新 (Documentation)

🔗 关联任务 / Related Issue

• Closes 部分提供商的openai兼容接口使用流式输出时会在含usage的SSE帧之后追加无usage的cost元数据帧导致new-api的缓存命中统计异常 #6272

✅ 提交前检查项 / Checklist

• 人工确认: 我已亲自整理并撰写此描述，没有直接粘贴未经处理的 AI 输出。
• 非重复提交: 已搜索现有 Issue/PR，未见有人处理。
• Bug fix 说明: 关联 Issue 部分提供商的openai兼容接口使用流式输出时会在含usage的SSE帧之后追加无usage的cost元数据帧导致new-api的缓存命中统计异常 #6272，已在该 issue 下方评论说明修复思路。
• 变更理解: 已理解修复仅在 OpenAI Chat Completions 流路径生效；最后一帧含 usage 时由 handleLastResponse 更新，与本修复语义不冲突。
• 范围聚焦: 仅改 relay/channel/openai/relay-openai.go（新增 ~15 行）+ 新增测试文件。
• 本地验证: 见下方运行证明。
• 安全合规: 无敏感凭据，符合项目规范。

📸 运行证明 / Proof of Work

新增回归测试 3 个（relay/channel/openai/relay_openai_stream_usage_test.go）：

```
=== RUN TestOaiStreamHandler_UsageExtractedWhenCostMetadataFrameFollows
--- PASS: TestOaiStreamHandler_UsageExtractedWhenCostMetadataFrameFollows (0.00s)
=== RUN TestOaiStreamHandler_UsageFromLastChunkWhenNoCostFrame
--- PASS: TestOaiStreamHandler_UsageFromLastChunkWhenNoCostFrame (0.00s)
=== RUN TestOaiStreamHandler_NoUsageAnywhereUsesEstimation
--- PASS: TestOaiStreamHandler_NoUsageAnywhereUsesEstimation (0.00s)
PASS
ok github.com/QuantumNous/new-api/relay/channel/openai
```

验证修复前的失败行为（临时 stash 修复代码后跑同一测试）：

```
TestOaiStreamHandler_UsageExtractedWhenCostMetadataFrameFollows
expected: 42 actual: 0
Messages: prompt_tokens should come from the chunk that carries real usage
--- FAIL
```

修复前 prompt_tokens=0（被估算路径覆盖），修复后正确拿到上游声明的 42。整个 relay/channel/openai 包测试也全过。

Summary by CodeRabbit

• Bug Fixes

  • Improved usage tracking for OpenAI streaming responses.
  • Preserved accurate usage data when later stream frames contain empty usage details.
  • Continued supporting usage estimation when upstream responses provide no usage information.

• Tests

  • Added coverage for usage extraction, cost metadata frames, and fallback estimation scenarios.

[coderabbitai]

Walkthrough

OaiStreamHandler now captures valid usage from intermediate Chat Completions SSE frames without allowing later empty cost metadata frames to overwrite it. Tests cover intermediate usage, final-chunk usage, and estimation when usage is absent.

Changes

OpenAI streaming usage handling

Layer / File(s)	Summary
Capture valid intermediate usage relay/channel/openai/relay-openai.go	Chat Completions stream scanning validates and stores usage as soon as a valid usage frame is encountered.
Validate streaming usage scenarios relay/channel/openai/relay_openai_stream_usage_test.go	SSE test helpers and cases verify preservation after cost metadata, final-chunk extraction, and estimation without upstream usage.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Sequence Diagram(s)

sequenceDiagram
  participant UpstreamSSE
  participant OaiStreamHandler
  participant UsageState
  UpstreamSSE->>OaiStreamHandler: sends SSE frame containing usage
  OaiStreamHandler->>UsageState: validates and stores usage
  UpstreamSSE->>OaiStreamHandler: sends later cost metadata frame
  OaiStreamHandler-->>UsageState: preserves stored usage

Loading

Possibly related PRs

• QuantumNous/new-api#2160: Also updates OaiStreamHandler to extract usage from non-final streaming frames.
• QuantumNous/new-api#6201: Also changes streaming usage handling in OaiStreamHandler.

Suggested reviewers: calcium-ion

Poem

I’m a rabbit guarding tokens bright,
Catching usage frames mid-flight.
Cost clouds come hopping through,
But true counts stay safe and true.
SSE streams now end just right!

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The changes match issue #6272 by retaining the first valid usage frame so later usage-less cost frames do not overwrite it.
Out of Scope Changes check	✅ Passed	The diff appears limited to the streaming usage fix and related regression tests, with no unrelated code changes.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the main fix: preserving the first valid OpenAI stream usage when later cost metadata frames appear.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

---

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[coderabbitai]

🧹 Nitpick comments (1)

relay/channel/openai/relay_openai_stream_usage_test.go (1)

89-91: 📐 Maintainability & Code Quality | 🔵 Trivial | 💤 Low value

Use assert for non-fatal value checks.

As per coding guidelines, "New or substantially rewritten Go backend tests must use testify/require for setup and fatal assertions and testify/assert for non-fatal value checks." Both of these locations use require.Equal for final value assertions; please switch them to assert.Equal (and make sure to add the github.com/stretchr/testify/assert import).

• relay/channel/openai/relay_openai_stream_usage_test.go#L89-L91: replace require.Equal with assert.Equal for token assertions.
• relay/channel/openai/relay_openai_stream_usage_test.go#L115-L116: replace require.Equal with assert.Equal for token assertions.

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@relay/channel/openai/relay_openai_stream_usage_test.go` around lines 89 - 91,
Replace the final token-value require.Equal assertions with assert.Equal in
relay/channel/openai/relay_openai_stream_usage_test.go at lines 89-91 and
115-116, and add the github.com/stretchr/testify/assert import; retain require
for setup and fatal assertions.

Source: Coding guidelines

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Nitpick comments:
In `@relay/channel/openai/relay_openai_stream_usage_test.go`:
- Around line 89-91: Replace the final token-value require.Equal assertions with
assert.Equal in relay/channel/openai/relay_openai_stream_usage_test.go at lines
89-91 and 115-116, and add the github.com/stretchr/testify/assert import; retain
require for setup and fatal assertions.

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 682b1475-ca44-4a64-be0b-865f72b97e15

📥 Commits

Reviewing files that changed from the base of the PR and between 5a6c53d and f14fae3.

📒 Files selected for processing (2)

• relay/channel/openai/relay-openai.go
• relay/channel/openai/relay_openai_stream_usage_test.go

[xiaoyuyu6420]

Closing in favor of #6070, which addresses the same root cause (first valid usage chunk overwritten by subsequent non-standard frames like OpenCode's x-opencode-type: cost) and was opened earlier.

On closer review, #6070 also covers the applyUsagePostProcessing body swap (passing the usage-bearing frame instead of lastStreamData), which this PR missed. Without that, channels that re-extract cached_tokens from the body (DeepSeek, Zhipu, Moonshot) would still not be fully fixed here, so #6070 is the more complete fix.

Redirecting review effort there.